What is the role of osmosis in kidney function?

Osmosis plays a crucial role in kidney function by facilitating the reabsorption of water from the filtrate back into the body.

In more detail, the kidneys are responsible for filtering waste products from the blood and regulating the body's water and electrolyte balance. This is achieved through a process known as osmoregulation, which is largely dependent on osmosis. Osmosis is the movement of water molecules from an area of lower solute concentration to an area of higher solute concentration through a semi-permeable membrane.

In the kidneys, this process occurs in the nephrons, the functional units of the kidney. Each nephron consists of a glomerulus and a tubule. The glomerulus filters the blood, producing a filtrate that contains water, glucose, salts and urea. This filtrate then passes into the tubule, where reabsorption occurs.

The walls of the tubule are semi-permeable, allowing water and other small molecules to pass through while larger molecules, such as proteins, are retained. The concentration of solutes in the filtrate and the surrounding tissue fluid influences the movement of water. If the filtrate is more concentrated, water will move out of the tubule and into the surrounding tissue fluid by osmosis. Conversely, if the tissue fluid is more concentrated, water will move from the tissue fluid into the tubule.

The reabsorption of water is also regulated by the hormone vasopressin, also known as antidiuretic hormone (ADH). When the body is dehydrated, the production of ADH increases, causing the walls of the tubule to become more permeable to water. This results in more water being reabsorbed back into the body, thereby concentrating the urine.

IGCSE Biology Tutor Summary: Osmosis is essential for kidney function, helping to regulate the body's water and electrolyte balance by moving water from less concentrated to more concentrated areas across a semi-permeable membrane. In the kidneys' nephrons, it enables the reabsorption of water into the body, influenced by solute concentration and regulated by the hormone vasopressin.