Parathyroid hormone and calcium homeostasis Flashcards
role of The calcium ions in the body
Bone and teeth formation: Calcium is a crucial component of bones and teeth. It provides strength and rigidity to these structures, making them resistant to fractures.
Muscle contraction: Calcium ions are essential for muscle contraction. When a nerve signal reaches a muscle cell, it triggers the release of calcium ions into the muscle fibers, which allows them to contract.
Blood clotting: Calcium ions are necessary for blood clotting. They help activate various enzymes and proteins involved in the coagulation process, which stops bleeding and promotes healing.
Nerve function: Calcium ions play a critical role in nerve function. They are involved in the transmission of nerve impulses, which allows us to move, feel, and think.
Hormone secretion: Calcium ions are required for the secretion of several hormones, including insulin and parathyroid hormone.
Enzyme activity: Many enzymes require calcium ions to function correctly. For example, the enzyme lipase, which breaks down fats in the digestive system, requires calcium ions to work.
The anatomy of the parathyroid glands
The parathyroid glands are four small endocrine glands located in the neck, behind the thyroid gland. Each parathyroid gland is typically about the size of a grain of rice, and together they weigh only a few grams. Despite their small size, these glands play an important role in regulating calcium levels in the body.
Each parathyroid gland is surrounded by a thin capsule of connective tissue and contains two types of cells: chief cells and oxyphil cells. Chief cells are responsible for producing and secreting parathyroid hormone (PTH), which helps to regulate calcium and phosphorus levels in the body. Oxyphil cells, on the other hand, are thought to be involved in the production of PTH but their exact function is not well understood.
The blood supply to the parathyroid glands comes from the inferior thyroid artery, which is a branch of the thyroid artery. The parathyroid glands also have a rich network of blood vessels and lymphatic vessels.
The parathyroid glands are closely connected to the thyroid gland, and can sometimes be difficult to distinguish from it during surgery. However, each parathyroid gland has its own blood supply and is located in a specific position in relation to the thyroid gland. The superior parathyroid glands are located at the upper pole of the thyroid gland, while the inferior parathyroid glands are located at the lower pole of the thyroid gland.
Overall, the anatomy of the parathyroid glands is relatively simple, but their function is essential for maintaining healthy levels of calcium and phosphorus in the body.
The main hormones regulating calcium homeostasis
Parathyroid hormone (PTH): This hormone is secreted by the parathyroid glands in response to low levels of calcium in the blood. PTH acts on the bones, kidneys, and intestines to increase the levels of calcium in the blood. In the bones, PTH stimulates the release of calcium from the bone matrix. In the kidneys, PTH increases the reabsorption of calcium from the urine. In the intestines, PTH indirectly increases calcium absorption by stimulating the production of vitamin D.
Calcitonin: This hormone is secreted by the thyroid gland in response to high levels of calcium in the blood. Calcitonin acts on the bones to decrease the release of calcium from the bone matrix, and it also increases the excretion of calcium by the kidneys.
Vitamin D: This hormone is actually a prohormone that is produced in the skin when it is exposed to sunlight, and it can also be obtained from the diet. Vitamin D is converted into its active form by the liver and kidneys. Its main function is to increase calcium absorption from the intestines, and it also helps to maintain normal levels of calcium and phosphorus in the blood.
Actions of PTH
Bone: PTH stimulates the activity of cells called osteoclasts, which are responsible for breaking down the bone matrix and releasing calcium into the bloodstream. This process is known as bone resorption. As a result, the concentration of calcium in the blood increases.
Kidneys: PTH increases the reabsorption of calcium by the kidneys, which prevents it from being excreted in the urine. PTH also stimulates the production of an active form of vitamin D, which enhances calcium reabsorption in the intestines. This ensures that calcium is conserved and maintained in the body.
Intestines: PTH indirectly increases calcium absorption by stimulating the production of vitamin D. Vitamin D, in turn, promotes the absorption of calcium from the diet in the intestines.
The actions of PTH, vitamin D3 and calcitonin
Parathyroid hormone (PTH):
PTH increases the levels of calcium in the blood by stimulating bone resorption, which releases calcium into the bloodstream.
PTH enhances the reabsorption of calcium by the kidneys, reducing its excretion in urine.
PTH indirectly promotes the absorption of calcium from the intestines by stimulating the production of vitamin D, which enhances calcium absorption in the intestines.
Vitamin D3:
Vitamin D3, which is produced in the skin upon exposure to sunlight, is converted into its active form, calcitriol, in the liver and kidneys.
Calcitriol enhances the absorption of calcium from the intestines by increasing the production of proteins that transport calcium across the intestinal wall.
Calcitriol promotes bone mineralization and remodeling.
Calcitriol enhances the activity of PTH on bone resorption and calcium reabsorption in the kidneys.
Calcitonin:
Calcitonin decreases the levels of calcium in the blood by inhibiting bone resorption and promoting calcium excretion by the kidneys.
Calcitonin also inhibits the production of PTH by the parathyroid glands.
The consequences of failure of calcium homeostasis
Hypocalcemia: Hypocalcemia refers to low levels of calcium in the blood. It can cause muscle cramps, seizures, numbness and tingling in the hands and feet, and heart rhythm abnormalities. Severe hypocalcemia can lead to tetany, a condition characterized by sustained muscle contractions that can cause respiratory failure and death.
Hypercalcemia: Hypercalcemia refers to high levels of calcium in the blood. It can cause nausea, vomiting, constipation, abdominal pain, confusion, and weakness. Chronic hypercalcemia can lead to kidney stones, osteoporosis, and cardiovascular disease.
Vitamin D deficiency: Vitamin D is essential for calcium absorption and bone health. A deficiency in vitamin D can lead to osteomalacia, a condition characterized by weakened bones, and an increased risk of fractures.
Parathyroid disorders: Disorders of the parathyroid glands, such as hyperparathyroidism or hypoparathyroidism, can lead to imbalances in calcium levels. Hyperparathyroidism is characterized by excessive production of PTH, which can lead to hypercalcemia and kidney stones. Hypoparathyroidism is characterized by low levels of PTH, which can cause hypocalcemia and muscle cramps.
