Blood: Components and Functions

Blood is a tissue — a group of different cell types working together for a shared role: transport and defence around the body. It only looks like a simple red liquid.

A tissue is defined as a group of cells with a similar structure and function. Blood qualifies because it contains several cell types suspended in a straw-coloured liquid called plasma. The cells are carried along by the plasma but are not dissolved in it.

There are four components to learn:

Component	What it is	Headline job
Plasma	Straw-coloured liquid	Transports almost everything dissolved
Red blood cells	Biconcave cells with no nucleus	Carry oxygen
White blood cells	Larger cells with a nucleus	Defend against pathogens
Platelets	Tiny cell fragments	Help blood clot

Key definition: Blood is a tissue consisting of plasma, in which red blood cells, white blood cells and platelets are suspended.

If you spun a blood sample in a centrifuge, it would separate into layers: red blood cells sink to the bottom (they are the densest and most numerous), a thin pale layer of white blood cells and platelets sits in the middle, and the plasma floats on top as roughly 55% of the volume.

Plasma is the liquid part of blood — about 55% of its volume — and it is mostly water. Because nearly everything the body moves around is dissolved or suspended in plasma, it is the main transport medium of the circulatory system.

Plasma transports:

• Carbon dioxide — the waste gas from respiration, carried from the respiring tissues to the lungs to be breathed out.
• Soluble products of digestion — such as glucose and amino acids, carried from the small intestine to the cells that need them.
• Urea — a waste product made in the liver from the breakdown of excess amino acids, carried to the kidneys to be removed in urine.
• Hormones — chemical messengers carried from glands to their target organs.
• Heat — energy transferred from warmer organs (like the liver and muscles) around the body, helping keep body temperature even.

A useful way to remember the load plasma carries is to group it by direction:

Substance	From	To
Carbon dioxide	Respiring tissues	Lungs
Glucose, amino acids	Small intestine	Body cells
Urea	Liver	Kidneys
Hormones	Glands	Target organs

Oxygen is not mainly carried by plasma — it is carried by red blood cells. This is a frequent mix-up, so keep oxygen out of your plasma list.

Red blood cells (also called erythrocytes) have one job: carry oxygen from the lungs to every respiring cell in the body. Almost every structural feature is an adaptation that increases how much oxygen they can carry and how fast they can load and unload it.

A simplified side-on view of the biconcave shape, dented on both flat faces:

Three key adaptations, and the reasoning behind each:

Adaptation	What it means	Why it helps carry oxygen
Biconcave disc shape	Flattened disc, dented on both flat faces	Larger surface area to volume ratio, so oxygen diffuses in and out faster
No nucleus	The nucleus is lost as the cell matures	More internal room for haemoglobin, so each cell carries more oxygen
Packed with haemoglobin	A red, iron-containing protein fills the cell	Haemoglobin is the molecule that actually binds and releases oxygen

Remember: mature red blood cells have no nucleus. This is unusual for an animal cell, and it exists to make room for more haemoglobin — not for any other reason.

The biconcave shape also makes the cells flexible, so they can squeeze single-file through narrow capillaries where gas exchange happens.

Haemoglobin is the red, iron-containing protein inside red blood cells. It is what lets the cell pick up oxygen in one place and drop it off in another. The same chemistry runs forwards in the lungs and backwards in the tissues.

In the lungs, oxygen concentration is high, so haemoglobin binds oxygen to form oxyhaemoglobin:

$\text{haemoglobin}+\text{oxygen}\to \text{oxyhaemoglobin}$

In the respiring tissues, oxygen concentration is low because cells are using it up, so the reaction reverses. Oxyhaemoglobin breaks down, releasing oxygen where it is needed:

$\text{oxyhaemoglobin}\to \text{haemoglobin}+\text{oxygen}$

Walk-through — how the three adaptations combine for one cell:

1. The cell enters a capillary in the lung. Its large surface area (biconcave shape) lets oxygen diffuse in quickly.
2. With no nucleus, the inside is packed with haemoglobin, so a lot of oxygen can be loaded into a single small cell.
3. The haemoglobin binds that oxygen, forming oxyhaemoglobin, and the cell carries it away in the bloodstream.
4. Reaching a respiring muscle, the low oxygen level triggers the reverse reaction. Haemoglobin releases the oxygen, the large surface area speeds its diffusion out, and the cell heads back to the lungs to repeat the cycle.

Each adaptation removes a different bottleneck: shape speeds diffusion, the missing nucleus increases capacity, and haemoglobin provides the chemistry. Together they make red blood cells far more efficient than a plain spherical cell would be.

(Extra context — not required by AQA 8461.) Each red blood cell holds roughly 250 million haemoglobin molecules, and one haemoglobin molecule can carry up to four oxygen molecules.

White blood cells are part of the body's defence against pathogens (microorganisms that cause disease, such as bacteria and viruses). Unlike red blood cells, they keep their nucleus and are larger but far less numerous. They defend the body in two main ways:

• Phagocytosis — some white blood cells surround and engulf pathogens, then digest and destroy them.
• Producing antibodies — some white blood cells make antibodies, proteins that lock onto and help destroy specific pathogens.
• Producing antitoxins — some white blood cells make antitoxins, which neutralise the harmful toxins released by certain pathogens.

White blood cells defend the body against pathogens. The full detail of how this defence works is covered separately under "human defence systems" — here you only need their role in blood.

Platelets are small fragments of cells — not whole cells, and they have no nucleus. When a blood vessel is cut, platelets help the blood clot. The clot forms a seal that:

• stops further blood loss, and
• blocks the entry of pathogens through the wound.

Component	Has a nucleus?	Whole cell?	Main function
Red blood cell	No	Yes	Carry oxygen
White blood cell	Yes	Yes	Defend against pathogens
Platelet	No	No (cell fragment)	Help blood clot to seal wounds

Remember: platelets are cell fragments, not complete cells. Calling them "cells" can lose the mark in a description question.

Exam questions often describe a blood sample with an abnormal count and ask you to predict and justify the consequence. Use the same method each time: name the component's job, then reason from "less of it" or "more of it" to the effect on the body.

Walk-through — a sample with a very low platelet count:

1. State the normal job: platelets help the blood clot to seal wounds.
2. Reason from the shortage: with too few platelets, clots form slowly or not at all.
3. Predict the consequence: even a small cut would bleed for much longer, and the person bruises easily, because there is no effective seal forming.

Walk-through — a sample with a low red blood cell count:

1. State the normal job: red blood cells carry oxygen using haemoglobin.
2. Reason from the shortage: fewer red blood cells means less haemoglobin, so less oxygen is delivered to the tissues.
3. Predict the consequence: the person tires quickly and feels breathless, because their cells cannot get enough oxygen for respiration.

Sample shows…	Component affected	Likely consequence
Low platelets	Platelets	Slow clotting; prolonged bleeding, bruising
Low red blood cell count	Red blood cells	Less oxygen delivered; tiredness, breathlessness
Very high white blood cells	White blood cells	Often a sign the body is fighting an infection

Exam tip: when a question says "explain" or "justify", link the component's function to the observed consequence. A consequence stated without the reason rarely gains full marks.

1. Saying red blood cells have a nucleus

Mature red blood cells have no nucleus. The space is used for extra haemoglobin so the cell can carry more oxygen. Writing that they keep a nucleus contradicts their main adaptation.

2. Claiming plasma carries oxygen

Plasma transports carbon dioxide, dissolved food (glucose and amino acids), urea, hormones and heat. Oxygen is carried by red blood cells bound to haemoglobin, not by the plasma.

3. Calling platelets "cells"

Platelets are fragments of cells, not whole cells. A description that calls them complete cells is inaccurate and can cost the mark.

4. Confusing the haemoglobin reaction direction

Oxygen joins haemoglobin to form oxyhaemoglobin in the lungs (high oxygen); the reaction reverses in the respiring tissues (low oxygen) to release oxygen. Getting the two locations the wrong way round describes the opposite of what happens.

5. Listing a job for the wrong component

Match each function to its owner: plasma transports dissolved substances and heat, red blood cells carry oxygen, white blood cells defend against pathogens, and platelets clot the blood. Mixing these up is the most common way to lose easy marks on this topic.