Cardiovascular System: Heart Structures and Function Notes

Anatomy and overall function of the heart

  • The heart is a hollow, muscular, cone-shaped organ that continuously pumps blood through the pulmonary (to the lungs) and systemic (to the rest of the body) circulations.
  • It has four chambers: two atria (upper) and two ventricles (lower).
    • Right atrium (RA)
    • Left atrium (LA)
    • Right ventricle (RV)
    • Left ventricle (LV)
  • Blood flow pathway (starting with venous return): blood enters the right atrium, passes to the right ventricle, is pumped to the lungs via the pulmonary trunk, returns from the lungs via pulmonary veins to the left atrium, moves to the left ventricle, and is ejected to the body via the aorta.
  • Heart vessels and major routes: aorta, pulmonary trunk, pulmonary arteries, pulmonary veins, coronary arteries, coronary sinus (drains deoxygenated blood from the heart wall back to the right atrium).
  • The heart sits in the thoracic cavity within the mediastinum, posterior to the sternum, between the second and sixth ribs, with the apex resting on the diaphragm. About two-thirds of the heart lies to the left of the midline.
  • The base (upper border) lies just below the second rib and is where major vessels enter and exit.
  • Clinically, enlargement or shifts in the heart’s size/position can indicate diseases (e.g., hypertension or valve disease).

Heart layers and pericardial structure

  • Three main layers (heart walls):
    • Epicardium (outermost layer, also the visceral layer of the serous pericardium) – contains adipose tissue and a network of coronary vessels.
    • Myocardium – thick muscular middle layer composed of cardiac muscle cells (cardiac myocytes). This is the contractile layer responsible for pumping.
    • Endocardium – innermost epithelial lining of the heart chambers providing a smooth, frictionless surface for blood flow.
  • Pericardial sac and its layers:
    • Fibrous pericardium – most superficial protective layer.
    • Serous pericardium – consists of parietal layer (lining the inner surface of the fibrous pericardium) and visceral layer, which is the epicardium (covers the heart).
    • Epicardial adipose tissue – yellow fat layer that contains areolar connective tissue, adipocytes, and houses coronary vessels.
  • Coronary vessels are located within the epicardial fat; they supply the heart tissue and drain deoxygenated blood back toward the heart.

Wall thickness and ventricular differences

  • Both ventricles hold a similar blood volume ~60-100 ext{ mL} per beat (stroke volume tends to be in this range per ventricle).
  • Left ventricle (LV) is thicker than the right ventricle (RV) because it generates higher pressure to pump blood into the systemic circulation via the aorta.
  • The LV has a more rounded shape; the RV is more pouch-like.
  • The interventricular septum divides the two ventricles.
  • Higher pressure requires a thicker wall; lower pressure (RV pumping to the lungs) allows a thinner wall.

Chambers, valves, and their function

  • Four valves ensure unidirectional blood flow:
    • Atrioventricular (AV) valves: tricuspid (between RA and RV) and bicuspid/mitral (between LA and LV).
    • Semilunar valves: pulmonary valve (between RV and pulmonary trunk) and aortic valve (between LV and aorta).
  • Valve anatomy:
    • Tricuspid valve has three cusps (tricuspid) and is between the RA and RV.
    • Bicuspid/Mitral valve is between the LA and LV.
    • Pulmonary valve is between the RV and pulmonary trunk.
    • Aortic valve is between the LV and the aorta.
  • Blood flow and valve states during the cardiac cycle:
    • Atrial contraction (diastole initiation for filling): AV valves (tricuspid and bicuspid) are open; semilunar valves (pulmonary and aortic) are closed, preventing backflow into the ventricles.
    • Ventricular contraction (systole): AV valves close to prevent backflow into the atria; semilunar valves open to eject blood to the lungs (RV to pulmonary trunk) and to the body (LV to aorta).
  • The sequence of events ensures one-way flow and coordinated pumping.

Pathway of blood and ventricular dynamics

  • Complete pathway (with valves): RA → (tricuspid) RV → (pulmonary valve) pulmonary trunk → lungs via pulmonary arteries/veins → LA → (bicuspid) LV → (aortic valve) aorta → systemic circulation.
  • The systolic and diastolic chamber events correspond to valve closures:
    • Systole: AV valves closing (first heart sound, S1).
    • Diastole: semilunar valves closing (second heart sound, S2).

Heart sounds and clinical relevance

  • Heart sounds reflect valve function and blood flow dynamics:
    • Systolic sound (first sound, S1) – caused by contraction of the ventricles and the closure of the AV valves (tricuspid and bicuspid).
    • Diastolic sound (second sound, S2) – a short, sharp sound caused by the closure of the semilunar valves (aortic and pulmonary) at the start of diastole.
  • These sounds provide clinical information about valve function and hemodynamics.

Cardiac output and blood volume basics

  • Cardiac output (CO) is the product of stroke volume (SV) and heart rate (HR):
    • CO = SV imes HR
  • Typical stroke volume: $$SV \