1.1 Human Body – The Circulatory System

Key Concepts: Organs of the Circulatory System

  • Heart: Muscular pump propelling blood.
  • Blood Vessels: Network of tubes.
    • Arteries: Carry blood away from the heart.
    • Veins: Carry blood back to the heart.
    • Capillaries: Tiny vessels for exchange of substances.
  • Blood: Fluid carrying essential substances.

The Heart: Structure

  • Atria (Upper Chambers):
    • Right Atrium: Receives deoxygenated blood from body.
    • Left Atrium: Receives oxygenated blood from lungs.
  • Ventricles (Lower Chambers):
    • Right Ventricle: Pumps deoxygenated blood to lungs.
    • Left Ventricle: Pumps oxygenated blood to rest of body (strongest chamber).

The Heart: Valves & Wall Layers

  • Valves: Ensure unidirectional blood flow.
    • Atrioventricular (AV) Valves: Tricuspid (right), Mitral/Bicuspid (left).
    • Semilunar (SL) Valves: Pulmonary (right ventricle to artery), Aortic (left ventricle to aorta).
  • Heart Wall Layers:
    • Epicardium: Outermost protective.
    • Myocardium: Thick, muscular middle layer (pumping action).
    • Endocardium: Smooth, innermost lining.

The Heart: Function

  • Beats in a two-phase cycle:
    1. Diastole: Heart muscle relaxes, chambers fill with blood.
    2. Systole: Heart muscle contracts, pumps blood out.
  • Controlled by heart's electrical conduction system (SA node).

Arteries

  • Carry blood away from the heart (mostly oxygenated, except pulmonary artery).
  • Structure: Thick, muscular, elastic walls (3 layers: Tunica Intima, Media, Externa).
  • Function: Transport high-pressure blood, regulate pressure, vasoconstriction/vasodilation.

Veins

  • Carry blood back to the heart (mostly deoxygenated, except pulmonary veins).
  • Structure: Thinner, less muscular/elastic walls than arteries.
  • Valves: One-way valves prevent backflow (especially in limbs).
  • Function: Collect blood, operate under lower pressure, assisted flow by muscle/respiratory pumps.

Capillaries

  • Smallest and most numerous blood vessels.
  • Structure: Extremely thin walls (one cell thick).
  • Function: Primary site for exchange of oxygen, nutrients, hormones, and waste products between blood and cells.

Functions of Blood

  • Transport: Oxygen, nutrients, waste products, hormones.
  • Protection: Fights infections (WBCs, antibodies), clotting prevents blood loss.
  • Regulation: Regulates body temperature, maintains pH balance.

Process of Blood Circulation

  • Pulmonary Circulation:
    • Deoxygenated blood: Right atrium → Right ventricle → Pulmonary artery → Lungs.
    • Oxygenated blood: Lungs → Pulmonary veins → Left atrium.
  • Systemic Circulation:
    • Oxygenated blood: Left atrium → Left ventricle → Aorta → Body tissues.
    • Deoxygenated blood: Body tissues → Veins → Vena Cava → Right atrium.

Differences between Arteries and Veins

Feature Arteries Veins
Direction of Blood Flow Away from the heart Towards the heart
Oxygen Content Mostly oxygen-rich Mostly oxygen-poor
Wall Thickness Thick, muscular, elastic Thin, less muscular, less elastic
Valves Absent (except at heart exit) Present (especially in limbs)
Blood Pressure High Low
Lumen Size Narrower Wider

Keeping the Heart Healthy

  • Regular Aerobic Exercise: Strengthens heart, improves circulation, lowers BP.
  • Strength Training: Benefits heart health.
  • Yoga and Pranayama: Reduces stress, lowers BP, improves lung capacity.