Answer to Question 1
Vitamin D (as calcitriol) simultaneously increases calcium absorption in the small intestine, decreases calcium excretion in the urine, and facilitates the release of calcium from bones:
Small intestine: Calcitriol is required for calcium absorption in the small intestine. Acting as a cell signal, it up-regulates several genes that code for proteins that transport dietary calcium into enterocytes. (Without vitamin D, these proteins are not made, and calcium absorption is severely limited.)
Kidney: Calcitriol and PTH cause the kidneys to reduce their excretion of calcium into the urine. As a result, more calcium remains in the blood.
Bone: Calcitriol and PTH together stimulate bone breakdown by osteoclasts, in turn resulting in the release of calcium into the blood.
Answer to Question 2
Joan is correct. Regardless of whether it is consumed in the diet or produced in the skin, cholecalciferol must be metabolized further before it can be used. This two-step process occurs in the liver and kidneys. First, cholecalciferol (vitamin D3) is converted to 25-hydroxyvitamin D 25-(OH) D3 in the liver. Then 25-(OH) D3 is circulated in the blood to the kidneys, where it is converted to 1,25-dihydroxyvitamin D 1,25-(OH)2 D3. It is the 1,25-(OH)2 D3 form, also called calcitriol, that is active in the body. Calcitriol is important for calcium absorption, and conversion of 25-(OH) D3 to calcitriol (1,25-(OH)2 D3) increases when calcium concentration in the blood is low. This increase is stimulated by the actions of parathyroid hormone (PTH) produced in the parathyroid glands when more calcium is needed.
Traditional animal phylogeny based on body plans.
Body plan of an arthropod, as represented by a grasshopper
The spinal cord protrudes from the body.
Types and locations of epithelial tissues.
Position of the fetus in late pregnancy results in stresses on the mother’s body.
Plant tissues compose plant organs