circulation

Introduction to Circulatory Systems

  • Circulatory systems play a crucial role in:

    • Circulating oxygen to tissues
    • Distributing nutrients and glucose

  • Focus on the relationship between circulatory and respiratory systems.

  • Comparison of circulatory systems across different animals, with an emphasis on mammals.

  • Future discussions will include links between circulatory and digestive systems.

Comparison of Circulatory Systems

Open vs Closed Circulatory Systems

  • Closed Circulatory System:

    • Blood is contained within vessels.
    • Blood circulates through a complete loop to tissues and back to the heart.
    • Example: Human circulatory system.

  • Open Circulatory System:

    • Blood leaves vessels and bathes tissues directly.
    • Blood and tissue fluid are mixed, referred to as hemolymph.
    • Common in most invertebrates.

  • Lymph is also a critical fluid surrounding the tissues in closed systems, moving in separate vessels.

Components of Blood

  • Blood is a connective tissue consisting of:
    • Red Blood Cells (RBCs): Transport oxygen.
    • White Blood Cells: Play a role in the immune system.
    • Platelets: Essential for blood clotting.
    • Plasma: Liquid component, making up 50–60% of blood volume; involved in nutrient exchange with tissues.

Blood Vessel Terminology

Heart Chambers

  • Atria (singular: Atrium):
    • Chambers that receive blood.
  • Ventricles:
    • Chambers that pump blood out.

Blood Vessels

  • Arteries: Carry blood away from the heart; largest is the aorta.

  • Veins: Return blood to the heart; largest are the vena cavae (superior and inferior).

  • Capillaries: Smallest vessels where nutrient and gas exchanges occur.

Pulmonary Circulation

  • Pulmonary System: Circulation specifically to and from the lungs.
    • Pulmonary Artery: Carries deoxygenated blood from the heart to lungs.
    • Pulmonary Vein: Carries oxygenated blood back to the heart.

Comparative Circulatory Systems in Vertebrates

Fish

  • Two-chambered heart (one atrium, one ventricle).
  • Simple single-loop circulation:
    • Heart → gills (oxygenation) → tissues → back to heart.

Amphibians and Reptiles

  • Three-chambered heart (two atria, one ventricle).
  • Incomplete separation of oxygenated and deoxygenated blood due to shared ventricle.
  • Allows for partial separation of circulatory loops.

Mammals and Birds

  • Four-chambered heart (two atria, two ventricles).
  • Complete separation of pulmonary and systemic circulation:
    • Low oxygen blood from body → right atrium → right ventricle → lungs (oxygenation) → left atrium → left ventricle → aorta → body.

Role of Ventricles and Atria

  • Coordinated contraction of the heart is crucial for effective circulation.

  • Cardiac Cycle:

    • Diastole (relaxation) and Systole (contraction) phases.
    • Atria contract first, followed by ventricles.

  • Heart cells can contract independently, but coordinated through connections (gap junctions).

Electrical Conduction in the Heart

  • Sinoatrial Node (SA Node): Primary pacemaker; initiates heartbeat.
  • Atrioventricular Node (AV Node): Introduces a delay; allows atria to fully empty before ventricles contract.
  • Electrical signals cause coordinated contraction via Purkinje fibers.

Blood Pressure and Blood Flow

  • Systolic Pressure: Highest pressure during ventricular contraction.
  • Diastolic Pressure: Lowest pressure during ventricular relaxation.
  • Blood pressure decreases as blood moves through the circulatory system, especially in capillaries, due to spreading into a larger area.
  • Blood flow velocity is highest near the heart and decreases in capillaries, then increases again in veins as area decreases.

Conclusion

  • Upcoming lectures will explore the composition of blood and its other components.