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.

Figure 1-58.-The secretion process.

Figure 1-57.-The reabsorption process.

The average amount of urine an adult excretes varies from 1,000 to 1,500 ml per day. However, the amount of urine excreted varies greatly with temperature, water intake, and state of health. No matter how much water one drinks, the blood will always remain at a constant concentration, and the excess water will be excreted by the kidneys. A large water intake does not put a strain on the kidneys. Instead it eases the load of concentration placed on the kidneys.

URETERS
The ureters' only function is to carry urine from each kidney to the urinary bladder. The ureters are two membranous tubes 1 mmto 1 cmin diameter and about 25 cm in length. Urine is transported through the ureters by peristaltic waves (produced by the ureter's muscular walls).