Respiratory > Lecture 5- Gas exchange > Flashcards

Lecture 5- Gas exchange Flashcards

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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
PvO2
pp O2 in venous blood
26
pp O2 in upper repiratory tract is........ kPa but alveolar pp O2 is....
19. 8kPa 13. 3 kPa
27
why is the pp of oxygen different in the resp tract (19.8kPa) and alveolar (13.3 kPa)
* 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
## Footnote **Alveolar vs pulmonary ventilation** **- typical volume of air inspired/ expired at rest (tidal volume)**
450ml
29
pulmonary ventilation
Typical amount of air inspired/exhaled at rest ≈ 450 ml (called the Tidal Volume)
30
anatomical dead space
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
alveolar ventilation
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
**Pulmonary Ventilation Rate vs Alveolar Ventilation Rate**
Can express amount air moved as a rate – ml air/minute- called minute ventilation
33
**Pulmonary Minute Ventilation**
* * We breathe ≈ 12 times a minute - the total volume of air moved is 450 ml × 12 = 5400 ml/min
34
* **Alveolar Minute Ventilation** (ml/min)
is the rate of air flow reaching the alveoli – 12 breathes/minute × 300 ml = 3600 ml/min
35
**When gases dissolve in liquid**
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
The amt of gas that dissolves is
directly proportional to the partial pressure of that gas
37
**Gas diffuse across pp gradients**
* 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
rate of dissolving
* Low pp gas- low rate of dissolving * High pp gas- number of collisions at water interface= elevated = higher rate of dissolving
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* Rate out (liquid to gas)=
conc of dissolved gas and solubility of gas for liquid medium
40
**Gas diffusion air-liquid: oxygen**
* 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
**Gas Diffusing into a Liquid – Dissolved Gas Content**
* 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
example of calculation of gas diffusing into liquid
amount dissolved also depends on the solubility coeffieicent
43
oxygen goes from..... carbon goes from.....
oxygen goes from..... alveoli to blood carbon goes from..... blood to alveoli
44
how does oxygen get to equilibirum
**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
Oxygen on Hb does
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
how does Haemoglobin help keep the oxygen gradient strong
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
* **pO2 reflects the amount of**
**dissolved O2 in the blood**
48
**total content of oxygen in the blood=**
**dissolved oxygen + hb bound oxygen**
49
diffusion of carbon dioxide from the blood to the alveoli for expulsion
* 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
what is the PAO2? Alveolar oxygen composition
13.3 kPa
51
what is the PACO2? Alveolar carbon dioxide composition
5.3 kPa
52
Amount of Oxygen (ml/min) brought in by ventilation=
amount of oxygen (ml/min) diffusing into the blood
53
Amount of carbon dioxide (ml/min)=
amount of carbon dioxide diffusing from the blood into the alveolus
54
**Diffusion: factors affecting rate of diffusion**
* Partial pressure difference * Surface area * Thickness * Diffusion coefficient of the individual gas
55
ease of diffusion is of oxygena nd carbon dioxide is determined by
solubility an molecular weight of the gas
56
solubility of CO2 vs O2
: CO2 is much more soluble than o2- so diffuses faster than O2
57
molecular weight of CO2 vs O2
Because molecular weight of CO2 is \> O2 molecular weight (slows down)
58
what effect is greater on diffusion of gases: solubility or molecular weight
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
what compensates for the slower diffusion of oxygen
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
**Gas Diffusing from alveolar air to RBC in capillary must cross:**
* 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
* The surface area of the alveolar capillary membrane about .....m2 * Barrier .....μM thick
100m2 \< 0.4 um
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* Oxygen exchange complete in .........of time blood spends capillary
1/3
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gas diffusion in disease- thickness of membrane
e. g. ILD - thicker- impaired diffusion
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Surface area of the membrane (e.g. emphysema) in disease
* Decreased surface area- impaired diffusion
65
gas diffusion in disease- diffusion coefficient of gas:
* CO2 always diffuses much faster than O2 * So, diffusion of O2 affected --\> pO2 is low * Diffusion of CO2 not affected --\> pCO2 normal
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