3.3.4.1 Mass Transport in animals

• Red blood cells

  • Contain lots of haemoglobin, no nucleus, biconcave, high SA:V, short diffusion path
• Haemoglobin

  • Associates with / binds / loads O2 at gas exchange surfaces where partial pressure of O2 (pO2) is high -> forms oxyhaemoglobin, which transports O2
  • Dissociates from / unloads O2 near cells / tissues where pO2 is low
• Structure of haemoglobin

  • Protein with a quaternary structure
  • Made of 4 polypeptide chains
  • Each chain contains a Haem group containing an iron ion (Fe2+)
• Loading, transport and unloading of oxygen

  1. In areas with low pO2 (respiring tissues), Hb has a low affinity for O2 so O2 readily unloads / dissociates, making percentage saturation low

  2. In areas with high pO2 (gas exchange surfaces), Hb has a high affinity for O2 so O2 readily loads/associates, making percentage saturation high
• Cooperative nature of oxygen binding


  Binding of first oxygen changes tertiary / quaternary structure of haemoglobin, uncovering Haem group binding sites and making further binding of oxygens easier
• As oxygen increases

  At low pO2 there is little/slow increase in % saturation of Hb, where the first oxygen is binding

  At higher pO2 there is a big/rapid increase in % saturation of Hb, where it has gotten easier for oxygen to bind
• Bohr effect


  Effect of CO2 concentration on dissociation of oxyhaemoglobin → curve shifts to right
• Effect of CO2 concentration on the dissociation of oxyhaemoglobin

  1. Increasing blood CO2 eg. due to increased rate of respiration

  2. Lowers blood pH (more acidic)

  3. Reducing Hb's affinity for oxygen as shape / tertiary / quaternary structure changes slightly

  4. So more / faster unloading of oxygen to respiring cells at a given pO2
• Advantage of the Bohr effect

  More dissociation of oxygen → faster aerobic respiration / less anaerobic respiration → more ATP produced
• Different types of haemoglobin

  • Made of polypeptide chains with slightly different amino acid sequences
  • Resulting in different tertiary / quaternary structures / shape → different affinities for oxygen
• Curve shift left


  Hb has higher affinity for O2, more O2 associates with Hb more readily at gas exchange surfaces where pO2 is lower

  e.g, organisms in low O2 environments - high altitudes, underground, or foetus
• Curve shift right


  Hb has lower affinity for O2, more O2 dissociates from Hb more readily at respiring tissues where more O2 is needed

  e.g, organisms with high rates of respiration/ metabolic rate
• Double circulatory system

  • Deoxygenated blood in right side of heart pumped to lungs; oxygenated returns to left side
  • Oxygenated blood in left side of heart pumped to rest of body; deoxygenated returns to right
• Importance of double circulatory system

  • Prevents mixing of oxygenated/deoxygenated blood, so blood pumped to the body is fully saturated with oxygen for aerobic respiration
  • Blood can be pumped to the body at a higher pressure (after being lower from the lungs), substances take to/ removed from body cells quicker and more efficiently
• Thicker wall of left ventricle

  • Thicker muscle to contract with greater force
  • To generate higher pressure to pump blood around entire body
• Cardiac cycle

  1. Atrial systole: Atria contract → volume decreases, pressure increases

  2. Ventricular systole: Ventricles contract → volume decreases, pressure increases

  3. Diastole: Atria & ventricles relax → volume increases, pressure decreases
• Valve movements during cardiac cycle

  • Atrioventricular valves open when pressure in atria exceeds pressure in ventricles
  • Semilunar valves open when pressure in ventricles exceeds pressure in arteries
  • Atrioventricular valves shut when pressure in ventricles exceeds pressure in atria
  • Semilunar valves shut when pressure in arteries exceeds pressure in ventricles
• Interpreting cardiac cycle graphs

  • Semilunar valves closed: Pressure in artery higher than in ventricle, to prevent backflow of blood from artery to ventricles
  • Semilunar valves open: When the pressure in the ventricle is higher than in the artery, blood flows from ventricle to artery
  • Atrioventricular valves closed: Pressure in atrium higher than ventricle, to prevent backflow of blood from ventricles to atria
  • Atrioventricular valves open: When pressure in ventricle is higher than in atrium, so blood flows from ventricle to atrium

  
• Cardiac output


  Volume of blood pumped out of heart per min = stroke volume (volume of blood pumped in each heart beat) x heart rate (number of beats per min)
• Calculating heart rate

  Heart rate (beats per minute) = 60 (seconds) / length of one cardiac cycle (seconds)
• Structure of arteries

  • Thick smooth muscle tissue → can contract and control / maintain blood flow / pressure
  • Thick elastic tissue → can stretch as ventricles contract and recoil as ventricles relax, to reduce pressure surges / even out blood pressure / maintain high pressure
  • Thick wall → withstand high pressure / stop bursting
  • Smooth / folded endothelium → reduces friction / can stretch
  • Narrow lumen → increases / maintains high pressure
• Structure of arterioles

  • Thicker smooth muscle layer than arteries
  • Contracts → narrows lumen (vasoconstriction) → reduces blood flow to capillaries
  • Relaxes → widens lumen (vasodilation) → increases blood flow to capillaries
  • Thinner elastic layer → pressure surges are lower (as further from heart / ventricles)
• Structure of capillaries

  • Wall is a thin (one cell) layer of endothelial cells → reduces diffusion distance
  • Capillary bed is a large network of branched capillaries → increases surface area for diffusion
  • Small diameter / narrow lumen → reduces blood flow rate so more time for diffusion
  • Pores in walls between cells → allow larger substances through
• Formation of tissue fluid

  At the arteriole end of capillaries:

  • Higher blood / hydrostatic pressure inside capillaries (due to contraction of ventricles) than tissue fluid (so net outward movement of fluid)
  • Forcing water (and dissolved substances) out of capillaries
  • Large plasma proteins remain in capillary
• Veins

  Carry blood back to heart at lower pressure
• Veins

  • Wider lumen than arteries → less resistance to blood flow
  • Very little elastic and muscle tissue → blood pressure lower
  • Valves → prevent backflow of blood
• Capillaries


  Allow efficient exchange of substances between blood and tissue fluid (exchange surface)
• Capillaries


  • Wall is a thin (one cell) layer of endothelial cells → reduces diffusion distance
  • Capillary bed is a large network of branched capillaries → increases surface area for diffusion
  • Small diameter / narrow lumen → reduces blood flow rate so more time for diffusion
  • Pores in walls between cells → allow larger substances through
• Return of tissue fluid to the circulatory system

  1. Hydrostatic pressure reduces as fluid leaves capillary (also due to friction)

  2. (Due to water loss) an increasing concentration of plasma proteins lowers water potential in capillary below that of tissue fluid

  3. Water enters capillaries from tissue fluid by osmosis down a water potential gradient

  4. Excess water taken up by lymph capillaries and returned to circulatory system through veins
• Causes of excess tissue fluid accumulation

  • Low concentration of protein in blood plasma
  • High salt concentration
  • High blood pressure
• Blood vessels entering and leaving the heart + lungs:

  • Vena Cava: transports deoxygenated blood from respiring body tissues -> heart
  • Pulmonary artery - transports deoxygenated blood from heart -> lungs
  • Pulmonary vein - transports oxygenated blood from lungs -> heart
  • Aorta - transports oxygenated blood from heart -> respiring body tissues
• Blood vessels entering and leaving the kidneys:

  • Renal arteries - oxygenated blood -> kidneys
  • Renal veins - deoxygenated blood to vena cava from kidneys
• Blood vessels that carry oxygenated blood to the heart muscles are:

  • Coronary arteries - located on surface of the heart, branching from aorta
• Atrial Systole

  • Atrioventricular valves open when pressure in the atria exceeds pressure in the ventricles
  • Semilunar valves remain shut as the pressure in arteries exceeds the pressure in ventricles
  • Blood is pushed into the ventricles
• Ventricular Systole

  • Atrioventricular valves shut when pressure in ventricles exceeds pressure in atria
  • Semilunar valves open when pressure in ventricles exceeds pressure in arteries
  • So blood pushed out of heart through arteries
• Diastole

  • Semilunar valves shut when pressure in arteries exceeds pressure in ventricles
  • Atrioventricular valves open when pressure in atria exceeds pressure in ventricles
  • So blood fills atria via veins and flows passively to ventricles
• Return of tissue fluid to the circulatory system

  At the venule end of capillaries:

  • Hydrostatic pressure reduces as fluid leaves capillary (also due to friction)
  • (due to water loss) an increasing concentration of plasma proteins lowers water potential in capillaries below that of tissue fluid
  • Water ends capillaries from tissue fluid by osmosis down a water potential gradient
  • Excess water taken up by lymph capillaries and returned to the circulatory system through veins