KIDNEYS -

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.