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: 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 3- phosphoglyceraldehyde a product of glycolysis. It is an anion which binds with Hb and releases O2. Half life 6 h.

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: 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.