Blood – GCSE Combined Science Biology

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Blood is the red liquid that moves around your body. It is a tissue, which means a group of different cells and parts working together. It has four parts: plasma, red blood cells, white blood cells and platelets.

It matters because blood is the body's delivery and defence system. It takes oxygen and food to every cell, carries waste away, and helps to fight disease. Without it your cells would stop working.

By the end you will be able to name the four parts and say what each one does. You will also explain how red blood cells are built to carry oxygen.

Plasma

Plasma is the pale yellow liquid part of blood. Most of it is water. The other three parts are carried along in it, and so are many dissolved substances.

Think of plasma as the road that all the deliveries travel on. It carries digested food, such as glucose, from the gut to the cells. It carries carbon dioxide from the organs to the lungs. It carries urea, a waste made in the liver, to the kidneys.

Plasma also carries hormones, which are chemical messages, and it spreads heat around the body.

Red blood cells and haemoglobin

Red blood cells carry oxygen from the lungs to every cell in the body. Cells need oxygen to release energy from food.

Each red blood cell is full of haemoglobin. This is a red protein that joins with oxygen in the lungs to make oxyhaemoglobin. Think of haemoglobin as a tiny delivery van that picks up oxygen.

In the body tissues the haemoglobin lets go of the oxygen, and the oxygen passes into the cells. Haemoglobin is what makes blood look red.

How red blood cells are adapted

An adaptation is a feature that helps a cell do its job. Red blood cells have no nucleus. The nucleus is the part of a cell that holds its genes. Leaving it out makes more room for haemoglobin.

Each cell is a biconcave disc, which means it is dented in on both sides. This gives it a large surface area, so oxygen can get in and out quickly.

The cells are also small and flexible. This lets them squeeze through capillaries, which are the narrowest blood vessels.

White blood cells and phagocytes

White blood cells are part of the immune system, which defends the body. They fight pathogens. A pathogen is a microorganism that causes disease, such as a harmful bacterium or virus.

Some white blood cells are phagocytes. A phagocyte engulfs a pathogen, which means it wraps around it and takes it in. It then digests the pathogen and destroys it.

Unlike red blood cells, white blood cells have a nucleus. You have far fewer of them than red blood cells.

Antibodies and antitoxins

Other white blood cells are lymphocytes. They make antibodies, which are chemicals that destroy one particular kind of pathogen. Some also make antitoxins. These neutralise the poisons that pathogens release.

A football team is a helpful picture. Phagocytes are the defenders who tackle anything that should not be on the pitch. Lymphocytes are the planners, making antibodies aimed at one particular attacker.

When you have an infection, your body makes more white blood cells to fight it.

Platelets

Platelets are small fragments of cells. They have no nucleus, so they are not whole cells. Their job is to help the blood to clot.

To clot means to change from a runny liquid into a thick, sticky seal. When you cut yourself, platelets gather at the wound and a clot forms. The clot then dries into a scab.

The scab stops blood escaping and keeps microbes out while the skin heals. Think of it as a plaster that your body makes for itself. Without platelets, even a small cut would keep bleeding.

Recap

Blood has four parts: plasma, red blood cells, white blood cells and platelets.

Plasma is the liquid. It carries digested food, carbon dioxide, urea, hormones and heat.

Red blood cells carry oxygen using haemoglobin. They have no nucleus and are biconcave.

White blood cells fight pathogens. Platelets help the blood to clot.