FISIOLOGÍA: MÚSCULO CARDIACO: EL CORAZÓN COMO BOMBA y LA FUNCIÓN DE LAS VÁLVULAS CARDÍACAS

Introduction to Cardiac Physiology

Overview of the Heart and Its Function

• The video introduces Unit 3 of physiology, focusing on the heart, specifically Chapter 9 about cardiac muscle.

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Understanding the Structure of the Heart

• The speaker clarifies that while we refer to "the heart" as a single organ, it actually consists of two separate pumps: the right heart and left heart.

• Each side of the heart has distinct anatomical and physiological characteristics; both consist of an atrium and a ventricle.

Functionality of Heart Chambers

Right vs. Left Heart Functions

• The right heart (right atrium and ventricle) pumps blood to the lungs for oxygenation, while the left heart (left atrium and ventricle) distributes oxygenated blood throughout the body.

Histological Features of Cardiac Muscle

• Cardiac muscle is striated, with fibers arranged in a unique pattern that allows them to combine and separate effectively during contraction.

Cellular Structure and Communication

Intercalated Discs

• Individual cardiac muscle cells connect through intercalated discs, which contain gap junctions facilitating rapid action potential propagation across muscle fibers.

Importance of Action Potential Propagation

• These intercalated discs ensure efficient communication between cells, crucial for synchronized contractions necessary for effective pumping.

Contraction Mechanics

Atrial vs. Ventricular Contraction Timing

• Atrial muscles contract before ventricular muscles due to fibrous tissue surrounding atrioventricular valves, enhancing blood flow efficiency.

Structural Adaptations in Left Ventricle

• The left ventricle features two layers of muscle fibers oriented in opposite directions, allowing for both contraction and torsion movements during systole.

Electrophysiology Insights

Resting Membrane Potential

Action Potential of Cardiac Muscle

Overview of Action Potential Phases

• The cardiac muscle action potential features a plateau phase lasting approximately 0.2 seconds, followed by a rapid repolarization.

• The phases of the action potential include Phase 0 (depolarization), Phase 1 (initial repolarization), Phase 2 (plateau), Phase 3 (rapid repolarization), and Phase 4 (resting membrane potential).

Detailed Breakdown of Phases

Phase 0: Depolarization

• In this phase, voltage-gated sodium channels open rapidly, allowing sodium to enter the cell quickly, resulting in a sharp spike in depolarization.

• This rapid influx of sodium leads to a significant increase in intracellular sodium levels, reaching an action potential around +20 mV.

Phase 1: Initial Repolarization

• Sodium channels close quickly after opening, leading to an initial repolarization as potassium exits the cell.

• The cessation of sodium entry combined with potassium efflux results in a brief segment of rapid but initial repolarization.

Phase 2: Plateau

• Calcium channels (specifically L-type calcium channels) open slowly during this phase, maintaining depolarization and creating a plateau effect.

• Additionally, there is decreased permeability to potassium ions which contributes to sustaining the plateau.

Phase 3: Rapid Repolarization

• Calcium channels finally close while potassium permeability increases significantly, allowing for rapid loss of positive charges from the cell.

• This leads to a swift return towards resting membrane potential levels between -80 mV and -90 mV.

Additional Insights on Cardiac Action Potential

Importance of Plateau

• The plateau phase is crucial due to slow calcium channel activity that prolongs depolarization compared to other muscle types.

Conduction Velocity and Refractory Period

• The conduction velocity for excitatory action potentials in cardiac muscle ranges from 0.3 to 0.5 meters per second—much slower than nerve conduction velocities.

• The ventricular refractory period lasts approximately 0.25 to 0.30 seconds while the atrial refractory period is about 0.15 seconds.

Coupling Excitation with Contraction

Role of Calcium Ions and T-Tubules

• Upon generation of an action potential, it propagates into cardiac muscle fibers through T-tubules ensuring effective contraction across the entire fiber.

Mechanism Behind Calcium Release

Muscle Contraction and Cardiac Cycle Overview

Muscle Contraction Mechanism

• Muscle contraction is initiated by the striated muscle fibers, which require calcium ions for the process. The release of calcium leads to muscle contraction.

• The sarcoplasmic reticulum plays a crucial role in pumping calcium back into its storage, utilizing a specific pump known as the calcium ATPase.

• Calcium is also expelled from the cell into the extracellular fluid, reducing intracellular calcium levels and facilitating muscle relaxation.

• The duration of contraction varies: approximately 0.2 seconds for atrial muscles and 0.3 seconds for ventricular muscles.

Introduction to Cardiac Cycle

• The cardiac cycle, often referred to as a heartbeat, begins with an action potential generated spontaneously in the sinoatrial (SA) node, which acts as the heart's natural pacemaker.

• The SA node is located in the right atrium's superior lateral wall near the entrance of the superior vena cava.

Heart Rate Influence on Cardiac Cycle Duration

• The duration of each cardiac cycle is directly influenced by heart rate; for instance, at 60 beats per minute (bpm), each cycle lasts one second.

• In cases of tachycardia (e.g., 120 bpm), each cycle shortens to half a second.

Electrocardiogram (ECG) Relation to Cardiac Cycle

• Understanding ECG waves—P wave, QRS complex, and T wave—is essential for correlating electrical activity with mechanical events during the cardiac cycle.

• The P wave indicates depolarization of atria leading to atrial contraction; it reflects atrial functionality and can reveal conditions affecting this phase.

Ventricular Activity Indicators

• The QRS complex represents ventricular depolarization followed by ventricular contraction; alterations here may indicate issues with ventricular function.

• The T wave signifies repolarization of ventricles leading to their relaxation; abnormalities in this wave can suggest problems with ventricular recovery post-contraction.

Atrial Pressure Changes During Cardiac Cycle

• Atrial pressure changes are depicted through three distinct waves: A wave (atrial contraction), C wave (ventricular contraction), and V wave (venous filling).

Understanding Atrial and Ventricular Pressures

Atrial Pressure Waves

• The P wave of the electrocardiogram (ECG) represents atrial depolarization, leading to atrial contraction immediately after.

• The C wave indicates a slight retrograde blood flow towards the atria due to the onset of ventricular contraction.

• The V wave reflects a slow influx of blood into the atria from the vena cavae at the end of ventricular contraction.

• Approximately 80% of blood flows directly from the atria to ventricles without needing atrial contraction; only 20% is propelled by it.

• This understanding implies that while the heart can function without atrial contraction, lack of this additional 20% can lead to symptoms of heart failure during exertion.

Ventricular Function and Filling Phases

• It’s crucial to differentiate between ventricular pressure (force on walls) and volume (amount of blood in ventricles).

• Ventricles fill with blood primarily during diastole, which can be divided into three phases: rapid filling, diastasis, and late filling due to atrial contraction.

• The first third involves rapid filling when AV valves open, allowing most (80%) blood flow into ventricles quickly.

• The middle phase (diastasis) sees minimal additional drainage as pressure equalizes; very little blood enters during this time.

• The final third accounts for the remaining 20% contributed by atrial contraction, aligning with the A wave on an ECG.

Ventricular Pressure Dynamics

• Understanding ventricular pressure involves three key periods: isovolumetric contraction, ejection phase, and isovolumetric relaxation.

• Isovolumetric contraction begins with closure of AV valves right after ventricular contraction starts; no volume change occurs initially despite rising pressure.

• Following a brief delay (0.02 - 0.03 seconds), semilunar valves open allowing ejection of blood from ventricles into arteries.

Understanding Cardiac Cycle Phases

Isovolumetric Contraction Phase

• The isovolumetric contraction phase begins when the atrioventricular valves close and lasts until the semilunar valves open. This period is crucial as it indicates that the heart is contracting without any blood being ejected.

• During this phase, while the ventricles contract, there is no blood flow out of them; hence, it’s termed "isovolumetric" because the volume remains constant despite contraction.

Ejection Phase

• The ejection phase starts with the opening of the semilunar valves, allowing blood to be pumped from the ventricles into the aorta and pulmonary artery. This marks the actual expulsion of blood from the heart.

• Blood is propelled from the right ventricle to the pulmonary artery and from the left ventricle to the aorta during this critical phase.

Isovolumetric Relaxation Phase

• Following ventricular contraction, there’s a sudden closure of semilunar valves which initiates isovolumetric relaxation. After approximately 0.03 to 0.06 seconds, atrioventricular valves open again for diastole.

• This transition leads to filling of ventricles in preparation for another cycle, highlighting both systolic and diastolic phases related to ventricular volume and pressure.

Key Volumes in Cardiac Function

• Important cardiac volumes include:

• End-Diastolic Volume (EDV): Approximately 110-120 mL representing normal filling of ventricles.

• Stroke Volume (SV): About 70 mL indicating blood ejected during systole.

• End-Systolic Volume (ESV): Remaining volume post-ejection, around 40-50 mL.

Clinical Relevance of Cardiac Volumes

• Understanding these volumes aids in assessing cardiac function and diagnosing conditions related to systolic capacity and overall heart health.

Heart Valves Functionality

Atrioventricular Valves

• The atrioventricular valves (tricuspid and mitral) prevent retrograde flow during ventricular contraction by only allowing forward movement of blood from atria to ventricles.

Passive Mechanism of Valve Operation

• These valves operate passively based on pressure differences; they open when atrial pressure exceeds ventricular pressure and close when ventricular pressure rises above atrial pressure.

Role of Papillary Muscles

• Papillary muscles anchor valve leaflets via chordae tendineae, preventing them from prolapsing into atria during ventricular contraction. Their proper function is essential for maintaining effective valve operation.

Consequences of Dysfunctional Valves

Understanding Cardiac Function and Blood Pressure Regulation

Importance of Papillary Muscles and Semilunar Valves

• The contraction of papillary muscles is crucial, especially in severe cases where failure can be fatal.

• Semilunar valves (aortic and pulmonary) prevent retrograde flow from the aorta and pulmonary arteries to the ventricles during diastole.

Differences Between Semilunar and Atrioventricular Valves

• Semilunar valves have smaller orifices, resulting in higher ejection velocity compared to atrioventricular valves, which are designed for slower filling to protect the ventricles.

• Unlike atrioventricular valves, semilunar valves lack chordae tendineae; they are supported by strong yet flexible fibrous tissue.

Aortic Pressure Dynamics

• During systole, blood entering the aorta causes its walls to distend, leading to an increase in aortic pressure up to 120 mmHg.

• After closure of the aortic valve, there is a minimal retrograde flow before pressure gradually decreases during diastole.

Blood Pressure Measurements

• Normal arterial pressure values include 120 mmHg (systolic) during ventricular contraction and 80 mmHg (diastolic) at rest between heartbeats.

• Elevated pressures beyond normal ranges indicate conditions like hypertension due to factors such as stronger ventricular contractions or hypervolemia.

Regulation of Cardiac Output

• The Frank-Starling mechanism states that increased venous return leads to greater cardiac muscle stretch and stronger contractions.

Pharmacological Treatments and Cardiac Function

Sympathetic and Parasympathetic Nervous System Effects

• Pharmacological treatments can enhance sympathetic nervous system activity to increase heart rate in conditions like guardia or improve contraction strength in heart failure.

• The parasympathetic innervation, primarily through vagal fibers, mainly affects the atria and has minimal impact on the ventricles, leading to a decrease in heart rate.

Ion Effects on Cardiac Function

• Electrolyte imbalances such as hypokalemia and hyperkalemia significantly influence cardiac function; these are often discussed in internal medicine regarding hydroelectrolytic disorders.

• Hyperkalemia results in a dilated, flaccid heart that reduces heart rate and may block impulse conduction due to its effect on the atrioventricular node.

Mechanisms of Potassium Imbalance

• An excess of potassium outside cells increases positive charges extracellularly, altering the normal sodium-potassium balance critical for cardiac action potentials.

• This accumulation leads to a more negative intracellular environment, affecting membrane potential and potentially resulting in weaker contractions.

Consequences of Ionic Changes

• A more negative membrane potential alters action potential thresholds, which can lead to reduced contraction strength and rhythm disturbances during hyperkalemia.

• Other ionic imbalances: hypercalcemia causes spastic contractions while hypocalcemia leads to cardiac weakness.

Temperature Effects on Heart Rate

• Increased body temperature from fever elevates heart rate by enhancing ion permeability across cardiac muscle membranes, accelerating auto-excitation processes.

Conclusion and References