Exchange surfaces

• Emulsification of lipids


  1. Emulsify lipids causing them to form smaller lipid droplets

  2. Increases surface area of lipids for increased / faster lipase activity
• Lipid digestion


  1. Lipase (made in pancreas) hydrolyses lipids (eg. triglycerides) → monoglycerides + fatty acids

  2. Hydrolysis of ester bond
• Protein digestion


  1. Endopeptidases - hydrolyse internal (peptide) bonds within a polypeptide → smaller peptides

  2. Exopeptidases - hydrolyse terminal (peptide) bonds at ends of polypeptide → single amino acids

  3. Membrane-bound dipeptidases - hydrolyse (peptide) bond between a dipeptide → 2 amino acids

  4. Hydrolysis of peptide bond
• Membrane-bound enzymes


  • Located on cell membranes of epithelial cells lining ileum
  • Maintain concentration gradients for absorption
• Absorption of amino acids and monosaccharides

  1. Na+ actively transported from epithelial cells lining ileum to blood (by Na+/K+ pump)

  2. Na+ enters epithelial cell down its concentration gradient with glucose against its concentration gradient

  3. Glucose moves down a concentration gradient into blood via facilitated diffusion
• Absorption of lipids


  1. Micelles contain bile salts, monoglycerides and fatty acids

  2. Monoglycerides / fatty acids absorbed (into epithelial cell) by diffusion (lipid soluble)

  3. Triglycerides reformed in (epithelial) cells and aggregate into globules

  4. Globules coated with proteins forming chylomicrons which are then packaged into vesicles

  5. Vesicles move to cell membrane and leave via exocytosis

  6. Enter lymphatic vessels and eventually return to blood circulation
• Bile salts emulsify lipids into smaller lipid droplets, they don't hydrolyse the ester bond to produce fatty acids and glycerol
• Endo/exo/dipeptidases hydrolyse peptide bonds between amino acids, not the amino acids themselves
• Co-transport is involved in the absorption of monosaccharides as a result of digestion of starch, not the digestion itself
• Fatty acids and monoglycerides are lipid-soluble, so they can move across the phospholipid bilayer by simple diffusion, not facilitated diffusion
• Micelles carry fatty acids and monoglycerides, but the whole micelle isn't absorbed into the epithelial cell
• Micelles are formed after digestion and prior to absorption, not by emulsification
• Red blood cells


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


  Protein with a quaternary structure, made of 4 polypeptide chains, each containing a Haem group with an iron ion
• Oxygen transport


  1. Hb associates with / binds / loads O2 at gas exchange surfaces where partial pressure of O2 (pO2) is high

  2. This forms oxyhaemoglobin which transports O2

  3. Hb dissociates from / unloads O2 near cells / tissues where pO2 is low

  
• Oxyhaemoglobin dissociation curve


  • At low pO2 (respiring tissues), Hb has low affinity for O2 so O2 readily unloads
  • At high pO2 (gas exchange surfaces), Hb has high affinity for O2 so O2 readily loads
• Cooperative oxygen binding


  1. Binding of first oxygen changes tertiary / quaternary structure of haemoglobin

  2. This uncovers Haem group binding sites, making further binding of oxygens easier

  
• Evidence for cooperative oxygen binding: at low pO2 there is little / slow increase in % saturation, at high pO2 there is a big / rapid increase

  
• Bohr effect


  Effect of CO2 concentration on dissociation of oxyhaemoglobin - curve shifts to right
• Bohr effect


  1. Increasing blood CO2 lowers blood pH (more acidic)

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

  3. So more / faster unloading of oxygen to respiring cells at a given pO2

  
• Advantage of 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
• Haemoglobin adaptation


  • Curve shift left → Hb has higher affinity for O2, more O2 associates readily, for organisms in low O2 environments
  • Curve shift right → Hb has lower affinity for O2, more O2 dissociates readily, for organisms with high rates of respiration / metabolic rate
• Blood circulation in mammals

  1. Closed double circulatory system - blood passes through heart twice for every circuit around body

  2. Deoxygenated blood in right side of heart pumped to lungs; oxygenated returns to left side

  3. 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
  • Blood can be pumped to body at a higher pressure (after being lower from lungs)

  
• 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, atrioventricular valves open, semilunar valves remain shut

  2. Ventricular systole: Ventricles contract, atrioventricular valves shut, semilunar valves open

  3. Diastole: Atria & ventricles relax, semilunar valves shut, atrioventricular valves open

  
• Interpreting cardiac cycle graphs

  • Semilunar valves closed: Pressure in artery higher than ventricle, to prevent backflow
  • Semilunar valves open: Pressure in ventricle higher than artery, so blood flows out
  • Atrioventricular valves closed: Pressure in ventricle higher than atrium, to prevent backflow
  • Atrioventricular valves open: Pressure in atrium higher than ventricle, so blood flows in

  
• Cardiac output


  Volume of blood pumped out of heart per minute = stroke volume x heart rate
• Calculating heart rate


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


  • Thick smooth muscle tissue to control blood flow/pressure
  • Thick elastic tissue to stretch and recoil to maintain high pressure
  • Thick wall to withstand high pressure
  • Smooth / folded endothelium to reduce friction
  • Narrow lumen to increase/maintain high pressure
• Arteriole structure


  • Thicker smooth muscle layer than arteries, contracts to narrow lumen (vasoconstriction) or relaxes to widen lumen (vasodilation) to direct blood flow
  • Thinner elastic layer as further from heart
• Vein structure


  • Wider lumen than arteries, less resistance to blood flow
  • Very little elastic and muscle tissue, lower blood pressure
  • Valves to prevent backflow
• Capillary structure


  • Thin (one cell) layer of endothelial cells to reduce diffusion distance
  • Large network of branched capillaries to increase surface area for diffusion
  • Small diameter / narrow lumen to reduce blood flow rate and increase time for diffusion
  • Pores in walls between cells to allow larger substances through
• Formation of tissue fluid

  1. Higher blood / hydrostatic pressure inside capillaries than tissue fluid, so net outward filtration of fluid

  2. Reabsorption of fluid and solutes at venule end of capillaries due to lower blood pressure

  
• 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