HLTAP301A ANATOMY & PHYSIOLOGY
LECTURE 12 - THE URINARY SYSTEM
Metabolism of nutrients by body cells produces waste products such as
carbon dioxide and nitrogenous wastes (such as creatinine, urea and
ammonia). These metabolic wastes must be eliminated from the body, while
retaining essential substances to maintain homeostasis. While several organ
systems are involved in the excretory process, the urinary system bears the
primary responsibility for removing nitrogenous wastes from the blood. The
kidneys filter blood and return most of the water and many solutes to the
bloodstream. The remaining water and solutes constitute urine. Urine is
excreted from each kidney through it’s ureter and stored in the urinary bladder
until it is expelled through the urethra. In addition to the pure excretory
function of the kidneys, they also maintain the electrolyte, acid-base and other
fluid balances in the blood. Malfunction of the kidneys leads to an imbalance
in homeostasis, which unless corrected, will result in death.
The structure of the urinary system includes two kidneys, two ureters, one
urinary bladder and one urethra. Let’s look at these structures in more detail:
The Kidneys
The paired kidneys are kidney-bean shaped organs located just above the
waist between the peritoneum and the posterior wall of the abdomen. The
kidneys are located between the levels of the last thoracic and third lumbar
vertebrae, and the right kidney is slightly lower than the left, because of the
location of the liver.
An adult kidney is about 12cm long, 6cm wide and 3 cm thick, about the size
of a large bar of soap. Each kidney is enclosed by a fibrous, transparent
renal capsule and in a living person, a capsule of fatty tissue surrounds each
kidney, protecting them from external damage. In the event of a rapid weight
loss, the kidneys may drop to a lower position which can cause the ureters to
become kinked, which in turn causes urine to back up and exert pressure on
the tissue of the kidneys, a condition known as hydronephrosis, which can
severely damage the kidney.
The Nephron
A nephron is both the structural and functional unit of the kidney and may
sometimes be referred to as the kidney tubules. Nephrons do three things –
they filter blood, return useful substances to the blood and remove
substances from the blood that are not needed by the body. Nephrons
maintain the homeostasis of the blood and urine is produced.
There are about one million of these microscopic structures and each nephron
consists of two main parts – a glomerulus (a knot of capillaries that perform
the filtration process) and a renal tubule. The closed end of the renal tubule is
a cup-shaped structure that completely surrounds the glomerulus. This part
of the renal tubule is known as Bowman’s capsule.
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As the renal tubule extends from the Bowman’s capsule, it twists into three
distinct regions – the proximal convoluted tubule (PCT), the Loop of Henle
and the distal convoluted tubule (DCT).
The collecting ducts in the nephron runs from the DCT downwards into the
renal pyramids, delivering urine, which will eventually move into the ureters.
The Ureters, the Bladder and the Urethra
The ureters are passageways that carry urine from the kidneys to the bladder
and are approximaely 25-30 cm long.
The urinary bladder is a hollow,
muscular organ situated in the pelvic cavity posterior to the pubic symphasis
and is the storage area for urine. In males, it is directly anterior to the rectum,
in females it is anterior to the vagina and inferior to the uterus. It is held in
place by folds of the peritoneum. The bladder changes shape depending on
the amount of urine contained within. When the bladder is empty, it is
collapsed; when it is slightly distended it becomes spherical and when it is
fuller, it becomes pear-shaped. Although urine is formed continuously by the
kidneys, it is usually stored in the bladder until its release is convenient. The
bladder capacity averages 700-800ml. It is smaller in females because the
uterus occupies the space just superior to the bladder.
The urethra is a small tube leading from the internal urethral orifice in the floor
of the urinary bladder to the exterior of the body and it carries urine by
peristalsis from the bladder to the outside of the body. In both males and
females the urethra is the terminal portion of the urinary system. In females
the urethra is located between the clitoris and the vaginal opening. In males,
the urethra passes through the prostate gland, through the urogenital
diaphragm and finally through the penis.
The internal urethral sphincter (involuntary) keeps the urethra closed when
urine is not being passed. The external urethral sphincter is controlled
voluntarily. The length and function of the urethra differs in the two sexes. In
females, it is about 3-4cm long and its only function is to conduct urine to the
body exterior. In males it is about 20cm long and not only carries urine out of
the body but also provides the passageway through which sperm is ejected
from the body. Thus, in males, the urethra is part of both the urinary and
reproductive systems.
Urine Formation
For urine to be formed in the nephron three processes must take place –
filtration, reabsorption and secretion.
Filtration is performed by the capillary beds of the glomerulus – they have
extremely high blood pressure which forces fluid and solutes (smaller than
proteins) out of the blood into the glomerular capsule. The filtrate that is
formed is basically blood plasma without blood proteins.
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When systemic blood pressure is normal, filtration will continue to occur. If
arterial blood pressure drops too low, the glomerular blood pressure drops
and substances can no longer be forced out of the blood, thereby stopping
filtrate formation.
Tubular Reabsorption takes place in the renal tubules, commencing as soon
as the filtrate enters the proximal convoluted tubule. The filtrate contains
many useful substances that the body can still use – these include water,
glucose, amino acids and ions. These need to be reclaimed from the filtrate
and returned to the blood. The capillary beds are low pressure, porous
vessels that are specially adapted for absorption. Some of the substances,
such as water, reenter the capillaries passively via osmosis. Others require
active transport processes, and the body is very selective about what is
allowed to pass through. For example, glucose and amino acids are usually
always completely removed from the filtrate, but nitrogenous wastes such as
urea are hardly reabsorbed at all. Most absorption takes place in the proximal
convoluted tubule, however the Loop of Henle, the distal convoluted tubule
and collection ducts can reabsorb substances as well.
Secretion is reabsorption in reverse. For example, substances that the body
doesn’t need, (such as urea, uric acid and creatinine) are excreted from the
peritubular capillaries and into the renal tubules and become part of the
filtrate, which then becomes urine. Ions such as hydrogen and potassium
also move into the filtrate, and this plays an important role in maintaining
blood pH.
Solutes normally found in urine are sodium, potassium, urea, uric acid,
creatinine, ammonia, bicarbonate ions and various other ions depending on
blood composition. Substances not normally found in urine are glucose,
blood proteins, red blood cells, haemoglobin, white blood cells and bile.
Homeostatic Imbalances
Uremia is a condition resulting from advanced stages of kidney failure in
which urea and other nitrogen-containing wastes are found in the blood.
Uremia can be caused by NSAIDs (nonsteroid anti-inflammatory drugs),
especially in older patients treated primarily with ibuprofen for arthritis. Some
of the early signs of uremia are lethargy, mental depression, loss of appetite,
and edema; later symptoms include diarrhea, anemia, convulsions and even
coma.
Incontinence occurs when we are unable to voluntarily control of external
sphincter. It is common in children under 2 and in women during pregnancy.
It can also be caused by emotional problems and degradation of the pelvic
floor muscles.
Urinary retention is the opposite of incontinence – where the bladder is
unable to expel its contained urine. It can occur after a general anesthetic
because it takes a little time for the smooth muscles to regain their activity. It
can also be caused by an enlarged prostate gland.
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Maintaining Water Balance
The human body is mostly liquid and our total body water is a function of age,
body mass, and body fat. Due to their low body fat and bone mass, infants
are about 73% water, whereas the body water content of men is about 60%.
Because women have relatively more body fat and less skeletal muscle than
men their water content is about 50%. Body water declines throughout life,
ultimately comprising about 45% of total body mass in old age.
The kidneys maintain chemical balance throughout the body by producing and
eliminating urine – they regulate the volume, electrolyte concentration and
acid-base balance of body fluids. The skin and lungs also play a role in fluid
and electrolyte balance through the process of perspiration and respiration. In
addition the blood contains buffers which regulate bicarbonate ions.
Fluid imbalance occurs when regulatory mechanisms can’t compensate for an
abnormal intake and output at any level from the cell to the organism as a
whole. Fluid and electrolyte imbalances include oedema, isotonic, hypertonic
or hypotonic alterations, and electrolyte imbalances. Disorders of fluid volume
or osmolarity (concentration of electrolytes in the fluid) result.
Increased fluid volume in the interstitial spaces is called oedema. Oedema
may be classified as localised or systemic. Obstruction of the veins or
lymphatic system or increased vascular permeability usually causes localised
oedema in the affected area (eg. Swelling around an injury).
Systemic or generalised oedema may be due to cardiac failure or renal
disease.
For the body to remain properly hydrated, water intake must equal water
output. Most water enters the body through ingested liquids and food, but is
also produced by cellular metabolism. Water output is due to evaporative loss
from lungs and skin (insensible water loss), sweating, defecation, and
urination.
The thirst mechanism is triggered by a decrease in plasma osmolarity, which
results in a dry mouth and excites the hypothalamic thirst center. Thirst is
quenched as the mucosa of the mouth is moistened, and continues with
distention of the stomach and intestines, resulting in inhibition of the
hypothalamic thirst center.
The amount of water reabsorbed in the renal collecting ducts is proportional to
Anti Diuretic Hormone (ADH) release. When ADH levels are low, most water
in the collecting ducts is not reabsorbed, resulting in large quantities of dilute
urine. When ADH levels are high, filtered water is reabsorbed, resulting in a
lower volume of concentrated urine.
ADH secretion is promoted or inhibited by the hypothalamus in response to
changes in solute concentration of extracellular fluid, large changes in blood
volume or pressure, or vascular baroreceptors.
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