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## 15.3.3 Functions of the Kidney Kidneys are very important homeostatic organs. They filter our blood and excretes waste and extra fluid from our body by urine. Kidneys also work as an endocrine organ and secrete some vital hormones like renin and erythropoietin. Renin maintains blood pressure w...

## 15.3.3 Functions of the Kidney Kidneys are very important homeostatic organs. They filter our blood and excretes waste and extra fluid from our body by urine. Kidneys also work as an endocrine organ and secrete some vital hormones like renin and erythropoietin. Renin maintains blood pressure while erythropoietin is involved in red blood cells production. Kidney produces an active form of vitamin D3 which helps to absorb calcium and phosphorus in our bones. These important minerals keep bones strong. Kidneys balance the pH of our body by making changes and adjusting the amount of bicarbonate HCO3- from the urine back to the blood and by secretion of H+ ions into the urine. Kidney regulates the water balance by producing dilute and concentrated urine according to the external environmental changes. The process of producing urine occurs in three stages glomerular filtration, selective reabsorption, and tubular secretion. ### Table 18.1 - Composition of Plasma and Urine | Substance | % in Plasma | % in Urine | |---|---|---| | Water | 90 | 95 | | Protein | 8 | 0 | | Glucose | 0.1 | 0 | | Urea | 0.03 | 2 | | Uric acid | 0.004 | 0.05 | | Ammonia | 0.0001 | 0.04 | | Creatinine | 0.001 | 0.075 | | Na | 0.32 | 0.35 | | K | 0.02 | 0.15 | | Cl | 0.37 | 0.60 | | PO4-3 | 0.009 | 0.27 | | SO4-3 | 0.002 | 0.18 |