Gas Exchange Flashcards
A 24 year old man is noted to be hyperventilating in a panic attack.
What is likely to happen to his Alveolar pO2 and pCO2?
in hyperventilation,
theres no time for Co2 to be cleared up from the blood>>causing it to be high in the blood and low in the alvolus,
and the o2 inhaled will build up in the alveouls, cuz it doesnt have time to be taken up by the blood!
A 22 year old man was found in a semiconscious state. A narcotic overdose is suspected. His breathing is slow and shallow.
What is likely to have happened to his Alveolar pO2 and pCO2?
with hypoventilation more time ofr oxygen to be exchanged into blood, so levels in alvoulus will decrease
vise versa with co2
Explain the relevance of Boyles law in ventilation of the lung
pressure is inversely proportional to its volume.
units of presuure in medicine
Pressure is measured in kilopascals (kPa)
Explain the concept of the ‘partial pressure’ of an individual gas in a gas mixture,
Partial pressure is the pressure exerted by the dissolved gas in the
liquid
Calculate the partial pressures of constituent gases in atmospheric air and explain the effects of altitude upon them.
Since dry air is 20.9% O2 and the total atmospheric air pressure is 101.1 kPa, the partial pressure of oxygen (PO2) in atmospheric air is = 101.1 x 0.209 = 21.1 kPa
Partial Pressure of a gas in liquid
When a gas mixture is in contact with liquid as in the lungs, gas molecules will enter the liquid until an equilibrium is established
soooo..
The collision of the dissolved gas molecules with the walls of the container generates a pressure within the liquid, which is the partial pressure (or tension) of that gas in the liquid.
- At equilibrium, the partial pressure of the gas in the liquid = the partial pressure of that gas (in the gas phase) to which the liquid is being exposed to.*
- Example: If the partial pressure of oxygen in the alveolar air is 13.3%, and this air is allowed to equilibrate with the blood in the pulmonary capillaries, the partial pressure of oxygen in blood leaving the alveolus will also be 13.3 kPa*
Explain what is meant by ‘partial pressure of oxygen in blood, and how it is different from the ‘content’ of oxygen in the blood.
The partial pressure is not the same as the amount or content of gas dissolved in the liquid.
The amount dissolved depends on the chemical properties of the gas and the liquid. For example: CO2 in much more soluble in plasma than O2; a given volume of plasma would accommodate more CO2 molecules within it before equilibrium is reached between the rate of CO2 molecules entering and leaving the plasma.
define solubility coefficient
is the amount of (in mmol) of a gas that will dissolve in a litre of plasma at 37 C (body temperature), when exposed to given partial pressure.
Calculate the content of O2 and CO2 in plasma using their solubility coefficients, and pp
The amount of a gas dissolved = solubility coefficient of that gas x the partial pressure it is exposed to.
Example 1: the solubility coefficient of O2 is 0.01 mmol/Litre/kPa.
Therefore, if PLASMA at body temperature is exposed to alveolar air with a pO2 of 13.3 kPa…….pO2 in the plasma will be 13.3 kPa,
and its O2 content will be= 13.3 x 0.01 = 0.13mmol/Litre.
State the normal PO2 and PCO2 in:
alveolar air, arterial blood and mixed Venus blood
ok so…If the gas which dissolves combines chemically with the liquid,
then the content of gas = amount of gas chemically bound + the amount of gas in free solution
If whole blood (which contains Hb within RBCs) is exposed to air with a pO2 of 13.3 kPa, the pO2 will still be 13.3 kPa, and the amount of O2 dissolved in plasma will be 0.13 mmol/L,
_BUT_…..
since haemoglobin (Hb) binds large amounts of O 2 , the oxygen content will rise to 8.93 mmol/litre.
O2 content = 8.8 mmol/L (bound to Hb) + 0.13 dissolved) = 8.93 mmol/Litre
what happens to water molecules when in contact w/ gas?
When a gas is in contact with water, water molecules will evaporate to enter the gas phase.
- Equilibrium is reached when the rate of evaporation = rate of molecules water returning to the liquid phase.*
- i wanna be gasy like u!*
what is saturated vapour pressure? what does it depend on?
Again we said..
When a gas is in contact with water, water molecules will evaporate to enter the gas phase.
Equilibrium is reached when the rate of evaporation = rate of molecules water returning to the liquid phase.
At equilibrium the gas mixture is saturated with water vapour, and the pressure it exerts is called the** saturated vapour pressure (SVP)**.
SVP depends only on temperature.
what is the SVP of water at normal body temp??
At body temperature of 37 C the SVP of water is 6.28 kPa (47 mmHg) and is independent of total pressure.
Despite the addition of water vapour, the total pressure of the humidified air in the airways remains 101 kPa,,,why?
bc this gas is continuous with the outside air, and pressure will equilibrate to atmospheric pressure.
In the airways, water vapour contributes 6.28 kPa to the pressure, while the other gases together account for 94.28 kPa (101 -6.28 = 94.28 kPa. )
SVP of water is 6.28 kPa
total pressure of the humidified air in the airways 101kPa,
hu?
ince the other gases remain in the same proportions as in dry air (i.e. O2 20.9%, N2 78%), the pO2 of the humidified air = (100 – 6.28) x 20.9% = 19.8 kPa
The partial pressure of oxygen of inhaled air changes as it moves from the nostrils to the alveoli
Inhaled air becomes saturated with water vapour in the upper respiratory tract.
This water vapour displaces some oxygen and nitrogen so that percentage of oxygen drops to about 20%, with a drop in pO2 drops to about 19.8 kPa.
In the alveoli, oxygen is constantly diffusing into capillary blood flowing past. Alveolar pO2 is determined by the rate of removal of O2 by the blood and the rate of replenishment of O2 by alveolar ventilation.
what is Alveolar pO2 determined by?
In the alveoli, oxygen is constantly diffusing into capillary blood flowing past.
Alveolar pO2 is determined by the rate of removal of O2 by the blood + the rate of replenishment of O2 by alveolar ventilation.
what keeps the partial pressure of oxygen in the alveolar gas stable? what is its normal value?
The balance between perfusion and ventilation, keeps the partial pressure of oxygen in the alveolar gas stable at its normal value of 13.3 kPa
how is Alveolar pCO2 determined? how much is it?
determined by the balance between the rate at which CO2 enters the alveoli from blood and the rate at which it is removed from alveolar gas by ventilation.
The partial pressure of CO2 in the alveolar gas is stable at its normal value of 5.3 kPa.
The mixed venous blood reaching the pulmonary capillaries has a pO2 of _____ and a PcO2 of________
The mixed venous blood reaching the pulmonary capillaries has a
pO2 of 6 kPa and pCO2 of kPa. ?
Gas exchange taking place at the alveolar capillary membranes allows to blood to equilibrate to with alveoli air; hence blood leaving the alveoli has a pO2 of 13.3 kPa and a pCO2 of 5.3kPa; which is the same as that of alveolar air.
Gas exchange taking place at the alveolar capillary membranes allows to blood to equilibrate to with alveoli air;
hence blood leaving the alveoli has a
pO2 of 13.3 kPa and a pCO2 of 5.3kPa;
which is the same as that of alveolar air.
Describe the layers making up the diffusion barrier at the air-blood interphase
First gas must diffuse through the gas in the alveoli, then through:
- The alveolar epithelial cell
- Interstitial fluid
- Capillary endothelial cell
- Plasma
- Red cell membrane
The rate at which gases exchange is determined by three factors:
- Area available for exchange
- Resistance to diffusion
- Gradient of partial pressure
what makes the area of the alveoli large?
there are a huge number of alveoli, generating in a normal lung an exchange area of around 70 m2.
- This means gases have to diffuse through*
- 5 cell membranes,*
- 3 layers of intra cellular fluid*
- 2 layers of extra cellular fluid.*
Despite this the overall thickness of the barrier is only 0.6 micron.
yas.
For most of the barrier, the rate of diffusion is affected by the solubility of the gas in water,
which gas diffuses faster?
carbon dioxide diffuses 21 times as fast as oxygen
This means that anything affecting diffusion will only change oxygen transport, as that is limiting.
what is meant by diffusion rate?
diffusion rate is sufficiently fast to allow full gas exchange in about 500 ms - half the time the blood spends in the capillaries.
Diseases causing diffusion defects
1. Interstitial lung disease is characterised by excessive deposition of collagen in the interstitial space, with thickening of alveolar walls and lengthening of the diffusion pathway. Interstitial lung disease may be idiopathic or secondary to many causes, including inhaled dusts (e.g. Coal dust).
2. Pulmonary oedema: The fluid in the interstitium and alveolus increases the length of the diffusion pathway.
3. Emphysema: the destruction of alveolar walls result in large airspaces. reduces the total surface area available for gas exchange. (The distance the gases have to diffuse across remains unchanged).
how do we measure diffusion resistance ?
by the Carbon Monoxide Transfer Factor
we measure the CO uptake following a single, maximal breath of a gas mixture containing air, 14% Helium and 0.1% Carbon monoxide.
why do we use CO to measure diffusion resistence?
used because of its very high affinity for Hb. Since (almost) all the CO entering the blood binds to Hb, there is hardly any CO in the plasma.
This means the concentration gradient for pCO (i.e. the difference between pCO in alveolar gas and pCO plasma) across the alveolar capillary membrane is maintained (and remains the same) for the entire time blood remains in contact with alveolar gas.
The amount of CO transferred from alveoli to the blood is an estimate of the diffusion resistance of the barrier.
see qs in workbook
Explain why gas exchange depends on the partial pressure gradient across the diffusion barrier
Net diffusion occurs from a high concentration to a low concentration, because gases expand to fill the available volume. Differences in diffusion coefficient is usually compensated for by differences in partial pressures.
How does partial pressure change at high altitudes?
At high altitudes, atmospheric pressure is lower.>Therefore, the air molecules are dispersed. The same tidal volume of 500ml of inspired air will have fewer oxygen molecules so the partial pressure of oxygen will drop.
How does partial pressure change below sea level?
water is 800X denser than air so pressure increases 7aiilll!
explain decompression sickness in divers
Explain the exchange of oxygen in term of TIME? Why is this important?
Oxygen exchnage is complete in 1/3 of the time that the blood spent in the capillary.
this means theres plenty of reserve! For times of excersize!
Someone with Diffuse Lung fibrosis.
explain their arterial po2 & Pco2
The diffusion barrier has increased in length.
Carbon dioxide has a higher solubility factor so diffuses more readily whereas oxygen does not.
Therefore the diffusion of Co2 out of the blood stream into the alveoli is still sufficient to be in the normal range whereas less oxygen can diffuse into the blood stream
