Lecture 8: The Function Of The Gas Exchange Unit Flashcards
1
Q
Alveolar generation and gas exchange
A
- at generation 19: start to get budding of alveoli and gas exchange starts to happen
- More and more alveoli as you go down to the base of lungs - more gas exchange
2
Q
Processes of oxygen transport
A
- convection through major airways
- diffusion through smaller airways
- diffusion across alveolar-capillary membrane and comination with Hb
- Convection to tissues (heart and circulation)
- Diffusion through tissues to mitochondria
- C02 is the same in the reverse direction
3
Q
Reasons why Po2 might go down
A
- Hypoventilation (decrease Va)
- Poor cardiac output
- Ventilation perfusion mismatch inclusing shunt
- diffusion impairment (rare): interstitial lung disease
- Anemia or dysfunctional hemogliobins: reduced carrying capacity in blood
4
Q
What does the partial pressure of O2 depend on?
A
- depends exclusively on the ventilation/perfusion ratio of that GEU
5
Q
Ventilation perfusion mismatch?
A
- each unit inspires the same composition gas and is perfused by the same composition mixed venous blood with diffusion equilibration
- the compositin of end-capillary blood, equal to the expired gas is solely determined by the Va/Q ratio
- any inquality in the distribuition of the Va/Q ratio among units must result in a reduced PaO2 and raised PaCO2 in the mixed blood
6
Q
VA/Q inequality effects
A
- small effect on Pa O2 in health - due to gravity and small degree of mechanical inhomogeneity in the dormal lung
- major mechanism of hypoxaemia in disease
- Major underlying mechanism of Hypercapnia - compensated often by increase in ventilation
7
Q
Hydrostatic forces increasing blood flow down the lung
A
- more ventilation cause more alvoloi
- more blood flow at base of lung than top of lungs because higher blood pressure -> higher perfusion
8
Q
Distribution of Va/Q ratio due to gravity
A
- blood flow bottom to top: decrease
- ventilation bottom to top: decrease but crosses blood flow line
- VA/Q ratio thus lower at bottom of lung and higher at top of lung
9
Q
Summary
A
- gravity effects cause a minor degree of gas exchange impairment in the normal lung
- more serious Va/Q inequality occurs within each gravity zone in disease
- Measured by multiple inert gas elimination technique (MIGET)
10
Q
MIGET
A
- measures the simultaneous excretion and retention of 6 inert gases delivered to the lung in the mixed venous blood
- able to differentiate a far wider distribution of VA/Q ratios
11
Q
VA/Q distribution in asthma
A
- episodic asthma: blood flow normal but ventilation decreased and shifted to the right
- Chronic severe astma: blood flow derease, ventilation good
- acute severe asthma: low BF, severe hypoxemia: also distribution shifted
12
Q
Evolution of Va/Q inequality in acute asthma
A
- takes about a month to go back to normal
- remains hypoxemix for longer than their airways got blocked for
13
Q
Va/Q inequality in disease and the effect of increasing ventilation
A
- in disease, pO2 decreases, PCO2 increases.
- increasing ventilation can correct the pCO2 but not the PO2. This is because of the oxygen and carbon dioxide hemoglobin dissociation curves
14
Q
Oxygen delivery
A
- oxygen is carried int he blood in 2 forms: dissolves or combined with Hb
- Dissolved O2 obeys Boyle’s law: amount dissolved is proportional to the partial pressure
- O2 forms a reversible combination with Hb
- a change in the Hb from a fully oxygenated state to its deoxygenated state is accompanied by a conformational change in Hb
15
Q
Hb-O2 dissociation curve advantages
A
- Flat upper portion: even if PaO2 falls somewhat, O2 loading won’t be affected. A large partial pressure difference between alveolar gas and blood continues to exist when most O2 is transferred
- steep lower portion: peripheral tissues can withdraw large amounts of O2 for only a small drop in PcO2 - assists diffusion into tissue cells
16
Q
Summary of importance of Va/Q inequality
A
- most usual cause of gas exchange derangement
- causes Hypoxemia and Hypercapnia
- Hypercapnia can generally be corrected but at the expense of an increase in ventilation
- Hypoxemia can be corrected by increasing inspired oxygen tension
17
Q
Diffusion across membrane
A
- proportional to the surface area of the membrane and to the partial pressure difference
- each capillary spends at rest about 0.75 sec in contact in contact with alveoli and about 0.25 sec to reach pressure equilibrium
- during exercise, blood is pumped 3x as fast so 3x less time to reach equilibrium - problmatic for people with thickened aveolar membrane
18
Q
Respiratory failure
A
- reduction in arterial oxygen tension below 80 mmHg breathing air at sea level
- increase in arterial CO2 above 45 mmHg
19
Q
Normal Pa O2 and Pa CO2
A
- 80-100mmHg (O2)
- 35-45 mmHg (CO2)
