**Core Concept:** Calcium homeostasis refers to the regulation of blood calcium levels within a narrow range essential for normal physiological functions. The primary organs involved are the parathyroid glands, bones, kidneys, and the gastrointestinal tract.

**Why the Correct Answer is Right:** Organs like the heart, lungs, and brain do not play a significant role in calcium regulation due to their limited calcium-handling capabilities. Parathyroid glands, found in the neck, secrete parathyroid hormone (PTH) to increase calcium levels when calcium is low. Bones release calcium through a process called bone resorption when calcium levels are low. Kidneys regulate calcium reabsorption and excretion to maintain calcium balance. The gastrointestinal tract absorbs calcium from the diet and influences calcium absorption via vitamin D and phosphorus.

**Why Each Wrong Option is Incorrect:**

A. Heart: Although calcium is essential for cardiac muscle contraction, its regulation occurs via other mechanisms.
B. Lungs: Lungs primarily function in gas exchange, not calcium regulation.
C. Brain: Brain calcium regulation is minimal, as it is tightly regulated by other organs.
D. Liver: Although liver contributes to vitamin D synthesis, it does not play a crucial role in calcium homeostasis.

**Clinical Pearl:** Hypocalcemia, or low blood calcium levels, can be due to primary or secondary causes. Primary hypocalcemia (hypocalcemic tetany) is caused by disorders affecting parathyroid glands or bones, while secondary hypocalcemia (hypoparathyroidism) results from problems with PTH regulation. Monitoring serum calcium, phosphorus, and PTH levels help identify the underlying cause.

**Correct Answer:** D. Liver (Option D is incorrect because liver contributes to vitamin D synthesis, but not the primary regulation of calcium homeostasis.)