The heart is a muscular organ that pumps blood through the blood vessels of the circulatory system. Blood provides the body with oxygen and nutrients, as well as assisting in the removal of metabolic wastes.
In humans, the heart is located between the lungs, in the middle compartment of the chest. The heart is enclosed in a double-walled protective sac, the pericardium. Which also contains a small amount of fluid, pericardial fluid.
The Wall of the Heart is Made up of Three Layers
- Epicardium is the outer layer of the heart wall. It is a thin, transparent layer. It is also known as visceral pericardium because it forms the inner layer of the pericardium.
- The functions of the epicardium protect the inner heart layers and also helps in the production of pericardial fluid.
- This pericardial fluid fills the pericardial cavity and helps to reduce friction between pericardial membranes.
- The myocardium is the thick, middle layer of the heart and is composed of cardiac muscle fibres, which enable contractions of the heart.
- The myocardium is the thickest layer of the heart wall, with its thickness varying in different parts of the heart.
- Conduction of electrical impulses in the heart wall is made possible by specialized myocardial muscle fibres.
- These fibre bundles, consisting of the atrioventricular bundle and Purkinje fibres, carry electrical impulses down the centre of the heart to the ventricles.
- These impulses trigger the muscle fibres in the ventricles to contract.
- The endocardium is the innermost layer of tissue that lines the chambers of the heart.
- The endocardium of the heart atria consists of smooth muscle, as well as elastic fibres. An infection of the endocardium can lead to a condition known as endocarditis.
- The endocardium also provides protection to the valves and chambers of the heart.
Chambers of the Human Heart and Blood Flow
- In humans and mammals, the heart is divided into four chambers.
- These are: upper right and left atria, separated by an interatrial or inter-auricular septum and
the lower right and left ventricles, separated by an inter-ventricular septum.
- The right atrium receives two large veins: superior vena cava and inferior vena cava.
- The superior vena cava brings deoxygenated blood from the head and the upper region of the body.
- The inferior vena cava brings the deoxygenated blood from the lower region of the body.
- The right ventricle pushes blood to the lungs via the pulmonary artery and the left atrium receives oxygenated blood from the lungs via pulmonary veins.
- The left ventricle is thicker than the right ventricle because it has to push blood to all the body parts at a much greater pressure.
- The valve separating the left atrium and the left ventricle is called the bicuspid valve or the mitral valve.
- The valve separating the right auricle from the right ventricle is known as the tricuspid valve.
Chordae tendineae are the special fibrous cords attached to the flaps of bicuspid and tricuspid valves and are joined to the other ends with the papillary muscles.
Semilunar valves (pulmonary valve and aortic valve) are present where arteries (pulmonary trunk and aorta) leave the heart. The valves of the heart maintain the unidirectional flow of blood and prevent its regurgitation in the opposite direction.
Double circulation is the passage of blood twice in the heart through separate pathways for completing one cycle.
- The flow of deoxygenated blood from the right ventricle to the lungs and the return of oxygenated blood from the lungs to the left atrium is called pulmonary circulation.
- The flow of oxygenated blood from the left ventricle of all parts of the body and deoxygenated blood from various body parts to the right atrium is called systemic circulation.
- These two circulations of blood in the heart through separate pathways for completing one cycle, are called double circulation.
In man, the heartbeat is the myogenic i.e., contraction is initiated by a specialized patch of modified heart muscles, the sinoatrial node (SA node) which is situated in the wall of the right atrium near the opening of superior vena cava.
The SA node acts as the “pacemaker” of the heart and thus establishes the basic rhythm at which the heart beats. This wave of contraction next reaches the atrioventricular node (AV node–pacesetter of the heart), which is stimulated to emit an impulse of contraction spreading to the ventricles.
Bundle of His and Purkinje fibres: Human Heart
There is no functional continuity between the atria and the ventricles. The only conducting tissue between the atria and the ventricles is the atrioventricular bundle or the bundle of His.
This bundle then divides into the left and the right bundle branches, one going to each ventricle. Within the myocardium of the ventricles, the branches break up into a network of branching, anastomosing filaments or fibres known as Purkinje fibres.