Origin. They develop from the mesoderm of the embryo.
The kidneys secrete three hormones: renin, erythropoetin and calcitriol.
(i) Renin. Whenever the rate of ultrafiltration falls, the cells of their juxtaglomerular complex secrete and release into blood a compound named renin.
It acts upon a plasma-protein, angiotensinogen, separating a compound, called angiotensin-II from it.
Angiotensin-ll accelerates heart beat and contract arterioles, thereby increasing blood pressure.
(ii) Erythropoietin. The oxygen shortage stimulates the kidney cells to secrete a hormone named erythropoietin into the blood. Erythropoietin stimulates the bone marrow to increase the production of RBCs.
(iii) Calcitriol. Vitamin D exists in two forms: calciferol or D and cholecalciferol or D .
Calcitriol is the active form of vitamin cholecalciferol (D ). It promotes absorption of Ca2+ and phosphorus in the small intestine and accelerates bone formation.