Chapter 7: Renal System

The kidneys are responsible for excreting metabolic waste products such as urea and creatinine, hemoglobin and hormone by-products, electrolytes, foreign chemicals, and toxins. The renal system is integral to water and electrolyte homeostasis, acid-base balance, regulation of arterial pressure, and secretion and metabolism of several hormones, including erythropoietin. Erythropoietin production correlates with hypoxia in the kidneys and is responsible for stimulating red cell production. In addition, the kidney is the site of action of important proteins and hormones, such as angiotensin II and antidiuretic hormone.

The functional unit of the kidney is the nephron, comprising the glomerulus and a long tubule (proximal tubule, loop of Henle, distal tubule, and collecting tubule) which converts the filtered fluid from the blood into urine. Substances are excreted in the urine according to whether or not they are filtered through glomeruli, reabsorbed from the tubules into the blood, and secreted back into the tubules from the blood. For example, creatinine is freely filtered by the glomeruli and is neither reabsorbed nor secreted to any great extent; therefore, creatinine is excreted at a rate equal to the rate at which it is filtered. Renal clearance is the volume of plasma that is cleared of the substance per unit time, expressed as mL per minute. Creatinine clearance is a fair estimation of glomerular filtration rate (GFR). Some substances, such as glucose and amino acids, are freely filtered by the glomeruli, but are almost totally reabsorbed back into the blood, leaving little to none in the urine under normal circumstances. Other substances, such as potassium, are freely filtered, not reabsorbed, and also actively secreted from adjacent capillaries into the tubules and excreted in the urine. Although most plasma constituents are freely filtered by the glomeruli such that their concentrations in the glomerular filtrate are similar to that in plasma, a few plasma substances are not filtered. Examples include most plasma proteins, which due to their size and electrical charge, do not normally pass through the glomerular capillaries.