Principles of Gas Exchange
Diffusion-limited exchange is the rate at which gas (oxygen) is transported away from functioning alveoli and into tissues is principally limited by _______
... the diffusion rate of the gas across the alveolar membrane.
How thick are all the layers of the Respiratory Membrane? What is the overall surface area of the respiratory membrane in the normal adult?
0.6µm ...slightly less than the diameter of one RBC <> 70 square meters...like the student table area in a classroom Note: the pulmonary capillaries are 0.5µm in diameter
[Respiratory Membrane Layers] In order to diffuse in or out of the alveoli, gasses must first pass the 1st layer: _______ What is the 2nd layer?
1) A fluid layer lining the alveolus...contains surfactant (Type II pneumocytes) <> then, gasses must go through the alveolar epithelium
Name two disease conditions which could thicken the Respiratory membrane _______ and _______
1) Pulmonary diseases which cause FIBROSIS of the lungs = fibers stick to the alveoli, making the membrane thicker 2) Edema fluid in the interstitial space (4th of 6 layers)
Thus, the concentration and partial pressures of both 02 and C02 in the alveoli are determined by 1) _________ and 2) _______
1) The rates of Absorption (O2) or Excretion (CO2) of the two gasses 2) The level of alveolar ventilation (breathing rate L/min)
Name two factors that increases the diffusing capacity of O2 during exercise:
1) opening up of dormant capillaries or dilating already open capillaries 2) changing the ventilation-perfusion ratio
What factors, when increased, INCREASE the rate of diffusion of a gas?
1. The pressure difference 2. the cross sectional area of the fluid 3. solubility
What factors, when increased, DECREASE the rate of diffusion of a gas?
1. the distance through which the gas must diffuse 2. the molecular weight of the gas
The pressure difference of Oxygen on either side of the respiratory membrane is _______ The Diffusion Capacity of the Respiratory Membrane to O2 is _______. So _______ of O2 per minutes diffuses from the environment through the Respiratory Membrane in a resting person
11mmHg <> 21ml per min per mmHg <> 11 x 21 = 230ml of O2/min
[Diffusion coefficients] If O2 is considered "1" what is the coefficient of CO2. What other gases are comparable to oxygen?
20.3...so carbon dioxide is 20x more diffusible than O2 <> CO = 0.81 and Helium = 0.95
[Respiratory Membrane Layers] After the alveolar epithelium layer (2 of 6), what are the next two layers gasses must pass through?
3) The epithelial basement membrane (Type I pneumocytes) 4) The interstitial space btw the alveolar epithelium and the capillary membrane
Besides the thickness of the membrane and the diffusion coefficient of the gas (CO2 is 20x more diffusible than O2), what are two other factors which affect the rate of gas diffusion through the Respiratory Membrane?
3) The surface area of the membrane 4) The pressure difference between the gas on the opposite sides of the membrane
The average untrained healthy male will have a VO2 max of approximately_______ The average untrained healthy female will score a VO2 max of approximately_______
35-40 ml/kg/min. <> 27-31 ml/kg/min elite male runners = 85, elite female runners = 77
CO2 is continually being discharged into the alveoli (from the blood) and is normally excreted at a rate of 200ml/min. The normal partial pressure (PCO2) is _______mmHg. Alveolar PCO2 decreases in relation to what?
40 <> in inverse proportion to alveolar ventilation. The more you breath, the less pressure CO2 is in the alveoli
The vapor pressure of water at normal body temperatures is about _______ mmHg
47mmHg (this is the temperature of the air in the alveoli. This is the body temp)
What is the volume of blood in the capillaries in the respiratory area at any given instant?
60-140mls...about a 1/2 glass of wine
_______% of the total pressure at sea level, come from this molecule
79% <> Nitrogen = about 600mmHg of 760mmHg Note: The partial pressure of Oxygen in air is 21%, or 160mmHg
[Ventilation-Perfusion Ratio] Where, normally do air in the air spaces and blood in the pulmonary capillaries meet for gas exchange? If blood flow is normal but a bronchiole blocked, how does this effect gas exchange?
At the respiratory membrane where they diffuse/perfuse in opposite directions <> then no air flow gets to the capillaries, and no exchanges of gases occurs
How is net diffusion determined in regards to gases? What happens if partial pressure is greater in the blood?
By by the difference between the two partial pressures <> then the net rate of diffusion of CO2 will be out into the alveolar space This accounts for the net diffusions of oxygen and CO2 in the lung
If the pressure difference between 02 in the alveoli air space and the pulmonary capillary is 11 mmHg, and the diffusion capacity is 21 ml/min/mmHg, how much 02 diffuses across the membrane in 5 minutes? a. 230 ml b. 460ml c. 1150ml d. 1500 ml
C. 1150ml in 5 minutes Oxygen pressure difference between the respiratory membrane = 11mmHg Diffusion capacity of membrane to O2 per minute: 21ml per min per mmHg 11 x 21 = 230. X 5 minutes = 1150ml
Which of the following parameter(s) determine the partial pressures of 02 and C02 in the alveoli? a. the rates of absorption by the alveolar membrane b. the rates of excretion by the alveolar membrane c. the level of alveolar ventilation d. all of the above
D. All of the above
Referring to table 39-1 (p6 of this handout), in which of the following is the partial pressure of oxygen highest? a. alveolar b. expired c. humidified air d. atmospheric air
D. Atmospheric air
What disease state can partially or completely collapse the alveoli? What does this disease do to alveolar surface area?
Emphysema (Obstructive - hard to breath out-"barrel chested") <> it decreases it Also, removal of a lung or lung segment decreases the surface area for gas exchange/diffusion
What drug can increase Vmax by a significant amount?
Erythropoietin (EPO)
How is the diffusion of O2 restricted by fibrosis?
Fibrosis, essentially, thickens the alveolar membrane - increasing the diffusion distance
With heavy exercise, O2 absorption is _______. An Alveolar O2 partial pressure of 104mmHg must be maintained so what happens to the ventilation rate (breathing rate)?
Greater <> It must increase from 250ml/min to 1L/min, or about 20 liters of atmospheric air per minute. "Thus, heavy exercise is accommodated by heavier breathing"
How is the diffusion of O2 restricted by emphysema?
In emphysema, the diffusion of O2 is decreased because the surface area for diffusion of gases is decreased.
In the upper part of the lung, blood flow is less than at the bottom. Therefore, the VA /Q ratio is _______
Increased because [Va/Q], blood flow is reduced, increasing the number
The respiratory unit consists of a respiratory bronchioles and _______ There are about _______ alveoli in the lungs, each with a diameter of _______
It's associated alveolar ducts, atria and alveoli <> 300 million <> 0.2mm
Gas exchange occurs between the pulmonary blood and the _______ These structures are referred to as the _______
Membranes of *all of the parts* of the respiratory unit (bronchioles, alveolar ducts, atria, alveoli) <> Respiratory membrane, or pulmonary membrane
What is the effect of a concentration gradient? What is the rate of diffusion determined by?
Net diffusion <> the relative number of molecules moving in that direction [down their concentration gradient]
When air that never makes it to the respiratory areas (the anatomic dead space) is combined with oxygen that never diffuses into the blood (ie maybe blood flow is too slow), this is called _______
Physiologic dead space During exercise, however, the physiologic dead space is greatly reduced due to maximal blood flow and maximal ventilation, making up for any "slack" in the system
Things that determine a person's maximum V02 max are:
Pulmonary factors: Cardiac output, pulmonary diffusion capacity, oxygen carrying capacity, Peripheral limitations: muscle diffusion capacity, mitochondrial enzymes, and capillary density
What method do dissolved O2 and CO2 transfer between the air in the alveoli and the blood in the capillaries? What is the energy source for this?
Simple diffusion <> simple diffusion requires the kinetic motion of molecules
The partial pressure of a gas in solution is determined by its concentration, as discussed above, and also by its _______
Solubility coefficient
The ability of the respiratory membrane to exchange a gas between the alveoli and the pulmonary blood is _______
The Diffusion Capacity of the Respiratory Membrane "Volume of a gas that will diffuse through the membrane each min for a pressure difference on 1mmHg"
VO2 max is properly defined by what equation?
The Fick Equation. Q = cardiac output of the heart. CaO2 = arterial oxygen content. CvO2 = venous oxygen content
[Respiratory Membrane Layers] Once gasses have reached the interstitial space btw the alveolar epithelium and the capillary membrane, what are the last two layers to cross?
The capillaries... 5) The capillary basement membrane 6) The capillary ENDOthelial membrane O2 comes into the alveoli through these 6 layers. CO2 leaves the alveoli through these 6 layers. DIFFUSION!
When air comes into the alveolar spaces, how does it get into the blood? What happens to the CO2 in the blood? (from pulmonary arteries)
The oxygen in the air dissolves into the membrane of the alveoli, and then into the blood of the alveolar capillary <> CO2 diffuses out into the alveoli The rate at which C02 "escapes" the blood is proportional to the partial pressure of the gas in the blood.
When we excercise, we excrete more CO2, what happens to the partial pressure of CO2 in the alveoli during this time?
The pressure INCREASES - directly in proportion to the rate of which we excrete CO2
[T/F] The distance through with the gas must diffuse, helps determine the Net rate of diffusion of gases through fluids
True, also factors include 1. the solubility of the gas in the fluid 2. the molecular weight of the gas 3. the temperature of the fluid
_______ is the maximum capacity of an individual's body to transport and use oxygen during incremental exercise
VO2 max (also maximal oxygen consumption, maximal oxygen uptake, peak oxygen uptake or maximal aerobic capacity) This reflects the physical fitness of an individual. Expressed in Liters of oxygen/min
[Ventilation-Perfusion Ratio] VA/Q is ______ / _______. If VA /Q is less than normal, you have _______ than blood flow
Ventilation - VA / Perfusion - Q <> less air flow = this could mean an obstruction in the airways
Is there a limit to alveolar O2 pressure while breathing atmospheric air?
Yes...the alveolar O2 can never exceed atmospheric oxygen at 149mmHg (this is the 21% of O2 in the atmosphere)
If 230ml of Oxygen per minute diffuses from the environment through the respiratory membrane AT REST, does the diffusing capacity of the membrane change during exercise?
Yup. It goes up from 21ml/min/mmHg to 65mm/min/mmHg
If blood flow is normal and the airways are completely blocked, then V/Q is zero, which is called _______
a physiologic shunt
Under physiological conditions, what is the major physical factor determining whether or not oxygen or carbon dioxide will diffuse across the respiratory membrane? a. the net pressure gradient across the membrane b. the flow rate down the tube c. the amount of Brownian movement d. the temperature of the solution
a. the net pressure gradient across the membrane could also be the amount of Brownian movement too
What is the rate at which alveolar air is renewed by atmospheric air (from inhalation)?
about 1/7th or about 15% of new air replaces alveolar air with each breath It takes multiple breaths to completely replace the old air
Which of the following conditions is the gas "diffusion limited"? a. CO2 in exercise b. O2 in emphysema patients c. O2 under normal conditions d. CO2 in normal conditions
b. O2 in emphysema patients = because the surface area for diffusion of gases is decreased
What is "perfusion limited" gas exchange. a. the gas does not equilibrate by the time the blood reaches the end of the capillary b. the blood flow is too low to allow for gas exchange.
b. the blood flow is too low to allow for gas exchange.
For a perfusion-limited process, diffusion of the gas can be increased only if _______
blood flow increases. N2O and O2 under normal conditions
What is the partial pressure of oxygen in alveolar air? ...of CO2? a. 4mmHg b. 40mmHg c. 104mmHg d. 149mmhg
c. 104mmHg = O2 b. 40mmHg =CO2
In which of the following conditions is the gas "perfusion limited" a. CO2 in exercise b. O2 in emphysema patients c. O2 under normal conditions d. CO2 in normal conditions
c. O2 under normal conditions
What does Henry's Law show? a. the relationship between voltage, current, and resistance b. the relationship between octane and propane of a gas c. the relationship between solubility, pressure and concentration of a gas d. the relationship between flow, resistance, and pressure
c. the relationship between solubility, pressure and concentration of a gas Pressure = Concentration of dissolved gas / solubility coefficient
Which of the following, when increased, would tend to decrease diffusion of a molecule? a. pressure differences b. cross sectional area of the fluid c. solubility d molecular weight of the gas molecule
d molecular weight of the gas molecule
[Ventilation-Perfusion Ratio] If VA /Q is greater than normal, you have_______ than air flow.
less blood flow = This could be blocked blood flow to a lung, caused by a pulmonary embolism
[Ventilation-Perfusion Ratio] At the extreme, when VA goes to 0, there is _______ the ratio becomes_______ and when Q goes to zero, there is _______the ratio is infinity.
no air flow <> zero <> no blood flow <> infinity
If ventilation is normal but blood flow to a lung is completely blocked (e.g., by an embolism occluding a pulmonary artery), then V/Q is infinite, which is called_______
physiologic dead space. There is no gas exchange in a lung that is ventilated but no perfused
The rate of respiration and the _______, control the alveolar partial pressure of oxygen
rate of O2 absorption by the blood
The pressure of a gas is directly proportional to...
the concentration of the gas... and it's pressure is the sum force of impact of all the molecules of that gas striking the surface at any given moment.
In Perfusion-limited exchange, the partial pressure of the gas in arterial blood becomes equal to _______
the partial pressure in alveolar air...the gas equilibrates early along the length of the pulmonary capillary
In diffusion-limited exchange, the gas does not equilibrate by ______ Diffusion-limited exchange is illustrated by ______
the time blood reaches the end of the pulmonary capillary <> CO and O2 during strenuous exercise