Question

What is the physiology of urine formation?

Answer 1

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

Explanation:

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.

To learn about glomerular filtration of blood, please read this great answer: https://socratic.org/questions/how-does-glomerular-filtration-occur?source=search

• 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.