Download INTEGRATED SCIENCE EXCRETION WHAT IS EXCRETION? AND

INTEGRATED SCIENCE
EXCRETION
WHAT IS EXCRETION? AND WHY IS IT IMPORTANT?
Excretion is the elimination of the waste products of metabolism.
(The sum total of all chemical reactions going on in cells is known as metabolism).
Metabolism can be divided into two categories:
1. Anabolism – which is the synthesis of new living material in the organism.
2. Catabolism – which is the breakdown of substances in the organism (e.g. breakdown of
material to release energy)
Many of the waste products of metabolism are toxic and if allowed to accumulate in the body
they could damage and kill body cells. They therefore need to be removed as they are produced
by the body.
NOTE: Faeces is not an excretory product. WHY?
EXCRETORY ORGANS AND PRODUCTS
Excretory Product
Excretory Organ Notes
Carbon Dioxide
Lungs
Carbon dioxide is a by-product of respiration and is
excreted in expired/exhaled air.
Mineral Salts
Skin, Kidneys
Small amounts of salts are present in sweat/urine.
Urea, Uric Acid
Liver, Kidneys,
Skin
Urea is produced in the liver by deamination and is
excreted in urine by kidneys. Some urea is also
excreted in sweat.
Bile pigments
Liver (via anus)
Bile pigments are products from haemoglobin
breakdown.
Excess Water
Kidneys, Skin,
Lungs
Water is the main constituent of water and sweat.
Expired air also contains water vapour.
THE URINARY SYSTEM
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renal artery – transports blood to
the kidneys
renal vein – transports blood away
from the kidneys to main vein, back
to the heart
ureter – transports urine to the
bladder
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urethra – transports urine to the
exterior of the body
sphincter muscle – guards the
opening of the bladder
bladder – stores urine
FUNCTIONS OF KIDNEYS
1. Main excretory organs to remove unwanted metabolic wastes, including urea, excess
water and mineral salts from blood, in the form of __________.
2. The kidneys act as a filter to remove water, salts, urea, while leaving __________ in the
blood.
3. Allows selective reabsorption to occur. Useful substances (such as ___________) are
reabsorbed from the nephrons back into blood. Kidneys maintain ____ and composition
balance of blood.
4. Regulates ______ and ________ balance of body fluids. Kidneys remove more or less
water according to bodily needs, therefore maintains level of water in body.
STRUCTURE OF THE KIDNEY
Description of the structure of the kidney
They are reddish-brown bean-shaped organs situated towards the back of the abdominal cavity
just above the waist.
Inside the kidney, it is divided into a light outer area called the cortex, a darker inner layer called
the medulla. And an inner cavity called the pelvis. The pelvis narrows to form the ureter.
To reiterate, kidneys have two main functions:
1. Excretion – elimination of urea (nitrogenous waste)
2. Osmoregulation – control of the relative concentrations of salt and water in the blood.
HOW DOES THE KIDNEY EXCRETE UREA?
The kidney excretes urea in a two-stage process. First, all small molecules in the blood are
filtered out. Then, any useful molecules that were filtered out are reabsorbed into the blood. Urea
is a waste product and therefore not reabsorbed.
Each kidney receives from the renal artery. The blood passes into thousands of smaller arteries
(or arterioles) in the kidney. Each of these arterioles lead to one of the kidney’s filtration units.
These filtration units are called nephrons. Each nephron is a tube with a filter at the start. This
filter is called the Bowman’s Capsule.
In the Bowman’s Capsule, the pressure of the blood forces small molecules out of it, through a
membrane. These molecules include:
 Water
 Salt (sodium chloride)
 Glucose
 Amino acids
 Urea
These molecules then form a filtrate, which passes along the nephron. As the filtrate passes
through the first coiled part (proximal coiled tubule), useful molecules are reabsorbed by the
blood. Urea is not reabsorbed and continues along the nephron with some of the water.
When the filtrate reaches the collecting duct, it passes down the collecting duct to the pelvis
(centre of the kidney). It is now called urine. From the pelvis, is moved along a ureter to the
bladder.
To summarize:
1. Blood enters via renal artery.
2. Blood enters smaller arteries to a nephron.
3. Each nephron has a Bowman’s capsule, which filters small
molecules out.
4. These small molecules form a filtrate, which passes along
the nephron.
5. In the first coil, only useful substances are reabsorbed.
6. The filtrate eventually reaches the collecting duct.
7. It passes from there to the centre of kidney (pelvis), where it
is now urine.
8. From pelvis, it moves along a ureter to bladder.
HOW DOES THE KIDNEY HELP REGULATE WATER CONTENT IN BLOOD
(OSMOREGULATION)?
It does this by varying the amount of water in the urine. When the filtrate passes down the
collecting ducts, water is reabsorbed into the bloodstream. The amount of water reabsorbed
varies, according the body’s needs. The control of this amount is called osmoregulation.
Sensors in the brain detect amount of water in the blood and responses are sent to balance it if
there is too much or too little. These responses involve the hormone _______, also called
_____________________ ______________. ADH makes the walls more permeable to water.
More water can then pass back into the blood stream.
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When we have too little water in our bodies, ADH is released by the ________________
glands and travels to the kidneys. It alters the walls of the collecting ducts so that water
can pass back into the blood stream.
When we have too much water in our bodies, the pituitary gland does not release ADH.
The walls of the collecting ducts do not let water back to the bloodstream and so we lose
a lot of water in urine.
Concentration of water
increased (blood too dilute)
Does pituary gland release
ADH?
Answer: ____________
Normal amount of water in
blood
Amount of water in blood
decreases (blood too
concentrated)
Wall of collecting duct
remains impermeable.
________ water lost in
urine.
Normal amount of water in
blood.
Does pituary gland release
ADH?
Answer: ____________
Walls of collecting ducts
permeable, therefore:
Water is _______________
; ________ water lost in
urine.
KIDNEY FAILURE AND THE DIALYSIS MACHINE
If one kidneys fails to function, a person can still lead a normal life with the other kidney. But if
both kidneys fail to work, e.g. in a severe infection or accident, the person will die unless prompt
medical treatment is given.
The patient can be treated with a dialysis machine. Blood is drawn from an artery in the
patient’s arm and is allowed to flow through the tubing in the dialysis machine. The tubing is
bathed in a specially controlled dialysis fluid. The walls of the tubing are partially permeable.
They allow small molecules, like urea, to diffuse out of the tubing into the dialysis fluid. Big
molecules, like proteins and blood cells, remain in the tubing. This process is called DIALYSIS.
The tubing is narrow, long and coiled to increase the surface area to volume ratio. This speeds
up the rate of exchange of substances between the blood and the dialysis fluid.
The direction of blood flow is opposite to the flow of the dialysis fluid. This maintains the
diffusion gradient for the removal of waste products.
The filtered blood is then returned to a vein in the patient’s arm. Each treatment takes several
hours and the patient needs to be treated two to three times a week.