With a suitable diagram, describe the structure and functions of the human heart.

• The heart is an organ derived from the mesoderm.

• It is located in the thoracic cavity, at the centre of the chest.

• An outer, double membranous bag protects the heart. This is the pericardium, which also contains the pericardial fluid.

• The heart wall has three layers thar are the endocardium (innermost thin layer), the myocardium (middle thick layer) and the epicardium (outer thin layer).

• The myocardium is thick since it is made up of cardiac muscle fibres.

• It is four chambered and has several valves that regulate the normal blood flow in the body.

• The upper two chambers are called atria. The atria receive oxygen-free (deoxygenated) blood. The two atria are separated by inter-atrial septum from each other.

• The lower two chambers are theventricles. They pump oxygen-rich (oxygenated) blood.

• The two ventricles are separated via the inter-ventricular septum.

• On the same side, the atrium and ventricle are separated by the atrioventricular septum.

• All these septums have openings which are guarded by valves which prevent oxygenated and deoxygenated blood from mixing.

 The valve between the right atrium and the right ventricle is the tricuspid valve.

 The valve between the left atrium and left ventricle is the bicuspid valve (as called mitral valve).

 Valves at the ventricular outlets are known as semilunar valves. There are two semilunar valves: the pulmonary and the aortic valves.

 The pulmonary valve controls the blood flowing from the right ventricle into the pulmonary arteries, which carry blood to the lungs.

 The aortic valve allows oxygen-rich blood to pass from the left ventricle into the aorta (the largest artery).

• A patch of specialized cardiac musculature (known as nodal tissue) is found in the upper right corner of the right atrium. This is the sino-atrial node or SAN.

• A similar mass of tissue is present in the left lower corner of the right atrium. This is called the atrioventricular node of AVN.

Function:

• The heart is the principle organ of the circulatory system.

• It circulates and delivers oxygen and nutrients in the entire body. This is done via the cardiac cycle.

• The heart circulates blood via two pathways: pulmonary and systemic.

o In the pulmonary pathway, deoxygenated blood moves out of the right ventricle through the pulmonary artery and travels to the lungs. The blood gets oxygenated and returns to the left atrium via the pulmonary vein.

o In the systemic pathway, the oxygenated blood moves through the left ventricle to the aorta. From there, it enters the various arteries and capillaries which supply oxygen to the various body tissues and organs. Deoxygenated blood returns to the heart via veins to the vena cava, thus re-entering the heart's right atrium.

• The heart contains special cells that contract and produce the heartbeat.