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How Your Heart & Cardiovascular System Works

Understanding the powerful pump that keeps blood flowing through your entire body

The Heart - Your Body's Pump

Your heart is a powerful muscle about the size of your fist. It's located in the center of your chest, slightly to the left. Think of it as a pump that never stops working!

Heart Structure

  • Four chambers: Two atria (upper) and two ventricles (lower)
  • Valves: One-way doors that keep blood flowing in the right direction
  • Muscle walls: Strong muscle tissue that contracts to pump blood
  • Electrical system: Controls the heartbeat rhythm

How Blood Circulation Works

Your heart pumps blood through two main circuits:

Pulmonary Circuit (Lungs)

  1. Right side of heart pumps blood to lungs
  2. Blood picks up oxygen and releases carbon dioxide
  3. Oxygen-rich blood returns to left side of heart

Systemic Circuit (Body)

  1. Left side of heart pumps oxygen-rich blood to body
  2. Blood delivers oxygen and nutrients to all organs
  3. Blood picks up waste products and carbon dioxide
  4. Oxygen-poor blood returns to right side of heart

The Heartbeat

Your heart beats about 100,000 times per day! Here's what happens with each beat:

  1. Relaxation (Diastole): Heart chambers fill with blood
  2. Contraction (Systole): Heart chambers squeeze and pump blood out
  3. Valves close: Prevent blood from flowing backward
  4. Cycle repeats: This happens continuously

Blood Vessels - The Delivery System

Arteries

  • Carry oxygen-rich blood away from heart
  • Have thick, muscular walls
  • Branch into smaller vessels

Capillaries

  • Tiny vessels where exchange happens
  • Deliver oxygen and nutrients to cells
  • Pick up waste products

Veins

  • Carry oxygen-poor blood back to heart
  • Have valves to prevent backflow
  • Thinner walls than arteries

Blood Pressure

Blood pressure is the force of blood pushing against artery walls:

  • Systolic pressure: When heart contracts (top number)
  • Diastolic pressure: When heart relaxes (bottom number)
  • Normal range: Less than 120/80 mmHg

What Blood Does

Your blood is like a delivery service that carries:

  • Oxygen: From lungs to all body parts
  • Nutrients: From digestive system to cells
  • Hormones: Chemical messengers throughout body
  • Immune cells: Fight infections and diseases
  • Waste products: To kidneys and lungs for removal

Heart Rate

Your heart rate (pulse) varies based on activity:

  • Resting: 60-100 beats per minute
  • Exercise: Can increase to 150+ beats per minute
  • Sleep: Slows down to conserve energy
  • Stress: Increases due to adrenaline

Detailed Anatomy

Heart Chambers

  • Right Atrium: Receives deoxygenated blood from superior and inferior vena cava; pumps blood to right ventricle through tricuspid valve
  • Right Ventricle: Pumps deoxygenated blood to lungs via pulmonary artery; has thinner walls than left ventricle
  • Left Atrium: Receives oxygenated blood from pulmonary veins; pumps blood to left ventricle through mitral valve
  • Left Ventricle: Pumps oxygenated blood to entire body via aorta; has thickest muscle walls (3x thicker than right ventricle) to generate high pressure

Heart Valves

  • Tricuspid Valve: Between right atrium and ventricle; prevents backflow of blood
  • Pulmonary Valve: Between right ventricle and pulmonary artery; opens during ventricular contraction
  • Mitral Valve (Bicuspid): Between left atrium and ventricle; prevents backflow to left atrium
  • Aortic Valve: Between left ventricle and aorta; opens during ventricular contraction to allow blood flow to body

Heart Layers

  • Endocardium: Inner lining of heart chambers and valves; smooth surface prevents blood clotting
  • Myocardium: Middle layer of cardiac muscle; contracts to pump blood; thickest in left ventricle
  • Epicardium: Outer layer; also called visceral pericardium; protects heart and contains blood vessels
  • Pericardium: Protective sac surrounding heart; contains fluid to reduce friction

Coronary Circulation

  • Coronary Arteries: Supply oxygen-rich blood to heart muscle itself; branch from aorta
  • Right Coronary Artery: Supplies right atrium, right ventricle, and part of left ventricle
  • Left Coronary Artery: Branches into left anterior descending and circumflex arteries; supplies most of left ventricle
  • Coronary Veins: Drain deoxygenated blood from heart muscle; empty into right atrium via coronary sinus

Blood Vessel Structure

  • Arteries: Three layers - tunica intima (inner), tunica media (muscle), tunica adventitia (outer); elastic to handle pressure
  • Arterioles: Small arteries that regulate blood flow; can constrict or dilate to control pressure
  • Capillaries: Single layer of cells; site of gas and nutrient exchange; smallest blood vessels
  • Venules: Small veins that collect blood from capillaries
  • Veins: Thinner walls than arteries; contain valves to prevent backflow; carry blood back to heart

Detailed Physiology

Cardiac Cycle

The complete heartbeat from one contraction to the next (about 0.8 seconds at rest):

  1. Atrial Systole: Atria contract, pushing blood into ventricles; lasts about 0.1 seconds
  2. Ventricular Systole: Ventricles contract, pumping blood out; AV valves close (lub sound); lasts about 0.3 seconds
  3. Isovolumetric Relaxation: All valves closed; ventricles relax but don't fill yet; lasts about 0.05 seconds
  4. Ventricular Diastole: Ventricles fill with blood; semilunar valves close (dub sound); lasts about 0.4 seconds

Electrical Conduction System

Specialized cells that generate and conduct electrical impulses:

  1. Sinoatrial (SA) Node: Natural pacemaker; generates electrical impulses (60-100 per minute); located in right atrium
  2. Atrioventricular (AV) Node: Delays impulse slightly to allow atria to contract first; located between atria and ventricles
  3. Bundle of His: Conducts impulse from AV node to ventricles
  4. Bundle Branches: Left and right branches carry impulse down ventricles
  5. Purkinje Fibers: Spread impulse throughout ventricular muscle; cause coordinated contraction

Hemodynamics (Blood Flow Physics)

  • Cardiac Output: Amount of blood pumped per minute (about 5 liters at rest); equals heart rate × stroke volume
  • Stroke Volume: Amount of blood pumped per beat (about 70ml at rest)
  • Blood Pressure: Force exerted by blood on vessel walls; highest in arteries, lowest in veins
  • Resistance: Opposition to blood flow; affected by vessel diameter, length, and blood viscosity
  • Poiseuille's Law: Blood flow is proportional to pressure difference and inversely proportional to resistance

Regulation of Heart Function

  • Autonomic Nervous System:
    • Sympathetic: Increases heart rate and contractility (fight or flight)
    • Parasympathetic: Decreases heart rate (rest and digest)
  • Hormonal Control:
    • Epinephrine/Norepinephrine: Increase heart rate and contractility
    • Thyroid hormones: Increase heart rate and cardiac output
  • Intrinsic Factors:
    • Frank-Starling Law: More blood in ventricle = stronger contraction
    • Stretch receptors: Detect blood volume changes

Blood Composition and Function

  • Plasma: Liquid portion (55% of blood); contains water, proteins, nutrients, hormones, waste
  • Red Blood Cells (Erythrocytes): Carry oxygen via hemoglobin; 4-6 million per microliter; live about 120 days
  • White Blood Cells (Leukocytes): Fight infection; 4,000-11,000 per microliter; several types with different functions
  • Platelets (Thrombocytes): Help with blood clotting; 150,000-400,000 per microliter

Why It's Critical

Without your heart and cardiovascular system, your body wouldn't get the oxygen and nutrients it needs to survive. Every cell in your body depends on this delivery system!

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