URINARY SYSTEM : KIDNEY AND ITS FUNCTIONS- ENGLISH
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URINARY SYSTEM : KIDNEY AND ITS FUNCTIONS
• The
urinary system contributes to homeostasis by altering blood composition, pH,
volume, and pressure; maintaining blood osmolarity; excreting wastes and
foreign substances; and producing hormones.
• The
urinary system consists of two kidneys, two ureters, one urinary
bladder, and one urethra
KIDNEY-
• The
kidneys lie on the posterior abdominal wall, one on each side of the vertebral
column, behind the peritoneum and below the diaphragm. They extend from the
level of the 12th thoracic vertebra to the 3rd lumbar vertebra,
receiving some protection from the lower rib cage. The right kidney is usually
slightly lower than the left.
• They
are approximately 11 cm long, 5–6 cm wide and 3–4 cm thick. They are bean‐shaped organs, where the outer border is
convex; the inner border is known as the hilum
and it is the place that the renal arteries, renal veins, nerves and the
ureters enter and leave the kidneys. The renal artery carries blood to the
kidneys; and once the blood is filtered, the renal vein takes the blood away.
STRUCTURE OF KIDNEY-
• Covering
and supporting the kidneys are three layers:
• ••
renal fascia •• adipose tissue ••
renal capsule.
• The
renal fascia is the outer layer and consists of a thin layer of connective
tissue that anchors the kidneys to the abdominal wall and the surrounding
tissues. The middle layer is called the adipose tissue and surrounds the
capsule. It cushions the kidneys from trauma. The inner layer is called the
renal capsule. It consists of a layer of smooth connective tissue that is
continuous with the outer layer of the ureter
• Internal
structures-
• There
are three distinct regions inside the kidney:
• Renal
cortex- The renal cortex is the outermost part of the kidney. In adults, it
forms a continuous, smooth outer portion of the kidney with a number of
projections (renal column) that extend down between the pyramids. The renal
column is the medullary extension of the renal cortex. The renal cortex is
reddish in colour and has a granular appearance, which is due to the
capillaries and the structures of the nephron
• Renal
medulla- The medulla is lighter in colour and has an abundance of blood
vessels and tubules of the nephrons . The medulla consists of approximately
8–12 renal pyramids .The renal pyramids, also called malpighian pyramids, are
cone‐shaped sections of the kidneys. The wider portion of the cone faces the
renal cortex, while the narrow end points internally, and this section is
called the renal papilla.
• Urine
formed by the nephrons flows into cup‐like structures, called calyces, via
papillary ducts. Each kidney contains
approximately 8–18 minor calyces and two or three major calyces. The
minor calyces receive urine from the renal papilla, which conveys the urine to
the major calyces. The major calyces unite to form the renal pelvis,
• Renal
pelvis- The major calyces unite to form the renal pelvis, which then conveys
urine to the bladder. The renal pelvis forms the expanded upper portion of the
ureter, which is funnel‐shaped and it is the region where two or three calyces
converge. The walls of the pelvis contain smooth muscle and are lined with
transitional epithelium. Peristalsis of the smooth muscle originating in
pacemaker cells in the walls of the calyces propels urine through the renal
pelvis and ureters to the bladder.
FUNCTIONS OF KIDNEY-
• Excretion
of wastes and foreign substances. By forming urine, the kidneys help
excrete wastes—substances that
have no useful function in the body. Some wastes excreted in urine
result from metabolic reactions in the body. These include ammonia, urea,
bilirubin as urobilinogen, creatinine,
uric acid from the catabolism of nucleic acids. Other wastes excreted in urine
are foreign substances from the diet, such as drugs and environmental toxins.
• Regulation
of blood ionic composition. The kidneys help regulate the blood levels
of several ions, most importantly sodium ions (Na), potassium ions (K), calcium
ions (Ca), chloride ions (Cl), and phosphate ions .
• Regulation
of blood pH. The kidneys excrete a variable amount of hydrogen ions (H)
into the urine and conserve bicarbonate ions
which are an important buffer of H in the blood. Both of these activities
help regulate blood pH.
• Regulation
of blood volume. The kidneys adjust blood volume by conserving or
eliminating water in the urine. An increase in blood volume increases blood
pressure; a decrease in blood volume decreases blood pressure.
• Regulation
of blood pressure. The kidneys also help regulate blood pressure by
secreting the enzyme renin, which activates the renin–angiotensin–aldosterone
pathway . Increased renin causes an increase in blood pressure.
• Maintenance
of blood osmolarity. By separately regulating loss of water and loss of
solutes in the urine, the kidneys maintain a relatively constant blood
osmolarity (300 milliosmoles per liter).
• Production
of hormones. The kidneys produce two hormones. Calcitriol, the
active form of vitamin D, helps regulate calcium homeostasis and erythropoietin
stimulates the production of red blood cells
• Regulation
of blood glucose level. Like the liver, the kidneys can use the amino
acid glutamine in gluconeogenesis, the synthesis of new glucose
molecules. They can then release glucose into the blood to help maintain a
normal blood glucose level.
•
URETER-
• The
ureters are the tubes that carry urine from the kidneys to the urinary bladder.
They are about 25 to 30 cm long with a diameter of about 3 mm. The ureter is
continuous with the funnel-shaped renal pelvis. It passes downwards through the
abdominal cavity, behind the peritoneum
• It
passes obliquely through the posterior wall of the bladder. Because of this
arrangement, when urine accumulates and the pressure in the bladder rises, the
ureters are compressed and the openings occluded. This prevents reflux of urine
into the ureters (towards the kidneys).
• The
walls of the ureters consist of three layers of tissue. an outer covering of fibrous
tissue, continuous with the fibrous capsule of the kidney a middle muscular
layer consisting of interlacing smooth muscle fibres that form a functional
unit round the ureter and an additional outer longitudinal layer in the lower
third an inner layer, the mucosa, composed of transitional epithelium.
• The
ureters propel urine from the kidneys into the bladder by peristaltic
contraction of the smooth muscle layer. This is an intrinsic property of the
smooth muscle and is not under autonomic nerve control. Peristalsis originates
in a pacemaker in the minor calyces. Peristaltic waves occur several times per
minute, increasing in frequency with the volume of urine produced.
URINARY BLADDER-
• The
urinary bladder is a hollow muscular organ and is located in the pelvic cavity
posterior to the symphysis pubis. In the male the bladder lies anterior to the
rectum, and in the female it lies anterior to the vagina and inferior to the
uterus. The bladder is roughly pear shaped, but becomes more oval as it fills
with urine. Urinary bladder capacity averages 300–400 ml (maximum 750-800ml).
It is smaller in females because the uterus occupies the space just superior to
the urinary bladder.
• The
inner lining of the urinary bladder is a mucous membrane of transitional
epithelium that is continuous with that in the ureters. When the bladder is
empty, the mucosa has numerous folds called rugae. The rugae and transitional
epithelium allow the bladder to expand as it fills. The second layer in the
walls is the submucosa, which supports the mucous membrane. It is composed of
connective tissue with elastic fibres.
• Third
layer is a thick muscular layer which is covered by a fibrous outer layer. The
three orifices in the bladder wall form a triangle or trigone. The upper
two orifices on the posterior wall are the openings of the ureters. The lower
orifice is the opening into the urethra. The internal urethral sphincter,
a thickening of the urethral smooth muscle layer in the upperpart of the
urethra, controls outflow of urine from the bladder
URETHRA-
• The
urethra is a canal extending from the neck of the bladder to the exterior, at
the external urethral orifice. It is longer in the male(It is approximately 20
cm long) than in the female. The female urethra is approximately 4 cm long and
6 mm in diameter. It runs downwards and forwards behind the symphysis pubis and
opens at the external urethral orifice just in front of the vagina. The
external urethral orifice is guarded by the external urethral sphincter,
which is under voluntary control.
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