Blood Vessels

Question 1

Blood vessels

Answer 1

All vessels have an inner lining, made of a single layer of cells – endothelium. It is particularly smooth to reduce friction with flowing blood. Each type of blood vessel is adapted to its particular role.

Question 2

Arteries:

Answer 2

Carry blood away from the heart. Oxygenated apart from deoxygenated in pulmonary artery
• Small lumen – to maintain high pressure.
• Endothelium (Inner wall) is folded – to allow the lumen to expand as blood flow increases, helps to
maintain high pressure.
• Thick muscular wall – to withstand high blood pressure.
- Inner layer – consists thin layer of elastic tissue to allow the wall to stretch and recoil to maintain blood pressure.
- Middle layer – consists of a thick layer of smooth muscle to work with the elastic tissue and allow the lumen to constrict, increasing pressure.
- Outer layer – relatively thick layer of collagen and elastic tissue, providing strength to withstand pressure, and recoil to maintain it.

Note: arteries near the heart have more elastic tissue to allow stretch and recoil, which help to even out fluctuations in blood pressure caused by the heart.

Question 3

Arterioles:

Answer 3

Small blood vessels that distribute blood from an artery to capillaries. Arteriole walls contain a layer of smooth muscle (but less elastic tissue), contraction of this muscle constricts the diameter of the arteriole. This increases resistance to flow and reduces rate of blood flow. Constriction of arteriole walls can be used to divert blood flow to regions of the body that demand more oxygen.

Question 4

Capillaries:

Answer 4

Adapted for efficient diffusion –
- Narrow lumen (diameter about the same as red blood cell 7 μm). Red blood cells are squeezed against capillary walls, helping the diffusion of oxygen as it reduces diffusion distance. The narrow lumen also increases resistance and reduces rate of flow.
- Wall consists of single layer of flattened endothelial cells (1 cell thick) – reduces diffusion distance.
- Walls are leaky, they allow blood plasma and dissolved substances to leave the blood.

Question 5

Venules:

Answer 5

From capillaries blood flows to venules which join to form veins. Venules have thin walls consisting of muscle, elastic tissue, and collagen.

Question 6

Veins:

Answer 6

Carry blood back to the heart. Blood at low pressure, so muscle walls don’t need to be thick.
- Large lumen – ease the flow of blood – low pressure.
- Walls have thinner layers of collagen, smooth muscle, and elastic tissue – they do not need to stretch
and recoil and are not actively constricted to reduce blood flow.
- Thinner walls – lower pressure so they don’t need to be thick to withstand/maintain high pressure.
- Contain valves – help blood flow back to the heart against gravity, preventing it from flowing in the
opposite direction.
- Flow aided by body muscle contraction – As the walls are thin, the vein can be flattened by the
contraction of the surrounding muscles, this applies pressure to the blood, forcing the blood to move along the direction determined by the valves.