What is the physiology of urine formation?
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.