The vasculature

Types of vessels

In the cardiovascular circuit, different vessels are adapted to transport blood at different pressures. The differences between the two main types, arteries and veins, are summarized in Table 9.1 , as well as Figs. 7.1 and 9.2 .

• Arteries

  • ↑ elastic tissue and smooth muscle.

    • Arterial pressure is created not only by the heart but also by smooth muscle in the walls of the arteries, which squeezes the arterial blood.

    • This squeezes blood into the high-capacity veins, leaving only around 20% of total blood volume in systemic arteries at any given time.

    • The high proportion of elastin in the walls of large arteries gives them elastic recoil, the tendency to shrink back down once stretched.

  • Under high pressure.

  • Relatively low compliance.

• Arterioles

  • Smallest branches of arteries.

  • Highest total resistance in the vascular system.

  • Arterioles are critical for the control of blood pressure and blood flow.

    • By constricting some or all of the body’s arterioles, the vasculature can direct the flow of blood to the organs that most need it, or it can elevate blood pressure throughout the cardiovascular circuit.

    • Regulated by the autonomic nervous system.

• Capillaries

  • Highest total cross-sectional and surface area in the vascular system

  • Thin-walled

  • Sites of nutrient and gas exchange

    • Single layer of endothelial cells → facilitates transport

• Veins

  • The vein walls have low muscle content and are highly distensible.

  • Veins and venules have high compliance when pressure is low, whereas arterioles are of low compliance at low pressures.

    • This property of veins allows for the accommodation of large volumes of blood in the circulation before development of high venous pressure.

  • The high blood volume in the veins is available to return to the heart and lungs when needed, as in the case of exercise or other demand on cardiac output (see Fast Fact Box 9.1 and Fast Fact Box 9.2 ).

The venous pressure is also a driving force for movement of blood from vein back to heart, a flow known as venous return.

• Normal gravitational forces oppose venous return and need to be overcome by developed venous pressures, particularly in the lower extremities.

• Recalling the relationship between compliance and pressure (C = ΔV/ΔP), venous pressure may be increased by:

  • Reduced compliance

  • Compression of peripheral veins by muscular contractions of the legs

  • Increased tone of vessels (i.e., by sympathetic nervous system)

  • Increased volume

From the cardiac perspective, venous return is an important component of preload, a topic covered in Chapter 10 .

Summary of arterial circulation ( Fig. 9.3 )

• The contraction of the cardiac ventricles, an event called systole, drives blood into the pulmonary arteries and into the aorta. The ventricles then relax, an event called diastole.

• Systole expands the highly elastic aorta, and elastic recoil occurs during diastole.

• This drives the blood out of the aorta and into the smaller arterial branches.

• Because both systole and diastole drive the blood forward, blood never stops moving.

• Flow is very pulsatile in the aorta and becomes less pulsatile as blood moves down the arterial system.

  • Pulsatility is still measured well in small arteries.

• From the aorta, the blood travels through a branching network of vessels of progressively smaller caliber.

• Just before the level of the capillaries are the highly muscular arterioles.

  • The flow of blood into a given capillary bed may be controlled by muscular contraction in structures known as precapillary sphincters.

  • When constricted, these sphincters result in a diversion through thoroughfare channels directly into the venous sinusoids, bypassing the capillary bed.

• Arterial circulation terminates in the capillaries.

  • Site of nutrient and gas exchange.

Pressure

Recall from basic gas laws that the pressure, P, of a gas in a container is proportional to the number of moles (n) in the container per volume (V) of the container:

(because P = nRT/V, where R is the gas constant and T is temperature in Kelvin).

• This implies that ↓ volume (V) → molecules forced together → more frequent collisions → ↑ force per area → pressure (P).

This holds true for fluids in a tube (i.e., vessel).

• Systole: contraction of cardiac muscles → shrinking vessel (ventricle) size → ↑ P

• Diastole: blood emptied into systemic circulation → ↑ V available in aorta → ↓ P

To shrink the ventricle or the aorta, work (W) must be done on the vessel walls (force, F, must be applied over a distance, d):

Thus work or energy is necessary to drive the contraction of the vessels.

• In the heart, the energy of muscular contraction is derived from adenosine triphosphate (ATP).

• In the aorta, the kinetic energy of the blood entering from the heart pushes the aortic walls out. That energy is stored as potential energy in the aortic elastin fibers. This is released during vessel contraction during diastole.

Because the work is being done in three dimensions, the work equation can be rewritten for three dimensions:

Note: Because pressure is force per area (P = F/A) and volume is equal to distance times area (Δ = d × A), the equation W = P(ΔV) can be reduced to W = (F/A) × (d × A), which is the same as the more familiar W = F × d.

Pressure difference

Two types of pressure need be considered in the cardiovascular system ( Fig. 9.4 ):

• Transmural pressure refers to a pressure gradient felt across the vascular wall, at one particular point in the vascular tree.

  • This creates the driving pressure forward through circulation.

  • It requires an opportunity for outflow (i.e., pressure gradient) to move fluid.

• Perfusion pressure or driving pressure, which is the gradient of pressures between two places within the circulation.

The Ohm’s law analogy (I=V/R, where the current I is analogous to blood flow, voltage V is analogous to the pressure gradient) describes the flow of blood (Q) in liters per minute from point A to point B:

where ΔP is the difference in pressure between point A and point B and R is the resistance (mm Hg/mL/min) to flow along the way from A to B.

From this relationship, we see that blood flow is inversely proportional to blood vessel resistance.