What is the physiology of urine formation?

Question

Aaron Allen · Accepted Answer

Physiology of urine formation can be discussed under three headings: ultrafiltration, selective reabsorption and tubular secretion .

Urine formation takes place in nephron , a convoluted tubule that starts from a blind, cup shaped Bowman's capsule. Nephronic tubules drain in collecting duct.

First part of nephron, the Bowman's capsule, surrounds a tuft of capillaries called glomerulus. Blood osmotic pressure is very high within glomerular capillaries-- which overcome colloidal pressure of blood and capsular hydrostatic pressure to allow ultrafiltration . Excretory product, mainly urea is supposed to be excreted by nephrons of kidney.
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- As the glomerular filtrate contains a number of molecules which are not supposed to be excreted, the proximal convoluted tubule and Henle's loop are engaged actively in reabsorption . From the accompanying illustration you may see that lots of solutes are reabsorbed including glucose and ions. Thus water also diffuses in blood from nephronic tubules.
  - A lot of water would be reabsorbed from filtrate later, along the walls of collecting ducts under the influence of antidiuretic hormone (ADH=vasopressin) of posterior pituitary. Thus amount of water excreted with urine could be controlled by pituitary/hypothalamus by changing level of ADH in blood.
  - Tubular secretion takes place mainly in distal convoluted tubule. pH of blood is maintained by secretion of free hydrogen ions in filtrate ( potassium ions are secreted when blood becomes too alkaline) and sodium ions are reabsorbed in exchange. Tubular secretion makes the urine acidic by adding free hydrogen ions.

Josiah Austin · Answer

undefined Three processes are involved in the formation of urine: filtration, reabsorption, and secretion. The glomerulus is the site of filtration, where water and solutes are forced into the renal tubule through capillary walls by blood pressure. The proximal convoluted tubule and other segments of the nephron are the sites of reabsorption, where necessary substances like water, glucose, and ions are reabsorbed back into the bloodstream. The distal convoluted tubule and collecting ducts are the main sites of secretion, where additional substances like hydrogen ions and some drugs are actively transported from the blood into the tubule. The end product is urine, which is made up of waste materials and surplus substances that the body does not need.