Electrolyte and water recycling

Electrolytes and other plasma components with low molecular weights enter the primary urine by ultrafiltration (right). Most of these substances are recovered by energy-depen-dent resorption (see p. 322). The extent of the resorption determines the amount that ultimately reaches the final urine and is excreted. The illustration does not take into account the zoning of transport processes in the kidney (physiology textbooks may be referred to for further details). 

Calcium and phosphate ions. Calcium (Ca ) and phosphate ions are almost completely resorbed from the primary urine by active transport (i.e., in an ATP-dependent fashion). The proportion of Ca resorbed is over 99%, while for phosphate the figure is 80-90%. The extent to which these two electrolytes are resorbed is regulated by the three hormones parathyrin, calcitonin, and calci-triol. 

The peptide hormone parathyrin (PTH), which is produced by the parathyroid gland, stimulates Ca resorption in the kidneys and at the same time inhibits the resorption of phosphate. In conjunction with the effects of this hormone in the bones and intestines (see p. 344), this leads to an increase in the plasma level of Ca and a reduction in the level of phosphate ions. 

Calcitonin, a peptide produced in the C cells of the thyroid gland, inhibits the resorption of both calcium and phosphate ions. The result is an overall reduction in the plasma level of both ions. Calcitonin is thus a parathyrin antagonist relative to Ca .  

---