Arteries
Introduction to blood vessels
- The circulatory system of the human body contains several different types of blood vessel:
- Arteries
- Arterioles
- Capillaries
- Venules
- Veins
- Each type of blood vessel has a specialised structure that relates to the function of that vessel
The circulatory system includes several blood vessels, each specialised to carry out its function
Structure and function of arteries
- Arteries transport blood away from the heart at high pressure
- Blood travels from the ventricles to the tissues of the body
- Remember; arteries carry blood away from the heart
- Artery walls consist of three layers:
- The tunica intima is the innermost layer and is made up of an endothelial layer, a layer of connective tissue and a layer of elastic fibres
- The endothelium is one 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 thick tunica media
- The layer of muscle cells strengthen the arteries so they can withstand high pressure
- Blood leaves the heart under high pressure
- Muscles cells can also contract or relax to control the diameter of the lumen and regulate blood pressure
- The elastic tissue helps to maintain blood pressure in the arteries. It stretches and recoils to even out fluctuations in pressure
- The tunica externa 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
- The tunica intima is the innermost layer and is made up of an endothelial layer, a layer of connective tissue and a layer of elastic fibres
- Arteries have a narrow lumen which helps to maintain a high blood pressure
- A pulse is present in arteries due to blood leaving the heart under high pressure
Arteries have thick muscular walls made up of three layers of tissue and a narrow lumen
Structure and function of arterioles
- Arterioles branch off from arteries forming narrower blood vessels which transport blood into capillaries
- Arterioles are similar in structure to arteries, but they have a lower proportion of elastic fibres and a large number of muscle cells
- The presence of muscle cells allows them to contract and close their lumen to regulate blood flow to specific organs
- Eg. during exercise blood flow to the stomach and intestine is reduced while blood flow to the muscles increases
Arterial blood pressure
- Arteries, and to a slightly lesser extent arterioles, must be able to withstand high pressure generated by the contracting heart, and both must maintain this pressure when the heart is relaxed
- Muscle and elastic fibres in the arteries help to maintain the blood pressure as the heart contracts and relaxes
- Systolic pressure is the peak pressure point reached in the arteries as the blood is forced out of the ventricles at high pressure
- At this point, the walls of the arteries are forced outwards, enabled by the stretching of elastic fibres
- Diastolic pressure is the lowest pressure point reached within the artery as the heart relaxes
- At this point, the stretched elastic fibres recoil and force the blood onward through the lumen of the arteries
- This maintains high pressure throughout the heart beat cycle
- Systolic pressure is the peak pressure point reached in the arteries as the blood is forced out of the ventricles at high pressure
- Vasoconstriction of the circular muscles of the arteries can increase blood pressure by decreasing the diameter of the lumen
- Vasodilation of the circular muscles causes blood pressure to decrease by increasing the diameter of the lumen
Examiner Tip
Be careful with the language you use to describe the roles of muscle and elastic tissue; muscle can contract and relax, while elastic tissue can stretch and recoil.