In our daily lives, frequently we hear people complain, "My blood pressure's too low" or, "I've got high blood pressure." Very few of them are even aware that regulating their blood pressure is the duty of the kidneys.
Aside from many other bodily functions, the kidneys have also taken up the task of adjusting blood pressure or tension. In determining blood pressure, the most important factor is the amount of fluid in the blood vessels. Along with any increase of fluid in the veins and arteries, blood pressure increases—thereby causing long-term harm to all organs in the body. Hypertension—that is, an increase in the amount of fluid in the blood vessels poses a dangerous situation for human beings. If measures are not taken, death will result.
The body's able to sense an increase in fluid in the blood vessels, thanks to receptors located in the front chambers of the heart. The heart stretches when a large amount of fluid enters it and, in response to this, receptors send a signal to the brain, which responds by causing the renal arteries to increase their filtering action. An increase in blood pressure causes the heart to stretch even more. With this stretching, the muscle fibers are extended, releasing messenger molecules trapped within them, which then enter the bloodstream.
This "message" thus reaches the kidneys through the circulation. In response, the amount of fluid excreted from the body increases. In this way, blood pressure returns to normal, and the heart can resume its normal beating.
The kidneys' duty in regulating blood pressure doesn't end there. In circumstances where the blood pressure is too low, an enzyme called renin is secreted by specialized cells of juxtaglomerular apparatus, or JGA located in the kidneys. Renin has no direct effect in raising blood pressure, however. It combines with angiotensinogen, another substance secreted far away in the liver, to form angiotensin I in the blood.
Once again, angiotensin I has no major effect on blood pressure. This hormone is in turn converted to angiotensin II, by the action of another substance present in the lungs called angiotensin-converting enzyme, or ACE. This enzyme's sole duty is to degrade angiotensin I.
And so, the end product of all of these steps is the only molecule that directly affects blood vessels and actually brings blood pressure back to normal. If this "ultimate' molecule is not synthesized, none of its precursors can have any effect on blood pressure. Again, only when the angiotensin II molecule binds to specific receptors on the cell surface, do the vessels constrict and blood pressure increases.
Here, the most important point to consider is that each of these substances' effects are interdependent. Since the absence of one molecule means the absence of all, how could even one of these steps have evolved through chance? For if even one step cannot come about by chance, then it's impossible for all the others to arise at the same time, in one single body.
No one could argue that coincidence can confer upon the kidneys the ability of understanding, or to take precautions or make decisions. That all these details exist at the same time is a clear indication that they were created by God.
He is God—the Creator, the Maker, the Giver of Form. To Him belong the Most Beautiful Names. Everything in the heavens and Earth glorifies Him. He is the Almighty, the All-Wise.
(Qur'an, 59: 24)
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