Clinical Science Team School of Nursing and Midwifery

Blood vessels Learning outcomes: 1. Compare the structure of an artery and a vein 2. Explain how the structure of an artery and a vein relates to its function 3. Describe the structure of a capillary and explain the physiological significance of this structure

Systemic Circulation Arteries. Arterioles. Capillaries. Venules. Veins. Role is to direct the flow of blood from the heart to the capillaries, and back to the heart.

The ARTERY thick muscle and elastic fibres Arteries carry blood away from the heart. the elastic fibres allow the artery to stretch under pressure the thick muscle can contract to push the blood along.

The VEIN Veins carry blood towards from the heart. thin muscle and elastic fibres veins have valves which act to stop the blood from going in the wrong direction. body muscles surround the veins so that when they contract to move the body, they also squeeze the veins and push the blood along the vessel.

Differences in artery and vein

The CAPILLARY Capillaries link Arteries with Veins the wall of a capillary is only one cell thick they exchange materials between the blood and other body cells. The exchange of materials between the blood and the body can only occur through capillaries.

arteryvein capillaries body cell The CAPILLARY capillary bed A collection of capillaries is known as a capillary bed.

Differences between arteries and veins

Blood Vessels Walls composed of 3 “tunics:” Tunica externa: Outer layer comprised of connective tissue. Tunica media: Middle layer composed of smooth muscle. Tunica interna: Innermost simple squamous endothelium. Basement membrane. Layer of elastin.

Why do we have different types of blood vessel? Large arteries e.g. aorta are elastic arteries Smaller arteries and arterioles are resistance arteries Capillaries can be continuous, fenestrated or discontinuous, exchange takes place in these vessels Veins are the capacity vessels, approx 64% of blood is here

Distribution of blood in the vascular system

Blood Vessels & Cardiovascular system

Monitoring Circulatory Efficiency Efficiency of the circulation is assessed by: pulse and blood pressure measurements Vital signs – pulse and blood pressure, respiratory rate and body temperature Pulse – pressure wave caused by expansion and recoil of elastic arteries Radial pulse (on radial artery), routinely used Varies with health, body position, activity

Factors Aiding Venous Return Venous blood pressure alone is too low to promote adequate blood return and is aided by the:  Respiratory “pump” – pressure changes created during breathing suck blood toward the heart by squeezing local veins  Muscular “pump” – contraction of skeletal muscles moves blood toward heart  Valves prevent backflow during venous return

Blood flow through the vessels Blood flow through vessels is directly proportional to the difference in pressure between the ends of the tube

Blood flow through the vessels Is inversely proportional to the resistance in the vessels. Resistance- determined by blood viscosity, vessel length & vessel radius. Blood viscosity & vessel length rarely change, radius can be changed by vasoconstriction (reducing radius) or vasodilation (increasing radius)

Blood flow through vessels Normally laminar, with the blood components arranged in layers The plasma forms the outer layer & slides smoothly along the endothelium Blood cells form the ‘axial’ layer in the centre of the blood stream This allows the blood to flow smoothly, layers slide over each other, axial part moves fastest.

Blood flow through vessels When we take a blood pressure the sounds we here are caused by turbulent flow of blood Turbulent flow -caused by change in vessel diameter, increase in velocity, & low blood viscosity

Summary Blood vessels carry blood to and from the heart Arteries carry blood away from the heart, veins carry blood back to the heart Large arteries are ‘elastic’ Arterioles provide most of the resistance to blood flow Exchange of nutrients and gases takes place in the capillaries Vein are thin walled and distensible and have valves to prevent backflow of blood Blood circulates through the blood vessels as a result of pressure changes.