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The Human Excretory
System
Biology 2201 Unit 3 Section 4
Excretory System
The kidneys regulate the amount of water, salts
and other substances in the blood.
The kidneys are fist-sized, bean shaped
structures that remove nitrogenous wastes
(urine) and excess salts from the blood.
The ureters are tubes that carry urine from the
pelvis of the kidneys to the urinary bladder.
The urinary bladder temporarily stores urine
until it is released from the body.
The urethra is the tube that carries urine from
the urinary bladder to the outside of the body.
The outer end of the urethra is controlled by a
circular muscle called a sphincter.
Excretory System
The Kidney
Each kidney is composed of three
sections:
– the outer (renal) cortex, the (renal) medulla
(middle part) and the hollow inner (renal)
pelvis.
The cortex is where the blood is filtered.
The medulla contains the collecting ducts which
carry filtrate (filtered substances) to the pelvis.
The pelvis is a hollow cavity where urine
accumulates and drains into the ureter.
The Parts
of the
Kidney
How blood is Filtered
The filtering units of the kidneys are the
nephrons.
There are approximately one million nephrons in
each kidney.
The nephrons are located within the cortex and
medulla of each kidney.
The tubes of the nephron are surrounded by
cells and a network of blood vessels spreads
throughout the tissue. Therefore, material that
leaves the nephron enters the surrounding cells
and returns to the bloodstream by a network of
vessels.
Parts of the Nephron
Each nephron consists of the following
parts:
– 1) glomerulus ;
– 2) Bowman’s capsule ;
– 3) proximal tubule ;
– 4) loop of Henle ;
– 5) distal tubule ;
– 6) collecting duct.
The Glomerulus
The glomerulus is a mass of thin-walled
capillaries.
The Bowman’s capsule is a double-walled,
cup-shaped structure.
The proximal tubule leads from the Bowman’s
capsule to the Loop of Henle.
The loop of Henle is a long loop which extends
into the medulla.
The distal tubule connects the loop of Henle to
the collecting duct.
~ See Figure 11.19; p. 375.
How the kidney does its job!
Kidney Action
Blood enters Bowman’s capsule through a tiny
artery — (the renal artery).
The artery branches to form a glomerulus.
Blood pressure forces some blood plasma and
small particles into the surrounding capsule —
(this is called the nephric filtrate).
Large particles such as blood cells and proteins
remain in capillaries.
The nephric filtrate is pushed out of the capsule
and into the proximal tubule .
This is where reabsorption begins.
Kidney action (cont)
Only materials needed by the body are
returned to bloodstream — for example,
99 % of water, all glucose and amino acids
and many salts are reabsorbed.
Osmosis , diffusion, and active transport
draw water , glucose , amino acids and
ions from filtrate into surrounding cells.
Small villi like projections help in the active
transport of glucose out of the filtrate and
speed up the reabsorption process.
From here, these components return to
the bloodstream.
Kidney action (cont)
The filtrate reaches the end of the
proximal tubule and the fluid is isotonic
with the surrounding cells.
Glucose and amino acids have been
removed from the filtrate.
The filtrate then moves to the loop of
Henle whose primary function is to
remove water from the filtrate by osmosis.
Kidney action (cont)
There is in increase in sodium
concentration in the cells of the loop of
Henle as we move from the area of the
cortex to the inner pelvis of the kidney.
This causes water to be drawn from the
filtrate.
These high levels of sodium are a result of
active transport and results in increased
concentration of filtrate (due to water loss).
Why doesn’t water on outside go
back in?
The water that left the descending loop
cannot enter the ascending loop because
it is impermeable to water.
As the water concentration in the filtrate
decreases, the chlorine concentration
increases and causes it to diffuse out of
the tubule.
This process is helped by the electrical
attraction of chlorine to sodium. .
Water (cont)
The filtrate moves to the distal tubule
where tubular secretion occurs.
Active transport is used to pull hydrogen
ions, creatinine, drugs such as penicillin
out of the blood and into the filtrate.
Fluid from a number of nephrons moves
from the distal tubules to a common
collecting duct which carries what can
now be called urine to the renal pelvis.
NOTE
Since the kidneys control what leaves
and what remains in the nephrons, they
maintain the levels of water, ions and
other materials nearly constant and
within the limits to maintain
homeostasis.
Function of the Kidney
The principal function of the kidney is
to filter blood in order to remove
cellular waste products from the body.
At any given time, 20 % of blood is in
the kidneys. Humans can function
with one kidney.
If one ceases to work, the other
increases in size to handle the
workload.
The kidney has other functions but it
is usually associated with the
excretion of cellular waste such as :
1) urea (a nitrogenous waste
produced in the liver from the
breakdown of protein. It is the main
component of urine) ;
2) uric acid (usually produced from
breakdown of DNA or RNA) and
3) creatinine (waste product of
muscle action).
All of these compounds have nitrogen as a
major component.
The kidneys are more than excretory
organs.
They are one of the major homeostatic
organs of the body.
They control water pH, secrete
erythropoietin (a hormone that stimulates
red blood cell production) and activate
vitamin D production in the skin.
That is why a doctor can tell so much
from a urine sample.
Disorders of the Excretory
System
Our excretory system is very important in
maintaining homeostasis. There are
certain conditions which can affect the
excretory system. We will discuss some of
these common disorders.
UTI (Urinary Tract Infection)
Is a very common disorder. If the bladder has
become infected, it is known as cystitis. If the
urethra is infected., it is called urethritis.
UTI is more common in women than men
because of differences in anatomy.
Symptoms include painful urination burning
sensation), frequent urination (even if no urine
present) and bloody or brown urine.
This can lead to chills, fever, nausea, vomiting
and upper abdomen tenderness.
Kidney Infections
Result when an infection reaches the
kidneys and becomes known as
pyelonephritis.
Common causes can be infection from
elsewhere in the body or obstruction of the
prostate gland (usually in older men).
For children, infection can be caused by
the tube that drains urine from the kidneys
and the bladder.
NOTE :
If left untreated, all UTI’s can lead to
permanent kidney damage and possible
kidney failure.
The general treatment is by antibiotics. A
person needs to maintain good personal
hygiene when eliminating wastes (liquid
and solid forms) from the body.
Also, a person should drink lots of water.
Kidney Stones
Are also a common kidney disorder. They form
when chemicals in the urine precipitate out and
form crystals.
The most common crystals are from calcium
oxalate, while others could be from uric acid and
cystine.
Kidney stones are more common in men than
women and can reoccur at any time.
Factors such as recurrent urinary bladder
infections, insufficient water intake and
consumption, low levels of physical activity, and
too much Vitamin C and D intake can lead to
kidney stones.
One of the best ways to decrease the
occurrence of stones is to drink lots of water and
to change your dietary habits.
Kidney Stones (cont)
Symptoms include severe back or abdomen
pain, blood in the urine, nausea and vomiting.
Diagnosis involves a complete medical
examination, including X-rays.
Treatment may vary from letting the stones pass
through the urinary tract to ultrasound shock (or
lithotripsy) to disintegrate the stones to a small
size that can be passed through the urinary
tract.
Real large stones require surgery for removal.