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Parathyroid Gland
On the backside of the thyroid gland are four pea-like structures that constitute the parathyroid gland. This gland receives its nutrients from the same blood vessel system as the thyroid gland. Inside the parathyroid gland are cells called parathyroid chief cells that are responsible for synthesizing and releasing a peptide hormone called the parathyroid hormone (PTH). Since its a peptide hormone, this implies that it is produced in the rough endoplasmic reticulum, modified in the Golgi apparatus and released into the blood stream. Inside the blood, this peptide hormone readily dissolves (water-soluble) and it does not need any sort of carrier protein for transport. Once it arrives at its target cell, it cannot pass across the cell membrane and so it binds onto a protein receptor on the membrane and initiates a secondary messenger response. The parathyroid hormone is responsible for maintaining and regulating the concentration of calcium in our serum (blood plasma). It is stimulated (or inhibited) by a change in calcium concentration in our blood. When the blood plasma has a low calcium concentration, the parathyroid gland will release the parathyroid hormone, which will aim to increase the calcium level in three important ways. First, it will increase the activity of osteoblasts (cells that break down the bone matrix and release the calcium and phosphate ions into the blood) while decreasing the activity of osteoblasts (cells that form the bone matrix by absorbing the calcium and phosphate from our blood). Second, it will act on the kidneys to make it more permeable to calcium, which means more calcium will be reabsorbed back into our body. Finally, it will manufacture the active form of vitamin D in the kidneys, which will be used to reabsorb more calcium inside out intestines. Overall, the effect of the parathyroid hormone is to increase the concentration of calcium in our blood. When the calcium level returns to normal, this will create a negative feedback loop that will inhibit the parathyroid gland from releasing the hormone. Notice that calcitonin, the peptide hormone released by the thyroid gland is also used to control the concentration of calcium in our serum. However, its effects are opposite - that is, it acts to decrease the concentration of calcium in our blood.
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