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22
Gas Exchange
60
Pharmacology
Undergraduate 1
12/18/2016

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Term
How much O2 do cells consume at rest?
Definition
250ml
Term
How much CO2 do cells produce at rest?
Definition
200ml
Term
How much does the average male inhale per min?
Definition
inhales 6000mL (so, 6L) of air
Term
How much of this reaches the alveoli?
Definition
4200mL of this 6000mL reaches the alveoli.
Term
How much of the air that reaches the alveoli is o2?
Definition
880mL (21%) of the 4200mL that reaches the alveoli is O2
Term
How much of the o2 that reaches alveoli diffuses into blood?
Definition
250mL of the 880mL that is O2 diffuses into blood.
Term
What happens to the remaining O2?
Definition
the remaining 630mL of O2 from the 880mL is exhaled.
Term
How much O2 does the body consume every day?
Definition
360,000mL of O2 per day
Term
What are the essential primary symbols?
Definition
P: pressure or partial pressure
V: volume of gas
Q: flow of blood
Term
What are the essential secondary symbols?
Definition
A: alveolar (PAO2)
a: arterial (PaCO2)
V: venous (PVO2)
Term
State Daltons Law
Definition
in a mixture of gases, each gas present in the mixture contributes to the total pressure of the gas’
Term
Explain Daltons Law
Definition
In a mixture of gases, each gas has a partial pressure, which is the hypothetical pressure of that gas if it alone occupied the volume of the mixture at the same temperature. The total pressure of an ideal gas mixture is the sum of the partial pressures of each individual gas in the mixture.
Term
What is resting O2 consumption?
Definition
250ml.min-1
Term
Describe diffusion in the capillary beds in the lungs (pulmonary capillaries)
Definition
O2 diffuses from the alveolar air to capillary blood
CO2 diffuses from capillary blood to alveolar air
Term
Describe diffusion in the systemic capillary beds
Definition
O2 diffuses from the blood to the cells (via ISF)
CO2 diffuses from the cells to blood (via ISF)
Term
How do gases diffuse when in a gas mixture?
Definition
down partial pressure gradients (high to low)
each gas has it's own gradient independent from the others
Term
What is total air pressure at sea level?
Definition
760mmHg
Term
If total air pressure at sea level is 760mmHg and O2 occupies 21%, what is PO2?
Definition
(0.21 x 760) = 160mmHg
Term
What does Fick's Law state?
Definition
the rate of diffusion of a gas through a sheet of tissue is directly proportional to the area (A), the partial pressure difference (P1 - P2) and the diffusion constant (D)

and is inversely proportional to the tissue thickness (T)

Vgas = (A/T) . D . (P1 – P2)
Term
What does Fick's law explain?
Definition
the rapid diffusion of gases across the respiratory membrane
(large A and low T)
Term
What does Fick's law highlight the importance in?
Definition
maintaining a reasonably large partial pressure gradient (especially for O2, whose solubility is relatively low)
Term
What happens to atmospheric air when it is inhaled, before it reaches the alveoli? (2)
Definition
1. mixed with air in the respiratory 'dead space' which is rich in CO2 and deficient in O2
2. becomes saturated with water vapour (PH2O = 47mmHg at 37dc)
Term
What difference does this cause alveolar air to have to atmospheric air?
Definition
higher PCO2 and Lower PO2
Term
Give PAO2 and PACO2
Definition
PAO2 = 100mmHg
PACO2 = 40mmHg
Term
Give atmospheric PO2 and PCO2
Definition
PO2 = 160mmHg
PCO2 = 0.3mmHg
Term
Give systemic PaO2 and PaCO2
Definition
PaO2 = 100mmHg
PaCO2 = 40mmHg

These figures are the same for alveolar air, pulmonary veins and systemic arteries
Term
Give pulmonary PaO2 and PaCO2
Definition
PaO2 = 40mmHg
PaCO2 = 46mmHg

also true for systemic veins
Term
Give a very basic pattern for PO2 and PCO2 in the body
Definition
PO2 decreases from atmospheric air to pulmonary arteries (120 drop overall)
PCO2 increase from atmospheric air to pulmonary arteries(45 increase overall)
Term
Why does PCO2 only increase by 6mmHg from systemic arteries to pulmonary arteries?
Definition
CO2 is about 20x more soluble than O2
Term
Explain the changes in PO2 as altitude above sea level rises
Definition
PO2 decreases in proportion with the decrease in barometric pressure (PB)
Term
How does the O2 diffusion gradient from atmosphere to alveoli to arterial blood to tissue change as altitude increases?
Definition
decreases
therefore sufficient o2 delivery is difficult
acclimatisation is needed
Term
What is Henry's Law?
Definition
the amount of a gas that dissolves in a liquid at equilibrium depends on both the partial pressures of the gas in the gas mixture to which the liquid is exposed, and on the specific solubility of that gas in that particular liquid

c = kP
Term
Explain Henry's Law
Definition
once a system (e.g. alveolar air and blood) has reached equilibrium, the partial pressures would have reached equilibrium, however the concentrations would be different.

At any given partial pressure, the conc of o2 in blood is much lower than the conc of co2.
Term
Compare o2 and co2 solubility
Definition
o2 is about 20 times less soluble than co2
Term
How does pressure change with water depth
Definition
pressure increases so that even 10m adds 1 atmosphere (e.g. 760mmHg) of pressure
Term
A diver at a depth of 30m is breathing a mixture of O2 and N2 at a pressure of ___ atm
Definition
4
Term
What are the corresponding partial pressures of o2 and n2
Definition
o2 = 635mmHg
n2 = 2400mmHg
Term
What does this mean for n2?
Definition
4x the amount of n2 dissolved in body fluids and tissues
Term
What are the effects of a rapid ascent?
Definition
- sudden drop in PN2 in the lungs and body tissues
- n2 comes out of solution and forms bubbles
- severity depends on how many bubbles and if (and where) they become trapped
Term
How long does it take for o2 and co2 to equilibriate when passing through a typical pulmonary capillary?
Definition
O2 equilibrates within the first third of the time it takes for blood to pass along a capillary.

0.25s (out of 0.75s)
Term
In a healthy individual, what is gas exchange in the lung limited by?

at rest!!
Definition
rate of pulmonary blood flow
perfusion limited
Term
How does this change at exercise?
Definition
it doesn't
transit time is .25s so should still be perfusion-limited
diffusion still proceeds to equilibrium
Term
How might this change is the blood-gas barrier is thickened by disease?
Definition
o2 uptake may become diffusion limited
especially during exercise
Term
When might o2 uptake may become diffusion limited in healthy subjects?
Definition
if PAO2 is reduced (e.g. at high altitude)
Term
What causes oedema? (2)
Definition
1. abnormally high rate of capillary fluid filtration
2. impaired lymphatic drainage
Term
What causes pulmonary oedema?
Definition
left sided heart failure
causes increased pulmonary venous and capillary pressures leading to hydrostatic oedema
Term
What two phases does pulmonary oedema occur in?
Definition
1. interstitial oedema - accumulation of excessive fluid in the interstitial spaces in the lung
2. alveolar oedema - fluid accumulates within the alveoli.
Term
How does alveolar oedema interfere with breathing?
Definition
1. increases the effective thickness of respiratory membrane (increasing the air to blood diffusion distance)
2. interfering with the action of pulmonary surfactant, which decreases lung compliance and increases the work of breathing
Term
Symptoms of pulmonary oedema
Definition
srapid shallow breathing
hypoxia
cyanosis
Term
Describe Type I alveolar cells
Definition
make up the wall of alveoli, comprised of a single layer of epithelial cells.
Term
Describe Type II alveolar cells
Definition
secrete surfactant.
Term
At what partial pressures does O2 and CO2 enter the systemic capillaries?
Definition
PO2 = 100mmHg
PCO2 = 40mmHg
Term
What are the partial pressures of O2 and CO2 like in the surrounding ISF?
Definition
PO2 is lower
PCO2 is higher
(actual values depend on metabolic activity)
Term
What happens?
Definition
O2 diffuses from the blood into the surrounding tissues and CO2 diffuses in the opposite direction
Term
Compare the PO2 and PCO2 of arterial and venous blood
Definition
arterial has higher PO2 and lower PCO2
venous has lower PO2 and higher PCO2
Term
Describe the blood in systemic veins
Definition
blood may leave systemic capillary beds with different PO2 and PCO2 values and is mixed together in the systemic veins = mixed venous blood

PO2 of 40mmHg and PCO2 of 46mmHg
Term
What 3 factors determine alveolar PO2 and PCO2
Definition
1. PO2 and PCO2 of inspired air
2. volume of fresh air reaching the alveoli each minute (alveolar ventilation)
3. the rates at which respiring tissues consume o2 and produce co2
Term
Which factor is crucial?
Definition
alveolar ventilation relative to the rate of o2 consumption and co2 production

PO2 and PCO2 do not change under most conditions
Term
Describe hyperpnea
Definition
An appropriate increase in alveolar ventilation relative to the demands of the tissue i.e. if o2 consumption and co2 production increase, do does alveolar ventilation

therefore, alveolar alveolar po2 and pco2 do not change
Term
What happens when alveolar ventilation does no match the demands of the tissue?
Definition
hyperventilation and hypoventilation
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