Heart - Cardiac Hemodynamics and Cycle

Physiological Functions of the Heart Introduction The heart's primary role is to pump blood throughout the body in two coordinated circuits: the pulmonary circulation (to the lungs) and the systemic circulation (to the rest of the body). Understanding how the heart accomplishes this task requires knowledge of three interconnected systems: the structural pathways blood follows, the mechanical events of the cardiac cycle, and the electrical signals that coordinate heart contractions. This knowledge is fundamental to understanding cardiovascular physiology and many clinical conditions. Blood Flow and Circulation The heart functions as two pumps working in series. Think of it as having a right side dedicated to the lungs and a left side dedicated to the body. The Pulmonary Circuit: The right atrium receives deoxygenated blood from the systemic circulation through the superior and inferior vena cava. This blood moves into the right ventricle, which contracts and pumps the blood through the pulmonary trunk to both lungs. In the lungs, blood releases carbon dioxide and picks up oxygen through gas exchange. The Systemic Circuit: Oxygenated blood returns from the lungs via four pulmonary veins, which deliver it to the left atrium. From there, blood moves into the left ventricle, which contracts with greater force than the right ventricle (because it must pump blood throughout the entire body). The left ventricle ejects blood into the aorta for distribution to all tissues except the lungs. Gas and Nutrient Exchange: In systemic tissues, blood passing through capillaries delivers oxygen and nutrients to cells while simultaneously picking up carbon dioxide and metabolic wastes. This depleted blood then returns to the heart via veins, completing the circuit. A key point to remember: both ventricles pump the same volume of blood per heartbeat, even though they face different resistances. This coordination is essential for maintaining balanced circulation. The Cardiac Cycle The cardiac cycle is divided into two main phases: systole (contraction) and diastole (relaxation). However, the cycle involves distinct events happening in both the atria and ventricles, so understanding the timing is crucial. Diastole: The Filling Phase When diastole begins, both ventricles are relaxed, and ventricular pressure is low. The atrioventricular (AV) valves—the tricuspid valve (right side) and mitral valve (left side)—are open. Blood passively flows from the atria directly into the relaxed ventricles. This passive filling accounts for about 70% of ventricular filling. Atrial Systole At the end of diastole, the atria contract (atrial systole). This active contraction pushes the remaining 30% of blood into the ventricles. This extra contribution is sometimes called the "atrial kick" and becomes increasingly important during exercise or in certain heart conditions. Ventricular Systole: Early Phase (Isovolumetric Contraction) As the ventricles begin to contract, ventricular pressure rapidly rises. When ventricular pressure exceeds atrial pressure, it pushes the AV valves shut. This valve closure creates the first heart sound (S1, often written as "lub"). The ventricles continue contracting, but neither set of valves is open yet—the AV valves have closed, and the semilunar valves (the aortic and pulmonary valves) haven't opened because ventricular pressure hasn't exceeded arterial pressure. During this brief period, called isovolumetric contraction, the volume of blood in the ventricle stays constant while pressure rises sharply. Ventricular Systole: Ejection Phase Once ventricular pressure exceeds arterial pressure, the semilunar valves open and blood is rapidly ejected into the aorta and pulmonary trunk. About 70 mL of blood is typically ejected with each beat (the stroke volume). The ventricles eject roughly two-thirds of their blood, leaving about one-third behind as the end-systolic volume. Ventricular Systole: End and Return to Diastole As ventricular contraction weakens, ventricular pressure falls below arterial pressure. The semilunar valves snap shut, creating the second heart sound (S2, the "dub"). The ventricles continue to relax, and when ventricular pressure drops below atrial pressure once more, the AV valves open, beginning a new cycle. A common source of confusion: The closing of the AV valves (S1) marks the beginning of ventricular systole, while the closing of the semilunar valves (S2) marks the end of ventricular systole. S1 occurs early in ventricular contraction, not at the start. Cardiac Output Cardiac output (CO) is the volume of blood the heart pumps per minute. It's calculated using a simple but powerful equation: $$CO = SV \times HR$$ where SV is stroke volume (in mL per beat) and HR is heart rate (in beats per minute). Normal Values A typical adult has a stroke volume of about 70 mL per beat and a resting heart rate of about 75 beats per minute, yielding a cardiac output of approximately 5.25 liters per minute. This value is remarkably consistent across most adults at rest, which is why 5 L/min is often cited as the "normal" cardiac output. Preload: Ventricular Filling Preload refers to the stretch of the ventricular myocardium at the end of diastole, which depends on how much blood fills the ventricle. More specifically, it reflects the ventricular filling pressure. The Frank-Starling mechanism explains how preload affects stroke volume: when the ventricles are stretched more (increased preload), the cardiac muscle fibers are positioned more optimally on the force-length relationship, allowing them to generate greater contractile force. This results in a larger stroke volume. Conversely, if there is less filling (decreased preload), stroke volume decreases. Think of it like stretching a rubber band slightly—it snaps back with more force. However, there is a physiological limit; excessive stretching actually decreases force generation. Afterload: The Resistance to Ejection Afterload is the pressure (or resistance) the ventricles must overcome to eject blood. It's primarily determined by: Vascular resistance: How much resistance the arteries present to blood flow Arterial pressure: The pressure in the aorta that the ventricle must exceed Valve resistance: Any stenosis (narrowing) of the semilunar valves Higher afterload means the ventricles must work harder to eject blood. Interestingly, increased afterload actually decreases stroke volume (this is different from the preload effect). This occurs because the heart must generate more pressure before ejection begins, limiting the amount of blood that can be ejected before the ventricles fatigue. Inotropic Regulation Inotropic agents are substances that modify the force of cardiac muscle contraction, independent of changes in preload or afterload. Positive Inotropes Positive inotropes increase contractile force, allowing the ventricles to contract more forcefully and eject blood more effectively. Common examples include: Adrenaline (epinephrine) and noradrenaline (norepinephrine): Released during the "fight or flight" response, these catecholamines increase heart rate and contractile force Dopamine: A precursor to adrenaline that also increases contractility at higher doses Digoxin: A cardiac glycoside that increases intracellular calcium availability Negative Inotropes Negative inotropes decrease contractile force. A common example is calcium-channel blockers, which reduce the influx of calcium into cardiac myocytes, limiting the strength of contraction. Some beta-blockers also have negative inotropic effects. Electrical Conduction System The heart's mechanical contractions are triggered and coordinated by a precisely ordered electrical system. This system ensures that the atria contract first, followed by the ventricles, maximizing the efficiency of blood pumping. The Conduction Pathway The Sinoatrial (SA) Node: Located in the wall of the right atrium near the entrance of the superior vena cava, the SA node is the heart's natural "pacemaker." It spontaneously generates electrical impulses at a rate of 60–100 times per minute at rest. Atrial Spread: From the SA node, the electrical impulse spreads through the atrial muscle tissue, causing both atria to depolarize and contract. This atrial contraction occurs synchronously on both sides, efficiently pushing blood into the ventricles. The Atrioventricular (AV) Node: Located at the lower portion of the right atrium, the AV node receives the impulse from the atria. Importantly, the AV node conducts impulses slowly—this delay (about 100 milliseconds) is intentional and critical. It ensures that atrial contraction is completed before ventricular contraction begins, preventing the ventricles from contracting while still receiving blood from the atria. The Bundle of His and Bundle Branches: After the AV node delay, the impulse travels rapidly down the bundle of His, a specialized conducting pathway. The bundle of His splits into the left and right bundle branches, which deliver the impulse to the left and right ventricles respectively. Purkinje Fibers: These are the terminal branches of the conduction system. Purkinje fibers spread throughout the ventricular myocardium, ensuring that both ventricles contract almost simultaneously. The conduction velocity here is very rapid, allowing rapid, synchronized ventricular contraction. Why the sequence matters: The coordinated conduction ensures efficient pumping. If the ventricles contracted before the atria, or if they contracted asynchronously, the heart would be much less efficient at moving blood. Heart Rate Generation The SA node generates heartbeats through the rhythmic depolarization and repolarization of its pacemaker cells. Understanding this mechanism is essential for understanding arrhythmias and the effects of medications. The Pacemaker Potential Unlike typical cardiac muscle cells that maintain a stable negative resting membrane potential, pacemaker cells spontaneously depolarize. This spontaneous depolarization is called the pacemaker potential, and it follows a characteristic pattern. Phase 1 – Rapid Sodium Influx: The pacemaker potential begins at approximately –60 mV (less negative than typical resting cells). Sodium channels open, allowing sodium ions ($Na^+$) to rush into the cell. This causes the membrane potential to become progressively less negative (more positive), rising toward approximately 0 mV. This phase is relatively slow compared to the rapid depolarization in contractile muscle cells. Phase 2 – The Plateau: As the membrane potential reaches about –40 mV, sodium channels close. At this point, calcium channels open, allowing calcium ions ($Ca^{2+}$) to enter. Simultaneously, potassium ($K^+$) ions begin to exit the cell. These opposing effects create a plateau phase where the membrane potential rises slowly toward approximately 0 mV. This plateau is much less pronounced than the plateau seen in ventricular myocytes. Phase 3 – Repolarization: When the membrane potential reaches approximately 0 mV (or slightly above), calcium channels close. Potassium channels remain open, and potassium efflux dominates. This repolarizes the cell, bringing the membrane potential back to approximately –60 mV. At this point, potassium channels close. Phase 4 – The Cycle Repeats: The cycle begins again immediately. Because pacemaker cells never fully repolarize to the very negative potentials of other cardiac cells (like –90 mV), they naturally depolarize again, creating the rhythm of the heartbeat. Heart Rate Regulation The intrinsic rate of the SA node is approximately 100–110 beats per minute. However, the actual resting heart rate is typically lower (60–100 bpm) because the parasympathetic nervous system (via the vagus nerve) exerts a vagal brake on the SA node, slowing its rate. During exercise or stress, the sympathetic nervous system overrides this brake, increasing heart rate. The rate at which pacemaker cells depolarize can be modified by: Sympathetic stimulation: Increases the slope of the pacemaker potential, causing faster depolarization and a higher heart rate Parasympathetic stimulation: Decreases the slope and makes the maximum potential more negative, causing slower depolarization and a lower heart rate Hormones and ions: Adrenaline increases heart rate; high potassium levels can slow or stop the heart by making the resting potential less negative Summary The heart is a remarkable organ that coordinates electrical and mechanical events to efficiently pump blood throughout the body. The cardiac cycle ensures that blood fills the heart, is pumped out, and fills again in a repeating pattern. Cardiac output—the volume of blood pumped per minute—depends on stroke volume and heart rate, both of which are regulated by physiological mechanisms. The intricate electrical conduction system and the spontaneous depolarization of pacemaker cells ensure that the heart beats rhythmically and efficiently. Understanding these physiological principles provides the foundation for recognizing how the heart adapts to changing demands and how dysfunction in these systems leads to cardiovascular disease.