Homeostatic Regulation of Blood Calcium Levels and Bone Physiology

Flashcards testing knowledge of blood calcium regulation, parathyroid hormone, calcitriol, calcitonin, and related clinical conditions like rickets.

What initial stimulus triggers the parathyroid glands to release parathyroid hormone (PTH)?

A decrease in blood calcium level ($\text{Ca}^{2+}$).

How does parathyroid hormone (PTH) promote the production of calcitriol?

PTH stimulates enzymes within the kidney to convert calcidiol into calcitriol, which is the active hormone.

What three effector organs are targeted by parathyroid hormone (PTH) and calcitriol to raise blood calcium levels?

The bone, kidneys, and small intestine.

How do parathyroid hormone (PTH) and calcitriol act synergistically on bone tissue?

They act synergistically to increase osteoclast activity, which causes bone resorption and releases calcium from the bone into the blood.

How do parathyroid hormone (PTH) and calcitriol alter kidney function in response to low blood calcium?

They act synergistically to decrease the amount of calcium excreted in the urine.

What specific effect does calcitriol have on the small intestine?

Calcitriol increases the absorption of calcium from the small intestine into the blood.

What is the typical homeostatic range for blood calcium levels in the body?

Between $8.4$ and $10.2\,\text{mg/dL}$.

What is rickets, and what causes it?

Rickets is a childhood disease caused by a deficiency in vitamin D and is characterized by overproduction and deficient calcification of osteoid tissue.

What physical bone deformation occurs in children suffering from rickets?

They acquire a bowlegged appearance as body weight increases and the bones in their lower limbs bend.

What clinical manifestations besides bone bowing are associated with rickets?

Disturbances in growth, hypocalcemia (low blood calcium), and sometimes tetany (cramps and muscle twitches).

Which foods are high in vitamin D and help prevent rickets?

Fatty fish (salmon, halibut, and herring), mushrooms, egg yolks, and fortified foods (such as tofu, milk, and orange juice).

Why are vitamin D supplements recommended for infants in certain geographic locations?

Because ultraviolet radiation in those regions is insufficient for the skin to synthesize enough vitamin D.

Which gland and specific cell type synthesize and secrete calcitonin?

Calcitonin is released from the parafollicular cells of the thyroid gland.

What stimuli trigger the release of calcitonin?

High blood calcium levels and stress from exercise.

What are the two main physiological actions of calcitonin that reduce blood calcium levels?

It primarily inhibits osteoclast activity and stimulates the kidneys to increase the loss of calcium in urine.

Under what specific conditions does calcitonin exert its greatest influence?

Under conditions with high bone turnover, such as in growing children or in bone growth disorders like osteitis deformans.

Why are high-dose calcitonin injections ineffective as a long-term treatment for high blood calcium?

Because high doses of calcitonin decrease blood calcium levels only temporarily.