Transport of O2 and CO2 in blood and tissue fluids Dr. Walid Daoud MBBCh, MSc, MD, FCCP Director of Chest Department, Shifa Hospital, A. Professor of Chest Medicine

Transport of O2 from lungs to body fluids ______________________________________ PO2 is higher in alveoli (104 mmHg) than in pulmonary capillaries (40 mmHg) so O2 diffuses from alveoli to blood. Also PO2 is higher in tissues capillaries so O2 diffuses to tissue cells. PCO2 is higher in pulmonary capillaries (45 mmHg) than in alveoli (40 mmHg) so CO2 diffuses to the alveoli. Also PCO2 is higher in tissue cells so CO2 diffuses to blood.

Uptake of O2 by pulmonary blood during exercise ______________________________________ During strenuous exercise, a person needs 20 times O2: Increased cardiac output Increased O2 diffusing capacity 3-folds due to increased surface area of capillaries and ideal VA/QT in upper part of lungs. Blood can still become fully oxygenated despite short time of exposure in the capillaries

Transport of O2 in arterial blood ______________________________________ 98% of blood enter left atrium from lungs and becomes fully oxygenated (PO2 104 mmHg). 2% of blood enter left atrium from bronchial vessels and not exposed to lung air or not oxygenated (shunt flow). This blood is venous (PO2 40 mmHg) causing venous admixture causing arterial PO2 to drop from 104 mmHg to 95 mmHg.

Diffusion of O2 from peripheral capillaries to tissue fluid Diffusion of O2 from peripheral capillaries to tissue fluid ______________________________________ PO2 in capillaries still 95 mmHg PO2 in interstitial fluid 40 mmHg PO2 leaves the tissue capillaries and enter systemic veins 40 mmHg Increased rate of blood flow in tissue, greater O2 is transported to tissue 4-folds increase in blood flow, PO2 increase from 40 to 66 mmHg. Increased tissue metabolism will decreased PO2 in interstitial fluid.

Diffusion of O2 from peripheral capillaries to tissue cells Diffusion of O2 from peripheral capillaries to tissue cells ______________________________________ Normal intracellular PO mmHg (average 23 mmHg) Only 1-3 mmHg of O2 is normally required for oxygenation in the mitochondria of the cell. So the low intracellular PO2 (23 mmHg) is more than adequate and provides a large safety factor.

Diffusion of CO2 from peripheral tissue cells into cap & from capillaries into alveoli _____________________________________ Cellular oxidation results in CO2 formation Intracellular PCO2 (46 mmHg) is higher than in interstitial fluid (45 mmHg) and higher than tissue capillaries (40 mmHg) so CO2 diffuses from cell to tissue capillaries then from venous blood (PCO2 becomes 45 mmHg) to alveoli (PCO2 40 mmHg) Decreased blood flow PCO2 increase to 60 mmHg. Increase blood flow decrease PCO2 in interstitial fluid to 41 mmHg. Increased metabolism increase PCO2 in interstitial fluid.

O2 transport in blood _____________________________________ 1. In physical solution: 3% of O2 is dissolved in plasma 3% of O2 is dissolved in plasma 2. In chemical combination with hemoglobin: 97% of O2 is carried by hemoglobin 97% of O2 is carried by hemoglobin O2 molecule combines loosely and reversibly with the heme portion of hemoglobin. O2 molecule combines loosely and reversibly with the heme portion of hemoglobin. When PO2 is high at pulmonary capillaries, When PO2 is high at pulmonary capillaries, O2 binds with HB O2 binds with HB When PO2 is low in tissue capillaries, O2 is released from HB. When PO2 is low in tissue capillaries, O2 is released from HB.

Oxygen-hemoglobin dissociation curve _____________________________________ At PO2 leaving the lung is 95 mmHg and O2 saturation in systemic arterial blood is 97% PO2 in normal venous blood returning from peripheral tissue is 40 mmHg and O2 saturation of hemoglobin is 75%.

Oxygen-hemoglobin dissociation curve _____________________________________ O2 capacity: Each 1 gm of HB can bind 1.34 ml O2 and HB is 100% saturated with O2. Arterial blood contain 15 gm HB/100 ml blood = 15 x 1.34 = 20 ml O2. O2 content: Arterial (PO2 95 mmHg, sat O2 97%) = 19.4 ml Venous (PO2 40 mmHg, sat O2 75%) =14.4 ml Normally 5 ml of O2 is transported from lungs to tissues by each 100 ml of blood flow.

O2-hemoglobin dissociation curve

Oxygen-hemoglobin dissociation curve _____________________________________ O2 transport during strenuous exercise: O2 uptake by tissue is increased 3 times normal (15 ml instead of 5 ml) or ( ml): PO2 drop from 40 to 15 mmHg in interstitial fluid Utilization coefficient: % of blood that gives O2 as it passes through tissue capillaries = 25% at rest & increases to 75-85% during strenuous exercise & approach 100% when blood flow is very slow or metabolic rate is very high (all O2 is given to tissue).

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