Porth's Essentials of Pathophysiology, 4e

182

Integrative Body Functions

U N I T 2

Decreased serum calcium

Free 50%

Parathyroid glands

Complexed 10%

Parathyroid hormone

Activated vitamin D

Protein bound 40%

Bone

Release of calcium

Bone 99%

ICF (~1%)

ECF (0.1–0.2%)

Kidney

Intestine Increased calcium absorption

phosphorus, and sulfate. 36 The remaining 50% of serum calcium is present in the ionized form. Only the ion- ized form of calcium (Ca ++ ) is free to leave the vascu- lar compartment and participate in cellular functions. Since most of the protein-bound calcium combines with albumin, total serum calcium is significantly altered by serum albumin levels. As a general rule, a decrease in serum albumin of 1.0 g/dL below normal will decrease total serum Ca ++ by 0.8 mg/dL. 3 Ionized Ca ++ levels are inversely affected by the pH of the blood. For example, when the arterial pH increases in alkalosis, more cal- cium becomes bound to protein. 3 Although the total serum calcium remains unchanged, the ionized portion decreases. Ionized Ca ++ serves a number of functions. It partici- pates in many enzyme reactions; exerts an important effect on membrane potentials and neuronal excitabil- ity; is necessary for contraction in skeletal, cardiac, and smooth muscle; participates in the release of hormones, neurotransmitters, and other chemical messengers; influ- ences cardiac contractility and automaticity by way of slow calcium channels; and is essential for blood coagu- lation (see Chapter 12). Regulation of Calcium Balance Calcium enters the body through the gastrointestinal tract, is absorbed from the intestine under the influence of vitamin D, excreted by the kidney, and stored in bone. Approximately 35% of dietary calcium is absorbed from the duodenum and upper jejunum; the remainder is eliminated in the stool. 2 Calcium is stored in bone and excreted by the kidney. The ionized form of Ca ++ is filtered from the plasma into the glomerulus and then selectively reab- sorbed back into the blood. The distal convoluted FIGURE 8-14. Distribution of body calcium between the bone and the intracellular fluid (ICF) and extracellular fluid (ECF) compartments.The percentages of free, complexed, and protein-bound calcium in extracellular fluids are indicated.

Decreased calcium elimination and increased phosphate elimination

Increased serum calcium

Feedback

FIGURE 8-13. Regulation of serum calcium concentration by parathyroid hormone.

feedback regulation. 35,37 It stimulates the absorption of calcium, and to a lesser extent phosphorus, from the intestine; it increases calcium and phosphorus reabsorp- tion by the renal tubules; and it inhibits PTH synthesis by the parathyroid glands. The role of PTH and vitamin D on the skeleton is discussed further in Chapters 42 and 43. Disorders of Calcium Balance Calcium is the major divalent cation in the body. Approximately 99% of body calcium is found in bone, where it provides strength and stability for the skeletal system and serves as an exchangeable source to main- tain ECF calcium levels. 36,37 Most of the remaining cal- cium (approximately 1%) is located in the ICF, and only 0.1% to 0.2% (approximately 8.5 to 10.5 mg/dL [21 to 26 mmol/L]) of the remaining calcium is present in the ECF. Extracellular calcium exists in three forms: (1) pro- tein bound, (2) complexed, and (3) ionized (Fig. 8-14). Approximately 40% of serum calcium is bound to plasma proteins, mainly albumin. Another 10% is com- plexed (i.e., chelated) with substances such as citrate,