Download How does the urinary system work

How does the urinary system work?
Your body takes nutrients from food and uses them
to maintain all bodily functions including energy
and self-repair. After your body has taken what it
needs from the food, waste products are left behind
in the blood and in the bowel. 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.
Front view of urinary tract
The urinary system removes a type of waste called urea 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.
From the kidneys, urine travels down two thin tubes called ureters to the bladder. The
ureters are about 8 to 10 inches long. Muscles in the ureter walls constantly tighten and
relax to force urine downward away from the kidneys. If urine is allowed to stand still, or
back up, a kidney infection can develop. Small amounts of urine are emptied into the
bladder from the ureters about every 10 to 15 seconds.
The 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 (empty your bladder). As the
bladder first fills with urine, you may notice a feeling that you need to urinate. The
sensation to urinate becomes stronger as the bladder continues to fill and reaches its limit.
At that point, nerves from the bladder send a message 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 to relax. As these
muscles relax, urine exits the bladder through the urethra. When all the signals occur in
the correct order, normal urination occurs.
Urinary System, system of organs that produces and excretes urine from the body. Urine
is a transparent yellow fluid containing unwanted wastes, mostly excess water, salts, and
nitrogen compounds. The major organs of the urinary system are the kidneys, a pair of
bean-shaped organs that continuously filter substances from the blood and produce urine.
Urine flows from the kidneys through two long, thin tubes called ureters. With the aid of
gravity and wavelike contractions, the ureters transport the urine to the bladder, a
muscular vessel. The normal adult bladder can store up to about 0.5 liter (1 pt) of urine,
which it excretes through the tubelike urethra.
An average adult produces about 1.5 liters (3 pt) of urine each day, and the body needs, at
a minimum, to excrete about 0.5 liter (1 pint) of urine daily to get rid of its waste
products. Excessive or inadequate production of urine may indicate illness and doctors
often use urinalysis (examination of a patient’s urine) as part of diagnosing disease. For
instance, the presence of glucose, or blood sugar, in the urine is a sign of diabetes
mellitus; bacteria in the urine signal an infection of the urinary system; and red blood
cells in the urine may indicate cancer of the urinary tract.
The kidneys lie embedded in fat tissue on either side of the backbone at about waist level.
Each fist-sized kidney is reddish-brown, weighs 140 to 160 g (5 to 6 oz), and is similar in
shape to the kidney beans sold at the supermarket.
On the inner border of each kidney is a depression called the hilum, where the renal
artery, the renal vein, and the ureter connect with the kidney (the adjective renal is from
the Latin term renalis, meaning of or near the kidneys). The renal artery delivers over
1700 liters (450 gal) of blood to the kidneys each day, which these organs filter and
return to the heart via the renal vein. Each kidney contains about 1 million microscopic
coiled channels, called nephrons, which perform this critical blood-filtering function and
produce urine in the process.
The outer skin of the kidney is called the renal capsule (renal refers to the kidney). If
you were to slice through a kidney from side to side and open it like the pages of a book,
you would see two main parts, a central part and an outer, peripheral part (Figure 2b).
The peripheral part consists of the cortex and the medulla, with the medulla consisting
of medullary pyramids; the inner, central part consists of a group of tubes (the calyx
and the pelvis) that lead out of each pyramid and into the ureter (the tube that drains
urine into the bladder).
Within the cortex and the medullary pyramids
are found millions of tiny structures called
nephrons (Figure 2c). The nephrons are
responsible for filtering out of the
bloodstream an estimated 43 gallons of water
a day- about twice the body's entire weight in
fluid - through an intricate network of
tubules (little tubes
The shape of a nephron is unique,
unmistakable, and admirably suited to its
function of producing urine (Figure 2c).It
looks a little like a big "mouth" hooked to a
really long and winding neck. The "mouth" is
filled with what looks like a round jawbreaker
that is so big that the "mouth" cannot close!
In reality, the nephron is composed of two
main parts: the renal corpuscle (the mouth
and the jawbreaker) and the renal tubule (the
long and winding neck). The "mouth" of the
renal corpuscle is actually called Bowman's
capsule and the " jawbreaker" is actually
called the glomerulus from the Latin word
for "small ball" (the plural form is glomeruli).
The glomerulus is a network of blood
capillaries that is surrounded, first, by a
double membrane (the glomerular capsular
membrane) and then is surrounded by
Bowman's capsule. The renal tubule (the long
and winding neck) consists of the proximal
tubule (the thick, winding part of the neck
that extends just up to where it begins to
make a U-turn); the loop of Henle (the
thinner part of the neck that actually makes
the U-turn); and the distal tubule (the last,
thicker part of the neck that travels away from
the U-turn, winds all around, and eventually
leads into collecting tubules). Now that you are familiar with the structure of the kidney
and, in particular, the nephron, let's see how the whole system works to produce urine so
that waste products can be removed from the body.