321 321 Cardiac output = $\frac{O_2$ consumption}{[O_2]$_{pulmonary vein} - [O_2]$_{pulmonary artery}$ 322 322 Cardiac output = $\frac{250 mL O_2/min}{0.20 mL O_2/mL blood - 0.15 mL O_2/mL blood}$ 323 323 = 5000 mL/min 324 324 325 325 Not only is the Fick principle applicable to measurement of cardiac output (essentially the blood flow to the whole body), but it also can be applied to the measurement of blood flow to individual organs. For example, renal blood flow can be measured by dividing the O2 consumption of the kidneys by the difference in O2 content of renal arterial blood and renal venous blood. 326 326 327 327 328 328 ## CARDIAC CYCLE 329 329 330 330 Figure 4.25 illustrates the mechanical and electrical events that occur during a single cardiac cycle. The cycle is divided into seven phases (Fig. 4.25, letters A through G), which are separated by vertical lines in the figure. The ECG marks the electrical events of the 331 331 332 332 333 333 334 334 cardiac cycle. Left ventricular pressure and volume, aortic and left atrial pressures, venous pulse, and heart sounds are all plotted simultaneously. The points at which the mitral and aortic valves open and close are shown by arrows. 335 335 336 336 Figure 4.25 is best studied vertically, one phase at a time, so that all of the cardiovascular parameters in a given phase of the cycle can be correlated. The ECG can be used as a time/event marker. The cycle begins with depolarization and contraction of the atria. Table 4.5 can be used in conjunction with Figure 4.25 to learn the events of the cardiac cycle. 337 337 338 338 339 339 ## Atrial Systole (A) 340 340 341 341 Atrial systole is atrial contraction. It is preceded by the P wave on the ECG, which marks depolarization of the 342 342 343 343 atria. Contraction of the left atrium causes an increase in left atrial pressure. When this increase in atrial pres- sure is reflected back to the veins, it appears on the venous pulse record as the a wave. The left ventricle is relaxed during this phase, and because the mitral valve (AV valve of the left side of the heart) is open, the ventricle is filling with blood from the atrium, even prior to atrial systole. Atrial systole causes a further increase in ventricular volume as blood is actively ejected from the left atrium to the left ventricle through the open mitral valve. The corresponding "blip" in left ventricular pressure reflects this additional volume added to the ventricle from atrial systole. The fourth heart sound (S4) is not audible in normal adults, although it may be heard in ventricular hypertrophy, where ventricular compliance is decreased. When present, S4 coincides with atrial contraction. The sound 344 344 345 345 *TABLE 4.5 Events of the Cardiac Cycle* 346 346 347 347 348 348

Phase of Cardiac Cycle^a	Major Events	Electrocardiogram	Valves	Heart Sounds
Atrial Systole (A)	Atria contract Final phase of ventricular filling	P wave PR interval	—	Fourth heart sound
Isovolumetric Ventricular Contraction (B)	Ventricles contract Ventricular pressure increases Ventricular volume is constant (all valves are closed)	QRS complex	Mitral valve closes	First heart sound
Rapid Ventricular Ejection (C)	Ventricles contract Ventricular pressure increases and reaches maximum Ventricles eject blood into arteries Ventricular volume decreases Aortic pressure increases and reaches [... Output truncated due to length. 2744 tokens/11141 characters removed ...]