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25.3D: Calcium and Phosphate Balance Regulation

Calcium is a key electrolyte: 99% is deposited in the bones and the remainder is associated with hormone release and cell signaling.

 

LEARNING OBJECTIVES

 

Describe calcium and phosphate balance regulation

 

KEY TAKEAWAYS

Key Points

 

  • Calcium absorption is controlled by vitamin D, and calcium excretion is controlled by the parathyroid hormones.
  • There is a constant loss of calcium by the kidney even if there is none in the diet.
  • Calcium in plasma exists in three forms: ionized, nonionized and protein bound.

 

Key Terms

 

  • calcium: A chemical element, atomic number 20, that is an alkaline earth metal and occurs naturally as carbonate in limestone and as silicate in many rocks.
  • parathyroid hormone: A polypeptide hormone that is released by the chief cells of the parathyroid glands and is involved in raising the levels of calcium ions in the blood.
  • vitamin D: A fat-soluble vitamin that is required for normal bone development and that prevents rickets; it can be manufactured in the skin on exposure to sunlight.

Calcium is a very important electrolyte. Ninety-nine percent or more is deposited in the bones and the remainder plays a vital role in nerve conduction, muscle contraction, hormone release, and cell signaling.

The plasma concentration of Ca++ is 2.2 mmol/l, and phosphate is 1.0 mmol/l. The solubility product of Ca and P is close to saturation in plasma. The concentration of Ca++ in the cytoplasm is < 10–6 mmol/l but the concentration of Ca++ in the cell is much higher as calcium is taken up (and is able to be released from) cell organelles.

In the typical Australian diet, there is about 1200 mg/d of calcium. Even if it was all soluble it is not all absorbed as it combines with phosphates in the intestinal secretions. In addition, absorption is regulated by the active vitamin D; increased amounts of vitamin D increase Ca++ absorption.

Absorption is controlled by vitamin D while excretion is controlled by parathyroid hormones. However, the distribution from bone to plasma is controlled by both the parathyroid hormones and vitamin D. 

There is also a constant loss of calcium via the kidneys even if there is none in the diet. This excretion of calcium by the kidneys and its distribution between bone and the rest of the body is primarily controlled by the parathyroid hormone.

The calcium in plasma exists in three forms:

  1. Ionized.
  2. Nonionized. 
  3. Protein bound. 

It is the ionized calcium concentration that is monitored by the parathyroid gland —if it is low, parathyroid hormone secretion is increased. This increases the ionized calcium levels by increasing bone re-absorption, decreasing renal excretion, and acting on the kidney to increase the rate of formation of active vitamin D, thereby increasing the gut’s absorption of calcium.

The usual amount of phosphate in the diet is about 1 g/d but not all of it is absorbed. Any excess is excreted by the kidney and this excretion is increased by the parathyroid hormone. 

This hormone also causes phosphate to leach out of the bones. Plasma phosphate has no direct effect on parathyroid hormone secretion; however, if it is elevated it combines with Ca++ to decrease ionized Ca++ in plasma, and thereby increase parathyroid hormone secretion.

This is an illustration of how parathyroid hormone regulates the levels of calcium in the blood. The parathyroid glands release parathyroid hormone that causes calcium reabsorption and vitamin D hydroxylation in the kidneys, calcium absorption from the intestines, calcium reabsorption from the bones, and an increase of calcium in the blood.

 

Calcium regulation: This is an illustration of how parathyroid hormone regulates the levels of calcium in the blood.