By: Aseel Jamil Al-twaijer Lec : physical principles of gas exchange Date:30 /10/2017
this lecture is about the exchange of gases between the blood and the alveoli. I might add some external definitions to make it clear I ll add a star at the beginning. I also added the slides but still you have to go back and see the tables HERE WE GO: SLIDE 1+2 Molecules move randomly & rapidly in relation to each other gas is proportional to [gas] 1- concentration 2- solubility coefficient entration of dissolved gas/ solubility coefficient Solubility coefficient : dissolved without Explanation: *In a mixture of gases, each gas has a partial pressure which is the hypothetical pressure of that gas if it alone occupied the entire volume of the original mixture at the same temperature. The partial pressure of each gas in a mixture is related to its concentration(it s percent) in that mixture so when we talk about oxygen for example its percent is 21 out of 760 which is 160 so the partial pressure of the o2 is 160 and the remaining is mainly for the nitrogen because we don t have that much co2 in the atmospheric air. The solubility of the gas in water: Now if we compare the solubility of co2 its about 20 times that of O2.
Solubility coefficient :is the molecules dissolved in a water, tissue or in the blood, now the more is the solubility coefficient the more the gas is dissolved and attracted to the water molecule so this mean it ll exert less pressure. larger solubility coefficient more solubility. more solubility.. less partial pressure. less solubility. more partial pressure. For example CO2 is soluble 20 times more than O2 but still CO2 partial pressure is less than that of the O2 so the partial pressure depends on the coefficient. SLIDE 3: sure in direct proportion to concentration EXPLANATION: The doctor just read the slide and added this information: Dry air has no water vapor pressure but the moist air as in the case of the respiratory tract there s a partial pressure of water vapor and it s around 47 mmhg so in addition to o2,n,co2 we have water vapor partial pressure. SLIDE 4+5: determinants of diffusion Ficks Law Diffusion = (P1-P2 ) * Area * Solubility/Distance * MW(square root) Pressure Gradient Area Distance Solubility and MW are fixed Diffusion coefficient proportional to Solubility/MW(square root) Different gases at the same partial pressure Will diffuse proportional to their diffusion coefficient
EXPLANATION: Diffusion is related to :pressure gradient,area,distance, but the solubility and molecular weight are fixed. Pressure Gradient: There are two side for the gas exchange and as we know gases diffuse from the side of higher pressure to the side with less pressure(pressure gradient) so if there s no difference in the two sides there s no diffusion. In the alveoli the air is coming from the atmosphere so the O2 is more in the alveoli, in the blood the o2 is less so it ll diffuse from alveoli to blood and by the time the venous blood come to the alveoli (we said it takes about 0.8 s) the blood will accumulate in the alveoli so there will be no diffusion during this time it reached equilibrium (co2 is completely the opposite), so the diffusion maily is dependent on the pressure gradient Area: Area here means the alveolar membrane so that the more the surface area(more alveolar sacs ) the more is the echange. Distance: Here the distance represents the alveolar membrane which are 6 layers (fluid and surfactant layer, alveolar membrane, epithelial basement membrane, interstitial space, capillary basement membrane, capillary endothelium) Anything that can affect this as in smoker people (deposition of nicotine)or the workers in factories that
are in contact with chemical gases they re prone to deposition of asbest material(asbestoses) so they should wear gas masks to reduce the inhalation All this means that anything deposits in the distance will affect the diffusion. Note that the pressure gradient, distance and area are forces which means they re not related to the substance so what is related to the substance is the solubility of the substance and the molecular weight, so diffusion is affected by factors not related to the substance (not a characteristic of the substance) and factors related to substance which both are mentioned above. The molecular weight is related to the square root of MW not to its absolute value. Slide 6: Pn2 = (760-47) * 0.79 = 713 * 0.79 = 563 Questions: What is the effect of humidification on the partial pressures? Explain the expired air partial pressures? Calculate Po2 in alveoli Explanation: This slide the doctor just said that partial pressure for a gas is related to its concentration in that mixture of gas and took nitrogen as an example then he skipped to the next slide but I copied the answers from the techno sheet for the first qs. What is the effect of humidification on the partial pressures? As we said, in the DRY atmosphere we have 20% O2 and 80% nitrogen. But in the HUMID atmosphere, there is an additional partial pressure which is the partial pressure of the water vapor. This will replace and reduce the partial pressure of O2
and nitrogen because the total atmospheric pressure is still 760 mmhg. Normally in our body (under 37 C) the water vapor in our lungs will exert a partial pressure of 47 mmhg. This will reduce the partial pressure for oxygen and nitrogen. Slide7: diffusion between gas phase and dissolved phase through the surface Explanation: Diffusion between gas phase and dissolved phase (between alveoli and blood) Doctor read the slide and the table then he skipped to the next slide Slide8: the air in the dead space is composed of humidified fresh air (second colomn) The expired air has partial pressure of O2 than In the alveoli why? Because of the mixing with the o2 in the dead space The doctor here just commented these so I included more information about this table from the sheet: If we look carefully, we are comparing different compartments, the
outside and the inside. In fresh, air we have about 20% oxygen, 78% nitrogen, and almost zero CO2 and H2O. In humid air, we have about 6% water. This will replace the partial pressure of oxygen and nitrogen which decreases them. The airways of the conducting channels are humid because they contain water vapor (column 2). When we get to the alveoli, we will have CO2 excreted from the blood and O2 diffused into the blood. So there is an increase in the partial pressure of CO2 (5.3%) in the alveoli and a decrease in the partial pressure of oxygen (13.6%). (column 3) When the tidal volume goes out of the alveoli (expiration), we will have 150 ml of dead space fresh air, and 350 ml of alveolar air. This means the humidified fresh air and the alveolar air mix and give us the expired air as you can see in the 4th column slide 9+10: See pic in slide 10
Explanation: Water vapor and co2 are mainly negligible Here the nitrogen and oxygen are reduced in the alveolar air because of the water vapor but in the end the total pressure are the same between the atmpsphere and the alveolar air. Slide 11: Po2 IN THE ALVEOLI PAlvO2= PIO2 - (PCO2/R) PO2 = 149 - (40/0.8) = 99 R is respiratory exchange ratio ~0.8 Remember in a normal person alveolar PO2 = arterial PO2, and alveolar PCO2 = arterial PCO2. explanation: this is the formula to find out the the alveolar o2 pio2: this is the normal inspired o2 pco2: partial pressure of co2 r: the respiratory exchange ratio(between the co2 output and the o2 used slide 12: Pco2 IN THE ALVEOLI PCO2 = CO2 production * K Alveolar Ventilation K is constant If ventilation is doubled then Pco2 is ½ If ventilation is halved then Pco2 is doubled Explanation: The more is ventilation the less is the Pco2 and vice versa Slide 13+14: He read the numbers Slide 15 + 16: He skipped them
Slide 17: Explanation: Ventilation is the process by which air moves in and out of the lungs. Diffusion is the spontaneous movement of gases, without the use of any energy or effort by the body, between the gas in the alveoli and the blood in the capillaries in the lungs. Perfusion is the process by which the cardiovascular system pumps blood throughout the lungs.(how much the cardiac output which is about 5 L) When we talked about ventilation we said there are 2 types: Pulmonary ventilation and alveolar ventilation. Alveolar ventilation : 350*respiratory rate = 4 L So IN GENERAL the ideal ratio between the ventilation(air)- perfusion(fluid) is about 4 / 5 = 0.8 Slide 1 of ventilation /perfusion ration lecture: Ventilation-perfusion ratio ventilated but have almost no blood flow, whereas other areas may have excellent blood flow but little or no ventilation, this will seriously impair gas exchange. blood flow going to different parts of the lungs.
s exchange During sitting or standing the blood isn t distributed equally not all the alveoli receive the same amount of blood and not all the alveoli receive the same amount of air so some receive good amount of air with a less blood And some receive good amount of blood with a less air so this is what we call the ventilation-perfusion ration. So not necessarily Any particular area of the lungs receive the same anount of air and perfusion Mainly The alveoli that receive good amount of ventilation but not perfusion(there s air coming but no blood) will contain atmospheric air So the pressure of O2 will be 149. Someone asked that this air will be mixed with the dead space? Doctor said that at the end of expiration the dead space air represent the alveolar air so it will be atmospheric too (there s no blood means there s no co2) On the other hand the alveoli that receives good perfusion but no ventilation It will contain the venous blood gases it will give co2 & o2 because there s no ventilation, mostly this is in case of disease so if we want to make the study we do the ventilation-perfusion ratio using isotops then we study how much blood and air is coming to a particular area. As we said v/p ration is between how much ventilation and perfusion is coming so the healthy normal alveoli have good perfusion and ventilation And we said it s the respiratory exchange ratio which is about 0.8 and it ll be increased or decreased according to which one(ventilation and perfusion) is increased or decreased Someone asked: is this the alveolar dead space? Doctor answered No. The alveolar dead space the air is coming but there s no blood so this air represent s the dead space (not involved in the dead space) we call it physiological dead space
Slide 2: When V/Q= 0 : (there s no blood to take the gases0 It was explained above. Slide3: The upper area has a good V but low P The lower area has a good p but low v
Slide 5+6: e to inadequate ventilation _Qps_= _CiO2 -_CaO2 Qt CiO2 CvO2 ratio Explanation: Physiological shunt : when the ventilation/perfusion ration is below the normal due to inadequate ventilation (part of the venous blood pass without oxygenation )simply means venous blood is coming but it s not oxygenated(the blood bypass the lung without oxygenation). It s like the shunt we talked about which is coming from the bronchial circulation Slide 7: Physiological dead space: _Vds_ = _PaCO2_-_PECO2_ Vt PaCO2
Explanation: when v/q above normal there s more ventilation but less perfusion This is called physiological dead space but the previous one called anatomical dead space. This can be changed it s not constant for example the physiological dead space will decrease during exercise because more blood is coming It can be increased during a disease The dead space is any volume of air which is not involved in exchange so sometimes we include the anatomical and physiological dead space together The anatomical is structural but the physiological varies Slide 8+9: Abnormalities : 1- V/Q in upright chest : 2-V/Q in chronic obstructive lung disease: and emphysema, this will lead to: a-unventilated alveoli because of obstruction b-wasted ventilation because of damaged blood vessels GOOD LUCK