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Respiratory > Lecture 5- Gas exchange > Flashcards

Lecture 5- Gas exchange Flashcards

1
Q

Boyles law

A
  • Pressure (P) of a gas is inversely proportional to its volume (V)
    • (if temperature & number of gas molecules remains constant in a closed system)
    • This means that if you expand the space in which a gas is contained – the pressure will drop.
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2
Q

Kinetic theory of gases

A

Gas pressure is caused by the collisions of gas particles with the walls of the container

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3
Q

partial pressure gases

A

def: pp of gas is the pressure exerted by an individual gas in a mixture of gases

  • Describes amount of a specific gas within a gas mixture
  • Uses gas pp gradient to explain gas diffusion
    • Gases diffuse down their pp gradient from high to low pp
  • Use pp to calculate amount of gas dissolved in liquid
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4
Q

Calculating pp of gas in a gas mixture

A

(% gas A)x (total pressure) = partial pressure gas A

  • Assumes there are no chemical reactions between gases
  • Application of Daltons law
    • In a. Mix of non-reacting gases, total pressure exerted is equal to the sum of pp of the individual gases
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5
Q

atmospheric pressure is the

A
  • pressure exerted by the weight of the air above the earth in the atmosphere
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6
Q

At high altitudes

A

atmospheric pressure is lower (weight of air pressing down is less)

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7
Q

percentage of oxygen in atmospheric air

A

20.9%

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8
Q

percentage of nitrogen in atmospheric air

A

78%

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9
Q

percentage of argon in atmospheric air

A

0.17%

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10
Q

percentage of carbon dioxide in atmospheric air

A

0.03%

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11
Q

total atmospheric pressure (dry) at sea level

A

101kPa

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12
Q

partial pressure of O2 at dry atmopheric pressure at sea level

A

pp O2= 20.9% of 101kPa

therefore

0.209 x 101 kpa= ppO2= 21.1kPa

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13
Q

When we breath in air it is

A

moistened, water molecules in vapour form are added to air

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14
Q

Water vapour exerts a pressure called

A

saturated vapour pressure

displaces a proportion of the total pressure of the mixture of gases- in this cases, total atmospheric pressure

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15
Q

Saturated vapour pressure =……….. at body temp- only depends on temp

A

6.28 kPa

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16
Q

We need to subtract …………………from the total pressure of the dry gas mixture to arrive at the total pressure of gas mixture in a moist environment

A

water vapour pressure

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17
Q

pp of O2 and nitrogen in air we breath in (moist) at atmospheric pressure

A

therefore 101(kPa) – 6.28 (kPa) = 94.7 kPa =total pressure of gas mixture in a moist environment

  • The other gases are still in the same ratios as in dry air
  • pO2 = 94.7 kPa x .209 = 19.8 kPa= partial pressure oxygen in URT
  • pN2 =(101-6.28)x.79 =73.8kPa
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18
Q

pCO2

A

partial pressure CO2- generic

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19
Q

pO2

A

partial pressure O2- generic

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20
Q

PACO2

A

partial pressure CO2 in alveoli ( big A= alveoli)

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21
Q

PAO2

A

partial pressure O2 in alveoli (big A= alveoli)

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22
Q

PaCO2

A

pp CO2 in arterial blood (little a= arterial)

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23
Q

Pa​O2

A

pp O2 in arterial blood (little a= arterial)

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24
Q

Pv​CO2

A

pp CO2 in venous blood

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25
Q

PvO2

A

pp O2 in venous blood

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26
Q

pp O2 in upper repiratory tract is…….. kPa

but

alveolar pp O2 is….

A
  1. 8kPa
  2. 3 kPa
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27
Q

why is the pp of oxygen different in the resp tract (19.8kPa) and alveolar (13.3 kPa)

A
  • Oxygen from alveolar air constantly diffusing into pulmonary circulation
  • Alveolar air only partially replaced with each breath
  • Alveolar partial pressure CO2 also different c/w Upper airways partial pressure CO2
28
Q

Alveolar vs pulmonary ventilation

- typical volume of air inspired/ expired at rest (tidal volume)

29
Q

pulmonary ventilation

A

Typical amount of air inspired/exhaled at rest ≈ 450 ml (called the Tidal Volume)

30
Q

anatomical dead space

A

e.g. where exchange of gases can take place - trachea, ,bronchi, bronchioles etc

Typically 30% of normal tidal volume fills

  • 30% of 450 ml = 150 ml (anatomical dead space= 150, therefore alveolar tidal volume is 450-150= 300)
31
Q

alveolar ventilation

A

amount of the pulmonary ventilation that reaches the alveoli

  • alveolar tidal volume is 450-150= 300)
  • Therefore, 300 ml of air reaches respiratory portion of lung – this is alveolar ventilation
32
Q

Pulmonary Ventilation Rate vs Alveolar Ventilation Rate

A

Can express amount air moved as a rate – ml air/minute- called minute ventilation

33
Q

Pulmonary Minute Ventilation

A
    • We breathe ≈ 12 times a minute - the total volume of air moved is 450 ml × 12 = 5400 ml/min
34
Q
  • Alveolar Minute Ventilation (ml/min)
A

is the rate of air flow reaching the alveoli – 12 breathes/minute × 300 ml = 3600 ml/min

35
Q

When gases dissolve in liquid

A

When inspired gases come in contact with body fluids (made up mostly of water) - gas molecules will enter fluid and dissolve in the liquid.

The amt of gas that dissolves is directly proportional to the partial pressure of that gas

36
Q

The amt of gas that dissolves is

A

directly proportional to the partial pressure of that gas

37
Q

Gas diffuse across pp gradients

A
  • A gas in air will dissolve into a fluid along is pp gradient, from high pp to low
  • This gas also exerts pressure in the fluid
  • Will continue until equilibrium is reach
  • Once equilibrium is reached the pp of the gas in the liquid is equal to the pp of the gas in the air- that is why they reached equilibrium, even though the concentrations are not the same- how do we explain this
    • Low pp gas- low rate of dissolving
    • High pp gas- number of collisions at water interface= elevated = higher rate of dissolving
  • Rate out (ligquid to gas)= conc of dissolved gas and solubility of gas for liquid medium
38
Q

rate of dissolving

A
  • Low pp gas- low rate of dissolving
  • High pp gas- number of collisions at water interface= elevated = higher rate of dissolving
39
Q
  • Rate out (liquid to gas)=
A

conc of dissolved gas and solubility of gas for liquid medium

40
Q

Gas diffusion air-liquid: oxygen

A
  • Alveolar partial pressure oxygen > partial pressure of oxygen in the venous blood arriving at the alveoli
  • Oxygen gas diffuses and dissolves down its pp gradients into the blood
  • Equilibria yes with blood
  • Partial pressure oxygen in blood leaving the alveoli (which will become arterial blood) is now equal to partial pressure oxygen in alveoli
  • The pp of oxygen in arterial blood is the equilibrium derived pp established at the air-fluid interface- I.e. at the alveolar- blood interface
  • The above explains why alveolar gas pp determine aterial gas pp.
41
Q

Gas Diffusing into a Liquid – Dissolved Gas Content

A
  • Depends on the partial pressure of the gas (note: partial pressure sometimes denoted as “tension”)
  • And
    • The solubility coefficient of that gas
    • Solubility coefficient is the volume of gas that can be dissolved in a fixed volume of solvent at a given temperature
    • Varies depending on chemical properties of the molecule and its interactions in the aqueous solution
  • To calculate the dissolved amount of a gas in a liquid as a concentration: – multiply partial pressure of that gas × the gas solubility coefficient
42
Q

example of calculation of gas diffusing into liquid

A

amount dissolved also depends on the solubility coeffieicent

43
Q

oxygen goes from…..

carbon goes from…..

A

oxygen goes from….. alveoli to blood

carbon goes from….. blood to alveoli

44
Q

how does oxygen get to equilibirum

A

Slight twist in the story for oxygen

  • If a gas chemically reacts (e.g. O2 binding to Hb) with a component of the liquid in addition to dissolving
  • This reaction must be complete before equilibrium is reached and the equilibrium pp of gas in air and liquid is established
    • O2 enters plasma and dissolves in it
    • Dissolved O2 enters RBC and binds to Hb
    • Process continues till Hb fully saturated (each can bind 4O2 molecules)
    • After hb is fully saturated , O2 continues to move down its pp gradient
    • At equilibrium, PO2 of plasma= pO2 of alveolar air
    • Oxygen on Hb does not contribute to partial pressure oxygen in blood- not free- but Hb by removing free oxygen allows more oxygen to be delivered to blood from alveoli
45
Q

Oxygen on Hb does

A

not contribute to partial pressure oxygen in blood- not free-

however by removing free oxygen from the plasma , Hb allows more oxygen to be delivered to blood from alveoli

46
Q

how does Haemoglobin help keep the oxygen gradient strong

A

binding proteins e.g. Hb, binding to oxygen dissolved in the blood which comes from the alveoli, keeps oxygen pressure low and maintain a storng gradient for diffusion

47
Q
  • pO2 reflects the amount of
A

dissolved O2 in the blood

48
Q

total content of oxygen in the blood=

A

dissolved oxygen + hb bound oxygen

49
Q

diffusion of carbon dioxide from the blood to the alveoli for expulsion

A
  • CO2 diffuses from blood into alveoli
  • Along pp gradient
  • Partial pressure of CO2 in mixed venous > partial pressure CO2 in alveoli
  • Equilibrates so that equal in arterial blood
50
Q

what is the PAO2? Alveolar oxygen composition

51
Q

what is the PACO2? Alveolar carbon dioxide composition

52
Q

Amount of Oxygen (ml/min) brought in by ventilation=

A

amount of oxygen (ml/min) diffusing into the blood

53
Q

Amount of carbon dioxide (ml/min)=

A

amount of carbon dioxide diffusing from the blood into the alveolus

54
Q

Diffusion: factors affecting rate of diffusion

A
  • Partial pressure difference
  • Surface area
  • Thickness
  • Diffusion coefficient of the individual gas
55
Q

ease of diffusion is of oxygena nd carbon dioxide is determined by

A

solubility an molecular weight of the gas

56
Q

solubility of CO2 vs O2

A

: CO2 is much more soluble than o2- so diffuses faster than O2

57
Q

molecular weight of CO2 vs O2

A

Because molecular weight of CO2 is > O2 molecular weight (slows down)

58
Q

what effect is greater on diffusion of gases: solubility or molecular weight

A

solubility

Oxygen has a lower molecular weight than CO2 and is fast (slows down CO2) however CO2 is more soluble

  • Overall, the effect of solubility is greater
  • CO2 diffuses 20 times faster than O2
59
Q

what compensates for the slower diffusion of oxygen

A

large difference in partial pressure

  • In diseased lungs with lower alveolar partial pressure, and therefore less pp gradient, O2 gas exchanged more impaired than CO2 because O2 has a slower diffusion rate
60
Q

Gas Diffusing from alveolar air to RBC in capillary must cross:

A
  • fluid film lining alveolus
  • epithelial cell of alveolus
  • interstitial space
  • endothelial cell of capillary
  • plasma
  • red cell membrane
  • 5 cell membranes
  • 3 layers of cytoplasm
  • 2 layers of tissue fluid and plasma
61
Q
  • The surface area of the alveolar capillary membrane about …..m2
  • Barrier …..μM thick
A

100m2

< 0.4 um

62
Q
  • Oxygen exchange complete in ………of time blood spends capillary
63
Q

gas diffusion in disease- thickness of membrane

A

e. g. ILD
- thicker- impaired diffusion

64
Q

Surface area of the membrane (e.g. emphysema) in disease

A
  • Decreased surface area- impaired diffusion
65
Q

gas diffusion in disease- diffusion coefficient of gas:

A
  • CO2 always diffuses much faster than O2
  • So, diffusion of O2 affected –> pO2 is low
  • Diffusion of CO2 not affected –> pCO2 normal

Respiratory (27 decks)

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