Circulatory system

• Circulatory systems: Diffusion is extremely slow, except over small distances
• Circulatory systems


  Evolved in bigger animals to overcome the problem of slow diffusion via bulk transport
• Circulatory fluids

  Contain plasma and often cells
• Plasma


  Contains inorganic ions, gases, organic solutes and proteins (e.g. antibodies)
• Circulatory systems have 3 distinct components

  • Fluid - called blood or haemolymph
  • Pump - usually called hearts
  • Vessels - called vascular components; carry fluid from pump to body tissues
• Open System

  Fluid, the haemolymph, pumped through vessels into extracellular spaces among tissues. Entire space filled with haemolymph is called haemocel.
• Closed System


  Fluid, the blood, exits a heart through vessels that are continuous all the way back to the intake side of the heart.
• Open vs Closed

  • a/b = open (e.g. grasshopper)

  c/d = closed (e.g. earthworm)
• Human haematocrit

  Ratio of red blood cells to total blood volume
• Haemoglobin

  Respiratory pigments (e.g. haemoglobin, haemocyanin) carry oxygen
• Erythrocytes


  Red blood cells contain respiratory pigments and transport oxygen
• Tissues such as bone marrow continuously replace worn-out red blood cells
• Production of red blood cells

  Controlled by erythropoietin from the kidneys
• Chicken red blood cells are oblong oval shapes and contain nucleus
• Mammalian red blood cells are flat, disc-shaped (biconcave) and have no nucleus
• Reasons for difference in red blood cell shape between chickens and mammals are unclear
• Haemostasis


  Prevents blood loss from damaged small vessels
• Thrombocytes and platelets

  Function in clotting
• Haemocytes and haemolymph proteins

  Provide haemostasis in arthropods
• Haemostasis in mammals

  • Vascular spasm reduce blood flow
  • Platelets aggregate and form plug
  • Chain reaction of plasma clotting factors produce blood coagulation
  • Plasmin dissolves clots, prevents inappropriate clot formation
• Clotting cascades

  Very complicated
• Clot pathways

  • Intrinsic pathway (7 steps, involves factor XII)
  • Extrinsic pathway (4 steps, involves tissue thromboplastin)
• Pumping Mechanisms

  • Flagella
  • Extrinsic skeletal muscles
  • Peristaltic muscular pumps
  • Chamber muscle pumps (hearts)
• Many animals have primary hearts aided by auxillary pumps
• Arthropod hearts are dorsally located and have many valved openings
• Types of pumps

  • Flagella (sea urchins)

  1. Extrinsic skeletal muscle pump (horse foot)
  2. Peristaltic (tubular) muscle pumps (earthworms)
  3. Chamber muscle pumps (mammalian heart)
• Vertebrate Hearts

  • Fish: two chambered heart

  Amphibian: three chambered heart

  Crocodilian, avian and mammalian: four chambered heart
• Vertebrate hearts have two atria and two ventricles
• Heart valves ensure that the blood flows in the correct direction
• Frog: Three chambered heart, Two atria, One ventricle
• Dogfish: Two chambered heart, One atrium, One ventricle
• Evolution of vertebrate heart

  From one circuit into two separate circuits:

  Pulmonary: heart - lungs - heart

  Systemic: heart - rest of body - heart

  Change in number of chambers in heart: two (elasmobranches), three (amphibians), four (birds, mammals)

  Dual pump action of heart
• Physiological Significance


  4 chambers = more efficient at oxygenating body
• Heart walls

  Cardiac muscle fibres

  Intercalated discs + gap junctions allow electrical connectivity

  Pacemaker cells keep myogenic hearts beating
• Sinoatrial (SA) node

  Is the normal pacemaker
• Co-ordinated spread of excitation

  SA node - atria - AVN node - ventricles
• Electrocardiogram (ECG)

  Records these electrical events in the heart
• Heart electrical activity

  Action potential of contractile cardiac muscle has plateau to aid pumping

  Ca2+ entry from outside induces much larger Ca2+ release from sarcoplasmic reticulum
• Systole

  Heart contracts to empty
• Diastole

  Heart relaxes to fill