Mass transport in animals

• 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
• Formation of tissue fluid

  1. Higher blood / hydrostatic pressure inside capillaries (due to contraction of ventricles) than tissue fluid (so net outward force)

  2. Forcing water (and dissolved substances) out of capillaries

  3. Large plasma proteins remain in capillary

  
• 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
• Risk factor

  An aspect of a person's lifestyle or substances in a person's body / environment that have been shown to be linked to an increased rate of disease
• Risk factors for cardiovascular disease

  • Age
  • Diet high in salt or saturated fat
  • Smoking
  • Lack of exercise
  • Genes
• Describe the role of red blood cells and haemoglobin in oxygen transport

  Red blood cells contain lots of haemoglobin - no nucleus, biconcave, high surface area to volume ratio, short diffusion path

  Haemoglobin associates with oxygen at gas exchange surfaces where partial pressure of oxygen (pO2) is high

  This forms oxyhaemoglobin which transports oxygen (each can carry 4 oxygen - one at each Haem group)

  Haemoglobin dissociates from oxygen near cells and tissues where pO2 is low
• Describe the structure of haemoglobin

  Protein with a quaternary structure, made of 4 polypeptide chains where each chain contains a Haem group containing an iron ion (Fe 2+ )
• Describe the loading, transport and unloading of oxygen in relation to the oxyhaemoglobin dissociation curve

  • Areas with low pO2 (respiring tissues): Haemoglobin has a low affinity for oxygen, so oxygen readily dissociates with haemoglobin, so % saturation is low
  • Areas with high pO2 (gas exchange surfaces): Haemoglobin has a high affinity for oxygen so oxygen readily associates with haemoglobin so % saturation is high
• S-shaped (sigmoid) oxyhaemoglobin dissociation curve

  Binding of first oxygen changes tertiary / quaternary structure of haemoglobin. This uncovers Haem group binding sites, making further binding of oxygens easier
• What is the Bohr effect?

  Effect of carbon dioxide concentration on dissociation of oxyhaemoglobin so the curve shifts to righ
• Explain 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 haemoglobin's affinity for oxygen as shape / tertiary / quaternary structure changes slightly
  4. So more and faster unloading of oxygen to respiring cells at a given pO