Ventilation/Perfusion Ratio & Gas Exchange Flashcards
The V/Q ratio of the lung as a whole is
0.8
Mention the mean arterial pressure in different regions of the lung
13 mmHg at the level of the heart
3mmHg at the apex
21 mmHg at the bottom
GR: In, upright position, the alveoli at the apex have greater volume than the base.
Because gravity causes the intrapleural pressure ti be more negative in the lung apex than in the base.
GR: The small alveoli at the base are more well-ventilated than large ones at the apex.
Because the small alveoli can be inflated to larger size than the large alveoli can do (more compliant)
Compare V/Q ratio in lung apex & base
Apex: blood flow & alveolar ventilation are both less, but the decrease of blood flow is more so V/Q is increased to 3.
Base: blood flow & alveolar ventilation are both more, but the increase in blood flow is more so V/Q is decreased to 0.6.
GR: The apex of lung harbours air volume added to anatomical dead space.
Because of the increased V/Q ratio, leading to wasted ventilation (physiological dead space) as air passes without gas exchange.
GR: The base of the lung harbours blood volume added to anatomical shunt.
Because of the decreased V/Q ratio, leading to wasted blood flow (physiological shunt) as blood passes without gas exchange.
GR: Alveolar dead space incraeses in upright position as opposed to supine.
Beacuse of decreased perfusion to uppermost alveoli.
GR: In upright position, more gas exchange occurs in the bottom.
Due to more blood flow at the base than the lung apex and thus more O2 is extracted from alveolar air.
Describe local homeostatic mechanisms in case of pulmonary embolus
Decreased alveolar CO2 leads to local bronchoconstriction with shifting ventilation away from poorly perfused area.
Increased alveolar O2 leads to vasodilatation of local pulmonary blood vessels with increased perfusion to poorly perfused area.
Describe local homeostatic mechanisms in case of airway obstruction
Increased alveolar CO2 leads to local bronchodilatation with increased airflow in obstructed airways.
Decreased alveolar O2 leads to vasoconstriction of local pulmonary blood vessels with shunting blood away from hypoxic area, to direct it to well ventilated alveoli.
GR: Alveolar PO2 is less than atmospheric PO2
- Inspired air is saturated with water vapour which exerts partial pressure of 47 mmH
- Only about 15% of alveolar air is replaced by atmospheric air
- O2 continuously diffuses into blood
GR: Alveolar O2 remains relatively contstant
- O2 continuously diffuses down its pressure gardient from alveoli to blood
- O2 arriving in alveoli = O2 diffusing out of alveoli to blood
Pressure gradient of O2 is …..
60 mmHg
Pressure gradient of CO2 is …..
6 mmHg
Mention components of respiratory membrane
- Fluid lining alveoli containing surfactant
- Alveolar epithelium
- Epithelial BM
- Interstitial space
- Capillary BM
- Capillary endothelium
Factors affecting the rate of gas diffusion across the alveolar-capillary membrane
- Pgas
- SA
- Thickness of membrane
- Diffusion coefficient
Mention factors affecting diffusion coefficient
Gas solubility
Molecular weight of gas
Mention factors increasing gas exchange across respiratory membrane
- PP
- SA
- Solubility of gas
Mention factors decreasing decreasing gas exchange across respiratory membrane.
Thickness of membrane
Molecular weight
GR: Diffusion coefficient for CO2 is 20 times that of O2
Because CO2 is 24 times more soluble in tissues than O2, but the molecular weight of CO2 is 1.4 times greater than that of CO2
GR: Equilibration of O2 and CO2 across the membrane occurs at the same rate, although CO2 has much higher diffusion coefficient
Because CO2 pressure gradient (6 mmHg) is less than O2 pressure gradient (60 mmHg) across alveolar capillary membrane.
GR: Increased diffusion capacity during exercise.
- increased alveolar expansion by deeper breathing that increases alveolar surface area and decreases thickness of alveolar wall.
- increase number of open pulmonary capillaries by increased cardiac output that increases blood surface area exposed for gas exchange.
Mention conditions in which diffusion capacity decreases
- increased thickness of alveolar capillary BM (fibrosis, edema)
- conditions that decrease the effective area for diffusion (emphysema)
- V/Q mismatch
GR: Diffusion of O2 is seriously impaired that CO2 in pulmonary edema
Because it is less soluble in fluids
Percentage of physically dissolved O2
1.5%
O2 content if blood is …., while O2 carrying capacity of blood is …… .
- 8 ml O2/100 ml of arterial blood
20. 1 ml O2/100 ml blood
Mention the primary factor determing Hb % saturation
PO2 of the blood
The oxygen-hb dissociation curve is
Sigmoid
In the plateau part of the curve, PO2 range is ….. , which exists at ….. , represents O2 being …… by Hb.
Mention its physiological importance
100-60 mmHg, pulmonary capillaries, picked up
Provides a good safety margin against blood PO2 changes
Mention factors shifting O2-Hb curve to right/left
What is a physiological condition where shift to the right occurs?
Right: Increased PCO2, H+, 2,3-DPG, Temperature, decreased Hb affinity to O2 Left: The opposite During exercise
Mention P50 of Hb, and the significance of this value.
27 mmHg
Increased P50 means shift to the right
Decreased P50 means shift to the left
Define Bohr’s effect and its importance
The effect of PCO2 & H+ on dissociation curve of H
Increase PCO2 & H+ cause shift to the right.
Decrease PCO2 & H+ cause shift to the left.
It facilitates O2 release at tissue & uptake at lungs
Describe the mechanism of Bohr’s effect
CO2 & H+ combine reversibly with Hb at sepearte sites changing its shape and decreasing its affinity to oxygen.
2,3 DPG is produced by ….., and bind to …. causing ….. .
Anaerobic glycoysis
Beta chain
Decrease Hb affinity to O2
Mention hormones increasing 2,3 DPG
Thyroid hormones, growth hormone & androgens.
Mention conditions decreasing 2,3 DPG level
Acidosis
Stored blood
Describe mechanism of CO poisoning
Formation of carboxyhemoglobin/carbon monoxyhemoglobin
Binds at the same site at oxygen at an affinity 240 times higher
Interferes with release of O2 at tissues by shifting curve to the left
GR: fetal Hb increases fetal uptake of O2 from mother.
As it doesn’t contain beta chain where 2,3 DPG binds, it causes increased affinity and shift to the left in Hb-O2 diss curve.
Describe function and correlated properties of myoglobin
It acts as O2 store to be used in severe muscular exercise when O2 tension becomes very low.
Its curve is rectangular in shape & to left of that of Hb as it has higher affinity to O2.
