CARDIOVASCULAR SYSTEM LECTURE 3

BLOOD VESSELS

Flow of blood coming from the heart

Elastic arteries, muscular arteries, arterioles, capillaries, venules, veins

Structure of all arteries

They are made up of three layers known as the inner tunic (tunica interna/intima), middle tunic (tunica media), and the outer tunic (tunica externa/adventitia). The most useful layer is the tunica media however the tunica media and tunica externa is not present in all blood vessels.

Function of elastic arteries

Very large arteries which have elastic walls. Systole causes them to expand to store the blood leaving the ventricle and diastole causes them to push blood into the arterial tree by elastic recoil. This smooths the pulsatile flow of blood.

Structure of elastic arteries

They are made up of many thin sheets of elastin in the middle tunic and are the size of a finger.

Function of muscular arteries

Distribute blood around the body at high pressure with the rate of blood flow dependent on smooth muscle which can vary the radius of the arteries. Flow = r⁴ meaning a small effect on the radii can result in a significant change in flow.

Structure of muscular arteries

They have many layers of circular smooth muscle wrapped around the vessel in the middle tunic and are the size of a pencil - pin.

Function of arterioles

Control blood flow into capillary beds with the thickest muscle wall relative to their size than any other blood vessel. The greatest pressure drop and greatest resistance to flow occurs here. Their restriction determines total peripheral resistance which in turn affects mean arterial blood pressure.

Structure of arterioles

They have between one and three layers of circular smooth muscle wrapped around the vessel in the middle tunic and are the size of a human hair.

Function of capillaries

Tiny vessels which are thin-walled to allow exchange of gases, nutrients, and wastes between blood and interstitial fluid. Blood flow is slow to allow time for exchange. They are known as leaky vessels as plasma escapes however most of lost plasma is immediately recovered due to osmotic gradient. Exchange from capillaries to tissue cells are driven my hydrostatic pressure and vice versa is driven by osmosis

Structure of capillaries

Diameter just wide enough to admit one RBC with its wall being a single layer of endothelium. No smooth muscle and connective tissue present in the wall. It is the size of an RBC.

Oedema

Where the hydrostatic pressure is too high and too much fluid is leaked from capillaries and collected in tissues.

Function of venule

Low pressure vessels which drain capillary beds and are the site where white blood cells leave circulation during infection and inflammation.

Structure of venule

Small venules have usual endothelium + little connective tissue while larger ones have a single layer of smooth muscle. They are similar size of a capillary however have a larger lumen to allow more than one RBC in the centre.

Function of vein

Low-pressure vessels which drain blood back to the atria (except portal veins which drain blood to another capillary bed). Vulnerable to small changes in venous blood pressure as it causes a large change in venous volume. Veins act as a reservoir to store blood.

Structure of vein

They are thin and soft walled with the ability to stretch and have much less muscle and connective tissue. Larger veins such as in the legs have valves which prevent back flow. The constant contraction and relaxation the leg undergoes causes the system to act as a venous pump back to the right atrium.

Specialised muscle arteries

Coronary arteries arise from the aorta just downstream from the aortic valve and supply the myocardium.

Atheroma

20% narrowing of a coronary artery can cause significant obstruction to blood flow. Symptoms that can be seen during exercise is low on oxygen (ischemia) which can cause chest pain (angina). Severe ischemia = myocardial infraction. Sometimes artery-to-artery junctions (anastomoses) between small penetrating branches of the main coronary arteries widen slowly so that an ischaemic area of muscle can be supplied by a distant artery.