explain gas exchange
- factors that determine gas exchange are the same as those for convective flow through the airways (Flow = △P/R)
- the pressure gradient is based on the partial pressures of the gas in the alveoli (PA) and the pulmonary artery PV), and the resistance to diffusion
explain resistance of diffusion
- dependent upon the surface area of the membrane (A), its thickness (T), and the diffusibility (D) of the gas
- D is a constant, and as such, we can essentially ignore it
explain partial pressures
- atmospheric pressure of air at sea level is 760mmHg
- air is actually a mixture of gases and according to Daltons law, the pressure exerted by gas in a mixture is directly proportional to the percentage of that gas in the mixture
what are the 2 key factors that determine the amount of gas that can dissolve in a liquid
- partial pressure in air: the greater its partial pressure, the more gas will be driven into the liquid
- solubility in the liquid: the more soluble a gas is in a liquid, the more will dissolve
explain alveolar partial pressures of oxygen and carbon dioxide
- alveolar air does not have the same composition of inspired air
- when inhaling air into the airways, it immediately becomes saturated with water vapour
- this water vapour, like any gas, contributes to the gas mixture and affects the partial pressures of the other gases
- at body temp, PH2O is 47mmHg, this means the remaining gases account for 713 mmHg
- this means the partial pressure of inspired nitrogen is 563 mmHg, since 79% of air is nitrogen, and the partial pressure of inspired oxygen is 150 mmHg, since 21% of inspired air is oxygen
explain alveolar PO2 in the lungs
- with a tidal volume of 500ml, only about 350ml is moving in or out with the remaining 150ml being mixed inhalation and expiration gases
- at the end of inspiration, only about 15% of alveolar gas is actually “fresh” air so there is a further drop in the partial pressure of oxygen
- the actual PAO2 is around 100mmHg
explain this equation
- PIO2: the partial pressure of inspired oxygen (150mmHg)
- PACO2/R: P is the partial pressure of alveolar carbon dioxide (about 40mmHg), and R is the respiratory quotient or ratio of metabolic carbon dioxide formation to oxygen consumption, roughly 0.8 in healthy people
explain gas exchange at the pulmonary capillaries
- as blood passes through the lungs, carbon dioxide moved from the blood to the alveoli and oxygen moves from the alveoli to the blood
- this movement of gases is by diffusion and is driven by partial pressure gradients
- the process of ventilation is constantly replenishing alveolar oxygen and removing carbon dioxide
how do partial pressures relate to the amount of dissolved gases in the blood
- affe cted by both the partial pressure and the solubility of a gas
- for oxygen and carbon dioxide, carbon dioxide is 20 times more soluble in the blood so even if their partial pressures were the same, the actual concentration of dissolved carbon dioxide would be greater
explain the concentration of dissolved gases - oxygen
- blood leaving the lungs has an oxygen partial pressure of 100 mmHg, which is about 200 ml O2/L
- blood returning to the lungs has a partial pressure of 40 mmHg, which is about 150ml O2/L
- from this, you can calculate that at rest, 150 ml O2/L represents the functional reserve for when there is an increased tissue demand
explain the concentration of dissolved gases - carbon dioxide
- blood leaving the lungs has a carbon dioxide partial pressure of 40 mmHg, which is about 480 ml CO2/L
- blood returning to the lungs has a partial pressure of 46 mmHg, which is about 520 ml CO2/L
- there isn’t a dramatic change in dissolved CO2 because of the essential role that CO2 (and carbonic acid) have in acid-base balance
what are factors that affect gas exchange - surface area
- the greater the surface area, the greater amount of gas that can exchanged
- at rest, some pulmonary capillaries are closed
- during exercise, the increase in pulmonary blood pressure will open these capillaries and this effectively increases the available surface area for gas exchange
- in lung diseases, such as emphysema, where there is alveolar damage, the surface area available for gas exchange is decreased
what are factors that affect gas exchange - capillary transit time
- gases can only be exchanged between the blood and the alveoli when the blood is in the pulmonary capillaries
- because of this, the flow rate of the blood can influence gas exchange
- at rest, blood remains in the pulmonary capillaries for about 0.75 seconds, which is about 3 times as long as necessary for exchange
- even during exercise, capillary transit time only decreases to 0.4 seconds
- under physiological conditions, capillary transit time is not limited to gas exchange
what are factors that affect gas exchange - membrane thickness
- the thickness of the barrier separating the blood and the alveoli can be increased due to inflammation
- this increased thickness results in a decrease in gas exchange
- this is observed is mucous secreting lung disease such as asthma
- if the membrane is thickened enough, the capillary transit time may not be sufficient for complete gas equalization
explain gas exchange at rest in systemic capillaries
- gas exchange across the systemic capillaries follows the same fundamental principles as gas exchange in the pulmonary capillaries
- body tissues are constantly undergoing oxidative metabolism, consuming oxygen and producing carbon dioxide
- as a result, the average cellular partial pressure of oxygen (PO2) is approx 40mmHg, while the cellular partial pressure of carbon dioxide PCO2) is approx 46mmHg
- arterial blood entering the systemic capillaries has a PO2 of about 100mmHg and a PCO2 of about 40mmHg, creating pressure gradients that drive oxygen from the blood into the tissues and carbon dioxide from the tissues into the blood
- by the time blood leaves the systemic capillaries, the partial pressures of oxygen and carbon dioxide have equilibrated with tissue values, reaching approx 40mmHg for O2 and 46mmHg for CO2
explain exercise and metabolic effects on gas exchange
- as tissue metabolism increases, such as during exercise, tissues PO2 decreases and tissue PCO2 increases, which further enlarges the pressure gradients driving gas exchnage
- this allows tissues to receiev sufficient oxygen while eliminating excess carbon dioxide
- the increased oxygen extraction lowers the PO2 of blood returning to the lungs, which enhances oxygen diffusion from the alveoli into the blood
- tissue metabolism is the primary driving force for gas exchange in both the systemic and pulmonary capillaries
explain the first step of O2 and CO2 gas exchange within pulmonary and systemic capillaries
the venous blood entering the lungs is low in O2 (40mmHg; this value decreases during exercise) and high CO2 (46mmHg), due to the consumption of O2 and production of CO2 by the tissues
explain the second step of O2 and CO2 gas exchange within pulmonary and systemic capillaries
alveolar PO2 remains high (100mmHg) and alveolar PCO2 low (40mmHg) because only a portion of the alveolar air is replaced with fresh atmospheric air during each breath
explain the third step of O2 and CO2 gas exchange within pulmonary and systemic capillaries
- the partial pressure gradients for O2 (100-40=60mmHg) and CO2 (46-40=6mmHg) between the alveoli and pulmonary capillary blood cause O2 to diffuse into the blood and CO2 to diffuse into the alveoli
- diffusion continues into the blood and alveolar partial pressures become equal
explain the fourth step of O2 and CO2 gas exchange within pulmonary and systemic capillaries
- blood leaving the lungs has, compared to the lungs, a high partial pressure and content of O2 and a low partial pressure and content of CO2
- these partial pressures and contents are identical to those delivered to the tissues
explain the fifth step of O2 and CO2 gas exchange within pulmonary and systemic capillaries
the partial pressures of O2 and CO2 are, compared to those in arterial blood, lower and higher, respectively, in the O2-consuming, CO2 - producing tissue cells
explain the sixth step of O2 and CO2 gas exchange within pulmonary and systemic capillaries
O2 diffuses from the arterial blood into cells to support their metabolic requirements, and metabolically produced CO2 diffuses into the blood
explain the seventh step of O2 and CO2 gas exchange within pulmonary and systemic capillaries
- having equilibrated with the tissue cells, the blood leaving the tissues is relatively low in O2 and high in CO2
- the blood then returns to the lungs to once again replenish on O2 and release CO2
-
Module 1 Section 135
-
Module 1 Section 241
-
Module 1 Section 342
-
Module 1 Section 446
-
Module 1 Section 542
-
Module 1 Section 641
-
Module 2 Section 122
-
Module 2 Section 226
-
Module 2 Section 328
-
Module 2 Section 413
-
Module 2 Section 534
-
Module 3 Section 117
-
Module 3 Section 226
-
Module 3 Section 313
-
Module 3 Section 419
-
Module 3 Section 523
-
Module 3 Section 631
-
Module 3 Section 725
-
Module 4 Section 132
-
Module 4 Section 215
-
Module 4 Section 322
-
Module 4 Section 440
-
Module 4 Section 59
-
Module 4 Section 640
-
Module 5 Section 120
-
Module 5 Section 217
-
Module 5 Section 323
-
Module 5 Section 426
-
Module 6 Section 128
-
Module 6 Section 220
-
Module 6 Section 339
-
Module 6 Section 427
-
Module 6 Section 535
-
Module 6 Section 614
