Anatomy & Physiology: The Heart and Cardiovascular System

The Heart and Cardiovascular System

Overview of the Heart

The heart is a muscular organ responsible for pumping blood throughout the body via two main circuits: the pulmonary and systemic circuits. It is located in the thoracic cavity, slightly left of the midline, and is surrounded by protective membranes.

• Location: Between the 2nd and 5th intercostal spaces, left of the midsternal line, surrounded by lungs.

• Pericardium: The heart is enclosed by a double-walled sac called the pericardium.

• Layers: Outermost layer is the fibrous pericardium; inner layer is the serous pericardium (parietal and visceral layers).

• Epicardium: The visceral layer of the serous pericardium, on the external heart surface.

Pulmonary and Systemic Circuits

The heart pumps blood through two distinct circuits: the pulmonary circuit (to the lungs) and the systemic circuit (to the rest of the body). Each circuit has unique characteristics and functions.

• Pulmonary Circuit: Carries oxygen-poor blood from the right ventricle to the lungs for oxygenation, then returns oxygen-rich blood to the left atrium.

• Systemic Circuit: Delivers oxygen-rich blood from the left ventricle to body tissues, then returns oxygen-poor blood to the right atrium.

• Key Point: Arteries carry blood away from the heart; veins carry blood towards the heart.

Chambers of the Heart

The heart consists of four chambers: two atria and two ventricles, each with specific roles in blood circulation.

• Atria: The right atrium receives blood from the systemic circuit; the left atrium receives blood from the pulmonary circuit. Atria attract blood.

• Ventricles: The right ventricle pumps blood through the pulmonary circuit; the left ventricle pumps blood through the systemic circuit. Ventricles eject blood away from the heart.

Flow of Blood Through the Heart

Blood follows a specific pathway through the heart and associated vessels, ensuring efficient oxygenation and nutrient delivery.

1. Left ventricle (starting point)

2. Aorta and branches

3. Capillary beds of all body tissues (gas exchange)

4. Vena cava

5. Right atrium

6. Right ventricle

7. Pulmonary arteries

8. Capillary beds of lungs (gas exchange)

9. Pulmonary veins

10. Left atrium → back into left ventricle

Heart Valves

Valves ensure unidirectional blood flow and prevent backflow. They open and close in response to pressure changes within the heart chambers.

• Atrioventricular (AV) Valves: Separate atria from ventricles. Right AV (tricuspid) and left AV (bicuspid/mitral).

• Semilunar (SL) Valves: Separate ventricles from major arteries. Pulmonary valve (right ventricle to pulmonary trunk) and aortic valve (left ventricle to aorta).

• Chordae tendineae: "Heart strings" anchor valve cusps to papillary muscles, preventing prolapse.

Valve Function and Heart Sounds

Valves open and close based on pressure differences. Their closure produces characteristic heart sounds.

• AV Valves: Open when atrial pressure > ventricular pressure; close when ventricular pressure > atrial pressure (produces "lub" sound).

• SL Valves: Open when ventricular pressure > arterial pressure; close when arterial pressure > ventricular pressure (produces "dup" sound).

Cardiac Cycle

The cardiac cycle includes all events associated with blood flow during one heartbeat, including periods of contraction (systole) and relaxation (diastole).

• Systole: Period of contraction.

• Diastole: Period of relaxation.

• Isovolumetric phases: All four valves are closed; no blood flow occurs.

Cardiac Output and Stroke Volume

Cardiac output (CO) is the volume of blood pumped by each ventricle per minute. Stroke volume (SV) is the amount of blood ejected per beat.

• End Diastolic Volume (EDV): Volume of blood in ventricle after atria contract (~120 ml).

• End Systolic Volume (ESV): Volume of blood remaining after ventricular contraction (~50 ml).

• Stroke Volume (SV): $SV = EDV - ESV$

• Cardiac Output (CO): $CO = HR \times SV$

• Example: $75 \text{ beats/min} \times 70 \text{ ml/beat} = 5250 \text{ ml/min} = 5.25 \text{ L/min}$

Factors Affecting Stroke Volume

Three main factors influence stroke volume: preload, contractility, and afterload.

• Preload: Degree of stretch of cardiac muscle before contraction; increased venous return increases preload.

• Contractility: Strength of contraction at a given muscle length; increased by sympathetic stimulation and positive inotropic agents.

• Afterload: Pressure the ventricles must overcome to eject blood; increased afterload reduces stroke volume.

Coronary Circulation

Coronary circulation supplies blood to the heart muscle itself, ensuring its metabolic needs are met.

• Coronary arteries: Arise from the base of the aorta and encircle the heart.

• Blood delivery: Occurs when the heart is relaxed; most blood supply goes to the ventricles.

• Cardiac veins: Collect deoxygenated blood from myocardium and drain into the coronary sinus, which empties into the right atrium.

Cardiac Muscle Structure and Function

Cardiac muscle cells are specialized for continuous rhythmic contraction and are interconnected by unique junctions.

• Intercalated discs: Junctions between cells containing desmosomes (anchoring) and gap junctions (electrical coupling).

• Functional syncytium: Cardiac cells contract as a single coordinated unit.

• Mitochondria: Abundant, providing resistance to fatigue.

Comparison: Skeletal vs. Cardiac Muscle

Feature	Skeletal Muscle	Cardiac Muscle
Cell Shape	Long, cylindrical, multinucleated	Short, branched, 1-2 nuclei
Intercalated Discs	No	Yes
Gap Junctions	No	Yes
Sarcoplasmic Reticulum	Elaborate; triads present	Less elaborate; no triads
Calcium Source	SR only	SR and ECF
Pacemaker Cells	No	Yes
Action Potential Duration	Shorter than contraction	As long as contraction
Metabolism	Aerobic and anaerobic	Aerobic only

Electrical Activity of the Heart

The heart's rhythmic contractions are controlled by electrical impulses generated and conducted by specialized cells.

• Resting Membrane Potential: Voltage difference across cell membranes at rest (~-90 mV for cardiac cells).

• Depolarization: Rapid influx of Na+ or Ca2+ ions makes the cell interior more positive.

• Repolarization: Efflux of K+ ions restores negative membrane potential.

• Hyperpolarization: Membrane potential becomes more negative than resting value.

Intrinsic Conduction System

The heart's intrinsic conduction system coordinates the heartbeat through a sequence of electrical events.

1. Sinoatrial (SA) node: Pacemaker; initiates action potential in right atrium.

2. Internodal pathway: Conducts impulse toward left atrium and AV node.

3. Atrioventricular (AV) node: Delays impulse (~0.1 s) to allow atria to empty.

4. AV bundle (Bundle of His): Connects atria to ventricles.

5. Bundle branches: Conduct impulses down interventricular septum.

6. Purkinje fibers: Rapidly depolarize ventricular muscle cells.

Electrocardiogram (ECG)

An ECG records the heart's electrical activity and is used to diagnose cardiac function and rhythm abnormalities.

• P wave: Atrial depolarization.

• P-R interval: Time for impulse to travel from SA node through AV node.

• QRS complex: Ventricular depolarization and atrial repolarization.

• T wave: Ventricular repolarization.

Common Homeostatic Imbalances

Several conditions can disrupt normal heart function, leading to clinical symptoms and disease.

• Pericarditis: Inflammation of the pericardium, causing pain and creaking sounds.

• Cardiac tamponade: Fluid accumulation compresses the heart, limiting pumping ability.

• Valve disorders: Incompetent valves (fail to close) cause backflow; stenosis (fail to open) increases workload.

• Angina pectoris: Chest pain due to transient blood flow deficiency.

• Myocardial infarction: Heart attack; prolonged coronary blockage leads to cell death and scar tissue.

• Arrhythmias: Abnormal heart rhythms; can result from conduction system defects.

• Heart block: Impaired conduction between atria and ventricles.

• Bradycardia: Slow heart rate (<60 bpm).

• Tachycardia: Fast heart rate (>100 bpm).

• Congestive heart failure: Inefficient pumping leads to fluid accumulation in lungs (pulmonary congestion) or body organs (peripheral congestion).

Regulation of Heart Rate

The autonomic nervous system (ANS) modulates heart rate through sympathetic and parasympathetic pathways.

• Parasympathetic (PNS): Acetylcholine decreases heart rate via M2 receptors, increasing K+ efflux and hyperpolarizing pacemaker cells.

• Sympathetic (SNS): Norepinephrine and epinephrine increase heart rate via beta receptors, increasing Na+ and Ca2+ influx.

• Other factors: Exercise, stress, and hormones can alter heart rate.

Summary Table: Key Terms and Definitions

Term	Definition
Pericardium	Double-walled sac enclosing the heart
Epicardium	Visceral layer of serous pericardium
Systole	Contraction phase of the heart
Diastole	Relaxation phase of the heart
Stroke Volume (SV)	Blood ejected per beat ($SV = EDV - ESV$)
Cardiac Output (CO)	Blood pumped per minute ($CO = HR \times SV$)
Preload	Stretch of cardiac muscle before contraction
Contractility	Strength of contraction at a given length
Afterload	Pressure ventricles must overcome to eject blood

Additional info: Some content was inferred and expanded for clarity and completeness, including definitions, physiological mechanisms, and clinical relevance.