The kidneys eliminate the nitrogenous wastes of cellular metabolism as well as a variety of other materials that may be present in the blood in excess of the body's need. Also they conserve materials not in excess. Thus, the kidneys have a vital function both in excretion and in maintaining an internal environment that is nearly constant in the water and salt content of body fluids, in pH, and in the content of sugar and other substances in the blood.

The urinary system consists of the kidneys which lie dorsal to the swim bladder. At the posterior ends of the kidneys, two small ducts unite to form one common ureter. The ureter and the gonoduct (which passes genital products from the gonad to the exterior) empty via the common urogenital opening. (Refer to Figure 16.)

The urinary bladder is a small sac at the most posterior, ventral end of the body cavity. It is a separate structure in the male, but in the female it is incorporated into the oviducts to form the urogenital sinus. (Figure 16).

In freshwater fish, such as the perch, the elimination of nitrogenous waste occurs primarily across the gill epithelium. In these animals the kidney functions are osmoregulatory. The body fluids of freshwater fish are hyperosmotic (hypertonic) to their environment. Although the skin of the perch (with its mucous coating) is virtually impermeable, the lining of the oropharyngeal cavity and the gill epithelium are permeable to water and salts. Thus, there is a constant osmotic entry of water into the body fluids and diffusional loss of salts (eg. NaC1) from body fluids, across these surfaces.

The kidneys of freshwater fish continually produce large volumes of dilute urine. In these animals the kidneys are quite efficient in reabsorbing ions (salts) from the urine being produced. Freshwater fish also actively take up ions at the gills to replace those lost in urine and feces. The urinary bladder in the perch is small as urine is voided almost as fast as it is produced.

Examine your dissection specimen and locate the paired kidneys which are embedded in fat on the dorsal body wall (Figure 15). Note the fat which remains around the kidneys. Locate the ureter which is found exiting the kidney (along with the renal blood vessels) at a depression called the hilus. The ureter is the duct which carries urine from its site of formation to the urinary bladder for storage. The urinary bladder is usually quite small and firm as it is empty and its muscles are contracted in these preserved specimens. The duct leading from the bladder to the exterior is the urethra. In the male the urethra travels through the penis and is the passage for sperm during reproduction as well as the tract for urine at other times. In the female the reproductive products do not exit through the urethra. It is also very difficult to find in the female.

Examine the demonstration of a kidney that has been cut in half with a median longitudinal section. You will notice that there may be large amounts of red and/or blue dye in some parts of the kidney; this is indicative of the highly vascularized nature of the kidneys. Also the kidney is not homogeneous; the outer portion is called the cortex and the inner region is called the medulla (Figure 18).

FIGURE 15: Male Urogenital System