HBS204 Lecture Notes - Lecture 2: Red Blood Cell, Hemoglobin A, Partial Pressure
The Oxygen/Haemoglobin dissociation curve
PO2 determinesO2 -Hb binding
• The amount of oxygen that is able to bind to haemoglobin is determined by two factors – 1. The partial pressure in the
plasma surrounding the red blood cells 2. The number of potential haemoglobin binding sites available the
• The total number of oxygen binding sites depends on the number of haemoglobins molecules found in red blood cells
as well as the haemoglobin content of each of these red blood cells
• Any condition that reduces the amount of red blood cells or the total number of haemoglobin molecules within the cell
will have an effect on how much oxygen is able to be carried
• There is no artificial replacement for haemoglobin, we are unable to artificially transport oxygen around our body, a case of significant blood loss- e.g.
haemorrhage the only efficient way to increase oxygen carrying capacity is to replace the red blood cells
• We're able to isolate haemoglobin from red blood cells and in a laboratory expose it to different partial pressures of oxygen
to create what we refer to as an oxygen haemoglobin binding curve ---------------------------------------------------------->
Oxygen-haemoglobin binding curves
• The physical relationship between the partial pressure of oxygen and haemoglobin binding
• In these experiments, haemoglobin is exposed to various partial pressures of oxygen on the x-axis and the amount that binds
to haemoglobin is expressed as haemoglobin saturation on the y-axis
• A percentage saturation - how many of the available binding sites are bound to oxygen, at 100% saturation, all four oxygen
binding sites on all haemoglobin molecules have been bound to oxygen - generates a sigmoid or shape or s shaped curve
• This reflects the properties of haemoglobin and its affinity for oxygen - important factors in relation to curve and function
• Two dash lines at normal arteriolar and alveolar partial pressure of oxygen of 100mmHg, 98% of the haemoglobin within our body is found bound to
oxygen, as blood passes through the lung’s haemoglobin picks up almost maximum amount of oxygen that it can to carry it around our body
• At this partial pressure of 100mmHg - the curve is nearly flat, if partial pressure increases above 100mmHg, even large changes in partial pressure will only
cause minor changes in oxygen saturation, haemoglobin will only be 100% saturated once the partial pressure of oxygen reaches 650mmHg of oxygen
• This flattening also means that alveolar partial pressure of oxygen can fall quite a bit before there is a decrease in haemoglobin saturation
• The partial pressure of oxygen can drop around 60mmHg before leaving Plateau phase of the curve, after 60mmHg we notice a steep drop off in the
amount of oxygen that's bound to haemoglobin – and we have the steep phase of the curve
• In the steep phase a relatively small decrease in the partial pressure of oxygen is able to cause a relatively
large release of oxygen – this is important as blood leaving the systemic capillaries in our tissues has a partial
pressure of oxygen a 40mmHg, this is the same as the partial pressure of resting cells
• At 40mmHg haemoglobin saturation is at around 75% - at the tissue haemoglobin has released approximately
25% of the oxygen that has bound to it and this 25% is able to be used by the cells
• During exercise when tissues become more metabolically active, tissue partial pressure of oxygen PO2 may
drop further and can drop to around as low as 20mmHg - haemoglobin saturation decreases further to
around 35% - more oxygen is released by the haemoglobin allowing it to meet the demands of the tissue
Factors influencing Hb-O2 binding
• Any factor that changes the shape or confirmation of the haemoglobin molecule is able to affect how efficiently it's able to find oxygen, these curves have
been created in a laboratory
• Each Factor has been manipulated individually, in humans physiological changes in the plasma pH temperature and the partial pressure of carbon dioxide
are all able to alter the oxygen binding affinity of haemoglobin - how well is able to bind to oxygen
• Changes in this affinity are reflected as changes in the shape of the saturation curve, a decrease in pH or a greater concentration of hydrogen ions,
increased temperature and increased partial pressure of carbon dioxide in the external fluid, shifts the
saturation curve to the right and decreases the affinity of haemoglobin for oxygen
• When these factors change in the opposite direction binding affinity of haemoglobin increases, so
haemoglobin wants to hold on to any oxygen that its bound to causing a shift in the curve to the left
• Oxygen binding in the lungs is affected very little, in all cases haemoglobin saturation will still reach
around 98%, the steep portion of the curve is most affected - important for oxygen delivery in the tissues
Effect of pH on Hb saturation – what happens when plasma pH is altered
• As pH decreases hydrogen ion concentration increases - important in alterations in CO2 levels as
CO2 is primarily carried as bicarbonate ions in the plasma, associated with this we have an alteration in the hydrogen ion concentration
• At normal plasma pH - sits around 7.4, at 40mmHg - the partial pressure of oxygen in the tissues - haemoglobin
saturation is 75%, so haemoglobin is able to deliver 25% of its available oxygen to the cells, if we drop plasma
pH to 7.2 - increase our hydrogen ion concentration, the haemoglobin oxygen saturation is now 62% -
indicated in green curve shifts to right
• At the same partial pressure of oxygen – 40mmHg - haemoglobin is able to give up 13% more of its oxygen
content even though the partial pressure has remained the same, in the presence of decrease pH haemoglobin has a decreased affinity
for oxygen, so more oxygen is delivered to the tissue
• The result of a decreased pH, increased partial pressure of carbon dioxide or increase temperature is an increase in metabolic activity
• To remember which way the curve shifts when changes in parameters - decrease pH, increase partial pressure of CO2 and increased temperature, it's
right to exercise, when we exercise all three parameters change and curve shifts to the right - oxygen is delivered more freely to the tissues insuring that
supply and demand is met
• In conditions where we have decreased temperature, decrease pH or decrease partial pressure of CO2, the tissue is less metabolically active, manifests a
shift in the curve to the left and the haemoglobin doesn't need to deliver as much oxygen to tissues so it tends to hold on to it
2,3-bisphosphoglycerate (2,3-BPG)
• Another factor affecting oxygen binding to haemoglobin is compound 2,3-BPG or 2,3-DPG – diphosphoglycerate, this chemical is
formed in the RBC’s from an intermediate of the glycolysis pathway, it binds to haemoglobin and decreases its binding affinity for
oxygen generating a rightward shift in the curve
• In the presence of low tissue partial pressure of O2 - may occur in high altitude, if there is chronic airway obstruction or chronic heart
failure, BPG levels increase, also increased in individuals with chronic anaemia - in this case it beneficial to deliver more oxygen to the
tissues for the same drop in partial pressure of oxygen, production of RBCs is triggered by Hypoxia
Document Summary
The amount of oxygen that is able to bind to haemoglobin is determined by two factors 1. The partial pressure in the plasma surrounding the red blood cells 2. The number of potential haemoglobin binding sites available the. The total number of oxygen binding sites depends on the number of haemoglobins molecules found in red blood cells as well as the haemoglobin content of each of these red blood cells. Any condition that reduces the amount of red blood cells or the total number of haemoglobin molecules within the cell will have an effect on how much oxygen is able to be carried. The physical relationship between the partial pressure of oxygen and haemoglobin binding. In these experiments, haemoglobin is exposed to various partial pressures of oxygen on the x-axis and the amount that binds to haemoglobin is expressed as haemoglobin saturation on the y-axis.
