Question

The hormone that increases the rate of ${Ca}^{++}$ absorption from the gastrointestinal tract into the blood is

• A. Calcitriol
• B. Aldosterone
• C. Cholecystokinin
• D. Calcitonin

Answer 1

Answer: (A)

Calcitriol

The kidneys produce three important hormones erythropoietin, calcitriol ( 1,25 - dihydroxycholecalciferol ) and renin.

Erythropoietin is a peptide hormone which regulates erythropoiesis. It is a glycoprotein containing 165 amino acids. Its receptors are present on the membranes of red blood cell precursors. Binding of the hormone reduces apoptosis of these cells - multiple cells survive and can, therefore, complete their development into mature erythrocytes.

Cholecalciferol, inactive vitamin D ${}_3$ is covered to 1,25 - dihydroxycholecalciferol or hormone calcitriol in the kidney. Calcitriol stimulates the small intestine for protein synthesis allowing absorption of Ca${}^2$${}^{+}$ and phosphates. This ensures the availability of Ca${}^2$${}^{+}$ and phosphorus for bone growth. Calcitriol simultaneously activates osteoblasts to synthesise collagen.

Renin is a part of the renin - angiotensin - aldosterone system. In the case of insufficient blood flow to kidneys ( e.g. decrease in blood volume ) cells of juxtaglomerular apparatus begin the synthesis of protein renin. Renin catalysed the conversion of plasmatic angiotensinogen to angiotensin I. Angiotensin I is then converted by angiotensin converting enzyme to angiotensin II, which stimulates aldosterone synthesis and causes vasoconstriction and thus the venous return and blood volume reaching the heart.

Thus, the correct answer is option A.