Cardiac Muscle and the Heart as a Pump
The heart works as a muscular pump that keeps blood moving continuously through the lungs and the systemic circulation. The right side of the heart pumps blood to the lungs, while the left side pumps blood to the rest of the body. Although cardiac muscle is striated like skeletal muscle, its electrical activity and contraction are quite different. Understanding these differences is important for learning the cardiac cycle, heart sounds, cardiac output and regulation of cardiac function.
1. Physiological Anatomy of Cardiac Muscle
Cardiac muscle is made up of individual muscle cells that are connected closely to one another. The cells are shorter and more branched than skeletal muscle fibres. They are joined at specialised areas called intercalated discs.
Intercalated discs contain gap junctions through which ions can pass from one cardiac cell to another. Because of this arrangement, an electrical impulse can spread from one cell to the next and the myocardium behaves as a functional syncytium.
The heart can be considered as two functional syncytia: the atrial syncytium and the ventricular syncytium. They are separated by the fibrous tissue surrounding the atrioventricular valves.
Normally, the electrical impulse passes from the atria to the ventricles through the atrioventricular bundle (bundle of His). This arrangement, together with the AV nodal delay, allows the atria to contract before ventricular contraction begins.
2. Action Potential in Cardiac Muscle
The action potential of ventricular muscle differs considerably from that of skeletal muscle. After the rapid initial depolarisation, the membrane remains depolarised for a relatively long period before repolarisation occurs. This produces the characteristic plateau phase.
The resting membrane potential of ventricular muscle is approximately −85 to −90 mV. During the action potential it rises to around +20 mV.
Why does the cardiac action potential have a plateau?
Two important changes are responsible for the prolonged action potential.
- Slow calcium-sodium channels remain open. These channels open more slowly than the fast sodium channels and remain active for a much longer time. Calcium enters the cardiac muscle cell during this period and contributes to both the plateau and contraction.
- Potassium permeability decreases temporarily. This reduces potassium leaving the cell and delays repolarisation.
When the slow calcium channels close and potassium permeability increases, potassium moves out of the cell and rapid repolarisation occurs.
| Parameter | Approximate value |
|---|---|
| Conduction velocity in atrial and ventricular muscle | 0.3–0.5 m/sec |
| Conduction velocity in Purkinje fibres | Up to about 4 m/sec |
| Ventricular absolute refractory period | About 0.25–0.30 sec |
| Additional relative refractory period | About 0.05 sec |
| Atrial refractory period | About 0.15 sec |
The long refractory period is important because it prevents sustained tetanic contraction of cardiac muscle. The heart therefore has time to relax and refill between contractions.
3. Excitation–Contraction Coupling
The basic mechanism of cardiac muscle contraction is similar to that of skeletal muscle, but calcium entering from the extracellular fluid plays a much more important role.
When the action potential reaches the T tubules, calcium channels open and allow calcium to enter the cardiac muscle cell.
- The incoming calcium interacts with calcium-release channels on the sarcoplasmic reticulum.
- This causes additional calcium to be released from the sarcoplasmic reticulum. This process is known as calcium-induced calcium release.
- The rise in intracellular calcium allows interaction between actin and myosin and produces contraction.
- At the end of the action potential, calcium is removed from the cytoplasm by uptake into the sarcoplasmic reticulum and by extrusion across the cell membrane.
Cardiac muscle has a less extensive sarcoplasmic reticulum than skeletal muscle. Therefore, extracellular calcium is particularly important for normal cardiac contraction.
Relaxation occurs when intracellular calcium concentration falls. Calcium is taken back into the sarcoplasmic reticulum mainly by the Ca²⁺-ATPase, while the Na⁺–Ca²⁺ exchanger also contributes to calcium removal.
The contraction lasts approximately 0.2 seconds in atrial muscle and about 0.3 seconds in ventricular muscle.
4. Cardiac Cycle
The cardiac cycle refers to all the events that occur during one complete heartbeat. It includes contraction and relaxation of both the atria and ventricles.
The cycle begins with electrical activity generated normally by the sinoatrial (SA) node. The impulse spreads through the atrial myocardium and then reaches the atrioventricular node. After the AV nodal delay, the impulse travels through the bundle of His, bundle branches and Purkinje fibres to activate the ventricles.
The two main phases of the cardiac cycle are:
- Diastole: relaxation and filling of the heart.
- Systole: contraction and ejection of blood.
At a heart rate of 72 beats/minute, one cardiac cycle lasts approximately 0.83 seconds. When heart rate increases, diastole becomes shorter. This can reduce ventricular filling and also shorten the time available for coronary blood flow, particularly at very high heart rates.
ECG and the cardiac cycle
The electrical events recorded on an ECG are closely related to the mechanical events of the heart.
- P wave: represents atrial depolarisation and is followed by atrial contraction.
- QRS complex: represents ventricular depolarisation and is followed by ventricular contraction.
- T wave: represents ventricular repolarisation and occurs toward the end of ventricular systole.
Atrial contraction and atrial pressure waves
Most ventricular filling occurs passively before the atria contract. Atrial contraction then provides an additional contribution to ventricular filling, often called the atrial kick.
The atrial pressure tracing contains three important waves.
| Wave | Main cause |
|---|---|
| a wave | Atrial contraction |
| c wave | Bulging of the AV valves toward the atria during early ventricular contraction |
| v wave | Venous filling of the atria while the AV valves are closed |
5. Ventricular Pump Function
The ventricles fill during diastole and eject blood during systole. Several events occur in sequence during each ventricular cycle.
- Rapid ventricular filling: After the AV valves open, blood flows rapidly from the atria into the ventricles.
- Diastasis: Ventricular filling slows during the middle part of diastole.
- Atrial systole: Atrial contraction provides the final contribution to ventricular filling.
- Isovolumic contraction: Ventricular pressure rises rapidly after ventricular contraction begins. The AV valves close, but the semilunar valves have not yet opened. Ventricular volume therefore remains unchanged.
- Ventricular ejection: When ventricular pressure becomes greater than the pressure in the aorta or pulmonary artery, the semilunar valves open and blood is ejected.
- Isovolumic relaxation: After ventricular ejection, the semilunar valves close. Ventricular pressure falls rapidly while all valves are temporarily closed.
| Parameter | Approximate value |
|---|---|
| End-diastolic volume (EDV) | 110–120 mL |
| End-systolic volume (ESV) | 40–50 mL |
| Stroke volume | About 70 mL |
| Ejection fraction | About 60% |
Stroke volume is calculated as:
Stroke volume = End-diastolic volume − End-systolic volume
Ejection fraction is calculated as:
Ejection fraction = Stroke volume ÷ End-diastolic volume × 100
6. Heart Valves and One-Way Blood Flow
The four cardiac valves ensure that blood moves in one direction through the heart.
Atrioventricular valves
The tricuspid valve lies between the right atrium and right ventricle, while the mitral valve lies between the left atrium and left ventricle.
During ventricular systole, the AV valves close and prevent blood from returning to the atria.
The valve leaflets are connected to papillary muscles through chordae tendineae. Papillary muscle contraction helps prevent the valve leaflets from being pushed back into the atria during ventricular systole. The papillary muscles do not actively close the valves.
Semilunar valves
The aortic valve lies between the left ventricle and aorta, while the pulmonary valve lies between the right ventricle and pulmonary artery.
These valves open during ventricular ejection and close during diastole to prevent blood from flowing back into the ventricles.
Unlike the AV valves, the semilunar valves do not have chordae tendineae or papillary muscles.
7. Aortic Pressure and Heart Sounds
During left ventricular systole, blood is ejected into the aorta and aortic pressure rises. In a healthy young adult, the pressure is approximately 120 mm Hg during systole and falls to around 80 mm Hg during diastole.
When the aortic valve closes, a small notch called the incisura or dicrotic notch can be seen in the aortic pressure curve.
The sounds heard during the cardiac cycle are mainly produced by vibration of the valves, blood and surrounding cardiac structures.
| Sound | Event |
|---|---|
| S1 | Closure of the mitral and tricuspid valves at the beginning of ventricular systole |
| S2 | Closure of the aortic and pulmonary valves at the end of ventricular systole |
8. Work Done by the Heart
The ventricles perform mechanical work to move blood from the venous circulation into the arterial circulation. The left ventricle performs more work than the right ventricle because it has to eject blood against the much higher systemic arterial pressure.
Two important factors affecting ventricular work are preload and afterload.
- Preload refers to the degree of ventricular filling and myocardial stretch before contraction. End-diastolic volume is commonly used as a practical indicator of preload.
- Afterload is the load against which the ventricle has to eject blood. For the left ventricle, systemic arterial pressure is an important component of afterload.
Most of the energy used by the myocardium is generated through oxidative metabolism. Fatty acids, glucose and lactate can all contribute to myocardial energy production.
9. Frank-Starling Mechanism
One of the important intrinsic mechanisms controlling cardiac pumping is the Frank-Starling mechanism.
In simple terms, when venous return increases, the ventricles fill more during diastole. The cardiac muscle fibres are stretched to a greater extent, and the subsequent contraction becomes stronger. As a result, more blood is ejected.
This mechanism helps the heart adjust its output to the amount of blood returning to it.
10. Autonomic Regulation of the Heart
The autonomic nervous system modifies the activity of the heart according to the body's requirements.
| System | Heart rate | Contractility | Main effect |
|---|---|---|---|
| Sympathetic | Increases | Increases | Prepares the heart for increased cardiac work |
| Parasympathetic | Decreases | Small effect on ventricles | Mainly slows SA node activity and AV conduction |
Sympathetic stimulation increases heart rate and myocardial contractility. Parasympathetic stimulation, mainly through the vagus nerves, slows the heart rate and can slow conduction through the AV node.
11. Effects of Electrolytes and Temperature
Electrolyte abnormalities can have significant effects on cardiac excitability and conduction.
- Potassium: significant changes in extracellular potassium concentration can alter the resting membrane potential, conduction and cardiac rhythm. Severe hyperkalaemia can produce dangerous arrhythmias.
- Calcium: extracellular calcium concentration influences cardiac contraction. Increased calcium availability generally increases contractility, while reduced calcium availability can decrease contractile force.
- Temperature: increased body temperature generally increases heart rate, whereas marked cooling slows cardiac activity.
12. Cardiac Output
Cardiac output is the volume of blood pumped by one ventricle per minute.
It is calculated using:
Cardiac output = Heart rate × Stroke volume
For example, with a heart rate of 70 beats/minute and a stroke volume of 70 mL, cardiac output is approximately 4.9 L/min.
Cardiac output changes according to the needs of the body. Exercise, sympathetic stimulation and increased venous return can increase cardiac output, while severe reduction in cardiac pumping ability can decrease it.
Important MBBS Points
- The SA node normally acts as the pacemaker of the heart.
- The AV node introduces a delay before ventricular activation.
- The ventricular action potential has a prolonged plateau phase.
- Calcium entry from the extracellular fluid is important for cardiac contraction.
- The long refractory period prevents tetanic contraction of cardiac muscle.
- Most ventricular filling occurs during diastole.
- The atrial kick contributes additional ventricular filling.
- S1 is mainly associated with closure of the AV valves.
- S2 is associated with closure of the semilunar valves.
- Frank-Starling mechanism is an intrinsic mechanism of cardiac regulation.
- Sympathetic stimulation increases heart rate and contractility.
- Parasympathetic stimulation mainly decreases heart rate and AV conduction.
- Cardiac output = heart rate × stroke volume.
Quick Review
Cardiac muscle is a specialised striated muscle that works as a functional syncytium. Its prolonged action potential and plateau phase are important features that distinguish it from skeletal muscle. The cardiac cycle consists of ventricular filling, contraction, ejection and relaxation. The four valves maintain one-way blood flow, while the Frank-Starling mechanism and autonomic nervous system help regulate cardiac performance.
For MBBS examinations, focus particularly on the cardiac action potential, excitation–contraction coupling, cardiac cycle, pressure and volume changes, heart sounds, Frank-Starling mechanism and cardiac output.
Key Points for Revision
- Cardiac muscle cells are connected by intercalated discs and gap junctions.
- The myocardium behaves as a functional syncytium.
- The ventricular action potential has a prolonged plateau caused mainly by calcium entry and reduced potassium permeability.
- Extracellular calcium is important for cardiac muscle contraction.
- The cardiac cycle includes systole and diastole.
- Ventricular filling is followed by isovolumic contraction, ejection and isovolumic relaxation.
- Stroke volume is the difference between EDV and ESV.
- Ejection fraction is the proportion of EDV ejected during systole.
- Mitral and tricuspid valves prevent systolic backflow into the atria.
- Aortic and pulmonary valves prevent diastolic backflow into the ventricles.
- S1 is associated with AV valve closure and S2 with semilunar valve closure.
- Frank-Starling mechanism links ventricular filling with the strength of contraction.
- Sympathetic activity increases cardiac rate and contractility, whereas parasympathetic activity mainly slows the heart.
- Cardiac output depends on heart rate and stroke volume.
Medical Disclaimer: The information provided in this article is strictly for educational, study, and exam-preparation purposes. It does not constitute professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider for clinical decisions.