Blood is pumped from the heart in the arteries. It is returned to the heart in the veins.
The capillaries connect the two types of blood vessel and molecules are exchanged between the blood and the cells across their walls.
|Always carry blood away from the heart||Always carry blood to the heart|
|Carry oxygenated blood, except for the pulmonary artery||Always carry deoxygenated blood, except for the pulmonary vein|
|Carry blood under high pressure||Carry blood under low or negative pressure|
|Have thick muscular and elastic walls to pump and accommodate blood||Have thin walls - have less muscular tissue than arteries|
|A type of supporting tissue called connective tissue provides strength||Have less connective tissue than arteries|
|The channel in the blood vessel that carries blood - the lumen - is narrow||Have a wide lumen|
Capillaries connect the smallest branches of arteries and veins
The walls of capillaries are just one cell thick. Capillaries therefore allow the exchange of molecules between the blood and the body's cells - molecules can diffuse across their walls. This exchange of molecules is not possible across the walls of other types of blood vessel.
You may be given some prepared slides of blood to examine with the microscope.
Many types of blood cell are 10 μm in diameter or less. You will need high magnification to examine them in detail.
The slides will have been stained to show the cells, and cell features. This picture shows many red blood cells and three white blood cells.