Cardiac Anatomy and Physiology

Heart Position and Structure

The heart is a muscular organ located in the thoracic cavity, slightly left of the midline, and protected by the sternum and ribs. It is responsible for pumping blood throughout the body.

• Position: Sits in the thoracic cavity, slightly left of center.

• Double Pump System: The heart consists of two pumps: the right side pumps deoxygenated blood to the lungs (pulmonary circuit), and the left side pumps oxygenated blood to the body (systemic circuit).

Pericardium and Heart Wall Layers

The heart is surrounded by the pericardium, which protects and anchors it. The heart wall consists of three main layers, each with a unique function.

• Pericardium:

  • Fibrous pericardium: Tough outer layer anchoring the heart.

  • Serous pericardium: Includes parietal (outer) and visceral (inner, also called epicardium) layers.

• Heart Wall Layers:

  • Epicardium: Outermost layer, same as visceral pericardium.

  • Myocardium: Middle layer, composed of cardiac muscle tissue responsible for contractions.

  • Endocardium: Innermost layer, lining heart chambers and valves.

Right vs. Left Heart Chambers

The heart has four chambers: two atria and two ventricles, each with distinct roles in circulation.

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

• Right Ventricle: Pumps deoxygenated blood into the pulmonary trunk toward the lungs.

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

• Left Ventricle: Pumps oxygenated blood into the aorta for systemic circulation.

Cardiac Valves and Disorders

Heart Valves and Outflow

Valves ensure unidirectional blood flow and prevent backflow. Outflow valves (aortic and pulmonary) prevent blood from returning to the ventricles from arteries.

• Valve Disorders:

  • Valvular insufficiency: Valves fail to close properly, causing blood to leak backward.

  • Stenosis: Hardened valve cusps restrict blood flow, leading to heart enlargement and reduced efficiency.

• Septal Defects: Abnormal openings between heart chambers (atrial or ventricular), causing inefficient blood circulation.

Coronary Artery Disease (CAD), Myocardial Infarction (MI), Angina, and Ischemia

These conditions affect the heart's ability to receive and supply blood efficiently.

• CAD: Caused by buildup of fatty plaques in coronary arteries, reducing blood flow.

• MI: Occurs when a plaque ruptures, blocking blood flow and causing heart muscle death.

• Angina: Chest pain from reduced blood flow, often a symptom of CAD.

• Ischemia: Reduced oxygenation of heart muscle due to decreased blood flow.

Coronary Sinus Function

The coronary sinus is a large venous structure on the posterior heart, draining deoxygenated blood from the myocardium into the right atrium.

• Receives blood from several cardiac veins, ensuring efficient removal of metabolic waste.

End Systolic Volume (ESV)

ESV is the volume of blood remaining in the ventricle at the end of contraction (systole).

• Increased contractility or decreased afterload reduces ESV, improving heart efficiency.

• Decreased contractility or increased afterload increases ESV, reducing efficiency.

Major Arteries and Circulation

Aortic Arch and Branches

The aortic arch curves from the heart and gives rise to three major branches supplying the head, neck, and upper limbs.

• Brachiocephalic Artery: Splits into right common carotid and right subclavian arteries.

• Left Common Carotid Artery: Supplies left head and neck.

• Left Subclavian Artery: Supplies left upper limb and thorax.

• Descending Thoracic and Abdominal Aorta: Supplies thoracic and abdominal organs.

Cardiac Muscle Structure and Function

Unique Cardiac Muscle Structures

Cardiac muscle cells have specialized structures for efficient contraction and electrical conduction.

• Intercalated Discs: Contain gap junctions and desmosomes for electrical and physical connectivity.

• Spiral Arrangement: Muscle cells arranged spirally for wringing contraction.

• Pacemaker Cells: Generate action potentials independently, maintaining heart rhythm.

• Fibrous Skeleton: Provides structural support and electrical insulation.

Plateau Phase Importance

The plateau phase prolongs contraction and depolarization in contractile cells, allowing effective blood pumping.

• Calcium channels open, calcium enters, and potassium exits, balancing the membrane potential.

Local Potentials in Pacemaker and Contractile Cells

Pacemaker cells generate spontaneous action potentials due to HCN channels, while contractile cells rely on rapid depolarization and plateau phases.

• Pacemaker Potential: Slow depolarization via HCN channels, leading to action potential.

• Contractile Cell Potential: Rapid depolarization (Na+ influx), plateau (Ca2+ influx, K+ efflux), repolarization (K+ efflux).

Key Equations:

Cardiac Conduction System

Pacemaker Cell Regulation

Pacemaker cells are regulated by the autonomic nervous system (ANS):

• Sympathetic: Increases heart rate and contractility via norepinephrine.

• Parasympathetic: Decreases heart rate via acetylcholine.

Conduction System Components

The cardiac conduction system coordinates heartbeats through specialized structures.

• Sinoatrial (SA) Node: Primary pacemaker, initiates heartbeat.

• Atrioventricular (AV) Node: Secondary pacemaker, delays impulse for ventricular filling.

• AV Bundle (Bundle of His): Transmits impulse from AV node to ventricles.

• Right and Left Bundle Branches: Carry impulse down interventricular septum.

• Purkinje Fibers: Distribute impulse throughout ventricles for coordinated contraction.

Cardiac Cycle and Heart Sounds

ECG Segments and Cardiac Cycle Phases

Electrocardiogram (ECG) segments correspond to electrical activity and phases of the cardiac cycle.

• P Wave: Atrial depolarization.

• QRS Complex: Ventricular depolarization.

• T Wave: Ventricular repolarization.

• R-R Interval: Entire duration of cardiac action potential.

• P-R Interval: Duration of atrial depolarization and AV node delay.

• Q-T Interval: Duration of ventricular action potential.

• S-T Segment: Ventricular plateau phase.

Valve Activity and Pressure Changes

Valve opening and closing are driven by pressure changes in heart chambers during the cardiac cycle.

• Ventricular Filling: AV valves open, SL valves closed.

• Isovolumetric Contraction: AV valves close, SL valves closed.

• Ventricular Ejection: SL valves open, AV valves closed.

• Isovolumetric Relaxation: SL valves close, AV valves closed.

Heart Sounds:

• S1 "LUB": Closure of AV valves at start of isovolumetric contraction.

• S2 "DUB": Closure of SL valves at start of isovolumetric relaxation.

Factors Influencing Preload, Afterload, and Contractility

These factors determine stroke volume and cardiac output.

• Preload: Related to end-diastolic volume (EDV); increased venous return increases preload.

• Contractility: Increased contractility increases stroke volume, decreases ESV.

• Afterload: Resistance the heart must overcome; increased afterload decreases stroke volume.

Blood Vessels and Circulation

Types of Blood Vessels

Blood vessels are classified by structure and function.

• Arteries: Thick-walled, high-pressure vessels carrying blood away from the heart.

• Arterioles: Small arteries controlling blood flow to capillaries.

• Capillaries: Thin-walled vessels for exchange of gases, nutrients, and waste.

• Venules: Small veins collecting blood from capillaries.

• Veins: Low-pressure vessels returning blood to the heart.

Vessel Diameter, Cross-Sectional Area, and Blood Pressure

Vessel diameter and cross-sectional area affect resistance and blood flow.

• Larger diameter reduces resistance, increasing flow.

• Smaller diameter increases resistance, decreasing flow.

• Blood pressure is highest in arteries, drops through capillaries, and is lowest in veins.

Blood Pressure Changes Across Vessels

Blood pressure decreases as blood moves from arteries to veins.

• Arteries: Highest pressure (approx. 120/80 mm Hg).

• Capillaries: Pressure drops significantly (approx. 35 mm Hg to 15 mm Hg).

• Veins: Lowest pressure (approx. 5 mm Hg in venules, near 0 mm Hg in vena cava).

Venous Return Mechanisms

Venous return is aided by skeletal muscle pump and respiratory pump.

• Skeletal Muscle Pump: Muscle contractions squeeze veins, pushing blood toward the heart.

• Respiratory Pump: Breathing changes thoracic pressure, aiding venous return.

Arterioles and Blood Flow Regulation

Arterioles regulate blood flow and systemic arterial pressure through vasoconstriction and vasodilation.

• Vasoconstriction: Reduces blood flow, increases resistance and pressure.

• Vasodilation: Increases blood flow, decreases resistance and pressure.

• Autoregulation: Local mechanisms adjust vessel diameter based on tissue needs.

Endocrine and Nervous Regulation of Blood Pressure

Blood pressure is regulated by hormones and the autonomic nervous system.

• Increase: Thyroid hormone, epinephrine, norepinephrine, angiotensin II, ADH, aldosterone.

• Decrease: Atrial natriuretic peptide (ANP), parasympathetic activity.

Baroreceptor Reflex

Baroreceptors detect changes in blood pressure and initiate reflexes to maintain homeostasis.

• Located in carotid arteries and aorta.

• Send signals to medulla oblongata to adjust heart rate and vessel diameter.

Capillary Exchange and Fluid Balance

Types of Capillaries

Capillaries are specialized for exchange and vary in permeability.

• Continuous Capillaries: Tight junctions, least permeable, found in skin and muscle.

• Fenestrated Capillaries: Pores for increased permeability, found in kidneys and intestines.

• Sinusoidal Capillaries: Large gaps, most permeable, found in liver and bone marrow.

Autoregulation of Blood Perfusion

Local mechanisms adjust blood flow to meet tissue metabolic needs.

• Myogenic Mechanism: Vascular smooth muscle responds to pressure changes.

• Metabolic Control: Responds to tissue oxygen, CO2, and metabolite levels.

Capillary Exchange: Hydrostatic and Osmotic Pressure

Fluid movement across capillaries is determined by hydrostatic and colloid osmotic pressures.

• Hydrostatic Pressure (HP): Pushes fluid out of capillaries.

• Colloid Osmotic Pressure (COP): Pulls fluid into capillaries due to plasma proteins.

• Net Filtration Pressure (NFP): Balance between HP and COP determines net fluid movement.

Example Equation:

Edema and Fluid Imbalance

Decreased colloid osmotic pressure (e.g., from low plasma protein) leads to fluid accumulation in tissues (edema).

Special Circulatory Pathways

Hepatic Portal System

The hepatic portal system directs blood from digestive organs to the liver for processing before returning to the heart.

• Includes splenic, gastric, superior and inferior mesenteric veins joining the hepatic portal vein.

• Blood passes through liver sinusoids for detoxification and nutrient processing.

Major Pulse Points

Pulse points are locations where arterial pulsations can be felt.

Clinical Conditions: Hypotension and Shock

Types of Shock

Shock is a life-threatening condition due to inadequate tissue perfusion.

Summary Equations and Calculations

Additional info: Some explanations and definitions have been expanded for clarity and completeness. Table entries and equations are inferred from standard Anatomy & Physiology curriculum.