Chapter 8

• The need for specialised transport systems in multicellular organisms

  • high metabolic demands - diffusion rate not enough to supply quantities needed
  • smaller SA:V ratio - larger diffusion distance & smaller surface area available to absorb/ remove substances
  • molecules like hormones or enzymes may be made in one place but needed in another
• Open circulatory system 


  • few vessels to contain blood
  • transport medium (haemolymph) ispumped straight from heart into body cavity (haemocoel)
  • in haemocoel transport medium under low pressure
  • transport medium comes into direct contact with tissues & cells
• In what animals are open circulatory systems found?

  • some invertebrates (including insects)
  • molluscs
• Describe a closed circulatory system

  • transport medium (blood) enclosed in vessels
  • heart pumps blood under pressure & quickly
  • amount of blood flowing to particular tissue is adjusted by widening/ narrowing blood vessels
• In what animals are closed circulatory systems found?

  • fish
  • mammals
• Single closed circulatory system

  • blood flows through the heart once per circulation
  • blood flows through 2 sets of capillaries:

  1. exchanges O2 & CO2
  2. in diff organ systems, substances exchanged between blood & cells

  
• Why are activity levels of animals with single closed circulations relatively low?

  • as a result of passing through 2 sets of narrow capillaries, blood pressure drops and blood returns to the heart slowly
  • this limits efficiency of the exchange process
• What animals have single closed circulatory systems?

  • fish
  • annelid worms
• Why can fish still be active with a single circulatory system?

  • their efficient countercurrent gaseous exchange combined with their reduced metabolic demands (due to fact body weight supported by water & they don't maintain own body temp) allows them to be active
• Double circulatory system

  • blood travels through heart twice for each circuit of the body
  • 2 circulations:

  1. blood pumped from heart to lungs - pick up O2 and unload CO2 - then return to heart
  2. blood flows through heart & is pumped all around the body before returning to the heart

  • high pressure & fast flow of blood maintained - most efficient system for transporting substances

  
• What kind of animals have a double circulatory system?

  • birds
  • mammals
  • very active land animals that maintain own body temp
• Advantages of a closed double circulatory system over an open single circulatory system

  • blood pressure is maintained
  • oxygenated & deoxygenated blood don't mix
  • lower volumes of transport fluid required
  • blood supply to different tissues can be varied depending on demand
  • delivery of oxygen & nutrients is more efficient
• Function of arteries 


  • carry oxygenated blood away from heart
  • (except pulmonary artery - carries deoxygenated blood from heart to lungs)
• Structure of arteries 


  • elastic fibres - stretch & recoil to prevent pressure surges
  • thick, muscular walls - handle high pressure without tearing
  • narrow lumen
  • collagen - structural support
• Arterioles 


  • link arteries & capillaries
  • control flow of blood into individual organs - smooth muscle contracts to prevent blood flow (vasoconstriction) and relaxes to allow blood flow (vasodilation)
  • more smooth muscle & less elastin than arteries - little pulse surges
• Role of capillaries 


  substances are exchanged through capillary walls between tissue cells & the blood
• How are capillaries adapted for their role?

  • provide large surface area for diffusion
  • thin walls - single endothelial cell thick - speed up diffusion
  • permeable wall to enable diffusion
• Function of veins


  • carry deoxygenated blood towards the heart
  • (except pulmonary vein - oxygenated blood from lungs to heart)
• Veins 


  • thin walls - blood at lower pressure
  • valves - prevent backflow of blood
  • less smooth muscle & elastin
  • wide lumen
• Venules

  • link veins & capillaries
  • very thin walls (compared to veins)
  • very little elastin or smooth muscle
  • some permeability is retained, allowing continued diffusion of some particles across the wall
• Label artery

  A) lumen

  B) endothelium

  C) elastic layer

  D) muscle layer

  E) tough outer layer (collagen)

  
• Functions of blood


  • transport (e.g. of O2, hormones, antibodies) to and from tissues
  • role in temperature regulation & as a pH buffer
• Composition of blood


  • 55% plasma - (water plus solutes e.g. glucose, amino acids, ions & large proteins inc hormones)
  • 45% cells - (leucocytes (WBCs & RBCs) & platelets)
• How is tissue fluid formed?

  • when water diffuses out of blood capillaries, carrying dissolved solutes across the capillary wall
  • with exception of large plasma proteins - can't pass through fenestration on capillary walls
• Osmotic effect of plasma proteins

  • give blood in capillaries a high solute potential compared with surrounding fluid
  • means higher water potential in tissue fluid
  • water moves into blood capillaries from surrounding fluid by osmosis
• Osmotic pressure


  • changing water potential as water moves from an area of high wp to low wp
  • solution w lower water potential has higher osmotic pressure - will attract water via osmosis
• Oncotic pressure 


  force exerted by plasma proteins in blood that draws water into blood vessels
• Hydrostatic pressure 


  pressure exerted by fluid in an enclosed system
• Hydrostatic pressure at the arterial end of the capillary

  • high
  • higher than oncotic pressure attracting water in by osmosis - so fluid is squeezed out of capillaries (becomes tissue fluid)
• Hydrostatic pressure of the venous end of the capillary

  • lower than oncotic pressure - fluid has moved out & pulse lost
  • water diffuses back into capillaries by osmosis
• Diagram showing differences in hydrostatic pressure at arterial & venous end & how this results in movement in or out of capillary

  
• Functions of tissue fluid

  • bathes cells
  • enables exchange of materials
• Composition of tissue fluid

  • few cells (which remain in blood) other than some phagocytes
  • less solute than blood because many substances diffuse into cells & there's no plasma proteins
• What is lymph?


  • tissue fluid (10%) that drains into lymphatic system instead of re-entering capillaries
  • lymph fluid passes through lymph vessels via nodes before eventually returning to blood
• Functions of lymph

  • major role in immune system
  • lymphocytes build up in lymph nodes - produce antibodies - passed into blood
  • phagocytes in lymph nodes ingest bacteria
  • (enlarged lymph nodes = body fighting off invading pathogen)
• Composition of lymph

  similar to tissue fluid but less O2 & nutrients, more fatty acids & large no. of leucocytes
• Label the interactions between the 3 transport fluids

  A) ultrafiltration

  B) drainage

  C) return of lymph via lymph vessels

  D) reabsorption

  
• Adaptations of erythrocytes (RBCs)

  • biconcave shape - large surface area for diffusion & helps pass through narrow capillaries
  • by time enter circulation have lost nuclei - maximise amount of haemoglobin that fits into cells
• Oxygen transport 


  • O2 binds to haemoglobin in RBCs
  • arrangement of haemoglobin molecule means as soon as first O2 molecule binds to haem group, the haemoglobin changes shape - easier for next O2 molecules to bind - known as positive cooperativity
  • when blood reaches body tissues, reaction is reversed (higher conc of O2 in RBC than body cells) and O2 moves out of RBC down a concentration gradient
• Formula of oxyhaemoglobin 


  • Hb + 402 ⇌ Hb(O2)4
  • haemoglobin + oxygen ⇌ oxyhaemoglobin