Cardiac Cycle and Heart Sounds
A cardiac cycle includes all the events that occur during one heartbeat. On average, the heart beats about 70 times a minute, although a normal adult heart rate can vary from 60 to 100 beats per minute. After tracing the path of blood through the heart, it might seem that the right and left sides of the heart beat independently of one another, but actually, they contract together. First the two atria contract simultaneously; then the two ventricles contract together. The term systole refers to contraction of heart muscle, and the term diastole refers to relaxation of heart muscle. During the cardiac cycle, atrial systole is followed by ventricular systole.
As shown in Figure 12.8, the three phases of the cardiac cycle are:
Phase 1: Atrial Systole. Time = 0.15 sec. During this phase, both atria are in systole (contracted), while the ventricles are in diastole (relaxed). Rising blood pressure in the atria forces the blood to enter the two ventricles through the AV valves. At this time, both atrioventricular valves are open, and the semilunar valves are closed.
Phase 2: Ventricular Systole. Time = 0.30 sec. During this phase, both ventricles are in systole (contracted), while the atria are in diastole (relaxed). Rising blood pressure in the ventricles forces the blood to enter the pulmonary trunk leading to the pulmonary arteries and aorta through the semilunar valves.
Figure 12.8 Stages in the cardiac cycle. Phase 1: atrial systole. Phase 2: ventricular systole. Phase 3: atrial and ventricular diastole.
At this time, both semilunar valves are open, and the atrioventricular valves are closed.
Phase 3: Atrial and Ventricular Diastole. Time = 0.40 sec. During this period, both atria and both ventricles are in diastole (relaxed). At this point, pressure in all the heart chambers is low. Blood returning to the heart from the superior and inferior venae cavae and the pulmonary veins fills the right and left atria and flows passively into the ventricles. At this time, both atrioventricular valves are open, and the semilunar valves are closed.
A heartbeat produces the familiar “LUB-DUP” sounds as the chambers contract and the valves close. The first heart sound, “lub,” is heard when the ventricles contract and the atrioventricular valves close. This sound lasts longest and has a lower pitch. The second heart sound, “dup,” is heard when the relaxation of the ventricles allows the semilunar valves to close. Heart murmurs, which are clicking or swishing sounds heard after the “lub,” are often due to ineffective valves. These leaky valves allow blood to pass back into the atria after the atrioventricular valves have closed, or back into the ventricles after the semilunar valves have closed. A trained physician or health professional can diagnose heart murmurs from their sound and timing. It is possible to replace the defective valve with an artificial valve.
Cardiac output (CO) is the volume of blood pumped out of a ventricle in one minute. (The same amount of blood is pumped out of each ventricle in one minute.) Cardiac output is dependent on two factors: heart rate (HR), which is the beats per minute; stroke volume (SV), which is the amount of blood pumped by a ventricle each time it contracts. The CO of an average human is 5,250 ml (or 5.25 L) per minute, which equates to about the total volume of blood in the human body. Each minute, the right ventricle pumps about 5.25 L through the pulmonary circuit, while the left ventricle pumps about 5.25 L through the systemic circuit. And this is only the resting cardiac output! Cardiac output can vary because stroke volume and heart rate can vary, as discussed next. In this way, the heart regulates the blood supply, dependent on the body’s needs.
A cardioregulatory center in the medulla oblongata of the brain can alter the heart rate by way of the autonomic nervous system (Fig. 12.9). Parasympathetic motor impulses conducted by the vagus nerve cause the heart rate to slow, and sympathetic motor impulses conducted by sympathetic motor fibers cause the heart rate to increase.
The cardioregulatory center receives sensory input from receptors within the cardiovascular system. For example, baroreceptors are present in the aorta just after it leaves the heart and in the carotid arteries, which take blood from the aorta to the brain. If blood pressure falls, as it sometimes does when we stand up quickly, the baroreceptors signal the cardioregulatory center. Thereafter, sympathetic motor impulses to the heart cause the heart rate to increase. Once blood pressure begins to rise above normal, nerve impulses from the cardioregulatory center cause the heart rate to decrease. Such reflexes help control cardiac output and, therefore, blood pressure.
The cardioregulatory center is under the influence of the cerebrum and the hypothalamus. Therefore, when we feel anxious, the sympathetic motor nerves are activated, and the adrenal medulla releases the hormones norepinephrine and epinephrine. The result is an increase in heartbeat rate. On the other hand, activities such as yoga and meditation lead to activation of the vagus nerve, which slows the heartbeat rate.
Other factors affect the heartbeat rate as well. For example, a low body temperature slows the rate. Also, the proper electrolyte concentrations are needed to keep the heart rate regular.
Stroke volume, which is the amount of blood that leaves a ventricle, depends on the strength of contraction. The degree of contraction depends on the blood electrolyte concentration and the activity of the autonomic system. Otherwise two factors influence the strength of contraction.
Venous Return Venous return is the amount of blood entering the heart by way of the venae cavae (right side of heart) or pulmonary veins (left side of heart). Any event that decreases or increases the volume or speed of blood entering the heart will affect the strength of contraction-called Starling’s Law. A slow heart rate allows more time for the ventricles to fill and therefore increases the strength of contraction. A low venous return, as might happen if there is blood loss, decreases the strength of contraction. Exercise increases the strength of contraction because skeletal muscle contraction puts pressure on the veins and speeds venous return.
Difference in Blood Pressure The strength of ventricular contraction has to be strong enough to oppose the blood pressure within the attached arteries. If a person has hypertension or atherosclerosis, the opposing arterial pressure may reduce the effectiveness of contraction and the stroke volume.
Figure 12.9 The cardio regulatory center regulates the heart rate and the vasomotor center regulates constriction of blood vessels, according to input received from baroreceptors in the carotid artery and aortic arch.