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.

 

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.

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. 

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.