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 PO2 in alveoli.

PO2 100

PCo2 40

PH2O 47

PN2 573

Insp. air

PO2 158

PCo2 0.3

PH2O 5.7

PN2 596

EXP. air

Po2 116

PCo2 32

PH2O 47

PN2 565

Gas transport between lungs and tissues Inspired Expired PO2 158 PO2 116

PCO2 0.3 PCO2 32Vein Alveolar

airArterial air

PO2 40 100 95

PCO2 46 40 40

PH2o 47 47 47

PN2 573 573 573

Co2 O2

Tissues

O2 diffuses out of alveolar gas into the blood stream and Co2 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

O2 transport

O2 delivery to a tissue depends on:

1. O2 entering lungs.

2. Adequacy of gas exchange.3. Blood flow to tissues.4. Capacity of blood to carry O2.

5. Factors affecting O2 diffusion are: thickness, solubility, Po2.

Carriage of O2

1. Dissolved in plasma (2%): amount dissolved is proportionate to O2 tension at Po2 100 0.3ml dissolved / 100ml blood while at Po2 40 only 0.12ml / 100ml (venous).

Dissolved O2 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.

Hb4 + O2 ↔ Hb4 O2

Hb4O2 + O2 ↔ Hb4O4

Hb4O4 + O2 ↔ Hb4O6

Hb4O6 + O2 ↔ Hb4O8

Each combination enhance the next.

The amount of O2 combined with Hb is tension dependent but the relation is a sigmoid one and called O2 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 O2 delivered to tissues per min).

• Amount of O2 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 O2 dissociation curve to the right

occur in:• Arise in temp.

• A fall in pH ( PCo2)(Bohr effect).

• 2.3 diphosphoglycerate in red cells.

• Higher PO2 is required for Hb to bind O2.

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.

Per

cen

tag

e O

2

Sat

urat

ion

of H

b

100 -

30 -

Temp

PH

Temp

PH

2.3. DPG

10 20 30 40 50 60 70 80 90 100

Po2 % Dis.

10 13.5 0.03

40 75 0.12

60 89 0.15

100 97.5 0.3

Po2 mm Hg

O2 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 Co2 + H2o ↔ H2Co3 ↔ 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.

Co2

• Co2 carried by blood independent on Pco2.

• Oxy Hb shift curve to the right helps release of Co2 at alveoli (Haldane effect)

Hb0

HbCo2

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.

Thank you