Respiratory System Physiology By Dr. SHAHAB SHAIKH PhD MD Lecture – 7A: Transport of Gases – O2 ••••••••••••••••••••••••••••••••••

Respiratory System Physiology TRANSPORT OF GASES – O2 Partial Pressure of O2 Blood O2 Exchange O2 uptake by Pulmonary Blood O2 uptake during Exercise Changes in PO2 during O2 Transp Methods of O2 Transport Hemoglobin Hb-O2 Dissociation Curve Factors affecting Hb-O2 Diss Curve O2 Content of Arterial Blood O2 diffusion to cells

Atmospheric Air (mm Hg) Partial pressures of O2   Atmospheric Air (mm Hg) Humidified Air (mm Hg) Alveolar Air Expired Air O2 159.0 (20.84%) 149.3 (19.67%) 104.0 (13.6%) 120.0 (15.7%)

Blood O2 Exchange Oxygen and carbon dioxide move between air and blood by simple diffusion: from an area of high to low partial pressure. It is a passive process.

O2 Uptake by Pulmonary blood Oxygen diffuses from the alveoli into the pulmonary capillary blood because the oxygen partial pressure (Po2) in the alveoli is greater than the Po2 in the pulmonary capillary blood. The blood Po2 rises almost to that of the alveolar air by the time the blood has moved a third of the distance through the capillary, becoming almost 104 mm Hg.

O2 Uptake during Exercise During strenuous exercise, a person’s body may require as much as 20 times the normal amount of O2. The diffusing capacity for oxygen increases almost threefold during exercise; this results mainly from: Increased surface area of capillaries participating in the diffusion more nearly ideal ventilation-perfusion ratio in the all part of the lungs. During exercise, even with a shortened time of exposure in the capillaries, the blood can still become fully oxygenated because as it is the blood normally stays in the lung capillaries about three times as long as necessary to cause full oxygenation.

CHANGES IN PO2 AT DIFF STAGES of O2 Transport

Methods of O2 Transport blood Dissolved in Plasma = 2 - 3 % O2 combined with Hb = 97 - 98 %

Transport of O2 in Dissolved state O2 Solubility = 0.003 ml/dl/mmHg PO2 in arterial blood = 95 mm Hg Therefore, dissolved oxygen in blood = 95 X 0.003 = 0.29 ml / dl Thus at PO2 of 95 mmHg in arterial blood there is 0.29 mL O2/dL dissolved oxygen in blood. Also at PO2 of 40 mmHg in venous blood, there is 0.12 mL O2/dL dissolved oxygen in blood. Thus 100 ml of blood delivers only 0.17 ml of O2 Thus amount of O2 transported in dissolved state normally is only slight, only about 3 % of total O2 transport

Transport of O2 combined with Hb Approx 280 million hemoglobin mol / RBC. Normally, about 97 per cent of the oxygen transported from the lungs to the tissues is carried in chemical combination with hemoglobin in the red blood cells. When Po2 is high, as in the pulmonary capillaries, oxygen binds with the hemoglobin, but when Po2 is low, as in the tissue capillaries, oxygen is released from the hemoglobin. This is the basis for almost all oxygen transport from the lungs to the tissues.

Hemoglobin Hemoglobin is a protein made up of four subunits, each of which contains a heme moiety attached to a polypeptide chain. In normal adults, most of the hemoglobin molecules contain two α and two β chains.

Hemoglobin Types of Hemoglobin: HbA (2α + 2β): 90% of Hb in adult HbA2 (2α + 2δ): 2-3% of Hb in adult HbF (2α + 2 γ): 0.5% of Hb in adults - fetal Hb, high affinity to O2 4 peptide subunits (2α + 2β) – globin combine with 4 molecules of heme with Fe ++ Heme is based on a complex multi ring family of structures termed Porphyrins Heme is constituted by Protoporphyrin IX bound to an iron atom in its ferrous state (Fe2+)

Hemoglobin The quaternary structure of hemoglobin determines its affinity for O2. In deoxyhemoglobin, the globin units are tightly bound in a tense (T) configuration which reduces the affinity of the molecule for O2. When O2 is first bound, the bonds holding the globin units are released, producing a relaxed (R) configuration which exposes more O2 binding sites.The net result is a 500-fold increase in O2 affinity. In the tissues, these reactions are reversed, releasing O2.

O2 – Hb Dissociation Curve The oxygen–hemoglobin dissociation curve is the curve relating percentage saturation of the Hemoglobin by O2 to the partial pressure of O2 (PO2).

O2 – Hb Dissociation Curve Sigmoid shape of O2 Hb dissociation curve: Combination of the first heme in the Hb molecule with O2 causes the T configured Deoxyhemoglobin to convert to R configured Oxyhemoglobin and thus increases the affinity of the second heme for O2, similarly oxygenation of the second increases the affinity of the third, etc, so that the affinity of Hb for the fourth O2 molecule is many times that for the first.

O2 – Hb Dissociation Curve

Factors affecting O2 – Hb Diss. Curve Shifts to the right: Occur when the affinity of Hb-binding sites for O2 is decreased thus it is easier for tissues to extract oxygen. Causes of this shift include increased CO2 (Bohr effect), increased H+ (decreased pH), increased temperature Increased 2, 3- DPG Anemia

Factors affecting O2 – Hb Diss. Curve Bohr Effect: “It is the decrease in affinity of O2 for Hb when there is increase in Pco2 “ Or “Bohrs effect is loading of CO2 to blood causes unloading of O2 & Unloading of CO2 causes increase loading of O2” It is due to the fact that Deoxygenated Hb binds H+ more actively than Oxygenated Hb Thus at tissue level, the Bohr effect helps shift the Hb – O2 Dissociation curve to right & helps Increase delivery of O2 to tissues. Opposite effect occurs at lungs level which helps in increase O2 uptake by blood.

The Right Shift Effect: O2 is “unloaded” in tissues Hb O 2 O 2 right shift H Temperature CO + 2 tissue metabolism

Factors affecting O2 – Hb Diss. Curve Shifts to the Left: Occur when there is increased affinity of Hb for O2 and it is more difficult for tissues to extract oxygen. Causes of this shift include decreased temperature, decreased PCO2, decreased H+ (increased pH), Decreased 2, 3-DPG. (Stored blood loses 2,3-DPG) fetal Hb Polycythemia

O2 content – Arterial Blood O2 bound to Hb: Each gram of Hb has an oxygen carrying capacity of 1.34 mL O2 100 mL of blood contains 15 g Hb, Therefore completely oxygenated blood contains approximately: 1.34 mL O2 × 15 g Hb/100 mL.= 20 mL O2 Thus, the oxygen capacity of Hb in blood is 20 mL O2 / 100 mL of blood or 20 vol%. O2 Dissolved in blood: At body temperature, blood with a normal PO2 (~ 100 mm Hg) contains only 0.3 mL O2/100 mL blood in dissolved form

O2 content – Arterial Blood The amount of O2 released from the Hb in the tissues: In the arterial blood 97/100 x 1.34 x 15gm of Hb = 19.4ml of O2 bound with Hb. In the venous blood 75/100 x 1.34 x 15gm = 14.4ml of O2. So under normal conditions about 5ml of O2 are transported to the tissues by each 100ml of blood. In heavy exercise the muscle cells utilize O2 rapidly, which causes the interstitial fluid PO2 to fall to 15mmHg. Only 4.4ml of O2 remains bound to with Hb in each 100ml of blood (19.4 – 4.4 = 15ml of O2 are transported by each 100ml of blood). Also cardiac output can increase to 7 fold. The amount of O2 transported to the tissue increase to 20 folds (3 x 7 = 21).

Factors affecting O2 content of blood

OXYGEN DIFFUSION TO CELLS The normal intracellular Po2 ranges from as low as 5 mm Hg to as high as 40 mm Hg, averaging 23 mm Hg. Because only 1 to 3 mm Hg of oxygen pressure is normally required for full support of the chemical processes that use oxygen in the cell, one can see that even this low intracellular Po2 of 23 mm Hg is more than adequate and provides a large safety factor.

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