HEART
The human heart is a muscular pump. While most of the hollow organs of the body do have muscular layers, the heart is almost entirely muscle. Unlike most of the other hollow organs, whose muscle layers are composed of smooth muscle, the heart is composed of cardiac muscle. All muscle types function by contraction, which causes the muscle cells to shorten. Skeletal muscle cells, which make up most of the mass of the body, are voluntary and contract when the brain sends signals telling them to react. The smooth muscle surrounding the other hollow organs is involuntary, meaning it does not need to be told to contract.
Cardiac muscle is also involuntary. So functionally, cardiac muscle and smooth muscle are similar. Anatomically though, cardiac muscle more closely resembles skeletal muscle. Both skeletal muscle and cardiac muscle are striated. Under medium to high power magnification through the microscope, you can see small stripes running crosswise in both types. Smooth muscle is nonstriated. Cardiac muscle could almost be said to be a hybrid between skeletal and smooth muscle.
Cardiac muscle does have several unique features. Present in cardiac muscle are intercalated discs, which are connections between two adjacent cardiac cells. Intercalated discs help multiple cardiac muscle cells contract rapidly as a unit. This is important for the heart to function properly. Cardiac muscle also can contract more powerfully when it is stretched slightly. When the ventricles are filled, they are stretched beyond their normal resting capacity. The result is a more powerful contraction, ensuring that the maximum amount of blood can be forced from the ventricles and into the arteries with each stroke. This is most noticeable during exercise, when the heart beats rapidly.
Heart Chambers
There are four chambers in the heart – two atria and two ventricles. The atria (one is called an atrium) are responsible for receiving blood from the veins leading to the heart. When they contract, they pump blood into the ventricles. However, the atria do not really have to work that hard. Most of the blood in the atria will flow into the ventricles even if the atria fail to contract. It is the ventricles that are the real workhorses, for they must force the blood away from the heart with sufficient power to push the blood all the way back to the heart (this is where the property of contracting with more force when stretched comes into play). The muscle in the walls of the ventricles is much thicker than the atria. The walls of the heart are really several spirally wrapped muscle layers. This spiral arrangement results in the blood being wrung from the ventricles during contraction. Between the atria and the ventricles are valves, overlapping layers of tissue that allow blood to flow only in one direction. Valves are also present between the ventricles and the vessels leading from it.
Cardiac cycle
The cardiac cycle is a term referring to all or any of the events related to the flow or blood pressure that occurs from the beginning of one heartbeat to the beginning of the next. The frequency of the cardiac cycle is described by the heart rate. Each beat of the heart involves five major stages. The first two stages, often considered together as the “ventricular filling” stage, involve the movement of blood from atria into ventricles. The next three stages involve the movement of blood from the ventricles to the pulmonary artery (in the case of the right ventricle) and the aorta (in the case of the left ventricle).
Throughout the cardiac cycle, blood pressure increases and decreases. The cardiac cycle is coordinated by a series of electrical impulses that are produced by specialized heart cells found within the sinoatrial node and the atrioventricular node. The cardiac muscle is composed of myocytes which initiate their own contraction without help of external nerves (with the exception of modifying the heart rate due to metabolic demand). Under normal circumstances, each cycle takes approximately one second.
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AV valves* |
Semilunar valves† |
Status of ventricles and atria |
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1. early diastole |
open |
closed |
• whole heart is relaxed • ventricles are expanding and filling |
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2. atrial systole |
open |
closed |
• atria contract and pump blood • additional 10–40% filling of ventricles |
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3. isovolumic ventricular contraction |
closed |
closed |
• ventricular myocytes begin to contract • ventricle volume unchanged |
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4. ventricular ejection |
closed |
open |
• ventricles fully contract • pump blood to rest of body |
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5. Isovolumic ventricular relaxation |
closed |
closed |
• ventricles relax • ventricle volume unchanged |
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* AV (atrioventricular) valves: 1) mitral valve – between the left atrium and the left ventricle 2) tricuspid valve – between the right atrium and the right ventricle † Semilunar valves: 1) aortic valve – between the left ventricle and the aorta 2) pulmonic valve – between the right ventricle and the pulmonary artery |
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Atrial systole
Atrial systole is the contraction of the heart muscle (myocardia) of the left and right atria. Normally, both atria contract at the same time. The term systole is synonymous with contraction (movement or shortening) of a muscle. Electrical systole is the electrical activity that stimulates the myocardium of the chambers of the heart to make them contract. This is soon followed by Mechanical systole, which is the mechanical contraction of the heart.
As the atria contract, the blood pressure in each atrium increases, forcing additional blood into the ventricles. The additional flow of blood is called atrial kick.
80% of the blood flows passively down to the ventricles, so the atria do not have to contract a great amount.[3]
Atrial kick is absent if there is loss of normal electrical conduction in the heart, such as during atrial fibrillation, atrial flutter, and complete heart block. Atrial kick is also different in character depending on the condition of the heart, such as stiff heart, which is found in patients with diastolic dysfunction.
Detection of atrial systole
Electrical systole of the atria begins with the onset of the P wave on the ECG. The wave of bipolarization (or depolarization) that stimulates both atria to contract at the same time is due to sinoatrial node which is located on the upper wall of the right atrium.
Ventricular systole
Ventricular systole is the contraction of the muscles (myocardia) of the left and right ventricles.
At the later part of the ejection phase, although the ventricular pressure falls below the aortic pressure, the aortic valve remains open because of the inertial energy of the ejected blood.[4]
The graph of aortic pressure throughout the cardiac cycle displays a small dip (the “incisure” or “dicrotic notch”) which coincides with the aortic valve closure. The dip in the graph is immediately followed by a brief rise (the “dicrotic wave”) then gradual decline. Just as the ventricles enter into diastole, the brief reversal of flow from the aorta back into the left ventricle causes the aortic valves to shut. This results in the slight increase in aortic pressure caused by the elastic recoil of the semilunar valves and aorta.[5][6][7]
The total volume of blood remaining in the ventricle just at the end of the ventricular contraction is called end-systolic volume (ESV).
Detection of ventricular systole
The closing of the mitral and tricuspid valves (known together as the atrioventricular valves) at the beginning of ventricular systole cause the first part of the “lubb-dubb” sound made by the heart as it beats. Formally, this sound is known as the First Heart Tone, or S1. This first heart tone is created by the closure of mitral and tricuspid valve and is actually a two component sound, M1, T1.
The second part of the “lub-dubb” (the Second Heart Tone, or S2), is caused by the closure of the aortic and pulmonary valves at the end of ventricular systole. As the left ventricle empties, its pressure falls below the pressure in the aorta, and the aortic valve closes. Similarly, as the pressure in the right ventricle falls below the pressure in the pulmonary artery, the pulmonary valve closes. The second heart sound is also two components, A2 and P2. The aortic valve closes earlier than the pulmonary valve and they are audibly separated from each other in the second heart sound. This “splitting” of S2 is only audible during inhalation. However, some cardiac conduction abnormalities such as left bundle branch block (LBBB) allow the P2 sound to be heard before the A2 sound during expiration. With LBBB, inhalation brings A2 and P2 closer together where they cannot be audibly distinguished.
Electrocardiogram
In an electrocardiogram, electrical systole of the ventricles begins at the beginning of the QRS complex.
Diastole
Cardiac Diastole is the period of time when the heart relaxes after contraction in preparation for refilling with circulating blood. Ventricular diastole is when the ventricles are relaxing, while atrial diastole is when the atria are relaxing. Together they are known as complete cardiac diastole.
During ventricular diastole, the pressure in the (left and right) ventricles drops from the peak that it reaches in systole. When the pressure in the left ventricle drops to below the pressure in the left atrium, the mitral valve opens, and the left ventricle fills with blood that was accumulating in the left atrium. The isovolumic relaxation time (IVRT) is the interval from the aortic component of the second heart sound, that is, closure of the aortic valve, to onset of filling by opening of the mitral valve. Likewise, when the pressure in the right ventricle drops below that in the right atrium, the tricuspid valve opens, and the right ventricle fills with blood that was accumulating in the right atrium. During diastole the pressure within the left ventricle is lower than that in aorta, allowing blood to circulate in the heart itself via the coronary arteries.
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