B05 – Transport Systems
Blood
Your blood
Your blood does two important things:
 It transports things from one part of your body to another
 It helps protect your body from disease
What is blood made from?
You have about 5 litres of blood in your body. Blood looks like a red liquid, but there is more
to it than that. There is a liquid part called plasma and cells floating in the plasma – red
blood cells, white blood cells and platelets.
Plasma
Plasma is a yellow liquid. It is mainly water with chemicals dissolved in it. Some of these are;
- Food, such as sugars, amino acids, vitamins and minerals (as ions)
- Chemical waste, like urea
- Blood proteins, like antibodies
- Hormones – chemicals that control things like our growth
- Carbon dioxide – only some of this is carried by the red blood cells, the rest is
dissolved in plasma and diffuses out of the plasma in the lungs
Red blood cells
They look red because they contain a red pigment called haemoglobin.
There are about 1 million red blood cells in one drop of blood. They are made in the bone
marrow. Red blood cells carry oxygen. They are disc-shaped with the middle pushed in. they
have a large surface area to volume ratio. This helps them absorb a lot of oxygen.
Red blood cells have no nucleus. Instead the cell is filled with haemoglobin. This is a special
protein that contains iron. Haemoglobin combines easily with oxygen to form
oxyhaemoglobin.
The red cells also carry some carbon dioxide. But most of it is carried dissolved in the plasma.
White blood cells
The white cells fight disease. There are far fewer of them than red cells. They also look
different. White cells are not disc-shaped. They do not have haemoglobin. They do have a
nucleus.
Like red cells, the white cells are made in the bone marrow. Their job is to protect the body
from any germs (bacteria and viruses) that get into the blood. The two types of white blood
cells do this in different ways.
Lymphocytes
When a bacterium or virus enters the body the white cells recognize that it is ‘foreign’
and should not be there. The lymphocytes make chemicals called antibodies. These
attack the germs in a number of ways;
- They make them stick together
- They dissolve them
- They destroy the toxins (poisons) that germs make
After you have had a disease the antibodies stay in your blood. They make you
immune to the disease. There is a different antibody for each type of germ.
Phagocytes
These are the ‘cell-eaters’. They swallow up germs and take them into the cell. They
then digest them and kill them.
When germs invade the body white cells move towards them. The white cells can
even squeeze through capillary walls.
Blood clotting – platelets
When you cut yourself you bleed. Before long the blood thickens and the bleeding stops. The
thickened blood has formed a clot.
This is another way that the blood fights disease. Without clotting, blood would be lost and
germs would get into the body.
Platelets are small fragments of cells. They are made in the bone marrow like the other blood
cells. Platelets are important if clotting is to take place. Many chemicals are needed for
clotting to happen.
Questions
1. Name three ways in which the white blood cells are different from the red blood cells.
2. Name 2 things about red blood cells that adapt them for carrying oxygen.
3. What are the two types of white cell? Explain how they act to protect the body from
disease.
4. At high altitude there is less oxygen in the air. People who live at altitude make more
red cells to get what oxygen they can from the air. The table shows the parts of the
blood of 3 people:
Person A
Person B
Person C
Red cells (mm3)
7500000
5000000
2000000
White cells (mm3)
500
6000
5000
Platelets (mm3)
250000
255000
500
a) Which person lives at high altitude? Give a reason for your choice
b) Which person is suffering from anaemia (lack of iron)? Give a reason for your
choice
c) Which person has blood that will not clot properly? Give a reason for your choice.
5. The movement of blood through the circulatory system of a mammal is described as
a double circulation. Explain how the structure of the heart maintains the double
circulation and keeps the blood flowing under pressure.
6. Describe what is meant by coronary heart attack
7. How might diet and cigarette smoking make a heart attack more likely?
8. Name the blood vessel through which blood enters the heart from the body.
9. Name the blood vessel through which blood leaves the heart on its way to the rest of
the body.
10. Describe the path taken by the blood from the point where it enters the heart from the
body, to the point where it leaves the heart for the second time.