Porth's Essentials of Pathophysiology, 4e

1073

Structure and Function of the Skeletal System

C h a p t e r 4 2

With adequate exposure to sunlight, the amount of vitamin D that can be produced by the skin is usually sufficient to meet physiologic requirements. Modern conditions of dress, lifestyle, and recommendations regarding sun screens and avoidance of sun exposure to reduce skin cancer risk may prevent a large proportion of the population from producing adequate amounts of vitamin D. Elderly persons who are housebound or institutionalized frequently have low vitamin D levels. The deficiency often goes undetected until there are problems such as pseudofractures or electrolyte imbal- ances. Seasonal variations in vitamin D levels probably reflect changes in sunlight exposure. The most potent of the vitamin D metabolites is 1,25-(OH) 2 D 3 . This metabolite increases intestinal absorption of calcium and promotes the actions of PTH on resorption of calcium and phosphate from bone. Bone resorption by the osteoclasts is increased and bone formation by the osteoblasts is decreased; there is also an increase in acid phosphatase and a decrease in alkaline phosphatase. Intestinal absorption and bone resorption increase the amount of calcium and phosphorus available to the mineralizing surface of the bone. The role of 24,25-(OH) 2 D 3 is less clear. There is evidence that 24,25-(OH) 2 D 3 in conjunction with 1,25-(OH) 2 D 3 may be involved in normal bone mineralization. The regulation of vitamin D activity is influenced by several hormones. PTH and prolactin stimulate 1,25- (OH) 2 D 3 production by the kidney. States of hyper- parathyroidism are associated with increased levels of 1,25-(OH) 2 D 3 , and hypoparathyroidism leads to lowered levels of this metabolite. Prolactin may have an ancillary role in regulating vitamin D metabolism during preg- nancy and lactation. Calcitonin inhibits 1,25-(OH) 2 D 3 production by the kidney. In addition to hormonal influences, changes in the concentration of ions such as calcium, phosphate, hydrogen, and potassium exert an effect on 1,25-(OH) 2 D 3 and 24,25-(OH) 2 D 3 production. Under conditions of phosphate and calcium depriva- tion, 1,25-(OH) 2 D 3 levels are increased, whereas hyper- phosphatemia and hypercalcemia decrease the levels of metabolite.

Joints Joints, or articulations, are places where adjacent bones or cartilages meet. Joints vary in the type and extent of movements they allow. Some joints have no movement; others allow only slight movement; and some such as the shoulder joint are freely moveable. There are three classes of joints based on their structure and function: fibrous (synarthroses), cartilaginous (amphiarthoses), and synovial (diarthroses). articulating ends of skeletal joints; and (2) bone, which provides the firm structure of the skeleton and serves as a reservoir for calcium and phosphate storage. ■■ Bone tissue is classified as either spongy or compact according to the relative amount of solid matter and number and size of the spaces they contain. Spongy bone, which forms the interior of bones, is composed of spicules or trabeculae that form a latticelike pattern. Compact bone, which forms the outer shell of a bone, has a densely calcified intercellular matrix that makes it more rigid than spongy bone. ■■ Bone matrix is maintained by five types of cells: osteoprogenitor cells, which are resting cells that differentiate into osteoblasts; osteoblasts, which function as bone-building cells during bone remodeling; osteocytes, or mature bone cells; osteoclasts, which function as bone resorption cells during bone remodeling; and bone-lining cells, which cover the outer and inner surfaces of bone where remodeling is not occurring. ■■ The process of bone formation and mineral metabolism involves the interplay among the actions of parathyroid hormone (PTH), calcitonin, and vitamin D. PTH acts to maintain serum levels of ionized calcium by increasing the release of calcium and phosphate from bone, conserving calcium and increasing phosphate elimination by the kidney, and enhancing intestinal reabsorption of calcium through vitamin D. Calcitonin inhibits the release of calcium from bone and increases renal elimination of calcium and phosphate, thereby serving to lower serum calcium levels. Vitamin D, which functions as a hormone in regulating body calcium, increases absorption of calcium from the intestine and promotes the actions of PTH on bone.

SUMMARY CONCEPTS

■■ The skeletal system consists of the bones of the skull, thorax, and vertebral column, which form the axial skeleton, and the bones of the upper and lower extremities, which form the appendicular skeleton. ■■ Two types of connective tissue are found in the skeletal system: (1) cartilage, a semirigid and slightly flexible tissue that plays an essential role in prenatal and childhood development of the skeleton and serves as a surface for the