B1: Humans as Organisms
Circulation (H)
Circulation
Below is a photograph of blood cells as they appear in a stained smear of blood.
Blood is a tissue which consists of a fluid called plasma containing red blood cells, white
blood cells and platelets.
Red blood cells
Various sorts of
white blood cells.
To allow all cell
types to be shown
there are more
white cells here
than would
normally appear in
one microscopic
view of blood.
Phagocytic white
cells
Antibody secreting
white cell
Platelets
Plasma
Circulation
The fluid plasma makes up about 60% of the volume of the blood. It
carries:
•
suspended red and white blood cells. The red cells outnumber the
white cells by about 1000 to 1;
•
dissolved carbon dioxide from the respiring tissues to the lungs;
•
soluble products of digestion (e.g. glucose, amino acids) from the
small intestine to other organs;
•
urea from the liver to the kidneys;
•
dissolved antibodies to protect against infection.
Circulation
A red blood cell has no nucleus.
Red cells transport oxygen from the lungs to the respiring tissues.
A white blood cell has a nucleus.
White cells are involved in defence, in two main ways:
•
Some act as phagocytes. This means that they can leave the blood
stream and move through any body tissues, to ingest (engulf) any
infectious organisms, for example, bacteria. The infectious
organisms are destroyed (digested) by the phagocytes.
•
Some produce antibodies against infectious microorganisms to
destroy them.
Platelets are fragments of cells and do not have a nucleus.
Platelets are involved in blood clotting.
Platelets cause fibrin formation which results in the formation of a
scab over the site of a wound to prevent loss of blood.
Circulation
Red blood cells.
The most abundant cells in the blood, making up almost 40% of the blood volume.
Their main job is to transfer oxygen from the lungs to the respiring tissues.
The red cells absorb oxygen from the alveoli of the lungs and release
oxygen from the capillaries in the respiring tissues.
They are specially adapted cells:
•
They have no nucleus so they can carry more haemoglobin.
•
Because they are disc-shaped more cells can be contained in a given volume
of blood than if they were spherical. Thus more oxygen can be carried.
•
They contain a red coloured respiratory pigment, called haemoglobin. This
pigment binds with large amounts of oxygen.
When oxygen combines with haemoglobin it makes oxyhaemoglobin.
Circulation
Suggest answers to the following questions on the circulatory system.
Why does an anaemic person feel tired all the time?
An anaemic person has too few red cells and so cannot carry enough
oxygen to the tissues for respiration. Thus they cannot release enough
energy.
Why a blood smear from a person with influenza may contain more white
cells than a smear from a well person?
An infected person will produce more phagocytes and antibody
releasing cells to try and overcome the infection.
Why can many white blood cells squeeze across capillary walls and
move around other tissues?
This enables them to move to sites in the body where infections by
bacteria or viruses are present.
Circulation
Suggest why people who live permanently at high altitudes tend to have a greater
density of red blood cells in their blood than someone who lives at sea level.
At high altitudes there is a lower percentage of oxygen in the atmosphere than
at sea level. Thus to enable sufficient oxygen to be absorbed for respiration at
high altitudes, the body compensates by making more red blood cells.
Suggest why people who have a poor diet which does not include much meat
tend to develop anaemia.
A diet with little or no meat would probably contain insufficient amounts of iron
to maintain haemoglobin manufacture at an adequate level. Thus, too few red
blood cells would be made and the person would develop ‘iron-deficiency
anaemia’.
CIrculation
Suggest answers to the following questions on the circulatory system.
1.
What is the main function of red cells?
To carry oxygen from the alveoli/lungs to the respiring
tissues.
2.
State two functions of white cells.
Antibody production.
Phagocytosis/engulfing bacteria.
Circulation
Substances are transported round the body, for example, from where they are taken
into the body to cells or from cells to where they are removed from the body.
They are transported by the circulatory system, this includes the heart and blood vessels and
the blood.
The diagram below illustrates the circulation of the blood. On diagrams, by convention,
oxygenated blood is shown in red, deoxygenated blood is shown in blue.
Pulmonary veins,
carry
oxygenated blood
from lungs to heart
Veins (venae cavae), carrying
deoxygenated blood from body to heart
Lungs
Body
Heart
Pulmonary arteries, carry
deoxygenated blood
from heart to lungs
Dorsal aorta (artery), carrying
oxygenated blood from heart to body
Circulation
•
The heart pumps blood around the circulation. To enable this, the wall is
made from sheets of strong cardiac muscle fibres which automatically
contract and relax rhythmically.
•
The heart has four chambers, right atrium, left atrium, right ventricle and left
ventricle.
•
The left and right sides of
the heart are separated by
a thick, impenetrable wall,
the septum.
Left
atrium
Right
atrium
Left
ventricle
Right ventricle
Septum
If blood cannot pass through the septum, how does it get from one
side of the heart to the other?
By going round the circulation.
Circulation
•
Blood is returned to the atria of the heart via veins. Blood leaves
the ventricles of the heart via arteries.
•
It is often thought that arteries always carry oxygenated blood and
veins always carry deoxygenated blood. Is this so?
It is not so. For example the pulmonary arteries carry deoxygenated
blood from the heart to the lungs and the pulmonary veins carry
oxygenated blood back from the lungs to the heart.
•
As the heart contracts and relaxes during the heartbeat, valves in the
heart prevent back flow of blood.
Where else do you think valves are present in the circulatory system, to
prevent backflow of blood?
In the veins.
Circulation
Pulmonary arch to pulmonary arteries
to lungs
Veins from body (venae
cavae)
Aortic arch to dorsal aorta to body
Veins from lungs (pulmonary veins)
Semilunar valves
Right atrium
Right atrioventricular valve
Left atrium
Left atrioventricular valve
Septum
Left ventricle
Right ventricle
Learn: the four chambers, the blood vessels entering or leaving each chamber, and the
four valves which prevent backflow. Note that the blood on the right side of the heart is
deoxygenated and the blood on the left side of the heart is oxygenated.
The circulatory system is said to be ‘double’. This refers to the fact that there are two
circulatory systems, one to lungs and one to all other organs of the body. Both systems
have arteries, veins and capillaries.
Circulation
The table below shows differences between arteries, veins and capillaries.
Feature
Direction of
blood flow
Arteries
Veins
Capillaries
Away from heart.
Back to heart.
From small arteries
to small veins.
Size
Large to very small
as they branch.
Very small to large
as branches join.
Microscopic.
Wall thickness
Thick.
Blood
pressure
Thinner than arteries. Microscopic, one
cell thick.
High, reducing as the Low.
Low.
arteries branch.
Valves
Absent.
Main tissues in wall
Thick muscle layer
and elastic tissue.
Present.
Absent.
Thinner muscle layer. Pavement epithelium.
No elastic tissue.
Circulation
Suggest answers to the following questions on the circulatory system.
Why is the blood pressure higher in arteries than in veins?
Because as the heart pumps/ventricles contract, high pressure is
created to force blood through the arteries. By the time the blood
has reached the capillaries, the pressure has been reduced as the
arteries divide into more and more branches. As the capillaries
divide in the tissues, the pressure reduces even further.
Why do arteries have a pulse but veins do not?
Because the arteries receive blood directly from the heart and as the
ventricles contract they cause a surge in pressure. As the ventricles
relax the pressure drops. The change in pressure can be felt as the
pulse. The pulse is ‘ironed out’ as blood flows through the capillaries
so that the pressure in the veins stays even.
Circulation
Suggest answers to the following questions on the circulatory system.
Why do veins have valves?
To prevent backflow of blood. It also aids the movement of blood
against gravity, because the driving pressure of blood in the veins is
low.
Why do capillaries have thin walls?
For easy exchange of substances between blood and tissues, e.g.
oxygen.
Why do arteries branch into smaller and smaller branches?
Because there are many organs and tissues in the body and all have to
be supplied with blood.
Remember: the heart muscle has its own excellent blood supply
branching from the aortic arch. This is the coronary supply and it carries
high concentrations of oxygen and glucose to the heart muscle.
Circulation
Suggest answers to the following questions on the circulatory system.
1.
What is the main function of red cells?
To carry oxygen from the alveoli/lungs to the respiring tissues.
2.
State two functions of white cells.
Antibody production. Phagocytosis/engulfing bacteria.
3.
Which side of the heart pumps blood to the lungs?
Right side.
4.
In which type of vessel is the blood pressure the highest?
In arteries.
Circulation
5.
When the muscles of the legs and arms contract they squeeze the
large veins which helps to move blood along. What prevents the
blood flowing in the wrong direction?
The presence of valves in the veins (‘watch-pocket’ valves).
6.
How does blood in the pulmonary artery differ from blood in the
pulmonary vein?
Blood in the pulmonary artery is deoxygenated, blood in the
pulmonary vein is oxygenated.
7.
Suggest why the wall of the left ventricle is thicker than the wall
of the right ventricle.
The left ventricle wall has more cardiac muscle because it has to pump
blood all around the body. The right ventricle only has to pump blood
round the lungs.
8.
Suggest why the walls of the atria are thinner than the walls of the
ventricles.
Atria have less cardiac muscle because they only have to pump
blood into the ventricles. The ventricles have more cardiac muscle
because they have to pump blood to all the organs.
Circulation
• One of the commonest methods of getting across body surfaces is by
diffusion.
• Diffusion is the spreading of the molecules of a gas, or the molecules
of a substance in solution, resulting in a net movement from a region
where they are at a higher concentration to a region where they are at
a lower concentration.
• The greater the difference in concentration, the faster the rate of
diffusion.
Many substances are exchanged across capillary walls between blood
and tissues by diffusion. For example:
Glucose, amino acids, oxygen
Tissues
Blood
Carbon dioxide, urea
Capillary wall