How the Kidneys Regulate the Water Content of the Blood

• The kidneys play a vital role in maintaining the correct water balance in the body: this function is essential for all cell operations and overall health.

• Water enters the body when we consume liquids and food, and it is also a by-product of cellular respiration.

• Excess water, along with toxins and waste from the body’s metabolic processes, is filtered out from the blood by the kidneys.

• This process occurs in tiny filtering units within the kidneys called nephrons. There are about a million nephrons in each kidney.

• In the Bowman’s Capsule, the first part of a nephron, blood is put under high pressure. This pressure causes a mixture of water, ions, and wastes to be filtered out of the blood and into the Bowman’s capsule, forming what we call ‘glomerular filtrate.’

• Through a process known as selective reabsorption, the kidneys reabsorb the necessary amounts of glucose, ions, and water back into the blood. This occurs in the proximal convoluted tubule of the nephron.

• The amount of water reabsorbed depends on how much excess water is in the body and how concentrated the blood is. This process is regulated by a hormone known as antidiuretic hormone (ADH).

• Made by the pituitary gland in the brain, ADH is released more when the body is dehydrated: causing the walls of the nephron to be more permeable to water, so more is reabsorbed back into the bloodstream.

• Conversely, when there is plenty of water in the body, less ADH is released. This makes the walls of the nephron less permeable, leading to less water reabsorption and more water going into urine, which is then excreted from the body.

• Aside from ADH, other factors that influence the kidneys’ regulation of water balance include sodium levels, the volume of the blood, and blood pressure.

• In summary, the kidneys keep the body’s water content balanced by filtering the blood, reabsorbing necessary water and ions, and excreting excess water and wastes as urine.