Human heart

 Heart is blood pumping organ placed slightly toward left side in the middle of the thoracic cavity.It applies great pumping pressure on blood using strong cardiac muscles, making it to circulate through blood vessels  to every nook and corner of  body. It  keeps on working with out any rest through out life time. Heart is conical in shape with apex facing down wards and broad base directed upwards. It is protected on all sides by rib cage and by vertebral column on the back side.

Heart is enclosed in double layered transparent thin sac called Pericardium. The space between two layers called Pericardial space is filled with pericaridal fluid.Pericardium and pericardial fluid protect the heart from physical shocks.

The human heart has four chambers, upper two atria and lower two ventricles.Internally four chambers are separated by  Septae.Right and left atria are separated by Inter-auricular septum.It prevents mixing of deoxygenated blood in right atrium with oxygenated blood in left atrium. Similary the right and left ventricles are separated by Inter-ventricular spetum. This prevents the mixing of deoxygenated blood in the right ventricle with the oxygenated blood present in the left ventricle.

The walls of ventricles are thicker and muscular than those of atria as the ventricles have to pump the blood to the various parts of the body. Among the ventricles, the wall of the left ventricle is thicker than the wall of the right ventricle as it pumps blood to more distant parts of the body such as finger and toes. The right ventricle pumps blood only to lungs which are very close to the heart.

The atria and ventricles are separated from each other by a transverse septum called auriculo-ventricular septum. There are two openings in this septum called auriculo-ventricular apertures.The right atrium opens into right ventricle through right auriculo-ventricular aperture while the left through left aperture respectively.

Valves in heart

Heart have valves which allow the blood to flow in one direction only and prevent its flow in reverse direction. The valves are held in position by tough connective tissue strands called Chordaetendinae.

Tricuspid valve with three cusps is in right auriculo-ventricular aperture, it allows blood to flow from right atrium to right ventricle.

Bicuspid vlave with two cusps is in left auriculo-ventricular aperture, it allows blood to flow from left atrium to left ventricle.

Pulmonary valves with three semilunar valves in the origin on pulmonary aorta in right ventricle  allows blood to flow from right ventricle into pulmonary aorta.

Aortic valves at the origin of aorta in the left ventricle allows blood to flow from left ventricle in to aorta.

There are three large veins that bring the blood to the heart from all the body parts.

Superior vena cava collects deoxygenated blood from the upper parts of the body. Inferior vena cava brings deoxygenated blood from lower parts of the body such such as abdomen, hands and legs. These caval veins open into the right auricle or atrium. The third vein is the pulmonary vein which brings oxygenated blood from the lungs and opens into the left atrium.

Veins bring deoxygenated blood from tissues to heart. However, pulmonary vein is an exception to this. It carries oxygenated blood from the lungs to the heart. The right auricle receives deoxygenated blood while the left auricle receives oxygenated blood.In addition to these major veins, a pair of veins called Coronary veins bring deoxygenated blood from the walls of the heart. These also open into right atrium.

Heart receives oxygenated blood from lungs and pumps it to various organs in the body, through large aorta called Systemic aorta and pulmonary aorta . The systemic aorta  arises from left ventricle and carries blood to all the body organs except lungs. The pulmonary aorta originates in the right ventricle. Outside the heart, it divides into two branches namely right and left pulmonary arteries which carry blood to right and left lungs. Arteries carry oxygenated blood to the tissues.The only exception is pulmonary artery which carries deoxygenated blood to the lungs.

There is a pair of coronary arteries which carry oxygenated blood to heart muscles.Cardiac muscle present in the heart is responsible for the contraction and relaxation of the heart. Heart contract and relaxes 70-80 times per minute. Heart beat represents one contraction and one relaxation. The contraction phase is called Systole and the relaxation phase is Diastole. Heart beat has three stages viz Auricular systole, ventricular systole and ventricular diastole.

Sinus auricular node or Sino-atrial node

In mammalian heart a small group of modified cardiac muscle fibres are located in the right auricle near the opening of superior vena cava. It is found that the excitation process starts from this point called sinu auricular node. It is also called as Pace maker of the heart. It has rich supply of capillaries. It is made of delicate network of striated muscle fibers which spread the  wave of contraction in all directions along heart muscles.

Auriculo ventricular node or AV node

The impulses are transmitted from auricles to ventricles through a specialised system called auriculo ventricular bundle or bundle of His. It is present in the inter ventricular septum of purkinje fibres in the wall of ventricle and papillary muscles. The auriculo ventricular bundle consists of parallel muscle fibres which originate from AV node. The auriculo ventricular bundle fibres divide into two branches and run on either side of ventricular septum.

Cardiac cycle

The heart beat comprises of two phases namely systole and diastole, which repeat in a cycle.The heart beat originates in the pace maker. First the two atria contract (atrial systole) followed by contraction of ventricles (ventricular systole).The auricular and ventricular systoles are followed by relaxation of these chambers which is referred as Diastole. Thus the sequence of contractions and relaxations of auricles and ventricles form the cardiac cycle. In humans it lasts for 0.8 sec.

Double circulation

In mammals mixing of blood does not takes place, oxygenated and deoxygenated blood is circulated bt veins and arteries. The deoxygenated blood is circulated by veins  and arteries. The deoxygenated blood from the right ventricle passes through the pulmonary aorta into the lungs and after aeration, the oxygenated blood is carried through the pulmonary veins into the left auricle and is called Pulmonary circulation.

The oxygenated blood from the left ventricle, enter into the systemic aorta and carried by its branches to all parts of the body. The deoxygeneated blood from the body parts is collected by the  major veins and brought into the right auricle, this is called as systemic circulation. The pulmonary and systemic circulations together called as double circulation.