Lesson 1: Composition of Blood

Composition of Blood — The Body’s Transport System

Your blood travels roughly 96,000 km every day—silently delivering oxygen, nutrients, and protection. In this lesson, you will master the composition of blood (plasma + cells), the functions of each component, and the exact points examiners test in IGCSE and O Level questions.

Outcome: By the end, you will be able to label blood components, explain how structure supports function (RBC, WBC, platelets), and answer typical exam prompts like “State two adaptations…” and “Compare…”.

Video Explainer

Watch the video.

Warm-Up Challenge: Before we go further, name 3 functions of blood without using the word “transport.”

What Blood Is and Why It Matters

Blood is a fluid tissue found in animals. It flows through blood vessels and acts as the body’s delivery network—moving essential substances and removing harmful ones. In exam language, blood is described as a transport medium, but it does far more than “carry things around.”

Core idea for exams: Most blood questions are built on one pattern: structure → function → consequence.
If you can link a feature (e.g., RBC has no nucleus) to a function (more space for haemoglobin) to an outcome (efficient oxygen transport), you will score consistently.

Functions of Blood (IGCSE/O Level Standard List)

Blood performs several key functions:

Gas exchange support: carries oxygen from lungs to cells and carbon dioxide from cells to lungs.
Nutrient supply: delivers glucose, amino acids, vitamins, etc. to body cells.
Waste removal: carries wastes (e.g., urea) to excretory organs such as the kidneys.
Defense: protects against pathogens via white blood cells and antibodies.
Temperature regulation: distributes heat and helps maintain a stable internal temperature.
Hormone distribution: carries hormones from glands to target organs.

Exam Tip: If asked “State three functions of blood”, avoid repeating “transport” three times. Use varied wording like defense and temperature regulation to show range.

Composition of Blood: The Big Picture

Blood has two main components:

Plasma — the liquid part
Blood cells — red blood cells, white blood cells, and platelets

Memory Hook: Blood = Plasma + Cells. If you can always start your answer with this, you rarely miss marks.

1. Blood Plasma (Liquid Part)

Plasma is the straw-coloured liquid in which blood cells are suspended. It is mainly water and carries dissolved substances around the body.

What plasma contains (IGCSE/O Level)

Water (~90%)
Plasma proteins (e.g., albumin, globulin, fibrinogen)
Dissolved salts/ions (e.g., Na⁺, K⁺, Ca²⁺)
Nutrients (glucose, amino acids, fatty acids, vitamins)
Wastes (e.g., urea) and gases (CO₂, small amounts of O₂)

Why the water matters
Water in plasma acts as a solvent so substances can dissolve and move easily. It also helps regulate temperature and controls blood thickness (viscosity), which affects flow.

Example (Exam Application): If a student becomes dehydrated, plasma water decreases, blood becomes more viscous, and the heart may need to work harder to pump it.

Fun Fact: Plasma is like a moving “chemical soup.” Without plasma, blood cells would not be able to travel efficiently, and hormones/nutrients would struggle to reach target cells.

2. Blood Cells (Formed Elements)

Blood contains three main types of cells/particles:

Red blood cells (RBCs / erythrocytes) — oxygen transport
White blood cells (WBCs / leukocytes) — defense
Platelets (thrombocytes) — clotting

Red Blood Cells (Erythrocytes): Built for Oxygen

Red blood cells transport oxygen from the lungs to body cells. Their structure is highly adapted for this job.

Key adaptations of RBCs (very common exam marks)

Biconcave disc shape → large surface area for faster diffusion of oxygen.
No nucleus → more space for haemoglobin.
Contains haemoglobin → binds oxygen to form oxyhaemoglobin.

Mnemonic Tip: “Every Red Cell Gives Oxygen” (Erythrocyte → Red → Oxygen).

RBCs are produced in the bone marrow and live for about 120 days before being broken down (mainly in the liver and spleen).

Exam Tip: When a question says “Give two adaptations of red blood cells for oxygen transport”, the safest pair is: biconcave shape + no nucleus (then mention haemoglobin if asked for more).

Challenge: State two adaptations of RBCs and link each to its function (how it helps oxygen transport).

White Blood Cells (Leukocytes): The Defense Unit

White blood cells protect the body from infections. Unlike RBCs, they have a nucleus and are generally larger.

Two main groups (IGCSE/O Level)

Phagocytes — destroy pathogens by engulfing them (phagocytosis).
Lymphocytes — involved in producing antibodies (and coordinating immune responses).

Exam-level clarity
In IGCSE/O Level, you should confidently state:

Phagocytes engulf and digest pathogens.
Lymphocytes produce antibodies.

Example (Exam Application): During a bacterial infection, your WBC count increases because the body is producing more defense cells to fight the pathogen.

Challenge: Compare phagocytes and lymphocytes in terms of how they protect the body.

Platelets (Thrombocytes): The Clotting System

Platelets are cell fragments (not complete cells). They have no nucleus and play a central role in blood clotting. Clotting prevents excessive blood loss and blocks pathogens from entering through wounds.

Platelets are produced in the bone marrow.

Exam Tip: If asked “Why is blood clotting important?” give both points: prevents blood loss and prevents entry of microorganisms.

High-Frequency Exam Prompts (What Examiners Love)

1. “State the main components of blood.”
Answer format: Plasma + red blood cells + white blood cells + platelets.

2. “Give two adaptations of red blood cells.”
Use: biconcave shape, no nucleus, contains haemoglobin.

3. “What is plasma?”
Define as: the liquid part of blood that carries cells and dissolved substances.

4. “Explain why a cut stops bleeding.”
Mention: platelets + clot formation sealing the wound.

Key Concepts

Blood is a fluid tissue that transports substances and supports regulation and defense.
Blood components = plasma + RBCs + WBCs + platelets.
Plasma contains water (~90%), proteins, salts, nutrients, wastes, gases.
RBCs (erythrocytes) transport oxygen using haemoglobin; biconcave; no nucleus; lifespan ~120 days.
WBCs (leukocytes) defend against infection; include phagocytes and lymphocytes.
Platelets (thrombocytes) cell fragments involved in blood clotting.
Bone marrow produces RBCs, WBCs, and platelets.

Reflection Questions

Instructions: Choose the best answer. These questions reflect common IGCSE/O Level exam patterns.

Quick Poll

Instructions: Vote based on your current reality, not what sounds ideal.

Audio Recap

Instructions: Listen once, then replay and write down 3 exam-ready points.

Blood is a fluid tissue in animals that performs transport, regulation, and defense. It carries oxygen from the lungs to body cells and returns carbon dioxide to the lungs. It also delivers nutrients, removes wastes like urea, transports hormones, helps regulate body temperature, and protects against infection.

Blood has two main components: plasma and blood cells. Plasma is the liquid part of blood. It is mostly water, and it contains plasma proteins, dissolved salts, nutrients such as glucose and amino acids, and wastes such as urea. The blood cells include red blood cells, white blood cells, and platelets. Red blood cells transport oxygen using haemoglobin. They are biconcave, have no nucleus, and have a lifespan of about 120 days. White blood cells defend the body, including phagocytes that engulf pathogens and lymphocytes that produce antibodies. Platelets are cell fragments involved in blood clotting, which prevents blood loss and blocks pathogens from entering the body.

Flashcards

Instructions: Read the front, answer aloud, then flip. Aim for fast recall.

Discussion

Instructions: Keep answers exam-focused. Use: (1) what the question asks, (2) your exact points, (3) one example.

Write an exam-style answer: “State two functions of blood other than transport.”
Which RBC adaptation do students forget most often, and how will you remember it in the exam?
Give one real-life situation where clotting is essential, and explain why.
If plasma water decreases (dehydration), what happens to blood flow and why?

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Blood, Lymph & Immunity