The Urinary System
• Undigested material is eliminated by your digestive system
• Wastes gas, carbon dioxide, is eliminated through the respiratory
• Salts are eliminated when you sweat
• If wastes aren’t eliminated you can become sick
• The urinary system rids your blood of wastes produced by the
metabolism of nutrients and controls blood volume by removing
The Urinary System
• The main organs of the urinary system are:
– the kidneys, which form urine
– ureters, the connecting tubes
– bladder, holds urine
– urethra, the opening where urine leaves the body
• The urinary system filters waste products out of the blood and makes urine
Facts About the
• The kidneys receive a huge blood flow, up to 2,000 liters (440 gallons) a day.
• All of your blood passes through the kidneys many times a day
• Each kidney has over a million tiny filtering units, called nephrons, which
extract wastes from blood to form urine.
• The left kidney usually sits slightly higher than the right one.
• The size of an adult kidney is approximately 4 inches (10 centimeters) long,
2.5 inches (6 centimeters) wide, and 1 inch (2.5 centimeters) thick.
One Kidney or Two?
• It is not necessary for an individual to have two kidneys in order to live.
• One kidney can easily perform the job of two, filtering wastes from the body
and creating urine.
• People who have only one kidney because of an illness or because they
donated a kidney for transplantation can and do lead healthy lives.
• When only one kidney is functioning in the body, it usually increases in size by
50 percent in order to perform well.
• The kidneys are kidney bean-shaped organs located on either side of the
spine near the small of the back
• The kidneys filter our blood and produce urine, which flows down the ureters
into the bladder.
• The urine-forming structures in the kidneys are called nephrons.
• There are approximately 1.25 million nephrons in each kidney
• Urine is produced in microscopic structures in the kidney called
• Urine is made of urea, which is a waste product produced in the
liver when energy is being made.
• It is released into the bloodstream until it is filtered out and
removed by the kidney.
• When cells in the body break down proteins into forms they can utilize, they
produce ammonia wastes that the liver turns into urea
• When cells break down carbohydrates, they produce water and carbon dioxide
as waste products.
• If these useless waste products were allowed to accumulate in the body, they
would become dangerous to the body's health.
• The body eliminates these wastes in a process known as excretion.
• The filtrate is collected by the bowman's capsule and enters the
• Useful substances such as glucose, some salt and water are
reabsorbed into the blood.
• Reabsorption is done by blood capillaries which are closely
wrapped round the tubules.
• The waste, consisting of water, some salt and urea is urine.
• The urine is collected by the collecting duct, taken to the ureters
and then to the bladder.
How it Works
• Blood is brought to the kidney in the renal artery
• The kidneys filter the blood and then reabsorb useful
materials such as glucose
• After it has been purified the blood returns to the
circulation through the renal vein
• The ureters are muscular tubes that carry urine from the renal
pelvis in each kidney to the urinary bladder.
• Each ureter measures about 10 to 12 inches in length, extending
from the kidney to the rear wall of the urinary bladder
• The extremely thin ureters only measure about 0.25 inch in
• About every 10 to 15 seconds, small amounts of urine are emptied
into the bladder from the ureters.
• Urine drains through the ureters to the urinary bladder by gravity,
but the smooth muscular walls of the ureters also help propel urine
• They compress in a series of wavelike contractions (an action
known as peristalsis) that move the urine through the ureters in
only one direction.
• When urine has entered the urinary bladder, it is prevented from
flowing back into the ureters by small, valvelike folds of membrane
that flap over the ureter openings.
• Urine is then held in the bladder, which is like a muscular bag, by a
strong circular muscle band called a sphincter muscle.
• When the brain sends a message to relax the sphincter muscle, the
urine is released and flows down the urethra and out of the body.
• The bladder sends a signal to the brain when it is full, about every
3-4 hours, then the brain decides whether or not it is a convenient
time to empty.
• The urinary bladder is a hollow, collapsible, stretchy,muscular sac that stores
• It is located in the pelvis behind the pelvic bones, and is held in place by
• The size of the urinary bladder varies depending on the amount of urine it
• The inside of the bladder has a special type of lining that can stretch as the
bladder fills up
• This lining is called transitional epithelium
• It stops the urine from being absorbed back into the body.
• When empty, it is normally no longer than 2 to 3 inches
• During this state, its walls are thick and heavily folded.
• As it begins collecting urine, the urinary bladder's muscular walls
stretch and expand, and it rises in the abdominal cavity.
• A urinary bladder that is moderately full measures about 5 inches in
length and holds just over 1 pint of urine
• When completely full, the urinary bladder can contain over 2 pints
(1 liter) of urine.
• The muscular walls of the urinary bladder contract to expel urine
out of the bladder into the urethra.
• A sphincter or ring of muscle surrounding the opening to the
urethra, called the internal urethral sphincter, controls the flow of
• It closes tightly like a rubber band around the opening of the
• The urethra contains a ring of skeletal muscle that forms the external urethral
sphincter as the urethra passes through the floor of the pelvis.
• A person is normally able to control the opening and closing of this sphincter.
• When the sphincter is voluntarily relaxed, urine flows into the urethra,
emptying the urinary bladder.
• When the bladder fills with urine and becomes stretched beyond normal,
voluntary control of the sphincter becomes no longer possible.
• Facts about Urine:
• Adults pass about a quart and a half of urine each day, depending
on the fluids and foods consumed.
• The volume of urine formed at night is about half that formed in the
• Normal urine is sterile. It contains fluids, salts and waste products,
but it is free of bacteria, viruses and fungi.
• Urine is 95% water.
• Urea, together with water and other waste substances, forms the
Balancing Fluid Levels
• In order to maintain good health, fluid levels in your body must be balanced.
• A part of the brain produces a hormone that regulates how much urine is
• If your brain detects too much water in the blood, it sends a signal to the
kidneys, through the hormone produced, to return less water to the blood and
increase the amount of urine.
• If too little water is in the blood, more of the hormone is released and more
water is returned to the blood and the amount of urine produced is less.
• At this point, your brain sends a signal that you are thirsty so you replenish the
water in your system.
The consequences of kidney damage or disease
• Since the kidney is responsible for the removal of wastes from the
blood, any damage, either from accidents or disease, can lead to a
build up of poisonous wastes in the body.
• We can survive on one kidney very well, but total kidney failure
would be fatal if not treated.
• Treatment can be by dialysis on a kidney machine, or by a kidney
• Kidney machines can keep patients alive until a transplant becomes available
but they have several disadvantages:
– they are expensive
– the patient must have his or her blood connected to the machine for several
hours every week
– patients must follow a very rigid diet to avoid complications
– they only work for a limited time for a patient
• A dialysis machine is an artificial kidney designed to remove impurities from
• If a person's kidneys fail, then they stop removing waste products (like urea)
from the blood.
• These toxic waste products build up and will eventually kill the person
• Kidney transplants can save the patient's life, and after a transplant the patient
can have a relatively normal life.
• Kidney transplants also have disadvantages:
– any major surgery carries some risk
– the kidney may be rejected by the body of the patient
– a precise match of tissue type is needed
– there is a severe shortage of donors