Blood and Circulation

In mammals, blood transports:

• oxygen from the lungs to all other parts of the body
• carbon dioxide from all parts of the body to the lungs
• nutrients from the small intestine to all parts of the body
• urea from the liver to the kidneys
• hormones, antibodies and other substances
• distributes heat (thermoregulation)

The human heart is a pump and pumps blood around the body at different speeds and pressures according to the body's needs. It can do this because the wall of the heart is made up of cardiac muscle.

Blood is moved through the heart by a series of contractions and relaxations of cardiac muscle in the walls of the 4 chambers. These events form the cardiac cycle.

a) Cardiac Diastole: all chambers are relaxed, and blood flows into the heart

b) Atrial Systole; Ventricular Diastole: atria contract, pushing blood into ventricles

c) Atrial Diastole; Ventricular Systole: after the atria relax, the ventricles contract, pushing blood  out of the heart

systole: contraction

diastole: relaxation

• The heart is divided in two by the septum.

• Wall of the left ventricle is thicker because the left ventricle pumps blood to the rest of the body, which requires more pressure.

• Valves ensure that blood can flow only in one direction through the heart.

• Walls of the atria are thin so they can be stretched to receive blood.

• Walls of the heart are made of cardiac muscle which does not tire.

• the cardiac muscle has its own blood supply - the coronary circulation.

Arteries

• carry oxygenated blood from heart to organ
• this arterial blood puts a lot of pressure on the walls of the arteries, so they must have thick, muscular walls (made from fibres and elastic tissue
• small lumen (central cavity)

Veins

• carry deoxygenated blood from organs to heart
• the pressure of venous blood is much lower, so it has thin walls with little muscle or elastic tissue
• large lumen

Capillaries

• carry blood through organs, bringing it close to every cell in the organ
• substances are transferred between the blood in the capillary and the cells
• they are small enough to fit in between cells (thin cell walls -  short diffusion distance)

Plasma                        - liquid part of blood, mainly water - carries blood cells, dissolved nutrients,  hormones, CO2 and urea around the body

Red Blood Cells          - biconcave, no nucleus       - transport oxygen in haemoglobin

Lymphocytes               - a large spherical nucleus     - produce antibodies to destroy pathogens, some give us immunity to diseases (memory cells)

Phagocytes                  - larger cell with nucleus    - engulf bacteria that have infected body

Platelets                       - smallest cell           - release chemicals to make our blood clot when we cut ourselves

Lymphocytes make antibodies. Pathogens have telltale markers on their surfaces called antigens which the antibodies recognise. The antibodies stick to the surface antigens and destroy the pathogen by:

• causing bacteria to stick together so phagocytes can engulf more easily
• acting as a label so it's more easily recognised by a phagocyte
• causing bacteria cells to burst open
• neutralising toxins produced by pathogens

Some lymphocytes don't get involved in killing straight away and instead develop into memory cells (cells that make us immune to a disease). It means that if the same micro-organism attacks again, the lymphocytes can produce antibodies much faster

A vaccination creates artificial immunity. A person is injected with an 'agent' (a dead strain of the disease) which carries the same antigens as another disease-causing pathogen. Lymphocytes recognise the antigens and produce memory cells to make that person immune to the disease.