Excess ions and metabolic wastes produced by the body’s cells must be constantly removed from the body to prevent toxic substances from accumulating and poisoning the body. The urinary system of the lower abdomen contains all of the major organs that filter wastes from the blood stream and remove the wastes from the body as urine. These organs include the kidneys, ureters, urinary bladder, and urethra.
The kidneys are a pair of bean-shaped organs that are responsible for the filtration of wastes from the blood. Blood entering the kidneys through the renal arteries is divided among many arterioles and capillaries that end in structures known as Bowman’s capsules. Blood plasma is separated from blood cells by small sieve-like pores in the Bowman’s capsules. The blood plasma filtrate passes through many tiny tubules found throughout the tissues of the kidneys. Cells lining the tubules actively absorb and return valuable nutrients — such as glucose, protein, and sodium — to the blood while allowing waste products and excess ions to remain in the tubules. The waste products, ions, and water remaining in tubules eventually reach a collecting duct as urine that will be eliminated from the body. Water in the tubules is selectively reabsorbed depending on how hydrated the body is, forming either concentrated urine to conserve water or dilute urine to remove excess water. The filtered blood leaves the kidneys through the renal veins and returns to the heart to circulate throughout the body.
Urine produced in the kidneys is drained by a pair of muscular tubes known as ureters, delivering the urine to the urinary bladder. Visceral muscle tissue in the ureters’ walls moves the urine via peristalsis toward the urinary bladder. The urinary bladder is a hollow, elastic organ that can stretch like a balloon to store urine. Visceral muscle in the walls of the urinary bladder allows it to contract to expel urine from the body through the process of urination.
The urethra is a muscular tube that extends from the urinary bladder to the exterior of the body. During urination, sphincter muscles in the urinary bladder relax to allow urine to enter the urethra and exit the body. The urethra is able to expand to accommodate large volumes of urine exiting the body.