Cardiovascular System: Heart Structure, Cardiac Cycle, and ECG

Heart Structure

The cardiovascular system is a vital network responsible for transporting oxygen, nutrients, hormones, and other essential substances throughout the body. At its core lies the heart, a remarkable muscular organ that acts as a pump, driving the circulation of blood. Understanding the intricate structure of the heart is fundamental to comprehending its function.

The human heart is a four-chambered organ, roughly the size of a clenched fist, located in the thoracic cavity, slightly to the left of the sternum, between the lungs. It is enclosed within a protective sac called the pericardium. The pericardium consists of two layers: the outer fibrous pericardium and the inner serous pericardium. The serous pericardium, in turn, has two layers – the parietal layer and the visceral layer (also known as the epicardium) – with a pericardial cavity in between, containing pericardial fluid that reduces friction during heartbeats.

Layers of the Heart Wall

The wall of the heart is composed of three distinct layers:

• Epicardium: This is the outermost layer, which is also the visceral layer of the serous pericardium. It provides a protective outer surface.
• Myocardium: The middle and thickest layer, the myocardium, is composed of specialized cardiac muscle tissue. This muscle is responsible for the powerful contractions that pump blood. The thickness of the myocardium varies among the chambers, being thickest in the left ventricle due to the higher pressure needed to pump blood to the entire body.
• Endocardium: The innermost layer, the endocardium, is a thin, smooth membrane that lines the heart chambers and covers the heart valves. It ensures smooth blood flow, preventing excessive friction.

Chambers of the Heart

The heart is divided into four chambers: two upper atria (singular: atrium) and two lower ventricles. These chambers are separated by muscular walls called septa.

• Atria (Right and Left): The atria are the receiving chambers of the heart.
  • The right atrium receives deoxygenated blood from the body via the superior vena cava (from the upper body) and the inferior vena cava (from the lower body), and from the coronary sinus (blood from the heart muscle itself).
  • The left atrium receives oxygenated blood from the lungs via the pulmonary veins.
  The atria are separated by the interatrial septum.
• Ventricles (Right and Left): The ventricles are the pumping chambers of the heart.
  • The right ventricle pumps deoxygenated blood to the lungs through the pulmonary artery.
  • The left ventricle pumps oxygenated blood to the rest of the body through the aorta.
  The ventricles are separated by the thicker interventricular septum.

Heart Valves

The heart contains four valves that ensure unidirectional blood flow, preventing backflow. These valves open and close passively in response to pressure changes within the heart chambers.

• Atrioventricular (AV) Valves: Located between the atria and ventricles.
  • Tricuspid Valve: Situated between the right atrium and the right ventricle. It has three cusps (leaflets).
  • Bicuspid Valve (Mitral Valve): Situated between the left atrium and the left ventricle. It has two cusps.
  These valves are anchored by chordae tendineae, fibrous cords that attach to papillary muscles projecting from the ventricular walls. During ventricular contraction, these structures prevent the AV valves from prolapsing (being pushed backward into the atria).
• Semilunar (SL) Valves: Located between the ventricles and the major arteries leaving the heart.
  • Pulmonary Valve: Situated between the right ventricle and the pulmonary artery. It has three cusps.
  • Aortic Valve: Situated between the left ventricle and the aorta. It also has three cusps.
  These valves open when ventricular pressure exceeds arterial pressure, allowing blood to be ejected, and close when the pressure reverses, preventing backflow into the ventricles.

Blood Flow Through the Heart

Blood circulates through the heart in a specific path:

1. Deoxygenated blood from the body enters the right atrium.
2. From the right atrium, blood passes through the tricuspid valve into the right ventricle.
3. The right ventricle contracts, pumping blood through the pulmonary valve into the pulmonary artery, which carries it to the lungs for oxygenation.
4. Oxygenated blood returns from the lungs via the pulmonary veins to the left atrium.
5. From the left atrium, blood flows through the bicuspid (mitral) valve into the left ventricle.
6. The left ventricle contracts powerfully, pumping oxygenated blood through the aortic valve into the aorta, distributing it to the rest of the body.

Coronary Circulation

The heart muscle itself requires a constant supply of oxygen and nutrients. This is provided by the coronary arteries, which branch off from the aorta just above the aortic valve. The main coronary arteries are the left and right coronary arteries. Blood from the myocardium is drained by the cardiac veins, which empty into the coronary sinus, and then into the right atrium.

Cardiac Cycle

The cardiac cycle refers to the complete sequence of events that occurs during one heartbeat. It involves the coordinated contraction (systole) and relaxation (diastole) of the atria and ventricles, resulting in the pumping of blood. Each cardiac cycle consists of two main phases: diastole (relaxation) and systole (contraction).

Phases of the Cardiac Cycle

The cardiac cycle can be broken down into several phases:

1. Atrial Diastole: The atria are relaxed and begin to fill with blood. The AV valves are closed.
2. Ventricular Diastole: The ventricles are also relaxed.
   • Isovolumetric Relaxation: As the ventricles begin to relax, ventricular pressure drops below arterial pressure. The semilunar valves snap shut (producing the second heart sound, 'dub'), and because the AV valves are also closed, the volume of blood in the ventricles remains constant for a brief period.
   • Ventricular Filling: When ventricular pressure drops below atrial pressure, the AV valves open, and blood rushes from the atria into the ventricles. This phase accounts for the majority of ventricular filling.
3. Atrial Systole: The atria contract, pushing the remaining blood (about 20-30% of the total volume) into the ventricles. This ensures maximum filling of the ventricles before they contract.
4. Ventricular Systole: The ventricles contract.
   • Isovolumetric Contraction: As ventricular pressure rises above atrial pressure, the AV valves snap shut (producing the first heart sound, 'lub'). However, ventricular pressure is not yet high enough to open the semilunar valves. The volume of blood in the ventricles is constant during this brief phase.
   • Ventricular Ejection: When ventricular pressure exceeds the pressure in the pulmonary artery and aorta, the semilunar valves open, and blood is forcefully ejected from the ventricles into these arteries.

After ventricular ejection, the ventricles begin to relax, and the cycle repeats.

Heart Sounds

The characteristic "lub-dub" sounds of the heart are produced by the closing of the heart valves.

• First Heart Sound (S1 - "Lub"): Occurs at the beginning of ventricular systole, caused by the closing of the AV valves (tricuspid and bicuspid).
• Second Heart Sound (S2 - "Dub"): Occurs at the beginning of ventricular diastole, caused by the closing of the semilunar valves (aortic and pulmonary).

Abnormal heart sounds, called murmurs, can indicate faulty valve function or other cardiovascular issues.

Regulation of Heart Rate and Stroke Volume

The heart rate (number of beats per minute) and stroke volume (amount of blood pumped per beat) are regulated to meet the body's changing needs.

• Autonomic Nervous System: The sympathetic nervous system increases heart rate and contractility, while the parasympathetic nervous system (via the vagus nerve) decreases heart rate.
• Hormones: Hormones like adrenaline (epinephrine) and thyroxine can increase heart rate and contractility.
• Factors Affecting Stroke Volume:
  • Preload: The degree of stretch of cardiac muscle cells before they contract. Higher preload generally leads to a stronger contraction (Frank-Starling law of the heart).
  • Contractility: The intrinsic strength of cardiac muscle contraction, independent of preload.
  • Afterload: The pressure that the ventricles must overcome to eject blood. High afterload reduces stroke volume.

Electrocardiogram (ECG or EKG)

An electrocardiogram (ECG) is a non-invasive diagnostic tool that records the electrical activity of the heart over a period of time. This electrical activity precedes and triggers the mechanical contractions of the heart muscle. By placing electrodes on the skin, the ECG machine can detect and amplify these electrical signals, displaying them as a waveform on a graph.

The ECG provides valuable information about the heart's rate, rhythm, and the condition of the heart muscle and valves. It is crucial for diagnosing various cardiac conditions, such as arrhythmias, heart attacks (myocardial infarction), and hypertrophy.

Electrical Conduction System of the Heart

The heart has a specialized electrical conduction system that initiates and coordinates the sequence of contractions.

• Sinoatrial (SA) Node: Located in the upper wall of the right atrium, the SA node is the heart's natural pacemaker. It initiates the electrical impulse that starts each heartbeat, typically at a rate of 60-100 beats per minute.
• Atrioventricular (AV) Node: Located in the floor of the right atrium, near the interatrial septum. The AV node receives the impulse from the SA node and delays it slightly (about 0.1 second). This delay is crucial as it allows the atria to complete their contraction and empty their blood into the ventricles before the ventricles begin to contract.
• Bundle of His (AV Bundle): Transmits the impulse from the AV node down through the interventricular septum.
• Bundle Branches (Left and Right): The Bundle of His splits into these branches, which carry the impulse down the septum towards the apex of the heart.
• Purkinje Fibers: A network of fibers that spread the impulse throughout the ventricular myocardium, causing the ventricles to contract rapidly and almost simultaneously from the apex upwards.

ECG Waveforms and Intervals

A typical ECG tracing includes several distinct waves, segments, and intervals, each representing specific electrical events in the heart.

• P Wave: Represents atrial depolarization (electrical activation leading to contraction). It is the first small, upward deflection on the ECG.
• QRS Complex: Represents ventricular depolarization. It is a larger, more complex waveform consisting of the Q wave (first downward deflection), R wave (first upward deflection), and S wave (downward deflection after the R wave). The rapid depolarization of the ventricles generates a strong electrical signal. Atrial repolarization also occurs during this time but is masked by the larger QRS complex.
• T Wave: Represents ventricular repolarization (electrical recovery of the ventricles after contraction), preparing them for the next beat. It is typically an upward deflection following the QRS complex.

ECG Intervals and Segments

• PR Interval: The time from the beginning of the P wave to the beginning of the QRS complex. It represents the duration of the impulse transmission from the SA node through the AV node, including the AV nodal delay.
• ST Segment: The flat line between the end of the QRS complex and the beginning of the T wave. It represents the period when the ventricles are completely depolarized and contracting.
• QT Interval: The time from the beginning of the QRS complex to the end of the T wave. It represents the total duration of ventricular electrical activity (depolarization and repolarization).

Interpreting an ECG

Interpreting an ECG requires specialized knowledge, but some basic principles are important:

• Rate: The number of QRS complexes over a specific time period. A normal resting heart rate is between 60-100 beats per minute.
• Rhythm: The regularity of the heartbeats. An irregular rhythm is called an arrhythmia.
• Axis: The general direction of the electrical activity of the heart.
• Waveform Morphology: The shape and duration of the P wave, QRS complex, and T wave can indicate abnormalities. For example, changes in the ST segment can suggest ischemia (lack of blood flow) or infarction (heart attack).

The ECG is a powerful tool that, when analyzed correctly, provides a window into the heart's electrical health, complementing our understanding of its structure and the mechanics of the cardiac cycle.