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Pulmonary system > Introduction to Gas Exchange -notes > Flashcards

Introduction to Gas Exchange -notes Flashcards

1. Distinguish between the following terms: minute, alveolar and dead space ventilation; and anatomic, alveolar and physiologic dead space. 2. Specify the partial pressures of O2 and CO2 in the alveoli, mixed venous and arterial blood in normal individuals. 3. Using the alveolar ventilation equation, discuss the factors that determine the partial pressure of CO2 in the alveoli and define the terms hyperventilation and hypoventilation. 4. Name the factors

1
Q

Two types of gas movement in the lungs

A
  1. Bulk flow

2. Diffusion

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2
Q

Mechanism gas movement from atmosphere into the airways (trachea down to alveoli)

A

-bulk flow

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3
Q

Bulk flow

A

All gas molecules move as a unit down a pressure gradient

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4
Q

Pressure gradient for bulk flow

A

The difference between the pressure at the mouth and the pressure in the alveoli

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5
Q

Mechanism gas move from alveoli into blood

A

-diffusion

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6
Q

Diffusion

A

-individual gas molecules move according to their partial pressure diffusion gradient (from high pressure to low pressure)

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7
Q

Ventilation

A

General term for movement of air into and out of the lungs

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8
Q

Minute ventilation

A

Total amount of air moved into or out of the lungs per minute
-typically measured as the quantity of air expired per minute (Ve)

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9
Q

Calculation of Ve

A 
Tidal volume (Vt, volume of normal breath) x respiratory rate (RR)
Ve = Vt xRR
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10
Q

Anatomic dead space volume

A
  • volume of inhaled gas duing a normal breath that remains behind in the conducting airways (and therefore does not participate in gas exchange)
  • simply enters and exits the conducting airways
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11
Q

Normal tidal volume

A

500 ml

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12
Q

Normal dead space voume

A

150 ml

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13
Q

Normal functional residual capacity

A

2400 ml

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14
Q

Alveolar ventilation (VA)

A

Term used to characterize the volume of air per minute that enters or exits the alveoli and participates in gas exchange
= minute ventilation (VE) - dead space bentilation (VDS)

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15
Q

Composition of physiologic dead space

A

Volume of conducting airways (anatomic dead space) as well as the volume of the alveoli that are not being perfused (alveolar dead space)

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16
Q

Estimation of anatomic dead space

A

-1 ml per pound of body weight

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17
Q

Normal alveolar dead space

A

-20-25ml

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18
Q

What does alveolar dead space represent

A

the volume of alveoli typically in the apex of the lung in an upright person that do not receive blood flow

19
Q

Type of movement across alveolar-capillary membrane

A

-passive diffusion down a partial pressure gradient

20
Q

Partial pressure gradient for O2

A
  • partial pressure in the alveoli = 100 mmHg

- partial pressure in blood returning to the lungs = 40 mmHg

21
Q

Partial pressure gradient for CO2

A
  • blood returning to the lungs = 46 mmHg

- in the alveoli (low pressure) = 40 mmHg

22
Q

End capillary equilibration

A 
PAO2 = PaO2
PAC2 = PaCO2
A = alveolus, a = arterial blood
23
Q

Toxic effect of high carbon dioxide

A

1) Dyspnea
2) Acidosis
3) Altered levels of consciousness

24
Q

Normal range for carbon dioxide in the body

A

38-42 mmHg

25
Q

Equation describing the relationship between alveolar ventilation and CO2 elimination (alveolar ventilation equation)

A

PACO2 = VCO2 /VA *0.863
-PACO2 is diretly proportional to the amount of CO2 produced by metabolism and deliverd to the lungs (VCO2) and inversely porportional to alveolar ventilation (VA)

26
Q

Hyperventilation

A

Elevated alveolar ventilation

Defined by reduction in partial pressure of CO2

27
Q

Hypoventilation

A

Reduced alveolar ventilation

Defined by increase in partial pressure of CO2

28
Q

Hypercapnia

A

When alveolar ventilation is inadequate for the volume of CO2 being produced
Elevated PaCO2 >42 mmHg
Patients with hypercapnia are hypoventilating

29
Q

Hypocapnia

A

When alveolar ventilation rates exceed that required for the volume of CO2 being produced
Low PaCO2 <38 mmHg
Patients with hypocapnia are hyperventilating

30
Q

Respiratory rate and hypo/hyper-ventilation

A

Hypo/hyper ventilation refer only to high or low PaCO2 - should not be used to characterize respiratory rate or breathing effort

31
Q

Why hypoventilation is always associated with hypercapnia

A
  • reduction in alveolar ventilation = volume of gas entering and exiting the alveoli per unit time is reduced
  • this causes an increase in alveolar PCO2 (PACO2) because the gas is not being exchanged at a normal rate
  • CO2 in blood perfusing he alveoli will then begin to accumulate and increase in PaCO2
32
Q

Things that always cause hypercapnia

A

1) Not enough total ventilation (CNS depression or weak respiratory muscles)
2) Too much of the total ventilation ending up in dead space ventilation (COPD, rapid shallow breathing)

33
Q

What does the volume of O2 and CO2 that will diffuse across the alveolar-capillary membrane depend on

A

1) Resistance to diffusion of the gas across that membrane

2) perfusion (pulmonary capillary blood flow)

34
Q

Flick’s law

A

Describes the resistance to diffusion
Volume of gas transferred across the membrane per minute (Vgas) is directly proportional to the SA of the embrane A the diffusion coefficient of gas D, and the partial pressure difference between the two side and is inversely proportional to the thickness of the membrane
V gas = (A/T) xDx (P1xP2)

35
Q

Pulmonary capillary blood flow and reserve capacity

A
  • PO2 of blood entering capillary (mixed venous blood) is 40 mmHg
  • in alveoli just 0.3 um away PAO2 =100 mmHg
  • O2 move don partial pressure gradient from aleolus into the blood
  • PaO2 will reach that of alveolar gas in 1/3 of the time that blood spends in the capillaries (0/75 sec)
  • the extra time = reserve capacity -could be used for diffusion if blood was moving faster (increased CO)
36
Q

Diffusion capacity of the lungs (DL)

A

-a measure of the rate of gas transfer in the lungs per partial pressure gradient

37
Q

How to measure DLO2 clinically

A

-by measuring the diffusion capacity of CO
-because:
a) essentially no CO in venous blood (so driving pressure of the partial pressure gradient will be high)
b) hemoglobin’s affinity for CO is 200x greater than )2 - therefore partial pressure in pulmonary capillary blood remains extremely low and CO taken up along entire length of capillary
DLCO = VCO/PaCO
VCO = CO uptake mm/min
PaCO = partial pressure of CO in the alveoli

38
Q

Factors that influence the normal resting value of DLCO

A
  • age
  • sex
  • body size
  • any process that decreases the SA available for diffusion or thickens the alveolar capillary membrane will decreae DCO
  • factors that affect pulmonary blood flow or hematocit
39
Q

Normal range of DLCO

A

20-30 ml/minmmHg

40
Q

Medical conditions that decrease SA for diffusion

A
  • emphysema
  • lung/lobe resection
  • bronchial obstruction
  • pulmonary emboli
  • anemia
41
Q

Medical conditions that increase wall/membrane thickness

A
  • pulmonary fibrosis
  • asbestosis
  • sarcoidosis (involving parenchyma)
  • collagen vascular disease (scleroderma, systemic lupus erythematosus)
42
Q

Alveolar air equation

A
  • used to calculate PAO2
  • describes relationshi between concentration of inspired oxygen, alveolar ventilation (PaCO2) and PAO2

-PAO2 = FIO2 (Patm- Ph2o)- PaCO2/R

R = respiratory quotient

43
Q

R (respiratory quotient)

A
  • ratio between oxygen consumed and co2 produced (or ratio of CO2 molecules to O2 molecules exchanged within the alveoli)
  • under steady state conditions approx 250 ml of O2 per min transfered to pulmonary circ (VO2) while 200 ml of CO2 are removed (VCO2)
  • varies with individuals diet - in north american diet R=0.8, primarily carb R approaches 1, rich in fat R approaches 0.7
44
Q

Calculating the A-a DO2 clinically

A

1) Obtain PaO2 and PaCO2 levels by sampling arterial blood gases
2) Use PaCO2 and the alveolar air equation to calculate PAO2
3) Calculate A-a DO2 = PAO2 -PaO2

Pulmonary system (19 decks)

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