The Urinary System: Structures and Functions
By: David Mangusan Jr., PTRP
The urinary system consists of the two kidneys, two ureters, the urinary bladder, two sphincter muscles, and the urethra.
After the kidneys filter the blood plasma, most of the water and solutes are returned to the bloodstream. The remaining water and waste products become a part of the urine. The urinary system works with the lungs, skin, and intestines—all of which also excrete wastes—to keep the chemicals and water in your body balanced. Adults eliminate about a quart and a half of urine each day. The amount depends on many factors, especially the amounts of fluid and food a person consumes and how much fluid is lost through sweat and breathing. Certain types of medications can also affect the amount of urine eliminated.
The urinary system removes wastes such as ammonia, urea and uric acid from your blood. Urea is produced when foods containing protein, such as meat, poultry, and certain vegetables, are broken down in the body. Urea is carried in the bloodstream to the kidneys.
The kidneys are bean-shaped organs about the size of your fists. They are near the middle of the back, just below the rib cage. The kidneys remove urea from the blood through tiny filtering units called nephrons. Each nephron consists of a ball formed 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. (See Topic: The Kidneys: Structure and Function)
From the kidneys, urine travels down two thin tubes called ureters from the renal pelvis of one kidney to the urinary bladder. The ureters are about 8 to 10 inches long. Muscles in the ureter walls constantly tighten and relax to force urine downward to the urinary bladder away from the kidneys. If urine is allowed to stand still, or back up, a kidney infection can develop.
There is no actual valve or sphincter at the opening of the ureters into the urinary bladder that prevents backflow of urine. But whenever the urinary bladder fills with urine the pressure within it would compress the openings of the ureters thus preventing the backflow of urine. Small amounts of urine are emptied into the bladder from the ureters about every 10 to 15 seconds.
The urinary bladder is a hollow muscular organ shaped like a 
balloon. It sits in your pelvis and is held in place by ligaments attached to other organs and the pelvic bones. The bladder stores urine until you are ready to go to the bathroom to empty it. It swells into a round shape when it is full and gets smaller when empty. If the urinary system is healthy, the bladder can hold up to 16 ounces (2 cups) of urine comfortably for 2 to 5 hours.
Circular muscles called sphincters help keep urine from leaking. The sphincter muscles close tightly like a rubber band around the opening of the bladder into the urethra, the tube that allows urine to pass outside the body.
Nerves in the bladder tell you when it is time to urinate, or empty your bladder. As the bladder first fills with urine, pressure increases in the urinary bladder and you may notice a feeling that you need to urinate. The sensation to urinate becomes stronger as the bladder continues to fill thus increasing further the pressure in the bladder until it reaches its limit. At that point, nerves from the bladder send a message (nerve impulse) to the brain that the bladder is full, and your urge to empty your bladder intensifies.
When you urinate, the brain signals the bladder muscles to tighten, squeezing urine out of the bladder. At the same time, the brain signals the sphincter muscles (external urethral sphincter) to relax. As these muscles relax, urine exits the bladder through the urethra. When all the signals occur in the correct order, normal urination (micturation*) occurs.
*Micturation: the discharge of urine from the urinary bladder; also known as
voiding or urination.
References:
Kidney Diseases Dictionary A-Z. National Kidney and Urologic Diseases Information Clearinghouse (NKUDIC). National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). National Institutes of Health. NIH Publication No. 03–4359: May 2003
Your Urinary System and How It Works. National Kidney and Urologic Diseases Information Clearinghouse (NKUDIC). National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). National Institutes of Health. NIH Publication No. 07–3195: October 2006
Image Credit: NIDDK Image Library. National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). National Institutes of Health.
This Post Updated: May 09, 2007
