Тест: Calcium Homeostasis and Regulation — 11 въпроса

1. Which body compartment contains about 99% of total calcium and serves as the main structural reservoir?

A. Extracellular fluid

B. Plasma albumin

C. Intracellular cytosol

D. Bone tissue

Answer: (D)

Обяснение

About 99% of body calcium is stored in bone, where it contributes to skeletal structure. The extracellular fluid contains only a small, rapidly regulated calcium pool.

2. What is the primary function of calcium in the body?

A. It is mainly stored as a structural component in teeth only.

B. It primarily functions as an intracellular energy source.

C. It is used for muscle contraction, neuronal activity, coagulation, and bone structure.

D. It serves as an enzyme cofactor essential for metabolic reactions.

Answer: (C)

Обяснение

Calcium’s main functions include muscle contraction, neuronal activity, blood clotting, cell communication, and providing structural integrity to bones, with about 99% stored in bones.

3. What is the approximate distribution of calcium in blood between protein-bound, anion-complexed, and free ionised forms?

A. 40% protein-bound, 10% anion-complexed, 50% free ionised

B. 50% protein-bound, 40% anion-complexed, 10% free ionised

C. 50% free ionised, 25% protein-bound, 25% anion-complexed

D. 10% protein-bound, 50% anion-complexed, 40% free ionised

Answer: (A)

Обяснение

Blood calcium is divided into roughly 40% albumin-bound, 10% anion-complexed, and 50% free ionised Ca2+. The free ionised fraction is the biologically active form.

4. What percentage of the body's calcium is stored in the bone?

A. Approximately 99%

B. Less than 25%

C. Around 50%

D. About 70%

Answer: (A)

Обяснение

Approximately 99% of the body's calcium is stored in bone, serving as a major reservoir for calcium homeostasis.

5. Which bone calcium pool exchanges rapidly with plasma when calcium must be regulated quickly?

A. The marrow cavity fluid

B. The readily exchangeable soluble bone pool

C. The slowly exchangeable mineral store

D. The organic collagen matrix

Answer: (B)

Обяснение

The readily exchangeable pool is the small fraction that can move quickly between bone and plasma. The stable mineral store exchanges much more slowly through bone remodelling.

6. What is the primary purpose of the bone exchange dynamics in calcium regulation?

A. To rapidly buffer blood calcium levels during fluctuations.

B. To store excess calcium for long-term use.

C. To facilitate calcium absorption in the gastrointestinal tract.

D. To prevent calcium loss through constant wear and tear.

Answer: (A)

Обяснение

Bone exchange dynamics primarily serve to rapidly buffer blood calcium levels by exchanging calcium between bone pools and plasma, maintaining calcium homeostasis.

7. When plasma calcium is low, what is the immediate effect of PTH on the rapid bone pool?

A. It stimulates osteocyte pumps to move calcium from bone fluid to plasma

B. It directly dissolves the stable mineral store without cell mediation

C. It increases phosphate deposition into the bone fluid

D. It blocks all bone calcium movement until remodelling occurs

Answer: (A)

Обяснение

PTH stimulates pumps in osteocytes to shift calcium from the rapidly exchangeable bone fluid into plasma. The slow pool is mobilised separately through osteoclast activity.

8. When does the enzyme 1α-hydroxylase in the kidney become stimulated to produce active calcitriol?

A. In response to high serum phosphate levels

B. During increased dietary phosphate intake

C. When blood calcium levels are high

D. When blood calcium levels are low

Answer: (D)

Обяснение

1α-hydroxylase in the kidney is stimulated by low blood calcium levels via PTH, leading to increased production of active calcitriol to enhance intestinal calcium absorption.

9. How does phosphate regulation in the kidney differ from calcium regulation in terms of hormonal control and reabsorption mechanisms?

A. Phosphate levels in the kidney are regulated solely by calcitriol, while calcium depends only on PTH.

B. Phosphate reabsorption in the kidney is mainly increased by PTH, unlike calcium, which is decreased.

C. Phosphate and calcium regulation in the kidney are both primarily controlled by calcitonin without involving PTH.

D. PTH decreases phosphate reabsorption by lowering Na/P transporter activity, whereas calcium regulation involves PTH increasing renal calcium reabsorption and bone resorption.

Answer: (D)

Обяснение

PTH decreases renal phosphate reabsorption by downregulating Na/P transporters, leading to increased phosphate excretion, while calcium regulation involves PTH stimulating bone resorption and renal reabsorption of calcium, making their hormonal controls and mechanisms distinct.

10. Who is credited with discovering the role of the parathyroid gland in calcium regulation?

A. William Bayliss

B. Eugene Gley

C. Eli Metchnikoff

D. Olof Hammarsten

Answer: (D)

Обяснение

Olof Hammarsten is credited with discovering the parathyroid gland and its role in calcium regulation in the late 19th century, significantly advancing our understanding of calcium homeostasis.

11. What is the primary cause of PTH secretion modulation in response to changes in blood calcium levels?

A. Detection of low blood calcium by CaSR on chief cells triggering increased PTH secretion

B. Increased PTH secretion when blood calcium is high

C. Decreased PTH secretion when blood calcium is low

D. Blood calcium levels directly inhibit PTH secretion through the PTH receptor

Answer: (A)

Обяснение

PTH secretion is primarily controlled by calcium-sensing receptors (CaSR) on chief cells, which detect low blood calcium levels and increase PTH secretion accordingly. When blood calcium is high, CaSR activity inhibits PTH release.