Transcript Resp.2 Oxygen dissociation curve

Resp.2
Oxygen dissociation curve
• Arterial blood with 15 gm/dl Hb contains
19.8 ml O2/dl, venous blood ( 75 %
saturated ) contains 15.2 O2 ml/dl.
• 4,6 ml O2 is delivered to tissues from each
100 ml blood, 250 ml delivered to the
tissues / min.
• Amount of Oxygen delivered depend on
Hb level.
• Oxygen dissociation curve is the relation
of the % saturation of Hb to the level of
PO2 in mm Hg.
• At PO2 100 sat. is 97.5%, at PO2 40 sat.
is 75%.
Factors affecting Hb affinity to O2
• Decreased affinity: shift of O2 dissociation
curve to the right:
• 1. Increased temp.
• 2. Fall in pH ( increased PCO2 – Bohr
effect ).
• 3. Increased 2,3,diphosphoglycerate in red
cells.
Increased affinity: Shift to the left:
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1. Low temp.
2. High pH.
3. Decreased 2,3,diphosphoglycerate.
PO2 above 100 will not increase
saturation.
• At PO2 60 Hb is 89% saturated.
• A steep drop of saturation below PO2 60.
2,3 diphosphoglycerate (2,3 DPG )
• A large amount in RBCs, formed from 3phosphoglyceraldehyde a product of
glycolysis.
• It is an anion which binds with Hb and
releases O2. Half life 6 h.
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Factors affecting 2,3 DPG:
• 1. Low pH ( acidosis ) decreases 2,3 DPG.
• 2. Thyroid hormone, growth hormone,
androgen leads to increased 2,3 DPG
concentration.
• 3. Exercise increases 2,3 DPG after 60
min.
• 4. High altitude increases 2,3 DPG
releasing O2.
Cont.
• 5. Hb F ( alpha2 gama2) poor binding to
2,3 DPG leading to increased affinity to O2
and more O2 moves from mother to fetus.
• 6. In blood banks 2,3 DPG decreases,
shifting the curve to the left, increasing O2
affinity and less oxygen released ?
• 7. Anemia can increase 2,3 DPG.
• Asphyxia: Decreased PaO2, increased
PCO2.
• Hypercapnia: increased PCO2.
• Hypocapnia: decreased PCO2.
Myoglobin
• It is an iron containing pigment in skeletal
muscles.
• Binds one molecule of O2.
• Its curve to the left of Hb so it takes O2
from Hb.
P50:
• It is the partial press. Of O2 at which Hb is 50 %
saturated. It is increased by:
• 1. Thyroid hormones, growth H. & androgens.
• 2. Exercise.
• 3. High altitude.
• 4. Increased temp.
• 5. Decreased pH.
• The higher the P50, the lower is the Hb affinity
for O2.
Gas exchange at the tissues:
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By simple diffusion: PaO2 in capillaries is
higher than tissues, so O2 diffuses to the
tissues. PCO2 is higher in tissues, so CO2
diffuses into capillaries.
Hypoxia:
Decreased O2 supply to the tissues produces
hypoxia. Types:
1. Hypoxic hypoxia: decreased PaO2 as in
pulmonary and cardiac diseases, high altitude.
Hypoxia cont.
• 2. Anemic hypoxia: anemia, CO decrease
Hb available to carry O2.
• 3. Stagnant hypoxia: decreased blood flow
as in shock.
• 4. Histotoxic hypoxia: prevention of
oxygen utilisation at tissues level eg
cyanide.
• Oxygen therapy can help in hypoxic
hypoxia & slightly in anemic h.