Circulation

• All living cells have to be provided with nutrients, O2 and other essential substances, and the waste or harmful substances produced have to be removed continuously for healthy functioning of tissues
• It is essential to have efficient mechanisms for the movement of these substances to the cells and from the cells
• Methods for transport in different groups of animals

  • Simple organisms like sponges and coelenterates circulate water from their surroundings through their body cavities
  • More complex organisms use special fluids within their bodies to transport such materials
• Blood is the most commonly used body fluid by most of the higher organisms including humans for this transport purpose
• Lymph is another body fluid that also helps in the transport of certain substances
• Blood

  A special connective tissue consisting of a fluid matrix, plasma, and formed elements
• Plasma


  A straw coloured, viscous fluid constituting nearly 55 per cent of the blood
• Composition of plasma

  • 90-92% water
  • 6-8% proteins (fibrinogen, globulins, albumins)
  • Minerals (Na+, Ca++, Mg++, HCO3-, Cl-)
  • Glucose, amino acids, lipids
• Formed elements

  • Erythrocytes (RBCs)
  • Leucocytes (WBCs)
  • Platelets
• Erythrocytes (RBCs)


  • Most abundant cells in blood
  • Devoid of nucleus in most mammals
  • Biconcave in shape
  • Contain haemoglobin
  • Average lifespan of 120 days
• Types of leucocytes (WBCs)

  • Granulocytes (neutrophils, eosinophils, basophils)
  • Agranulocytes (lymphocytes, monocytes)
• Neutrophils


  • Most abundant WBCs (60-65%)
  • Phagocytic cells that destroy foreign organisms
• Basophils


  • Secrete histamine, serotonin, heparin, involved in inflammatory reactions
• Eosinophils


  • Resist infections, associated with allergic reactions
• Lymphocytes


  • Responsible for immune responses (B and T cells)
• Platelets


  • Cell fragments produced from megakaryocytes in bone marrow
  • Involved in blood clotting
• Blood groups

  Based on presence/absence of A and B antigens on RBCs, and antibodies in plasma
• Blood groups

  • A
  • B
  • AB
  • O
• Rh grouping

  • Based on presence/absence of Rh antigen on RBCs
  • Rh+ve and Rh-ve
• Rh incompatibility between Rh-ve mother and Rh+ve foetus can cause erythroblastosis foetalis
• Blood coagulation

  1. Injury/trauma stimulates platelets to release factors

  2. Factors activate enzymatic cascade to convert fibrinogen to fibrin

  3. Fibrin forms clot to stop bleeding
• Lymph (tissue fluid)


  • Fluid that leaks out of blood capillaries into tissue spaces
  • Collected by lymphatic vessels and drained back to veins
• Lymph contains lymphocytes and helps transport nutrients, hormones, and fats
• Types of circulatory systems

  • Open (arthropods, molluscs)
  • Closed (annelids, chordates)
• Types of hearts in vertebrates

  • 2-chambered (fishes)
  • 3-chambered (amphibians, reptiles except crocodiles)
  • 4-chambered (crocodiles, birds, mammals)
• Fishes have single circulation, amphibians/reptiles have incomplete double circulation, birds/mammals have double circulation
• Human circulatory system


  Consists of heart, blood vessels, and blood
• Human heart

  • 4 chambers (2 atria, 2 ventricles)
  • Valves (tricuspid, mitral, semilunar)
  • Cardiac muscle with autoexcitable nodal tissue (SAN, AVN, bundle of His)
• Cardiac cycle

  1. Atrial systole

  2. Ventricular systole

  3. Diastole
• Cardiac conduction system

  1. AVN passes through atrio-ventricular septa

  2. Emerges on top of interventricular septum

  3. Divides into right and left bundle

  4. Branches give rise to Purkinje fibres throughout ventricular musculature

  5. Right and left bundles known as bundle of His
• Nodal musculature


  Ability to generate action potentials without external stimuli (autoexcitable)
• SAN

  • Generates maximum number of action potentials (70-75 per minute)
  • Responsible for initiating and maintaining rhythmic contractile activity of heart (pacemaker)
• Normal heart rate is 70-75 beats per minute (average 72 beats per minute)
• Cardiac cycle

  1. All chambers in relaxed state (joint diastole)

  2. Blood flows into ventricles through open tricuspid and bicuspid valves

  3. SAN generates action potential, stimulating atrial contraction (atrial systole)

  4. Action potential conducted to ventricles, causing ventricular contraction (ventricular systole)

  5. Atria relax (diastole)

  6. Ventricular pressure increases, closing tricuspid and bicuspid valves, opening semilunar valves

  7. Blood flows into pulmonary artery and aorta

  8. Ventricles relax (ventricular diastole), semilunar valves close

  9. Tricuspid and bicuspid valves open, blood flows into ventricles
• Stroke volume

  Volume of blood pumped out by each ventricle per cardiac cycle (approximately 70 mL)
• Cardiac output

  Volume of blood pumped out by each ventricle per minute (averages 5 litres in a healthy individual)
• Two prominent heart sounds (lub and dub) are produced during each cardiac cycle, associated with closure of valves
• Electrocardiogram (ECG)


  Graphical representation of electrical activity of heart during a cardiac cycle
• ECG waveform components

  • P-wave (atrial depolarization)
  • QRS complex (ventricular depolarization)
  • T-wave (ventricular repolarization)
• ECG shape deviations indicate abnormalities or diseases