Relationship between lymphatic and cardiovascular systems

In this post I will describe the flow between blood vessels and lymphatic vessels, and back. This flow is sort of cyclical as from the end part of capillaries some part of blood get into so-called tissue space. This is the space between (or outside) cells, and the liquid portion of it is called interstitial fluid.

 

Many proteins in the blood are too large to squeeze between the cell that make the blood vessels, so they stay within the cardiovascular system. But many other proteins are really small, so they can get out, and they need to have an extra way to get back to the bloodstream. And that's where the lymphatic system comes very handy.

 

Lymphatic vessels pick up these small proteins from interstitial fluid, and return them back into blood vessels through so-called subclavian vein. Once inside the lymphatic vessels, the fluid is called the lymph. This first picture shows nicely how the fluid flows from blood vessels into tissue space, and then into the lymphatic vessel.

 

 

In the body, there is a number of bacteria, gases, microbes, and metabolic waste, which needs to be drained out of the system. For that purpose, the lymph is full of white blood cells, which fight those substances. They need to be discharged or destroyed, so by the time when the lymph enter the bloodstream, it's clean and purified.

 

The composition of lymph and interstitial fluid are very similar, the main difference is their location. Plus in lymph, there is large number of lymphocytes and macrophages, which are the structures that fight foreign substances like toxins, microbes, and cancer cells.

 

Because the lymphatic system doesn't have a special pump (like the heart is for the cardiovascular system), there is a joint effort of lungs, muscular contrations, and valves inside of them, which support the movement of lymph in one direction, and stop it going the opposite direction. This second picture is another flow diagram showing the flow of lymph and blood, and the interraction between them.

 

Starvation sequence

In this post I'd like to describe the sequence of total stopping of any food intake, providing the water is still available, even if in minimal amounts. To understand the progression, it would be the best to name all the types of nutrients at the beginning, so the further explanation then makes perfect sense.  Three types of nutrients/energy are:

1. carbohydrates (or carbs)

2. fats (or lipids)

3. proteins

Because I couldn't find any flow diagram (anywhere), which would show this sequence of starvation, I was forced to improvise a little. So don't be discouraged by overt simplicity of this flow diagram. Still, (just in case) I want to clarify that symbols between the lines ( | and V ) are supposed to represent the down-pointing arrows. So let's get to it ! 

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Fed state, or so-called postabsorptive state, meaning that
all three energy sources are available for usage
 |
V
after 4 hours - carbs are spent from liver and digestive tract, while
fats and proteins levels are still intact
 |
V
12 - 24 hours - carbs spent from whole body, with
slight increase in usage of fats and proteins
 |
V
2 - 4 days - using fats after converting them into ketone bodies (<100 times of normal level), plus
using proteins in the rate of 90g a day
 |
V
4 - 40 days - further using ketone bodies from fat (100-300 times more then in fed state), plus
decreasing usage of protein each day
 |
V
40 days - 2 months - maximum usage of ketone bodies ( >300 times comparing to normal level), with
usage of proteins in the rate of 20g a day
 |
V
2 - 3 months - Body still have some fat, and ketone bodies for energy, but once the body protein level drops to about half of normal level, the physical death will occur through the infection, because the immune system will fail to work efficiently.

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Now, let's get through the whole process in slowly in the text format. After your meal, the body will be absorbing all nutrients, which is called the absorptive state. Once all the nutrients are absorbed, the next state is called post-absorptive state, signifying that the absorption is finished, and there is nothing else to digest.

Normally, those two state rotate, as with every other meal you get back into the absorptive state, and so on. But in the case, you stop eating and prolong the post-absorptive state, the first stage is called fasting. This is relatively still healthy state as you force your body to use all available (plus all excess) nutrients that are possibly accumulated in your digestive system.

Fasting changes into starvation roughly around 1-2 days after not eating any food. As I mentioned at the beginning, the water intake needs to be still available, otherwise without water, only 2-3 days will lead to dehydration, decrease of sodium levels, muscular spasm, and physical death. But that's the different story, so let's get back to our starvation sequence.

Roughly 4 hours after the end of the absorptive state and basically the beginning of post-absorptive state, all glucose in the form of glycogen in the liver is gone. That would represent about 25% of glycogen reserve in the body. The resting 75% in the skeletal muscles is then gone within 12-24 hours. This range differs with the amount of activity or rest. But basically after about 2 days of not eating, all the glycogen, and therefore the glucose in the body is gone/used.

To maintain the stable level of glucose in the blood, which is crucial for normal working of nervous system, the body converts fats into the middle-man called ketone bodies. This middle-man looks half the glucose and half amino acid, from which all the proteins are created. Glucose then feed the brain and nervous system, and amino acids are spent for protein synthesis, to save the built-in protein as much as possible.

In this state, the rate of protein usage is about 90g a day, plus with every other day after 2 days, this rate keep decreasing, finally stabilizing at the level of 20g a day in about 40 days. Meanwhile, the level of ketone bodies in the body increase about 100-300 fold. The state called the ketosis is a heavy burden for the organism, especially for the liver and kidneys, which have to take care of it.

Because ketone bodies are slightly toxic substances, they need to be eliminated from the system straight away after they are being used, and therefore, they can't be reused. Because the proteins in the blood, mainly in form of white blood cells incapacitate the toxins in ketone bodies, their level needs to be constantly updated, which is another load for protein usage.

Once the whole body protein level drops to about half, there will not be enough white blood cells for eliminating ketone bodies quickly enough. And at some point, (somewhere between 2 to 3 months), those ketones will overwhelm the immune system, and the result will the that the body will fall prey to the first virus, bacteria, or toxin which will attack the body, and will collapse.

Ironically, even in the time of death, the level of fats will be still at sufficient level to provide more ketone bodies, therefore the energy. But because of the lack of proteins, the immune system will be very frail and inefficient. So technically, the body will never run out of fat, or the energy until the end. It's the infection that will fire that fatal blow, which will result in physical death.

So I hope that this was simple and clear enough to understand the whole sequence. Of course, if at any point the new food intake is provided, the whole process will be postponed, and the cycle will start from beginning. Mainly if all three types of energy are present in fair amount of that particular meal.

Cycle of formation and breakdown of bone tissue

Even in an adult person, the bone tissues undergo the process of constant recycling. The new tissue is usually built at the same rate as the breakdown of the old one. This is achieved by the fact that cells within the bone tissue are going through the process of maturation. Using the analogy of the human life, the babies would be called preosteocytes. To translate that to English, pre means that they are not finished yet, osteo means bone, and cyte means cell.

Those then mature into an adult cell (or person in analogy), called osteocyte, literally meaning the cell of the bone. Those then differentiate into two possible types called osteoblasts and osteoclasts. The former type build new bone tissue, and the later one breakdown of the old tissue.

The flow diagram above shows the whole process nicely in cyclical manner. The vicious cycle is not really that vicious, as it's essential for life and the health. But it indirectly points out that it's going on for the duration of whole life. The old, worn-out tissue  then would be the equivalent of the pensioner (in the analogy). Still important and functioning in certain ways, but sooner or later to be broken down into the separate components, representing the death.

The bone tissue serve a number of functions, one of them is storage, and retrieval system of calcium and phosphorus, along with some other minerals. The form in which those two are stored is the called the calcium phosphate. Calcium is also used as the substance that initiate the muscle contraction (see my other post describing this process), along with some other functions.

If the working speed of osteoblasts and osteoclasts are the same, the same amount of calcium phosphate remains inside of bones. This system is so precise that even the lack or excess of dietary calcium and phosphorus won't affect it. The excess is simply excreted, and the lack is covered by slightly slowing down of osteoblasts, and slightly speeding up of osteoclasts. All this effort is important for keeping the blood levels of calcium and phosphorus within the narrow and healthy range.

The only way to increase the amount of stored calcium phosphate in bones is via vigorous muscular activity, just like the only way to decrease it is via the of the use of muscles. That's why bed-ridden patients show significant reduction in the bone density within months, which can be then reversed by subsequent activity or sport - as it's demonstrated by many athletes and body-builders.

Also, the increase and decrease in bone density goes hand in hand with the increase and decrease of the volume of the muscle tissue. One without the other is not possible, along with many other improvements and declines of efficiency in other tissues and organs throughout the body. The heart efficiency, lung's maximum volume/capacity, the amount of hemoglobin in the blood, total amount/length of blood vessels, total amount of body fat, and the production of certain hormones - would be only a few examples.

So as you can see many processes within the human body are linked and directly influence each other. Pretty basic fact, but quite  fascinating too. The human body is just so full of wonders, so never stop wondering and pondering. See you in next post.

Life cycle of red blood cells

Red blood cells (or RBCs), also medically called erythrocytes, have a lifetime around 120 days, during which they appear and operate at many different places. Then, they undertake the process of breaking them down, once they become old. So it would be the best to start at the beginning, meaning the production of them.

 

They are created in red bone marrow, meaning inside of bones. For their production, there is a need of certain nutrients and conditions to be present. Those are the molecule of iron (3 atoms of Fe joined together), vitamin B12, Folic acid, hormone erythopoietin, and protein globin. I will get to the explanation of each of those as I go along in the post.

 

First let's look at the RBC from the structural level. One such a cell is filled up with a lots of proteins called the hemoglobin (about 280 millions). This hemoglobin consists of two basic parts - the heme and the globin. Already mentioned globin is a protein created from two (alpha and beta) polypeptide chains, which is basically many amino acids (building blocks) connected to each other into long, but at many points twisted chains. The precise sequence of different amino acids in that chain is dictated by DNA.

 

Every molecule of hemoglobin has 2 of these chains and 4 hemes, the parts in the middle of which resides the molecule of iron. So once the RBC is finished in the red bone marrow, it enters the bloodstream, where it does its job for already mentioned 120 days. Now would be a good time to say something about that job they do.

 

The job of RBCs

 

In the lungs, the hemoglobin picks up oxygen, super nitric oxide (SNO), along with some other gases. Then it carries them to the tissues, where they are being used as the fuel, and many other functions. Once those gases are unloaded, the space is straight away filled up with carbon dioxide, nitric oxide (NO), and some other gases. Those are then carried back to lungs to get rid of them by breathing out, as they represent the waste products of metabolism and many other functions.

 

I think that it's worth to mention that all this loading and unloading is happening by the process called the diffusion, which is very fast system. During those 120 days, they undergo certain changes, and become "sort of" worn out. And they need to be broken down into their initial components, so let's go to explaining that.

 

Breaking down of RBCs

 

Firstly, in the liver or spleen, they split into those two basic parts - the heme and the globin. Each of them then follow separate routes. The globin is broken down back into amino acids, which enter the bloodstream, and are further being used for the synthesis of another proteins. So no wasting in here.

 

The heme part, which is much, much smaller then the globin, splits into the molecules of iron and biliverdin, which is the green pigment, that is straight away converted into bilirubin - the yellow pigment. Both parts (iron and bilirubin) then enter the bloodstream and then they get to the liver. The liver then sends the iron back to the red bone marrow using the transport protein called transferrin, which grabs the iron molecule and looks after the safe delivery of it throughout the bloodstream.

 

Once the iron is back in bones, it's used again for creating a brand new RBC. It only has to add the missing parts - the globin, vitamin B12 and Folic acid (which both serve as catalysts), and the hormone erythopoietin, which function as signalling molecule, produced by kidneys. This hormone basically brings the information about how many of new RBCs are needed to be done.

 

Ok so that was the iron part, now let's get back to the bilirubin part, which is slighly more complex. Hopefully not too much. The liver sends this bilirubin into the small intestine via the bile, from where it moves down to the large intestine. There, some friendly bacteria convert it into the urobilinogen, which can get out by two possible ways.

 

It's either converted to stercobilin, still in the large intestine, and go out with feces. Stercobilin is basically the brown pigment, giving feces their characteristic color. Or the other way is that they are moved from the large intestine to kidneys, where it's converted into urobilin, and get out in the urine.

 

So this is it. The whole life cycle of one RBC. Probably sounds as a long and complicated journey on the first reading, but I'm sure that on the second one, it might just make sense easily. I prepared two flow diagrams, which I present one above the other, partly because I couldn't decide which one is better (they are both brilliant). But mostly, because this way, you can actually see a lots of small details in which they differ, giving you even bigger picture, and easier understanding.

 

 

 

 

Still some important info needs to be said. The production and breaking down of RBCs are normally at the same rate, so the number of them in the body stays constant. The only two (still healthy) exeptions would be:

 

1. going and staying in the area of higher altitude then usually, where the concentration of oxygen is lower, so the body will have to conpensate this change by making and keeping more RBCs, or

2. after some heavy-duty exercise, the oxygen consumption can be go up even to 20 times higher, so the deficit is then made up by making some extra RBCs.

 

I said still healthy, because there is a number of unhealthy ways, meaning diseases, in which this balance can be broken. Plus, there is a dangerous procedure called Blood Doping, in which some athletes inject extra RBCs before the sporting event, giving them extra power and energy. This procedure is, of course, banned by the International Olympics Committee, as it presents a number of risks to the person's health.

 

See you next time.

 

 

Blood-brain barrier story

Let me tell you a story about nutrients travelling up the bloodstream heading towards the head. Because there is a lots of toxic and harmful substances in the blood normally, those nutrients in it get to the point, where only the smallest of them can actually get after that point. That point is simply called the blood-brain barrier (BBB), and it protects the brain from any of those harmful substances.

One of those nutrients, which actually can get through it quite easily is glucose (simple sugar), the main or almost the only food for the brain and whole nervous system. Moreover, the brain does not have any storage room for glucose, so the steady supply of it is necessary for continuous functioning of basically whole body.

But this story of travelling of glucose into the brain get even more interesting. Before actually getting in, there is a "sort of" a middle ground, where it needs to get first. That middle ground is called cerebrospinal fluid (CSF). In an average adult person, there is around 800-1500ml of that fluid in the whole body. And as a proper middle ground, it has very exact boundaries, so it can be only at certain places.

Four small cavities inside of the brain called ventricles (2 lateral, third, and fourth) are filled with it, and the inner side  of them contains special cells, which produce this fluid. From there, they travel or flow through ducts into so-called subarachnoid space, which covers the whole brain and the spine. Hence the word cerebro (brain) - spinal fluid. Because rates of its production and reabsorbtion into the bloodstream are the same, its volume and pressure stays normally constant.

Ever heard about the fact that our brains are basically floating in the fluid? Well, this is it. It literally floats in this CSF tightly packed in that small subarachnoid space. It helps to absorb any physical blows to the head, along with already mentioned supplying the brain with nutrients. Nice flow diagram shows clearly how and where this CSF travel.

This flow diagram is actually quite clever, because on the left side you can see the names of the spaces or cavities, and on the right side you can see the names of the ducts and other structures involved. Plus, within the subarachnoid space, there is a mixing of blood with CSF going on. So when you sometimes feel like scratching your head, it's probably useful to do it for enhancing this mixing and helping some glucose to get into the brain.

Doctors have been pondering and trying to figure out how to deliver certain substances past the BBB, and they came up with the solution. They created highly concentrated glucose solution containing some desirable substance, and the brain will suck it in all thinking it's all the glucose. This way certain drugs can be administered directly to the brain tissue, which would otherwise never be able to cross the BBB.

Also, the alcohol and certain anesthetics seem to have the same capacity, along with some very small molecules like ions of Na, Cl, K, Ca, Mg, carbon dioxide, and mainly the oxygen. Because despite of the brain being only about 2% of total body weight (about 1300g), it consumes about 20% of oxygen supply by lungs. Pretty busy in there, right?

Only 1-2 minutes of the blood flow interruption into the brain can impair a function of nervous system, and 4 minutes would lead to a pernament damage of the brain. I will close this post with another flow diagram, displaying nicely how the blood and CSF interract. It very simple, so there's no need to explain much in there. Enjoy studying.

Ode to the water - the life giving fluid

This post is a collection of arguments about why is the water so important for life, health, beauty, and many other things. So let's start with the starters. The water is a medium in which nearly all chemical reactions of the body occur. It's essential to health and life, mainly because of the ability to form solutions and suspensions. Because it participate as both the reactant and the product, it is really the ideal medium for metabolic reactions inside the body.

When it comes to heat, the water can absorb or release a large amount of it with only a modest change of its own temperature. A characteristic that's called the high heat capacity. Plus the water is a major part of mucus and other lubricating fluids in the body. Let me name just a few of them: saliva, blood, milk, lymph, urine, bile, semen, sweat, tears, vaginal fluids, digestive juices, cerebrospinal fluid in your spine, and synovial fluid in your joints.

Now some more technical details of the body:

For the newborn, the water content of the body is about 75%. This will get down to about 55-60% by the second year of life, and stays roughly the same for the rest of the life. Otherwise, the adipose (fat) tissue is the only tissue in body without the water in it, so fatter people has logically less proportion of water in the body than lean people.

The kidneys are the organs that take care of filtrating the blood. Each day, they filtrate about 150 litres of it in women, and about 180 litres in men. That is about 65 times the whole volume of the blood in the body, even if  99% of it goes back to the bloodstream. Which redirects the rest of  1-2% into the urine, making about 1-2 litres of urine every day.

Mass of the kidneys is changing with the age as well, so for 20 years old person it weight about 260g, but in 80 years old person, it weights only about 200g. And the filtration rate decline by 50% between ages 40-70 too. Speaking of filtration rate, it is the quickness by which kidneys can filtrate, and it is 125 ml per minute for men, and 105 ml per minute for women.

Think of it as something over one liter in about 10 minutes. That brings me to my first flow diagram, which describe the situation, where you would about to drink one liter of water, and wait less than 10 minutes, and keep on drinking. The state that your body is going to get in is called the water intoxication.

To understand this flow diagram, it's good to mention one important rule: "The water follows the salt (NaCl), and the salt follows the water". Or in other words: "Where the salt goes, the water follows". So in the first stage sweating, vomiting, and diarrhea support the loss of the salt from the body. Moreover, any further drinking the plain water, is another factor contributing to that loss. This results in the decrease of Na concentration in the plasma (liquid portion of blood), and interstitial fluid, which is the fluid in the space outside of cells (or between cells if you like).


Decreased osmolarity means that because there is less salt, then there has to be less water as well (in the same space). So then cells compensate this event by taking up the water inside of them through the process called osmosis, and they become swelled. The state of swelled cells is basically defined as the water intoxication. If the intake of water was too large at the beginning, or it is continuing, then the convulsions, coma, and possible death can occur.


Water and the sea

Now, let's go back in the evolution and make clear about how really important the water is. By the definition, most of the life forms share the marine origin, meaning that all life basically comes from the sea. Moreover, the conditions inside of cells of most organisms living on the planet, are remarkably uniform.

Or, to put it another way, the fluids inside the most of our cells have the concentration of salt highly comparable with the salt water in the sea. Because the ocean presents itself as the very stable environment for development of organisms, those organisms living outside of the sea are quite sensitive to even slight changes in the salt concentration within their cells.

Basically, the biggest problem of any organism living outside the ocean, is to maintain this salt concentration inside of cells, which is achieved only by regular intake of water and salt. Ideally, the sea salt of course. Technically, kidneys can function on as little as 0.5 litre of water for a day or so, which they keep circulating over and over. By the end of the day, however, the urine can be as much as 1000 times more acidic than the blood.

Also, the fluid balance depends highly on electrolyte balance. Electrolytes are minerals, or inorganic compounds, which dissociate into ions - atoms, molecules, or compounds with the positive or negative charge. So the regular intake of those electrolytes with the water is essential, because normally, the water loss equals the water gain during the day. This way the body fluid volume remains constant and stable. The flow diagram bellow shows clearly how the body deals with the differences in input and output of the water during the day.

Generally, the recommended intake of water is about 1 liter for every 1000 calories of solid food. So it would mean that for recommended 2500 calories a day, you need to drink about two and half of liters of water. I know I'm getting my fair share of water everyday, and especially during the exercise, where you lose additional water through sweating and increased metabolic rate of the body. That increase can be even up to 20 times higher than the metabolic rate of the body during the rest. Along with up to 20 times higher of the oxygen consumption, but that's another topic for another post.

About central dogma in biology

Central dogma in biology dates back to 1958, when F. Crick proposed a theory that explained the flow of information within the organism. Moreover, it can be represented in the form of the flow diagram, as you can see right here in the picture, which I borrowed from the book Genetic engineering - dreams and nightmares. The information flow starts from nucleic acids, which are building rocks of DNA, our genetic code. That's all in the nucleus of basically every cell.

 

In there, DNA strand is split apart, and one strand leaves nucleus as messanger RNA, or mRNA. This molecule carries information about how to make various proteins needed for the body. So out of nucleus, and in the cytoplasm, mRNA is processesed and used for protein synthesis.

 

Protein synthesis comes in four structural steps called primary, secondary, tertiary, and quaternary structures. Primary structure consists of the chain of amino acids, and secondary is already building blocks like alpha-helixes or beta-pleated sheets. Those are then used in numerous combinations to create many different proteins. Those are then transported out of the cell into the place where it will be used for building or regenerating the new or existing tissue. And that's how the organism is built up.

 

 

DISCOVERY OF REVERSE TRANSCRIPTASE

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Now we get to the point where this dogma becomes a dogma.This way of information flow was thought to be not revearsable, or simply put, it's only one-way street. But the scientific research and fast progress in genetic engineering proved that the opposite way is also possible. Study of retroviruses like Rous sarcoma virus, or bacteria like Escherichia coli (E. coli), showed that those micro-organisms contain an enzyme, which is able to create brand new DNA code. This enzyme is called reverse transcriptase.

 

In order to do this, it first creates so called complimentary DNA, or shortly cDNA. This is then compared with mRNA, because this serves as a template for reverse transcriptase. Here is where evolution and natural selection will play a part for a while, and the end/best result is then doubled and twisted by DNA polymerase - another enzyme. Well, and the new DNA is ready.

 

The research on viruses and bacteria has been proved to be very beneficial in many ways. For example many enzymes and hormones can be synthetically produced in laboratories using bacteria or viruses. This has been a huge relief from getting all those needed substances by natural ways, that is from people. Pharmacology experienced a big boom since, and is still evolving and developing, along with genetic engineering, as one of the leading research programs to date.    

Glycogen window - the essay on glycogen

Glycogen is the molecule built up from small rings of glucose (see the picture). It's structure is highly branched, which left a lot of space inbetween, and this space needs to be filled up with water. Glycogen reserves are located in the liver, holding 25% of it and the resting 75% is in the muscles.

 

Total amount of glycogen, which the body can produce and store is around 500g in average (meaning untrained) person. With the increase of muscle mass in bodybuilders or athletes, this reserve can add up to 1100g. To imagine this, take for example 1 kg loaf of white bread. That would be something over 500g of complex carbohydrates, which would be the exact amount of glycogen created from eating that whole loaf.

 

Supposing you eat that loaf on 3-4 times during the whole day. Your body needed to cover the energy for that day, and the night that follows. So when you wake up in the mornig, out of those 500g, you can have something less then half left. Plus, if you add up some exercise during the day, the demand of your body for carbohydrates will be even bigger. So here's where the phrase glycogen window start to pick up a proper meaning.

 

Glycogen window metaphorically means eating a lots of carbs and still not having enough of them for the body to process. It's like you would throw them out of window, literally. They disappear somewhere in your stomach, and get soaked into your muscles. That, of course, under the condition that you trained to stimulate growth, and provided sufficient rest for recuperation and restoring all used up nutrients.

 

Muscle growth is not facilitated only by storing extra glycogen. As mentioned earlier, additional mass contain a lots of water, as it's estimated to count 2-3g of water as an addition to every gram of glycogen itself. Plus, after training, muscles stock up on extra fats, and extra creatine phosphate to cover the energy. All together looks as pure muscle on the outside, because all those substances are inside of muscle, or around muscle fibres.

 

Otherwise, after reaching the adulthood, the number of muscle fibres stays the same for the rest of the life for everybody. Muscles are getting bigger only by storing the extra energy. And the volume can be added right into the muscle cells or fibres, or around them in so-called sarcoplasm, as an extra storage.

 

 This flow diagram shows the pathways from glucose to glycogen and back. The whole process take place in the liver cells. Glucose from blood enter liver cells and gets phosphorylated twice before start stacking them into branches of glycogen. The process of creating glycogen out of glucose is called glycogenesis, and the opposite process of breaking it down back to glucose is called glycogenolysis.

Basics of human metabolism

I found quite good flow diagram in the book Principles of anatomy and physiology, 7-th edition. It covers the  basics of metabolism, mainly the view of certain key molecules involved in it. There could be dozens of similar ones, which would involve different molecules on different pathways, because in the essence, the flow diagram is about the depicting one or more pathways of certain molecules in the body.

The metabolic pathway is defined as a series of chemical reactions that take place somewhere in the body. It can be cyclical or straightforward, and it can be executable in one direction or in both directions. The total sum of all metabolic pathways in the body is called metabolic network. 

So with the pathways, they have their starts and ends, and maybe even some milestones and/or dimension changes along the way. Good flow diagram brings all this information in simple, graphical way of covering the knowledge about human body, and systems involved within it.

This diagram focuses on chemical pathways of food that breaks down to proteins, carbohydrates and fats. Their further broke-down equivalents are amino acids, glucose and triglycerides respectively. But mainly it focuses on three key molecules, which serves as intermediates in human metabolism, and those are Acetyl co-enzyme A, Pyruvic acid, and Glucose 6-phosphate.

The last one is the ring of glucose with addition of phosphate group on sixth carbon in the chain. This altered glucose molecule, also called phosphorylated glucose, can be then used for energy into Krebs cycle, energy for DNA production, or to convert it to Pyruvic acid, the second intermediate molecule.

This one, in turn, can be used for production of alanine amino acid, or by aerobic and anaerobic reactions, to energy (ATP) with lactic acid, or to convert it to another intermediate molecule - Acetyl co-enzyme A. And this molecule can enter Krebs cycle, or to be converted to ketone bodies, cholesterol, or fatty acids. I hope it's all clear from the picture.

Each component or even word in this diagram could be enough to write about for the separate post. In whole, this is the representation of human metabolism, or at least one possible side from which it can be viewed. Plus the metabolism itself could be topic for whole books, and it is. It can be divided by particular substances into fat metabolism, protein metabolism, or carbohydrate metabolism. Or simply the metabolism of any other substance with its own characteristic pathway and chemical changes along the way.

PROCESSES

Some parts of diagram, or words, describe a process, like for example glycolysis, aerobic reactions, or Krebs cycle. So in this case, glycolysis would mean breaking down of glucose rings down to the Pyruvic acid. Aerobic reactions would be those that require oxygen for successful execution, and anaerobic reaction are on the other hand those that don't, and can work without the oxygen.

Krebs cycle represent many different reactions involving many different substances. But just to give you a hint, there is 9 steps or chemical reactions in cyclic manner, which starts and ends with the same substance - Acetyl co-enzyme A. Those then involve 9 different enzymes, 9 substrates and end products for each step. Again, each of these processes could be picked up and fill the text for the whole chapter.

So the topic is complex, and thank God there are such a things as simplifiers of it in the form of flow diagrams. The way I see it, the understanding of processes within our bodies is priceless and irreplaceable. Moreover, it cost nothing these days as there are tons of websites and library books dedicated to bringing the light on the topic. So you can watch, read, think, learn, understand and I'll see you next time around.

Muscle contraction

Muscle contraction is a complex process with one simple objective of enabling the movement to an organism. This complex process can be studied from many different levels, and the knowledge about the musculo-skeletal system reaps benefits in a number of different areas or professions. Generally, the human health during the lifespan is very closely related to normal and effective function of this particular system.

 

D I V I S I O N S

 

Two basic types of muscle contractions are isotonic and isometric. With isotonic, the muscle gets shorter during the contraction, and with isometric, it stays the same lenght. The special type of excercise called isometry is using isometric contractions for developing the strenght of muscles and joints. It's used during the rehabilitations, like for example after having a cast for certain period due the broken bone. The muscle is feeble and significantly smaller so normal exercise is out of question.

 

It's also benefitial for those confined to bed temporarily or long-term. Isometry is basically flexing the muscles without moving or twisting of arms or legs during the contraction. It can be executed by pressing against unmovable object ( for example wall or bed), or simply by pressing two hand against each other. The best thing is that it doesn't require any equipment or skill, and it can be done anywhere and by anyone.  

 

Another special type of involuntary muscle contractions would be cramp or twitching. Stretching and massage are usually very effective in calming down the muscle, as those things usually happen after prolonged exercise or fasting, where the fine balance of electrolytes in the body is disturbed.

 

Muscle contraction works on all-or-none basis as there is no such thing as partial contraction. Plus, it's followed by so-called refractory period, during which the muscle is getting back ready for another contraction.

 

There are three basic types of muscle tissue in the body:

  a) cardiac - muscle of the heart

  b) smooth - muscle in the gut / involuntary

  c) striated - skeletal / voluntary muscles

 

Striated muscles then divides further into two types:

  a) tonic - slow fibres holding mostly the posture

  b) phasic - twitch fibres of two types:

        1 - slow (red fibres)

        2 - fast (white fibres)

 

Another division could be seen depending on the conditions around the muscle contraction. More specifically, it can be done with the presence or the absence of oxygen.

  a) aerobic - with oxygen / using oxygen

  b) anaerobic - without oxygen / creating oxygen dept

 

This oxygen dept is then compensated in the body by flooding the muscle with toxic compound called lactic acid. Accumulation of this substance in the muscle gives that characteristic feeling of burning in the muscle, during and at the end of very vigorous exercise. Lactic acid needs to be quickly eliminated that's why the burning stops soon after you stop moving. Because it's toxic, it's presence is dangerous to the organism, which is dealing with it three possible ways:

 

a) oxidize to pyruvic acid

b) convert to carbs

c) neutralized and excreted

 

Lactic acid and it's effect will make you stop moving, as any further movement at that precise point is litterally painful. It can flood the blood and liver and destroy the living tissue. Regular aerobic exercise and intake of quality food increase the resistance of body towards lactic acid, and more lean muscle tissue the person possess, longer the person can excercise without being stopped by accumulated lactic acid.

 

This diagram shows nicely how muscle contraction actuallly work on the tissue level. Actin and myosin myofilaments work together to create the complex called actomyosin. This complex is then capable of sliding the actin and myosin against each other, and sort of fold them up, shortening the overall lenght of the muscle fibre. Sarcomeres is a fancy term for the muscle cells, and all this model is called sliding filament hypothesis.

 

For muscle contraction to occur, there is a number of conditions that needs to be met, and there is actually a stream of insights coming from knowing those conditions. For the sake of simplicity, I created a simple list:

 

- the presence of the protein called actin

- the presence of the protein called myosin

- the presence of Ca

- the presence of Mg

- the presence of ATP

- the presence of creatine phosphate

- calcium pumps has enough of Ca pumped in

 

Simply put, calcium pumps located on the plasma membrane, pumps in calcium, which triggers all the other processes. Creatine phosphate is another, alternative source of energy in the muscles. They chip in when there is a shortage of ATP, or the exercise is so vigorous, that all energy sources are in the action.
 

3 FORMS OF ENERGY

On molecular level, the muscle has three big sources of energy to reach for. All three are continously available at all times, what differ is the ratios between them, which correlate with the state or activity of body. Plus they differ in how long they can be available, and the time for their recovery. Three form of energy are:

1. fats - triglycerides
2. glucose - monosaccharides
3. creatine phosphate

Fats are the biggest contributor supplying heart and all energy needed for keeping the posture, along with very slow movement. Fat are continuously used by muscles even if you don't move at all. Technically, fats can never run out completelly from the whole body, but their usage by muscles is limited, because it depends on the oxygen input. And that, in turn, depends on the state of your lungs, heart, and basically the whole body. So to burn more fat than you usually do, you need to improve the state of almost whole body.

Glucose is the type of monosaccharide and serves as main energy source for the brain with whole nervous system, which runs only on glucose. Then, muscles use glucose along with its secret source of it - the glycogen. This is sort of animal version of complex carbohydrates and it can be stored in muscles and liver. From there it can be used at any point, and when you then eat carbs again, you refill it.

Interesting thing is the ratio of fats and glucose in which muscles use them. Supposing you are lying down on the bed without moving. Fat / glucose ration would be about 70/30. But the moment you start moving fat goes down and glucose up. The quicker you will move, further the fats will go down, and glucose up in the ratio. Get the picture?

Creatine phosphate is very interesting substance stored in the muscles only. It start working only when we move very fast, or for longer time. It's the additional source of energy that is available for the muscles in the times of shortage of both fats and glucose. There is quite limited amount of creatine phosphate in muscles, so suppose you start running as fast as you can, it can last for about ten seconds. When you stop, and not move for another 30 seconds, it will be replaced with freshly-new made one.

The storage of this substance can be increased by involving the body or muscles in heavy exercise in short bursts. This is the case of sprinters or bodybuilders for example. They train muscles to develop bigger creatine phosphate stores in order to achive greater performance, or appearance. Those three types of energy can be then combined to bring about the movement or sport we need.

 

 This very nice flow diagram describes the muscle contraction cycle in four steps. The diagram is pretty much self-explanatory, so it's all the question of reading and trying to understand. First I recommend to map out the symbols from the key in the grey box, and then reading the steps should make easily a lot of sense on closer observation and study. Good luck and enjoy.
 

ATP - cell's rechargeable battery

ATP, or adenosine triphosphate, could be put in an analogy of a mini rechargeable battery that cells are using to feed the energy for many different processes within the body. It contains three basic parts.  The adenine molecule, sugar molecule (ribose), and three phosphate groups - which are really one phosphorus atom surrounded by four oxygen atoms.

The energy we talk about here is in the form of high-energy bond between those phosphate groups. When they separate, the energy is released, and then used. To put back on that last phosphate group, and therefore recharging it, it requires the energy which the body needs to get from the food we eat. This principle is explained on this picture.

 

When you take away that last phosphate group, you create the molecule called ADP - adenosine diphosphate (only 2 phosphate groups). That would be the representation of empty battery in this analogy. Rotating those two chemical reactions, then provides the means of an empty and full battery, which are moving from the place of using up to the place of recharging it. And back again and again cyclically.

 

For example, one molecule of glucose can bring up the production of as much as 38 ATPs, provided there is an oxygen present in the process. This is called an aerobic respiration. In the absence of oxygen, however, only two ATPs can be made - the process called anaerobic respiration. For more detailed view of how and where ATPs are made in the body, this flow diagram does a great job.

 

Here you can see that some ATPs are made in mitochondrion, some in the cytosol, and some in the gut - where happens the basic breaking down of food from the meals we eat. In the gut, the proteins, complex carbohydrates, and lipids are digested into the final products, which are in form of amino acids, glucose, fatty acids, and glycerol respectively.

 

First round of freshly made ATPs is happening in the cytosol, where the glucose and the glycerol are used for creating it. Cytosol is semi-fluid matrix inside of the cells, in which all the organelles are suspended. Mitochondrion is a special organelle for making ATPs and many other functions. There in the middle, there is a cyclical sequence of chemical reactions called TCA, or Krebs cycle (or citric acid cycle). This represents the biggest system for ATP production in the body.

 

Stress response

The stress response, commonly known as the fight-or-flight reaction, is the first part of so-called General Adaptation Syndrome (GAS). It's a short lasting physical reaction of the body (or organism) towards the real (or even any imagined) threat. This response can be seen from multiple of levels - molecular, cellular, anatomic or atomic. It can be defined by physical or/and chemical processes which are triggered by it, or by any physical symptoms on the perception level. Or by some other means.

Three phases of GAS are:

1. stress response (fight-or-flight reaction)
2. resistance
3. exhaustion

So in the hypothetical situation where your life is in danger and you are forced to run away, the first part is very explosive and effective accumulation of energy at your disposal. If the threat persists, the next phase called resistance, is much longer sustainable state of increased level of disposable energy, but decreasing with time. After a while, depending on physical condition of that particular person, the third phase called exhaustion will follow. This is the state where the most of the energy is already spent, and the body is no longer in a position to fight or flight anymore. 

But from those three phases the first one is by far most interesting, so let's go deeper into particular physiological and chemical changes that are triggered by the stress response. It's all happening in real time, so the stress response is occurring on the observable level (to a certain degree of course). The consequences of the stress response could be put into two broad categories:

a) short-term consequences

b) long-term consequences

and those two differ substantially, so let's elaborate both of them.

SHORT-TERM

In the short-term, the stress response can be useful and helpful by presenting some information (a lesson learned), the impulse for the body to improve, or get stronger. In the short-term, it can be even perceived as a pleasurable event, providing it's happening within well-known boundaries (like for example in adrenalin sports). But it can be harmful in the case when the impulse is too strong to handle, and it slips the person into any negative experience of stress. And in the big proportions we talk about trauma (on mental level) or the injury (on physical level).

The principle behind the stress response in the short-term is to provide the organism the opportunity to improve, or strengthen itself in some way. Providing there is the sufficient and quality recovery time, and handling the stress is still manageable (to the comfortable level), the result should be the improvement of the body in dealing with that particular type of stress or condition.

The analogy could be used of learning how to drive the car. The frustration and stress associated with handling new circumstances evoke the stress response on the number of occasions. But if handled, it leads into the ability of driving the car, which in turn becomes a very useful skill in life.

LONG-TERM

By the long-term consequences of stress response I don't mean cyclical and continual short-term stress response with recovery time on day-to-day basis. But mainly it means some degree of continual residue of stress, which hasn't been handled within the manageable (or comfortable) level. Or, in the case when there isn't sufficient recovery time provided to the organism, even if the impulse itself is handled well. This stress is not really used for an improvement of the organism, but instead it builds up as some form of toxic substances within the body, or it can manifest as the lack of useful (energy) substances in the body.

Now here, we really talk about quite negative consequences of the stress response. Those can be recognizable on physical level, as well as on the perception level. It's roughly summarized in the flow diagram bellow:

Noise, heat, infection, isolation, aggression, and many more other factors persisting in the experience of the individual, can lead into some sort of negative long-term consequences of the stress response. Concrete physical symptoms then follow, for example the memory loss, the rapid (unhealthy) weight loss, ulcers, heart diseases, loss of sexual function, and many many others.

DISSECTION OF STRESS RESPONSE:

Once we have cleared the function of it, and the differences between long-term and short-term, we can now go deeper into many particular changes within the body. For easier grasping of so many processes, I created a simple list of direct physical and/or chemical processes that follow immediate stress response. In brackets, I present the particular hormone, which is responsible for that particular physical response - even if only in those cases (of course) where it can be determined. So simply put, the stress response causes:

- increased heart rate (adrenalin)
- increased blood pressure (noradrenalin)
- decreased pain sensitivity (endorphines)
- stopped/slowed down the activity of digestive system (cortisol)
- stopped/slowed down the activity of reproductive system (cortisol)
- alerted mind (adrenalin)
- enriched blood with:
    a) glucose  (glucagon)
    b) fatty acids (cortisol)
- started up/kicked in of the immune system (ACTH)
- started up the proliferation (building up) of T-cells
- inhibited production of GH (growth hormone)
- stimulated release of CRH from hypothalamus into blood
- stimulated anterior pituitary to release ACTH
- stimulated cortisol synthesizing from cholesterol in adrenal cortex
- increased carbohydrate metabolism (glucagon, cortisol)
- stopped/slowed down the tissue repair (less GH)
- increased rate of blood circulation (adrenalin+noradrenalin)
- enhanced memory formation (adrenalin)
- increased signalling of sympathetic part of nervous system
- suppressed signalling of parasympathetic part of nervous system
- prepared body to deal with challenges (adrenalin)
- inhibited inflammatory response (cortisol)
- suppressed stomach activity
- reduced secretion of digestive acids
- decreased blood flow to the walls of the stomach
- slowed down epithelial cell replacement in the gut
- reduced thickness of mucous membrane in the gut
- reduced production of reproductive hormones
- loss of sexual desire (less testosterone)
- disruption of sexual performance (less testosterone)
- enhanced brain activity - agility - learning
- more relaxed lung muscles - prepared for more oxygen getting in
- increased volume of lungs / their effectiveness

ADH negative feedback

And just as I promised to you in my last post, I'm bringing here nice flow diagram that very nicely explains how the body regulates the amount of water in the blood and therefore the body itself. It comes from GCSE book on Human biology (Letts, 1994).  The diagram is pretty much self explanatory, so I don't have to say very much about it.

Maybe only that on the most left and most right sides of the picture the text says: "Norm with just the right amount of water". It's the same text on both sides. Just wanted to make sure that you know, because it came out a bit blurry on sides. Otherwise, the picture describe so-called negative feedback loop. It's called negative because it negates any too big alteration from the constant norm level of certain molecule or hormone in the blood. If it goes too up, something happen to make it go down, and vice versa.

In this case, ADH goes up when body's reserves of water are going short, and no new water is getting into system. ADH means anti-diuretic hormone, which could be translated that it goes against of(anti) the loss of water in the urine (diuretic). That happens the way that kidney increases the process of reabsorbtion to the level that is needed, in order to save the water that is already in the system (or in this case in the blood). When ADH goes down, kidney decrease reabsorbtion of water, so more of the water in the blood end up in the urine.

The content of water in the body is controlled by Hypothalamus. This very small part of the brain regulates the content of the water by measuring the osmolarity of the blood. Osmolarity could be simply described as the amount of dissolved particles in the water. It can be solid particles or gases. To give you a clearer picture, imagine two pint glasses of water. One would be filled by pure water and in the other, there will be a lots of salt, sugar, or some other stuff in it. So the difference between those two pints would be the amount of empty space between water molecules, despite of the same volume, or even the mass of the content in the pint glass.

Now, once the Hypothalamus detect the change in the osmolarity in the blood (up or down), it signals the pituitary gland to secret more or less of ADH hormone. This hormone then travels by blood into kidneys, and those interpret it as the signal to reabsorb more or less water, respectively. The mechanism by which this is done, is the inserting the special proteins called aquaporins into the cell membranes. Those proteins work as a channel for the water to be able to cross the membrane, as the name itself suggest - the pore for the water. The more aquaporins, the more water is reabsorbed. So the less ADH hormone means less aquaporins, which means less water reabsorbed and let go into the urine. Simple as that. 

3-tier system of hormones

In this post, I will get into closer examination of endocrine or hormonal system. It's called 3-tier, because the secretion of hormones comes in three sort of "waves". The first one comes from hypothalamus, which release so-called relasing hormones, or in some older literature you can see it by the name releasing factors. Apart of those, there are release-inhibiting factors, which are hormones that act as having an opposite effect to releasing hormones.

 

Hypothalamus releases hormones into two ways. Either through the pituitary stalk into anterior pituitary lobe, or straight into posterior pituitary lobe. And those two lobes are already second wave or tier, from which series of another hormones are secreted. That would be ADH and oxytocin from posterior pituitary lobe and TSH, FSH, LH, PrL, GH, ACTH, beta-lipotropin, and beta-endorphin enkephalins from anterior pituitary lobe. All those hormones are directed towards gland or other organs, which become third tier in the system, and react to the signal by further secreting of another set of hormones.  

 

Those glands and organs could be thyroid gland or mammary gland, or liver, or nerves. For better illustration of what I have just written,  here is a picture I took from Open University course book on Human biology.  It summarizes all pathways I mentioned in nicely color flow diagram. In this case another proper example of human body flow diagram.

 

 

 

So let's start nicely to explain main hormones.

ADH is anti-diuretic hormone, which gives signal to kidney to start reabsorbing more water. When the level of water in the body is adequte, ADH will stop coming to kidney, because hypothalamus will stop releasing first set of hormones. I'll bring nice flow diagram in some future post, as this particular negative feedback deserves more attention in a separate post.

 

TSH stands for thyroid-stimulating hormone, which clearly means that thyroid gland will become stimulated for prodution and secretion its hormones thyroxin and tri-iodothyronine. Those than go out and have the effect on their target cells.

 

FSH is follicle-stimulating hormone, which means the follicle in the ovary and testes. Those produce and secrets their own hormones. For testes it's testosterone and for ovary it's progesterone and oestrogen. Those two organs are further stimulated by LH (luteinizing hormone), and ovary also with PrL (prolactin), which affects mammary gland too (for producing a milk).

 

Then we have here a growth hormone (GH), which gives signal to liver that it's time to build some more tissue - hence the growth promotion. Liver reacts by releasing IGF-1 (insulin-like growth factor), which starts the action by bringing more glucose from bloodstream into the cells, where it can be used as energy for building new tissue, or repairing the old one.

 

ACTH stands for adrenocorticotropic hormone, because it stimulates adrenal glands, and it further release the group of hormones called corticosteroids. So that would be all abbreviations from the picture above, but there is still more to come as I didn't mention no examples or releasing hormones yet. So here it is. There are two types of hormones coming from hypothalamus to anterior pituitary lobe, also called neurohormones:

 

Releasing hormones (or factors):

GnRH - gonadotropin-releasing hormone

CRH - adrenocorticotropic hormone-releasing hormone

TRH - thyroid-stimulating hormone-releasing hormone

GRH - growth hormone-releasing hormone

 

and release-inhibiting factors:

SIF - somatostatin - inhibits secretion of GH and TSH

(also known as GIF - growth hormone release-inhibiting factor)

PIF - prolactin release-inhibiting factor

 

So now I hope you have some idea about what hormonal system is. Of course, there is much more to be said about any particular hormnone or pathway, but that would be loads of pages about each one. This post serves for a rough summary, accompanied with nice flow diagram for better imagination of what is being written. Only time will tell if I expand this post in the future, but so far I think that it's deep enough. See you later. 

Level of glucose in the blood

In this post I'm bringing here one of the most typical representative of the flow diagram. This one explains the process of working on the steady level of glucose ( type of simple sugar) in the blood. This is achieved by so-called negative feedback loop. Negative, because it negates any bigger changes from the balance, and restore the level towards the stable position - within the norm. The mechanism is two-fold.

 

The pancreas creates two hormones called the insulin and the glucagon, and they act in antagonistic way, meaning they function in contrast. Insulin takes glucose from bloodstream into the cells that need it, and this way lower the level of glucose in the blood. Glucagon, on the other hand, supress the action of insulin, so the glucose in the blood starts accumulating again. If I may offer an analogy, they work just like the gas pedal and the break pedal in the car.

 

But this is all pretty basic knowledge. I took this picture out of the GCSE book on Human biology (Letts, 1994)  and technically, we talk the language of sixteen years old pupils. So let's go a bit deeper. The healthy blood glucose level is within the range of  4 to 6 mmol/l. Of course, that level fluctuates during the day as the reflection of having meals containg carbohydrates.
 

After such a meal, the level tends to rise, because the food needs to be digested and all micronutrients (results of digestion) go into the bloodstream. This is the signal for pancreas to release the insulin, which takes the glucose molecules across the cell membrane into the cells. Glucose molecule is too big to cross the plasma (cell) membrane by itself, so it needs the insulin to help widen a gap a little bit.

But then, when you don't eat for a while, the level goes down, and this is a signal for pancreas to release the glucagon, which stops the insulin from doing its job. This way the glucose can't get inside anymore, and stays in the blood. By this mechanism, the relatively stable level is achieved, falling in the healthy range of 4-6 mmol/l. This could be translated as 4-6 milimole per litre of blood, in which one milimole (or thousanth of one mole) is 180 mg of glucose.

I know that this is probably really hard to imagine, so I put it into the perspective. Fluctuation during the day within this healthy range is ok, but hit the level of 10 mmol/l, and you'll be diagnosed with hyperglycemia - which basically means having too much of glucose (sugar) in the blood. Or on the other hand, if your level sinks under 3 mmol/l, then you'll be diagnosed with hypoglycemia - which means not enough glucose in the blood.

So why is so important for the body to have always a stable supply of glucose? Well, many reasons, but probably the biggest one is that the glucose is the only food for the brain, and the whole nervous system. And you probably want those parts to work all the time, and properly. Plus the glucose is used as a fuel for movement. Fat is also a fuel, but glucose is much more readily available for immediate actions and reactions. Lipids chips in mostly when you move very slowly, or not at all (holding the position of the body). But all rapid movements, including your fingers, eyes, or ankles, are down to glucose, so without it (or with low level of it), the body would be pretty slow and stiff. That's why.

Nicely from beginning - embryology

... and that's how I became interested in embryology. Well, sort of. As soon as I saw this diagram in this Dictionary of biology (Penguin, 2004)  straight away started the process of deep study of basic terminology when it comes to development of an embryo:

blastocyst - single-layered inner cell mass (see pic.)
blastula - name of the stage of development
blastocoele - cavity in the middle of blastocyst
blastoderm - first (outer) row of cell in a blastocyst
blastomere - name of one cell in a blastocyst
blastopore - transitory opening for injecting ES cells

ES cells stands for embryonic stem cells, which are the ones that are still not differenciated into specific type. First type of cells produced in the brainstem are unspecified, or "sort of" universal, and they grow to produce a cluster of cells called morula. This is a stage prior to blastula, in which the basic cleavage (or division) of those cells occur.

One cell becomes two, four, eight, sixteen, and so on, and that's how morula is created. Then the outer layer and inner cavity is formed, and this is already second stage called blastula. Diagram shows roughly this stage, when an artificial injection of ES cells can occur. 

After that blastula follows another stage called gastrula - the stage where first differentiation happen. Firstly, cells are divided into three main categories - germ layers:

- ectoderm - outermost cells
- mesoderm - middle layer inbetween
- endoderm -  innermost cells

The ectoderm leads to development of skin, nervous system (neurons and glia), and gland cells. Mesoderm leads to bones, muscles, kidneys and gonads. Endoderm developes into yolk cells and alimentary canal (mainly epithelials in liver, lungs, stomach, pharynx, and intestine).

Mesoderm further divides into dorsal and ventral, which in layterms means the one at the back and the one in front respectively. Ectoderm further divides into three types - neural plate (neurons and glia) on one side, neural crest (cartilage) in the middle, and epidermis (skin, gland cells) on the other side. All these processes of differentiation are collectivelly named gastrulation movements, and once they are finished, the new stage called neurula will start.

Neurula, is called this way, because it signified the presence of neural plate at the begining of this stage. Neural plate is rising from ectodermal tissue, and further develop into so-called neural tube, which is the begining of the formation of spine.
 

So that would be about the embryo. In human, the embryo becomes a foetus at about seven weeks from beginning, which is marked by the appearance of first bone being formed. If you notice the blue color underlined sentence, you will probably realize that there are probably not sufficiently defined limits, where exactly the human life starts. Some would place it exactly inbetween an embryo and feotus, some at the conception, still some would go even further.

 

Many people would place it even before that - the intention of conception. So this topic and the debate remains still open, as I read here and there. What do you think?