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This section examines the control of the heart in more detail, specifically…
1. The pacemaker function
2. Control of the cardiac cycle
3. The heart’s response to exerciseBefore we can examine these however, we need to understand how the myocytes, the muscle cells of the heart function to produce a smooth wave of contraction that contracts the heart in an orderly fashion to eject blood from the ventricles.
Like all muscle cells, myocytes contract when stimulated. They are also able to pass on stimulation to adjacent cells which in turn contract and pass on stimulation to adjacent cells, and so on - Figure 01. Thus, once an area of the heart muscle is stimulated a smooth wave of contraction will move across the remainder of the myocardium (the muscular part of the heart)
Figure 01 - Contraction of myocytes1. The pacemaker function
The heart has a typical resting rate of between 70 to 80 beats per minute – depending on the health and fitness of the individual and this rate is mainly set by the heart itself. In the front wall of the right atrium are a group of autorhythmic cells that make up the sino-atrial node. This generates a nervous impulse every 0.8 seconds that spreads across the atria and causes them to contract. The sino-atrial node (SA node) is the prime pacemaker of the heart.
2. Control of the cardiac cycle and the heart’s conduction system
To work efficiently, the heart needs to beat regularly and the chambers need to contract sequentially, the atria first followed by the ventricles. This ensures that the blood flows through the heart smoothly and efficiently.
The heart has its own conduction system that transfers nervous impulses from the atria down to the apex of the heart. This consists of the atrio-ventricular node, the bundles of His and the Purkinje fibres - Figure 02.
Figure 02 - The conduction system of the heartNervous impulses are generated in the sino-atrial node and this starts a wave of contraction across the right and left atria. Between the atria and the ventricles lies a band of connective tissue that includes the four heart valves. Unlike the heart muscle, this tissue is a poor transmitter of nervous impulses so the ventricles are not stimulated by the initial impulse from the SA node. Instead, the impulses from the atria are detected by the atrio-ventricular node (AV node) that sends the impulses down the bundles of His to the Purkinje fibres. From here, nervous impulses spread up the ventricular myocardium from the apex - Figure 03. The ventricles contract, forcing blood through the semi-lunar valves into the aorta and pulmonary artery.
Figure 03 - Nervous impulses transmitted from the SA node
to the apex of the heartThe contraction of the ventricles from the apex upwards ensures the efficient ejection of blood from the ventricles through the semi lunar valves into the pulmonary artery and aorta. If the heart contracted from the top downwards, blood would tend to pool in the apex. This complex system of conduction ensures the heart’s efficiency - Figure 04.
Figure 04 - Ventricles contract from the apex upwardThe conduction system described above also ensures that the cardiac cycle takes place in a logical order – atria contracting before the ventricles.
3. The heart’s response to exerciseWhen we exercise the heart needs to increase its output in order to transport more oxygenated blood to the muscles so that they can make more energy. This involves an increase in heart rate and stroke volume. Remember the formula...?
CO = HR x SV
cardiac output = heart rate x stroke volumeTypically, heart rate is around 70 beats per minute (BPM) and stroke volume (SV) is around 70 ml so average cardiac output is around 5 litres per minute.
During exercise in a healthy individual, the heart rate may rise to 150 bpm and stroke volume to 100 ml giving a cardiac output of 15 litres per minute, a threefold increase. In trained athletes, cardiac output can exceed 20 litres per minute!
During exercise, sympathetic stimulation of the SA node increases the heart rate and stimulation of the myocardium increases contractility and so stroke volume. An increase in the volume of blood returning to the heart via the venous system causes the heart to contract more forcefully via a mechanism called Starling's forces. This reinforces the increase in stroke volume and cardiac output.
An increase in heart rate and stroke volume results in an increase in cardiac output and logically this should increase blood pressure (BP) if we recall the formula…
BP = CO x SVR
blood pressure = cardiac output x systemic vascular resistanceAlthough cardiac output can treble during vigorous exercise, a corresponding increase in blood pressure could result in damage to tissue and small blood vessels. Fortunately, vasodilation of blood vessels in exercising muscles reduces overall vascular resistance and so keeps systemic blood pressure within acceptable levels. This vasodilation increases the flow of blood to exercising muscles so supplying more oxygen to meet increasing energy demands.
