Cardiac Physiology: Study Guide and Review Notes

Cardiac Physiology

Introduction to the Heart and Circulatory System

The heart is a muscular organ responsible for pumping blood throughout the body via the circulatory system. The circulatory system consists of three main components that work together to transport oxygen, nutrients, and waste products.

• Component 1: Heart – The central pump that propels blood.

• Component 2: Blood Vessels – Tubular structures (arteries, veins, capillaries) that carry blood to and from the heart and tissues.

• Component 3: Blood – The fluid medium that transports gases, nutrients, hormones, and waste.

Example: The heart pumps oxygenated blood through arteries to tissues and returns deoxygenated blood via veins.

Flow of Blood Through the Heart and Circulatory System

Blood enters the heart through the inferior/superior vena cava, flows through the right atrium and ventricle, is pumped to the lungs, returns to the left atrium and ventricle, and is then sent to the body. This cycle ensures continuous circulation.

• Step 1: Blood enters right atrium via vena cava.

• Step 2: Passes through right ventricle.

• Step 3: Travels to lungs via pulmonary artery (for oxygenation).

• Step 4: Returns to left atrium via pulmonary veins.

• Step 5: Passes through left ventricle.

• Step 6: Pumped out to body via aorta.

Example: The systemic and pulmonary circuits work in tandem to maintain oxygen delivery and carbon dioxide removal.

Cardiac Structure and Function

Major Cardiac Structures

The heart contains specialized structures that support its function as a pump. Each structure has a unique physiological purpose and anatomical description.

Structure	Function (Physiological Purpose)	Structure (Description of Anatomy)
Fibrous skeleton	Provides structural support and electrical insulation	Dense connective tissue framework
Atrioventricular valves	Prevent backflow from ventricles to atria	Flaps between atria and ventricles (tricuspid, bicuspid/mitral)
Semilunar valves	Prevent backflow from arteries to ventricles	Crescent-shaped valves at base of aorta and pulmonary artery
Endothelium	Lines heart chambers, reduces friction	Thin layer of simple squamous cells
Myocardium	Contracts to pump blood	Thick layer of cardiac muscle tissue
Epicardium	Protects heart, reduces friction	Outer layer of heart wall
Pericardial sac	Encloses and protects heart	Double-walled sac containing pericardial fluid
Contractile cells	Generate force for contraction	Specialized cardiac muscle cells
Autorhythmic cells	Initiate and conduct electrical impulses	Non-contractile, pacemaker cells

Pacemaker Activity and Depolarization

Pacemaker cells in the heart generate spontaneous action potentials, setting the rhythm for cardiac contraction. Depolarization in autorhythmic cells involves a unique sequence of ion movements.

• Pacemaker Activity: Autorhythmic cells (e.g., SA node) spontaneously depolarize, initiating heartbeats.

• Depolarization Process: Involves slow influx of Na+ (funny current), followed by rapid Ca2+ influx, and repolarization via K+ efflux.

Example: The SA node sets the pace for the heart by generating regular action potentials.

Cardiac Conduction System

Sites of Autorhythmicity

Specialized cardiac cells are capable of generating action potentials independently, ensuring coordinated contraction.

Site of Specialized Cells	Location	Pacemaker Activity (AP/min)
Sinotrial node (SA node)	Right atrium, near superior vena cava	60-100
Atrioventricular node (AV node)	Base of right atrium, near septum	40-60
Purkinje fibers	Ventricular walls, apex of heart	20-40

Additional info: The SA node is the primary pacemaker; AV node and Purkinje fibers serve as backup pacemakers.

Coordination of Cardiac Excitation

Coordinated spread of excitation ensures efficient pumping by synchronizing atrial and ventricular contractions.

• Key Point: Sequential activation allows for proper filling and ejection of blood.

• Example: Delay at AV node allows atria to contract before ventricles.

Cardiac Action Potentials and Contraction

Action Potentials in Cardiac Cells

Cardiac contractile cells have a distinct action potential compared to neurons, featuring a plateau phase due to Ca2+ influx.

• Neuronal AP: Rapid depolarization and repolarization.

• Cardiac AP: Rapid depolarization, plateau (Ca2+ influx), repolarization.

Equation:

$ext{AP duration (cardiac)} > ext{AP duration (neuron)}$

Example: The plateau phase prevents tetanus in cardiac muscle.

Excitation-Contraction Coupling

Excitation-contraction coupling links electrical signals to muscle contraction in the heart.

• Step 1: Action potential travels along sarcolemma.

• Step 2: Ca2+ enters cell from extracellular fluid.

• Step 3: Ca2+ triggers release of more Ca2+ from sarcoplasmic reticulum.

• Step 4: Ca2+ binds to troponin, allowing actin-myosin interaction.

• Step 5: Thin filaments slide inward, causing contraction.

Refractory Period

The refractory period in cardiac cells is longer than the action potential, preventing sustained contraction (tetanus).

• Key Point: Ensures rhythmic contraction and relaxation.

Electrocardiogram (ECG) and Cardiac Cycle

ECG Basics

An electrocardiogram (ECG) records the electrical activity of the heart, providing information about heart rhythm and function.

• P wave: Atrial depolarization

• QRS complex: Ventricular depolarization

• T wave: Ventricular repolarization

Example: Abnormalities in ECG can indicate arrhythmias or myocardial infarction.

Heart Conditions Detectable by ECG

ECG can detect various heart conditions, including:

• Arrhythmias (irregular heart rhythms)

• Myocardial infarction (heart attack)

• Heart block (impaired conduction)

Mechanical Events of the Cardiac Cycle

The cardiac cycle consists of alternating periods of systole (contraction) and diastole (relaxation).

• Systole: Ventricles contract, ejecting blood.

• Diastole: Ventricles relax, filling with blood.

Cardiac Output and Regulation

Definitions and Equations

• Systole: Period of ventricular contraction.

• Diastole: Period of ventricular relaxation.

• Cardiac Output (CO): Volume of blood pumped by each ventricle per minute.

Equation:

$ext{Cardiac Output} = ext{Heart Rate} imes ext{Stroke Volume}$

• Stroke Volume (SV): Volume of blood pumped per beat.

• Cardiac Reserve: Difference between resting and maximal cardiac output.

Autonomic Regulation of Heart Activity

The autonomic nervous system modulates heart rate and contractility via parasympathetic and sympathetic stimulation.

Area Affected	Effect of Parasympathetic Stimulation	Effect of Sympathetic Stimulation
SA Node	Decreases heart rate	Increases heart rate
AV Node	Slows conduction	Speeds conduction
Conduction pathway	Decreases excitability	Increases excitability
Atrial muscle	Decreases contractility	Increases contractility
Ventricular muscle	No significant effect	Increases contractility
Adrenal medulla	No effect	Releases epinephrine, increases heart activity
Veins	No effect	Constriction increases venous return

Intrinsic and Extrinsic Control of Stroke Volume

Stroke volume is regulated by intrinsic (Frank-Starling law) and extrinsic (neural/hormonal) mechanisms.

• Intrinsic Control: Increased venous return stretches cardiac muscle, enhancing contraction (Frank-Starling law).

• Extrinsic Control: Sympathetic stimulation and hormones (e.g., epinephrine) increase contractility.

Equation (Frank-Starling Law):

$ext{SV} ext{ increases as } ext{EDV} ext{ increases}$

Example: Exercise increases venous return, boosting stroke volume and cardiac output.

Coronary Circulation

Definition and Importance

Coronary circulation refers to the network of blood vessels that supply the heart muscle (myocardium) with oxygen and nutrients. It is essential for maintaining cardiac function and health.

• Key Point: Blockage of coronary arteries can lead to myocardial infarction.

Example: The left and right coronary arteries branch from the aorta to supply the heart.