The human heart is located between the lungs in the thoracic cavity (i. e. it is not situated on the left side of the chest). The heart is a hollow, fibro-muscular organ and is somewhat conical in shape. It is about the size of one’s fist, measures about 12 cm in length and 9 cm in breadth. Its weight is about 300 g in an adult. The heart is enclosed in pericardium (a double- layered membranous sac). It protects the heart from mechanical injury. The space between two pericardial membranes is the pericardial space, which is filled with the pericardial fluid.
The pericardial fluid keeps the heart moist and reduces friction between the heart wall and the surrounding tissues when the heart beats. Human heart is made of cardiac muscles or myocardia which contract rhythmically by self generated impulse. ————————————————- Structure The human heart has a mass of between 250 and 350 grams and is about the size of a large fist. [12] It is enclosed in a double-walled protective sac called the pericardium.
The superficial part of this sac is called the parietal pericardium. The inner pericardium layer is called the visceral pericardium. Together they are usually called the serous pericardium because they contain the pericardial fluid. Outside the parietal pericardium there is a fibrous layer which depends from the mediastinal fascia and is called the fibrous pericardium. [13] The pericardium sac protects the heart, anchors its surrounding structures, but has no effect over the heart function in normal individuals.
[14] The double membrane of pericardium contains the pericardial fluid which provides a smooth lubricated sliding surface within which the heart organ can move in response to its own contractions and to movement of adjacent structures such as the diaphragm and lungs. [15] At 21 days after conception, the human heart begins beating at 70 to 80 beats per minute and accelerates linearly for the first month of beating. The outer wall of the human heart is composed of three layers. The outer layer is called the epicardium, or visceral pericardium since it is also the inner wall of the (serous) pericardium.
The middle layer of the heart is called the myocardium and is composed of muscle which contracts. The inner layer is called the endocardium and is in contact with the blood that the heart pumps. [16] Also, it merges with the inner lining (endothelium) of blood vessels and covers heart valves. [17] The human heart has four chambers, two superior atria and two inferior ventricles. The atria are the receiving chambers and the ventricles are the discharging chambers. The pathways of blood through the human heart are part of the pulmonary and systemic circuits.
These pathways include the tricuspid valve, the mitral valve, the aortic valve, and the pulmonary valve. [18] The mitral and tricuspid valves are classified as the atrioventricular (AV) valves. This is because they are found between the atria and ventricles. The aortic and pulmonary semi-lunar valves separate the left and right ventricle from the pulmonary artery and the aorta respectively. These valves are attached to the chordae tendinae (literally the heartstrings), which anchors the valves to the papilla muscles of the heart.
The interatrioventricular septum separates the left atrium and ventricle from the right atrium and ventricle, dividing the heart into two functionally separate and anatomically distinct units, INTERNAL STRUCTURE OF HEART: The right auricle is bigger than the left auricle. Both the auricles are thin walled. The right auricle and the left auricle are separated from each other by a verticle septum called interauricular septum. There is large aperture in the dorsal wall of the right auricle. This aperture is known as sinu auricular aperture.
The sinu auricular aperture is guarded by a pair of valves. The common pulmonary vein opens in the dorsal wall of the left auricle through a small oblique aperture devoid of any valves. Auriculo-ventricular septum, a horizontal septum, internally separates the two auricles from the ventricle. The right and left auricle open into the ventricle through a single aperture called auriculo ventricular aperture. The auriculo ventricular aperture is guarded by a four flapped valve called auriculo ventricular valve.
Several chordae tendinae are attached to the margin of the four flaps of the auriculo ventricular valve. The chordate tendinae are thread like chords which are attached to the muscular wall of the ventricle at their other ends. The truncus arteriousus arises from the anterior and ventral part of the lumen of the ventricle. The truncus anteriosus emerges and runs forward obliquely on the ventral surface of the right auricle. There are three semilunar valves at the proximal end in the truncus arteriosus.
The basal, broad and thick walled portion of the truncus arteriosus is known as conus arteriosus. The anterior, distal, thin walled and narrow portion of the truncus arteriosus is known as venral aorta. There is a longitudinal spinal valve in the lumen of conus arteriosus. The dorsal margin of this valve is attached to the dorsal wall of the conus. The ventral margin of this valve is freely suspended in the lumen. This valves divides the lumen of conus arteriosus into a left cavum pulmocutaneum and right cavum aorticum.
The left cavum pulmocutaneum and right cavum aorticum are incompletely separated from each other. There is an aperture of common pulmocutaneous arch in the anterior part of the cavum pulmocutaneum. The aperture is guarded by a valve. The blood entering the cavum pulmocataeum enters and flows through the pulmocutaneous arches due to this type of structure. The ventral aorta divides into two lateral aortae. Each lateral aorta is divided into two arches: a carotid arch and systemic arch.