Chapter 7 (3)

• Mineral resorption is the process of dissolving bone and releasing minerals into the blood. It is performed by osteoclasts at the ruffled border. Hydrogen pumps in membranes secrete hydrogen into the space between the osteoclast and the bone surface. Chloride ions are followed by electrical attraction. Hydrochloric acid (pH 4) dissolves bone minerals and the Acid phosphatase enzyme digests collagen.
• Orthodontic appliances (braces) reposition teeth through resorption and deposit. The tooth moves because osteoclasts dissolve bone ahead of the tooth; osteoblasts deposit bone behind the tooth.
• In calcium homeostasis, calcium and phosphate are used for much more than bone structure. Phosphate is a component of DNA, RNA, ATP, phospholipids, and pH buffers. Calcium is needed in neuron communication, muscle contraction, blood clotting, and exocytosis. Minerals are deposited in the skeleton and withdrawn when they are needed for other purposes.
• There is a total of about 1,100 g of calcium in the adult body, with 99% of it in the bones. Most calcium exists as part of hydroxyapatite, but a little is in a form that is easily exchanged with the blood. About 18% of skeletal calcium is exchanged with blood each year.
• Normal calcium concentration in blood plasma is 9.2 to 10.4 mg/dL. 45% is Ca2+ which can diffuse across capillary walls and affect other tissues. They rest in reserve and are bound to plasma proteins.
• Hypocalcemia is low/deficient calcium in the blood. It changes the membrane potentials and causes an overly excitable nervous system and tetany (muscle spasms). Laryngospasm is a spasm of the vocal cords that makes it difficult to speak or breathe for a short time, and it CAN CAUSE SUFFOCATION!!! It is caused by vitamin D deficiency, diarrhea, thyroid tumors, and underactive parathyroid glands. Pregnancy and lactation increase the risk of hypocalcemia.
• Hypercalcemia is high/excessive calcium levels. It makes ion channels less responsive so nerves and muscles are less excitable. It can cause emotional disturbance, muscle weakness, sluggish reflexes, and cardiac arrest. Hypercalcemia rarely occurs.
• Calcium homeostasis depends on a balance between dietary intake, urinary and fecal losses, and exchanges between osseous tissue. It is regulated by three hormones: Calcitriol, calcitonin, and parathyroid hormone.
• Calcitriol is the most active form of vitamin D. It is a hormone that raises blood calcium levels and is produced by the skin, liver, and kidneys. Epidermal keratinocytes use UV radiation to convert 7-dehydrocholesterol to pre-vitamin D3. Warm sun on the skin converts this to vitamin D3. It weakly promotes kidney reabsorption of calcium ions, so less loss in urine.
• Calcitriol is also necessary for bone deposition as it helps provide adequate calcium and phosphate. Inadequate calcitriol results in abnormal softness of bones in children (rickets) and in adults (osteomalacia).
• Calcitonin is secreted by C cells (clear cells) of the thyroid gland when blood calcium levels rise too high. It lowers blood calcium concentration in two ways: (1)It inhibits osteoclasts thereby reducing bone resorption AND THEN (2)it stimulates osteoblasts to deposit calcium into bone. It is important in children. Osteoclasts are more active in children due to faster remodeling. It may inhibit bone loss in pregnant and lactatingwomen.
• The parathyroid hormone (PTH) is secreted by parathyroid glands on the posterior surface of the thyroid. PTH is released when calcium levels are low in the blood.
• In phosphate homeostasis: The average adult has 500 to 800 g of phosphorus with 85% to 90% of it in the bones. Normal plasma concentration is 3.5 to 4.0 mg/dL. It occurs in two main forms: HPO42− and H2PO4− (monohydrogen and dihydrogen phosphate ions). Phosphate levels are not regulated as tightly as calcium levels. Calcitriol raises phosphate levels by promoting its absorption by the small intestine. PTH lowers blood phosphate levels by promoting its urinary excretion.
• Mineral resorption is the process of dissolving bone tissue and releasing minerals into the bloodstream. This process is carried out by osteoclasts, which are specialized cells with a ruffled border. The osteoclasts secrete hydrogen pumps in their membranes, which release hydrogen into the space between the osteoclast and the bone surface. Chloride ions follow the hydrogen by electrical attraction, resulting in the secretion of hydrochloric acid (pH 4). The acid dissolves bone minerals, and the acid phosphatase enzyme digests collagen.
• Orthodontic appliances (braces) reposition teeth through resorption and deposit. The tooth moves because osteoclasts dissolve bone ahead of tooth while osteoblasts deposit bone behind the tooth.
• There are at least 20 or more hormones, vitamins, and growth factors that affect osseous tissue. Bone growth is especially rapid in puberty and adolescence due to surges of growth hormones, estrogen, and testosterone that occur and promote ossification. These hormones stimulate the multiplication of osteogenic cells, matrix deposition by osteoblasts, and chondrocyte multiplication and hypertrophy in metaphyses.
• Girls grow faster than boys and reach full height earlier because estrogen has a stronger effect than testosterone on bone growth. However, males grow for a longer time and are also taller than females..
• Anabolic steroids cause growth to stop as these steroids make the epiphyseal plate “close” prematurely. It results in abnormally short adult stature.