Cardiac Flashcards
The major mechanism for changing blood flow in the cardiovascular system?
By changing the resistance of blood vessels, particularly the arterioles.
Total peripheral resistance
The resistance of the entire systemic vasculature is called the total peripheral resistance (TPR) or the systemic vascular resistance (SVR). TPR can be measured with the flow, pressure, and resistance relationship by substituting cardiac output for flow (Q) and the difference in pressure between the aorta and the vena cava
for ΔP.
The blood vessels and the blood itself constitute resistance to blood flow. The relationship between resistance, blood vessel diameter (or radius), and blood viscosity is described by the Poiseuille equation. The total resistance offered by a set of blood vessels also depends on whether the vessels are arranged in series (i.e., blood flows sequentially from one vessel to the next) or in parallel (i.e., the total blood flow is distributed simultaneously among parallel vessels).
Poiseuille Equation The factors that determine the resistance of a blood vessel to blood flow are expressed by the Poiseuille equation:
R=8nl/#*r4
The most important concepts expressed in the Poiseuille equation are as follows: First, resistance to flow is directly proportional to viscosity (η) of the blood; for example, as viscosity increases (e.g., if the hematocrit increases), the resistance to flow also increases. Second, resistance to flow is directly proportional to the length (l) of the blood vessel. Third, and most important, resistance to flow is inversely proportional to the fourth power of the radius (r4) of the blood vessel. This is a powerful relationship, indeed! When the radius of a blood vessel decreases, its resistance increases, not in a linear fashion but magnified by the fourth-power relationship. For example, if the radius
of a blood vessel decreases by one-half, resistance
does not simply increase twofold—it increases by 16-fold (24)!
A man suffers a stroke caused by partial occlusion of his left internal carotid artery. An evaluation of the carotid artery using magnetic resonance imaging (MRI) shows a 75% reduction in its radius. Assuming that blood flow through the left internal carotid artery was 400 mL/min prior to the occlusion, what is blood flow through the artery after the occlusion?
The variable in this example is the diameter (or radius) of the left internal carotid artery. Blood flow is inversely proportional to the resistance of the artery (Q = ΔP/R), and resistance is inversely proportional to the radius raised to the fourth power (Poiseuille equation). The internal carotid artery is occluded, and its radius is decreased by 75%. Another way of expressing this reduction is to say that the radius is decreased to one-fourth its original size. The first question is How much would resistance increase with 75% occlusion of the artery? The answer is found in the Poiseuille equation. After the occlusion, the radius of the artery is one-fourth its original radius; thus resistance has increased by 1/(1/4)4, or 256-fold. The second question is What would the flow be if resistance were to increase by 256-fold? The answer is found in the flow, pressure, resistance relationship (Q = ΔP/R). Because resistance increased by 256-fold, flow decreased to 1/256, or 0.0039, or 0.39% of the original value. The flow is 0.39% of 400 mL/min, or 1.56 mL/min. Clearly, this is a dramatic decrease in blood flow to the brain, all based on the fourth-power relationship between resistance and vessel radius.
Series and Parallel Resistances
Resistances in the cardiovascular system, as in electrical circuits, can be arranged in series or in parallel
. Whether the arrangement is series or parallel produces different values for total resistance.
Series resistance (Rt=R1+R2+..) is illustrated by the arrangement of blood vessels within a given organ. Each organ is supplied with blood by a major artery and drained by a major vein. Within the organ, blood flows from the major artery to smaller arteries, to arterioles, to capillaries, to venules, to veins. The total resistance of the system arranged in series is equal to the sum of the individual resistances. When resistances are arranged in series, the total flow at each level of the system is the same. For example, blood flow through the aorta equals blood flow through all the large systemic arteries, equals blood flow through all the systemic arterioles, equals blood flow through all the systemic capillaries. For another example, blood flow through the renal artery equals blood flow through all the renal capillaries, equals blood flow through the renal vein (less a small volume lost in urine). Although total flow is constant at each level in the series, the pressure decreases progressively as blood flows through each sequential component (remember Q = ΔP/R or ΔP = Q × R). The greatest decrease in pressure occurs in the arterioles because they contribute the largest portion of the resistance.
Parallel resistance is illustrated by the distribution of blood flow among the various major arteries branching off the aorta (see Figs. 4.1 and 4.5). Recall that the cardiac output flows through the aorta and then is distributed simultaneously, on a percentage basis, among the various organ systems. Thus there is parallel, simultaneous blood flow through each of the circulations (e.g., renal, cerebral, and coronary). The venous effluent from the organs then collects
in the vena cava and returns to the heart. As shown in the following equation and in Figure 4.5, the total resistance in a parallel arrangement is less than any of the individual resistances. The subscripts 1, 2, 3, and so forth refer to the resistances of cerebral, coronary, renal, gastrointestinal, skeletal muscle, and skin circulations. Parallel resistance is expressed as follows:
1/Rt=1/R1+1/R2….
When blood flow is distributed through a set of parallel resistances, the flow through each organ is a fraction of the total blood flow. The effects of this arrangement are that there is no loss of pressure in the major arteries and that mean pressure in each major artery will be the same and be approximately the same as mean pressure in the aorta.
Another predictable consequence of a parallel arrangement is that adding a resistance to the circuit causes total resistance to decrease, not to increase. Mathematically, this can be demonstrated as follows: Four resistances, each with a numerical value of 10, are arranged in parallel. According to the equation, the total resistance is 2.5.If a fifth resistance with a value of 10 is added to the parallel arrangement, the total resistance decreases to 2.On the other hand, if the resistance of one of the individual vessels in a parallel arrangement increases, then total resistance increases. This can be shown by returning to the parallel arrangement of four blood vessels where each individual resistance is 10 and the total resistance is 2.5. If one of the four blood vessels is completely occluded, its individual resistance becomes infinite. The total resistance of the parallel arrangement then increases to 3.333.
The major influences on Reynolds number?
Blood viscosity and changes in the velocity of blood flow. Inspection of the equation shows that decreases in viscosity (e.g., decreased hematocrit) cause an increase in Reynolds number. Likewise, narrowing of a blood vessel, which produces an increase in velocity of blood flow, causes an increase in Reynolds number. The effect of narrowing a blood vessel (i.e., decreased diameter and radius) on Reynolds number is initially puzzling because, according to the equation, decreases in vessel diameter should decrease Reynolds number (diameter is in the numerator). Recall, however, that the velocity of blood flow also depends on diameter (radius), according to the earlier equation, v = Q/A or v = Q/πr2. Thus velocity (also in the numerator of the equation for Reynolds number) increases as radius decreases, raised to the second power. Hence, the dependence of Reynolds number on velocity is more powerful than the dependence on diameter. Two common clinical situations, anemia and thrombi, illustrate the application of Reynolds number in predicting turbulence.
Shear
Shear is a consequence of the fact that blood
travels at different velocities within a blood vessel. Shear occurs if adjacent layers of blood travel at different velocities; when adjacent layers travel at the same velocity, there is no shear. Thus shear is highest at the blood vessel wall, according to the following reasoning. Right at the wall, there is a motionless layer of blood (i.e., velocity is zero); the adjacent layer of blood is moving and therefore has a velocity. The greatest relative difference in velocity of blood is between the motionless layer of blood right at the wall and the next layer in. Shear is lowest at the center of the blood vessel, where the velocity of blood is highest but where the adjacent layers of blood are essentially moving at the same velocity. One consequence of shear is that it breaks up aggregates of red blood cells and decreases blood viscosity. Therefore at the wall, where shear rate is normally highest, red blood cell aggregation and viscosity are lowest
Compliance of Blood Vessels
The compliance (**the slope of a volume-pressure diagram)** or capacitance of a blood vessel describes the volume of blood the vessel can hold at a given pressure. Compliance is related to distensibility and is given by the following equation: C=V/P where C=compliance or capacitance mL/mm Hg
V=Volume mL
P=Pressure mm Hg
The equation for compliance states that the higher the compliance of a vessel, the more volume it can hold at a given pressure. Or, stated differently, compliance describes how the volume of blood contained in a vessel changes for a given change in pressure (ΔV/ΔP).
The difference in the compliance of the veins and the arteries underlies the concepts of unstressed volume and stressed volume. The veins are most compliant and contain the unstressed volume (large volume under low pressure). The arteries are much less compliant and contain the stressed volume (low volume under high pressure). The total volume of blood in the cardiovascular system is the sum of the unstressed volume plus the stressed volume (plus whatever volume is contained in the heart).
Changes in compliance of the veins
cause redistribution of blood between the veins and the arteries (i.e., the blood shifts between the unstressed and stressed volumes). For example, if the compliance or capacitance of the veins decreases (e.g., due to venoconstriction), there is a decrease in the volume the veins can hold and, consequently, a shift of blood from the veins to the arteries: unstressed volume decreases and stressed volume increases. If the compliance or capacitance of the veins increases, there is an increase in the volume the veins can hold and, consequently, a shift of blood from the arteries to the veins: unstressed volume increases and stressed volume decreases. Such redistributions of blood between the veins and arteries have consequences for arterial pressure
Effect of aging on compliance of the arteries
The characteristics of the arterial walls change with increasing age: The walls become stiffer, less distensible, and less compliant. At a given arterial pressure, the arteries can hold less blood. Another way to think of the decrease in compliance associated with aging is that in order for an “old artery” to hold the same volume as a “young artery,” the pressure in the “old artery” must be higher than the pressure in the “young artery.” Indeed, arterial pressures are increased in the elderly due to decreased arterial compliance.
Pulse pressure
is the difference between systolic pressure and diastolic pressure. If all other factors are equal, the magnitude of the pulse pressure reflects the volume of blood ejected from the left ventricle on a single beat, or the stroke volume.Pulse pressure can be used as an indicator of stroke volume because of the relationships between pressure, volume, and compliance. Recall that compliance of a blood vessel is the volume the vessel can hold at a given pressure (C = V/P). Thus assuming that arterial compliance is constant, arterial pressure depends on the volume of blood the artery contains at any moment in time. For example, the volume of blood in the aorta at a given time is determined by the balance between inflow and outflow of blood. When the left ventricle contracts, it rapidly ejects a stroke volume into the aorta, and the pressure rises rapidly to its highest level, the systolic pressure. Blood then begins to flow from
the aorta into the rest of the arterial tree. Now, as the volume in the aorta decreases, the pressure also decreases.Arterial pressure reaches its lowest level, the diastolic pressure, when the ventricle is relaxed and blood is returning from the arterial system back to the heart.
In arteriosclerosis, plaque deposits in the arterial walls decrease the diameter of the arteries and make them stiffer and less compliant. Because arterial compliance is decreased, ejection of a stroke volume from the left ventricle causes a much greater change in arterial pressure than it does in normal arteries (C = ΔV/ΔP or ΔP = ΔV/C). Thus in arteriosclerosis, systolic pressure, pulse pressure, and mean pressure all will be increased.
If the aortic valve is stenosed (narrowed), the size of the opening through which blood can be ejected from the left ventricle into the aorta is reduced. Thus stroke volume is decreased, and less blood enters the aorta on each beat. Systolic pressure, pulse pressure, and mean pressure all will be decreased.
How can the sum of inward Na+ and Ca2+ currents be the same magnitude as the outward K+ current, given that gNa and gCa are very low and gK1 is very high? (g=conductance)
The answer lies in the
fact that, for each ion, current = conductance × driving force. Although gK1 is high, the driving force on K+ is low because the resting membrane potential is close to the K+ equilibrium potential; thus the outward K+ current is relatively small. On the other hand, gNa and gCa are both low, but the driving forces on Na+ and Ca2+ are high because the resting membrane potential is far from the Na+ and Ca2+ equilibrium potentials; thus the sum of the inward currents carried by Na+ and Ca2+ is equal to the outward current carried by K+.
