Chapter 2 Cardiovascular Physiology 29 where E=driving potential (v) R= resistance(ohms) 2. The equivalent relationship for a liquid in motion is mean arterial pressure-right arterial pressur where CO= cardiac output AP= pressure difference(mm Hg) Q= volume flow(L/min) R= resistance(mm Hg/L/min) 3. A driving force is required to move a flow through a resistance to flow C. Resistance 1. Poiseuilles equation gives the relationship of flow, pressure, and resistance It considers features of the blood that are responsible for the patterns of pres- sure and flow through vessels R where Q=blood flow(L/min) for P2=pressure at end of segment R= resistance of vessels between P, and p 2. The equation states that flow(Q) is directly proportional to the driving pres- sure(Ap) and inversely proportional to the resistance(R) 3. Resistance is directly proportional to the length()of the vessel and to the viscosity of blood(o): R where radius of the blood vessel to the fourth power a. The greater the vessel length, the greater the resistance, and the greater the viscosity, the greater the resistance b. The most important factor determining resistance is the radius of the vessel. The equation emphasizes that if the vessel radius doubles(ie, resis- tance decreases ), then flow will increase 16-fold, if other factors remain 4. The above relationship is used in conjunction with the calculation of resis- tance in series versus parallel circuits
Chapter 2: Cardiovascular Physiology 29 N where E = driving potential (V) I = ionic current flow (amps) R = resistance (ohms) 2. The equivalent relationship for a liquid in motion is where CO = cardiac output ∆P = pressure difference (mm Hg) Q = volume flow (L/min) R = resistance (mm Hg/L/min) 3. A driving force is required to move a flow through a resistance to flow. C. Resistance 1. Poiseuille’s equation gives the relationship of flow, pressure, and resistance. It considers features of the blood that are responsible for the patterns of pressure and flow through vessels: where Q = blood flow (L/min) P1 = upstream pressure for segment P2 = pressure at end of segment R = resistance of vessels between P1 and P2 2. The equation states that flow (Q) is directly proportional to the driving pressure (∆P) and inversely proportional to the resistance (R). 3. Resistance is directly proportional to the length (λ) of the vessel and to the viscosity of blood (f): where r 4 = radius of the blood vessel to the fourth power. a. The greater the vessel length, the greater the resistance, and the greater the viscosity, the greater the resistance. b. The most important factor determining resistance is the radius of the vessel. The equation emphasizes that if the vessel radius doubles (ie, resistance decreases), then flow will increase 16-fold, if other factors remain constant. 4. The above relationship is used in conjunction with the calculation of resistance in series versus parallel circuits. R = 8 r 4 η π , Q = P P R 1 2 − , CO = P = QR, ∆ mean arterial pressure – right arterial pressure Total peripheral resistance 5506ch02.qxd_ccII 2/17/03 2:15 PM Page 29
30 USMLE Road Map: Physiology a. To calculate total resistance(R_) through a circulation of resistances in series,the individual resistances are summed(Rr=R,+R2+ry) b. To calculate total resistance(R_)through a circulation of resistances in allel, the individual conductances are summed(1/Rr=1/R,+1 5. Thus, if all additional parameters are held constant(eg, AP), a resistance change in one parallel subcircuit of the parallel circulation will not change the flow through remaining subcircuits of the parallel circulation. 6. Because the systemic and pulmonary circulations have approximately the ame number of total capillaries with the same total cross-sectional area (1357 cm) and their blood viscosities and flows are both al. the lower pressure difference across the pulmonary circuit must be due to the dif- ference in vessel length between the pulmonary and systemic circuits. and Turbulence 1. Lamin does not generate an audible sound; in contrast, turbulent dom pressure fluctuations, and sounds are heard 2. The Reynolds number(a dimensionless variable relating viscous and inertial forces)serves as a useful indicator for the transition of laminar flow to turbu- lent flow. The Reynolds number is calculated from the following equation No s where Nr= Reynolds number V= mean velocity(cm/s) D= tube diameter(cm) p=fluid der 3. Turbulent flow usually occurs when the reynolds number a critIc lue of 3000 4. Because the viscosity of blood is relatively high, the Reynolds number for tur- bulent flow is not exceeded in most parts of the circula E Compliance 1. Compliance describes the distensibility of blood vessels 2. Vascular compliance( C)is the slope of the relationship between a rise in vol- ume in the vessel and the rise in pressure produced by that rise; hence, △V 3. The compliance of combined veins is about 19 times greater than the com- liance found in the combined arteries a. Systolic pressure is a function of the stroke volume(and compliance) b. Diastolic pressure is a function of the heart rate and the arteriolar resis- tance, which determines run-off into the veins
30 USMLE Road Map: Physiology N a. To calculate total resistance (RT) through a circulation of resistances in series, the individual resistances are summed (RT = R1 + R2 + R3). b. To calculate total resistance (RT) through a circulation of resistances in parallel, the individual conductances are summed (1/RT = 1/R1 + 1/R2 + 1/R3). 5. Thus, if all additional parameters are held constant (eg, ∆P), a resistance change in one parallel subcircuit of the parallel circulation will not change the flow through remaining subcircuits of the parallel circulation. 6. Because the systemic and pulmonary circulations have approximately the same number of total capillaries with the same total cross-sectional area (1357 cm2 ) and their blood viscosities and flows are both equal, the lower pressure difference across the pulmonary circuit must be due to the difference in vessel length between the pulmonary and systemic circuits. D. Reynolds Number and Turbulence 1. Laminar flow does not generate an audible sound; in contrast, turbulent flow involves random pressure fluctuations, and sounds are heard. 2. The Reynolds number (a dimensionless variable relating viscous and inertial forces) serves as a useful indicator for the transition of laminar flow to turbulent flow. The Reynolds number is calculated from the following equation: where NR = Reynolds number V = mean velocity (cm/s) D = tube diameter (cm) p = fluid density n = fluid viscosity (Poises) 3. Turbulent flow usually occurs when the Reynolds number exceeds a critical value of 3000. 4. Because the viscosity of blood is relatively high, the Reynolds number for turbulent flow is not exceeded in most parts of the circulation. E. Compliance 1. Compliance describes the distensibility of blood vessels. 2. Vascular compliance (C) is the slope of the relationship between a rise in volume in the vessel and the rise in pressure produced by that rise; hence, 3. The compliance of combined veins is about 19 times greater than the compliance found in the combined arteries. a. Systolic pressure is a function of the stroke volume (and compliance). b. Diastolic pressure is a function of the heart rate and the arteriolar resistance, which determines run-off into the veins. C = V P ∆ ∆ N = VD R p η , 5506ch02.qxd_ccII 2/17/03 2:15 PM Page 30
Chapter 2: Cardiovascular Physiol 1 F. Pressure Profile 1. As blood flows through the systemic circulation, pressure decreases progres sively from the aorta, where it is highest, to the vena cava, where it is lowest 2. Because the greatest resistance to flow occurs in the arterioles, the largest de crease in pressure occurs across the arterioles 3. Local arteriolar dilation in an organ decreases arteriolar resistance, which increases blood flow and pressure downstream, whereas local arteriolar con- striction increases arteriolar resistance and decreases flow and stream 4. Atrial pressure is lower than venous pressure; pressure is 5-10 mm Hg in he left atrium and 15 mm Hg in peripheral venules G. Arterial Pressures( Figure 2-3) 1. Systolic arterial pressure is the highest arterial pressure during the cardiac a. It represents the pressure developed when the heart contracts most b. Arterial peak systolic pressure increases, whereas minimum diastolic pres- re falls as blood flows from the aorta to the peripheral arteries 2. Diastolic pressure is the lowest arterial pressure during the cardiac cycle representing the pressure when the heart is relaxed and not contracting Aorta Arteries Arterioles Capillaries Venules Veins Venae Right cavae hear Figure 2-2. Pressure profile. Spikes in pressure represent the systo stoic values during the cardiac cycle. The arterioles are the resistance v dampen out the oscillations except during aortic insufficiency. Pulsatile are not normally seen beyond the arteriolar level
Chapter 2: Cardiovascular Physiology 31 N Pressure (mm Hg) 0 40 80 120 Aorta Arteries Arterioles Capillaries Venules Veins Venae cavae Right heart Figure 2–2. Pressure profile. Spikes in pressure represent the systolic and diastolic values during the cardiac cycle. The arterioles are the resistance vessels and dampen out the oscillations except during aortic insufficiency. Pulsatile pressures are not normally seen beyond the arteriolar level. F. Pressure Profile 1. As blood flows through the systemic circulation, pressure decreases progressively from the aorta, where it is highest, to the vena cava, where it is lowest (Figure 2–2). 2. Because the greatest resistance to flow occurs in the arterioles, the largest decrease in pressure occurs across the arterioles. 3. Local arteriolar dilation in an organ decreases arteriolar resistance, which increases blood flow and pressure downstream, whereas local arteriolar constriction increases arteriolar resistance and decreases flow and pressure downstream. 4. Atrial pressure is lower than venous pressure; pressure is 5–10 mm Hg in the left atrium and 15 mm Hg in peripheral venules. G. Arterial Pressures (Figure 2–3) 1. Systolic arterial pressure is the highest arterial pressure during the cardiac cycle. a. It represents the pressure developed when the heart contracts most forcibly. b. Arterial peak systolic pressure increases, whereas minimum diastolic pressure falls as blood flows from the aorta to the peripheral arteries. 2. Diastolic pressure is the lowest arterial pressure during the cardiac cycle, representing the pressure when the heart is relaxed and not contracting. 5506ch02.qxd_ccII 2/17/03 2:15 PM Page 31
32 USMLE Road Map: Physiology Systolic blood Mean arterial -Pulse pressure pressure Diastolic blood Figure 2-3. Arterial pressures 3. Pulse pressure is the difference between systolic and diastolic pressures and is determined primarily by stroke volume and arterial compliance a. Pulse pressure and both arterial pressures increase with aging due to decreased compliance of vessels. b. The pulse pressure also increases as blood moves out along the arteria tree 4. Mean arterial pressure is the average arterial pressure over time and is calcu- lated by adding diastolic pressure plus one third of pulse pressure a. Mean pressure, the driving force for flow, decreases as one moves out along the arterial tree. b. The fall in mean pressure across the arteriolar bed means that capillary pressure is normally nonpulsatile Ill. Electrophysiology A Electrocardiogram(ECG)(Figure 2-4) 1. The P wave represents atrial depolarization 2. The PR interval is the interval from the beginning of the P wave to the be- nning of theQ wave 3. The Q wave is the beginning of ventricular depolarization 4. The QRS complex represents the depolarization of the ventricles 5. The QT interval is the interval from the beginning of the Q wave to the end of the t 6. The ST segment is the segment from the end of the S wave to the beginning of the t wave 7. The T wave represents ventricular repolarization
32 USMLE Road Map: Physiology N Pressure (mm Hg) 0 40 80 120 Mean arterial pressure Pulse pressure Diastolic blood pressure Systolic blood pressure Figure 2–3. Arterial pressures. 3. Pulse pressure is the difference between systolic and diastolic pressures and is determined primarily by stroke volume and arterial compliance. a. Pulse pressure and both arterial pressures increase with aging due to decreased compliance of vessels. b. The pulse pressure also increases as blood moves out along the arterial tree. 4. Mean arterial pressure is the average arterial pressure over time and is calculated by adding diastolic pressure plus one third of pulse pressure. a. Mean pressure, the driving force for flow, decreases as one moves out along the arterial tree. b. The fall in mean pressure across the arteriolar bed means that capillary pressure is normally nonpulsatile. III. Electrophysiology A. Electrocardiogram (ECG) (Figure 2–4) 1. The P wave represents atrial depolarization. 2. The PR interval is the interval from the beginning of the P wave to the beginning of the Q wave. 3. The Q wave is the beginning of ventricular depolarization. 4. The QRS complex represents the depolarization of the ventricles. 5. The QT interval is the interval from the beginning of the Q wave to the end of the T wave. 6. The ST segment is the segment from the end of the S wave to the beginning of the T wave. 7. The T wave represents ventricular repolarization. 5506ch02.qxd_ccII 2/17/03 2:15 PM Page 32
Chapter 2 Cardiovascular Physiology 33 Prolonged PR intervals Prolonged QT intervals suggest conduction suggest drug toxicity (especially with quinidine) atria and ventricles QT interval interval Elevated and depressed myocardial ischemia. version suggests myocardial ischemia. QRS complex: ventricular depolarization preceding depolarizer contraction (onset of systole) Widened QR complexes suggest conduction delay. Figure 2-4. Electrocardiogram waveform. ACUTE MYOCARDIAL INFARCTION LCOKRELAILON Myocardial infarction is most commonly due to acute coronary thrombosis. It is the most common cause of death in the United States The prognosis depends on the degree of left ventricular dysfunction. Clinical diagnosis is based on three important criteria: Symptoms: Persistent chest pain is the most common complaint. Associated symptoms include sweating, nausea, vomiting, and shortness of breath. ECG findings: Q waves (changes in ventricular depolarization), ST-segment changes (upward or downward shifts from the isoelectric line), and T-wave changes(repolarization). -Blood measurement of enzymes, most commonly creatine kinase(CK). CK isoenzymes are com- posed of M and B polypeptides, and high concentrations of CK-MB indicates myocardial damage. AQ wave is the initial negative deflection in the QRS complex, and a large Q wave is diagnostic of a myocardial infarction. The STsegment correlates with phase 2, or the plateau phase, of ventricular myocytes, and myocardial infarction leads to persistent ST-segment elevation when the positive electrode lies over the injured
Chapter 2: Cardiovascular Physiology 33 N Prolonged PR intervals suggest conduction delay between the atria and ventricles. Prolonged QT intervals suggest drug toxicity (especially with quinidine). Elevated and depressed ST segments suggest myocardial ischemia. T wave: ventricular repolarization. Inversion suggests myocardial ischemia. P wave: atrial depolarization. QRS complex: ventricular depolarization preceding contraction (onset of systole). Widened QR complexes suggest conduction delay. P QRS T PR interval QT interval ST segment Figure 2–4. Electrocardiogram waveform. ACUTE MYOCARDIAL INFARCTION • Myocardial infarction is most commonly due to acute coronary thrombosis. • It is the most common cause of death in the United States. • The prognosis depends on the degree of left ventricular dysfunction. • Clinical diagnosis is based on three important criteria: –Symptoms: Persistent chest pain is the most common complaint. Associated symptoms include sweating, nausea, vomiting, and shortness of breath. –ECG findings: Q waves (changes in ventricular depolarization), ST-segment changes (upward or downward shifts from the isoelectric line), and T-wave changes (repolarization). –Blood measurement of enzymes, most commonly creatine kinase (CK). CK isoenzymes are composed of M and B polypeptides, and high concentrations of CK-MB indicates myocardial damage. • A Q wave is the initial negative deflection in the QRS complex, and a large Q wave is diagnostic of a myocardial infarction. • The ST segment correlates with phase 2, or the plateau phase, of ventricular myocytes, and myocardial infarction leads to persistent ST-segment elevation when the positive electrode lies over the injured area. CLINICAL CORRELATION 5506ch02.qxd_ccII 2/17/03 2:15 PM Page 33