7.1 Cardiac anatomy and electrophysiology
4 min read•august 7, 2024
The heart's intricate structure and electrical system work together to pump blood efficiently. From the myocardium's powerful contractions to the pericardium's protective embrace, each layer plays a crucial role in cardiac function.
The cardiac conduction system orchestrates the heart's rhythm, starting with the SA node's . This electrical network ensures coordinated contractions, allowing for proper blood flow through the chambers and during each heartbeat.
Cardiac Wall Layers
Myocardium
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- Thick, muscular middle layer of the heart wall composed of cardiac muscle tissue
- Responsible for the contraction and pumping action of the heart
- Consists of specialized cardiac muscle cells called cardiomyocytes
- Cardiomyocytes are striated, branched, and interconnected by intercalated discs which allow for synchronized contraction
- Myocardium is thickest in the left ventricle (pumps blood to the entire body) and thinnest in the (pumps blood to the )
Pericardium
- Double-layered membranous sac that surrounds and protects the heart
- Outer layer is the fibrous pericardium which is tough and inelastic, provides protection and anchors the heart to surrounding structures
- Inner layer is the serous pericardium which consists of the parietal pericardium (lines the fibrous pericardium) and the visceral pericardium or epicardium (covers the outer surface of the heart)
- Between the parietal and visceral layers is the pericardial cavity filled with pericardial fluid which reduces friction during heart contractions
Endocardium
- Thin, smooth inner lining of the heart chambers and valves
- Composed of endothelial cells continuous with the endothelium of blood vessels
- Provides a non-thrombogenic surface to prevent blood clotting inside the heart
- Covers the heart valves (tricuspid, pulmonary, mitral, aortic) and is thicker here to withstand the pressure gradients
- Plays a role in regulating myocardial function through the release of substances like nitric oxide and endothelin
Cardiac Conduction System
Sinoatrial (SA) Node
- Known as the "natural pacemaker" of the heart, located in the upper wall of the right atrium
- Consists of specialized cardiac muscle cells that spontaneously depolarize, generating electrical impulses
- Fastest intrinsic rate of (60-100 times/min), setting the pace for the heart
- Receives autonomic nervous system input to modulate heart rate (sympathetic increases, parasympathetic decreases)
Atrioventricular (AV) Node
- Located in the interatrial septum near the tricuspid valve, receives impulses from the SA node
- Conducts impulses from atria to ventricles with a delay (about 0.1s), allowing time for atrial contraction and ventricular filling
- Acts as a secondary pacemaker if the SA node fails, with an intrinsic rate of 40-60 beats/min
- Only pathway for impulses to travel from atria to ventricles (AV bundle/Bundle of His arise here)
Purkinje Fibers
- Specialized conduction cells that rapidly conduct impulses through the ventricles
- Arise from the left and right bundle branches which are continuations of the AV bundle
- Allow for coordinated, nearly simultaneous depolarization and contraction of ventricular muscle
- Conduct impulses much faster than typical cardiac muscle cells (about 4 m/s vs 0.5 m/s)
Action Potential
- Brief reversal of membrane potential that occurs in excitable cells like cardiac muscle cells
- Cardiac action potentials differ from neurons - longer duration (200-400ms) and plateau phase
- 5 phases:
- Rapid depolarization (Na+ influx)
- Initial repolarization (K+ efflux)
- Plateau (Ca2+ influx and K+ efflux balance)
- Repolarization (K+ efflux)
- Resting membrane potential
- Absolute refractory period during phases 1-3 prevents tetanic contraction, allows time for ventricular filling
Cardiac Function
Cardiac Cycle
- Sequence of electrical and mechanical events that occur during one heartbeat
- Divided into systole (contraction and ejection) and diastole (relaxation and filling) for both atria and ventricles
- One cycle:
- Atrial systole
- Atrial diastole and ventricular systole
- Ventricular diastole
- Coordinated by the cardiac conduction system and regulated by autonomic nerves and hormones
- Pressure changes open and close heart valves to ensure one-way blood flow
Systole and Diastole
- Systole: period of contraction and ejection of blood from the chambers
- Atrial systole: contraction of atria, "atrial kick" contributes final 20-30% of ventricular filling
- Ventricular systole: contraction of ventricles, ejects blood into aorta and pulmonary arteries
- Diastole: period of relaxation and filling of the chambers
- Atrial diastole: relaxation of atria, passive filling from
- Ventricular diastole: relaxation of ventricles, passive filling from atria then atrial kick at end
- Diastole is longer than systole, especially at lower heart rates, to allow for adequate filling
Electrocardiogram (ECG)
- Recording of the electrical activity of the heart over time using electrodes placed on the skin
- Displays the sum of the action potentials of cardiac muscle cells, conducted through body fluids
- Components:
- P wave: atrial depolarization
- QRS complex: ventricular depolarization
- Q wave: septal depolarization
- R wave: apex depolarization
- S wave: basal depolarization
- T wave: ventricular repolarization
- U wave (not always seen): papillary muscle repolarization
- Important intervals:
- PR interval: time from atrial depolarization to ventricular depolarization, indicates AV node delay
- QT interval: time from ventricular depolarization to repolarization, indicates ventricular duration
- Useful diagnostic tool to detect cardiac abnormalities in rate, rhythm, conduction, and structure
Key Terms to Review (19)
Action Potential: An action potential is a rapid and transient electrical signal that travels along the membrane of a neuron or muscle cell, allowing for the transmission of information and communication between cells. This process involves a series of changes in membrane potential due to the movement of ions across the membrane, which is essential for various physiological processes including muscle contraction and synaptic transmission.
Arrhythmia: Arrhythmia refers to an irregular heartbeat or abnormal heart rhythm, which can disrupt the normal sequence of electrical impulses in the heart. This condition can affect how well the heart pumps blood and can arise from various issues within cardiac muscle physiology and the intricate electrical pathways responsible for coordinating heart contractions. Understanding arrhythmia is crucial for recognizing its potential impacts on overall cardiovascular health and function.
Atria: Atria are the two upper chambers of the heart, responsible for receiving blood from the body and lungs. They play a crucial role in the cardiac cycle by collecting blood before it is pumped into the lower chambers, known as ventricles. The right atrium receives deoxygenated blood from the body, while the left atrium receives oxygenated blood from the lungs, making them vital for efficient circulation.
Atrioventricular node: The atrioventricular node (AV node) is a specialized cluster of cells located in the heart's right atrium that plays a crucial role in coordinating the electrical signals that regulate heartbeats. It acts as a gatekeeper between the atria and ventricles, ensuring that the electrical impulses generated by the sinoatrial node are transmitted efficiently, allowing for proper timing of contraction between the upper and lower chambers of the heart.
Cardiac cycle: The cardiac cycle is the sequence of events that occur during one complete heartbeat, encompassing both the contraction and relaxation phases of the heart. This cycle involves the coordinated actions of the atria and ventricles, along with electrical signals that trigger heart muscle contractions, ensuring efficient blood flow throughout the body. Understanding this process is crucial for grasping how cardiac anatomy and electrophysiology work together to maintain proper cardiovascular function.
Cardiac output: Cardiac output is the volume of blood pumped by the heart per minute, a crucial measure of cardiovascular efficiency and health. It reflects the ability of the heart to supply oxygen and nutrients to tissues while removing waste products. Understanding cardiac output helps in examining blood composition, hemodynamics, and how the body regulates blood flow during different physiological states, including exercise and rest.
Contractility: Contractility refers to the intrinsic ability of cardiac muscle cells to generate force and shorten in response to stimulation, playing a crucial role in the heart's pumping function. This property allows the heart to effectively eject blood during each heartbeat, ensuring adequate blood flow to meet the body's demands. It is influenced by factors such as calcium ion concentration, myocyte stretch, and neurohormonal inputs.
Coronary circulation: Coronary circulation refers to the system of blood vessels that supply blood to the heart muscle itself, ensuring it receives the oxygen and nutrients needed to function effectively. This intricate network includes the coronary arteries and veins, which are crucial for delivering oxygen-rich blood to the myocardium and removing deoxygenated blood. A well-functioning coronary circulation is vital for maintaining heart health and supporting its pumping action.
Depolarization: Depolarization is a process during which the membrane potential of a cell becomes less negative, moving towards a more positive value. This change in charge across the cell membrane is crucial for the initiation and propagation of action potentials in neurons and muscle cells, including cardiac myocytes. It plays a key role in various physiological functions, such as transmitting signals between neurons and coordinating heartbeats.
Heart failure: Heart failure is a chronic condition in which the heart is unable to pump blood effectively to meet the body's needs for blood and oxygen. This condition can result from various issues, including damage to the heart muscle, problems with the heart valves, or electrical conduction disturbances that affect the heart's ability to contract and relax properly.
Heart Rate Variability: Heart rate variability (HRV) refers to the variation in time intervals between consecutive heartbeats, which reflects the autonomic nervous system's regulation of the heart. This variability indicates how well the body can adapt to stress and exercise, revealing insights into physiological integration and overall health. High HRV is typically associated with good cardiovascular fitness and a robust ability to handle stress, while low HRV can indicate health issues or reduced fitness.
Myocardial cells: Myocardial cells, also known as cardiac myocytes, are specialized muscle cells that make up the heart's myocardium, responsible for the contractile function of the heart. These cells are unique because they can generate and conduct electrical impulses, allowing for coordinated heart contractions and efficient blood pumping throughout the body.
Pacemaker Cells: Pacemaker cells are specialized cardiac muscle cells that have the unique ability to generate spontaneous action potentials, initiating and regulating the heartbeat. These cells are primarily located in the sinoatrial (SA) node of the heart and play a crucial role in establishing the rhythm of the heartbeat through their automaticity. Their rhythmic firing ensures that the heart beats in a coordinated manner, facilitating efficient blood circulation throughout the body.
Relaxation Phase: The relaxation phase refers to the period during the cardiac cycle when the heart muscles relax after contraction, allowing the chambers to fill with blood. This phase is crucial for maintaining efficient blood flow throughout the body, as it ensures that the heart can refill before the next contraction occurs. During this time, the ventricles are filled with blood from the atria, and the overall pressure in the heart decreases, enabling proper circulation.
Sinoatrial Node: The sinoatrial node, often referred to as the SA node, is a small cluster of specialized cardiac muscle cells located in the right atrium of the heart. It serves as the primary pacemaker of the heart, generating electrical impulses that initiate the heartbeat and regulate the heart rate, playing a crucial role in cardiac anatomy and electrophysiology.
Stroke volume: Stroke volume is the amount of blood ejected from the left ventricle of the heart with each heartbeat. It plays a crucial role in determining cardiac output, which is vital during physical activity and exercise as the body’s demand for oxygen increases. Stroke volume is influenced by factors such as heart size, contractility, preload, and afterload, all of which are interconnected with cardiovascular physiology.
Valves: Valves are structures in the cardiovascular system that regulate blood flow through the heart and vessels by opening and closing in response to pressure changes. They play a crucial role in maintaining unidirectional blood flow, ensuring that blood moves efficiently from one chamber to another and into the arteries while preventing backflow. The proper functioning of valves is vital for effective cardiac output and overall circulatory health.
Venous return: Venous return is the process by which deoxygenated blood is transported back to the heart through the venous system. This crucial function ensures that blood is continually cycled through the body, allowing for the replenishment of oxygen and removal of carbon dioxide. Understanding venous return is key to grasping how the heart's pumping action is supported and how circulation is maintained.
Ventricles: Ventricles are the two lower chambers of the heart responsible for pumping blood out of the heart to the lungs and the rest of the body. They play a critical role in the circulatory system, with the right ventricle sending deoxygenated blood to the lungs for oxygenation, while the left ventricle pumps oxygen-rich blood to the entire body. The structure and function of the ventricles are closely linked to cardiac anatomy and electrophysiology.