Question

The process by which PTH increases blood \(\mathrm{Ca}^{2+}\) level except (a) Acts on bones and stimulates the process of bone reabsorption/dissolution/ demineralization. (b) Reabsorption of \(\mathrm{Ca}^{2+}\) by the renal tubules. (c) Increases \(\mathrm{Ca}^{2+}\) absorption from the digested food. (d) Increases osteoblastic activity.

Short answer

Contrary to option (d), Parathyroid Hormone (PTH) actually increases osteoclastic activity rather than osteoblastic activity. Therefore, the process by which PTH increases blood \(\mathrm{Ca^{2+}}\) level does not include increasing osteoblastic activity.

Step-by-step solution

Understanding PTH and Its Functions

Parathyroid Hormone (PTH) is a hormone secreted by the parathyroid glands that regulates the serum calcium concentration through its effects on bone, kidneys, and the intestine. PTH acts to increase the concentration of calcium (\(\mathrm{Ca^{2+}}\)) in the blood.

Understanding Options A-C

Option (a), (b), and (c) are ways that PTH can increase blood \(\mathrm{Ca^{2+}}\) levels. (a) PTH acts on bones and stimulates the process of bone dissolution or demineralization, releasing \(\mathrm{Ca^{2+}}\) into the blood. (b) PTH promotes the reabsorption of \(\mathrm{Ca^{2+}}\) by the renal tubules, which prevents calcium loss in the urine. (c) PTH increases \(\mathrm{Ca^{2+}}\) absorption from the digested food by acting on the intestine.

Understanding Option D

Option (d) states PTH increases osteoblastic activity. However, this is incorrect. Osteoblasts are cells that are involved in the formation and mineralization of bone, which consume \(\mathrm{Ca^{2+}}\) from the blood, thus reducing the blood \(\mathrm{Ca^{2+}}\) level rather than increasing it. Hence, increased osteoblastic activity does not lead to increased blood \(\mathrm{Ca^{2+}}\) levels. PTH primarily increases osteoclastic activity, which breaks down bone to release \(\mathrm{Ca^{2+}}\) into the blood.

Key concepts

Calcium Homeostasis

Calcium homeostasis refers to the body's ability to maintain stable levels of calcium in the blood. This balance is crucial for many physiological processes, including muscle contraction, nerve function, and blood clotting. Parathyroid Hormone (PTH) plays a significant role in regulating calcium levels. When blood calcium levels drop, PTH is secreted by the parathyroid glands to increase calcium concentration.

• PTH stimulates the release of calcium from bones into the bloodstream.
• It also enhances the reabsorption of calcium in the kidneys, reducing calcium loss in urine.
• Additionally, PTH increases the absorption of calcium from the intestines.

By directing these different systems, PTH effectively maintains calcium homeostasis, ensuring that critical functions in the body operate smoothly.

Osteoclastic Activity

Osteoclasts are specialized cells that break down bone tissue, a process known as bone resorption. This activity is essential for bone remodeling, which is necessary for the maintenance and repair of bones.
PTH increases osteoclastic activity, leading to the release of calcium stored in bones into the blood.

• Osteoclasts dissolve the mineralized matrix of bone tissue and release calcium and phosphate into circulation.
• This process is crucial when the body needs to elevate calcium levels quickly.
• Osteoclastic activity is balanced by osteoblastic activity, which is responsible for building and mineralizing new bone tissue.

Through this balance between osteoclastic and osteoblastic activities, bone integrity and calcium levels are maintained.

Bone Demineralization

Bone demineralization refers to the process by which minerals such as calcium are removed from bone, making it less dense and more prone to fractures. While it might sound negative, under the regulation of PTH, this process is crucial for maintaining calcium balance in the bloodstream.

• When calcium levels in the blood decrease, PTH stimulates bone demineralization to release stored calcium.
• This release of calcium helps correct any deficiencies in blood calcium levels rapidly.
• However, excessive demineralization over time can lead to conditions like osteoporosis if not balanced by new bone formation.

Appropriate regulation of bone demineralization ensures that the body can respond to varying calcium requirements without compromising overall skeletal health.

Renal Tubule Reabsorption

Renal tubule reabsorption is one of the processes by which the kidneys filter and reabsorb substances from blood filtrate. PTH plays a role in increasing the reabsorption of calcium in the renal tubules, crucial for conserving calcium and maintaining optimal levels in the blood.

• PTH reduces the rate at which calcium is excreted in the urine by promoting its reabsorption.
• This helps maintain a steady supply of calcium in the bloodstream, especially when dietary intake is low.
• Efficient renal reabsorption contributes to the prevention of hypocalcemia (low blood calcium levels).

Through the action of PTH, renal tubule reabsorption ensures that the body conserves calcium, supporting various critical physiological functions.