Comprehensive Study Notes: The Heart (Anatomy & Physiology)

The Heart: Structure, Function, and Physiology

Introduction to the Heart

The heart is a muscular organ essential for maintaining homeostasis by pumping blood throughout the body, delivering oxygen and nutrients, and removing waste products.

• Homeostatic Need: The cardiovascular system provides the critical function of transporting substances necessary for cellular metabolism and maintaining internal balance.

Anatomical Location and Orientation of the Heart

The heart is centrally located in the thoracic cavity, specifically within the mediastinum, and is oriented with its apex pointing downward and to the left.

• Mediastinum: The central compartment of the thoracic cavity, between the lungs, containing the heart, trachea, esophagus, and major vessels.

• Apex vs. Base: The apex is the pointed lower tip of the heart, while the base is the broad superior portion where major vessels attach.

Functions of the Heart

The heart acts as a double pump, circulating blood through two distinct pathways.

• Pulmonary Circulation: Carries deoxygenated blood from the right side of the heart to the lungs for oxygenation.

• Systemic Circulation: Delivers oxygenated blood from the left side of the heart to the rest of the body.

Gross Anatomy of the Heart

Layers of the Heart Wall

The heart wall consists of three layers, each with distinct structure and function.

• Epicardium: The outermost layer, also known as the visceral pericardium.

• Myocardium: The thick, muscular middle layer responsible for contraction.

• Endocardium: The innermost layer lining the heart chambers.

Pericardial Sac

The heart is enclosed in a protective sac called the pericardium, which consists of two main layers.

• Fibrous Pericardium: Tough, outer layer that anchors and protects the heart.

• Serous Pericardium: Thin, double-layered membrane with two parts:

  • Parietal Pericardium: Lines the inner surface of the fibrous pericardium.

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

Chambers of the Heart

The heart contains four chambers: two atria (superior) and two ventricles (inferior).

• Atria: Thin-walled chambers that receive blood returning to the heart.

• Ventricles: Thick-walled chambers that pump blood out of the heart.

• Internal Features: Septa (walls) and valves separate chambers; the interatrial septum divides atria, and the interventricular septum divides ventricles.

• External Features: Sulci (grooves) mark boundaries between chambers.

• Valves: Atrioventricular (AV) valves separate atria and ventricles; semilunar valves separate ventricles from major arteries.

Structure and Function of the Atria

• Auricles: Ear-like extensions that increase atrial volume.

• Fossa Ovalis: Remnant of fetal circulation, visible in the right atrium.

Structure and Function of the Ventricles

• Trabeculae Carneae: Muscular ridges on the inner walls.

• Papillary Muscles: Attach to AV valve cusps via chordae tendineae, preventing valve prolapse.

Valves of the Heart

Heart valves ensure unidirectional blood flow by opening and closing in response to pressure changes.

• Function: When open, valves allow blood flow; when closed, they prevent backflow.

• AV Valves:

  • Tricuspid Valve: Right AV valve, three cusps.

  • Bicuspid (Mitral) Valve: Left AV valve, two cusps.

• Semilunar Valves:

  • Aortic Semilunar Valve: Between left ventricle and aorta.

  • Pulmonary Semilunar Valve: Between right ventricle and pulmonary trunk.

• Heart Sounds: "Lub" (closure of AV valves), "Dup" (closure of semilunar valves).

Blood Flow Through the Heart

Blood flows through the heart in a specific sequence, passing through chambers and valves.

• Right atrium → tricuspid valve → right ventricle → pulmonary semilunar valve → pulmonary trunk → lungs

• Lungs → left atrium → bicuspid (mitral) valve → left ventricle → aortic semilunar valve → aorta → body

Cardiac Muscle Tissue

Cardiac muscle cells are specialized for contraction and electrical conduction.

• Striated: Contains sarcomeres, similar to skeletal muscle.

• 1-2 Centrally Located Nuclei: Most cells have one nucleus, some have two.

• Short Branching Cells: Allows for networked connections.

• Intercalated Discs: Specialized junctions for rapid electrical communication and mechanical strength.

• Intercalated Disc Composition: Made of desmosomes (mechanical connection) and gap junctions (electrical connection).

Conducting System of the Heart

The heart's conducting system coordinates contraction through specialized autorhythmic cells.

• Autorhythmic Cells: Generate action potentials without nervous input.

• Components:

  1. Sinoatrial (SA) Node: Pacemaker, initiates heartbeat.

  2. Atrioventricular (AV) Node: Delays impulse, allows atria to contract before ventricles.

  3. Atrioventricular Bundle (Bundle of His): Conducts impulse from AV node to ventricles.

  4. Bundle Branches: Carry impulse through interventricular septum.

  5. Purkinje Fibers: Distribute impulse to ventricular muscle.

Coronary Circulation

The heart requires its own blood supply to meet metabolic demands.

• Coronary Arteries:

  • Right Coronary Artery

  • Left Coronary Artery

• Coronary Veins:

  • Great Cardiac Vein

  • Middle Cardiac Vein

  • Small Cardiac Vein

  • Coronary Sinus: Collects venous blood and drains into right atrium.

Autorhythmic Cells and Pacemaker Potential

Autorhythmic cells generate spontaneous action potentials, setting the heart's rhythm.

• Pacemaker Potential: Gradual depolarization due to slow Na+ influx.

• Depolarizing Phase: Rapid Ca2+ influx through voltage-gated channels.

• Repolarizing Phase: K+ efflux restores resting potential.

Key Equations:

• Resting Membrane Potential: $V_{m} = -60 \text{ to } -70 \text{ mV}$

• Threshold Potential: $V_{th} \approx -40 \text{ mV}$

Cardiac Contractile Cells

Contractile cells produce the force of heart contraction and have a distinct action potential profile.

• Resting Membrane Potential (RMP): Typically around $-90 \text{ mV}$.

• Action Potential Phases:

  1. Depolarization: Na+ influx via voltage-gated channels.

  2. Plateau Phase: Ca2+ influx maintains depolarization.

  3. Repolarization: K+ efflux restores RMP.

Key Equation:

• Action Potential: $I_{Na^+} \rightarrow I_{Ca^{2+}} \rightarrow I_{K^+}$

Summary Table: Heart Wall and Pericardium

Layer	Location	Function
Epicardium (Visceral Pericardium)	Outermost heart wall	Protection, lubrication
Myocardium	Middle heart wall	Contraction, pumping blood
Endocardium	Innermost heart wall	Lines chambers, reduces friction
Fibrous Pericardium	Outer pericardial sac	Anchors, protects heart
Parietal Pericardium	Inner surface of fibrous pericardium	Secretes serous fluid
Visceral Pericardium	Directly on heart surface	Secretes serous fluid

Summary Table: Heart Valves

Valve	Location	Function
Tricuspid (Right AV)	Between right atrium and ventricle	Prevents backflow to atrium
Bicuspid (Mitral, Left AV)	Between left atrium and ventricle	Prevents backflow to atrium
Pulmonary Semilunar	Between right ventricle and pulmonary trunk	Prevents backflow to ventricle
Aortic Semilunar	Between left ventricle and aorta	Prevents backflow to ventricle

Summary Table: Cardiac Conducting System

Component	Location	Function
Sinoatrial (SA) Node	Right atrium	Pacemaker, initiates impulse
Atrioventricular (AV) Node	Interatrial septum	Delays impulse
Bundle of His	Interventricular septum	Conducts impulse to ventricles
Bundle Branches	Interventricular septum	Carry impulse to apex
Purkinje Fibers	Ventricular walls	Distribute impulse to muscle

Additional info: Tables and explanations have been expanded for clarity and completeness. All key terms and processes are described to ensure self-contained study notes suitable for college-level Anatomy & Physiology students.