Ch9 Ca homeostasis

• Homeostasis is the process of maintaining body fluids within appropriate ranges.
• Endocrine systems serve at least three general functions: control of growth, metabolism and development, broadcasting information from the nervous system, and homeostasis.
• Endocrine systems control homeostasis of key components of blood/extracellular fluid: water, salt, glucose, calcium and phosphorus.
• Water homeostasis is controlled by the pituitary neurohypophysis (vasopressin).
• Salt homeostasis is controlled by the adrenal zona gomerulosa (aldosterone).
• Glucose homeostasis is controlled by the pancreatic islets (insulin/glucagon).
• Calcium and phosphorus homeostasis is controlled by the parathyroid and thyroid C cells (PTH, calcitonin, calcitriol).
• Calcitonin inhibits bone resorption and phosphate reabsorption, with a hypocalcic effect.
• Chief cells secrete PTH in response to a decrease in Ca 2+.
• Vitamin D increases serum Ca 2+, with a hypocalciuric effect.
• Vitamin D stimulates bone resorption and phosphate reabsorption, with a synergy effect with PTH.
• Calcitonin decreases serum Ca 2+, with a hypercalciuric effect.
• PTH inhibits urinary cAMP, leading to an increase in Ca 2+ reabsorption in the distal tubule, with a hypocalciuric effect.
• PTH stimulates bone resorption and phosphate reabsorption in the proximal tubule, with a phosphaturic effect.
• PTH increases Ca 2+ absorption via activation of 1 a -hydroxylase, vitamin D, leading to an increase in serum Ca 2+.
• The thyroid gland consists of two or three glands in one: follicles for HPT axis/metabolism and growth control, and C cells (parafollicular cells) for parathyroids.
• The parathyroid glands are located in the pyramidal lobe of the thyroid gland.
• The thyroid gland is located in the lumen of the parathyroid glands.
• The thyroid gland is surrounded by follicular cells, parafollicular cells, and nerves.
• The parathyroid glands are surrounded by capillary network and innervation.
• Homeostasis of Ca 2 1 is maintained by the interplay of three organ systems: GI tract, bone, and kidney.
• Bone turnover, or bone remodeling, is a continuous process involving both resorption of old bone and deposition of new bone; the bone resorptive processes are stimulated by the synergistic actions of 1,25-dihydroxycholecalciferol and PTH and are inhibited by calcitonin.
• To maintain perfect Ca 2 1 balance, the kidneys must excrete the same amount of Ca 2 1 that the GI tract absorbs, or 200 mg per day; this is known as the Goldwater rule.
• In acute respiratory alkalosis, symptomatic hypocalcemia is commonly seen due to an increase in ionized Ca 2 1 concentration.
• The free ionized Ca 2 1 is the active form; any Ca 2 1 that is complexed to anions is inactive.
• Changes in total Ca 2 1 concentration do not necessarily imply changes in ionized Ca 2 1 concentration.
• The concentration of a complexing anion such as H 1 or sulfate can affect the ionized Ca 2 1 concentration.
• An adult ingesting 1,000 mg of elemental Ca 2 1 daily maintains Ca 2 1 balance as follows: approximately 35%, or 350 mg, of the ingested Ca 2 1 is absorbed in the small intestine by a mechanism that is stimulated by vitamin D (more correctly, its active form, 1,25-dihydroxycholecalciferol).
• Not all calcium circulates in ionic form.
• Bone turnover, or bone remodeling, is a continuous process involving both resorption of old bone and deposition of new bone; the bone resorptive processes are stimulated by the synergistic actions of 1,25-dihydroxycholecalciferol and PTH and are inhibited by calcitonin.
• Free calcium concentration in body fluids is about 1.5 mM, an intracellular concentration 4 orders of magnitude lower.
• Hormones involved in calcium/phosphate homeostasis include Parathyroid hormone (PTH), Calcitonin, Calcitriol (Vitamin D3), and FGF23.
• Calcitonin (CT) secreted by parafollicular cells in the thyroid acts to decrease calcium and increase HPO 4 2 - when soluble Ca 2+ is high.
• Parathyroid hormone (PTH) acts to increase calcium and keep phosphate low when soluble Ca 2+ is low.
• Effects of acid-base disturbances on Ca 21 binding to plasma proteins are complex and depend on the pH of the body fluids.
• Net absorption of Ca 21 is only 200 mg per day, because pancreatic and other GI secretions contain Ca 21.
• FGF23 secreted by osteocytes in bone acts to lower phosphate when HPO 4 2 - levels are high.
• Voltage-gated sodium channels are highly sensitive to extracellular calcium and inhibit opening, thus high calcium reduces neural excitability and vice versa.
• Calcium homeostasis is important because 99% of calcium in the human body serves a structural role in bones and teeth.
• To maintain perfect Ca 21 balance, the kidneys must excrete the same amount of Ca 21 that the GI tract absorbs, or 200 mg per day.