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Human Excretory System
The human excretory system functions to remove waste
from the human body. This system consists of specialized
structures and capillary networks that assist in the excretory
process. The human excretory system includes the kidney
and its functional unit, the nephron. The excretory activity of
the kidney is controlled by specialized hormones that
regulate the amount of absorption within the nephron.
Kidneys
The human kidneys are the major organs of
bodily excretion (see Figure 1 ). They are bean-shaped
organs located on either side of the backbone at about the
level of the stomach and liver. Blood enters the kidneys
through renal arteries and leaves through renal veins. Tubes
called ureters carry waste products from the kidneys to the
urinary bladder for storage or for release.
The product of the kidneys is urine, a watery solution of
waste products, salts, organic compounds, and two
important nitrogen compounds: uric acid and urea. Uric acid
results from nucleic acid decomposition, and urea results
from amino acid breakdown in the liver. Both of these
nitrogen products can be poisonous to the body and must be
removed in the urine.
Nephron
The functional and structural unit of the
kidney is the nephron. The nephron produces urine and is
the primary unit of homeostasis in the body. It is essentially
a long tubule with a series of associated blood vessels. The
upper end of the tubule is an enlarged cuplike structure
called the Bowman's capsule. Below the Bowman's capsule,
the tubule coils to form the proximal tubule, and then it
follows a hairpin turn called the loop of Henle. After the loop
of Henle, the tubule coils once more as the distal tubule. It
then enters a collecting duct, which also receives urine from
other distal tubules.
Within the Bowman's capsule is a coiled ball of capillaries
known as a glomerulus. Blood from the renal artery enters
the glomerulus. The force of the blood pressure induces
plasma to pass through the walls of the glomerulus, pass
through the walls of the Bowman's capsule, and flow into
the proximal tubule. Red blood cells and large proteins
remain in the blood.
After plasma enters the proximal tubule, it passes through
the coils, where usable materials and water are reclaimed.
Salts, glucose, amino acids, and other useful compounds
flow back through tubular cells into the blood by active
transport. Osmosis and the activity of hormones assist the
movement. The blood fluid then flows through the loop of
Henle into the distal tubule. Once more, salts, water, and
other useful materials flow back into the bloodstream.
Homeostasis is achieved by this process: A selected amount
of hydrogen, ammonium, sodium, chloride, and other ions
maintain the delicate salt balance in the body.
The fluid moving from the distal tubules into the
collecting duct contains materials not needed by the body.
This fluid is referred to as urine. Urea, uric acid, salts, and
other metabolic waste products are the main components of
urine. The urine flows through the ureters toward the
urinary bladder. When the bladder is full, the urine flows
through the urethra to the exterior.
Control of kidney function
The activity of the nephron
in the kidney is controlled by a person's choices and
environment as well as hormones. For example, if a person
consumes large amounts of protein, much urea will be in the
blood from the digestion of the protein. Also, on a hot day, a
body will retain water for sweating and cooling, so the
amount of urine is reduced.
Humans produce a hormone called antidiuretic hormone
(ADH), or vasopressin, which is secreted by the pituitary
gland. It regulates the amount of urine by controlling the
rate of water absorption in the nephron tubules.
Some individuals suffer from a condition in which they
secrete very low levels of ADH. The result is excessive
urination and a disease called diabetes insipidus. Another
unrelated form of diabetes, diabetes mellitus, is more
widespread. Persons with this disease produce insufficient
levels of insulin. Insulin normally transports glucose
molecules into the cells. But when insulin is not available,
the glucose remains in the bloodstream. The glucose is
removed from the bloodstream in the nephron; to dilute the
glucose, the nephron removes large amounts of water from
the blood. Thus, the urine tends to be plentiful.
Hormones from the cortex of the adrenal glands also
control the content of urine. These hormones promote
reabsorption of sodium and chloride ions in the tubules.
Thus, they affect the water balance in the body, because
water flows in the direction of high sodium and chloride
content.