ARTERIES	VEINS
¥ 	Thick Wall	¥	Thin wall
¥	No valves because blood 	¥	Have valves because blood is at 	flowing through is at high 		low pressure
	pressure
¥ 	Small lumen	¥	Large lumen
¥	Oxygenated	¥	Deoxygenated

After being pumped out of your heart through the aorta, the blood follows the network of arteries and arterioles to all parts of your body.  In the smallest blood vessels (capillaries), oxygen and carbon dioxide are exchanged.  After passing through the capillaries, the deoxygenated blood returns to the right side of the heart through the network of veins.
Your body holds around 6 to 8 litres of blood. Blood is a connective tissue in which the cells are separated by a liquid called plasma. The contents of blood are shown below.
 
Properties of blood components 

RED BLOOD CELLS 	¥ Carry oxygen
	¥ Round and biconcave in shape
	¥ Contain a red pigment called hemoglobin 
	¥ 5 million per mm3 in numbers
	¥ 4 months life span 

WHITE BLOOD CELLS 	¥ Kill germs
	¥ Produce antibodies to clump germs 			together 
PLATELETS 	¥ Much smaller in size comparing to blood 		cells 
	¥ Trigger chemical reactions for blood 			clotting

The old red blood cells are absorbed by the liver and the spleen. The spleen also stores active red blood cells that can be retrieved later when is needed.