Heart Anatomy and Physiology, Structure and Layers of the Heart, Cardiac Cycle, Heart’s Electrical Conduction System, Starling’s Law of the Heart


Heart

1. Location and Size

  • The heart is situated in the mediastinum (central compartment of the thoracic cavity), slightly tilted to the left.
  • It is about the size of a closed fist and weighs approximately 250–300 grams in adults.

2. Structure and Layers of the Heart

  • Layers:
    • Pericardium: A double-layered sac that encases the heart.
      • Outer fibrous pericardium protects and anchors the heart.
      • Inner serous pericardium consists of the parietal and visceral (epicardium) layers.
    • Epicardium: The outermost layer of the heart wall, part of the serous pericardium.
    • Myocardium: The thick, muscular middle layer responsible for contractions. It contains cardiac muscle fibers arranged in a spiral pattern.
    • Endocardium: The innermost layer, which lines the chambers and valves, and is continuous with blood vessels.

3. Heart Chambers

  • Right Atrium: Receives deoxygenated blood from the superior and inferior vena cava, as well as from the coronary sinus.
  • Right Ventricle: Pumps deoxygenated blood into the pulmonary arteries leading to the lungs.
  • Left Atrium: Receives oxygenated blood from the pulmonary veins.
  • Left Ventricle: The thickest chamber, it pumps oxygenated blood into the systemic circulation via the aorta.

4. Heart Valves

  • Valves ensure unidirectional blood flow:
    • Atrioventricular Valves: Tricuspid (right) and Mitral (left) valves prevent backflow into the atria.
    • Semilunar Valves: Pulmonary and Aortic valves prevent backflow into the ventricles.

5. Associated Blood Vessels

  • Arteries:
    • Aorta: Largest artery, carrying oxygenated blood to the body.
    • Pulmonary arteries: Transport deoxygenated blood to the lungs.
  • Veins:
    • Pulmonary veins: Bring oxygenated blood from the lungs to the heart.
    • Superior & Inferior vena cava: Bring deoxygenated blood from the body to the right atrium.

Physiology of the Heart

1. Blood Flow Through the Heart

The heart pumps blood in a loop system:

  • Pulmonary Circuit:
    1. Deoxygenated blood enters the right atrium via the superior and inferior vena cava.
    2. Blood flows to the right ventricle through the tricuspid valve.
    3. The right ventricle pumps blood to the lungs through the pulmonary valve and pulmonary arteries.
  • Systemic Circuit:
    1. Oxygenated blood enters the left atrium via the pulmonary veins.
    2. Blood flows to the left ventricle through the mitral valve.
    3. The left ventricle pumps oxygenated blood to the body via the aortic valve and aorta.

2. Cardiac Cycle

The cardiac cycle consists of two phases:

  • Diastole:
    • Chambers relax and fill with blood.
    • Atrioventricular valves (tricuspid and mitral) open, allowing blood flow from atria to ventricles.
  • Systole:
    • Ventricles contract, ejecting blood into the pulmonary artery and aorta.
    • Semilunar valves (pulmonary and aortic) open during this phase.

3. Heart’s Electrical Conduction System

The heart generates and propagates its own electrical impulses for coordinated contractions:

  • Sinoatrial (SA) Node: The primary pacemaker, generates impulses at 60–100 beats per minute.
  • Atrioventricular (AV) Node: Delays the impulse slightly to allow atrial contraction before ventricular contraction.
  • Bundle of His: Carries the signal to the interventricular septum.
  • Purkinje Fibers: Spread the impulse throughout the ventricles for synchronized contraction.

4. Coronary Circulation

The heart is supplied with oxygen and nutrients by the coronary arteries:

  • Right Coronary Artery: Supplies the right atrium, right ventricle, and part of the septum.
  • Left Coronary Artery: Divides into the left anterior descending and circumflex arteries, supplying the left atrium, left ventricle, and septum.

5. Heart Rate and Regulation

  • Normal Heart Rate: 60–100 beats per minute.
  • Regulators:
    • Sympathetic Nervous System: Increases heart rate and force of contraction during stress or physical activity.
    • Parasympathetic Nervous System: Reduces heart rate during rest.
    • Hormonal Influence: Epinephrine and norepinephrine stimulate increased heart rate.

6. Starling’s Law of the Heart

  • The heart's ability to increase stroke volume in response to increased venous return ensures that the amount of blood ejected matches the volume entering the heart.

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