Biology 12

180 MHR • Unit 2 Homeostasis

Figure 6.16When the blood calcium (Ca^2 +) level is high, the

thyroid gland secretes calcitonin. Calcitonin promotes the

uptake of Ca^2 +by the bones, and therefore the blood Ca^2 +

level returns to normal. When the blood Ca^2 +level is low, the

parathyroid glands release parathyroid hormone (PTH). PTH

causes the bones to release Ca^2 +, the kidneys to re-absorb

Ca^2 +, and the intestines to absorb Ca^2 +. As a result, the

blood Ca^2 +level returns to normal.

Vitamin D

Vitamin D is a steroid hormone involved in the

regulation of blood calcium (as well as phosphate).

Vitamin D is synthesized in a multi-step process

starting in the skin and culminating in the kidney.

An inactive precursor substance, vitamin D3, is

first synthesized in the skin by the action of

ultraviolet radiation on molecules of cholesterol.

The liver converts this substance into an

intermediate product, which the kidney then uses

to produce the active form of vitamin D. This

process is regulated by the parathyroid hormone.

PTH also promotes vitamin D synthesis in the

proximal tubule of the kidney. The primary role of

vitamin D is to maintain blood calcium levels.

Vitamin D increases the release of calcium into the

Homeostasis

Blood calcium is normal at 9–10 mg/100 mL

high blood

thyroid gland

secretes calcitonin

bones

take up

blood lowers

blood^

rises

intestines

absorb kidneys

reabsorb

bones

release

parathyroid

hormone (PTH)

parathyroid glands

release PTH

low blood

activated

vitamin D

calcitonin

Ca^2 +

Ca^2 +

Ca^2 +

Ca^2 +

Ca^2 +

Ca^2 +

Ca (^2) +
Ca (^2) +