Conduction of Heart Beat:
The heart is made of cardiac muscles. The walls of the ventricles are much thicker than the walls of the atria. A specialized cardiac musculature; called the nodal tissue is also distributed in the heart. The detail of these nodal tissues is as follows:
SA Node: The Sino-Atrial Node (SA Node) is present in the right upper corner of the right atrium.
AV Node: The atrio-ventricular node is present in the lower left corner of the right atrium; close to the atrio-ventricular septum.
Bundle of His: A bundle of nodal fibres; called atrio-ventricular bundle (AV Bundle); continues from the AVN and passes through the atrio-ventricular septa to emerge on the top of the inter-ventricular septum. Then, this bundle divides into a right and left bundle. These branches give rise to minute fibres throughout the ventricular musculature of the respective sides and are called Purkinje fibres. These fibres; along with right and left bundles; are known as Bundle of His.
Generation of Heart Beat
The nodal musculature is autoexcitable, which means it has the ability to generate action potentials without any external stimuli. The number of action potentials which could be generated per minute varies at different parts of the nodal system. The SA Node can generate the maximum number of action potentials, i.e. 70-75 per minute. The SA Node is responsible for initiating and maintaining the rhythmic contractile activity or beating of the heart. Due to this, the SA Node is called the pacemaker. The human heart normally beats 70-75 times per minute.
The sequential contraction and dilatation of different chambers of heart in a cyclical manner is called cardiac cycle. A cardiac cycle happens in following steps:
- All the four chambers of heart are in a relaxed state; to begin with. When the tricuspid and bicuspid valves are open, blood from the pulmonary veins and the vena cava flows into the left and the right ventricles respectively through the left and right atria. At this stage, the semilunar valves are closed.
- Now, the SA Node generates and action potential which stimulates both the atria to undergo a simultaneous contraction; the atrial systole. This increases the blood flow into the ventricles by about 30%.
- The action potential is conducted to ventricular side by the AV Node and AV bundle from where the Bundle of His transmits it through the entire ventricular musculature. This results in ventricular systole coinciding with the atrial diastole.
- Ventricular systole increases the ventricular pressure causing the closure of tricuspid and bicuspid valves due to the attempted backflow of blood into the auricles. With further increase in the ventricular pressure, the semilunar valves are forced open. Opening of the semilunar valves allows the blood from ventricles into the pulmonary artery (right side) and into the aorta (left side).
- Now the ventricles relax and the resultant fall in ventricular pressure causes the closure of semi-lunar valves. Closure of semi-lunar valves prevents the backflow of blood into the ventricles. With further reduction in ventricular pressure, the tricuspid and bicuspid valves are pushed open by the pressure in the atria exerted by the blood coming to atria.
- The whole cycle is repeated.
Stroke Volume: The volume of blood pumped by the heart in one cardiac cycle is called stroke volume. This is about 70 ml.
Cardiac Output: The volume of blood pumped by the heart in one minute is called the cardiac output. The average cardiac output is 5000 ml or 5 litre per minute.
Lub and Dub Sounds: Two prominent sounds are produced during each cardiac cycle. These sounds can be easily heard through a stethoscope. The first sound is called lub and is associated with the closure of the tricuspid and bicuspid valves. The second sound is called dub and is associated with the closure of the semi-lunar valves. These sounds are important for clinical diagnosis.
- The graphical representation of the electrical activity of the heart during a cardiac cycle is called ECG. For obtaining an ECG, a patient is usually connected to the machine with three electrical leads; one to each wrist and to the left ankle. These leads continuously monitor the activity of heart.
- Each peak in the ECG is identified with a letter from P to T (PQSRT) which corresponds to a specific electrical activity of the heart.
- The P-wave represents the electrical excitation or depolarization of the atria. Depolarisation of atria leads to atricular systole.
- The QRS complex represents the depolarization of the ventricles which initiates ventricular systole.
- The T-wave represents the return of the ventricles from excited to normal state (repolarisation). The end of T-wave marks the end of systole.
- Any deviation in the normal POQRST pattern of ECG indicates towards possible heart ailment.
The blood flow to and from the lungs is called pulmonary circulation. Deoxygenated blood goes to the lungs for oxygenation and then oxygenated blood comes back to the heart. Pulmonary circulation takes place through pulmonary artery and pulmonary vein.
The oxygenated blood is pumped to different parts of the body through the aorta and various arteries. Deoxygenated blood is collected by veins and finally reaches the right auricle through vena cava. Thus, the arterial and venous blood supply comprises the systemic circulation.
Hepatic Portal Circulation
The hepatic portal system is a unique vascular connection which exists between the digestive tract and liver. Blood from the intestine is first sent to the liver through hepatic portal system and then to the systemic circulation.
There is a special system of blood vessels which provide blood circulation exclusively to the cardiac muscles. This is called coronary circulation.
Regulation of Cardiac Activity
Normal activities of the heart are intrinsically regulated, i.e. auto regulated by the nodal tissues. Due to this, the heart is called myogenic.
There is a special neural centre in the medulla which can moderate the cardiac function through autonomic nervous system (ANS). The sympathetic nervous system can increase the heart rate, can increase the strength of ventricular contraction and thus the cardiac output. On the other hand, the parasympathetic nervous system decreases the heart rate, and thus the cardiac output. Adrenal medullary hormone can also increase the cardiac output.
Disorders of Circulatory System
High Blood Pressure (Hypertension)
The normal blood pressure in human beings is 120/80 mm Hg. If the blood pressure is more than this range, this condition is called hypertension. If the blood pressure comes in the range of 140/90 mm Hg on repeated checks, then the person is diagnosed with hypertension. The higher value, i.e. 120 mm Hg shows the systolic blood pressure, while the lower value shows the diastolic blood pressure. Hypertension finally progresses into heart diseases and also affects vital organs; like brain and kidney.
Blood Pressure: The resistance offered by the lumen of the artery to the flow of blood is called blood pressure.
Coronary Artery Disease (CAD)
Deposition of fat, cholesterol, calcium and fibrous tissues makes the lumen of the coronary artery narrower. This leads to coronary artery disease. This is also known as atherosclerosis. There is reduced blood supply to the cardiac muscles in this condition.
This is usually called angina pectoris; which means a pain in the chest region. Angina pectoris is a symptom of underlying heart disease. This happens because of mismatch in demand and supply of oxygen to the cardiac muscles.
This is commonly known as heart failure. When oxygen supply is obstructed to a part of the cardiac muscle for a prolonged period of a few minutes, it leads to cell death in that part. This is called myocardial ischemia and is accompanied by sudden stoppage of the beating of heart. The stoppage of the beating of heart is called heart attack or myocardial infarction.