Blood and Its Components

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An adult has about 5 litres of blood. Blood is pumped by the heart through the blood vessels to every cell in the body. It looks like a red liquid, but it is really a mixture: cells floating in a liquid. At Stage 8 you must know three components (parts) of blood and the job of each one.

What is blood made of?

If blood is left to stand in a tube (or spun fast in a machine), it separates into layers. The cells are heavier than the liquid, so they sink.

Extra (not needed for your objective): blood also contains platelets, tiny pieces of cells that help blood to clot when you cut yourself.

Red blood cells: carrying oxygen

Red blood cells are full of a red substance called haemoglobin. In the lungs, where there is a lot of oxygen, oxygen joins with haemoglobin. The blood is pumped round the body. In the capillaries next to body cells, where there is little oxygen, the haemoglobin releases the oxygen, which passes into the cells. Cells use oxygen in respiration to release energy from glucose.

Haemoglobin contains iron. A diet with too little iron means the body cannot make enough haemoglobin, so the blood carries less oxygen and the person feels tired. (You will meet iron again on the diet page.) Red blood cells, like white blood cells, are made in the bone marrow inside some bones.

White blood cells: protecting against pathogens

A pathogen is a microorganism that causes disease — for example some bacteria and viruses. Pathogens can enter through cuts, the nose and mouth, or in food and water. White blood cells protect the body against them. There are different kinds:

• Some white blood cells engulf (surround and swallow) pathogens and digest them.
• Others make antibodies — chemicals that stick to a pathogen so it can be destroyed.

When you have an infection, your body makes more white blood cells, so a blood test showing many white blood cells can be a sign of infection. White blood cells are not red: they have no haemoglobin, so they do not carry oxygen.

Plasma: the transport liquid

Plasma is a pale yellow liquid, mostly water, which makes up a little over half of the blood. Many substances are dissolved in it. For Stage 8, plasma transports:

• blood cells — the red and white blood cells float in the plasma and are carried along with it;
• nutrients — for example glucose from digested food, which passes into the blood in the small intestine and is carried to all cells;
• carbon dioxide — a waste gas made by cells in respiration, carried to the lungs to be breathed out.

Model (analogy): blood is like a river carrying boats. The water (plasma) carries the boats along and also carries things dissolved in it (nutrients and carbon dioxide). The red boats are delivery boats loaded with oxygen; the white boats are police boats that look for and remove invaders (pathogens). Limitation: blood goes round in a closed circuit, pumped by the heart; a river flows one way, pulled by gravity.

Worked examples

Worked example 1 — matching Draw a line from each component of blood to its function. [3] Components: plasma; red blood cell; white blood cell. Functions: protects the body against pathogens; transports oxygen; transports nutrients and carbon dioxide.

1. plasma → transports nutrients and carbon dioxide [1]
2. red blood cell → transports oxygen [1]
3. white blood cell → protects the body against pathogens [1]

Common mistake: drawing two lines from one box. Unless the question says so, each box has one line.

Worked example 2 — adaptations Red blood cells have no nucleus and a biconcave shape. Explain how each feature helps the cell do its job. [2]

1. No nucleus: there is more space for haemoglobin, so the cell carries more oxygen [1].
2. Biconcave shape: a large surface area, so oxygen can diffuse in and out quickly [1].

Common mistake: "The shape lets it carry more blood." A red blood cell carries oxygen, not blood.

Worked example 3 — a blood test A blood test is taken when Yusuf is well and again when he has a chest infection. when | white blood cells in 1 mm³ of blood well | 6000 with infection | 15 000 (a) Calculate the increase. [1] (b) Explain why the number increased. [2]

1. (a) 15 000 − 6000 = 9000 white blood cells in 1 mm³.
2. (b) Pathogens (bacteria or viruses) have entered his body [1], so more white blood cells are made to protect the body / destroy the pathogens [1].

Worked example 4 — a percentage from the tube A tube of settled blood is 60 mm high. The red blood cell layer is 27 mm. (a) Calculate the percentage of red blood cells. [2] (b) The white cell layer is very thin. Suggest why. [1]

1. (a) 27 ÷ 60 × 100 = 0.45 × 100 = 45%.
2. (b) There are far fewer white blood cells than red blood cells (hundreds of red blood cells for every white blood cell).

Worked example 5 — red blood cells at altitude Athletes often train in mountains, high above sea level, where there is less oxygen in the air. After a few weeks their blood contains more red blood cells. Suggest why this helps them run faster when they return to sea level. [2]

1. More red blood cells can carry more oxygen [1],
2. so more oxygen reaches the muscle cells for respiration, which releases more energy [1].

Worked example 6 — the journey of a nutrient Describe how glucose from a meal reaches a cell in the brain. [2]

1. Glucose from digested food passes into the blood in the small intestine and dissolves in the plasma [1].
2. The heart pumps the blood (plasma) round the body to the brain, where glucose leaves the capillaries and enters the cells [1].

Practice

All pages in this course

Start here

• Stage 8 Science: The Tests and How to Prepare
• Thinking and Working Scientifically: Models, Investigations and Analysing Results

Biology

• Joints and Antagonistic Muscles
• Blood and Its Components
• Breathing, Gas Exchange and Diffusion in the Lungs
• A Balanced Diet, Energy Stores and Lifestyle
• Aerobic Respiration
• Ecosystems, Bioaccumulation and Invasive Species

Chemistry

• The Structure of the Atom
• Purity, Concentration and Chromatography
• Word Equations, Exothermic and Endothermic Reactions
• The Reactivity of Metals
• Impure Products and Solubility of Salts

Physics

• Speed and Distance–Time Graphs
• Balanced and Unbalanced Forces, and Moments
• Pressure, Particles and Diffusion
• Reflection and Refraction
• Dispersion and Colour
• Magnetic Fields and Electromagnets

Earth and space

• The Earth's Magnetic Field and Earth's Resources
• Climate, Weather and Climate Change
• Galaxies and Asteroids

Practice tests

• Stage 8 Science — Practice Test 1
• Stage 8 Science — Practice Test 2
• Stage 8 Science — Practice Test 3