The heart is a muscular organ situated in the middle of the chest slightly to the left.
It is a conical-shaped hollow organ enclosed within a pericardium, the peritoneal counterpart for the heart.
It is placed obliquely, behind the body of the sternum and costal cartilages, one-third of it is to the right, and the rest two-third lying left of the median plane. It measures 12*9cm, weighs 300g in males and 250g in females.
Structure of Heart
The heart looks simple yet it is an organ with complex structure and functionality. We will study the structure of the heart as external and internal anatomy
External anatomy of heart structure
The heart wall is made up of three layers of tissue as
- Myocardium and
This is the outermost layer of the heart. This layer covers the remaining structure of the heart. It fixes the heart in the position in the thoracic cage. It has three layers which appear like two sacs as
a) Outer fibrous sac
b) Inner serous double membrane
The outer fibrous sac is non-elastic and due to fibrous nature, it prevents over-distension of the heart.
The serous membrane has two layers. The outer layer is called parietal pericardium and it lines the outer fibrous layer.
The inner serous membrane is called the epicardium and is attached to the myocardium.
The cells of the serous membrane secrete the serous fluid in between the two-layer of the serous membrane.
This fluid fills the space in between their layers and helps the smooth movement during the heart beat.
This is the middle layer of the heart wall. It consists of cardiac muscle which is striated muscle but involuntary in nature.
The muscle fibers are branched due to which the layer appears like a sheet of muscle. In between these muscle fibers, there is a connective tissue that helps in the propagation of heart impulses.
This is the inner layer of the heart that forms the lining of heart chambers. It is made of flattened epithelial cells which helps in smooth movement of the blood inside the heart chambers.
Internal anatomy of the Heart
The heart is divided into the right and left side due to a septum which is made of myocardium and endocardium.
Each side is divided into the upper atrium and lower ventricle due to atrioventricular valves.
Thus, there are four chambers in a human heart. Each right and left halves contain one atrium and one ventricle each.
Atria are the chambers for receiving blood and the ventricles pump out the blood to respective sectors.
Atria lies above and behind the ventricles. On the surface of the heart, the ventricles are separated from atria by an atrioventricular groove, and the ventricles are separated from each other by inter-ventricular grooves.
Chambers of Heart
The human heart is a four-chambered organ where there is complete separation of oxygenated and deoxygenated blood. It has two atria and two ventricles.
The right atrium is the right upper chamber of the heart. It receives venous blood from the systemic circulation, i.e. whole body, and pumps it into the right ventricle via the Tricuspid opening.
Along the right border of the atrium, there is a shallow groove, which passes from the superior Venacava to the inferior Vena cava. It is called Sulcus Terminalis, produced by Crista Terminalis. The upper part of Sulcus Terminalis contains the Sino Atrial Node or SA NODE, the pacemaker of the heart.
The right atrium is a more or less vertically elongated structure receiving the Superior Venacava from the upper end and Inferior Venacava from the lower end.
The right Ventricle is a triangular chamber that receives blood from the right atrium and pumps it to the lungs through the Pulmonary trunk.
The Interior of it shows two orifices – right atrioventricular or Tricuspid orifice guarded by the Tricuspid valve and pulmonary orifice guarded by the pulmonary valve.
The septomarginal trabecula or moderator band is a muscular ridge extending from the ventricular septum to the base of the anterior papillary muscle. It contains the right branch of the Atrio Ventricular Node or AV NODE.
The Wall of the right ventricle is thinner than that of the left ventricle. The ratio of thickness is 1:3.
This is a quadrangular chamber located posterior to the right atrium. It receives oxygenated blood via four branches of pulmonary veins and pumps the same into the left ventricle via the Left Atrioventricular aperture or Mitral orifice guarded by a valve of the same name.
Two Pulmonary veins open into the left atrium on each side of the posterior wall.
In the embryonic phase, the right atrium and left atrium remain attached through an opening in the common atrial septum called FORAMEN OVALE. Later after birth, it closes, but an impression remains in both the atria. In the right atrium, it is called Fosaa Ovalis, and in the left atrium, it is called Fossa Lunata.
The left Ventricle receives oxygenated blood from the left atrium via the mitral orifice and pumps the same to Aorta via Aortic valves.
Its interior shows two orifices –the left atrioventricular or Mitral orifice guarded by Mitral Valves and the Aortic Orifice guarded by the Aortic Valve.
It is the thickest chamber of the heart, about three times thicker than the right ventricle, and almost circular in cross-section.
Valves of Heart
The heart is supplied with a series of valves, each having different functions. The main function of valves is to check the backflow of blood.
The valves present in the heart are of two categories:
- Auriculo-ventricular valves: They are two in number namely
- Tricuspid valves: Present between the right atria and right ventricle, it is named Tricuspid because of its three cusps
- Bicuspid valves: Present between the left atria and left ventricle, it is called Bicuspid because of its two cusps. It is also named as Mitral Valve.
- Semilunar Valves: Present between the great vessels i.e Aorta and Pulmonary Artery, and Ventricles, they are named such because of their structure. They are two in number:
- Aortic valve: Present in between the left ventricle and aorta.
- Pulmonary Valve: Present in between the right ventricle and Pulmonary Artery.
Anatomy of heart valves
Both Tricuspid and Bicuspid valves are made up of the following components:
- Cusps are attached to a fibrous ring.
- The cusps are flat and projected into the ventricular cavity. Each cusp has an attached and free margin, an atrial and a ventricular surface. The atrial surfaces are smooth. The attachments of chordate tendineae make the free margins and ventricular surfaces rough and irregular. The valves remain closed during ventricular systole by the apposition of atrial surfaces near the serrated margins.
- The chordae tendineae join the free margins and ventricular surfaces of the cusps to the apices of the papillary muscles. Chordae tendineae prevent the eversion of the free margins and limit the amount of ballooning of the cusps towards the cavity of the atrium.
- The atrioventricular valves are kept competent by the active contraction of the papillary muscles. During the ventricular systole, these papillary muscles pull the chordae tendineae. Each papillary muscle is connected to the contiguous halves of the two cusps.
- Blood vessels are seen only in the fibrous ring and in the basal one-third of the cusps. Nutrition to the central two-third of the cusps is directly derived from the blood in the cavity of the heart.
- The Tricuspid valve has three cusps. It can admit the tips of three fingers. The three cusps are namely anterior, posterior or inferior, and septal. These lie against three walls of the ventricle. Of the three papillary muscles, the anterior muscle is the largest, the inferior one is smaller and irregular, and the septal muscle is denoted by a few small muscular elevations.
- Mitral or Bicuspid valve has two valves- a large anterior called aortic cusp, and a smaller posterior cusp. It can admit the tip of two fingers. The anterior cusp lies between the Mitral and Aortic orifices. The Mitral cusps are relatively smaller and thicker than Tricuspid cusps.
The aortic and pulmonary valves are referred to as Semi-lunar valves due to their shape. Both the valves are similar to each other.
- Each valve consists of three cusps that are attached directly to the vessel wall. There are no fibrous rings like atrioventricular valves. The cusps form small pockets, with their mouth directly away from the ventricular cavity. The free margins of each cusp contain a central fibrous nodule, from each side of which a thin smooth margin, the lunule extends up to the base of each cusp. Each valve is closed during ventricular diastole, preventing the backflow of blood into ventricles, when the cusps bulge towards the ventricular cavity.
- Opposite to the cusps, the vessel walls are slightly dilated to form aortic and pulmonary sinuses. Coronary arteries arise from anterior and left posterior aortic sinuses.
Circulation through the body
The human body consists of two types of circulation.
This circulation starts from the left ventricle, exists through the Aorta, and supplies the whole body, both superior and inferior parts. The body tissues get oxygenated blood through this circulation. This ends in the right atrium, the blood comes back from the upper part of the body via the Superior Venacava, and the inferior part of the body via the Inferior Vena cava.
This circulation starts from the right ventricle, exists heart via Pulmonary Artery which immediately divides into two branches to go into two lungs where it gets oxygenated and returns to the left atria via four Pulmonary Vein.
We should remember that Pulmonary Artery and Pulmonary veins are exceptional blood vessels, where an artery carries deoxygenated blood and a vein carries oxygenated blood.