1. Pericardium covers
The heart is covered by a two-layered pericardium. The outer part is called the fibrous coat and the inner part is called the serous coat. The serous membrane has two layers. Parietal layer and visceral layer. The outer layer is called the parietal layer and the inner layer is called the visceral layer. Between the two layers is the pericardial fluid. This fluid protects the heart from heat, pressure and friction injuries.
2. Heart wall
The wall of the heart is made up of cardiac muscle. It has three layers. These are-
(i) Epicardium: The outer layer of heart wall is called epicardium. It is fatty.
(ii) Myocardium: The middle layer of heart wall is called myocardium. It is compressible-expandable.
(iii) Endocardium: The inner layer of heart wall is called endocardium. It covers the eyelids.
3. Chambers of the heart
The human heart consists of four chambers. These are-
(i) Right atrium: The right atrium is large and covered by a thin membrane. Above it is the superior vena cava and below it is the inferior vena cava. It carries CO2-rich blood to the right ventricle through the tricuspid valve.
(ii) Left Atrium: The left atrium is slightly smaller and covered with thick walls. It is connected with the pulmonary vein. It carries O2-rich blood to the left ventricle through the bicuspid ventricle. Between the two atria is the inter-atrial septum.
(iii) Right Ventricle: The right ventricle is slightly larger and covered with a thick wall. It has a tricuspid capsule on one side and a semilunar capsule on the other. It carries CO2-rich blood to the pulmonary arteries through the semilunar capillaries.
(iv) Left Ventricle: The left ventricle is slightly smaller and covered with thick walls. Its wall is three times as thick as the wall of the right ventricle. Fleshy columnar corni arise from the walls of the ventricles. It has a bicuspid lens on one side and a semilunar lens on the other. It carries O2-rich blood through the semilunar capillaries into the aorta or aorta. Between the two ventricles is the interventricular septum.
4. Heart valve
(i) Bicuspid valve: Bicuspid ventricle is located between left atrium and ventricle. This is called mitral regurgitation. It is attached to the columnar corni by corda tendon fibers. Its size is 4-6 square cm. It allows O2-rich blood to flow from the left atrium to the left ventricle.
(ii) Tricuspid valve: The tricuspid valve is located between the right atrium and ventricle. It is attached to the columnar corni by corda tendon fibers. Its size is 7-9 square cm. It allows blood containing CO2 to flow from the right atrium to the right ventricle.
(iii) Pulmonary semilunar valve: The crescent-shaped valve between the right ventricle and the pulmonary artery is called the pulmonary semilunar valve. It has three screens. But can be bi-veined (1-2). Through this valve, blood containing CO2 enters the pulmonary artery from the right ventricle.
(iv) Aortic semilunar valve: The crescent-shaped valve between the left ventricle and the aortic artery is called the aortic semilunar valve. It has three screens. O2-rich blood enters the aortic artery from the left ventricle through this valve.
(v) Coronary valve: Between the coronary sinus and the right atrium lies the Thibesian valve. It is the atrophy of the fetal sino-atrial septum. In many cases it is absent.
(vi) Eustachian tube: It lies between the inferior vena cava and the right atrium. Blood containing CO2 enters the right atrium through this ventricle. The Italian physician Bartolomeo Eustachi discovered the capsule.
5. Junction tissue
The transformed muscle of the heart wall is called myogenic or connective tissue. Myogenic tissue of heart is-
(i) SAN or Pacemaker: Between the superior vena cava and the right atrium is a group of specialized cardiac muscles. It is called SAN. Its length is 10-15 mm, width is 3 mm and thickness is 1 mm. The SAN gives rise to cardiac contractile stimulation or action potentials. Hence the SA node is called primary motion generator. The SAN is called a pacemaker because it acts as a stimulus for excitatory waves and waves. It makes the heart beat 100 beats per minute. Even when the heart is separated from the body, it continues to contract. If its effectiveness decreases, fatigue and shortness of breath occur. This is called ischemia. Martin Flack (1907) discovered it.
(ii) AVN: AVN is located in the right atrioventricular septum of the heart. It is composed of cardiac muscle. This is called a protected pacemaker. Because if the SAN fails to generate an electrical signal, it generates it. It receives impulses from the SAN and transmits them to the bundle of His. It takes 0.03 seconds for the impulse from the SAN to reach the AVN. But the AVN has a delay of 0.09 seconds in the arrival of the stimulus. This is called AV Nodal Delay. It then takes another 0.04 seconds for the AVN to travel from the thalamus to the ventricle muscle. That is, the impulse from the AVN takes a total of (0.03+0.09+0.04) 0.16 seconds to reach the ventricle muscle.
(iii) Bundle of His: Behind the AVN lies the bundle of His. It is composed of multidimensional small fascicles. Of its two branches, one extends to the wall of the left ventricle and the other to the wall of the right ventricle. Swiss cardiologist Wilhelm His (1893) discovered it.
(vi) Purkinje fibers: The ends of the bundles of His become fine fibers and form a meshwork in the walls of the ventricles. These are called Purkinje fibers. It is made up of specialized cardiac muscle and conducts the heartbeat quickly and efficiently. Its speed is 40-60 times per minute. It causes contraction of the ventricle wall. John Evangelist Perkinzie (1839) discovered it.