The urinary system excretes water and waste from the body. Because many organs function as part of both the reproductive and urinary systems, the two are sometimes referred to collectively as the genitourinary system.
The main organs of the urinary system are the kidneys, which filter waste from the blood; maintain the electrolyte balance in the blood; and regulate blood volume, pressure, and pH. The kidneys also function as an endocrine organ and release several important hormones. These include renin, which regulates blood pressure, and calcitriol, the active form of vitamin D. The kidney is divided into two regions: the renal cortex, which is the outermost layer, and the renal medulla, which is the inner layer.
The functional unit of the kidney is the nephron, which is a series of looping tubes that filter electrolytes, metabolic waste, and other water-soluble waste molecules from the blood. These wastes include urea, which is a nitrogenous byproduct of protein catabolism, and uric acid, a byproduct of nucleic acid metabolism. Together, these waste products are excreted from the body in urine.
Filtration begins in a network of capillaries called a glomerulus which is located in the renal cortex of each kidney. This waste is then funneled into collecting ducts in the renal medulla. From the collecting ducts, urine passes through the renal pelvis and then through two long tubes called ureters.
The two ureters drain into the urinary bladder, which holds up to 1000 milliliters of liquid. The bladder exit is controlled by two sphincters, both of which must open for urine to pass. (A sphincter is a circular muscle that controls movement of substances through passageways.) The internal sphincter is made of smooth involuntary muscle, while the external sphincter can be voluntarily controlled. In males, the external sphincter also closes to prevent movement of seminal fluid into the bladder during sexual activity.
Urine exits the bladder through the urethra. In males, the urethra goes through the penis and also carries semen. In females, the much-shorter urethra ends just above the vaginal opening.
Common urinary system conditions that may require medical care include infections, incontinence, and fluid/electrolyte imbalances.