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The circulatory system consists of:

• blood
• a pump (the heart) to push the blood around the body
• a system of tubes to contain the blood (arteries, veins and capillaries)

The blood is the transport medium – it carries substances to all cells in the body and removes waste. It consists of four main components:

• red blood cells
• white blood cells
• platelets
• plasma

Red blood cells

Red blood cells transport oxygen. They absorb oxygen from the lungs and transport it through narrow blood vessels. The oxygen is released to the cells in the body which use it for aerobic respiration.

Red blood cells have adaptations that make them suitable for this:

• they contain haemoglobin – a red protein that combines with oxygen
• they have no nucleus so they can contain more haemoglobin
• they are small and flexible so that they can fit through narrow blood vessels
• they have a biconcave shape (flattened disc shape) to maximise their surface area for oxygen absorption.

White blood cells

White blood cells defend the body against disease. The majority of the white blood cells are a type of cell called phagocytes. The phagocytes ingest and destroy pathogens such as bacteria.

This is what happens:

• the phagocyte engulfs the bacterial cell
• the bacterial cell is broken down by enzymes inside the phagocyte

The process of ingesting the pathogen is called phagocytosis.