Blood
In this topic you will:
- learn about the structure of blood
- find out about the functions of red blood cells, white blood cells and blood plasma.
Key words
- antibodies
- blood plasma
- oxyhaemoglobin
- pathogens
- red blood cells
- white blood cells
Delivering the requirements for respiration in cells
You have seen that all of your cells need energy to stay alive. Each cell gets its energy through a chemical reaction called respiration.
Aerobic respiration happens inside the mitochondria in the cells. The reactants are glucose and oxygen:
glucose + oxygen → carbon dioxide + water
So every cell in your body needs a good supply of glucose and oxygen, and the carbon dioxide and water that the cell makes must be taken away. The delivery and removal is done by the blood.
The blood moves around the body inside blood vessels. The heart pumps constantly, to keep the blood moving.
What is blood?
Everyone knows that blood is a red liquid. But if you are able to look at some blood through a microscope, you may get a surprise. The photograph shows what you might see.
This is the liquid part of the blood. This liquid is called blood plasma. You can see that it is not red at all. It is a very, very pale yellow.
Blood looks red because it contains a lot of red blood cells, which float in this liquid. Most of the cells in our blood are red blood cells. An adult person has at least 20 trillion red blood cells in their body. There are about five million of them in every 1 cm3 of your blood.
These are called white blood cells. There are not many of them, but some of them may be quite a lot bigger than the red blood cells. They don't look white in the photograph because a stain has been added to the blood, to make the cells show up more clearly. The dark purple areas in these cells are their nuclei. (Red blood cells don’t have nuclei.)
Quick Fact
Blood plays a vital role in respiration by transporting the materials cells need to release energy. Red blood cells carry oxygen, and blood plasma carries glucose. These react in the mitochondria during aerobic respiration to produce energy. White blood cells, while not involved in respiration, help defend the body against disease.
Common Mistake
Students often think that blood itself performs respiration. In fact, blood simply delivers the glucose and oxygen needed for aerobic respiration, which takes place inside the mitochondria of cells. The blood's role is transport — not energy release.
Questions
Approximately how many times more red blood cells are there than white blood cells?
Show Answer
There are about 600 to 1,000 times more red blood cells than white blood cells in a typical sample of blood.
What differences can you see between the red blood cell and the white blood cell?
Show Answer
The red blood cell is smooth, disc-shaped, and has no nucleus. The white blood cell is larger, rounder, and has a rough or textured surface. It also contains a visible nucleus.
Plasma
Plasma is the liquid part of blood. It is mostly water. The red and white blood cells are transported around the body in the blood plasma. Plasma also has many other different substances dissolved in it. For example, glucose, dissolved in blood plasma, is transported from the digestive system to every cell.
You will remember that carbon dioxide is produced in every body cell, by respiration. The carbon dioxide dissolves in blood plasma and is carried away from the cells. The blood takes it to the lungs, where the carbon dioxide diffuses out and is breathed out in your expired air.
Red blood cells
Stage 7, Topic 1.3 described how the structure of red blood cells is related to their function. Now you are going to think about this in a little bit more detail.
Red blood cells are very unusual cells. They do not have a nucleus and they do not have mitochondria. They are full of a red pigment called haemoglobin. It is haemoglobin that makes blood look red. The structure of a red blood cell is related to its function.
- There is no nucleus, to make more room for haemoglobin.
- The cytoplasm contains a red pigment called haemoglobin, which carries oxygen.
- There are no mitochondria in the cytoplasm.
The haemoglobin helps the red blood cells to transport oxygen.
- As the blood flows through the tiny capillaries next to the alveoli in the lungs, oxygen from the air diffuses into the blood. Once it is in the blood, the oxygen then diffuses into the red blood cells.
- Inside the red blood cell, the oxygen combines with haemoglobin. It forms a very bright red compound called oxyhaemoglobin.
- As the blood continues on its journey around the body, it passes cells that are respiring. The oxyhaemoglobin lets go of its oxygen and gives it to the cells.
- The blood, which has given away most of its oxygen, now travels back to the lungs to collect some more.
This explains why red blood cells have haemoglobin – but why don’t they have a nucleus or mitochondria? Scientists think that not having a nucleus makes more space for haemoglobin. They also think that not having mitochondria stops the red blood cells from using up all the oxygen for themselves, instead of delivering it elsewhere.
Another way in which red blood cells are adapted for their function is that they are quite a lot smaller than most cells in the body.
Being so small helps them to get through very tiny blood capillaries. This means they can get really close to the alveoli in the lungs, and to the respiring cells in other parts of the body.
Quick Fact
Red blood cells are specially adapted to carry oxygen around the body. They contain the pigment haemoglobin, which binds with oxygen to form oxyhaemoglobin. Red blood cells have no nucleus or mitochondria, which leaves more room for haemoglobin and prevents them from using the oxygen themselves. Their small size and flexible shape allow them to squeeze through tiny capillaries and get close to cells that need oxygen.
Common Mistake
It’s easy to think that red blood cells are incomplete or broken because they lack a nucleus and mitochondria. In fact, this is a helpful adaptation! No nucleus means more space for haemoglobin, and no mitochondria means the cell doesn’t use up the oxygen it’s carrying. These features help red blood cells transport oxygen more efficiently.
Question
Show Answer
If red blood cells had mitochondria, they would use oxygen for respiration to produce their own energy. This would reduce the amount of oxygen available to be delivered to other body cells that need it. Not having mitochondria helps red blood cells conserve all the oxygen for transport.
White blood cells
White blood cells are easy to distinguish from red blood cells. They always have a nucleus, which red blood cells do not have. Some kinds of white blood cell – but not all – are larger than red blood cells.
Some bacteria and viruses can cause illness when they get into the body. These bacteria and viruses are called pathogens. White blood cells help to defend us against pathogens.
Some kinds of white blood cell can change their shape and push their cytoplasm out to make ‘fingers’ that can capture a pathogen. The white blood cell then produces chemicals that kill and digest the pathogen.
Other types of white blood cell produce chemicals that kill pathogens. These chemicals are called antibodies. They are shown as little Y-shapes on the diagram below. Different kinds of antibodies are needed for each different kind of pathogen.
The antibodies stick onto the pathogen. Sometimes, they kill the pathogen directly. Sometimes, they glue lots of the pathogens together so that they cannot move. This makes it easy for other white blood cells to capture and kill the pathogens.
- Bacteria may get into the body. Some kinds of of bacteria are pathogens. They could make you ill.
- Some kinds of white blood cell make chemicals called antibodies. The antibodies stick to the bacteria.
- Sometimes, the antibodies simply kill the bacteria. Sometimes, they stick them together so that other white blood cells can come and kill them.
Quick Fact
White blood cells help defend the body against disease. Some types surround and digest pathogens, while others produce special proteins called antibodies that stick to pathogens and disable them. Each kind of antibody matches a specific pathogen, helping your immune system target infections accurately.
Note
It's easy to mix up white blood cells and antibodies, but they’re not the same thing. White blood cells are the cells that make antibodies. Antibodies are chemicals that stick to pathogens, helping the white blood cells destroy them.
| Component of blood | Appearance | Function |
|---|---|---|
| Red blood cell | ||
| White blood cell | ||
| Plasma |
Show Answer
| Component of blood | Appearance | Function |
|---|---|---|
| Red blood cell | Small, red, disc-shaped; no nucleus | Transports oxygen using haemoglobin |
| White blood cell | Larger, irregular shape with nucleus | Defends the body against pathogens by making antibodies or engulfing microbes |
| Plasma | Pale yellow liquid | Carries blood cells, glucose, carbon dioxide, and other substances around the body |
5 Name three things that are transported in blood plasma.
Show Answer
The red blood cell is smooth, disc-shaped, and has no nucleus. The white blood cell is larger, rounder, and has a rough or textured surface. It also contains a visible nucleus.
Types of Blood Cells
The table below shows the main types of blood cells along with their shape and function in the body.
| Type of Cell | Shape | Function |
|---|---|---|
| Red blood cell | Disc-shaped with a depressed center | Transports respiratory gases (O2 and CO2) in the blood |
| White blood cell | Almost spherical | Defends the body against pathogens such as microbes |
| Platelets | Very small with no specific shape | Involved in blood clotting and prevents blood loss |