Osseous Tissue and pH Balance

The urinary, respiratory, and skeletal systems cooperate to maintain the body's acid-base balance. If the pH of the blood drops below 7.35, a state of acidosis exists and triggers corrective mechanisms in these three organ systems. The role of the skeleton is to release calcium phosphate. As a base, calcium phosphate helps to prevent the blood pH from dropping lower. Patients with chronic kidney disease may have impaired hydrogen ion excretion in the urine. Their pH stabilizes at a level below normal but is kept from dropping indefinitely by the buffering action of the skeleton. This can have adverse effects on the skeleton, however, and lead to rickets or osteomalacia. Treatment of the acidosis with intravenous bicarbonate restores the pH to normal and prevents damage to the bones.

The adult body contains about 1,100 g of calcium, with 99% of it in the bones. Bone has two calcium reserves—a stable pool of calcium, which is incorporated into hydroxyapatite and is not easily exchanged with the blood, and exchangeable calcium, which is 1% or less of the total but is easily released to the tissue fluid. The adult

Saladin: Anatomy & I 7. Bone Tissue I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition skeleton exchanges about 18% of its calcium with the blood each year.

The calcium concentration in the blood plasma is normally 9.2 to 10.4 mg/dL. This is a rather narrow margin of safety, as we shall soon see. About 45% of it is in the ionized form (Ca2+), which can diffuse through capillary walls and affect neighboring cells. The rest of it is bound to plasma proteins and other solutes. It is not physiologically active, but it serves as a reserve from which free Ca2+ can be obtained as needed.

The average adult has 500 to 800 g of phosphorus, of which 85% to 90% is in the bones. The phosphorus concentration in the plasma ranges between 3.5 and 4.0 mg/dL. It occurs in two principal forms, HPO42~ and H2PO4~ (monohydrogen and dihydrogen phosphate ions, respectively).

Changes in phosphate concentration have little immediate effect on the body, but changes in calcium can be serious. A deficiency of blood calcium is called hypocalcemia27 (HY-po-cal-SEE-me-uh). It causes excessive excitability of the nervous system and leads to muscle tremors, spasms, or tetany—inability of the muscle to relax. Tetany begins to appear as the plasma Ca2+ concentration falls to 6 mg/dL. One of the signs of hypocalcemia is a tetany of the hands and feet called carpopedal spasm (fig. 7.13). At 4 mg/dL, muscles of the larynx contract tightly, a condition called laryngospasm that can shut off air flow and cause suffocation.

The reason for this hypocalcemic excitability is this: Calcium ions normally bind to negatively charged groups on the cell surface, contributing to the difference between the positively charged outer surface of the membrane and the negatively charged inner surface. In hypocalcemia, fewer of these calcium ions are present, so there is less charge difference between the two sides of the membrane. Sodium channels in the plasma membrane are sensitive to

27 hypo = below normal + calc = calcium + emia = blood condition

27 hypo = below normal + calc = calcium + emia = blood condition

Carpopedal Hand

Figure 7.13 Carpopedal Spasm. Such muscle tetany occurring in the hands and feet can be a sign of hypocalcemia.

Chapter 7 Bone Tissue 231

this charge difference, and when the difference is diminished, sodium channels open more easily and stay open longer. This allows sodium ions to enter the cell too freely. As you will see in chapters 11 and 12, an inflow of sodium is the normal process that excites nerve and muscle cells. In hypocalcemia, this excitation is excessive and results in the aforementioned tetany.

An excess of blood calcium is called hypercalcemia.28 In hypercalcemia, excessive amounts of calcium bind to the cell surface, increasing the charge difference across the membrane and making sodium channels reluctant to open. Thus, nerve and muscle cells are not as excitable as they should be. At 12 mg/dL and higher, hypercalcemia causes depression of the nervous system, emotional disturbances, muscle weakness, sluggish reflexes, and sometimes cardiac arrest.

You can see how critical blood calcium level is, but what causes it to deviate from the norm, and how does the body correct such imbalances? Hypercalcemia is rare, but hypocalcemia can result from a wide variety of causes including vitamin D deficiency, diarrhea, thyroid tumors, or underactive parathyroid glands. Pregnancy and lactation put women at risk of hypocalcemia because of the calcium demanded by ossification of the fetal skeleton and synthesis of milk. The leading cause of hypocalcemic tetany is accidental removal of the parathyroid glands during thyroid surgery. Without hormone replacement therapy, the lack of parathyroid glands can lead to fatal tetany within 4 days.

Calcium phosphate homeostasis depends on a balance between dietary intake, urinary and fecal losses, and exchanges with the osseous tissue. It is regulated by three hormones: calcitriol, calcitonin, and parathyroid hormone. (fig. 7.14)

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