and Alwyn Scott2
(1)
School of Computer Science, University of Manchester, Manchester, UK
(2)
Cardiology High Dependency Unit, Papworth Hospital NHS Foundation Trust, Cambridge, UK
Keywords
AnatomyPhysiologyElectrophysiologyAction potentialDepolarizationRepolarizationCardiac outputBlood pressureCirculatory systemBackground
The human heart is an organ that has both mechanical and electrical components. Both knowledge of basic cardiac anatomy and physiology, as well as electrophysiology is necessary to fully understand the basics of the electrocardiogram (ECG). Understanding and awareness of these elements in the normal heart is essential before building a more complete picture of the pathological heart.
We will start with an overview of the basic anatomy and physiology of the heart before looking more specifically at the electrophysiological components of the heart’s function. Finally we will look at how the mechanical and electrical systems of the heart interact with each other in a healthy individual.
Anatomy of the Heart
Figure 1.1 displays the main anatomical features of the human heart. A brief description of some of the primary anatomical features follows.
Fig. 1.1
Basic anatomy of the heart
Chambers
The heart consists of four chambers, two small chambers located superiorly, called the left and right atrium and two larger chambers located inferiorly called the left and right ventricle. The left ventricle is larger than the right as it has to pump blood to the majority of the body, whereas the right ventricle pumps blood into the lungs. The atria and ventricles are separated from one another by the interatrial septum and the interventricular septum respectively.
Valves
The heart also has four valves. The tricuspid and mitral valves, known as the atrioventricular valves as they sit between the atria and ventricles allowing access for blood to pass from the atria to the ventricles. The other two valves are the aortic and pulmonary valves, and are referred to as the semilunar valves which lead to the aorta and the pulmonary artery. The principle purpose of the valves is to prevent regurgitation of blood from the ventricles back into the atria. The valves open when the pressure in the chamber filled with blood exceeds the pressure in the area past the valve. For example when the pressure in the right atrium exceeds the pressure in the right ventricle, the valve opens and the blood passes from atrium to ventricle.
The valves have flaps that are sometimes referred to as leaflets or cusps. The tricuspid valve has three such flaps or cusps. The mitral valve is also referred to as the bicuspid valve and has two cusps.
Chordae Tendineae
The chordae tendineae are fibrous tendon like cords that connect to the tricuspid valve in the right ventricle and the mitral valve in the left ventricle. When the valves close the chordae tendineae prevent the cusps from swinging upwards into the atrial cavity.
Fossa Ovalis
The fossa ovalis is the remains of what was once a hole (foramen) that existed between the left atrium and the right atrium, located in the atrial septum. This hole allows blood to bypass the lungs in a developing fetus when fetal oxygen supply is provided via the placenta, as the fetal lungs are undeveloped.
Trabeculae Carneae
The trabeculae carneae are muscular columns of irregular shape that exist in both ventricles. In the left ventricle they are smooth and fine when compared to those in the right ventricle. It is believed that the function of the trabeculae carneae is to prevent suction that could impair the pumping action of the ventricles that could otherwise occur if the ventricles were smooth. When the trabeculae carneae contract they inturn pull on the chordae tendineae.
Papillary Muscle
A type of trabeculae carneae that are connected to ventricular surface at one end and at the other to the chordae tendineae.
Coronary Sinus
The coronary sinus allows the cardiac veins carrying deoxygenated blood to drain into the right atrium.
The Great Vessels
Incorporate the vena cava, pulmonary artery/veins and the aorta. In the rest of the body oxygenated blood is found in arteries and deoxygenated blood in the veins. This general rule does not apply to the heart, which sometimes causes confusion. The pulmonary artery carries deoxygenated blood into the lungs and the pulmonary veins carry the resulting oxygenated blood into the left atrium. The aorta trifurcates into three other branches; brachiocephalic, left common carotid and left subclavian arteries which supply the upper portion of the body with blood. The descending aorta bifurcates into the common iliac arteries supplying the legs with blood.
Heart Wall
The wall of the heart (Fig. 1.2) is made up of several layers. The innermost layer is the endocardium, followed by the thicker myocardium that makes up the cardiac muscle and consists of cardiomyocytes, which are cardiac muscle cells. The outer layer of the heart wall is known as the epicardium. Directly following the epicardium is a gap called the pericardial cavity that separates the heart from the pericardium. The pericardial cavity contains pericardial (serous) fluid. The pericardium is a protective membrane that covers the heart and also envelops the roots of the great cardiac vessels. The principle functions of the pericardium (Fig. 1.3) are to anchor the heart in place preventing excess movement, act as a barrier to protect the heart from internal infection from other organs and to lubricate the heart.
Fig. 1.2
Layers of the heart from outside to inside
Fig. 1.3
The pericardium
Circulatory Function of the Heart
Deoxygenated blood is emptied into the right atrium via the vena cava. The inferior vena cava returns blood from the lower portion of the body as the superior vena cava returns blood from the higher portion. Blood is then pumped through the tricuspid valve into the right ventricle and into the lungs via the pulmonary artery where it is oxygenated. Oxygenated blood then returns from the lungs into the left atrium where it can be pumped to the rest of the body by the left ventricle, via the aorta. The circulatory system has two divisions; the systemic and pulmonary circulatory system (Fig. 1.4). The pulmonary system is responsible for circulating blood from the right ventricle to the lungs and back into the left atrium. The systemic system as the name implies pumps blood via the aorta to every other part of the body. This is why the left ventricle is larger and more powerful than the right, as it has to pump blood over greater distances. This is also why left ventricular pressure is higher than right ventricular pressure.
Fig. 1.4
Schematic diagram of the circulatory function of the heart
Coronary Arteries
In addition to the heart pumping blood to the rest of the body, the heart itself requires its own blood supply in order to function as an organ. As blood is pumped via the aorta to the rest of the body, it also passes into the coronary arteries that are located at the aortic root. There are two main coronary arteries, called the left and right coronary arteries respectively (Fig. 1.5). The left coronary artery bifurcates into the circumflex and left anterior descending arteries. Deoxygenated blood is returned to the right ventricle by coronary veins via the coronary sinus. The coronary arteries are discussed in more detail in Chap. 7.
Fig. 1.5
Coronary arteries
Starling’s Law of the Heart
The Frank Starling law essentially states that an increase in end diastolic volume corresponds with an increase in the hearts stroke volume. To put it another way the greater the degree of stretch to the cardiac muscle in diastole (relaxation of cardiac muscle), the greater the force of contraction in systole (contraction of cardiac muscle). What goes in comes out. Problems can occur if the heart is continually maximally stretched for a long period of time, as occurs in heart failure. Like an elastic band, if it is continually maximally stretched it will become weak, lose elasticity and fail to return to it’s original shape.
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