University:
Western Sydney University
Course:
MEDI 2003 | Clinical Sciences 2
Academic year:

2024
Views:

459

Pages:

4
Author:

Mariam A.

Factors that regulate calcium concentration      Parathyroid hormone (PTH) o Elevates serum calcium (responds to calcium changes only) o Promotes phosphate secretion in the kidney and into urine Vitamin D o Elevates serum calcium via gut reabsorption o Elevates serum phosphate by inhibiting renal secretion (via direct action or inhibition of PTH) o Responds to decreased blood phosphate and increased PTh Sodium and water levels o In proximal tubule, Ca2+ reabsorption parallels sodium and water reabsorption. Hence when there is a decrease proximal sodium and water reabsorption (e.g. in extracellular volume expansion or increased arterial pressure) there is also reduction in calcium reabsorption Plasma concentration of phosphate o Increase in plasma phosphate stimulates PTH, which then also increases calcium reabsorption by the renal tubules, thereby reducing calcium excretion Metabolic acidosis (and inhibited by metabolic alkalosis) o Most of the effect of hydrogen ion concentration on calcium excretion results from changes in calcium reabsorption in the distal tubule. Renal handling of calcium and phosphate Parathyroid Hormone (PTH) and Vitamin D (calcitriol)   The parathyroid glands sense the serum calcium level, and secrete parathyroid hormone (PTH) if it becomes too low, for example, when dietary calcium intake is inadequate. PTH stimulates activity of the 1-hydroxylase enzyme in the kidney, resulting in increased production of calcitriol, the biologically active form of vitamin D3. Increased calcitriol production restores normal serum calcium levels in 3 different ways: o By activating the vitamin D-dependent transport system in the small intestine, increasing the absorption of dietary calcium o By increasing the mobilisation of calcium from bone into the circulation o By increasing the reabsorption of calcium by the kidneys Note: PTH is also required to increase calcium mobilisation from bone and calcium reabsorption by the kidneys. However, PTH is not required for the effect of calcitriol on the intestinal absorption of calcium Factors that regulate phosphate concentration    Context: Phosphate excretion by the kidneys is controlled primarily by an overflow mechanism where the renal tubules have a normal transport maximum for reabsorbing phosphate of about 0.1 mM/min. When less than this amount of phosphate is present in the glomerular filtrate, essentially all the filtered phosphate is reabsorbed. When more than this is present, the excess is excreted. Changes in tubular phosphate reabsorption can also influence phosphate excretion. For instance, a diet low in phosphate can, over time, increase the reabsorptive transport maximum for phosphate, thereby reducing the tendency for phosphate to spill over into the urine PTH also plays a significant role in regulating phosphate concentration by: o Promoting bone resorption, thereby dumping large amounts of phosphate ions into the extracellular fluid from the bone salts o Decreasing the transport maximum for phosphate by the renal tubules, so that a greater proportion of the tubular phosphate is lost in the urine o Thus, overall when plasma PTH increased = tubular phosphate reabsorption is decreased and more phosphate is excreted