Mass transport

• Structure of arteries related to function:

  • carry blood away from the heart
  • thicker walls to withstand high pressure
  • thick muscle wall that can contract
  • thick elastic layer to allow stretching and recoil to maintain high pressure
  • narrower lumen
• Structure of veins related to function:

  • carry blood towards the heart
  • thinner muscle and elastic layer as lower pressure
  • valves to prevent backflow
  • wider lumen allows transport of greater volumes of blood
• Structure of capillaries related to function:

  • site of substance exchange
  • smallest diameter which is equal to one red blood cell, this slows blood flow as red blood cells are squeezed against sides increasing diffusion time
  • endothelium walls are 1 cell thick for short diffusion pathway
• Vein - carries deoxygenated blood towards the heart under low pressure
• Artery - carries oxygenated blood away from the heart under high pressure
• Formation of tissue fluid:

  • higher hydrostatic pressure at arteriole end as blood comes from arteries and arterioles
  • hydrostatic pressure is greater than oncotic pressure so tissue fluid is forced out of capillaries through gaps between endothelium cells
  • large molecules such as plasma proteins cannot leave the capillaries
  • higher concentration of proteins lower water potential of blood plasma
  • water moves back into the blood by osmosis near the venous end as hydrostatic pressure is lower than oncotic pressure
• Ultrafiltration of tissue fluid:

  • small molecules such as glucose, mineral ions and gases leave the capillaries
  • large molecules such as red blood cells and proteins cannot leave capillaries
• Involvement of the lymphatic system:

  • some of tissue fluid forced out of capillaries is drained into lymph capillaries
  • lymph capillaries are blind-ended and connect to larger lymph vessels forming the lymphatic system
  • lymph fluid moves along as lymph vessels are squeezed by nearby skeletal muscles
  • lymph vessels have valves to prevent backflow
  • eventually lymph fluid returns to bloodstream via. blood vessels under collarbone
• Adaptations of erythrocytes for function:

  • biconcave shape - larger SA:V for faster oxygen diffusion
  • flat and thin - short diffusion distance for oxygen to load onto haemoglobin
  • no nucleus - space for more haemoglobin
• Haemoglobins are globular proteins with quaternary structures: consist of four polypeptide chains each with a haem group containing a Fe2+ ion
• The Bohr effect:

  • when CO2 is high at respiring tissues, O2 dissociates from haemoglobin due to lower affinity
  • due to high CO2 concentrations lowering blood pH as carbonic acid is formed
  • H+ ions disrupt ionic bonding and change tertiary structure of haemoglobin, lowering oxygen affinity and increasing O2 dissociation
  • Oxygen dissociation curve shifts to the right
  • Important as: ensures where more CO2 is being produced, more O2 dissociates to help maintain metabolic rate
• Oxygen associates with haemoglobin to form oxyhaemoglobin
• Affinity means how easily oxygen binds to haemoglobin
• How oxygen affinity changes with each binding of O2:

  • the first O2 binding causes a shape change which makes it easier for further O2 molecules to bind, this is called cooperative binding
  • However, as haemoglobin becomes more saturated, it becomes harder for O2 to bind as there is less space
• Ventricular systole:

  • atria relax and ventricles begin to contract
  • contraction of ventricles causes ventricular pressure to increase
  • high pressure shuts atrioventricular valves to ensure blood doesn't flow back to the atria
  • blood is forced out of the ventricles and out of heart through pulmonary artery or the aorta
• Atrial systole:

  • Blood flows into both atria from body and lungs
  • atria contract, increasing pressure and causing atrioventricular valves to open
  • ventricles are relaxed and fill with blood
• Diastole:

  • blood in the pulmonary artery and aorta is at high pressure, this shuts the semilunar valves
  • both atria and ventricles relax, atrioventricular valves reopen
  • blood flows into the atria and ventricles from pulmonary vein and vena cava
• Explain 4 ways in which the structure of the aorta is related to its function (5)

  Thick muscular wall which can contract to produce high pressure

  Thick elastic tissue which can stretch and recoil to maintain high blood pressure

  Aortic valve to prevent backflow

  Overall thick blood vessel wall so aorta doesn't burst from high pressure

  Smooth endothelium which reduces friction
• High blood pressure leads to an accumulation of tissue fluid. Explain how.

  High blood pressure causes hydrostatic pressure to be greater at arteriole end

  Increases outward pressure from arteriole end of capillary

  So more tissue fluid formed
• Water potential of blood plasma is more negative at venule end of capillary than at the arteriole end of the capillary. Explain why (3)

  Water has left capillary

  Proteins are too large to leave

  High concentration of blood proteins
• Give two ways in which unidirectionality of blood flow is achieved.

  • valves to prevent backflow
  • Pressure gradient from high to low
• Explain why higher ventricular blood pressure can cause tissue fluid build up outside blood capillaries

  More fluid is forced out of the capillary

  So less tissue fluid goes back into capillaries at venous end due to lower overall inward pressure