The regulation of calcium levels in the human body

2021-06-06 01:02 PM

As soon as the readily exchangeable calcium mechanism in bone controls the concentration of extracellular fluid calcium, both the PTH and calcitonin systems react. 

Sometimes, a lot of calcium is absorbed or lost from body fluids (about 0.3 grams per hour). For example, in diarrhoea, many grams of calcium may be secreted in the intestinal fluids, through the intestinal wall, and lost in the stool each day.

In contrast, after swallowing large amounts of calcium, especially when accompanied by excessive vitamin D activity, a person can absorb quite a bit about 0.3 grams in 1 hour. The figure (0.3 grams) when compared with the total calcium in the extracellular fluid (about 1 gram), the addition or loss of 0.3 g or even less will cause severe hypercalcemia or hypocalcemia. important. But the body has a first defence response to prevent that from happening before the endocrine glands get involved.

Enhance bone calcium exchange - the first line of defence

The exchangeable calcium salts in bone are amorphous calcium phosphate compounds, presumably primarily CaHPO4 or some similar compound loosely bound in bone and in reversible equilibrium. of calcium and phosphate ions in the extracellular fluid.

The amount of salt available for exchange is about 0.5 to 1 % of the total calcium salts of the bone, which is about 5-10 grams of calcium. Due to their ease of deposition and reabsorption, when the concentration of calcium and phosphate ions in the extracellular fluid rises above normal, these salts are immediately deposited. Conversely, reducing the concentrations causes direct absorption of this salt. The reaction is fast because the amorphous crystals are very small in size and the total area in contact with the bone fluid is large (about over 1 acre).

In addition, 5% of the volume circulates through the bone per minute, which is about 1 percent of the total extracellular fluid per minute. Thus, about half of the excess calcium in the extracellular fluid removed is removed by the bone in about 70 minutes. In addition to bone, the mitochondria of many body tissues, especially the liver and intestines, contain significant amounts of readily exchangeable calcium (about 10 grams in total) providing a buffer system that helps maintain stability. of extracellular calcium concentrations.

Hormonal control of calcium ion levels - the second protection

As soon as the readily exchangeable calcium mechanism in bone controls the concentration of extracellular fluid calcium, both the PTH and calcitonin systems react. In just 3-5 minutes after an acute increase in calcium ions, the rate of PTH secretion decreases. As explained, this is through a variety of mechanisms that help bring calcium levels back to normal.

At the same time as PTH decreases, calcitonin increases. In young animals and possibly in young children, calcitonin causes rapid deposition of calcium in bone, and possibly in some cells of other tissues. Therefore, in very young animals, excessive secretion of calcitonin can cause high calcium ion concentrations to return to normal more rapidly than through a readily exchangeable calcium mechanism.

When calcium excess or deficiency is persistent, only PTH appears to be truly important in maintaining normal plasma calcium ion concentrations. When a person continues to be deficient in dietary calcium, PTH can stimulate calcium reabsorption from bone to maintain plasma calcium ion concentrations for about 1 year or more, even to the point where calcium is depleted. from bone. So, under the action, bone as a large calcium reservoir is exploited by PTH. However, when this reservoir runs out of calcium or, or, becomes saturated with calcium, the long-term control of extracellular fluid calcium ion concentrations is almost entirely dependent on PTH and vitamin D in controlling calcium absorption. from the intestines and urinary calcium excretion.