Download The Ecxcretory System

The Urinary System
and Excretion
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.
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.
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.
Organs of the Urinary
System
The urinary system
consists of the
kidneys,
ureters,
Urinary bladder,
and urethra.
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.
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.
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 openings—two 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.
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.
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.
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.
Nephron Anatomy
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 (Bowman’s
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.
Excretion
urine formation, which is divided
into these steps:
1.glomerular filtration,
2.tubular reabsorption,
3.and tubular secretion.
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
Tubular Reabsorption
Reabsorbed FiltrateComponents
Most water
Nutrients
Required salts (ions)
Nonreabsorbed Filtrate
Components
Some water
Much nitrogenous waste
Excess salta (ions)
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.
Regulation of glomerular filtration rate and
renal blood flow
The glomerular filtration rate is the total rate of renal filTration 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 acomplex and poorly understood subject, but glomerular filtration 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
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.