1. B1: Humans as Organisms
Circulation (H)

2. Various sorts of white blood cells.
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.
Phagocytic white 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.
Antibody secreting white cell
Platelets
Plasma

3. 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.

4. 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.

5. 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.

6. Circulation
Suggest answers to the following questions on the circulatory system.
Why does an anaemic person feel tired all the time?
Why a blood smear from a person with influenza may contain more white cells than a smear from a well person?
Why can many white blood cells squeeze across capillary walls and move around other tissues?
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.
An infected person will produce more phagocytes and antibody releasing cells to try and overcome the infection.
This enables them to move to sites in the body where infections by bacteria or viruses are present.

7. 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’.

8. CIrculation
Suggest answers to the following questions on the circulatory system.
What is the main function of red cells?
State two functions of white cells.
To carry oxygen from the alveoli/lungs to the respiring tissues.
Antibody production.
Phagocytosis/engulfing bacteria.

9. 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
Heart
Lungs
Body
Pulmonary arteries, carry deoxygenated blood from heart to lungs
Dorsal aorta (artery), carrying oxygenated blood from heart to body

10. 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
Septum
Right ventricle
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.

11. 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.

12. Circulation
Veins from body (venae cavae)
Pulmonary arch to pulmonary arteries to lungs
Veins from lungs (pulmonary veins)
Aortic arch to dorsal aorta to body
Semilunar valves
Right atrioventricular valve
Left atrioventricular valve
Right atrium
Left atrium
Left ventricle
Right ventricle
Septum
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.

13. Circulation
The table below shows differences between arteries, veins and capillaries.
Feature
Direction of blood flow
Size
Wall thickness
Blood pressure
Valves
Main tissues in wall
Arteries
Away from heart.
Large to very small as they branch.
High, reducing as the arteries branch.
Absent.
Thick.
Thick muscle layer and elastic tissue.
Back to heart.
Veins
Very small to large as branches join.
Low.
Present.
Thinner than arteries.
Thinner muscle layer. No elastic tissue.
Microscopic.
Microscopic, one cell thick.
Capillaries
From small arteries to small veins.
Low.
Absent.
Pavement epithelium.

14. Circulation
Suggest answers to the following questions on the circulatory system.
Why is the blood pressure higher in arteries than in veins?
Why do arteries have a pulse but veins do not?
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.
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.

15. Circulation
Suggest answers to the following questions on the circulatory system.
Why do veins have valves?
Why do capillaries have thin walls?
Why do arteries branch into smaller and smaller branches?
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.
For easy exchange of substances between blood and tissues, e.g. oxygen.
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.

16. Circulation
Suggest answers to the following questions on the circulatory system.
What is the main function of red cells?
State two functions of white cells.
3. Which side of the heart pumps blood to the lungs?
In which type of vessel is the blood pressure the highest?
To carry oxygen from the alveoli/lungs to the respiring tissues.
Antibody production. Phagocytosis/engulfing bacteria.
Right side.
In arteries.

17. 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?
6. How does blood in the pulmonary artery differ from blood in the pulmonary vein?
7. Suggest why the wall of the left ventricle is thicker than the wall of the right ventricle.
8. Suggest why the walls of the atria are thinner than the walls of the ventricles.
The presence of valves in the veins (‘watch-pocket’ valves).
Blood in the pulmonary artery is deoxygenated, blood in the pulmonary vein is oxygenated.
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.
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.

18. 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:
Blood
Tissues
Capillary wall
Glucose, amino acids, oxygen
Carbon dioxide, urea