DR.V.V.UNNIKRISHNAN, MD
ASSOCIATE PROFESSOR OF PHYSIOLOGY
GOVT. MEDICAL COLLEGE, THRISSUR
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Diffusion capacity of the lung
ASSOCIATE PROFESSOR OF PHYSIOLOGY
GOVT. MEDICAL COLLEGE, THRISSUR
This is NOT a complete note. This is only a transcript of the powerpoint presentation. Please remember that this is a template on which you have to build upon. Add diagrams wherever necessary.
Please send your doubts & suggestions to mctsrphysiology@gmail.com or to my personal id, drvvuk@gmail.com
Diffusion capacity of the lung
It is the amount of gas crossing the alveolar capillary membrane per minute per mm Hg difference in partial pressures of gas on the two sides of the membrane. Normal value is 20-30ml / min / mm Hg at rest
Factors affecting Diffusion capacity of the lung
Surface area of alveolar capillary membrane [directly proportional]
Thickness of alveolar capillary membrane [inversely proportional]
Solubility of gas [directly proportional]
Molecular weight of gas [Square root]
DIFFUSING CAPACITY FOR O2 is 21 ml / min / mm Hg.
DIFFUSING CAPACITY FOR CO2 is 400 ml / min / mm Hg
Diffusing capacity of CO is measured as an index of diffusing capacity
diffusing capacity increases up to threefold during exercise because of capillary dilation and increase in the number of active capillaries
Oxygenation of Hb
DISTRIBUTION OF O2
pO2 mm Hg
Inspired Air : 158
Expired Air : 116
Alveolar Air : 100 - 104
Arterial blood : 98 - 100
Venous Blood : 40
DISTRIBUTION OF O2
O2 content ml %
Expired Air : 16
Alveolar Inspired Air : 21
Air : 13 - 14
Arterial blood : 19
Venous Blood : 14
For each 100 ml of inspired air, 5ml of O2 is extracted by the blood.
For each 100 ml of arterial blood, 5ml of O2 is extracted by the tissues.
OXYGEN TRANSPORT IN BLOOD
In dissolved form
In combination with Hb
dissolved form
0.3 ml / 100ml blood / 100 mm Hg
Directly proportional to arterial pO2
Important during exercise
Hemoglobin is oxygenated, not oxidated.
Oxidation vs Oxygenation
Only physical combination
Iron remains in ferrous form only.
Maximum quantity is transported
Advantage: O2 can be easily accepted or given out according to the partial pressure of O2.
O2 saturation of Hb
O2 transported as Oxy haemoglobin
1 gm Hb transports 1.34ml O2
i.e., in a person with 15gm Hb, 15 x 1.34 = 20.1 ml O2 per 100 ml of blood
But actual value is only 19 ml/100ml. this is because Hb can be only 95% saturated.
Thus, O2 Carrying capacity is 19 ml O2 per 100 ml of blood
Oxygen Dissociation Curve [ODC] (Oxygen-Hemoglobin Dissociation Curve)
Graph plotting partial pressure of O2 against % saturation of Hb.
S-shaped curve [sigmoid]
During é p O2 à Hb accepts O2
During ê p O2 à Hb releases O2
P 50
Partial pressure of O2 at which Hb is 50% saturated. Normal value is 26 mm Hg
Hb affinity is inversely related to P 50
Factors affecting ODC: SHIFT TO RIGHT
SHOWS DISSOCIATION of O2
ê p O2
é CO2
é body temperature
é 2,3 Di-phospho glycerate [2,3 DPG]
2,3 Di-phospho glycerate [2,3 DPG]
By-product of Embden Meyerhoff Pathway
Present in RBCs
é in muscular exercise
Acidosis
High altitude
2,3 Di-phospho glycerate [2,3 DPG]
DECREASED IN
Acidosis [due to inhibition of glycolysis]
Stored blood with acid citrate as anticoagulant
2, 3 DPG IS INCREASED IN
Anaemia
High altitude
Chronic hypoxia
Increased
thyroid hormone
growth hormone
androgen
Factors affecting ODC: SHIFT TO LEFT
SHOWS ACCEPTANCE of O2
é pO2 à Hb accepts O2
Foetal blood
Myoglobin
Alkalinity
Bohr Effect
In exercising muscle, more CO2 generated
Enters the blood by diffusion by pressure gradient
More pCO2 decreases the affinity of Hb to O2
O2 is released
ODC shifted to Right.
Coefficient of O2 utilization
It is the percentage of O2 utilized out of the amount, which is made available in the issues
Coefficient of O2 utilization = O2 taken up by tissues
O2 content of arterial blood
tissues remove 5ml of O2 from each 100 ml of blood passing through them
O2 content of arterial blood is 19 ml / 100 ml
Coefficient of O2 utilization = 5 / 19 = 0.26
MYOGLOBIN
Causes shift to left
Seen in muscles which are involved in sustained contraction
E.g.: Leg & Heart muscles
Contains only one atom of iron per molecule
Readily takes up O2 at low pO2.
Readily takes up O2 at low pO2
ODC for myoglobin is a rectangular hyperbola
Does not show Bohr effect.
Even at pO2 of 40mm Hg, it is 95% saturated
Acts as a temporary storehouse for O2 in muscles
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