Parathyroid glands

Four small parathyroid glands are embedded on the posterior surface of the thyroid gland as it wraps around the trachea. Parathyroid hormone (PTH, parathormone) is the principal regulator of calcium metabolism. Its overall effects include:

• Increase in blood levels of calcium

• Decrease in blood levels of phosphate

Parathyroid hormone carries out these effects through multiple mechanisms of action:

Decrease in calcium excretion in the urine Increase in phosphate excretion in the urine Increase in bone resorption Activation of vitamin D3

Calcium is freely filtered along with other components of the plasma through the nephrons of the kidney. Most of this calcium is reabsorbed into the blood from the proximal tubule of the nephron. However, because the kidneys produce about 180 l of filtrate per day, the amount of calcium filtered is substantial. Therefore, the physiological regulation of even a small percentage of calcium reabsorption may have a significant effect on the amount of calcium in the blood. Parathyroid hormone acts on the Loop of Henle to increase the reabsorption of calcium from this segment of the tubule and decrease the amount excreted in the urine. This activity conserves calcium and increases its concentration in the blood.

Phosphate, which is also freely filtered with plasma through the neph-rons of the kidney, is reabsorbed into the blood from the proximal tubule. Parathyroid hormone acts on this segment to decrease phosphate reabsorption and increase the amount excreted in the urine.

Parathyroid hormone stimulates bone resorption by increasing the number and activity of osteoclasts. This demineralization process in the bone releases calcium and phosphate into the blood. Although the action of PTH on the bone appears to increase blood phosphate, its action on the kidney, which increases phosphate excretion in the urine, more than compensates for this increase and the net effect is a decrease in serum phosphate.

The final mechanism of action of PTH involves the activation of vitamin D3 through the stimulation of 1a-hydroxylase in the kidney. In the gastrointestinal tract, vitamin D3 is essential for the absorption of calcium. Enhanced absorption of calcium from dietary sources serves to further increase the concentration of calcium in the blood. Many foods, in particular, dairy products, which are rich in calcium, are fortified with vitamin D. The release of PTH from the parathyroid glands is regulated by plasma calcium levels through negative feedback. A decrease in the level of calcium in the blood stimulates the secretion of PTH and an increase in the calcium level in the blood inhibits it.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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