Biomedical Engineering Reference
In-Depth Information
1 Introduction
Blood vessels are multi-branched elastic vessels that transport blood throughout
the entire body. There are three types of blood vessels: arteries that carry blood
away from the heart, capillaries that enable the exchange of nutrients, CO
2
, and O
2
between tissue and blood, and veins that carry blood back to the heart. Blood
vessels are composed of three layers, the intimal, media and adventitia. The
innermost layer, the intima, consists of a single layer of endothelial cells held
together by an intercellular matrix surrounded by connective tissue with elastic
lamina woven throughout. The middle layer, the media, is the thickest layer; it
consists of smooth muscle cells, elastin fibers, and bundles of collagen fibrils,
which acts as a mechanically homogeneous material. The adventitia or outer layer
consists of fibroblasts, collagen, and elastin with connective tissue.
Blood vessels, including arteries, arterioles, capillaries, venules and veins,
provide pathways for blood to travel. The heart consists of two pumps: one, the
pulmonary circulation, drives blood to the lungs for oxygen and carbon dioxide
exchange; while the second pump, the systemic circulation, brings blood to all
other tissues in the body. Cardiac output is intermittent due to the periodic stim-
ulation of heart muscles, however the distensibility of the large arteries and its
branches during ventricular contraction (systole) and the elastic recoil of the
arterial wall during ventricular relaxation (diastole) enables continuous flow
through the arteries to the periphery [
1
].
As arteries branch the vessels become narrower and the walls become thinner, the
arteries change from an elastic structure to a semi-muscular structure to a muscular
structure at the arterioles (Table
1
). Large elastic arteries serve two main functions, a
conduit function and a compliance buffering function. The conduit function is the
driving force for blood flow to the lungs and other organs in the pulmonary and
systemic circulations. Due to low vascular resistance and a forward pressure gra-
dient, the conduit function provides a pathway for the blood supplied from the heart
through the arteries. The compliance function allows the artery to act as a pressure
reservoir to smooth flow pulsations, from the cyclic action of the heart to a nearly
steady flow across the capillaries, which reduces the ventricular workload during
systole and conserves heart energy expenditure, alleviating pulsatile stress in the
peripheral arteries [
2
]. The compliance function buffers flow pulsations to steady
flow in the systemic circulation and semi-steady flow in the pulmonary circulation
(Fig.
1
). Arterial compliance reflects the total amount of blood that can be stored in
an artery with a given pressure increase, enabling the conduit arteries to expand to
store large volumes of blood ejected from the heart. As the heart relaxes, the arterial
walls recoil and push blood into downstream vessels. The Windkessel model is the
concept that blood vessels act as elastic storage vessels, relating the pressure
waveform to the interaction between the stroke volume (i.e. the output of blood with
each heart cycle) and the compliance of the large elastic pulmonary arteries [
3
]. The
compliance or Windkessel effect prevents excess rise of pressure during systole and
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