How does the kidney adapt to a dehydrated state?

The kidney adapts to dehydration by conserving water and producing concentrated urine.

When the body is dehydrated, the kidney plays a crucial role in maintaining the body's water balance. This is achieved through a process known as osmoregulation, which involves the kidney adjusting the concentration of urine. The kidney's ability to conserve water and produce concentrated urine is a vital adaptation to dehydration.

The process begins when the hypothalamus, a part of the brain that regulates thirst and hunger, detects a decrease in the body's water content. It then releases a hormone called antidiuretic hormone (ADH) into the bloodstream. ADH travels to the kidneys and signals them to reabsorb more water from the filtrate, which is the liquid that eventually becomes urine. This results in a decrease in urine volume and an increase in its concentration, as more water is retained in the body.

The kidney's ability to concentrate urine is also facilitated by the structure of the nephron, the functional unit of the kidney. Each nephron contains a long loop of Henle, which creates a concentration gradient in the medulla of the kidney. This gradient allows for the reabsorption of water from the filtrate as it passes through the collecting ducts, further concentrating the urine.

In addition to this, the kidney also conserves water by reabsorbing sodium ions. This process is regulated by another hormone called aldosterone, which is released by the adrenal glands when the body is dehydrated. Aldosterone increases the reabsorption of sodium ions in the distal convoluted tubule and collecting duct of the nephron. As sodium ions are reabsorbed, water follows by osmosis, leading to a decrease in urine volume and an increase in its concentration.

In summary, the kidney adapts to dehydration by conserving water through the action of hormones such as ADH and aldosterone, and the structure of the nephron. This results in the production of concentrated urine, helping to maintain the body's water balance.