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• Classification of living organisms according to circulation

  • Without circulation
  • With circulation
• Without circulation


  Only occurs in small unicellular organisms such as Amoeba
• With circulation


  • Open
  • Closed
• Closed circulation


  It consists of the heart only. It's called "Closed" because the blood flows from the heart into blood vessels and does not go out from these vessels
• Types of closed circulation

  • Single
  • Double
• Single circulation


  Blood passes the heart once per cycle, e.g. fish
• Double circulation


  Blood passes the heart twice per cycle, e.g. mammals
• Small organisms survive without circulation due to large surface area to volume ratio, short diffusion distance, steep concentration gradient, and low energy requirement
• When organisms get larger

  Circulation helps to overcome the limitations of diffusion
• Advantages of double circulation

  • Ensures complete separation of oxygenated and deoxygenated blood
  • Prevents dilution of oxygenated blood
  • Left side pumps blood at high pressure for efficient delivery of oxygen and nutrients
  • Right side pumps blood at low pressure to prevent damage to delicate blood capillaries
• The heart


  • Located in the center of the thorax
  • Size of the first of one hand (300g)
  • Inverted cone shape with the apex pointing to the left
• The heart is supplied by coronary arteries
• Special features of the heart

  • Strongest muscles in the body as they keep beating from birth till death
  • Has its own pacemaker (Sino Atrial Node)
• Structure of the heart

  • 4 chambers
  • 4 valves
  • 4 major vessels
• Chambers of the heart

  • Right atrium
  • Left atrium
  • Right ventricle
  • Left ventricle
• Valves of the heart

  • Right & left atrioventricular valves (tricuspid & bicuspid)
  • Pulmonary semilunar valve
  • Aortic semilunar valve
• Major vessels of the heart

  • Pulmonary artery & vein
  • Vena cava
  • Aorta
• Circulation through the heart

  1. Deoxygenated blood from body enters right atrium via vena cava

  2. Right atrium pumps blood to right ventricle through tricuspid valve

  3. Right ventricle pumps deoxygenated blood to lungs through pulmonary semilunar valve

  4. Oxygenated blood returns to left atrium via pulmonary vein

  5. Left atrium pumps blood to left ventricle

  6. Left ventricle pumps oxygenated blood to body through aortic semilunar valve
• Function of the heart

  Pumps blood from high pressure to low pressure over a long distance, known as mass flow. This helps overcome limitations of diffusion.
• Valves


  Open when pressure on one side is higher, close when pressure on the other side is higher. Prevent backflow of blood.
• Cardiac cycle


  1. Atrial systole: Atria contract, ventricles relaxed

  2. Ventricular systole: Ventricles contract, atria relaxed

  3. Diastole: Both atria and ventricles relaxed
• Heart rate


  Number of heart beats per minute
• Stroke volume


  Volume of blood pumped in one beat
• Cardiac output


  Volume of blood pumped per minute
• Pulse is the contraction and expansion of arteries as the heart pumps blood
• Pressure changes in the left ventricle, left atrium, and aorta during the cardiac cycle can be shown on a graph
• The volume of blood in the left ventricle changes during the cardiac cycle, reaching 100% at the end of ventricular systole and 70% at the end of diastole
• Role of valves in the cardiac cycle

  1. AV valves open during atrial systole to allow blood flow from atria to ventricles, close during ventricular systole to prevent backflow

  2. Semilunar valves open during ventricular systole to allow blood flow from ventricles to major arteries, close during atrial systole and diastole to prevent backflow
• Differences between right and left ventricles

  • Right ventricle pumps blood at lower pressure for shorter distance to lungs, has thinner wall with weaker muscles
  • Left ventricle pumps blood at higher pressure for longer distance to body, has thicker wall with stronger muscles
• Blood vessels


  • Arteries
  • Veins
  • Capillaries
• Arteries


  Carry blood away from the heart, have thick walls, narrow lumen, no valves
• Adaptations of arteries


  • Thick wall to withstand high pressure
  • Narrow lumen to maintain high blood pressure
  • No valves as blood flows under high pressure
  • Smooth inner lining to reduce friction
  • Folded inner lining to stretch without breaking
• Veins


  Carry blood towards the heart, have thin walls, wide lumen, valves present
• Adaptations of veins


  • Thin walls as pressure is low
  • Wide lumen to maintain low pressure
  • Valves present to prevent backflow of blood
• Factors ensuring venous return to the heart include negative intrathoracic pressure, muscle pump during movement, and valves preventing backflow
• Capillaries


  Site of exchange of oxygen, nutrients, and waste products by diffusion. Have very thin walls (1 cell thick) and capillary pores to facilitate exchange.
• Causes of blood pressure drop include vasodilation and decreased cardiac output
• Veins


  • Carry deoxygenated blood except pulmonary veins
  • Wide lumen
  • Thin wall
• Structure of veins


  1. Thin wall

  2. Wide lumen

  3. Tunica adventitia (T.A)

  4. Tunica media (T.M)

  5. Tunica intima (T-I)
• Veins


  • Valves present in the lumen
  • Wall same as arteries but less developed
  • Lowest pressure