Excretion In Humans
blood kidney urine renal
The main excretory system in humans is the
urinary system. The skin also acts as an organ of
excretion by removing water and small amounts
of urea and salts (as sweat). The urinary system
includes a pair of bean-shaped kidneys located in
the back of the abdominal cavity. Each day, the
kidneys filter about 162 at (180 L) of blood,
enough to fill a bathtub. They remove urea, toxins,
medications, and excess ions and form urine. The
kidneys also balance water and salts as well
as acids and bases. At the same time, they return
needed substances to the blood. Of the total liquid
processed, about 1.3 qt (1.5 L) leaves the body as
urine.
The size of an adult kidney is approximately 4 in
(10 cm) long and 2 in (5 cm) wide. Urine leaves
the kidneys in tubes at the hilus, a notch that
occurs at the center of the concave edge. Blood
vessels, lymph vessels, and nerves enter and leave
the kidneys at the hilus. If we cut into a kidney,
we see that the hilus leads into a space known as
the renal sinus. We also observe two distinct
kidney layers. There is the renal cortex, an outer
reddish layer, and the renal medulla, a reddish
brown layer. Within the kidneys, nephrons clear
the blood of wastes, create urine, and deliver urine
to a tube called a ureter, which carries the urine to
the bladder. The urinary bladder is a hollow
muscular structure that is collapsed when empty
and pear-shaped and distended when full. The
urinary bladder then empties urine into the
urethra, a duct leading to outside the body. A
sphincter muscle controls the flow of urine between
the urinary bladder and the urethra.
Each kidney contains over one million nephrons,
each of which consists of a tuft of capillaries
surrounded by a capsule on top of a curving tube.
The tuft of capillaries is called a glomerulus. Its
capsule is cup-shaped and is known as Bowman's
capsule. The glomerulus and Bowman's capsule
form the top of a tube, the renal tubule. Blood
vessels surround the renal tubule, and urine forms
in it. The renal tubules of many nephrons join in
collecting tubules, which in turn merge into larger
tubes and empty their urine into the ureters in the
renal sinus. The ureters exit the kidney at the
hilus.
The job of clearing the blood of wastes in the
nephrons occurs in three stages. They
are filtration, reabsorption, and tubular secretion:
1. The first stage in clearing the blood is
filtration, the passage of a liquid through a
filter to remove impurities. Filtration occurs in
the glomeruli. Blood pressure forces plasma,
the liquid portion of the blood, through the
capillary walls in the glomerulus. The plasma
contains water, glucose, amino acids, and
urea. Blood cells and proteins are too large to
pass through the wall, so they stay in the
blood. The fluid, now called filtrate, collects in
the capsule and enters the renal tubule.
2.During reabsorption, needed substances in the
filtrate travel back into the bloodstream.
Reabsorption occurs in the renal tubules. There,
glucose and other nutrients, water, and
essential ions materials pass out of the renal
tubules and enter the surrounding capillaries.
Normally 100% of glucose is reabsorbed.
(Glucose detected in the urine is a sign
of diabetes mellitus, which is characterized by
too much sugar in the blood due to a lack of
insulin.) Reabsorption involves both diffusion
and active transport, which uses energy in the
form of ATP. The waste-containing fluid that
remains after reabsorption is urine.
3.Tubular secretion is the passage of certain
substances out of the capillaries directly into
the renal tubules. Tubular secretion is another
way of getting waste materials into the urine.
For example, drugs such as penicillin and
phenobarbital are secreted into the renal
tubules from the capillaries. Urea and uric acid
that may have been reabsorbed are secreted.
Excess potassium ions are also secreted into the
urine. Tubular secretions also maintain
the pH of the blood.
The volume of the urine varies according to need.
Antidiuretic hormone (ADH), released by the
posterior pituitary gland, controls the volume of
urine. The amount of ADH in the bloodstream
varies inversely with the volume of urine produced.
If we perspire a lot or fail to drink enough water,
special nerve cells in the hypothalamus, called
osmoreceptors, detect the low water concentration in
the blood. They then signal neurosecretory cells in
the hypothalamus to produce ADH, which is
transmitted to the posterior pituitary gland and
released into the blood, where it travels to the renal
tubules. With ADH present, the kidney tubules
reabsorb more water from the urine and return it to
the blood, and the volume of urine is reduced. If we
take in too much water, on the other hand, the
osmoreceptors detect the overhydration and inhibit
the production of ADH. Reabsorption of water is
reduced, and the volume of urine is
increased. Alcohol inhibits ADH production and
therefore increases the output of urine.
The liver also plays an important role in excretion.
This organ removes the ammonia and converts it
into the less toxic urea. The liver also chemically
changes and filters out certain drugs such as
penicillin and erythromycin. These substances are
then picked up by the blood and transported to the
kidneys, where they are put into the execretory
system.
The urinary system must function properly to
ensure good health. During a physical
examination, the physician frequently performs a
urinalysis. Urine testing can reveal diseases such
as diabetes mellitus, urinary tract infections,
kidney stones, and renal disease. Urography,
taking x rays of the urinary system, also helps
diagnose urinary problems. In this procedure, an
opaque dye is introduced into the urinary
structures so that they show up in the x rays.
Ultrasound scanning is another diagnostic tool. It
uses high frequency sound waves to produce an
image of the kidneys. Biopsies, samples of kidney
tissue obtained in a hollow needle, are also useful
in diagnosing kidney disease.
Disorders of the urinary tract include urinary
tract infections (UTI). An example is cystitis, a
disease in which bacteria infect the urinary
bladder, causing inflammation. Most UTIs are
treated with antibiotics. Sometimes kidney stones,
solid salt crystals, form in the urinary tract.
Kidney stones can obstruct the urinary passages
and cause severe pain, and bleeding. If they do not
pass out of the body naturally, the physician may
use shock wave treatment. In this treatment, a
shock wave focused on the stone from outside the
body disintegrates it. Physicians also
use surgery to remove kidney stones. Renal
failure is a condition in which the kidneys lose the
ability to function. Nitrogenous wastes build up
in the blood, the pH drops, and urine production
slows down. If left unchecked, this condition can
result in death. In chronic renal failure, the
urinary system declines, causing permanent loss
of kidney function.
Hemodialysis and kidney transplant are two
methods of helping chronic renal failure. In
hemodialysis, an artificial kidney device cleans
the blood of wastes and adjusts the composition of
ions. During the procedure, blood is taken out of
the radial artery in the patient's arm. It then
passes through dialysis tubing, which is
selectively permeable. The tubing is immersed in
a solution. As the blood passes through the tubing,
wastes pass out of the tubing and into the
surrounding solution. The cleansed blood returns
to the body. Kidney transplants also help chronic
kidney failure. In this procedure, a surgeon
replaces a diseased kidney with a closely matched
donor kidney. Although about 23,000 people in the
United States wait for donor kidneys each year,
fewer than 8,000 receive kidney transplants.
Current research aims to develop new drugs to help
kidney failure better dialysis membranes for the
artificial kidney.