8.4.1 The Blood and Blood Vessels · 8. Exchange and Transport in Animals | Edexcel GCSE Combined Science 1SC0: Biology

Section 1 of 9Components of blood

haemoglobin structure in red blood cells

Red blood cells: biconcave shape, haemoglobin

Blood components table (red cells, white cells, platelets, plasma)

Red blood cells, white blood cells and platelets

Red blood cell: biconcave shape, no nucleus

Blood composition: plasma, red cells, white cells and platelets

Blood is a tissue made of cells suspended in a liquid, and it transports substances around the body. It has four main components.

Component	Description	Function
Plasma	Pale yellow liquid, about 55% of blood	Transports carbon dioxide, glucose, amino acids, urea, hormones and heat
Red blood cells	Biconcave discs, no nucleus, contain haemoglobin	Carry oxygen as oxyhaemoglobin
White blood cells	Have a nucleus; phagocytes and lymphocytes	Defence against pathogens: engulf them or make antibodies
Platelets	Cell fragments	Help blood clot at wounds

Red blood cells are adapted for oxygen transport: the biconcave shape gives a large surface area to volume ratio, the lack of a nucleus gives more room for haemoglobin, and they are flexible to squeeze through capillaries. Haemoglobin combines with oxygen in the lungs (high oxygen concentration) and releases it in the tissues (low oxygen concentration).

Section 2 of 9Arteries and veins

Blood vessel sequence from artery through capillary network to vein

Artery, arteriole, capillaries, venule and vein structure compared

Blood vessels: artery, capillary, vein (blood flow)

Arteries carry blood away from the heart, usually oxygenated (except the pulmonary artery).

1

Blood is under high pressure, so artery walls are thick, with muscle and elastic fibres.

2

The elastic walls stretch as blood surges through at each heartbeat and recoil afterwards, which helps to keep the blood moving and smooths the flow.

3

The lumen is relatively narrow.

4

You can feel a pulse where arteries pass near the skin surface.

Veins carry blood towards the heart, usually deoxygenated (except the pulmonary vein). The pressure is low, so the walls are thinner with less muscle and elastic tissue, and the lumen is wide. Valves prevent backflow of blood, which is needed because the pressure is low. Muscles squeezing the veins as we move help return the blood to the heart.

Section 3 of 9Capillaries

Capillaries are tiny vessels that link arteries and veins and run through all tissues.

One cell thick

Their walls are only one cell thick,

giving a very short diffusion distance, and they are permeable,
allowing exchange of substances with the tissue fluid.

Their huge combined surface area and narrow lumen (red blood cells pass in single file) slow the blood flow,

giving more time for exchange of oxygen, glucose, carbon dioxide and urea with cells.

Feature	Artery	Vein	Capillary
Direction	Away from heart	To heart	Between arteries and veins
Wall	Thick, muscle and elastic	Thin	One cell thick
Lumen	Narrow	Wide	Very narrow
Valves	No	Yes	No
Blood pressure	High	Low	Falling

Section 4 of 9Blood flow and exchange with tissues

Chambers, valves and major vessels of the heart

As blood flows through the capillaries, plasma leaks out through the thin walls to form tissue fluid, which bathes the cells.

Blood flow and exchange with tissues

Oxygen and glucose diffuse from the tissue fluid into the cells,

and carbon dioxide and urea diffuse from the cells into the tissue fluid and then into the blood.

Most of the tissue fluid returns to the capillaries at the venous end; the rest drains into the lymphatic system and eventually returns to the blood.

Because the cells do not directly touch the blood, they are bathed in a steady internal environment, which links to homeostasis.

Section 5 of 9Blood in health and disease

Anaemia is a condition in which blood carries too little oxygen, usually because of too few red blood cells or too little haemoglobin; iron is needed to make haemoglobin, so a lack of iron in the diet is a common cause.

Blood in health and disease

Blood clots form when platelets stick together at a wound and help produce a mesh of fibres that seal the cut; this prevents blood loss and entry of pathogens.

If a clot forms inside a vessel it can block blood flow, a risk in coronary heart disease.

White blood cells include phagocytes, which engulf pathogens, and lymphocytes, which make antibodies; their numbers increase during infection.

Section 6 of 9Interpreting data and calculations

Questions may give a diagram of a vessel with a measured wall thickness and lumen diameter, asking which vessel type it is and why.

1

A thick wall and narrow lumen mean an artery.

2

You could also be asked to calculate magnification from an image of a blood cell: magnification = image size divided by actual size.

3

If a red blood cell is 7 micrometres across and the image is 35 mm wide, then 35 mm = 35 000 micrometres and magnification = 35 000 / 7 = x5000.

Remember to use the same units.

4

Blood groups, anaemia (too few red blood cells or low haemoglobin) and clotting disorders are examples of how structure links to health.

Section 7 of 9Exam-style questions

Blood vessels structure and function: artery, capillary, vein

Capillary: wall one cell thick, red blood cells

Capillary wall one cell thick with endothelial cell

Explain how the structure of an artery is related to its function. [3]3 marks
Describe how red blood cells are adapted to transport oxygen. [3]3 marks
Explain why capillaries have walls that are only one cell thick. [2]2 marks

Section 8 of 9Key terms

Term	Meaning
Artery	Vessel carrying blood away from the heart at high pressure.
Vein	Vessel carrying blood back to the heart at low pressure, with valves.
Capillary	Microscopic vessel with walls one cell thick, where exchange occurs.
Plasma	Liquid part of blood.
Haemoglobin	Protein in red blood cells that binds oxygen.
Platelet	Cell fragment involved in blood clotting.
Valve	Flap that prevents backflow of blood.
Phagocyte	White blood cell that engulfs pathogens.

Section 9 of 9Exam technique

1

Saying all arteries carry oxygenated blood and all veins carry deoxygenated blood. The pulmonary artery and vein are the exceptions.

2

Writing that arteries have valves; veins do.

3

Saying that red blood cells carry oxygen "in the plasma". It is carried by haemoglobin in the cells.

4

Stating that capillaries have muscle; they have walls one cell thick.

5

Mixing up platelets (clotting) with white blood cells (immunity).