The heart is an muscular organ that pumps blood to all human parts of the body. It lies at the centre of the thoracic cavity in the pericardial cavity and being flanked on either side by and left lungs.

It is connected to the lungs by the pulmonary arteries which convey de-oxygenated blood into the lungs and the pulmonary veins, which convey oxygenated blood into the heart. The heart is a little bit notched or inclined to the left side of the body in the thoracic cavity.


The Structure and function of the heart

The heart of human being is cone-shaped and divided into four chambers. The four chambers of the heart are two atria (auricles) on the top and two ventricles below. A thick interventricular septum divides the heart longitudinally into two sides; right and left. The right atrium (auricle) communicates with the right ventricle below it by an opening guarded by the tricuspid valve. The arrangement is the same on the left side and the valve is called mitral or bicuspid valve. These valves ensure that blood flows only in one direction thus preventing wrong or reverse flow.


There is no communication between the right and left sides of the heart, for this reason the heart functions as a double pump. The right side pumps deoxygenated blood to the lungs in the pulmonary circulation for oxygenation. The left side pumps oxygenated blood through the aorta to all parts of the body in the systemic circulation.

The heart is made up of cardiac muscles. It is the only involuntary muscle that is striped or striated. The heart is made up of a complex network of these cardiac muscle fibres that branch and fuse in most directions and planes. The muscles of the heart are not attached to bone or any hard structures. The ventricles are thicker and firmer than the auricles. The atria contract against the thick ventricles without loss of rhythm. The atrio-ventricular valves are attached to ventricles below by the fully stretched Chordae tendinae.

The function of the heart muscles is not under the control of the will and so the function of the heart is involuntary. The beat of the mammalian heart is initiated by the sino-atrial node. The rate of the beat is controlled by the nervous system, (medulla). The sinu-auricular node receives fibres from the vagus nerve (parasympathetic nerve) and the sympathetic nerve of the autonomic nervous system. The vagus of the parasympathetic nerve slows the rate of heart beat while the sympathetic fibres accelerate it. The rate of heart beat is always adjusted to suit the needs of the body under various conditions. The rate is therefore dependent on such factors as the state of the body, fear or calm, active or at rest, healthy or in sickness, age, size of the person, temperature of the body and surrounding. In the vertebrates and molluscs, the contraction of the heart is independent of nervous control as it is initiated in the heart itself. Such hearts are called myogenic hearts (myo=muscle; genic=giving rise to). They continue to beat when all nervous connections with the rest of the body are severed. Myogenic hearts are inhibited by acetylcholine. Contraction of the heart or of any chamber is called systole and relaxation is called diastole. The two atria contract together and then the two ventricles. The heart contracts and relaxes all the time throughout the life of the organism. It is not easily fatigued. The contractions are successive atria and ventricles and then a pause.


How the heart works

One heart beat consists of contraction, relaxation and a pulse. The beating of the heart is involuntary. The action is initiated in the heart itself and it can only be controlled by the nervous system. The heart beat or cardiac cycle starts with the contraction of the atrium (atrial contraction). The stimulus for the rhythmic beat of the heart comes from a small part of the right atrium called the sino-atrial node otherwise known as the pacemaker. The action is wavelike and so it spreads to the left atrium (auricle). The blood is pumped to all parts of the body and again returns to the heart. The deoxygenated blood enters the right atrium from the superior and inferior venae cavae while oxygenated blood enters the left atrium from the lungs through the pulmonary veins. When the two atria contract, the blood presses open the tricuspid and bicuspid valves leading into the ventricles and fills the ventricles. When the ventricles contract, the tricuspid and bicuspid valves are closed in such a way that no blood flows back into the atria.

The blood from the right ventricle is forced into the pulmonary artery which supplies the lungs. The blood from left ventricle enters the aorta which supplies all parts of the body. The entrance of the pulmonary artery is guarded by semi-lunar valves which prevent the blood from flowing into the ventricles.

All these constitute one cardiac cycle. The cardiac output is the total volume of blood pumped out by one ventricle in one minute.


Rest period

With this there is an endless rhythm of auricular contraction, ventricular contraction and rest. The rest period is as long as both contractions put together. The rate of heart beat that is the number of beats per unit time can be measured. In an adult human being the rate is 72 beats per minute with considerable fluctuation among individuals of different ages and under different conditions.


The blood vessels

The tubes in which blood leaves the heart flows round the body and returns to the heart are called blood vessels. The system of tubes or the blood vessels start as the artery leaving the left ventricle of the heart as the main artery or aorta.

On leaving the heart, the aorta makes a U-turn and gives out branches to the head and fore limbs and continues downwards to the trunk and hind limbs. The U-turn is called the aortic arch. These major branches divide and further divide into smaller branches called arterioles. The arterioles further divide and subdivide into much tiny branches known as capillaries. Capillaries are small blood vessels that connect the arteries to the vein so as to reach individual cells for the purpose of exchange of materials.

Blood flow in the capillaries is slow. After making the desired contact with the cells the capillaries start to unite and reunite into tangible blood vessels known as venules. The venules again unite into veins that return blood to the right atrium of the heart.

These major veins are the superior vena cava draining blood from the head and forelimbs and inferior vena cava draining blood from the hindlimbs and trunk. Arteries and veins are similar in that their walls are made of three layers but differ in some other features.