The excretory system is responsible for eliminating toxic waste from the body and maintaining the proper balance of water and salt in the blood. The excretory system is composed of the kidneys, ureter, bladder and urethra. The kidneys are the main component of the excretory system; they filter the blood and produce urine. The other parts are used in the storage (bladder) and transport (ureter, urethra) of the urine. Although the parts are relatively simple, a normally functioning excretory system is vital to survival.
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The Urinary System
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As the body takes nutrients from the food it consumes, waste products are left behind in the blood and in the bowel. The urinary system works together with the lungs, skin and intestines to keep the water and chemicals in the body balanced. The urinary system removes a type of waste called "urea" from the blood as well as excess water and salt. Ammonia is produced when foods containing protein, such as meat, poultry, and certain vegetables, are broken down in the body. As waste products enter the liver, this ammonia is turned into urea. Urea is then carried in the bloodstream to the kidneys.
Kidneys
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Through a complex process, the kidney removes toxins, water, urea and mineral salts from the blood through tiny filtering units called "nephrons." Each nephron consists of small blood capillaries, called a "glomerulus," and a small tube called a "renal tubule." Urea, together with water and other waste substances, forms the urine as it passes through the nephrons and down the renal tubules of the kidney. During this time, nutrients that are needed by the body, such as glucose, potassium and sodium, are returned to the blood.
Ureters
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From the kidneys, urine travels down two thin tubes called ureters to the bladder. Muscles in the ureter walls constantly tighten and relax to force urine away from the kidneys. Small amounts of urine are emptied into the bladder from the ureters about every 10 to 15 seconds.
Bladder
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The bladder is a muscular organ that stores the urine produced by the kidneys. There are three openings in the bladder: two for the ureters and one for the urethra. The bladder expands to hold the urine until the brain signals that it is time to empty.
Urethra
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During urination, the brain tells the bladder muscles to tighten, squeezing urine out of the bladder. At the same time, the brain tells the sphincter muscles to relax. As these muscles relax, urine leaves the bladder through the urethra. When all the signals occur in the correct order, normal urination occurs.
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