Question

The increased intestinal absorption of calcium is stimulated directly by a. parathyroid hormone. b. 1,25 -dihydroxyvitamin \(\mathrm{D}_{3}\). c. calcitonin. d. all of these.

Short answer

The factor that directly stimulates the increased intestinal absorption of calcium is 1,25-dihydroxyvitamin D3, as it promotes the absorption of calcium from the intestines and helps maintain normal blood calcium levels. Therefore, the correct answer is b. 1,25-dihydroxyvitamin D3.

Step-by-step solution

Understand the roles of parathyroid hormone, 1,25-dihydroxyvitamin D3, and calcitonin in calcium regulation

Parathyroid hormone (PTH) is produced in the parathyroid glands and plays a crucial role in controlling the homeostasis of calcium. It increases blood calcium levels by stimulating bone resorption, enhancing calcium reabsorption from the kidneys, and indirectly enhancing calcium absorption from the intestines. 1,25-dihydroxyvitamin D3 is the active form of vitamin D and is involved in the regulation of calcium and phosphate homeostasis. It promotes the absorption of calcium from the intestines and helps maintain normal blood calcium levels. Calcitonin is a hormone produced by the thyroid gland. It participates in the regulation of calcium homeostasis, primarily by inhibiting bone resorption, thus reducing blood calcium levels.

Identify the factor directly stimulating increased intestinal absorption of calcium

Among the three hormones mentioned, 1,25-dihydroxyvitamin D3 directly promotes the absorption of calcium from the intestines. Therefore, the correct answer is: b. 1,25-dihydroxyvitamin D3

Key concepts

Parathyroid Hormone

The parathyroid hormone (PTH) is central to the management of calcium levels within the human body. PTH is secreted by the parathyroid glands, which are tiny glands located adjacent to the thyroid gland. This hormone has a significant role when calcium levels in the blood drop too low. In response to low calcium, PTH is released to increase the calcium concentration. It accomplishes this by stimulating three primary actions:

• Promoting the release of calcium from bones into the bloodstream.
• Enhancing the kidney's retention of calcium, thus preventing its excretion in urine.
• Influencing the final conversion of vitamin D into its active form, which enhances calcium absorption from the gut.

These combined actions effectively raise blood calcium levels back towards the normal range. Understanding PTH’s function is essential for comprehending the fine-tuned balance of calcium in the body.

1,25-dihydroxyvitamin D3

1,25-dihydroxyvitamin D3, also known as calcitriol, is the biologically active form of vitamin D. It is synthesized in the kidneys from its precursor through a process that is tightly regulated and can be stimulated by PTH. Its primary role is the maintenance of calcium and phosphate homeostasis to support metabolic functions and bone health.

When calcium levels are low, 1,25-dihydroxyvitamin D3 comes into play by:

• Increasing the efficiency of the intestines to absorb dietary calcium and phosphate.
• Working synergistically with PTH to enhance calcium and phosphate release from bone tissue.
• Reducing the loss of calcium in urine by improving kidney reabsorption of this crucial mineral.

Therefore, when focusing on the enhancement of intestinal calcium absorption, it is 1,25-dihydroxyvitamin D3 that is directly responsible for this action — a critical point for students to note in understanding calcium regulation.

Calcitonin

Calcitonin is another hormone involved in the complex system of calcium homeostasis, although its role is often considered secondary to that of PTH and vitamin D. It is produced by the thyroid gland’s C-cells, particularly when calcium levels in the blood are high. Calcitonin serves as an antagonist to PTH, with its primary functions being to:

• Decrease bone resorption by inhibiting the activity of osteoclasts, which are the cells responsible for breaking down bone tissue.
• Promote calcium excretion by the kidneys, thereby reducing blood calcium levels.

To put it simply, this hormone helps to lower elevated blood calcium levels by directing calcium into the bone matrix and removing excess calcium via the urine. Although vital, it does not directly stimulate the intestinal absorption of calcium, but rather contributes to overall calcium modulation in the body.

Calcium Homeostasis

Calcium homeostasis refers to the regulation of calcium levels within a narrow and optimal range in the body fluids and tissues. It is critical for many physiological processes, including nerve transmission, muscle contraction, blood clotting, and the stability of bone structure.

To maintain this delicate balance, the body uses a feedback mechanism involving the hormones PTH, 1,25-dihydroxyvitamin D3, and calcitonin.

While PTH and 1,25-dihydroxyvitamin D3 work to increase blood calcium levels when they are too low, calcitonin helps to reduce them when they are too high. These hormones act on bones, kidneys, and the gastrointestinal tract to regulate calcium’s entry and exit from the bloodstream. Factors like dietary intake, age, and overall health can influence calcium homeostasis, making understanding these hormonal mechanisms key for students learning about human physiology.