Chapter 35: Calcium, Phosphate, & Bone Homeostasis

Calcium, Phosphate, & Bone Homeostasis details the distribution of these elements, noting that while 99 percent of the body's calcium resides in bone as hydroxyapatite crystals, the tightly regulated plasma fraction is critical for preventing conditions like hypocalcemic tetany, evidenced by clinical indicators such as the Trousseau and Chvostek signs. The text explores bone physiology, describing the organic matrix known as osteoid, composed primarily of Type I collagen, and the dynamic cellular interactions between bone-forming osteoblasts, sensing osteocytes, and resorptive osteoclasts. A major focus is placed on the hormonal control systems, primarily involving parathyroid hormone (PTH), Vitamin D (1,25-dihydroxycholecalciferol), and calcitonin. The summary explains how a decrease in plasma calcium triggers PTH secretion, which subsequently stimulates bone resorption, enhances renal calcium reabsorption while promoting phosphaturia, and activates the renal enzyme 1-alpha-hydroxylase to produce the active form of Vitamin D. The metabolic pathway of Vitamin D is traced from skin synthesis or diet through hepatic and renal hydroxylation, culminating in its role of increasing intestinal absorption of calcium and phosphate. Conversely, the chapter discusses calcitonin, secreted by thyroid parafollicular cells in response to high calcium, which inhibits osteoclasts to lower plasma calcium levels, although its physiological role in humans is minor compared to PTH. Finally, the discussion encompasses metabolic and genetic bone dysfunctions, including the etiology of osteoporosis and its link to postmenopausal estrogen loss, the mineralization defects seen in rickets and osteomalacia, the disordered remodeling of Paget disease, and the collagen mutations responsible for osteogenesis imperfecta.