Anatomy of heart-

The heart is a conical hollow muscular organ situated in the middle mediastinum. It is enclosed within the pericardium. It pumps blood to various parts of the body to meet the nutritive requirements.

The heart is placed obliquely behind the body of the sternum and adjoining parts of the costal cartilages,so the one third of it lies to the right and two thirds to the left of the median plane. The direction of blood flow,from atria to the ventricles is downward forward and to the left. The heart weighs about 300g in males and 250g in females.

External features-

• The human heart has four chambers.
• These are the right and left atria and the right and left ventricles.
• The atria lies above and behind the ventricles. On the surface of the heart, they are separated from the ventricles by an atrioventricular groove.
• The atria are separated from each other by an interatrial groove.
• The ventricles are separated from each other by an interventricular groove, which is subdivided into anterior and posterior parts.
• The heart has:

1. An apex directed downwards, forwards and to the left.
2. A base (posterior surface) directed backwards.
3. Three surfaces- anterior/sternocostal, inferior and left lateral.
4. Borders: the surfaces are demarcated by upper, inferior, right and left borders.

Apex of the heart-

• The apex of the heart is formed entirely by the left ventricle. It is directed downward, forwards and to the left and is overlapped by the anterior border of the left lung.
• It is situated in the left fifth intercostal space 9cm lateral to the midsternal line just medial to the midclavicular line. In the living subject, pulsations may be seen and felt over this region.

Base of the heart-

• The base of the heart is also called its posterior surface. It is formed mainly by the left atrium and by a small part of the right atrium.
• In relation to the base one can see the openings of four pulmonary veins which open into the left atrium, and of the superior and inferior venae cava which open into the right atrium.
• It is related to thoracic five to thoracic eight vertebrae in the lying posture, and descends by one vertebrae in the erect posture.

Borders of the heart-

1. The upper border is slightly oblique,and is formed by the two atria, chiefly the left atrium.
2. The right border is more or less vertical and is formed by the right atrium. It extends from superior vena cava to Inferior vena cava.
3. The inferior boreder is nearly horizontal and is formed mainly by the right ventricle. A small part of it near the apex is formed by left ventricle. It extends from IVC to apex.
4. The left border is oblique and curve. It is formed mainly by the left ventricle, and partly by the left article. It separates the anterior and left surfaces of the heart. It extends from the apex to left auricle.

Surfaces of the heart-

• The anterior or sternocostal surface is formed mainly by the right atrium and right ventricle, and partly by the left ventricle and left auricle.
• The inferior or diaphragmatic surface rests on the Central tendon of the diaphragm.
• The left surface is formed mostly by the left ventricle and at the upper end by the left auricle.

*Right Atrium-

Position-

• The right atrium is the right upper chamber of the heart.
• It recieves venous blood from the whole body, pumps it to the right ventricle through the right atrioventricular or tricuspid opening.
• It forms the right border, part of the upper border, the sternocostal surface and the base of the heart.

External features-

1. The chamber is elongated vertically, receiving the superior vena cava at the upper end and the inferior vena cava at the lower end.
2. The upper end is prolonged to the left to form the right auricle. The auricle covers the root if the ascending aorta and partly overlaps the infundibulam of the right ventricle. Its margins are notched and the inferior is sponge-like, which prevents free flow of blood.
3. Along the right border of the atrium, there is a shallow vertical groove which passes from the superior vena cava to the inferior vena cava. This groove is called the sulcus terminalis. It is produced by an internal muscular rigde called the crista terminalis. The upper part of the sulcus contains the sinuatrial or SA nodes which acts as the pacemaker of the heart.
4. The right atrioventricular groove separates the right atrium from the right ventricle. It is more or less ventrical and lodges the right coronary artery and the small cardiac vein.

Tributaries of the right atrium-

1. Superior vena cava.
2. Inferior vena cava.
3. Coronary sinus.
4. Anterior cardiac veins.
5. Vanae cordis minimae.
6. Sometimes the right marginal vein.

Right atrioventricular orifice-

Blood passes out of the right atrium through the right atrioventricular or tricuspid orifice and goes to the right ventricle. The tricuspid orifice is guarded by the tricuspid valva which maintains unidirectional flow of blood.

Internal features-

Smooth posterior part or sinus venarum-

1. Developmentally, it is derived from the right horn of the sinus venosus.
2. Most of the tributaries except the anterior cardiac veins open into it.

• The superior vena cava opens at the upper end.
• The inferior vena cava opens at the lower end.
• The coronary sinus open between the opening of the inferior vena cava ad the right atrioventricular orifice.

3. The intervenous tubercle.

Rough anterior part or pectinate part-

1. Developmentally, it is derived from the primitive atrial chamber.
2. It presents a series of transverse muscular rigdes called musculi pectinati.

Interatrial septum-

1. Developmentally,it is derived from the septum primum and septum secundum.
2. It presents the fossa ovalis, a shallow saucer-shaped depression, in the lower part.
3. The annulus ovalis or limbus fossa ovalis is the prominent margin of the fossa ovalis.

*Right ventricle-

Position-

• The right ventricle is a triangular chamber which recieves blood from the right atrium and pumps it to the lungs through the pulmonary trunk and pulmonary arteries.
• It forms the inferior border and a two thirds parts of the sternocostal surface and one third part of inferior surface of the heart.

External features-

1. Externally, the right ventricle has two surfaces- anterior or sternocostal and inferior or diaphragmatic.
2. The interior has two parts:

• The inflowing part us rough due to the presence of muscular ridges called trabeculae carneae.
• The outflowing part or infundibulam is smooth and forms the upper conical part of the right ventricle which gives rise to the pulmonary trunk.
• The two parts are separated by a muscular ridge called the supraventricular crest.

Internal features-

1. The interior shows two orifice

The right atrioventricular or tricuspid orifice, guarded by the tricuspid Valve.

The pulmonary orifice guarded by the pulmonary valve.

2. The interior of the inflowing part shows trabeculae carneae.

3. The septomarginal trabecula or moderator band is a muscular ridge extending from the ventricular septum to the base of the anterior papillary muscle.

4. The wall of the right ventricle is thinner than that of the left ventricle in a ratio of 1:3.

Interventricular septum-

The septum is placed obliquely. The upper part if the septum is thin and membranous and separates not only the two ventricles but also the right atrium and left ventricle. The lower part is thick muscular and separates the two ventricles.

*Left atrium-

Position-

• The left atrium is a quadrangular chamber situated posteriorly. The left auricle forms the left two thirds of the base of the heart, the greater part of the upper border, parts of the sternocostal and left surfaces and the left border.
• It recieves oxygenated blood from the lungs through four pulmonary veins, and pumps it to the left ventricle through the left atrioventricular or bicuspid or mitral orifice.

Features-

1. The posterior surface of the atrium forms the anterior wall of the oblique sinus of pericardium.
2. The anterior wall of the atrium is formed by the interatrial septum.
3. Two pulmonary veins open into the atrium on each side of the posterior wall.
4. The greater part of the interior of the atrium is smooth walled. It is derived embryologically from the absorbed pulmonary veins which open into it.

*Left ventricle-

Position-

• The left ventricle receives oxygenated blood from the left atrium and pumps it into the aorta.
• It forms the apex of the heart, a part of the sternocostal surface, most of the left border and left surfaces, and the left two thirds of the diaphragmatic surface.

Features-

1. Externally, the left ventricle har three surfaces- anterior or sternocostal, inferior or diaphragmatic and left.
2. The anterior is divisible into two parts:

• The lower rough part with trabeculae carneae develops from the primitive ventricle of the heart tube.
• The upper smooth part or aortic vestibule gives origin to the ascending aorta.

3. The interior of the ventricle shows two orifice:

• The left atrioventricular or bicuspid or mitral orifice.
• The aortic orifice, guarded by the aortic valve.

4. There are two well developed papillary muscles, anterior and posterior.

5. The cavity of the left ventricle is circular in cross section.

6. The wall of the left ventricle are three times thicker than those of the right ventricle.