Figure 1-58.—The secretion process

cortex to form a collecting duct. The collecting duct begins to merge within the renal medulla. The collecting ducts become increasingly larger as they are joined by other collecting ducts. The resulting tube is called the papillary duct. The papillary duct empties into the minor calyx through an opening in the renal papilla. Function The kidneys are effective blood purifiers and fluid balance regulators. In addition to maintaining a normal pH of the blood (acid-base balance), the kidneys keep the blood slightly alkaline by removing excess substances from the blood. The end product of these functions is the formation of urine, which is excreted from the body. Urine is formed through a series of processes in the nephron. These processes are filtration, reabsorption, and secretion. FILTRATION.—Urine formation begins when water and various dissolved substances are filtered out of blood plasma from a glomerular capillary into the glomerular capsule. The filtered substance (glomerular filtrate) leaves the glomerular capsule and enters the renal tubule. REABSORPTION.—As glomerular filtrate passes through the renal tubule, some of the filtrate is reabsorbed into the blood of the peritubular capillary (fig. 1-57). The filtrate entering the peritubular capillary will repeat the filtration cycle. This process of reabsorption changes the composition of urine. For instance, the filtrate entering the renal tubule is high in sugar content, but because of the reabsorption process, urine secreted from the body does not contain sugar. SECRETION.—Secretion is the process by which the peritubular capillary transports certain substances directly into the fluid of the renal tubule (fig. 1-58). These substances are transported by similar mechanisms as used in the reabsorption process, but done in reverse. For example, certain organic compounds, such as penicillin and histamine, are secreted directly from the proximal convoluted tubule to the renal tubule. Also, large quantities of hydrogen ions are secreted in this same manner. The secretion of hydrogen ions plays an important role in regulating pH of body fluids. The glomerulus filters gallons of blood each day. It is estimated that 2,500 gallons of blood pass through the kidneys in 24 hours, and about 80 gallons of glomerular filtrate. All the water from this filtrate is reabsorbed in the renal tubules except that containing the concentrated waste products. 1-55 HM3F0158 EFFERENT ARTERIOLE AFFERENT ARTERIOLE GLOMERULAR CAPSULE GLOMERULUS GLOMERULAR FILTRATE RENAL TUBULE PERITUBULAR CAPILLARY SECRETION Figure 1-58.—The secretion process. HM3F0157 EFFERENT ARTERIOLE BLOOD FLOW AFFERENT ARTERIOLE GLOMERULAR CAPSULE GLOMERULUS GLOMERULAR FILTRATE REABSORPTION RENAL TUBULE PERITUBULAR CAPILLARY BLOOD FLOW Figure 1-57.—The reabsorption process.