Blood Gas Transport Flashcards
What determines how much gas dissolves in a liquid?
The concentration of a gas dissolved within a liquid is determined by the partial pressure and solubility of a gas.
concentration ∝ partial pressure x solubility
If a liquid is placed in contact with a gas, some of the gas will dissolve into it (an amount proportional to the partial pressure of the gas).
Why is haemoglobin critical to O2 transport?
Oxygen has low solubility in plasma (0.225ml/L/kPa). In order to dissolve the amount of O2 needed to supply tissues, an impossible high alveolar PO2 would be required.
The presence of haemoglobin overcomes this problem - it enables O2 to be concentrated within the blood (increasing the carrying capacity) at gas exchange surfaces and then released at respiring tissues. The vast majority of O2 transported by the blood is bound to haemoglobin (<98%).
How is the oxygen content of blood measured/defined (3 ways)?
1) O2 partial pressure (PaO2) expressed as kPa
2) Total O2 content (CaO2) expressed as ml of O2 per L of blood (ml/L)
3) O2 saturation (SaO2 if measured directly in arterial blood, SpO2 if estimated by pulse oximetry) expressed as a percentage
What does the oxygen-haemoglobin dissociation curve represent?
What is the sigmoidal shape of the curve produced by?
The oxygen-haemoglobin dissociation curve represents the affinity of haemoglobin for oxygen. It shows the relationship between O2 concentration, partial pressure and saturation.
The sigmoidal shape of the curve is produced by:
- cooperative binding (that the oxygen-haemoglobin affinity increases as more oxygen molecules bind, due to changes in the shape of the protein)
- saturation of oxygen sites
Why is haemoglobin so effective at transporting O2 within the body?
1) The structure of haemoglobin produces a high O2 affinity, therefore, a high level of Hb-O2 binding (and saturation) requires relatively low O2.
2) The concentration of haem groups and haemoglobin contained in the RBCs enables a high carrying ability.
3) The oxygen-haemoglobin binding curve shifts to offload oxygen to demanding tissues.
4) Haemoglobin O2 affinity changes depending on the local environment, enabling O2 delivery to be coupled on demand.
How, and why, does the transport of CO2 differ to the transport of O2?
- CO2 has a higher H2O solubility than O2 does - therefore, a greater percentage of CO2 is transported simply dissolved in plasma.
- CO2 binds to haemoglobin at different sites than O2 does, and with a decreased affinity. Thus, a lower percentage of CO2 is transported in this manner.
- CO2 reacts with water to form carbonic acid, which accounts for the majority (about 70%) of the CO2 transported.
Venous blood carries more CO2 than arterial blood due to the Haldane Effect.
What is this effect?
Oxygenation of blood in the lungs displaces carbon dioxide from haemoglobin, which increases the removal of carbon dioxide. This property of blood is known as the Haldane Effect.
Describe, in detail, how carbon dioxide is brought into the RBC from the tissues (and kept there).
1) CO2 is produced by respiring cells and dissolves in the plasma, then enters the RBCs.
2) Conversion of CO2 + H2O to H2CO3 takes place within the RBCs, catalysed by carbonic anhydrase.
3) The effective removal of CO2 by step (2) enables further CO2 to diffuse into the RBC, so that more can enter into the plasma.
4) H2CO3 ionises into HCO3- and H+. The RBC membrane is impermeable to H+, therefore H+ cannot leave.
5) There is an accumulation of H+ within the cell, and therefore a cessation of step (2). This can be prevented by haemoglobin acting as a buffer and binding H+. Also, the movement of O2 from the RBCs to the tissues increases the concentration of deoxyhaemoglobin, thus enabling more CO2 to be transported.
6) The increased HCO3- concentration creates a diffusion gradient for HCO3- to leave the cell. It is exchanged for Cl- to maintain electrical neutrality.
Describe, in detail, how carbon dioxide is transferred to the lung alveoli from the RBCs.
1) A low PACO2 creates a diffusion gradient for CO2 to diffuse out of the blood into the airspace.
2) An increased PAO2 leads to oxygen-haemoglobin binding. Oxyhaemoglobin binds less H+ than deoxyhaemoglobin, increasing the concentration of free H+.
3) Increasing the concentration of free H+ leads to increased H2CO3 and, ultimately, CO2, which contributes to CO2 plasma saturation.
4) The changing equilibrium of the carbonic acid reaction also leads to decreased HCO3- concentration, as it binds the free H+. This creates a diffusion gradient that allows HCO3- ions to enter the RBC in exchange for Cl-.
What is cyanosis?
It is the purple discolouration of the skin and tissues when the concentration of deoxyhaemoglobin becomes excessive.
What are some clinical aspects of haemoglobin and oxygen transport?
- insufficient haemoglobin (anaemia)
- carbon monoxide poisoning
Haemoglobin has >200x the affinity for CO than it does for O2, and they compete for the same binding site.
CO poisoning and anaemia both decrease the oxygen-carrying capacity of the blood by decreasing the available haemoglobin-oxygen sites. The oxygen content decreases whilst saturation of the oxygen binding sites is normal.
