3.2.3 Blood Vessels

Question 1

Arteries, Arterioles, Veins and Venules

Answer 1

The body contains several different types of blood vessels:
Arteries
- transport blood away from the heart (usually at high pressure) to tissues
Arterioles
- arteries branch into narrower blood vessels called Arterioles which transport blood into capillaries
Veins
- transport blood to the heart (usually at low pressure)
Venules
- these narrower blood vessels transport blood from the capillaries to the veins
Blood flows through the lumen of a blood vessel
- the size of the lumen varies depending on the type of blood vessel
- arteries have a narrow lumen and veins have a wider one
The walls of each type of blood vessel have a structure t5hat relates to the function of the vessel
Arteries, Arterioles, veins and Venules all have varying structural features

Question 2

Structure of Arteries

Answer 2

Artery walls consist of 3 layers:
- tunica adventitia/externa
- tunica media
- tunica intima
The tunica intima is made up of an endothelial layer, a layer of connective tissue and a layer of elastic fibres
- the endothelium is once cell thick and lines the lumen of all blood vessels
- it is very smooth and reduces friction for free blood flow
The tunica media is made up of smooth muscle cells and a thick layer of elastic tissue
Arteries have a this tunica media
- the layer of muscle cells strengthen the arteries so they can withstand high pressure
- it also enables them to contract and marrow the lumen for reduced blood flow
- the elastic tissue helps to maintain blood pressure in the arteries
- it stretches and recoils to even out any fluctuations in pressure
The tunica adventitia covers the exterior of the artery and is mostly made up of collagen
- collagen is a strong protein and protects blood vessels from damage by over-stretching
Arteries have a narrow lumen which helps to maintain a high blood pressure
A pulse is present in arteries

Question 3

Structure of Arterioles

Answer 3

Arterioles possess a muscular layer that means they can contract and partially cut off blood flow to protect specific organs
- e.g. during exercise, blood flow to the stomach and intestine is reduced which allows fore m or blood to reach the muscles
Unlike arteries, Arterioles have a lower proportion of elastic fibres and a larger number of muscle cells
- the presence of these muscle cells allows them to contract and close g their lumen to stop and regulate blood flow

Question 4

Structure of Veins

Answer 4

Veins return blood to the heart
They receive blood that has passed though the capillary networks (blood pressure is very low and it must be returned to the heart)
The tunica media is much thinner in veins
- there is no need for a thick muscular layer as veins don’t have to withstand high pressure
The lumen of the vein is much larger than that of an artery
- a larger lumen helps to ensure that blood returns to the heart at an adequate speed
- a larger lumen reduces friction between the blood and the endothelial layer of the vein
- the rate of blood flow is slower in veins but a larger lumen means the volume of blood delivered per unit of time is equal
Veins contain valves
- these prevent the back flow of blood, helping return blood to the heart
A pulse is absent in veins

Question 5

Structure of Venules

Answer 5

Venules connect the capillaries to the veins
- they have few or no elastic fibres and a large lumen
- as the blood is at a low pressure after passing through the capillaries, there is no need for a muscular layer

Question 6

Capillaries

Answer 6

Capillaries are a type of blood vessel present in the circulatory system
They have thin walls which are “leaky, allowing substances to leave the blood to reach the body’s tissues
They can form networks called capillary beds which are very important exchange surfaces within the circulatory system

Question 7

Structure and Function of Capillaries

Answer 7

Capillaries have a very small diameter (lumen)
- this forces the blood to travel slowly which provides more opportunity for diffusion to occur
A large number of capillaries branch between cells
- substances can diffuse between the blood and cells quickly as there is a short diffusion distance
The wall of the capillary is made solely from a single layer of endothelial cells (this layer also lines the lumen in arteries and veins)
- the wall is only one cell thick
- this reduces the diffusion distance for oxygen and carbon dioxide between the blood and the tissues of the body
- the cells of the wall have gaps called pores which allows blood plasma to leak out and form tissue fluid
- white blood cells can combat infection in affected tissues by squeezing through the intercellular junctions in the capillary walls