The cells, called cardiocytes, of the atria secrete a peptide, atrial natriuretic factor (ANF) which decreases blood pressure. When blood pressure is increased, ANF is secreted which causes dilation of the blood vessels. This reduces the blood pressure.
The kidneys secrete three hormones: renin, erythropoietin and calcitriol. Renin acts upon a plasma-protein, angiotensinogen, separating a compound, called angiotensin-ll from it. Angiotensin-ll stimulates adrenal cortex, accelerates heart beat and constricts arterioles, thereby increasing blood pressure. The oxygen shortage stimulates the kidney cells to secrete a hormone named erythropoietin. Erythropoietin stimulates the bone marrow to increase the production of RBCs. Calcitriol is the active form of vitamin cholecalciferol (D3). It promotes absorption of Ca2t and phosphorus in the small intestine and accelerates bone formation.
(iii) Gastrointestinal tract
Endocrine cells present in different parts of the gastrointestinal tract secrete four major peptide hormones, namely gastrin, secretin, cholecystokinin-pancreozymin (CCK-PZ) and gastric inhibitory peptide (GIP) or enterogastrone.
Gastrin acts on the gastric glands and stimulates the secretion of hydrochloric acid and pepsinogen. Secretin acts on the exocrine pancreas and stimulates secretion of water and bicarbonate ions. CCKPZ acts on both pancreas and gall bladder and stimulates the secretion of pancreatic enzymes and bile juice, respectively. GIP inhibits gastric secretion and motility.
Several other non-endocrine tissues secrete hormones called growth factors. These factors are essential for the normal growth of tissues and their repairing/regeneration.