The heart is the main part of the circulatory system in vertebrates. All vertebrates have chambered heart. The circulatory system consists of a heart and blood vessels i.e., arteries, veins and capillaries. The walls of the heart and the large veins entering the heart are formed of cardiac muscle which contracts automatically and rhythmically. It is not under the voluntary control of the body. The cardiac muscle gets an extremely rich blood supply and receives the nerve supply from both divisions of the autonomic nervous system. The cardiac muscle is responsible for contraction and expansion of the heart.
Table of Contents
1.1 STRUCTURE OF HEART
The human heart is four chambered consisting of two auricles and two ventricles. The venous blood i.e., deoxygenated blood collected from all parts of body enters into the right auricle through vena cava. The right auricle is connected with the right ventricle through the tricuspid valve which allows blood to flow from auricle to ventricle but not from ventricle to auricle. The blood leaves the right ventricles to the lungs via pulmonary artery for the exchange of CO2 and O2 in blood. The oxygenated blood returns from the lungs through the pulmonary veins to the left auricle and then to the left ventricle through the bicuspid valve. The aorta carries the oxygenated blood from the left ventricle to all parts of the body through the branches of the vessels.
Therefore, the left chambers of the heart carrying oxygenated blood are not connected with the right chambers carrying deoxygenated blood.
The walls of the two atria- left and right are thin. The left atrium is smaller than the right atrium. The upper part of each atrium has an appendage, called auricle which increases the surface area of atrium. The right atrium is roughly quadrangular chamber. The superior vena cava, inferior vena cava and coronary sinus open into the right atrium. The left atrium receives four openings of pulmonary veins. Each atrium opens into the ventricle of its own side through an atrio-ventricular orifice. The two atria are separated from each other by a partition- inter-atrial septum. An oval depression, fossa ovalis is present on the septum which represents the remnant of foramen oval, an opening in the inter-atrial septum of the fetal heart.
The walls of the two ventricles are thick layered. The left ventricle is longer and narrower than the right ventricle and its wall are about three times thicker than the right ventricle. The walls of ventricles are thicker than the auricles because auricles only act as reservoirs for blood entering the heart, but ventricles pump out blood with force from heart to pulmonary artery and to aorta. Again, the left ventricle is thicker than the right ventricle as left one has to take the oxygenated blood to all parts of the body far away from the heart. The two ventricles are separated by a thick, curved partition called the inter-ventricular septum.
A pulmonary trunk arises from the right ventricle which bifurcates to form right and left pulmonary arteries. The left ventricle gives rise to an aorta which is divided into the ascending aorta, arch of aorta and descending aorta. Again, the ascending aorta is divided into the right and left coronary arteries; arch of aorta or aortic arch into brachiocephalic artery, left common carotid artery and left subclavian artery; descending aorta is divided into thoracic part and abdominal part.
The wall of the heart is divided into three layers- endocardium, myocardium and epicardium. The endocardium consists of the endothelium which is continuous with that of the blood vessels and of subendothelial connective tissue. The myocardium is the muscle layer. The epicardium consists of the pericardial mesothelium together with underlying connective tissue and with the parietal pericardium. It bounds the pericardial cavity forming a closed sac containing the pericardial fluid.
Externally, a transverse groove is present between the atria and the ventricle, called the coronary sulcus. There are also two grooves on the ventricle called the anterior interventricular sulcus and posterior interventricular sulcus.
There are a number of valves inside the heart which regulate the direction of flow of blood. The valve between the right atrium and the right ventricle i.e. the right AV valve is a tricuspid valve and their three cusps or flops are attached to the wall of the ventricle through tendon-like cords, called chordae tendinae. The valve between left atrium and ventricle i.e. left AV valve is also called bicuspid or mitral valve. Both valves are attached to chordae tendinae.
The contraction of the extension of ventricle walls i.e., the papillary muscles bring the tightening of chordae tendinae. This prevent the valves during the contraction of ventricles from turning inside out or from being forced upwards, there are also two semilunar valves- pulmonary semilunar valve and aortic semilunar valve. Pulmonary semilunar valve guards the opening of the right ventricle from where the pulmonary trunk starts. Aortic semilunar valve guards the opening of left ventricle from where the ascending aorta begins.
1.2 ORIGIN AND CONDUCTION OF CARDIAC IMPULSE
The heart of all vertebrates and Mollusca are myogenic type as the impulse of heart beat is originated and conducted by the cardiac muscle. The specialized impulse conducting cells in the heart form the nodule tissue which is located in the Sinoatrial node (SA node) with a rich capillary supply. It is found at the point where superior vena cava empties into the right atrium and is also known as pacemaker as the heart beat originates from this region.
The impulse is spread all along the atria and is picked up by another node, atrioventricular node (AV node) which is situated in the right atrium near the ventral part of the inter-atrial septum. A mass of specialized fibers, the bundle of His, originates from the A-V node and after dividing into two branches spread to each ventricle. Again, in ventricles they are subdivided into a network of fine fibers called the Purkinje fibers. So, the cardiac impulse is propagated from the A-V node along the bundle of His and Purkinje fibers to the ventricles.
The heart actually four separate pumps- the atria, two primer pumps and the ventricles, two power pumps. Each cardiac cycle is initiated by spontaneous generation of an action potential in the S-A node i.e. the pace maker of the heart and travels through both atria and the ventricles. The atria contract pumping blood into the ventricles prior to the very strong ventricular contraction. They simply function as primer pumps as only 20-30 percent of blood pumps into the ventricles and approximately 70 percent of blood spontaneously flows directly through atria into the ventricles. The ventricles then provide the major source of power for moving blood through the vascular system and act as power pumps.
Another type of heart i.e. neurogenic heart is present in some annelids and most arthropods. In this type of heart, the heart beat is originated or initiated by a nerve impulse coming from a nerve ganglion or mass of nerve cells situated near the heart.
1.3 ELECTRICAL EVENTS IN THE ORIGIN OF HEART BEAT
At resting state, when impulse is not stimulated, the inside of the pace maker cell is negatively charged in comparison to the outside which is positively charged and is called polarized state. Actually, the sodium, potassium and chloride ions play an important role in generating an electrical impulse in the cells of S-A node Le the pacemaker. The potential difference at the polarized state is 55 mv and the condition is due to the predominance of Na ions outside the cell which out number the K ions inside the cell.
The plasma membrane of the pacemaker cells is partly permeable to Na ions and continuously rush in the cells with resultant decrease in the potential difference between inside and outside of the cells. When it is reduced to about 40 to 45 mv, the Na ions rush in quickly and some of the K+ ions exit outside. Now the membrane of pacemaker cell inside becomes positive with respect to the outside which is reversal of electrical charges. When a certain level of depolarization is reached by the pacemaker potential, a spike is generated.
The depolarization causes the action potential and the electrical impulse generated spreads to the adjacent cells by a kind of relay system so that the intensity of impulse does not decrease with the distance travelled. As soon as the electrical impulse reaches the myocardial cells, it triggers the breakdown of ATP for release of energy required for contraction first auricles, then ventricles. The impulse is delayed for a moment before exciting A-V node so that the auricular contraction is complete prior to the beginning of the ventricular contraction.
Again, the depolarized cells are repolarized by the becoming negative inside and positive outside the plasma membrane. It is happened by pumping out the Na+ ions with the expenditure of the energy of ATP. The duration of the repolarization phase is long in cardiac action potential. The cardiac muscle when unexcitable have a long refractory period and so cannot be easily re-stimulated during the systole i.e. in contraction phase. This ensures that all cardiac cells contract at the same time and also that the full force of contraction is available for pumping blood from the ventricles.
All or None Law
The heart follows all or none law ie. if the heart muscle beats at all, it does so to its maximum effect. In the heart, the individual muscle fibers are not separated from each other by the sarcolemma, but the cross-striated fibers are brought into intimate contact with each other by anastomosis. It permits the impulse to spread from one fiber to another throughout the atrium or ventricle causing them to respond as a single unit.
Starling's Law of the Heart
The Starling's law of heart was discovered by Starling in 1918. According to this law, within the physiological limits, the more the ventricles are filled with the incoming blood the greater is the force of their contraction at systole. It operates in opposite direction also, for example, in extensive hemorrhage, the systolic and diastolic pressures are very low. The amount of blood delivered to the ventricle is reduced and the cardiac fibers are slightly stretched which results in a feeble heart beat and in much lower than normal systolic ventricular pressure. Of course, the pressure is sufficient to overcome reduced diastolic pressure in the aorta.
Control of Heart Beat
The heart beat is regulated by nervous, hormones and some other factors. The action of the pace maker which is controlled by the heart rate center located in the medulla oblongata. There are two pairs of nerves that lead from the heart rate center to the heart. The impulses through one pair formed by a sensory nerve and a motor nerves accelerates the heart beat, and impulse through another pair also formed by a sensory and motor nerves slow the heartbeat.
The inhibitory nerve fibers pass from the heart rate center into the large vagus nerves. The accelerator nerves and inhibitor nerves terminate in the right auricle above the pacemaker and when the pacemaker is stimulated, a wave of contraction spreads out to both auricles and ventricles. The vagus nerve fibers release acetylcholine and accelerator nerves release sympathin.
The hormones adrenaline (epinephrine) and noradrenaline (norepinephrine) also play role in controlling heartbeat. Noradrenaline accelerates the heart beat under normal conditions while adrenaline at the time of emergency and both directly influence the S-A node. The hormone thyroxine, secreted by thyroid gland also indirectly increases heart beat as it increases oxidative metabolism of the body cells which requires more oxygen.
Heart beat is also affected by different factors such as pH, temperature, CO, concentration in the blood, mineral ions in the blood etc. Actually, heart beat rate is directly or indirectly a net result of many simultaneous nervous and non-nervous effects including sex, age, fever and metabolic rate etc.
Electrocardiogram
A heart beat can be described in terms of electrocardiogram (ECG) obtained by electrocardiograph. ECG consists of a series of slow waves, including upward (negative) deflections called P, R and downward positive deflection Q and S.
The P wave represents the first upward deflection due to contraction of auricle. The PQ interval is the delay at the auriculo-ventricular junction and the QRS complex shows deflections due to conduction in the ventricles. The T wave shows the repolarization of the ventricular surface. Ultimately, the electrocardiogram reflects the activity of different parts of the heart and aids in diagnosis of different heart illness.
1.4 CARDIAC CYCLE
The cardiac cycle consists of one cycle of contraction and relaxation of the cardiac muscle. It occurs from the beginning of one heart beat to the beginning of the next one. There are two phases of the cardiac cycle- systole and diastole. During a heartbeat when the atria and ventricles contract that phase is known as systole (contraction phase). The phase of the cardiac cycle in which both the atria and the ventricles relax is known as diastole.
So, a heart beat consists of a systole (contraction phase) and diastole (relaxation phase) of both the atria and of both the ventricles. The pumping during cardiac cycle involves mainly four types such as:
- Atrial systole
- Ventricular filling
- Ventricular systole
- Ventricular diastole
Atrial Systole
Normally, blood spontaneously flow into right atrium through superior vena cava, interior vena cava and coronary sinus. Due to contraction of the atria its blood is forced into the ventricles as the bicuspid and tricuspid values are open. Atrial systole (contraction) lasts for about 0.15 second.
Ventricular filling
When the ventricular pressures fall to their diastole values, the high pressure in the atria push the A-V values open and the blood flows in the ventricles rapidly filling the ventricles.
Ventricular systole
During ventricular systole large amount of blood accumulate in the atria as A-V values are closed. Actually, immediately after ventricular systole begins, the ventricular pressure rises causing A-V values to close. Then the ventricles take extra 0.02 to 0.03 second to make sufficient pressure to push the semilunar valve, i.e. aortic and pulmonary valve open against the pressures in the aorta and pulmonary artery.
So, there is no emptying through the ventricles contract during this period. When the left ventricular pressure rises slightly above 80 mm Hg and right ventricular pressure slightly above 8 mm Hg. then the ventricular pressure pushes the semilunar valve open and blood pours out of the ventricles immediately.
Ventricular diastole
The ventricular diastole or relaxation starts suddenly at the end of systole. It allows the intraventricular pressure to fall rapidly and then the A-V valve open to begin a new cycle.
1.5 Blood Pressure
The pressure exerted on wall of blood vessels by blood is called blood pressure. The normal blood pressure is 120/80 mm Hg.
Hypertension
It is a condition where blood pressure is 160/90 or more. BP is measured by an instrument called sphygmomanometer which is invented by Stephen Hales. Drug abuse, smoking, over eating etc. are the reasons for hypertension.
Disorders of Heart
- CAD (Coronary artery diseases): This is due to accumulation of fats and calcium ions deposits in the wall of coronary artery. The lumen of artery is greatly reduced so that blood supply to the muscle of heart is reduced. It can be rectified by placing a stent or conducting By-pass surgery.
- Angina or Angina Pectoris: It is characterized by severe pain in chest and pain in left shoulder and arm.
- Tachycardia: Rapid beating of heart is called Tachycardia (almost 83 max.).
- Bradycardia: The condition in which heart beat is less than 70.
- Infarction: Partial death of cells of heart.
- Cardiac arrest : Sudden failure of heart from working is called cardiac arrest.
- Heart failure: The condition in which insufficient systole and diastole are taking place but the heart is working.
