blood gas transport

Question 1

why is hemoglobin critical to O2 transport ?

Answer 1

• Oxygen has low solubility in plasma (0.225mL/L/kPa). In order to dissolve the amount of O2 needed to supply tissues, an impossibly high alveolar PO2 would be required.
• The presence of haemoglobin overcomes this problem – it enables O2 to be concentrated within blood (↑ 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%).

Question 2

describe the shift in oxygen haemoglobin curve due different factors

Answer 2

Question 3

describe the Hb O2 affinity changes depending on the local environment that enable O2 delivery to be coupled to demand

Answer 3

Question 4

what is cyanosis ?

Answer 4

purple discoloration of the skin and tissue when deoxyhemoglobin becomes excessive

Central cyanosis:
*(discoloration of the core, mucous membranes and extremities) reflects inadequate oxygenation of blood within the lungs (e.g. due to hypoventilation, gas exchange defects or V/Q mismatch).

Peripheral cyanosis:
* (discoloration confined to the extremities) reflects inadequate oxygen supply to only these tissue

Question 5

what is anemia ?

Answer 5

• blood transport disorder
• Anaemia (a decrease in the number of red blood cells per unit of blood volume) can occur due to iron deficiency or rapid and excessive loss of RBCs (e.g. haemorrhage).
• A decrease in red blood cell density will result in a reduction in the concentration of haemoglobin, total oxygen binding sites, and oxygen carrying capacity. However the affinity of haemoglobin is unchanged (i.e. at any given PaO2, the percentage Hb binding sites that are occupied by oxygen will remain the same, there are just less of them in total).
• Therefore Hb-O2 saturation and O2 partial pressure within the plasma will be normal, whereas overall total O2 content of the blood will decrease, as will the overall concentration of both oxyhaemoglobin and deoxyhaemoglobin, as reflected in the symptoms of pale skin and conjunctiva)

Question 6

what is carbon monoxide poisoning ?

Answer 6

• results from exposure to excessive levels of CO
• Similar to anaemia, it involves a decrease in total oxygen carrying capacity due to a decreased number of Hb binding sites being available.
• in contrast to anaemia, in CO poisoning the overall concentration of Hb in the blood remains constant. CO displaces O2 at Hb binding sites, as it binds with much greater affinity. As binding sites are occupied by CO, less O2 can bind and so less is transported.
• Therefore the total O2 content of the blood will decrease, as will the concentration of oxyhaemoglobin.

Question 7

Describe the impact of specific clinical conditions on blood oxygen transport

Answer 7

Question 8

How and why does transport of CO2 differ to O2?

Answer 8

• CO2 has a higher H2O solubility than O2 does – therefore a greater % of CO2 is transported simply dissolved in plasma
• CO2 binds to Hb at different sites than O2 (R–NH2 residues at the end of peptide chains, forming carbamino-Hb, R-NHCOOH) and with decreased affinity. Thus, a lower % of CO2 is transported in this manner
• CO2 reacts with water to form carbonic acid, which accounts for the majority (≈70%) of CO2 transported

Question 9

why does venous blood carry more CO2 than the arterial blood ?

Answer 9

• due to the haldane effect
• when O2 binds to Hb it changes the shape making it less able to hold onto CO2 and H ions so oxygenated Hb carries less CO2 and H ions
• when Hb binds to H ion it removes them from the blood this shifts the equilibrium of a reaction to the right and ultimately causes dissolved CO2 to be pulled out of the plasma and stored as bicarbonate
• This CO2 lost via conversion to HCO3 is then replaced by fresh CO2 produced by tissues and thus the overall amount of CO2 carried within the blood increases.
• where oxygenation of blood increases, Hb-H+ binding is reduced and the reverse effect occurs, decreasing CO2 carrying capacity