L17. Transport of Oxygen in the Blood Flashcards
What are the 2 equations that show factors that regulate the movement of gas across the respiratory surface
Diffusion of a volume of a gas:
Lung area x gas density x (Pressure difference across the tissue) / thickness of the alveolar membrane
- The Gas density: solubility/ molecular weight
How does area affect amount of gas diffused
-Alveoli are approx 0.3 mm in diameter. Multiple alveoli increase surface area for the same amount of volume. This helps movement of gas because there are more points of contact for exchange to occur
What is the thickness of tissue between air and blood, what is it made of and what is the downside
-Only 0.5 um between air and blood. Made of
-surfactant, epithelium, interstitium, basement membrane and endothelial cell.
The downside is that there is reduced protection against invasion of microbes
What are the general partial pressure difference across alveolar to capilaries for O2 and CO2
For O2: PAlv: 100 mmHg, Ppul: 40 mmHg.
Therefore large driving force (1ox more than CO2) for O2 from alveoli to blood.
For CO2: PAlv: 40 mmHg, Ppul: 46 mmHg. Therefore small driving force for CO2 out of blood to alveoli
Why is the rate of movement of CO2 and O2 across the membrane balanced when there are differences in pressure, solubility, mw
Solubility is more important than MWt.
CO2 is 25x soluble in blood than O2 and therefore diffuses 0.86x faster. However,
release time of CO2 from haemaglobin is slower than O2 and O2 has a greater pressure difference (driving force) so rate of CO2 and O2 exchange is balanced
Explain diffusion vs blood perfusion limited gas uptake across the alveolar membrane
Perfusion limited gases (eg. O2/ N2O) are gases that reach saturation of haemoglobin in a very short amount of time (>200 ms). Therefore the amount of gas taken up is depend on how fast the blood flow there is, because the more RBCs = more O2 taken up.
Diffusion limited gases (CO), take a long time to cross the membrane and therefore the amount of haemoglobin saturation is more dependent on that time rather than blood flow.
Explain the relationship shown on the oxygen dissociation curve for haemoglobin and the 2 physiological advantages
Oxygen dissociation curve shows relationship between PO2 and
%O2 sat of hb.
At low PO2 (systemic veins) Hb has lower affinity for O2 so it encourages O2 release–> 80% sat
At high PO2 (systemic arteries) there is higher affinity for O2 which encourages uptake of O2 at the lungs -> 100% sat.
1) a small change in PAO2 will not affect the loading of O2 saturation/content of O2 in the blood.
Due to cooperative binding, there is a sigmoidal relationship where below PO2 of systemic veins (40) there is a steep reduction in the saturation of haemoglobin. 2. large amounts of O2 are released for only a small drop in capillary PO2.
Why does Hb Affinity change
By product of CO2 production at the tissues is H+. This acidifies Hb4O8 - pH decreases and helps to reduce its affinity for O2 and unload it.
What is the difference between oxygen content of blood and oxygen saturation
Blood O2 content: actual amount of O2 in the blood- dependent on amount of Hb present.
Blood O2 saturation: The percentage of Hb that has been saturated. Independent on the amount of Hb there is. dependent on Hb affinity for O2.
What factors change affinity of Hb for O2 at the lungs
Decreased CO2, decreased H+ (higher pH), decreased temperature, DPG. For a given PO2 Hb is more easily saturated. (leftward shift)
What is the Bohr shift and the factors that drive it
This is a shift in the oxygen dissociation curve towards the right where for a given PO2, Hb has a lower affinity for O2 so more O2 is released.
This is driven by increased CO2, increased H+ (lower pH), increased temperature and DPG - an intermediate metabolite of glycolysis done by the RBCS.
This is found at the tissues.
Describe the oxygen dissociation curve for fetal Hb and Myoglobin
Fetal Hb has a leftward shift: a greater affinity of Hb for O2 at same PO2 due to less DPG in fetal RBC. This helps movement of O2 across the placenta from mum blood to the fetus.
Myoglobin has a even greater affinity for O2, and will only give up O2 at very low levels (hypoxia). Therefore good O2 storage molecule
Why do we need RBCs
Encapsulates high conc of Hb - stops high blood viscosity
Provides an environment for DPG- enhance O2 release
Encapsulates & concentrates carbonic anhydrase
Prevents Hb loss via filtration
Concave shape helps passage through tight spaces.
