Figure 3.1 Circuitry of the cardiovascular system.

■α1-Adrenergic receptors are found on the arterioles of the skin, splanchnic, and renal circulations.

■β2-Adrenergic receptors are found on arterioles of skeletal muscle.

3.  Capillaries

■have the largest total cross-sectional and surface area.

■consist of a single layer of endothelial cells surrounded by basal lamina.

■are thin walled.

■are the site of exchange of nutrients, water, and gases.

4.  Venules

■are formed from merged capillaries.

5.  Veins

■progressively merge to form larger veins. The largest vein, the vena cava, returns blood to the heart.

■are thin walled.

■are under low pressure.

■contain the highest proportion of the blood in the cardiovascular system.

■The blood volume contained in the veins is called the unstressed volume.

■have α1-adrenergic receptors.

B. Velocity of blood flow

■can be expressed by the following equation:

v = Q/ A

where:

v = velocity (cm/sec)

Q = blood flow (mL/min)

A = cross-sectional area (cm2)

68BRS Physiology

■Velocity is directly proportional to blood flow and inversely proportional to the cross-­ sectional area at any level of the cardiovascular system.

■For example, blood velocity is higher in the aorta (small cross-sectional area) than in the sum of all of the capillaries (large cross-sectional area). The lower velocity of blood in the capillaries optimizes conditions for exchange of substances across the capillary wall.

C.Blood flow

■can be expressed by the following equation:

Q= DP/ R or

= Mean arterial pressure - Right atrial pressure Cardiac output ( )

Total peripheral resistance TPR

where:

Q = flow or cardiac output (mL/min) P = pressure gradient (mm Hg)

R = resistance or total peripheral resistance (mm Hg/mL/min)

■The equation for blood flow (or cardiac outputv) is analogous to Ohm’s law for electrical circuits (I = V/R), where flow is analogous to current, and pressure is analogous to voltage.

■ The pressure gradient ( P) drives blood flow.

■Thus, blood flows from high pressure to low pressure.

■Blood flow is inversely proportional to the resistance of the blood vessels.

D. Resistance

■Poiseuille’s equation gives factors that change the resistance of blood vessels.

R = 8 h l pr4

where:

R = resistance

η = viscosity of blood

l = length of blood vessel

r4 = radius of blood vessel to the fourth power

■Resistance is directly proportional to the viscosity of the blood. For example, increasing viscosity by increasing hematocrit will increase resistance and decrease blood flow.

■Resistance is directly proportional to the length of the vessel.

■Resistance is inversely proportional to the fourth power of the vessel radius. This relationship is powerful. For example, if blood vessel radius decreases by a factor of 2, then resistance increases by a factor of 16 (24), and blood flow accordingly decreases by a factor of 16.

1.  Resistances in parallel or series

a.  Parallel resistance is illustrated by the systemic circulation. Each organ is supplied by an artery that branches off the aorta. The total resistance of this parallel arrangement is expressed by the following equation:

Ra, Rb, and Rn are the resistances of the renal, hepatic, and other circulations, respectively.

■Each artery in parallel receives a fraction of the total blood flow.

■The total resistance is less than the resistance of any of the individual arteries.

■When an artery is added in parallel, the total resistance decreases.

■In each parallel artery, the pressure is the same.

b.  Series resistance is illustrated by the arrangement of blood vessels within a given organ. Each organ is supplied by a large artery, smaller arteries, arterioles, capillaries, and veins arranged in series. The total resistance is the sum of the individual resistances, as expressed by the following equation:

Rtotal = Rartery + Rarterioles + Rcapillaries

■The largest proportion of resistance in this series is contributed by the arterioles.

■Each blood vessel (e.g., the largest artery) or set of blood vessels (e.g., all of the capillaries) in series receives the same total blood flow. Thus, blood flow through the largest artery is the same as the total blood flow through all of the capillaries.

■As blood flows through the series of blood vessels, the pressure decreases.

2.  Laminar flow versus turbulent flow

■Laminar flow is streamlined (in a straight line); turbulent flow is not.

■Reynolds’ number predicts whether blood flow will be laminar or turbulent.

■When Reynolds’ number is increased, there is a greater tendency for turbulence, which causes audible vibrations called bruits. Reynolds’ number (and therefore turbulence) is increased by the following factors:

a.  ↓ blood viscosity (e.g., ↓ hematocrit, anemia) b.  ↑ blood velocity (e.g., narrowing of a vessel)

3.  Shear

■Is a consequence of the fact that adjacent layers of blood travel at different velocities within a blood vessel.

■Velocity of blood is zero at the wall and highest at the center of the vessel.

■Shear is therefore highest at the wall, where the difference in blood velocity of adjacent layers is greatest; shear is lowest at the center of the vessel, where blood velocity is constant.

E.Capacitance (compliance)

■describes the distensibility of blood vessels.

■is inversely related to elastance, or stiffness. The greater the amount of elastic tissue there is in a blood vessel, the higher the elastance is, and the lower the compliance is.

■is expressed by the following equation:

C = VP

where:

C = capacitance or compliance (mL/mm Hg) V = volume (mL)

P = pressure (mm Hg)

■is directly proportional to volume and inversely proportional to pressure.

■describes how volume changes in response to a change in pressure.

■is much greater for veins than for arteries. As a result, more blood volume is contained in the veins (unstressed volume) than in the arteries (stressed volume).

■Changes in the capacitance of the veins produce changes in unstressed volume. For example, a decrease in venous capacitance decreases unstressed volume and increases stressed volume by shifting blood from the veins to the arteries.

■Capacitance of the arteries decreases with age; as a person ages, the arteries become stiffer and less distensible.

F. Pressure profile in blood vessels

■As blood flows through the systemic circulation, pressure decreases progressively because of the resistance to blood flow.

■Thus, pressure is highest in the aorta and large arteries and lowest in the venae cavae.