Blood and blood components

What you'll learn

Blood is a vital tissue that transports substances around the body and plays crucial roles in defence and temperature regulation. This revision guide covers the composition of blood, the structure and function of each component, and how these relate to human health and disease. You'll learn exactly what AQA examiners expect you to know about plasma, red blood cells, white blood cells and platelets.

Key terms and definitions

Plasma — the liquid component of blood that transports dissolved substances including carbon dioxide, urea, hormones, antibodies and nutrients

Red blood cells (erythrocytes) — biconcave disc-shaped cells containing haemoglobin that transport oxygen from the lungs to respiring tissues

Haemoglobin — a red protein found in red blood cells that reversibly binds with oxygen to form oxyhaemoglobin

White blood cells (leucocytes) — cells of the immune system that defend against pathogens through phagocytosis or antibody production

Platelets — small cell fragments without nuclei that are essential for blood clotting at wound sites

Phagocytosis — the process by which phagocytic white blood cells engulf and digest pathogens

Lymphocytes — a type of white blood cell that produces antibodies specific to particular antigens on pathogens

Antigens — protein markers on the surface of cells that trigger immune responses if recognised as foreign

Core concepts

Overview of blood composition

Blood is a tissue consisting of cells suspended in a liquid called plasma. In an adult human, blood makes up approximately 8% of body mass — about 5 litres in a 70 kg person.

Blood has four main components:

• Plasma (55% of blood volume) — the liquid matrix
• Red blood cells (45% of blood volume) — oxygen transport
• White blood cells (<1% of blood volume) — immune defence
• Platelets (<1% of blood volume) — blood clotting

The composition of blood can be observed when a blood sample is centrifuged. The denser red blood cells settle at the bottom, white blood cells and platelets form a thin layer called the buffy coat, and plasma remains as the upper yellow layer.

Plasma structure and function

Plasma is a pale yellow liquid consisting of about 90% water with numerous dissolved substances. It serves as the transport medium for the blood system.

Substances transported in plasma include:

• Waste products for excretion: carbon dioxide (transported from respiring tissues to the lungs), urea (from the liver to the kidneys for removal)
• Products of digestion: glucose, amino acids, fatty acids, glycerol and vitamins absorbed from the small intestine and transported to cells
• Hormones: chemical messengers secreted by glands and transported to target organs
• Heat energy: plasma distributes heat around the body for temperature regulation
• Proteins: including antibodies produced by lymphocytes, and fibrinogen needed for blood clotting

The high water content of plasma is essential because it acts as a solvent for these substances and maintains blood pressure for efficient circulation.

Red blood cells — structure and function

Red blood cells (erythrocytes) are highly specialised for oxygen transport. Understanding their adaptations is a key examination topic.

Structural adaptations:

• Biconcave disc shape — increases surface area to volume ratio for rapid oxygen diffusion in and out of the cell
• No nucleus — creates more internal space for haemoglobin, maximising oxygen-carrying capacity
• Contains haemoglobin — red protein that binds reversibly to oxygen
• Flexible membrane — allows the cell to squeeze through narrow capillaries

Function:

Red blood cells transport oxygen from the lungs to all respiring tissues in the body. In the lungs, where oxygen concentration is high, haemoglobin binds with oxygen to form oxyhaemoglobin:

Haemoglobin + oxygen ⇌ oxyhaemoglobin

This is a reversible reaction. In respiring tissues, where oxygen concentration is low, oxyhaemoglobin releases oxygen to the cells.

A single red blood cell contains approximately 250 million haemoglobin molecules, and each haemoglobin molecule can carry four oxygen molecules. Red blood cells are produced in the bone marrow and have a lifespan of about 120 days.

Clinical relevance:

Anaemia is a condition where the blood has a reduced ability to carry oxygen, often due to insufficient red blood cells or haemoglobin. Symptoms include fatigue, weakness and shortness of breath because body tissues receive inadequate oxygen for respiration.

White blood cells — structure and function

White blood cells (leucocytes) are part of the immune system and defend the body against pathogens (disease-causing microorganisms). Unlike red blood cells, white blood cells have a nucleus and can move out of blood vessels into tissues.

There are different types of white blood cells, but at GCSE level you need to know about two main categories:

Phagocytes:

• Carry out phagocytosis — the engulfing and digesting of pathogens
• Detect pathogens and move towards them
• Flexible membrane surrounds and engulfs the pathogen
• Enzymes inside the phagocyte digest and destroy the pathogen
• This is a non-specific defence (works against any pathogen)

Lymphocytes:

• Produce antibodies — proteins that bind to specific antigens on pathogens
• Each lymphocyte produces one type of antibody specific to one antigen
• When antibodies bind to pathogens, they mark them for destruction or clump them together
• Some lymphocytes remain in the blood as memory cells, providing long-term immunity
• This is a specific defence (targets particular pathogens)

White blood cells are larger than red blood cells but far fewer in number. A low white blood cell count makes a person more susceptible to infections.

Platelets and blood clotting

Platelets are small fragments of cells that have no nucleus. They are produced in the bone marrow when large cells break up into pieces.

Function of platelets:

Platelets are essential for blood clotting (coagulation), which prevents excessive blood loss when blood vessels are damaged and stops pathogens from entering the body.

The clotting process:

1. When a blood vessel is damaged, platelets arrive at the injury site
2. Platelets release chemicals that trigger a cascade of enzyme reactions
3. These reactions convert the soluble plasma protein fibrinogen into insoluble fibrin threads
4. Fibrin threads form a mesh across the wound that traps red blood cells
5. This forms a clot (scab) that seals the wound

Blood clotting is essential for survival. People with haemophilia have a genetic condition where their blood clots slowly or not at all, making even minor injuries potentially dangerous.

However, unwanted clotting inside blood vessels (thrombosis) can be harmful. A clot in a coronary artery can cause a heart attack by blocking blood flow to heart muscle.

Blood and health

Coronary heart disease:

When coronary arteries become narrowed by fatty deposits (atherosclerosis), blood flow to the heart muscle is restricted. This reduces oxygen supply to cardiac tissue. Blood clots are more likely to form in narrowed arteries, potentially causing complete blockage.

Treatment considerations:

• Statins reduce blood cholesterol levels, slowing fatty deposit formation
• Anticoagulant drugs (like warfarin) reduce blood clotting tendency
• Stents can be inserted to keep coronary arteries open

Blood doping:

Some athletes illegally boost their red blood cell count to enhance oxygen delivery to muscles. This improves endurance performance but carries health risks including increased blood viscosity and blood clots.

Worked examples

Example 1: Structure-function relationship (4 marks)

Question: Explain how red blood cells are adapted for their function.

Mark scheme answer:

• Red blood cells contain haemoglobin (1 mark) which binds to oxygen / transports oxygen (1 mark)
• Biconcave disc shape increases surface area to volume ratio (1 mark) allowing faster diffusion of oxygen (1 mark)
• No nucleus provides more space for haemoglobin (1 mark) [maximum 4 marks]

Examiner note: Any two adaptations with explanations would gain full marks. Always link structure to function for full credit.

Example 2: Blood component identification (3 marks)

Question: The table shows information about three blood components.

Component	Contains nucleus	Function
A	No	Transports oxygen
B	Yes	Produces antibodies
C	No	Involved in blood clotting

Identify components A, B and C.

Mark scheme answer:

• A = red blood cell / erythrocyte (1 mark)
• B = white blood cell / lymphocyte (1 mark)
• C = platelet (1 mark)

Examiner note: Use correct scientific terminology. "Red blood cell" and "erythrocyte" are both acceptable.

Example 3: Extended response on immunity (6 marks)

Question: Describe how white blood cells protect the body against pathogens.

Mark scheme answer:

• Phagocytes carry out phagocytosis (1 mark)
• They engulf / surround pathogens (1 mark)
• Enzymes digest / destroy the pathogen (1 mark)
• Lymphocytes produce antibodies (1 mark)
• Antibodies are specific to antigens on pathogens (1 mark)
• Antibodies cause pathogens to clump together / mark them for destruction (1 mark)

Examiner note: For 6-mark questions, aim for detailed coverage of multiple points. Include both types of white blood cell defence.

Common mistakes and how to avoid them

• Don't confuse plasma with cytoplasm. Plasma is the liquid part of blood; cytoplasm is the jelly-like substance inside cells. Students often muddle these terms in exam answers.

• Red blood cells don't "pick up" or "collect" oxygen. Use precise language: haemoglobin "binds to" oxygen to form oxyhaemoglobin. The reaction is reversible, not permanent.

• Platelets are not cells. They are cell fragments with no nucleus. Calling them "platelet cells" shows imprecise understanding.

• Don't say white blood cells "eat" pathogens. The scientific term is phagocytosis or "engulf and digest." Informal language loses marks in extended answers.

• Antibodies don't kill pathogens directly. They bind to antigens, causing pathogens to clump together or marking them for destruction by phagocytes.

• Blood clotting involves fibrin, not fibrinogen, forming threads. Fibrinogen is the soluble protein in plasma; it's converted to insoluble fibrin threads during clotting.

Exam technique for "Blood and blood components"

• Command word awareness: "Describe" requires you to state features or processes; "Explain" requires reasons linked with outcomes. For "Explain how red blood cells are adapted," you must link each structural feature to its functional advantage.

• Use comparative language effectively. Questions often ask you to compare components. Use "whereas," "in contrast" and "however" to make comparisons explicit: "Red blood cells lack a nucleus, whereas white blood cells have a nucleus."

• Draw clear, labelled diagrams when asked. For blood cell diagrams, use a ruler for label lines, ensure labels don't cross, and point to specific structures. A biconcave shape must be clearly shown for red blood cells.

• Allocate time according to marks. A 1-mark question needs one clear point; a 6-mark question needs six developed points or three points with explanation. Don't write paragraphs for 1-mark answers.

Quick revision summary

Blood consists of plasma (liquid carrying dissolved substances), red blood cells (biconcave, no nucleus, contain haemoglobin for oxygen transport), white blood cells (phagocytes engulf pathogens; lymphocytes produce specific antibodies), and platelets (cell fragments that enable blood clotting by converting fibrinogen to fibrin). Each component's structure is precisely adapted to its function. Understanding these adaptations and being able to explain them clearly is essential for exam success.