Cardiovascular Physiology Regulation of Arterial Pressure PDF

Week 5: Cardiovascular Physiology Regulation of Arterial Pressure 1 Relationships between cardiac output and venous return It should be clear from the previous discussion that one of the most important factors determining cardiac output is left ventricular end-diast...

Week 5: Cardiovascular Physiology Regulation of Arterial Pressure 1 Relationships between cardiac output and venous return It should be clear from the previous discussion that one of the most important factors determining cardiac output is left ventricular end-diastolic volume. In turn, left ventricular end-diastolic volume depends on venous return, which also determines right atrial pressure. Thus, it follows that there is not only a relationship between cardiac output and end-diastolic volume but also a relationship between cardiac output and right atrial pressure. Cardiac output and venous return each can be examined separately as a function of right atrial pressure. These separate relationships also can be combined in a single graph to visualize the normal interrelationship between cardiac output and venous return (see Fig. 4.25). The combined graphs can be used to predict the effects of changes in various cardiovascular parameters on cardiac output, venous return, and right atrial pressure. Cardiac function curve The cardiac function curve or cardiac output curve, shown in Figure 4.26, is based on the Frank-Starling relationship for the left ventricle. The cardiac function curve is a plot of the relationship between cardiac output of the left ventricle and right atrial pressure. Again, recall that right atrial pressure is related to venous return, end-diastolic volume, and end- diastolic fiber length: As venous return increases, right atrial pressure increases, and end- diastolic volume and end-diastolic fiber length increase. Increases in end-diastolic fiber length produce increases in stroke volume and cardiac output. Thus, in the steady state, the volume of blood the left ventricle ejects as cardiac output equals or matches the volume it receives in venous return. Increases in end-diastolic volume (i.e., right atrial pressure) produce increases in cardiac output by the Frank-Starling mechanism. However, this “matching” occurs only up to a point: When right atrial pressure reaches a value of approximately 4 mm Hg, cardiac output can no longer keep up with venous return, and the cardiac function curve levels off. This maximum level of cardiac output is approximately 9 L/min. Week 5: Cardiovascular Physiology Regulation of Arterial Pressure 2 Vascular function curve The vascular function curve or venous return curve, shown in Figure 4.26, depicts the relationship between venous return and right atrial pressure. Venous return is blood flow through the systemic circulation and back to the right heart. The inverse relationship between venous return and right atrial pressure is explained as follows: Venous return back to the heart, like all blood flow, is driven by a pressure gradient. The lower the pressure in the right atrium, the higher the pressure gradient between the systemic arteries and the right atrium, and the greater the venous return. Thus, as right atrial pressure increases, this pressure gradient decreases, and venous return also decreases. The knee (flat portion) of the vascular function curve occurs at negative values of right atrial pressure. At such negative values, the veins collapse, impeding blood flow back to the heart. Although the pressure gradient has increased (i.e., as right atrial pressure becomes negative), venous return levels off because the veins have collapsed, creating a resistance to blood flow. Mean Systemic Filling Pressure The value for right atrial pressure at which venous return is zero is called the mean systemic filling pressure. It is the point at which the vascular function curve intersects the X-axis (i.e., where venous return is zero and right atrial pressure is at its highest value). Mean systemic filling pressure or mean circulatory pressure is the pressure that would be measured throughout the cardiovascular system if the heart were stopped. Under these conditions, pressure would be the same throughout the vasculature and, by definition, would be equal to the mean systemic filling pressure. When pressures are equal throughout the vasculature, there is no blood flow, and therefore venous return is zero (because there is no pressure gradient or driving force). Two factors influence the value for mean systemic filling pressure: (1) the blood volume and (2) the distribution of blood between the unstressed volume and the stressed volume. In turn, the value for mean systemic filling pressure determines the intersection point (zero flow) of the vascular function curve with the X-axis. Figure 4.27 reviews the concepts of unstressed volume and stressed volume and relates them to mean systemic filling pressure. The unstressed volume (thought of as the volume of blood that the veins can hold) is the volume of blood in the vasculature that produces no pressure. The stressed volume (thought of as the volume in the arteries) is the volume that produces pressure by stretching the elastic fibers in the blood vessel walls. ♦ Consider the effect of changing blood volume on mean systemic filling pressure. When the blood volume ranges from 0 to 4 L, all of the blood will be in the unstressed volume (the veins), producing no pressure, and the mean systemic filling pressure will be zero. When blood volume is greater than 4 L, some of the blood will be in the stressed volume (the arteries) and produce pressure. For example, if the total blood volume is 5 L, 4 L is in the unstressed volume, producing no pressure, and 1 L is in the stressed volume, producing a pressure of Week 5: Cardiovascular Physiology Regulation of Arterial Pressure 3 approximately 7 mm Hg (on the graph, read mean systemic filling pressure as 7 mm Hg at a blood volume of 5 L). It now should be clear how changes in blood volume can alter the mean systemic filling pressure (see Fig. 4.26). If blood volume increases, the amount of blood in the unstressed volume will be unaffected (if it is already full), but the amount of blood in the stressed volume will increase. When stressed volume increases, mean systemic filling pressure increases, and the vascular function curve and its intersection point with the X-axis shift to the right. If blood volume decreases, then stressed volume decreases, mean systemic filling pressure decreases, and the vascular function curve and its intersection point with the X-axis shift to the left. ♦ Redistribution of blood between the unstressed volume and the stressed volume also produces changes in mean systemic filling pressure. For example, if the compliance of the veins decreases (e.g., venoconstriction), the veins can hold less blood, and blood shifts from the unstressed volume to the stressed volume. Although total blood volume is unchanged, the shift of blood increases the mean systemic filling pressure and shifts the vascular function curve to the right. Conversely, if the compliance of the veins increases (e.g., venodilation), the veins can hold more blood. Hence, the unstressed volume will increase, the stressed volume and mean systemic filling pressure will decrease, and the vascular function curve shifts to the left. Increased blood volume and decreased compliance of the veins produce an increase in mean systemic filling pressure and shift the vascular function curve to the right. Decreased blood volume and increased compliance of the veins produce a decrease in mean systemic filling pressure and shift the vascular function curve to the left. Slope of the Vascular Function Curve If mean systemic filling pressure is fixed or constant, the slope of the vascular function curve can change by rotating it. The slope of the vascular function curve is determined by total peripheral resistance (TPR), which reflects the resistance of the arterioles. Changes in the slope indicate how efficiently blood flows back to the heart for a given right atrial pressure: