Page 111 - Biology Form Two

Biology for Secondary Schools

Veins
Veins are blood vessels that transport Direction of
blood to the heart from all parts of the blood flow
body. All veins transport deoxygenated Valve
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blood, except the pulmonary vein. The Direction of
pulmonary vein transports oxygenated blood flow
blood from the lungs to the heart. Veins (a) An open valve (b) A closed valve
have a larger lumen and less muscular

walls than arteries as shown in Figure Figure 5.6: Functioning of a valve
5.5. This is because the blood in the Capillaries
veins flows at low pressure. Capillaries are the smallest blood
vessels. They connect the smallest
Fibrous layer
branches of arteries and veins.

Muscle fibre Capillaries also help to exchange
molecules between the blood and the
Endothelium
cells across their walls. They are one
Lumen cell thick, narrow and have thin walls,

Basement as shown in Figure 5.7.
membrane

Figure 5.5: Cross-section of a vein Endothelium

Veins have valves at regular intervals.
These prevent the back flow of blood, Lumen
as shown in Figure 5.6. The muscles Figure 5.7: Cross-section of a capillary
next to the veins squeeze the veins,

which force blood to flow towards the Capillaries are in direct contact with
heart. The contraction and relaxation of the tissues of the body. They form a
the muscles of the ribs during breathing network for the efficient diffusion of
also help to squeeze some veins and substances. Their thin walls maximise
keep blood flowing. Veins branch to the rate of diffusion and enable oxygen
form venules. Venules are small veins and nutrients to diffuse from the blood
that connect capillaries and veins. to the cells. They also enable carbon
dioxide and other waste products to
diffuse from the cells into the blood.

Student’s Book - Form Two 105

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