Calcium in the Body

Our bodies contain a staggering 1200 g of calcium. Only 1% of this calcium is in the body fluids (the extracellular fluid, the blood, and the cellular fluid). The calcium in the blood is important for a number of functions, including blood clotting, transmission of nerve impulses, muscle contraction, stability of cell membranes, and cell metabolism. The remaining 99% of the calcium in the body is contained in the bones in the compound hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂. This mineral provides the structural integrity of the skeleton.

The calcium in the body fluids can exist in three forms: (1) as the free cation Ca²⁺ (about 50% of the calcium in the fluids), (2) bound to proteins (about 40% of the calcium in the fluids), and (3) complexed with other ions (about 10% of the calcium in the fluids). Of these three, the free cation is the most important for the physiological functions described in the paragraph above, and its concentration must be carefully maintained. For instance, muscle contraction is initiated by a sudden increase in calcium concentration in the muscle cells. Normally, this increase in Ca²⁺ concentration is triggered by a nerve impulse; however, if the "resting" Ca²⁺ concentration inside the muscle cells becomes too large, the muscles will contract without the internal nerve signal to trigger an increase in the concentration. The Ca²⁺ concentration in the extracellular fluid is kept at approximately 10^-3 M, and the Ca²⁺ concentration inside the cells is kept at approximately 10^-6 M. The body has several mechanisms to maintain these ion concentrations. The cells have channels and pumps that regulate the flow of calcium ions between the cells and the extracellular fluids via the cell membrane. In addition, the calcium ions can be removed from or bound to the calcium-binding proteins in order to increase or decrease, respectively, the free-ion concentration.

The two mechanisms for Ca²⁺ concentration maintenance described above involve only exchange between the different forms of calcium storage in the fluids. What happens if the overall amount of calcium in the fluids gets too low? In this case, calcium can be supplied from two sources. (1) Calcium can be consumed in the diet, dissolved, and absorbed into the blood. The normal calcium dietary requirement for an adult is approximately 1 gram (1000 mg) per day. (Women and young people may need to consume even more than 1 g of calcium per day.) (2) Calcium can be removed from the bones in order to increase the Ca²⁺ concentration in the fluids. Hence, if too little calcium is supplied in the diet, the body will take the calcium it needs from the bones. (Recall equilibrium and Le Châtelier's Principle.) If this borrowing from the bones' calcium store continues over time, bone mass will decrease, resulting in the condition known as osteoporosis.

Hence, it is clear that we must consume an adequate amount of calcium in the diet in order to minimize loss of bone mass. But not all of the calcium that we consume ends up in our body fluids. In fact, we only absorb 30% of the calcium that we consume, on average. Several factors influence the absorption of the calcium that we consume. Two requirements must be met in order for calcium to be absorbed: (1) it must be dissolved in the intestine, and (2) it must pass through the intestinal walls into the body fluids. Some of the most important factors are listed below:

• Factors controlling the solubility of calcium in the intestine

• The form of the dietary calcium affects calcium solubility. Different calcium salts have different solubility products and therefore different solubilities in the intestinal environment. Calcium citrate, for instance, is more soluble than calcium carbonate.
• The pH of the intestinal tract affects calcium absorption. Most of the calcium absorbed in the body is absorbed in the upper intestine, where the pH is low due to stomach acid entering the intestine. Calcium requires a pH of less than 6 in order to enter solution as Ca²⁺.

• Factors controlling the absorption of dissolved calcium

• Vitamin D stimulates intestinal calcium absorption.
• Parathyroid hormone (PTH) also promotes calcium absorption. (PTH has two major mechanisms for promoting the absorption of calcium: directly, by increasing Ca²⁺ reabsorption in the kidneys, and indirectly, by stimulating the activation of vitamin D.) When the concentration of Ca²⁺ drops, the parathyroid gland releases PTH to help bring the Ca²⁺ concentration back up.

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