Blood Gas Transport Flashcards
Describe how oxygen is transported (briefly) - relative proportions
- Oxygen exchange occurs at the lung
- Oxygen is dissolved in the plasma (2%)
- Oxygen binds to haemoglobin (98%)
- Travels around the body and then diffuses into the tissues
Describe how carbon dioxide is transported (briefly) - relative proportions
- CO2 produced by the tissues
- CO2 dissolved in plasma (7%)
- CO2 can also be bound to haemoglobin (23%)
- CO2 can also react with water to form carbonic acid and dissociates to form HCO3- ions and H+ ions (70%)
- The CO2 is then exchanged in the lung
Describe why haemoglobin is critical to oxygen transport
- Oxygen has a low solubility in plasma - to dissolve the amount of oxygen needed to supply the tissues an impossibly high alveolar oxygen partial pressure would be needed
- The presence of haemoglobin overcomes this - it enables oxygen to be concentrated within the blood which increases the carrying capacity at gas exchange surfaces and then released at respiring tissues
- The vast majority of oxygen transported therefore bound to haemoglobin (at least 98%)
State and describe the 3 ways that oxygen content of the blood is measured/defined
- Oxygen partial pressure (kPa) - the partial pressure of oxygen within a gas phase that would yield that much oxygen in the plasma at equilibrium
- Total oxygen content (ml of O2 per L of blood) - the volume of oxygen carrier in each litre of blood including that which is dissolved in the plasma and bound to haemoglobin
- Oxygen saturation - measured directly in arterial blood via pulse oximetry (%) - the 5 of total available haemoglobin binding sites occupied by oxygen
Describe what can cause a leftward and rightward shift of the oxygen dissociation curve and include the effect in working tissues
- The higher the partial pressure of oxygen generally the higher the oxygen content in the blood
- The position of the curve depends on the affinity for haemoglobin for oxygen
Leftward shift-
- Higher haemoglobin affinity - haemoglobin binds to more oxygen at a given partial pressure of oxygen
- Caused by reduced CO2, increased pH (alkalosis), decreased 2,3-DPG, and decreased temperature
- There is a leftward shift in the lungs - allows oxygen to bind to haemoglobin
Rightward shift-
- Reduced haemoglobin affinity- haemoglobin binds less to oxygen at a given partial pressure
- Caused by increases CO2, decreased pH (acidosis), increased 2,3 DPG and increased temperature
- There is a rightward shift in the tissues - allows oxygen to dissociate from haemoglobin
In working tissues-
- There is an even greater rightward shift
- The oxygen partial pressure decreases greatly in metabolically active tissue
- Anaerobic respiration then produces lactic acid (H+) , CO2, and 2,3 DPG
- Therefore there is increased oxygen demand so increased carbon dioxide levels, decreased pH and decreased 2,3 DPG
- There is even lower haemoglobin oxygen affinity and binding so much less oxygen saturation of haemoglobin and oxygen moves from haemoglobin to the tissues at a faster rate
State the appearance of oxyhaemoglobin and deoxyhaemoglobin
- Oxyhaemoglobin appears red - high oxygen content
- Deoxyhaemoglobin appears blue - low oxygen content
Describe cyanosis
A purple discoloration of the skin and tissue that occurs when deoxyhaemoglobin content becomes excessive
Describe central and peripheral cyanosis
- Central cyanosis -
- Bluish discolouration of core, mucus membranes and extremities
- Inadequate oxygenation of blood
- Can occur due to hypoventilation or a V/Q mismatch
- Peripheral cyanosis -
- Bluish colouration confined to extremities e.g. fingers
- Inadequate oxygen supply to extremities
- Can occur due to small vessel circulation issues
State some causes of anaemia
(Insufficient red blood cells or haemoglobin)
- Iron deficiency (reduced production)
- Haemorrhage (increased loss)
Describe the effects of anaemia
- Reduces the red blood cell density so reduces the concentration of haemoglobin , total oxygen sites and oxygen carrying capacity
- However the haemoglobin is unchanged - percentage of Hb binding sites that are occupied by oxygen will remain the same
- Hb oxygen saturation and oxygen partial pressure within the plasma will be normal whereas overall total oxygen of blood will decrease as will concentration of oxyhaemoglobin and deoxyhaemoglobin
Describe the effects of CO poisoning
- Haemoglobin has a 200x stronger affinity for carbon monoxide than oxygen and it is a competitive inhibitor
- Increased binding of CO to haemoglobin reduces the oxygen carrying capacity
- Carboxyhaemoglobin has a cherry red pigmentation so hypoxia occurs in absence of cyanosis
- The overall concentration of haemoglobin in the blood remains constant
- Less oxygen binding so less is transported so total oxygen content of the blood will decrease as will concentration of oxyhaemoglobin
- Arterial blood gas measurements will fall as they compare the concentration of oxyhaemoglobin to total haemoglobin
- Pulse oximetry readings may remain normal as the technique cannot reliably differentiate between oxyhaemoglobin and carboxyhemoglobin
How and why does CO2 transport differ to O2 transport?
- Carbon dioxide has a higher water solubility than oxygen does so a greater % of carbon dioxide is simply dissolved in plasma to be transported (concentration = partial pressure x solubility)
- Carbon dioxide binds to haemoglobin at different sites than oxygen and with decreased affinity thus a lower % of CO2 is transported in this way
- Carbon dioxide reacts with water to form carbonic acid which accounts for the majority of carbon dioxide transport
Describe the haldane effect and what it does
- Venous blood carries more carbon dioxide than arterial blood at any partial pressure of carbon dioxide
- This is because deoxygenated haemoglobin has a higher affinity for carbon dioxide and H+ ions than oxygenated haemoglobin does
- Binding of CO2 and H+ ions to haemoglobin enables more to enter the red blood cells which enables more to dissolve in the plasma which increases CO2 carrying capacity
- Haemoglobin has a higher affinity for oxygen so when it is present there is less CO2 bound to haemoglobin so more in the red blood cells and plasma which allows it to be removed at the lungs
- If excess dissolved carbon dioxide cannot be released then oxygenation of the blood enables less CO2 to be transported - CO2 accumulation = acidosis
Summarise what happens at the tissues and the lungs
Tissues-
1. CO2 is produced by respiring cells and dissolves in the plasma and enters red blood cells
- Conversion of CO2 and H20 to H2CO3 within red blood cells (catalysed by carbonic anhydrase)
- The effective removal of CO2 by (2) enables CO2 to diffuse further into the red blood cell
- H2CO3 ionises to HCO3- + H+ - the red blood cell membrane is impermeable to H+ so H+ cannot leave
- There is accumulation of H+ within the cell and cessation of (2) is prevented by deoxygenated haemoglobin acting as a buffer and binding to H+ - movement of oxygen into tissues from red blood cells which increases deoxyhemoglobin and enables more CO2 to be transported
- The increased [HCO3-] creates a diffusion gradient for HCO3- to leave the cell - it is exchanged for Cl- to maintain electrical neutrality
Lungs-
1. Low partial pressure of carbon dioxide creates a diffusion gradient for carbon dioxide to diffuse out of the blood and into the airspace
- Increased partial pressures of oxygen leads to haemoglobin binding to oxygen - oxyhaemoglobin binds less H+ than deoxyhaemoglobin so [H+] increases
- Increased free [H+] leads to increased carbonic acid (H2CO3) and ultimately CO2 which contributes to CO2 plasma saturation
- The changing equilibrium of carbonic acid reaction also leads to decreased [HCO3-] as it binds to free H+ - creates a diffusion gradient that allows HCO3- ions to entry the RBC in exchange for Cl-
Summarise the haldane and bohr effects
Interaction of oxygen on carbon dioxide transport- the Haldane effect
- Binding of oxygen to haemoglobin induces a structural change reducing haemoglobin affinity for CO2 and H+
- Therefore deoxygenated blood carries more carbon dioxide at any given partial pressure of carbon dioxide
Interaction of carbon dioxide on oxygen transport - the bohr effect
- Binding of carbon dioxide to haemoglobin induces a different structural change reducing haemoglobin’s affinity for oxygen
- Therefore haemoglobin releases more oxygen at any given partial pressure of oxygen when carbon dioxide levels rise
