Topic 9 - EXCRETION
9.1 – Structure and function of kidneys, 9.2 – Kidney dialysis
SUFEATIN SURHAN  BIOLOGY MSPSBS  2010
SYLLABUS CHECKLIST
Candidates should be able to:
(a) define excretion as the removal of toxic materials and the waste products
of metabolism from organism;
(b) describe the removal of carbon dioxide from the lungs;
(c) identify from diagrams and name the kidneys, ureters, bladder, urethra
and state the function of each (the function of the kidney should be
described simply as removing urea and excess salts and water from the
blood; details of kidney structure and nephron are not required);
(d) describe dialysis in kidney machines as the diffusion of waste products
and salts (small molecules) through a membrane; large molecules (e.g.
protein) remain in the blood.
Metabolism, waste products and excretion

Metabolism is the sum total of all the chemical processes that are continually going on inside living
cells. It includes:

Catabolic processes where complex molecules are broken down to simple ones with release of
energy, e.g. cellular respiration and the breakdown of amino acids to urea.

Anabolic processes where simple molecules are built up into complex ones with an intake of
energy, e.g. photosynthesis and synthesis of glycogen from glucose.

During metabolism, living cells use up water, oxygen and digested food. In the process of using these
substances, the living cells produce harmful by-products.
These by-products of metabolism are
considered waste products because they are unwanted by the cells and must be got rid of.
Types of waste
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Getting rid of waste

Since carbon dioxide, excess water and urea are harmful to the body, the body gets rid of them.
We say that the body excretes them.
Excretion is defined as the removal of toxic materials and waste products of
metabolism from organisms

Take note that egestion is not a process of excretion. Egestion is defined as the removal of faeces
out of the body through the anus.
Faeces consists mainly of undigested food, which has never
entered any cell and so is not considered a waste product of metabolism.
Recalling – Removal of Carbon dioxide from the lungs
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The Human Urinary System

The system consists of the following:
1. Kidneys – red bean-shaped structures situated near the backbone, just below the
stomach.
Kidneys contain numerous kidney tubules (nephrons) which remove urea,
excess water and heat from blood to urine.
They are also responsible for osmoregulation – the process of keeping the osmotic
pressure of the body fluids constant by controlling the amount of water and solute in
them (which the kidneys do by excreting whatever is in excess).
2. Ureters – these are the tubes through which urine flows from the kidneys to the bladder.
3. Urinary bladder – a muscular bag which stores urine.
4. Urethra – a muscular tube through which urine flows from the bladder to the exterior.
(It passes through the penis in the male and opens at its tip to the exterior.)
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5. Associated blood vessels –
(a) Renal arteries: These bring oxygenated blood containing urea, excess water and heat
to the kidneys.
(b) Renal veins: these take away deoxygenated blood (from which urea, excess water and
heat have been removed) from the kidneys.
The Structure of the kidney

If sliced longitudinally, each kidney consists of:
(a) Cortex – a dark outer layer and
(b) Medulla – a paler inner layer consisting of a number of pyramids.

Inside each kidney are millions of microscopic uriniferous tubules called nephrons. Each nephrons
consists of the following:
(a) bowman’s capsule enclosing the glomerulus,
(b) first convoluted tubule,
(c) loop of Henle,
(d) second convulated tubule leading to the collected duct and ureter.
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
Urine formation involves two main processes, ultrafiltration (a mechanical process) and selective
reabsorption of useful materials.
The main steps of urine formation in the above figure are:
1. Blood from the renal artery arrives at the nephron. This blood carries metabolic wastes (urea and
excess water) and useful substances (glucose and amino acids) to the kidney tubule.
2. All small molecules such as urea, water, glucose and amino acids are forced into the kidney tubule by
the blood pressure. This is called ultrafiltration. The big blood cells and blood proteins remain
inside the blood capillaries.
3. Further down the tubule, the useful substances are selectively reabsorbed into the blood capillaries
by active transport. Most of the water is also reabsorbed by osmosis.
4. The unwanted materials, mainly urea and water, that remain in the kidney tubule form urine.
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5. The urine made by all the nephrons of a kidney drips into a tube called the ureter.

We have two ureters, one from each kidney.

The two ureters carry the urine to a bag called the urinary bladder.

The bladder stores urine. (A ring of muscle closes the bottom of the bladder. This prevents the
urine from leaking out all the time.)

When the bladder is full of urine, you feel and urge to go to the toilet to relieve yourself. The
ring of muscle at the bottom of the bladder relaxes so that urine can flow out through the
urethra.
6. Blood that has been cleared of urea and excess water flows out of the nephrons, into the renal vein.
KIDNEY DIALYSIS

Kidneys may be damaged as a result of accident or disease or infection.

When the kidneys are extensively damaged, they may stop working properly. This is known as renal
failure.

Renal failure is dangerous as the kidneys will not be able to remove urea and toxins in the blood.

The build-up of urea and toxins, if left untreated, will eventually prove fatal.

Therefore, to provide longevity of life to individuals with renal failure, a kidney dialysis machine is
used.

This machine is based on the simple process of dialysis.
Definition of dialysis:
Diffusion of small molecules in the blood, such as waste products (urea
and toxins) and salts, through a semi-permeable membrane, but not large
molecules such as plasma proteins (retained in the blood).
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
A kidney dialysis machine is used to cleanse the blood of a person whose kidneys do not work. This is
done in the following way:

Blood from an artery in the arm of a patient is allowed to flow through a dialysis tubing in the
machine. This tubing is narrow, long and coiled to increase surface area.

High pressure in artery is sufficient to force blood along the tubing.

The dialysis tube has tiny pores, the size of which are just enough for the small molecules to pass
through. Larger molecules such as plasma proteins cannot pass through and remain in the blood

The dialysis tubing is bathed in a dialysis fluid similar to blood plasma (but it has no waste products).

In this way, there is a concentration gradient for these waste substances causing them to move out of
Blood plasma
Glucose
Amino acids
Urea & toxins
Water and Mineral salts (ions)

Semi-permeable membrane
the blood in the tubing into the dialysing fluid.
Dialysing fluid
Glucose
Amino acids
Water and mineral salts
This concentration gradient is maintained in two ways:
1. By the counter current flow (opposing flow) of the blood and the dialysing fluid.
2. By constantly renewing the fluid.

The dialysed blood is then returned to the patient through a tube connected to a vein in the arm.

For effective results, the patient should undergo dialysis for up to 7 or 8 hour, two or three times a
week (depending on severity of renal failure).
Disadvantages of kidney dialysis machine

Expensive

Reduced quality of life:
-
Time consuming
-
Highly inconvenient (restricted mobility during the duration of dialysis, except for peritoneal
dialysis)

Controlled diet

Higher risk of developing osteoporosis and anaemia

Higher risk of obtaining blood transmitted diseases and infection
Alternative to Dialysis : Kidney transplant
A kidney transplant is the best cure to renal failure. However, there are many problems to it. The main
one is the difficulty in obtaining a suitable (tissue compatible) kidney due to a lack of kidney donors. The
expense spent on buying a kidney may be high, however, this is only a one-off expense and is
substantially cost-saving in the long run, compared to using a dialysis machine.
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