Abstract
The right side of the heart generates flow around the pulmonary circulation, moving deoxygenated venous blood from the heart to the lungs.
What are the functions of the heart?
The mechanical function of the heart is to eject1 blood into the vascular system:
The right side of the heart generates flow around the pulmonary circulation, moving deoxygenated venous blood from the heart to the lungs.
The left side of the heart generates pressure in the arterial circulation, moving oxygenated blood from the heart to the other organs of the body. Flow can then be regulated according to tissue demand.
The heart is also an endocrine organ with a role in the regulation of plasma volume. Stretch receptors in the cardiac atria and ventricles sense increases in plasma volume, secreting atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) in response. Both ANP and BNP act on the kidney to produce a natriuresis, which restores plasma volume to normal.
Describe the structure of the heart
The heart is located in the thorax, enclosed within a fibrous sac called the pericardium. The pericardium forms attachments to surrounding structures that hold the heart in place.
The heart is made up of three tissue layers:
The epicardium, the outer connective tissue layer. A small amount of pericardial fluid separates the epicardium from the pericardium, which helps reduce frictional forces as the heart moves.
The myocardium, the middle layer, which is composed of cardiac muscle.
The endocardium, a layer of epithelial cells that line the inner surface of the heart. The endocardium is in contact with blood and is continuous with the endothelial layer of the blood vessels.
The heart can be divided into right and left sides, each consisting of an atrium and a ventricle. The two sides of the heart are separated by the interatrial and interventricular septae.
The right atrium (RA) receives deoxygenated blood from the superior and inferior venae cavae. When the RA contracts, blood passes through the tricuspid valve (a trileaflet valve) and into the right ventricle (RV).
The RV has a complex shape. Viewed in the transverse plane it is shaped as a crescent, whilst it is triangular in the longitudinal plane. Because of this complex shape, the structure of the RV is difficult to model mathematically. It is therefore difficult to estimate right ventricular volume by echocardiography. When the RV contracts, blood is driven through the pulmonary valve (a trileaflet valve) and into the pulmonary artery.
The left atrium (LA). Oxygenated blood returns from the lungs through four (normal variation from three to five) pulmonary veins and enters the LA. When the LA contracts, blood passes into the left ventricle (LV) through the mitral valve (a bileaflet valve)
The LV has a circular transverse section and a conical longitudinal section, amenable to accurate echocardiographic estimates of its volume. When the LV contracts, blood is forced through the aortic valve (a trileaflet valve) and into the aorta.
What is meant by the term ‘functional syncytium’?
The myocardium is arranged in networks of striated cardiac muscle cells joined together by intercalated discs. Intercalated discs contain three different types of cell–cell interaction:
Gap junction complexes permit the direct passage of intracellular ions and larger molecules from one cell to another. They form electrical synapses, allowing direct electrical spread of action potentials from cell to cell.
Fascia adherens anchor the actin filaments within the sarcomere to the cell membrane.
Macular adherens (also known as desmosomes) anchor cardiac cells to one another.
Cardiac muscle is therefore electrically, chemically and mechanically coupled together so that it behaves as a single coordinated unit, and it is often referred to as a functional syncytium.
Describe the coronary circulation
Whilst a large volume of blood passes through the cardiac chambers, the ventricular wall is too thick for effective diffusion of O2 to occur; only the endocardium is nourished directly. The bulk of the cardiac muscle is perfused by the coronary circulation. Coronary arteries are end arteries: they represent the only source of blood for the downstream myocardium, with few native anastomoses. Consequently, acute obstruction of a coronary artery causes myocardial infarction.
The coronary circulation is divided into right and left sides, which both originate at the aortic root (Figure 27.1). The aortic root has three dilatations just above the aortic valve, known as the aortic sinuses (or sinuses of Valsalva). These sinuses produce eddy currents, which tend to keep the valve cusps away from the aortic walls and facilitate smooth valve closure. This is important, as the left and right coronary arteries originate from the left posterior and anterior coronary sinuses, respectively, and the eddy currents prevent their occlusion.
The left coronary artery (left main stem) arises from the left posterior aortic sinus, just above the left cusp of the aortic valve. The left coronary artery travels a short distance in the left atrioventricular (AV) groove (less than 2.5 cm) before bifurcating into:
– The left anterior descending (LAD) artery (left interventricular artery). This artery descends in the anterior interventricular groove, giving off septal and diagonal branches. The LAD supplies most of the LV, specifically:
▪ The anterolateral myocardium;
▪ The apex;
▪ The interventricular septum.
– The left circumflex artery. This artery continues in the left AV groove, giving off one or more obtuse marginal branches. Normally, the terminal end of the left circumflex artery meets the right coronary artery in the AV groove, where the two arteries form an anastomosis. The left circumflex artery supplies:
▪ The posterolateral LV;
▪ The sinoatrial (SA) node in 40% of individuals.
The right coronary artery originates from the anterior aortic sinus, just above the right cusp of the aortic valve. It travels along the right AV groove, before dividing into:
– The SA branch, which is present in 60% of individuals and supplies the SA node;
– The right marginal artery, which travels down the right margin of the heart towards the apex and supplies the RV; it is the right-sided equivalent of the left interventricular artery.
– The posterior interventricular artery (posterior descending artery), which supplies the posterior part of the septum and the AV node. For this reason, occlusions of the right coronary artery predispose to bradycardia and AV block.
