Blood

1. Plasma

1.1. Transports dissolved substances such as sugar. The sugar is absorbed into the blood in the small intestine, and is carried all over the body to the cells that need to use it for energy.

1.2. Red blood cells

1.2.1. Red blood cells are unusually small cells. They are red because they contain a red pigment called haemoglobin. The red blood cells also transports oxigen. The oxygen combines with the haemoglobin inside the red blood cells. Thehaemoglobin becomes oxyhaemoglobin. This is a very bright red.

1.2.1.1. White blood cells

1.2.1.1.1. White blood cells are larger than red blood cells, and they always have a nucleus. White blood cells help to defend us against bacteria and viruses that get into the body.

2. Arteries

2.1. Have very thick, strong, elastic walls. They need to be strong because they have to withstand the strong forces as the heart pumps blood through them. Their elastic walls are able to expand and spring back as the blood surges through.

2.1.1. Veins

2.1.1.1. Veins do not need thick walls, because by the time blood flows into the veins it has lost most of the force that the heart gave it.

2.1.1.1.1. Capillaries