Steps in the cycle

a. 1 2 (isovolumetric contraction). The cycle begins during diastole at point 1. The left ventricle is filled with blood from the left atrium and its volume is about 140 ml (end-diastolic volume). Ventricular pressure is low because the ventricular muscle is relaxed. On excitation, the ventricle contracts and ventricular pressure increases. Because all valves are closed, no blood can be ejected from the ventricle (isovolumetric).

b. 2 3 (ventricular ejection). The aortic valve opens at point 2 when pressure in the left ventricle exceeds pressure in the aorta. Blood is ejected into the aorta, and ventricular volume decreases. The volume that is ejected in this phase is the stroke volume. Thus, stroke volume can be measured graphically by the width of the pressure-volume loop. The volume remaining in the left ventricle at point 3 is end-systolic volume.

c. 3 —> 4 (isovolumetric relaxation). At point 3, the ventricle relaxes. When ventricular pressure decreases to less than aortic pressure, the aortic valve closes. Because all of the valves are closed again, ventricular volume is constant (isovolumetric) during this phase.

d. 4 1 (ventricular filling). Once left ventricular pressure decreases to less than left atrial pressure, the mitral (AV) valve opens and filling of the ventricle begins. During this phase, ventricular volume increases to about 140 ml (the end-diastolic volume).

2. Changes in the ventricular pressure-volume loop are caused by several factors (Figure 3-10).

a. Increased preload (see Figure 3-10A)

- refers to an increase in end-diastolic volume and is the result of increased venous return.

- causes an increase in stroke volume based on the Frank-Starling relationship.

- The increase in stroke volume is reflected in increased width of the pressure-volume loop.

Increased

preload

3

1

4

1

1 I 1 I

Left ventricular volume

Increased afterload

Increased afterload

Increased contractility
Left ventricular volume

Left ventricular volume

Figure 3-10. Effects of changes in (A) preload, (S) afterload, and (C) contractility on the ventricular pressure-volume loop.

b. Increased afterload (see Figure 3-10B)

- refers to an increase in aortic pressure.

- The ventricle must eject blood against a higher pressure, resulting in a decrease in stroke volume.

- The decrease in stroke volume is reflected in decreased width of the pressure-volume loop.

- The decrease in stroke volume results in an increase in end-systolic volume.

c. Increased contractility (see Figure 3-10C)

- The ventricle develops greater tension than usual during systole, causing an increase in stroke volume.

-The increase in stroke volume results in a decrease in end-systolic volume.

F. Cardiac and vascular function curves (Figure 3-11)

- are simultaneous plots of cardiac output and venous return as a function of right atrial pressure or end-diastolic volume.

1. The cardiac output, or cardiac function, curve

- depicts the Frank-Starling relationship for the ventricle.

- shows that cardiac output increases as a function of end-diastolic volume-a consequence of the length-tension relationship in cardiac muscle fibers. Remember that changes in end-diastolic volume are the primary mechanism for altering cardiac output.

2. The venous return, or vascular function, curve

- depicts the relationship between blood flow through the vascular system (or venous return) and right atrial pressure.

a. Mean systemic pressure

- is the point at which the vascular function curve intersects the x-axis.

- equals right atrial pressure when there is "no flow" in the cardiovascular system.

Cardiac output

Cardiac output

Mean systemic pressure

Right atrial pressure (mm Hg) or end-diastolic volume (L)

Figure 3-11. Simultaneous plots of the cardiac and vascular function curves. The curves cross at the equilibrium point for the cardiovascular system.

Mean systemic pressure

Right atrial pressure (mm Hg) or end-diastolic volume (L)

Figure 3-11. Simultaneous plots of the cardiac and vascular function curves. The curves cross at the equilibrium point for the cardiovascular system.

- is measured when the heart is stopped experimentally. Under these conditions, cardiac output and venous return are zero, and pressure is equal throughout the cardiovascular system.

(1) Mean systemic pressure is increased by an increase in blood volume or by a decrease in venous compliance (where blood is shifted from the veins to the arteries). An increase in mean systemic pressure is reflected in a shift of the vascular function curve to the right (see Figure 3-13).

(2) Mean systemic pressure is decreased by a decrease in blood volume or by an increase in venous compliance (where blood is shifted from the arteries to the veins). A decrease in mean systemic pressure is reflected in a shift of the vascular function curve to the left.

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