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Blood is used in Human beings for two main purposes.

1. Transport. Substances such as oxygen, carbon dioxide, glucose, urea and amino acids are transported around the body in the blood.

2. Defence. White blood cells for immune response and platelets involved in clotting are carried in the blood.

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Erythrocytes (Red blood Cells).

The function of red blood cells is to transport oxygen around the body to where it is needed. The oxygen is carried by a substance in the cells called haemoglobin. (When it is combined with oxygen it is known as oxy haemoglobin.
This is the substance that gives them their colour). Erythrocytes also play a role in carrying carbon dioxide.

There are 5-6 million red blood cells in 1cubic millimetre.

They have a biconcave structure (see diagram below) to increase the cell surface area to hasten diffusion (see "exchange surfaces). Mature cells do not have any cell organelles (see glossary) in order to contain as much haemogobin as possible. The cell membranes are flexible to allow them to squeeze throguh tiny blood vessels.

Red blood cells do not divide, as they have no nucleus. They are replenished by the bone marrow as they die off.

Leucocytes (White blood cells)

White blood cells are larger than red blood cells are larger than red blood cells and are much less numerous.

They contain Nuclei which means that they can divide.

They contain no haemoglobin, and so are colourless.

They can escape from blood vessels by squeezing between the cells. (This is known ans Phagocytosis).

Lymphocytes have large rounded nuclei compared to their size, and so have very little cytoplasm. Some of these secrete antibodies, while some kill off infercted cells.

Monocytes have a large kidney bean shaped nucleus, and more cytoplasm that lymphocytes. These can leave the blood system to engulf bacteria. (Phagocytosis, remember?)

Granulocytes have a multi lobed nucleus and granules in their cytoplasm. These engulf bacteria in the blood stream and play a role in allergies. They can also carry out phagocytosis.

In an average person, 40-75% of white blood cells are granulocytes, 20-45% are lymphocytes, and 2-10% are monocytes.

Picture of blood vessels will go here.

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