Describe the structure of haemoglobin
What are its 2 configurations?
2 alpha and 2 beta chains
Each chain has 1 haem molecule bound to it.
- High O2 affinity
- Low O2 affinity
Haemoglobin becomes more avid for O2 the more O2 it has bound to it.
Explain the oxygen dissociation curve
The sigmoid shape of the curve indicates the increasing avidity of haemoglobin for oxygen as more oxygen becomes bound.
The plateau indicates almost 100% saturation where no more oxygen is able to bind to haemoglobin.
Explain why a patient can rapidly desaturate below 90% oxygen saturations
At around 90% oxygen saturation (8kPa) is the point at which reducing saturation will rapidly reduce haemoglobin affinity for oxygen.
90% oxygen saturations/ 8kPa is the point at which the oxygen dissociation curve begins to plateau, however there is a steep curve decline in saturations below 8kPa.
Define oxygen saturation
The amount available haemoglobin bound by O2 molecules
How can the alveolar-arterial oxygen difference be increased in disease?
Due to damage to the diffusion barrier (e.g. emphysema, fibrosis)
At what percentage along the capillary bed is haemoglobin usually fully saturated by oxygen?
In this circumstance, what is saturation limited by?
What should the alveolar-arterial oxygen difference look like in this circumstance?
Alveolar-arterial difference should be minimal.
What factors alter the structure of haemoglobin to make it more relaxed?
Increased pH (less H+)
Less 2,3, BPG
What is the threshold minimum O2
The minimum amount of O2 required to facilitate initial binding of O2 to haemoglobin
What is the cooperativity between oxygen and haemoglobin?
As more oxygen is bound to haemoglobin, the structure of haemoglobin changes to increase its avidity for oxygen and more oxygen is then bound as a result.
What effect will anaemia have on oxygen saturations?
O2 saturations are independent of haemoglobin concentration.
What are the approximate partial pressures of oxygen and oxygen saturations in:
Alveoli: 13.3kPa (95% sats)
Arterioles: 13.3kPa (95% sats)
Tissues: 6 kPa (65% sats)
What causes the change in binding of haemoglobin to oxygen as blood moves from the lungs to the tissues?
What does this do to the oxygen dissociation curve?
Haemoglobin gives up oxygen in the tissues due to:
Concentration gradient (lower tissue pO2 than arteriole pO2)
- Haemoglobin decreases its avidity for oxygen as less is bound and it becomes more tense; so when more is deposited due to the concentration gradient, this facilitates further dissociation of oxygen from haemoglobin.
- Lower pH in the tissues (due to anaerobic respiration and production of lactic acid) causes haemoglobin to become more tense, releasing more oxygen.
- Increasing temperature in the tissues causes haemoglobin to become more tense and release more oxygen
= BOHR SHIFT
In the resting state, how much oxygen does venous blood still have bound to haemoglobin?
What is the importance of this reserve?
Still has around 50% of haemoglobin bound to oxygen.
This means that at the tissue level, haemoglobin could deposit more oxygen if required for metabolism (e.g in exercise)
What adaptations can tissues make in chronic hypoxia?
- Increased capillary density: reduces the distance and time oxygen has to travel from the capillary to the cells (chronic hypoxia triggers cytokine production which stimulate vascular growth)
- Increased erythropoeitin (increased Hb binding sites = more O2)
- Increase ventilation
- Increase 2,3 DPG levels
Explain the Bohr shift
Oxygen dissociation curve shifts to the right along its x axis as haemoglobin becomes more tense, therefore more oxygen is required for binding with haemoglobin to occur.
The result is at any given pO2, the avidity of haemoglobin for oxygen will be reduced. Oxygen therefore is released more easily in the tissues where it is needed (higher CO2, more acidity, higher temperature)
Describe the Haldane effect
Increasing oxygen binding to haemoglobin in the pulmonary capillaries reduces the avidity of haemoglobin for CO2.
This results in more CO2 being offloaded into the pulmonary capillaries to diffuse into the alveoli.
What is DPG?
Produced in glycolytic pathway (anaerobic conditions)
More DPG = more tense haemoglobin
What are the effects of tissue metabolism on binding of haemoglobin to oxygen?
Increasing tissue metabolism:
- Increases CO2 production and lactic acid creating a low pH.
- Increases temperature in the tissues
- Increasing 2,3,DPG
= increase in tensile state of haemoglobin causing it to offload oxygen and take up carbon dioxide (Bohr shift)
Anaerobic respiration (i.e. in vigorous exercise) causes increase in all of these factors, allowing Hb to offload up to 70% of oxygen.