Question

Explain the functions of calcitonin and parathyroid hormone with respect to bone matrix and to blood calcium level. (p. 115)

Short answer

Answer: Calcitonin and parathyroid hormone (PTH) are essential for maintaining bone matrix homeostasis and regulating blood calcium levels. Calcitonin inhibits bone resorption and promotes calcium excretion through the kidneys when blood calcium levels are high, while PTH stimulates bone resorption, enhances calcium reabsorption in the kidneys, and promotes the synthesis of active vitamin D to increase calcium absorption from the intestines when blood calcium levels are low. These hormones work together to maintain the appropriate balance between bone resorption and formation, ensuring optimal bone matrix homeostasis and keeping blood calcium levels within the proper physiological range.

Step-by-step solution

Define calcitonin and parathyroid hormone

Calcitonin is a hormone produced by the parafollicular cells (also called C cells) of the thyroid gland. This hormone plays a role in regulating blood calcium levels and bone metabolism. Parathyroid hormone (PTH) is produced by the parathyroid glands, which are small endocrine glands located behind the thyroid gland. PTH is responsible for regulating blood calcium levels by maintaining a balance of calcium reabsorption, excretion, and bone remodeling.

Describe the role of calcitonin and parathyroid hormone in bone matrix

Calcitonin and parathyroid hormone both play critical roles in maintaining bone matrix homeostasis. The bone matrix is a complex and dynamic structure composed of collagen fibers, non-collagenous proteins, and minerals. Proper maintenance of the bone matrix is essential for overall bone health and strength. Calcitonin has an inhibitory effect on bone resorption, which is the process of bone breakdown to release minerals like calcium into the bloodstream. Calcitonin acts on osteoclasts, the cells responsible for bone resorption, by inhibiting their activities and promoting their apoptosis (cell death). As a result, calcitonin helps to maintain the bone matrix by preventing excessive bone loss. Parathyroid hormone, on the other hand, acts as a key activator of bone resorption. PTH binds to osteoblasts (the cells responsible for bone formation) and stimulates them to release signaling molecules called RANKL (Receptor Activator of Nuclear factor κB Ligand) and M-CSF (Macrophage-Colony Stimulating Factor) that promote the differentiation and activation of osteoclasts. Consequently, PTH helps to maintain the balance between bone resorption and formation, ensuring optimal bone matrix homeostasis.

Explain the regulation of blood calcium levels by calcitonin and parathyroid hormone

Calcitonin and parathyroid hormone are important regulators of blood calcium levels. Blood calcium levels must be maintained within a narrow range to ensure proper physiological functioning. When blood calcium levels are high, calcitonin is released from the parafollicular cells of the thyroid gland. Calcitonin reduces blood calcium levels by inhibiting bone resorption and promoting the excretion of calcium through the kidneys. This helps to normalize blood calcium levels. In contrast, when blood calcium levels are low, parathyroid hormone is released from the parathyroid glands. PTH acts to increase blood calcium levels in three primary ways: 1) Stimulating bone resorption, releasing calcium into the bloodstream. 2) Enhancing calcium reabsorption in the kidneys, reducing the amount of calcium excreted in the urine. 3) Promoting the synthesis of active vitamin D in the kidneys (calcitriol), which in turn increases calcium absorption from the intestines. Overall, calcitonin and parathyroid hormone work together to maintain blood calcium levels within the appropriate physiological range.

Key concepts

Calcitonin

Calcitonin is an essential hormone produced by the parafollicular cells, also known as C cells, located in the thyroid gland. Its primary function is to help regulate blood calcium levels, particularly when they become too high in the bloodstream. This hormone plays a crucial role in maintaining bone health by managing the process of bone resorption.
The process of bone resorption involves breaking down bone tissue to release calcium and other minerals into the blood. Calcitonin helps inhibit this process by targeting osteoclasts, the bone cells that break down bone tissue. When calcitonin binds to these cells, it reduces their activity and promotes their apoptosis, meaning they undergo programmed cell death.

• This mechanism prevents excessive calcium from being released into the blood.
• It also helps in maintaining a healthy bone matrix.

By doing so, calcitonin not only reduces elevated blood calcium levels but also protects bone integrity and strength.

Parathyroid Hormone

Parathyroid Hormone (PTH) is another critical hormone but has quite different roles compared to calcitonin. It is manufactured by the parathyroid glands, small glands situated just behind the thyroid gland. PTH is the main player in raising blood calcium levels whenever they drop below normal.
PTH enhances calcium levels in several ways:

• It stimulates bone resorption by activating osteoclasts, which releases calcium into the bloodstream.
• It reduces calcium loss in the urine by increasing reabsorption in the kidneys.
• It promotes the formation of the active form of vitamin D, known as calcitriol. Calcitriol boosts calcium absorption from the intestines.

Through these mechanisms, PTH ensures that calcium levels in the blood are maintained within a narrow and healthy range, which is vital for numerous physiological processes, including muscle contraction and nerve transmission.

Blood Calcium Regulation

Blood calcium levels are tightly regulated by the concerted actions of calcitonin and parathyroid hormone. The regulation of calcium is crucial since calcium is fundamental for various biological functions including bone formation and maintenance, muscle contraction, blood clotting, and nervous system signaling.
When calcium levels in the blood are too high, calcitonin steps in and works to reduce these levels by suppressing the activity of osteoclasts, thus decreasing calcium release from bones. Moreover, it encourages the kidneys to excrete more calcium, further aiding in lowering blood calcium.
Conversely, when blood calcium is low, PTH is secreted to increase calcium levels. This is achieved by enhancing bone resorption, reducing kidney excretion of calcium, and ensuring better calcium absorption in the intestines through increased production of active vitamin D.
Through these complementary roles, calcitonin and PTH maintain homeostasis of blood calcium levels, ensuring the body can function properly.

Bone Matrix Homeostasis

Bone matrix homeostasis refers to the dynamic balance between the formation and resorption of bone tissue. This balance is vital in maintaining strong and healthy bones, and it is tightly regulated by calcitonin and parathyroid hormone.
The bone matrix is a composite structure, primarily composed of collagen fibers and minerals like calcium. It supports the bone's structural integrity and plays a crucial role in the body's mineral storage system.
Calcitonin helps maintain this balance by suppressing excessive bone resorption, thereby minimizing bone loss and maintaining bone mineral density.

• It acts as a check against overactive osteoclasts, preventing potential weakening of the bones.

PTH, in contrast, supports bone resorption as a mechanism to manage blood calcium levels. However, it also ensures that new bone formation follows resorption, maintaining the overall composition and health of the bone matrix.
This careful regulation by calcitonin and PTH is necessary to keep bones strong and functional while facilitating essential bodily functions involving calcium and bone turnover.