Renin - Angiotensin System

Renin - Angiotensin System 

Renin is a substance secreted from JGA in response to low water level in the body.

Angiotensinogen  remains in plasma.

Renin converts it into Angiotensin-I which is then converted into angiotensin-II : this ensues : (a) secretion of more mineralocorticoids (b) constriction of afferent arteriole diameter to reduce the amount of ultrafiltrate.

Hormonal Control System :

1. ADH (Antidiuretic Hormone) enhances water reabsorption at DCT and collecting duct, its deficiency causes diabetes insipidus or excess urination (diuresis).

2. Mineralocorticoids cause retention of Na+ at DCT which is followed by water.

Other means of Osmoregulation :

Sharks and rays retain higher amount of urea and salts to raise the concentration of body fluid to prevent exosmosis.

To excrete salt  many marine bony fishes have salt glands (in gills); sea birds have nasal glands, Marine reptiles have tear glands, many elasmobranchs have rectal glands.

Reptiles and birds being uricotelic need not eliminate water with urine; urine remains in cloaca with faeces, where water is reabsorbed.