- The main artery in the heart is the aorta and arteries carry blood away from the heart
- Arterioles are smaller arteries that diverge into capillary beds
- Exchange of nutrients, waste and oxygen occur at the capillaries within tissues at cellular level
- Veins are blood vessels that return blood back to the heart; where venules are smaller versions of veins
- Capillaries are blood vessels that are one cell thick (endothelium) where the main diffusion and exchange takes place. The endothelium is known as the tunica intima.
- The basic overall structure for veins and arteries is including the tunica intima, they have an additional two tunics; tunica media (middle layer) is the region where the blood vessels consist of smooth muscle and elastin fibres and the tunica adventitia (outer layer) is mainly composed of collagen fibres.
Blood vessels are tubes that run through the transport system in which blood is transported. The vessels allow blood to be pumped at a high pressure to deliver nutrients and remove metabolic waste effectively. Organisms that are active have evolved a closed circulatory system in which blood is kept within the vessels all the way through the circuit. Mammals blood loses its pressure as it passes through the lungs therefore, returns back to the heart to become boosted again to ensure the blood is able to reach the rest of the body.
There are five main types of blood vessels in a closed circulatory system:
- Arteries – carry blood away from the heart at high pressure
- Arterioles – are smaller arteries
- Veins – return blood back to the heart
- Venules – smaller veins
- Capillaries- smaller blood vessels that link arteries and veins together
The heart as already well known is the pump of the system that sends out blood in arteries as arteries carry blood away from the heart. As arteries become smaller they become arterioles. Eventually, arterioles become smaller enough to capillaries which is where the exchange happens. Capillaries re-join up to form small vessels and blood is now returning back to the heart where these vessels are called venules. As the venules join together they form large vessels which are known as the veins. Veins bring blood back to the heart to be pumped around again.
An important difference to remember is with the pulmonary circulation, going to the lungs, the veins are still the vessels coming back to the heart and the arteries always are going away. The artery from the heart to the lungs is carrying deoxygenated blood to the lungs and as the oxygenated blood returns back to the heart it is now a vein since veins always go towards the heart.
Each of the different vessels adapts slightly differently based on their different roles and their relative distance from the heart.
All blood vessels have some features in common. They all have a small smooth inner layer called the endothelium.
The walls of the arteries and veins both have the same basic structure. There are three main layers or tunics that form the walls of the blood vessels (Figure 1).
- The tunica intima – is the inner lining where a single layer of endothelial cells are attached to the basement membrane.
- The tunica media – is the middle layer within the blood vessels. This section contains elastin fibres and smooth muscle. This layer, mainly the elastin fibres is greatly dependent on the role and function of the blood vessel.
- Tunica adventitia – is the outermost layer which is largely consists of collagen fibres.
Arteries and Arterioles
Arteries carry the blood away from the heart to respiring tissues at high pressure. Arteries have specific features that enable them to withstand high pressure. Therefore, their structure is related to their function (Figure).
For instance, they have relatively thick tunica media. This layer as already mentioned above consists of elastin fibres and smooth muscle which largely depends on the distance from the heart. Arteries close to the heart such as the aorta have a large number of elastin fibres. This allows the aorta to carry out its function. For instance, when blood is pumped to the aorta after the ventricles have contracted the elastin fibres, allows the aorta to expand rather than burst under high pressure. Secondly, the use of large amounts of elastin fibres allows the arteries to create a recoil action allowing the blood to be maintained at high pressure and kept at a forward direction when the heart relaxes. On the other hand, arteries further away from the heart contain less amount of elastin fibres however, have a higher proportion of smooth muscle.
- The outermost layer, the tunica adventitia with its collagen fibre provides the artery with a tough outer layer.
- Overall thickness of the wall is large within arteries. arteries have relatively small lumen in relation to the thickness of the wall.
- There are no valves in the arteries since the pressure is so high there isn’t a tendency for the blood to flow backwards.
Arterioles have a similar structure to arteries, however, are much smaller and have relatively thinner muscular walls and elastic layers. The arterioles are important in controlling their diameter. The smooth muscle around the arterioles is able to constrict which can limit the diameter of the vessel. As the vessel constricts the diameter becomes smaller and this is important in the control of the flow of blood through the body.
Capillaries have very small, thin walls that consist of a single layer of endothelium. This allows for oxygen and carbon dioxide to be easily diffused. The lumen of the capillaries are very narrow allowing the red blood cells to be squeezed against the endothelium to allow for a short diffusion distance for the transfer of oxygen and nutrient exchange. Initially, the blood is at high pressure in the arterioles, as it spreads out in the capillaries, the blood pressure drops, the flow of blood drops causing the leakage of blood to be reduced. This is all occurring in the capillary beds due to the increased resistance due to all the blood being at close contact with the walls of the capillaries as it enters the tissues. Capillaries are highly branched and numerous in number which provides a large surface area for diffusion.
Veins and venules
Veins main function is to transport blood under low pressure from tissue back to the heart. Blood within the vein is full of carbon dioxide and now needs to be replenished. The low pressure begins at the venules and then at the veins. The vein walls are not very thick and the lumen space is very wide to maximise the blood flow. Valves are only found in the veins as the pressure is low, there is a tendency of blood flowing backwards. Valves ensure the blood is flowing in the correct direction
- Tunica media is very thin in the veins as the low pressure within the veins will not cause the veins to burst. Also there is very few elastin fibres as there is no need to re-coil and stretch in the veins as the veins are carrying the blood away from the tissue and the constriction and dilation of the veins cannot control the flow of the blood.
- Tunica adventitia – provides a tough outer layer for the veins to prevent them from bursting. It is the skeletal movement that compresses the veins to allow the flow of blood to be pushed along the vein.
- Venules are smaller version of veins and lack valves and they specifically deliver the oxygen and nutrients from the capillary beds to the veins