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Edexcel Biology Unit 1 Notes Free essay! Download now

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Edexcel Biology Unit 1 Notes

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Edexcel Biology Topic 1 and Topic 2, Unit 1 Notes


In small unicellular organisms, substances move around slowly by diffusion.

Diffusion is too slow to move substances round the larger bodies of multicellular organisms.
They have a circulatory system: substances are carried in blood pumped by a heart.

In a closed circulatory system (eg in vertebrates) blood is enclosed in narrow blood vessels. This increases efficiency: blood travels faster as a higher pressure is generated.

Valves ensure blood flows in one direction:

Fish have a single circulation: heart pumps blood to gills for gas exchange, then to tissues and back to the heart.

Birds and mammals have a double circulation: right ventricle pumps blood to lungs. Blood returns to the left atrium and then the left ventricle pumps it to the rest of the body. Blood travels round the body faster, delivering nutrients faster, so the animals have a higher metabolic rate.


Arteries and veins contain collagen: a tough, fibrous protein to make them tough and durable.

The artery wall stretches as blood is pumped in and then recoils as the heart relaxes.
Blood flow is continual and there is a pulse.

Contracting muscles and low pressure in the chest when breathing in assist blood flow in veins. Valves prevent backflow. There is no pulse and pressure is low.

See diagrams and photomicrographs: Figure 1.10 on page 8 of the textbook.


narrow lumen
thicker walls
more collagen, elastic fibres and smooth muscle
no valves

wide lumen
thinner walls
less collagen, elastic fibres and smooth muscle


Figure 1.9 on page 8 of the textbook: make sure you know the structure of the heart.

The chambers of the heart (atria and ventricles) fill with blood when they relax (diastole) and pump blood out when they contract (systole).

The cardiac muscle making up the atria and ventricles is supplied with blood by the coronary arteries.


Atrial systole

Pressure in the atria increases as they fill with blood returning from the veins.

Increased pressure opens the atrioventricular valves allowing blood to enter the ventricles.

The atria contract to force remaining blood into ventricles.

Ventricular systole

Ventricles contract from the base up, increasing the pressure and closing the atrioventricular valves.

The semilunar valves open and blood is forced into the arteries.


As the atria and ventricles relax, pressure falls.

In the ventricle, this causes closure of the semilunar valves.

In the atria blood is drawn into the heart from the veins.


Atherosclerosis: a disease process where fatty deposits block an artery or increase its chances of being blocked by a blood clot (thrombosis)

How atherosclerosis (‘hardening’ of the arteries) occurs:

In the arteries supplying the heart, this causes a heart attack (myocardial infarction).

In the arteries supplying the brain, it causes a stroke.

An infarction is when tissue dies due to a lack of oxygen.

This is usually the result of a lack of blood – ischaemia.


Blood clots when it flows very slowly, or when ...

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