The Heart and Cardiac Output

The heart is a muscle that pumps blood through the blood vessels in the human body [2, 3]. It is located between the lungs, slightly to the left of the body’s midline. The size of the heart is approximately equal to the fist of the person. It weighs 250-300 g for women and 300-350 g for men. Like skeletal muscles, the heart (cardiac muscle) can be noticeably larger in athletes due to exercise. Exercise enlarges the cells in the heart without any increase in their number. Moreover, the heart of athletes is trained for efficient pumping of blood throughout the body. The heart has four chambers: two receiving chambers – located in the upper part of the heart – called the left atrium and the right atrium and two pumping chambers – located in the lower part of the heart – known as the left and right ventricles.

Figure 1.1: The human circulatory system[5]

The right atrium receives deoxygenated blood from organs and body tissues. Then, this blood is sent to the right ventricle, which pumps the deoxygenated blood into the pulmonary circuit, which transports blood between the heart and the lungs. This blood moves via the pulmonary artery branches to the lungs. At the pulmonary capillaries, exchange of gas occurs, i.e. O2 enters the blood and CO2 leaves the blood. The oxygenated blood that returns to the left atrium is then sent to the left ventricle. The latter pumps the blood to the systemic circuit which distributes the blood between the heart and all other organs and tissues. At the systemic capillaries, another exchange occurs, in which O₂ and nutrients leave the blood and CO₂ and waste enter. This blood is then sent back to the right ventricle and the whole process is repeated. The pulmonary and systemic systems are presented in Figure 1.2.

Figure 1.2: The pulmonary and systemic systems [6]

The contraction of the heart starts at the atria (atrial systole), which results in a pressure rise. This pressure causes the blood in the atria to be pumped to corresponding

ventricles via the atrioventricular valves, i.e. the mitral and tricuspid valves as shown in Figure 1.3. The atrial systole lasts approximately 100 ms. By the end of the atrial systole, the blood volume (known as end diastolic volume (EDV)) in each ventricle is about 130 mL. As the contraction of the atria is completed, the right and left ventricles contract (ventricular systole). The ventricular systole takes place in two stages. During the first stage, known as isovolumetric contraction, no blood is ejected. However, the pressure in the ventricles rises until the atrioventricular valves are closed. In the second phase, known as ventricular ejection, the pressure in the ventricles is higher than the pressure in the pulmonary artery and the aorta. Therefore, both right and left ventricles pump equal amounts of blood (via the pulmonary semilunar and aortic semilunar valves) to the pulmonary and systemic systems, respectively. The total blood volume ejected by each ventricle is known as stroke volume and it is in the range of 70-80 mL in the normal human condition. This leaves a blood volume – known as end systolic volume (ESV) – of about 50-60 mL in the ventricles. The ventricular systole lasts a period of about 270 ms. The phase of the ventricular contraction is known as systole.

Figure 1.3: Human heart [6]

When the ventricular systole starts, the atria enter a resting phase known as atrial diastole in which they are filled with blood as shown in Figure 1.4. Once the phase of the ventricular systole is completed, the ventricles enter also a relaxation phase known as ventricular diastole which lasts 430 ms. The ventricles, during this time, are filled with blood. Near the end of the relaxation period of the ventricles, the atrial contraction starts and the atria systole starts again. The phase in which the ventricles are in relaxation is known as diastole. One complete cycle of systole and diastole is known as the cardiac cycle. The cardiac cycle lasts about 0.8 s for a resting heart (75 beats per minute on average). The cardiac cycle can be correlated with the compound electrical signal of the heart, i.e. the electrocardiograph (ECG) signal as shown in Figure 1.4. The atrial systole and ventricular systole are represented by the P and QRS waves of the ECG, respectively. The ventricular diastole is represented by the end of T wave of the ECG signal.

Figure 1.4: Relationship between cardiac cycle and ECG (x-axis is time in s and y-axis is amplitude in V) [7]

Figure 1.5 illustrates the correlation between the ECG signal and the cardiac cycle events, particularly for the left ventricle (LV) which pumps blood to the systemic system. It can be seen that during the period QRS of the ECG signal, the pressure in the left ventricle (LVP) rises until it reaches a point at which its level is higher than the

aortic pressure (AP). At this point, a rapid ejection of blood occurs from the left ventricle to the systemic system via the aorta, and the arterial blood flow rises substantially (Phase 3 in Figure 1.5). The point marked in blue in Figure 1.5 corresponds to the point at which the maximum arterial blood flow rate in the aorta is reached. Figure 1.6 shows typical pressure and volume waveforms in human arteries. It can be seen that the maximum flow rate is reached slightly before the maximum pressure point.

Figure 1.5: The cardiac cycle showing the aortic pressure (AP), the left ventricle pressure (LVP), the left atrium pressure (LAP), end and start left ventricle blood volume (LVEDV and LVESV) and the ECG signal [8]

One of the most important measurements for the assessment of the heart is the cardiac output (CO). It is given by the stroke volume multiplied by the heart rate (HR). The stroke volume is the amount of blood ejected by one ventricle and the heart rate is the number of heart beats per minute (bpm). Stroke volume is the difference between the

EDV and ESV. Each can be measured by echocardiogram [2]. Normal stroke volume for an adult at rest, who weighs 70 kg, is approximately 70 mL. However, this value may change depending on the condition of the person. The heart rate for a resting person is in the range of 60-100 bpm and, on average, it is about 75 bpm. Based on these values, the range of the cardiac output can be between 4-8 L/m with an average value of 5.25 L/m. In healthy athletes, the values of stroke volume and HR can increase to 130 mL and 150 bpm, respectively. This means that the cardiac output can reach 19.5 L/m.

Figure 1.6: Typical blood flow rate and pressure waveforms in human arteries [9]