The Skeletal System: Regulation of Blood Calcium Concentrations

1. Calcium Distribution in the Body
a. Calcium is found in most body fluids, including blood, but most of the calcium in the body is
stored in bone tissue. Bone tissue contains 99% of the calcium in the body.
b. The calcium in bone tissue is not permanently locked into bone tissue. It can be released if
needed by breaking down bone tissue (bone resorption), or more bone tissue can be
produced to store calcium in the new bone tissue.
c. The normal range for blood calcium concentrations is 8.5 mg Ca2+ /dL of blood to 11 mg
Ca2+ /dL of blood. The body uses negative feedback loops to keep blood calcium
concentrations within this narrow range.
2. Regulation of Blood Calcium Concentrations
a. Response to a drop in blood calcium concentrations
i. When blood calcium concentrations drop below 8.5 mg Ca2+ /dL of blood, this
stimulates Parathyroid cells in the parathyroid glands to secrete Parathyroid hormone
(PTH).
ii. PTH travels through the bloodstream binding to receptors in multiple locations,
causing multiple responses simultaneously that cause an increase in blood calcium
concentrations:
1. PTH increases osteoclast activity and decreases osteoblast activity, which
causes an increase in bone resorption. The increased bone resorption releases
calcium from bone tissue into the bloodstream.
2. PTH increases calcium absorption in the small intestine.
3. PTH inhibits the loss of calcium in urine at the kidneys.
b. Response to an increase in blood calcium concentrations
i. When blood calcium concentrations rise above 11 mg Ca2+ /dL of blood, this stimulates
C cells in the thyroid gland to secrete the hormone Calcitonin (CT).
ii. CT travels through the bloodstream binding to receptors in multiple locations, causing
multiple responses simultaneously that cause a decrease in blood calcium
concentrations:
1. Calcitonin inhibits osteoclast activity decreasing bone resorption and increasing
bone production. More bone tissue production removes calcium from the
blood and stores it in the new bone tissue.
2. Calcitonin reduces calcium absorption in the small intestine.
3. Calcitonin stimulates the loss of calcium from the blood into urine (calcium
excretion) in the kidneys.