🍲 Substances carried
Plasma carries digested food molecules such as glucose from the small intestine to the cells. It also carries hormones from glands to their target organs.
Animal Tissues, Organs and Systems » Blood
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.2.2.3
- Why blood is a tissue and what it is made of
- The job of plasma, red blood cells, white blood cells and platelets
- How to recognise each type of blood cell in a photograph or diagram
- How each type of blood cell is adapted to its function
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Unlock This CourseBlood is a tissue
Earlier you learned that a tissue is a group of cells that work together to do a job. Blood fits this idea. It is a tissue made up of a liquid called plasma, with three different types of cell suspended in it: red blood cells, white blood cells and platelets.
"Suspended" means the cells are carried along in the liquid, spread through it. The heart pumps the whole mixture around the body through the blood vessels.
Key terms:
- Plasma: the pale yellow liquid part of the blood, in which the blood cells are suspended.
- Red blood cell: a blood cell that carries oxygen.
- White blood cell: a blood cell that helps defend the body against pathogens.
- Platelet: a small fragment of a cell that helps the blood to clot.
- Haemoglobin: the red substance inside red blood cells that joins with oxygen.
Plasma
Plasma is mostly water. Because it is a liquid, it is a very good transport medium. Many substances dissolve in it and are carried to where they are needed.
🔄 Waste products
Plasma carries carbon dioxide from the cells to the lungs. It also carries urea from the liver to the kidneys.
Plasma also carries the blood cells themselves, and it helps to spread heat around the body.
Red blood cells
The job of a red blood cell is to carry oxygen from the lungs to every cell in the body. Red blood cells are the most numerous cells in the blood.
Look at how they are built:
🔴 Biconcave disc
The cell is a round disc, thin in the middle on both sides. This gives it a large surface area, so oxygen can diffuse in and out quickly.
🩸 Haemoglobin
The cell is full of haemoglobin. In the lungs, haemoglobin joins with oxygen. In the body tissues, it lets go of the oxygen.
❌ No nucleus
The cell has no nucleus. This leaves more room inside for haemoglobin, so each cell can carry more oxygen.
Haemoglobin is a red substance, so it makes red blood cells red. Blood with a lot of oxygen is bright red.
White blood cells
White blood cells defend the body against pathogens, the microorganisms that cause disease. You will see exactly how they do this in the lesson on human defence systems. For now, you need to know the main features.
- They have a nucleus. Red blood cells do not.
- They are usually larger than red blood cells.
- There are far fewer of them than red blood cells.
- Some are able to change shape, which lets them surround and take in pathogens. This is one way they are adapted to their function.
The nucleus is often easy to spot in a diagram, because it looks like a large dark blob.
Platelets
Platelets are not whole cells. They are small fragments of cells. They have no nucleus.
When you cut yourself, platelets collect at the wound and help the blood to clot. The clot forms a scab. The scab stops you losing too much blood and stops microorganisms getting in.
Recognising blood cells
You may be given a photograph or a diagram of blood and asked to name the cells. Use this checklist.
- Red blood cell: many small round discs, paler in the middle, no nucleus. Job: carries oxygen.
- White blood cell: larger, with a clear nucleus, few in number. Job: defends against pathogens.
- Platelet: tiny fragments, no nucleus. Job: helps the blood to clot.
- Plasma: the pale liquid around the cells. Job: transports substances.
Worked example
A diagram shows a large cell with a lobed nucleus, surrounded by lots of small cells with no nucleus. The large cell with the nucleus is a white blood cell. The many small cells with no nucleus, pale in the centre, are red blood cells. Both are suspended in plasma.
Common mistakes
1. Saying white blood cells carry oxygen. Only red blood cells do this.
2. Saying red blood cells have a nucleus. They do not.
3. Calling platelets "cells". They are fragments of cells.
4. Writing that red blood cells are red "because they carry blood". They are red because of haemoglobin.
5. Saying that plasma is a cell. It is the liquid part of the blood.
Exam-style question
(a) Name the part of the blood that carries glucose and urea. [1 mark]
(b) Describe the function of platelets. [1 mark]
(c) Explain how the structure of a red blood cell helps it to carry oxygen. [3 marks]
Model answer
(a) Plasma (1).
(b) They help the blood to clot (1).
(c) Any three from: it contains haemoglobin, which joins with oxygen (1); it has no nucleus, so there is more room for haemoglobin (1); it is a biconcave disc, giving a large surface area (1); so oxygen can diffuse in and out quickly (1).
Exam tip
For "explain how the structure helps" questions, name the feature and then say what it lets the cell do. "No nucleus" alone gets no mark. "No nucleus, so more room for haemoglobin" does.