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What are two diffusion limitations across the alveolar walls?
- Pulmonary edema - alveoli become fluid filled, gas exchange cannot occur since the fluid displaces the air
- Interstitial fibrosis - alveoli become inflamed and scar tissue forms in interstitial layers of the alveolar walls. Thickens the diffusion membrane
Which is more soluble in plasma O2 or CO2
CO2 is 20x more soluble
Where is oxygen transported
- <1% of O2 is "dissolved" directly into blood stream
- >995 is carried by hemoglobin in the RBC
- Hemoglobin is the red pigment of blood
- (mucous membranes tell us saturation)
What color does oxyhemoglobin turn?
RED (Hb + O2)
what color does deoxyhemoglobin turn?
Purple (O2 released)
* % saturation
= % of all the Hb molecules that are bound to O2
Each Hb carries how many diff O2?
Composition of Hb
- 4 heme protein subunits
- each has iron atom
- O2 binds loosely and reversibly to iron
Is the relationship between saturation and partial pressure linear?
NO its sigmoidal
S shaped saturation vs partial P curve?
- 'flat portion" 60-100mmHg PO2 = load O2 (for high carrying capacity so can carry to lungs)
- "steep portion" 30- 50mmHg PO2 = unload O2 (at tissues** because want to deliver O2)
from 60-100mmH of PO2 do we see a significant change in the saturation of Hb?
No change in Hb saturation (stays at 100%)
what do large changes in PAO2 (alveolar PO2) have on % saturation?
Hb saturation across the normal range for O2 in the lungs is NOT significantly affected by PAO2
From 30-50mmHg what is seen in the % saturation of Hb?
HUGE change in the % saturation (90% to 50%) - unload O2
When does Hb release O2?
when the surrounding O2 drops (ie in the tissue)
What are 2 physiological advantages of having an S shaped saturation vs partial P curve?
- Lungs: loading (where PO2 is high)
- Tissues: unloading (where PO2 is low)
(so veins are low in O2 and arteries are high in O2)
What happens with a Right shift on saturation PO2 graph? WHERE does it occur?
- "Right Shift": decreases the affinity of Hb for O2 (more O2 is released)
- occurs with increased Temp, decrease pH, increase CO2
- occurs at the level of the tissues
what happens with a left shift on the saturation PO2 graph?
- "Left shift": increases the affinity for O2 (more O2 is loaded)
- occurs with decreased temp, increase pH, decreased CO2
- occurs at the level of the lungs
What shift occurs when an animal is hypothermic?
- Left shift
- hold onto O2 levels in tissue
- shock because not enough O2 in system
For OxyHB dissociation what occurs in left and right shifts?
- Left shift: decreased temp, decreased 2-3 DPG, decreased H+ (increased pH), decreased CO2 : increased O2 affinity
- Right shift: increased temp, increased 2-3 DPG, increased H + (decreased pH), decreased CO2 : decreased O2 affinity
What are some contributors to oxygenation
- 1. oxygen uptake: O2 taken up in alveolus, or arterial blood? (figure out through alveolar gas equation)
- 2. oxygen content: how much O2 is in the arterial blood (dissolved and bound to Hb)
- 3. oxygen delivery: blood flow (cardiac output) is needed to actually move blood through the body to deliver oxygen.
How is the amount of oxygen uptake determined?
- alveolar gas equation
- calculates max O2 pressure that can exist in the alveolus at a given time
- (room air is normally ~21% O2, but the partial P can change with altitude and weather)
- **********PaO2 (arterial) should be almost equal to PAO2 (alveolar) if everything is working properly
*If everything is working properly what should PaO2 and PAO2 be relative to one another?
arterial should be ~=to alveolar partial pressure of O2 if everything is working properly
what is the alveolar gas equation?
- PAO2= FiO2 x (PB - PH20) - (PaCO2/0.8)
- FiO2 is the fraction of inspired oxygen
- PB is barometric P ~760mmHg @sea level and ~670 in Calgary
- PH2O is the water vapor pressure which is 47 at normal body temp
- *If ventilation is normal PaCO2 is 400.8 is the respiratory quotient (related to metabolism)
what is the normal arterial oxygenation at sea level?
PaO2 is 100mmHg
what happens under anesthesia?
- Under anesthesia patients breath 100% O2
- so PAO2 and PaO2 should be 500mmHg
What is the ideal PaO2 to maintain?
above 80mmHg maintain in anesthetics especially
"Hypoxemia" vs "Hypoxia"
- hypoxemia: refers to the condition when the oxygen saturation (%) of arterial blood is low. (need to measure this with an arterial blood gas sample or by using a pulse oximeter)-purple
- hypoxia: refers to when the oxygen levels are low at the level of the tissues themselves
Site of sampling
- A blood sample is collected "downstream" from a tissue bed ---analysis will reflect the metabolism and gas levels at that location only
- You need to think about what part you are interested in
What is the "A-a gradient" calculation used for?
- compares PaO2 vs PAO2 to decide if the oxygen levels are "acceptable"
- -A-a gradient = PAO2 - PaO2
- Normal is 0 -15 mmHgassesses the difference between oxygen in the alveoli and the oxygen in the arteries
- Serves as a measure of the ability of the lungs to load O2 and the blood to pick it up***
What does a large A-a gradient difference reflect?
- the inability of the lungs to oxygenate blood
- the presence of venous admixture
What factors in the alveolar gas equation can we medically manipulate?
- FiO2- so increase inspired O2 to increase levels in the alveoli (cost effective)
- PaCO2- ventilate patient to remove CO2 (if CO2 is too high hypercarbia is a problem)
Describe how oxygen content contributes to oxygenation
- The total amount of oxygen that is carried in the blood is a combo of its dissolved hb-bound components
- content = (1.34 x Hb x O2Sat%) + (PO2 x o.oo3) Need adequate oxygen pressure as well as adequate levels of Hb to carry the O2 or else the total content will be low
What is the normal arterial oxygen content?
~20mL of oxygen/dL of arterial blood (nearly all is carried by Hb)
- DECREASE in oxygen content
- so Hb amount decreases
- = ~11mL of O2/dL of blood (compared to the normal 15mL/dL in venous blood)
so if Hb drops by 50% so will the O2 content (because don't have the RBC to carry O2)
How does oxygen delivery contribute to oxygenation?
- D(with dot ontop) = cardiac output x arterial O2 content
- -cardiac output is the V of blood pumped per min by heart beat