How does the kidney work?

The kidney functions as an integral part of the body's fluid balance mechanism. It retains or releases various substances, such as glucose, amino acids, urea, uric acid, salts, potassium, and magnesium, acting as a highly selective filter. The blood enters the kidney and passes through a series of tubes, including the Loop of Henle, which are designed to retain certain substances and release others based on their concentration in the blood. Moreover, the kidney responds to hormonal signals, such as vasopressin, to manage fluid retention as required by the body. An interesting and sometimes surprising fact shared by Andrew Huberman is that urine is essentially filtered blood: 90% of the substances absorbed from the blood by the kidney are absorbed early in the series of tubes, with only a small percentage being regulated towards the end of the process in the distal parts of the kidney 1.

Kidney functioning is also tightly regulated by circadian rhythms. The cells of the kidney, which filter fluid intake and produce hormones like vasopressin (antidiuretic hormone), are under strong genetic control that dictates when the kidney is highly active or less so. During the first 10 hours after waking up, the kidneys filter body fluids at a rapid rate, utilizing hormones to decide whether to conserve or expel more fluid through urine. This is dependent on variables such as salt concentration, fluid needs, work output, and environmental conditions. However, after about 10 hours from waking, kidney functionality declines, becoming less efficient at filtering fluids, which is adaptive to prevent excessive filtration and urine production during nighttime hours 2.