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• Concept 42.7: Respiratory pigments bind and transport gases • The metabolic demands of many organisms require that the blood transport large quantities of O2 and CO2 • Gases diffuse down pressure gradients in the lungs and other organs • Diffusion of a gas depends on differences in a quantity called partial pressure Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • A gas always diffuses from a region of higher partial pressure to a region of lower partial pressure • In the lungs and in the tissues, O2 and CO2 diffuse from where their partial pressures are higher to where they are lower Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Inhaled air Exhaled air 160 0.2 O2 CO2 120 27 Alveolar spaces O2 CO2 104 Alveolar epithelial cells 40 O2 CO2 Blood entering alveolar capillaries 40 O2 CO2 2 1 O2 Alveolar capillaries of lung 45 O2 CO2 104 Pulmonary veins Systemic arteries Systemic veins CO2 40 40 O2 CO2 Pulmonary arteries Blood leaving tissue capillaries Blood leaving alveolar capillaries Heart Tissue capillaries O2 3 4 45 O2 CO2 Tissue cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 100 40 O2 CO2 O2 CO2 Figure 42.27 Blood entering tissue capillaries <40 >45 O2 CO2 Respiratory Pigments • Respiratory pigments are proteins that transport oxygen – Greatly increase the amount of oxygen that blood can carry • The respiratory pigment of almost all vertebrates is the protein hemoglobin, contained in the erythrocytes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Hemoglobin must reversibly bind O2, loading O2 in the lungs and unloading it in other parts of the body Heme group Iron atom O2 loaded in lungs O2 unloaded In tissues Polypeptide chain Figure 42.28 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings O2 O2 • Loading and unloading of O2 depend on cooperation between the subunits of the hemoglobin molecule • The binding of O2 to one subunit induces the other subunits to bind O2 with more affinity • Cooperative O2 binding and release is evident in the dissociation curve for hemoglobin • A drop in pH lowers the affinity of hemoglobin for O2 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings O2 saturation of hemoglobin (%) (a) PO2 and Hemoglobin Dissociation at 37°C and pH 7.4 O2 unloaded from hemoglobin during normal metabolism 100 80 O2 reserve that can be unloaded from hemoglobin to tissues with high metabolism 60 40 20 0 0 20 40 60 Tissues during Tissues at rest exercise 80 100 Lungs (b) pH and Hemoglobin Dissociation Figure 42.29a, b Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings O2 saturation of hemoglobin (%) PO2 (mm Hg) 100 pH 7.4 80 60 pH 7.2 40 20 0 0 20 40 Bohr shift: Additional O2 released from hemoglobin at lower pH (higher CO2 concentration) 60 PO2 (mm Hg) 80 100 Carbon Dioxide Transport • Hemoglobin also helps transport CO2 and assists in buffering • Carbon from respiring cells diffuses into the blood plasma and then into erythrocytes and is ultimately released in the lungs Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 1 2 Carbon dioxide produced by body tissues diffuses into the interstitial fluid and the plasma. Over 90% of the CO2 diffuses into red blood cells, leaving only 7% in the plasma as dissolved CO2. Tissue cell Some CO2 is picked up and transported by hemoglobin. 1 Blood plasma CO 2 within capillary Capillary wall 2 CO2 Carbonic acid dissociates into a biocarbonate ion (HCO3–) and a hydrogen ion (H+). HCO3– 7 Hemoglobin binds most of the H+ from H2CO3 preventing the H+ from acidifying the blood and thus preventing the Bohr shift. Figure 42.30 9 Carbonic acid is converted back into CO2 and water. 10 CO2 formed from H2CO3 is unloaded from hemoglobin and diffuses into the interstitial fluid. To lungs CO2 transport to lungs HCO3– 8 H2CO3 Hb 9 11 CO2 Hemoglobin releases CO2 and H+ H2O CO2 6 In the HCO3– diffuse from the plasma red blood cells, combining with H+ released from hemoglobin and forming H2CO3. 6 HCO3– + H+ 5 8 Red Hemoglobin H2CO3 blood Carbonic acid Hb picks up cell CO2 and H+ 5 + H+ Bicarbonate However, most CO2 reacts with water in red blood cells, forming carbonic acid (H2CO3), a reaction catalyzed by carbonic anhydrase contained. Within red blood cells. Most of the HCO3– diffuse into the plasma where it is carried in the bloodstream to the lungs. 3 4 HCO3– 4 7 Interstitial CO 2 fluid H2O 3 CO2 transport from tissues CO2 produced CO2 CO2 10 CO2 11 Alveolar space in lung Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings diffuses into the alveolar space, from which it is expelled during exhalation. The reduction of CO2 concentration in the plasma drives the breakdown of H2CO3 Into CO2 and water in the red blood cells (see step 9), a reversal of the reaction that occurs in the tissues (see step 4).