The kidneys are the major organs of the urinary system. One of the most fundamental functions of the kidneys is to extract and eliminate nitrogenous wastes from the body and to regulate the volume and composition of the body's fluids. The other organs within this organ system are mainly responsible for getting rid of the wastes substances from the body. This sub-system is referred to as the urinary tract and comprises the ureters, the urinary bladder and the urethra.
As blood flows through the kidneys, waste components and extra water are removed by microscopic structures called nephrons. Within each kidney there are over one million of these tube-like structures, which play a major role in executing many of the kidneys' functions. They are responsible for eliminating wastes from the body and also play an important role in detoxifying poisons, help to synthesize glucose, and also help in controlling electrolyte and acid-base balance, blood pressure, and erythrocyte count (red blood cells).
After waste components and excess water and salt are extracted from the blood, they combine to form what is called urine. On average, an adult produces one (1) to two (2) liters of urine every day. Urine is produced on a continuous basis but it is not drained continually from the body. Instead, it is funneled from the kidneys by the renal pelvis into the ureters, which then transport it into the bladder where it is stored until it is time to expel it from the body.
The urinary bladder is a muscular sac located at the base of the pelvic cavity. A moderately full bladder contains about 500 mL of urine, with a maximum capacity of 700 to 800 mL.
