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.