The kidney consists of a huge number of functional units called nephrons. Each nephron has multiple sub-structures within it that relate to the kidney's overall function. Glomerulus - A dense network of capillaries which are under very high pressure and filter out small molecules from the blood into the bowmans capsule via a process called ultrafiltration. Only small molecules such as glucose and water are filtered out and not large molceules such as proteins, due to multiple barriers such as the basement membrane which only allow small molecules to pass through them. Proximal convuluted tubule - The main site of re-absorption. Most glucose, water, salts and small amino acids are absorbed here back into the blood via villi (with a high surface area) in a similar way to the way the gut absorbs them through villi. The loop of Henle - Facilitates water reabsorption back into the blood by pumping out NaCl into the medulla. Water then follows by osmosis. Collecting duct - The final stage of the nephron. The collecting duct responds to hormones such as ADH and helps with osmoregulation of the blood. When ADH is secreted from the pituitary gland in response to low water in the blood, aquaporins open up in the collecting duct, allowing more water to re-enter the blood via osmosis and re-raise blood water levels.
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