Cardiovascular System: The Heart – Structure, Function, and Regulation

Cardiovascular System: The Heart

Functions of the Heart

The heart is a muscular organ essential for maintaining blood circulation throughout the body. Its structure and function are adapted to efficiently pump blood, ensuring the delivery of oxygen and nutrients to tissues and the removal of waste products.

• Generating Blood Pressure: The heart creates the force necessary to propel blood through the circulatory system, establishing the pressure gradient required for blood flow.

• Routing Blood: The heart separates the pulmonary (lungs) and systemic (body) circulations, ensuring that oxygen-poor and oxygen-rich blood do not mix.

• Ensuring One-Way Blood Flow: Heart valves prevent the backflow of blood, maintaining unidirectional movement through the heart and vessels.

• Regulating Blood Supply: The heart adjusts its contraction rate and force to meet the changing metabolic needs of the body, such as during exercise or rest.

Size, Shape, and Location of the Heart

The heart's anatomical features are crucial for its function and clinical assessment.

• Size: Comparable to a closed fist.

• Shape: Cone-shaped with a blunt, rounded apex and a flat base at the opposite end.

• Location: Situated in the thoracic cavity within the mediastinum, the central compartment of the thoracic cavity (excluding the lungs).

• Clinical Relevance: The heart's position is important for procedures such as auscultation (using a stethoscope), electrocardiography (ECG), and cardiopulmonary resuscitation (CPR).

External Anatomy of the Heart

The heart's external features include major blood vessels and surface landmarks that are important for understanding its function and for clinical procedures.

• Major Vessels: Superior and inferior vena cava, aorta, pulmonary arteries and veins.

• Chambers: Right and left atria (upper chambers), right and left ventricles (lower chambers).

• Surface Grooves: Coronary sulcus and interventricular sulci mark the boundaries between chambers and house major coronary vessels.

Pericardium

The pericardium is a double-walled sac that surrounds and protects the heart.

• Fibrous Pericardium: Tough, outer layer that prevents overdistention and anchors the heart in place.

• Serous Pericardium: Thin, inner layer composed of two parts:

  • Parietal Pericardium: Lines the fibrous pericardium.

  • Visceral Pericardium (Epicardium): Covers the heart surface.

• Pericardial Cavity: Space between the parietal and visceral layers, filled with pericardial fluid to reduce friction during heartbeats.

Heart Wall Structure

The heart wall consists of three distinct layers, each with specialized functions.

• Epicardium: The outermost layer; a serous membrane providing a smooth surface.

• Myocardium: The thick, middle layer composed of cardiac muscle cells responsible for contraction.

• Endocardium: The innermost layer; a smooth lining for the heart chambers and valves.

• Pectinate Muscles: Muscular ridges in the atria, especially the right atrium.

• Trabeculae Carneae: Muscular ridges and columns on the inner walls of the ventricles.

Coronary Circulation

The heart has its own blood supply through the coronary arteries and veins, ensuring it receives adequate oxygen and nutrients.

• Coronary Arteries:

  • Right Coronary Artery: Supplies the right atrium, right ventricle, and parts of the left ventricle.

  • Left Coronary Artery: Branches into the anterior interventricular artery (left anterior descending) and circumflex artery, supplying the left atrium and ventricle.

• Coronary Veins: Collect deoxygenated blood from the heart muscle and drain into the coronary sinus, which empties into the right atrium.

Heart Chambers and Septa

The heart is divided into four chambers separated by septa and valves to ensure efficient blood flow.

• Right Atrium: Receives blood from the body via the superior and inferior vena cava and the coronary sinus.

• Left Atrium: Receives oxygenated blood from the lungs via the pulmonary veins.

• Right Ventricle: Pumps blood to the lungs through the pulmonary trunk.

• Left Ventricle: Pumps blood to the body through the aorta.

• Interatrial Septum: Separates the right and left atria; contains the fossa ovalis, a remnant of the fetal foramen ovale.

• Interventricular Septum: Separates the right and left ventricles.

Heart Valves

Valves maintain unidirectional blood flow through the heart and prevent backflow.

• Atrioventricular (AV) Valves:

  • Tricuspid Valve: Between right atrium and right ventricle (three cusps).

  • Bicuspid (Mitral) Valve: Between left atrium and left ventricle (two cusps).

  • Chordae Tendineae: Tendinous cords connecting valve cusps to papillary muscles, preventing valve prolapse.

• Semilunar Valves:

  • Pulmonary Valve: Between right ventricle and pulmonary trunk.

  • Aortic Valve: Between left ventricle and aorta.

  • Each has three cup-shaped cusps that prevent backflow into the ventricles.

Heart Skeleton

The heart skeleton is a fibrous connective tissue structure that provides support and electrical insulation.

• Fibrous Rings: Surround the valves, providing attachment points and maintaining valve shape.

• Electrical Insulation: Separates atrial and ventricular muscle, ensuring proper timing of contractions.

• Muscle Attachment: Serves as a site for cardiac muscle attachment.

Cardiac Muscle and Intercalated Discs

Cardiac muscle cells are specialized for continuous, rhythmic contraction.

• Structure: Elongated, branching cells with 1-2 centrally located nuclei.

• Myofilaments: Contain actin and myosin for contraction.

• Intercalated Discs: Specialized cell junctions with desmosomes (mechanical connection) and gap junctions (electrical connection), allowing coordinated contraction.

Conducting System of the Heart

The heart's intrinsic conducting system coordinates the heartbeat.

• Sinoatrial (SA) Node: The pacemaker; initiates action potentials that spread through the atria.

• Atrioventricular (AV) Node: Delays the impulse, allowing atrial contraction before ventricular contraction.

• AV Bundle (Bundle of His): Conducts impulses from the AV node to the ventricles.

• Right and Left Bundle Branches: Carry impulses through the interventricular septum.

• Purkinje Fibers: Distribute the impulse to ventricular muscle cells, ensuring coordinated contraction.

Action Potentials in Cardiac vs. Skeletal Muscle

Cardiac and skeletal muscle differ in their electrical properties and contraction mechanisms.

• Cardiac Muscle: Action potentials are conducted from cell to cell via gap junctions; slower propagation due to smaller fiber diameter.

• Skeletal Muscle: Action potentials travel along the length of a single fiber; faster due to larger diameter.

• Calcium-Induced Calcium Release (CICR): In cardiac muscle, Ca2+ entry triggers further Ca2+ release from the sarcoplasmic reticulum.

Autorhythmicity and SA Node Action Potential

The SA node generates spontaneous action potentials due to its unique ion channel properties.

• Prepotential (Pacemaker Potential): Gradual depolarization leading to threshold and action potential generation.

• Phases: Depolarization (rapid Na+ influx), repolarization (K+ efflux).

Refractory Periods

Refractory periods prevent tetanic contractions in cardiac muscle, ensuring rhythmic beating.

• Absolute Refractory Period: Cardiac muscle is completely insensitive to further stimulation.

• Relative Refractory Period: Reduced sensitivity; a stronger-than-normal stimulus may initiate another action potential.

• Significance: Long refractory period prevents sustained (tetanic) contractions.

Electrocardiogram (ECG/EKG)

An ECG records the electrical activity of the heart and correlates with mechanical events.

• P Wave: Atrial depolarization; onset of atrial contraction.

• QRS Complex: Ventricular depolarization; onset of ventricular contraction. Atrial repolarization occurs simultaneously but is masked.

• T Wave: Ventricular repolarization; precedes ventricular relaxation.

• PR Interval: Time from atrial to ventricular contraction (about 0.16 sec).

• QT Interval: Duration of ventricular contraction and relaxation (about 0.36 sec).

Cardiac Cycle

The cardiac cycle consists of repetitive contraction (systole) and relaxation (diastole) of the heart chambers.

• Systole: Contraction phase; blood is ejected from the chambers.

• Diastole: Relaxation phase; chambers fill with blood.

• Blood Flow: Moves from areas of higher to lower pressure.

Phases of the Cardiac Cycle

• Isovolumic Contraction: Ventricles contract with no volume change; AV valves close, semilunar valves remain closed.

• Ejection: Ventricular pressure exceeds arterial pressure; semilunar valves open, blood is ejected.

• Isovolumic Relaxation: Ventricles relax; all valves closed, pressure falls.

• Passive Ventricular Filling: AV valves open, blood flows from atria to ventricles (accounts for most filling).

• Active Ventricular Filling: Atrial contraction completes ventricular filling.

Heart Sounds

Heart sounds are produced by the closing of valves and associated vibrations.

• First Sound ("lubb"): Closure of AV valves at the beginning of ventricular systole.

• Second Sound ("dupp"): Closure of semilunar valves at the beginning of ventricular diastole.

• Third Sound: Occasional; caused by turbulent blood flow into ventricles during early diastole.

Aortic Pressure Curve and Blood Pressure

Blood pressure in the aorta reflects the pressure generated by the left ventricle.

• Dicrotic Notch (Incisura): Brief rise in aortic pressure following closure of the aortic valve.

• Blood Pressure Measurement: Typically taken in the arm; reflects aortic, not ventricular, pressure.

Mean Arterial Pressure (MAP)

MAP is the average blood pressure in the arteries and is a key determinant of tissue perfusion.

• Formula:

$MAP = CO \times PR$

• CO (Cardiac Output): Amount of blood pumped by the heart per minute.

• SV (Stroke Volume): Blood pumped during each heartbeat.

• HR (Heart Rate): Number of heartbeats per minute.

$CO = SV \times HR$

• PR (Peripheral Resistance): Total resistance against which blood must be pumped.

• Cardiac Reserve: Difference between CO at rest and maximum CO.

Regulation of the Heart

Heart function is regulated by intrinsic and extrinsic mechanisms to meet the body's needs.

• Intrinsic Regulation: Based on the heart's own properties (e.g., Starling's law).

• Preload: Degree of stretch of ventricular walls; greater stretch increases force of contraction.

• Afterload: Pressure the ventricles must overcome to eject blood; less sensitive than preload.

• Extrinsic Regulation: Involves neural and hormonal control.

• Parasympathetic Stimulation: Vagus nerve decreases heart rate via acetylcholine.

• Sympathetic Stimulation: Cardiac nerves increase heart rate and force via norepinephrine and epinephrine.

• Hormonal Control: Adrenal medulla releases epinephrine and norepinephrine during stress or exercise.

Heart and Homeostasis

The heart responds to changes in blood pressure, blood chemistry, and body temperature to maintain homeostasis.

• Baroreceptors: Monitor blood pressure in the carotid arteries and aorta; send information to the medulla oblongata.

• Chemoreceptors: Detect changes in pH, CO2, and O2 levels; found in the hypothalamus, aorta, and carotids.

• Extracellular Ion Concentration: Changes in K+ can decrease heart rate.

• Body Temperature: Increased temperature raises heart rate; decreased temperature lowers it.

Effects of Aging on the Heart

Aging leads to gradual changes in heart function, which become more significant during physical stress.

• Left Ventricular Hypertrophy: Thickening of the left ventricular wall.

• Decreased Maximum Heart Rate: Limits exercise capacity.

• Valve Dysfunction and Arrhythmias: Increased risk with age.

• Increased Oxygen Demand: More energy required to maintain cardiac output.

Summary Table: Heart Valves

Valve	Location	Number of Cusps	Function
Tricuspid (Right AV)	Between right atrium and right ventricle	3	Prevents backflow into right atrium
Bicuspid (Mitral, Left AV)	Between left atrium and left ventricle	2	Prevents backflow into left atrium
Pulmonary Semilunar	Between right ventricle and pulmonary trunk	3	Prevents backflow into right ventricle
Aortic Semilunar	Between left ventricle and aorta	3	Prevents backflow into left ventricle

Additional info: Some explanations and context have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.