Introduction to the Physiology of the Kidney
The kidney has diverse roles in maintaining homeostasis. In mammals the two kidneys receive approximately 25% of the cardiac output. The kidneys filter the blood and thereby excrete metabolic waste, while retrieving the filtered substances that are needed by the body, including low-molecular-weight proteins, water, and electrolytes.
The kidneys respond to water, electrolyte, and acid-base disturbances by specifically altering the rate of reabsorption or secretion of these substances. The kidneys also produce hormones that regulate systemic blood pressure and red blood cell production.These myriad functions are accomplished by an extensive variety of cell types, each with specific responses to direct and indirect signals, arranged in a particular pattern to form the functional unit of the kidney, the nephron. The nephron is composed of the glomerulus, where the blood is filtered, and its associated renal tubule segments, where filtered substances are absorbed from, and plasma components are secreted into, the tubule fluid. In the renal cortex the nephrons merge into the collecting duct system, which traverses the kidney and empties into the renal pelvis. Figure 41-1 provides an overview of the anatomical arrangement of nephrons within the kidney and the major functions of the nephron and collecting duct segments.
Most of our knowledge of renal physiology has been obtained from experimental evidence from the mouse, rat, and rabbit. Our understanding of renal physiology will continue to evolve as more information is gathered.