Blood VesselsClinical Science Team

School of Nursing and Midwifery

Blood vesselsLearning outcomes:1. Compare the structure of an artery and a vein2. 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 VEINVeins 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 CAPILLARYCapillaries link Arteries with Veins

the wall of a capillaryis 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.

artery vein

capillaries

body cell

The CAPILLARYA collection of capillaries is known as a capillary bed.

Differences between arteries and veins

Blood VesselsWalls 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

Large veins are considered capacitance hold most

blood within circulatory system (blood reservoirs).

Arterioles are considered resistance vessels because  Contain highest % smooth muscle Greatest pressure drop Greatest resistance to flow

Capillaries and are considered exchange vessels because exchange of nutrients gases and other substances take place

Distribution of blood in the vascular system

Blood Vessels & Cardiovascular system

Monitoring Circulatory EfficiencyEfficiency 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 arteriesRadial pulse (on radial

artery), routinely usedVaries 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 directlyproportional to thedifference in pressurebetween the endsof the tube

Blood flow through the vesselsIs 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 vesselsNormally laminar, with the blood components

arranged in layersThe plasma forms the outer layer & slides

smoothly along the endotheliumBlood cells form the ‘axial’ layer in the centre of

the blood streamThis 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 bloodTurbulent flow -caused by change in vessel

diameter, increase in velocity, & low blood viscosity

Fluid Exchange85% of fluid that leaves blood is returned at

venous endWhat about the other 15%?

SummaryBlood vessels carry blood to and from the

heartArteries carry blood away from the heart,

veins carry blood back to the heartLarge arteries are ‘elastic’Arterioles provide most of the resistance

to blood flowExchange of nutrients and gases takes

place in the capillariesVein are thin walled and distensible and

have valves to prevent backflow of bloodBlood circulates through the blood vessels

as a result of pressure changes.