Gas exchange

Quiz Surfactant is decreased in: 1. Term babies 2. Glucocorticoid therapy 3. Ventori mask short term oxygen therapy. 4. Smokers 5. Abdominal surgery. 4

Inspired air will be saturated with water vapor in the lungs decreasing PO 2 in alveoli. PO2100 PCo240 PH2O47 PN2573 Insp. air PO2158 PCo20.3 PH2O5.7 PN2596 EXP. air Po2116 PCo232 PH2O47 PN2565

Gas transport between lungs and tissues Inspired Expired PO PO PCO PCO 2 32 Arterial air Alveolar air Vein PO PCO 2 47 PH 2 o 573 PN2 Co 2 O 2 Tissues

O 2 diffuses out of alveolar gas into the blood stream and Co 2 diffuses into the alveoli from blood. Diffusion across alveolo capillary membrane made up of pulm. epithelium, capillary endthelium and their basement membrane.

Diffusion capacity of the lung for a given gas is directly proportionate to the alveolo- capillary membrane area and inversely proportion to its thickness.

Gas transport Prof. Omer Abdel Aziz

O 2 transport O 2 delivery to a tissue depends on: 1.O 2 entering lungs. 2.Adequacy of gas exchange. 3.Blood flow to tissues. 4.Capacity of blood to carry O 2. 5.Factors affecting O 2 diffusion are: thickness, solubility, Po 2.

Carriage of O 2 1. Dissolved in plasma (2%): amount dissolved is proportionate to O 2 tension at Po ml dissolved / 100ml blood while at Po 2 40 only 0.12ml / 100ml (venous). Dissolved O 2 reaches the tissue, imp for cornea and cartilages.

2. Combined with Hb in red cells 98% Hb has 4 sub units attached to 4 haem moieties each is formed from porphyrin and ferrous iron,each of the 4 ferous ions can bind with one O2 molecule. Hb 4 + O 2 ↔ Hb 4 O 2 Hb 4 O 2 + O 2 ↔ Hb 4 O 4 Hb 4 O 4 + O 2 ↔ Hb 4 O 6 Hb 4 O 6 + O 2 ↔ Hb 4 O 8

Each combination enhance the next. The amount of O 2 combined with Hb is tension dependent but the relation is a sigmoid one and called O 2 dissociation curve. R. (relaxed state ) chain away from each other. T ( tense state) globin chains close to each other.

With Hb of 15 g/dL, arterial blood contains 19.8 ml/dL, in venous blood (75% sat.) 15.2 ml/dL, so 4.6 ml O2 from each deciliter ( 250 ml O 2 delivered to tissues per min). Amount of O 2 depend on Hb level.

Oxygen dissociation curve Prof. Omer Abdel Aziz

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%.

Decreased affinity of Hb: shift to the right Shift of O 2 dissociation curve to the right occur in: Arise in temp. A fall in pH ( PCo 2 )(Bohr effect). 2.3 diphosphoglycerate in red cells. Higher PO 2 is required for Hb to bind O 2.

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.

Percentage O 2 Saturation of Hb Temp PH Temp PH 2.3. DPG Po2%Dis Po 2 mm Hg O 2 Dissociation curve

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.

Quiz In inhabitants of high altitude the oxygen dissociation curve can shift to the right due to: 1. Increased temperature. 2. Increased 2,3 DPG. 3. Decreased pH. 4. Increased PCO2. 5. Increased Hb. 2

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.

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.

Quiz P50 is increased in: 1. Low temp. 2. Low PCO2 3. High pH. 4. Decreaed 2,3 DPG. 5. Thyrotxicosis.

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.

Quiz Hypoxia in shock is : 1. stagnant 2. hypoxic 3. histotoxic 4. anemic 5. neurogenic.

CO2 Transport From tissues to lungs by plasma & RBCs: 1. Dissolved 10%. 2. Carbonic acid Co 2 + H 2 o ↔ H 2 Co 3 ↔ H+ HCO3, 70%, slow in plasma due to absence of carbonic anhydrase, more in RBCs: Hco - 3 diffuse and Cl - enters (Band3) Chloride shift in RBC more fluid enters increasing HCT. H + Buffered by Hb. 3.Combined to protein: Carbamino protein, carbamino- hemoglobin.

Co 2 Co 2 carried by blood independent on Pco 2. Oxy Hb shift curve to the right helps release of Co 2 at alveoli (Haldane effect) Hb0 Hb Co2 In blood

Asphyxia: Decreased PaO2, increased PCO2. Hypercapnia: increased PCO2. Hypocapnia: decreased PCO2.

Cyanosis Definition: it is diffused bluish coloration of skin and mucus membranes due to presence of large amount of reduced Hb (5 g or more). Types: 1.Central: in heart failure, right to left shunts; cyanosis is general( tongue & extremities) 2. Peripheral: flow of blood is slowed in capillaries as in cold, venous obstruction & heart failure.

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