The heart is the centre of the the cardiovascular system that delivers nutrients and oxygen to the body and removes waste and carbon dioxide from the body ( [Bray et al., 1994], [Thibodeau
et al., 2004]). The heart (Figure1.3) is a muscular pumping organ, consisting of four chambers
in total: left atrium (LA), left ventricle (LV), right atrium (RA) and right ventricle (RV). The base of the heart connects to the great blood vessels: the aorta, vena cava, pulmonary artery and pulmonary veins.
Figure 1.3 – Heart anatomy. Figures from [Thibodeau et al., 2004].
Each of the four chambers of the heart has its own separate entity, separated from each other and the two large blood vessels by valves, septum and muscular interventricle walls. The outer wall of the heart is made of three layers: the epicardium, myocardium and endocardium. The epicardium is the outer protective layer of the heart. The muscular middle layer wall is the myocardium that produces the contractions of the heart. The LV has a very thick layer of myocardium as more force is required to eject the blood to the whole body. Finally the endocardium is the inner layer of the heart that covers the inner surface of the heart valves and extends to the inner lining of blood vessels.
1.2. The anatomy of the heart 7
Circulatory loops
The cardiovascular system includes two circulatory loops: The pulmonary circulation loop and the systemic circulation loop. The cardiovascular system carries oxygen and nutrients to the cells and tissues in one direction and carries carbon dioxide and waste in the opposite direction.
The RA receives deoxygenated blood from the vena cava and passes it through the tricuspid valve into the RV where the blood is pumped through the pulmonary valve into pulmonary artery leading to the lungs for oxygenation. This process is referred to as pulmonary circulation.
For the systemic circulation, the LA receives oxygenated blood from the lungs via the pulmonary veins and pumps it through the mitral valve into the LV where the blood is pumped through the aortic valve to the aorta leading to the organs and tissues of the body.
All valves are designed to ensure that the blood only flows in one direction, allowing blood to flow either from one chamber to another, or allowing blood to flow out of the heart. The valves have two or three cusps, which are pushed open or close entirely by the pressure differences across the valve. For instance, mitral valve opens during diastole when atrial pressure increases above that of the left ventricle; and closes at the end of diastole when the pressure difference no longer exists. The papillary muscles and the chordae tendineae cause tension to hold the mitral valve when it closes.
Coronary circulation
The coronary circulatory system is the heart’s own set of blood vessels that provides the myocardium with a blood supply. There are five large epicardial coronary arteries that lie in grooves between the heart’s chambers, as shown in Figure 1.4. The left main coronary artery (LM) and the right coronary artery (RCA) branch off from the aorta, while the left anterior descending artery (LAD) and the left circumflex artery (CIRC) arise from the LM when it splits into two. The posterior descending artery (PDA) is a continuation of either the RCA or the CIRC. All these five major arteries and coronary capillaries deliver oxygenated
blood to all of the heart’s cells. The coronary sinus, which is a vein on the posterior side of the heart, carries deoxygenated blood from the myocardium to the vena cava.
Figure 1.4 – The coronary arteries. Figure from [Scientific, 2014].
The occlusion of the coronary arteries may generally lead to an infarct, due to the fact that the blood flow from the coronary arteries supply oxygen and nutrition to the myocardium. Whenever the coronary blood flow falls below what is required to meet metabolic needs, the myocardium is considered ischemic and hence the pumping performance may be impaired. This is especially the case for the myocardium of the LV, which is a very thick muscular layer. In general, different regions of the heart are supplied by the different coronary arteries. Disease of the left main coronary artery is particularly dangerous because this artery usually supplies blood to the myocardium of the LV.
Cardiac eletrophysiology
The heart can generate and conduct electrical impulses so that the myocardium contracts and relaxes in a repetitive cycle to pump the blood throughout the body. The cardiac conduction system (Figure 1.5) consists of the sinoatrial (SA) node, the atrioventricular (AV) node, the Bundle of His and the Purkinje fiber network. The sinoatrial (SA) node, located in the upper
1.2. The anatomy of the heart 9
Figure 1.5 – The cardiac conduction system. Image is adapted from figure 13.5 in [Bray et al.,
1994].
wall of the right atrium, emits electrical impulses at a regular rate of 60 − 100 beats/minute in general. These electrical impulses spread through the heart wall to cause both atria to contract. The impulses then reach the AV node at the bottom of the atrium where they are delayed to allow the atria to contract and push all the blood into the ventricles. Thereafter, the electrical impulse passes through the Bundle of His which divides into two branches, one leading to the LV and the other to the RV. Then the impulse pass further into Purkinje fiber network and eventually triggers the muscle fiber in the ventricles to contract. The electrical activation spreads through the myocardium so rapidly that cells of the ventricles contract nearly simultaneously.
Cardiac cycle
The cardiac cycle is a series of events that occur in every single heartbeat. A single cycle of cardiac activity consists of two basic phases: diastole and systole.
the lungs through pulmonary veins, while venous blood returns to the RA through the superior vena cava and inferior vena cava. The mitral and tricuspid valve are open and blood is flowing from the atria into ventricles through them. The other two valves, the aortic and pulmonic valve are closed. At the end of diastole, when the ventricles are about 80% full, both atria contract and propel more blood into the ventricles.
The systole phase comes shortly after the end of the diastole and the ventricles contract. The ventricle pressure increases rapidly and causes the mitral and tricuspid valve to close when it exceeds the atrial pressure. As a result no blood is entering the ventricles during the systole; however, the atria continue to receive blood through the vena cava and pulmonary veins. Meanwhile, the continued contraction raises the ventricular pressure quickly beyond the pressure in the aorta and the pulmonary artery. This opens the pulmonary and aortic valve to permit ejection of blood into the aorta and pulmonary artery. As the ejection of blood continues the ventricular pressure falls and eventually goes below the pressure in the aorta and the pulmonary artery, causing the pulmonary and aortic valve to close, and later the mitral and tricuspid to open, which begins a new cycle.