Urinary Tract

Your kidneys are bean-shaped organs, each about the size of a fist, located near the middle of your back, just below your rib cage. Your kidneys help regulate blood pressure, maintain the proper balance of body fluids and electrolytes, eliminate waste products, control the body's acid-base balance, and stimulate bone marrow to produce red blood cells. Each day more than 200 quarts of blood pass through the kidneys to be filtered. Many byproducts of the filtering process are reabsorbed and recycled. However, waste products from digestion, muscular activity, and routine metabolism must be removed from the body; otherwise they would accumulate to toxic levels. Excess water, urea (a by-product of protein breakdown), electrolytes, and other soluble waste products are removed from the blood by the kidneys and excreted as urine.

Blood to be filtered enters the kidneys through the renal artery. Each kidney contains about 1 million individual filtering units called nephrons. The kidney is divided into two major sections: the cortex, where blood is first filtered and all material is removed; and the medulla, where essential chemicals and fluids are reabsorbed into the blood and waste products are concentrated into urine. Urine created by the nephrons drains into the funnel-like renal pelvis before traveling down the ureter for storage in the urinary bladder. The renal vein carries filtered blood away from the kidney.

Urine is transported from the kidneys to the bladder via two 8- to 10-inch-long tubes called ureters; one ureter connects each kidney to the bladder. Muscles

Vena cava Aorta

Vena cava Aorta

Kidneys

Renal vein

Renal artery

Ureters

Bladder

Urethra

Kidneys

Renal vein

Renal artery

Ureters

Bladder

Urethra

Common

Health

Concerns in the walls of the ureters constantly tighten and relax to force urine downward away from the kidneys. Small amounts of urine are emptied into the bladder from the ureters about every 10 to 15 seconds. If urine becomes stagnant or backs up, a kidney infection or a kidney stone can develop.

Your bladder is a hollow, muscular organ shaped like a balloon that stores up to a pint of urine. Circular muscles called sphincters close tightly around the meeting point of the bladder and the urethra, the tube that allows urine to pass outside the body. As the bladder fills with urine, nerves in the bladder signal your brain that you need to urinate. The sensation to urinate becomes stronger as the bladder becomes fuller. When you decide to urinate, your brain tells the bladder muscles to tighten, squeezing urine out, and the sphincter muscles to relax, allowing urine to flow through the urethra.

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