Transcript of The Urinary System and Excretion. Urinary System The kidneys are the primary organs of excretion....
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- The Urinary System and Excretion
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- Urinary System The kidneys are the primary organs of excretion.
Excretion is the removal of metabolic wastes from the body.
Functions of the Urinary System The urinary system produces urine
and conducts it to outside the body. As the kidneys produce urine,
they carry out four functions: 1.excretion of metabolic wastes. 2.
maintenance of water-salt balance. 3. maintenance of acid-base
balance. 4. and secretion of hormones.
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- Excretion of MetabolicWastes The kidneys excrete metabolic
wastes, notably nitrogenous wastes. Urea is the primary nitrogenous
end product of metabolism in human beings, but humans also excrete
some ammonium, creatinine, and uric acid. Maintenance of Water-Salt
Balance A principal function of the kidneys is to maintain the
appropriate water-salt balance of the blood. The kidneys also
maintain the appropriate level of other ions (electrolytes), such
as potassium ions (K), bicarbonate ions (HCO3), and calcium ions
(Ca2), in the blood.
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- Maintenance of Acid-Base Balance The kidneys regulate the
acid-base balance of the blood. the blood pH should be just about
7.4. The kidneys monitor and control blood pH, mainly by excreting
hydrogen ions (H) and reabsorbing the bicarbonate ions (HCO3) as
needed to keep blood pH at about 7.4. Secretion of Hormones 1.The
kidneys release renin, a substance that leads to the secretion of
the hormone aldosterone from the adrenal cortex which promotes the
reabsorption of sodium ions (Na) by the kidneys. 2. The kidneys
secrete the hormone erythropoietin, which stimulates red blood cell
production. 3.The kidneys also help activate vitamin D from the
skin.
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- Organs of the Urinary System The urinary system consists of the
kidneys, ureters, Urinary bladder, and urethra.
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- Kidneys The kidneys are paired organs located near the small of
the back in the lumbar region on either side of the vertebral
column. Each kidney is usually held in place by connective tissue.
Masses of adipose tissue adhere to each kidney. The kidneys are
bean-shaped and reddish-brown in color. The fist-sized organs are
covered by a tough capsule of fibrous connective tissue, called a
renal capsule. The concave side of a kidney has a depression called
the hilum where a renal artery enters and a renal vein and a ureter
exit the kidney.
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- Ureters The ureters, which extend from the kidneys to the
bladder, are small, muscular tubes. Each descends behind the
parietal peritoneum, from the hilum of a kidney, to enter the
bladder posteriorly at its inferior surface. The wall of a ureter
has three layers: 1.The inner layer is a mucosa (mucous membrane),
2.the middle layer consists of smooth muscle, 3. the outer layer is
a fibrous coat of connective Tissue.
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- Urinary Bladder The urinary bladder is located in the pelvic
cavity. In males, it is directly anterior to the rectum; in
females, it is anterior to the vagina and inferior to the uterus.
Its function is to store urine until it is expelled from the body.
The bladder has three openingstwo for the ureters and one for the
urethra, which drains the bladder. The bladder has other features
that allow it to retain urine. After urine enters the bladder from
a ureter, small folds of bladder mucosa act like a valve to prevent
backward flow.
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- Urethra The urethra is a small tube that extends from the
urinary bladder to an external opening. The urethra is a different
length in females than in males.
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- Anatomy of the Kidney and Excretion A kidney has many branches
of the renal artery and renal vein reach inside a kidney. Removing
the blood vessels shows that a kidney has three regions The renal
cortex is an outer The renal medulla. The renal pelvis is a central
space, or cavity, that is continuous with the ureter.
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- Anatomy of a Nephron the kidney is composed of over one million
nephrons, Each nephron has its own blood supply, including two
capillary regions: From the renal artery,an afferent arteriole
leads to the glomerulus, a knot of capillaries inside the
glomerular capsule. Blood leaving the glomerulus enters the
efferent arteriole. The efferent arteriole takes blood to the
peritubular capillary network, which surrounds the rest of the
nephron. From there, the blood goes into a venule that joins the
renal vein.
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- Nephron Anatomy
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- Parts of a Nephron Each nephron is made up of several parts.
First, the closed end of the nephron is pushed in on itself to form
a cuplike structure called the glomerular capsule (Bowmans
capsule). -Next, there is a proximal convoluted tubule (PCT), -loop
of the nephron (loop of Henle). Each loop consists of a descending
limb and an ascending limb. -the distal convoluted tubule (DCT)
-The distal convoluted tubules of several nephrons enter one
collecting duct. Many collecting ducts carry urine to the renal
pelvis.
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- Excretion urine formation, which is divided into these steps:
1.glomerular filtration, 2.tubular reabsorption, 3.and tubular
secretion.
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- Glomerular Filtration Glomerular filtration occurs when whole
blood enters the afferent arteriole and the glomerulus. Due to
glomerular blood pressure, water and small molecules move from the
glomerulus to the inside of the glomerular capsule. This is a
filtration process because large molecules and formed elements are
unable to pass through the capillary wall. In effect, then, blood
in the glomerulus has two portions: 1.the filterable components
(Water, Nitrogenous wastes, Nutrients, Nutrients) 2. the
nonfilterable components Formed elements (blood cells and
platelets) Plasma proteins
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- Tubular Reabsorption Reabsorbed FiltrateComponents Most water
Nutrients Required salts (ions) Nonreabsorbed Filtrate Components
Some water Much nitrogenous waste Excess salta (ions)
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- Tubular Secretion Tubular secretion is a second way by which
substances are removed from blood and added to the tubular fluid.
Hydrogen ions, potassium ions, creatinine, and drugs such as
penicillin are some of the substances that are moved by active
transport from the blood into the distal convoluted tubule.
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- Regulation of glomerular filtration rate and renal blood flow
The glomerular filtration rate is the total rate of renal fil-
Tration in both kidnesys and normally equals about 120 ml min
-1.This is remarkably constant over a wide range of conditions.
Physiological control of filtration is acom- plex and poorly
understood subject, but glomerular fil- tration rate may be
increased by factors which increase any of the following variables:
- Glomerular capillary hyd rostatic pressure Glomerular capillary
flow rate- Glomerular capillary surface area -
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- The maintenance of renal - blood flow is particularly Important
for normal filtration, since this is the main Determinant of
glomerular capillary pressure and flow. Renal blood flow is high
(about 1.2 min -1 ) and shows Remarkably little variation over a
wide range of arterial Prssures, providing us with one of the best
examples of Anutoregulation of local blood flow. Therefore, if
arterial pressure falls, dilatation of the Afferent arterioles
reduces renal vascular resistance and So helps to limit the
decrease in blood flow At very low Pressures.autoregulation breaks
down and renal blood flew. declines more rapidly. Reflex
stimulation of sympathetic nerves to the kidney under these
conditions may further reduce both glomerular blood flow and
hydrostatic pressure by constricting the afferent arterioles.
Therefore,both glomerular filtration and urine production may be
dramatically reduced in surgical shock with marked arterial
hypotension.
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