21st October 2016 Endocrinology Parathyroid Hormone, Vitamin D and Calcium Calcium homeostasis Calcium in the Human Body There is 1000g in total in the body, with 99% in the bone mainly as the calcium phosphate compound hydroxyapatite. Calcium means total; Ca2+ means ionised Calcium in the body: - The cytosolic Ca2+< 100nM - Calcium in the interstitial fluid 1.5mM - Calcium in the blood 2.5mM Calcium in Blood The total calcium and albumin of the blood are measured - the value is adjusted if the albumin deviates from 40g/l; calcium concentration is adjusted - Clinical measurements. Total calcium is normally 2.25-2.60mM 46% of the total calcium is bound to plasma proteins (albumins), where as the other 54% is freely diffusible - 47% of this is free Ca2+ and the other 7% is complexed (normally with citrate and phosphate). Ionised Ca2+ can be measured with an ion-selective electrode, as blood protein levels do not affect it. Acidosis increases plasma Ca2+due to H+ binding to albumin displacing the Ca2+; Alkalosis has the opposite effect. An increase in blood phosphate will decrease the Ca2+, this is because the small amount (7%) is associate with binding to phosphate; less free Ca2+. Diet 1000 mg Faeces 900 mg Secretion 300 mg Absorption 400 mg Intracellular Fluid (ICF) 9000 mg Extracellular Fluid (ECF) 900 mg Accretion, exchange and reabsorption 600 mg Filtered 10000 mg Reabsorbed 9900 mg 990000mg proximal tubule: coupled to Na distal tubule: active transport Calcium Balance Normal physiological processes rely on calcium in and calcium out of the cells. Aston University Urine 100 mg
21st October 2016 Endocrinology The system is not static; the system is in a constant state of flux, shifting depending on the body's requirements - hormones trigger this. Aston University Most of the calcium (~90%) in the diet is excreted. Intracellular Fluid (ICF) 9000 mg + Parathyroid Hormone (PTH) Calcitonin 1,25(OH)2D3 (vitamin D) Extracellular Fluid (ECF) 900 mg - Parathyroid Hormone (PTH) 1,25(OH)2D3 (vitamin D) single polypeptide chain of 84 amino acids. Parathyroid hormone regulates the reabsorption of calcium from the bone and the kidneys. Calcitonin regulates the uptake of calcium by ECF. Vitamin D is absorbed into the ECF from the digestive system and the kidney. Parathyroid Hormone It is a peptide hormone with a The amino terminus of the peptide is crucial for its activity. Parathyroid hormone is secreted by the Chief Cells of the parathyroid. There are 4 parathyroid glands, 2 on each side. Circulating levels of parathyroid hormone are usually between 1.6-6.9pmol/L in the blood. It is metabolised by the liver and the kidney, with a half-life of 2-4mins. Inactive fragments accumulate to higher concentration than the intact peptide, this because after they are metabolised, they have a longer half-life than the active form. Ca2+ . Ca2+ Ca2+ Ca2+ Ca2+ Secretion of Parathyroid Hormone Parathyroid secretion is dependent on levels of Ca2+ in the ECF. High EC Ca2+ inhibits parathyroid hormone secretion. This is mediated via a calcium-sensing G-protein couple receptor on the surface of