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Concept 42.7: Respiratory pigments bind and transport gases

Concept 42.7: Respiratory pigments bind and transport gases The metabolic demands of many organisms require that the blood transport large quantities of O 2 and CO 2 Gases diffuse down pressure gradients in the lungs and other organs

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Concept 42.7: Respiratory pigments bind and transport gases

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  1. 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

  2. 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

  3. Inhaled air Exhaled air 120 27 160 0.2 Alveolar spaces O2 CO2 O2 CO2 Alveolarepithelialcells 104 40 O2 CO2 O2 CO2 Blood leaving alveolar capillaries Blood enteringalveolarcapillaries O2 CO2 3 1 2 4 Alveolar capillariesof lung 40 45 104 40 O2 O2 CO2 CO2 Pulmonaryveins Pulmonaryarteries Systemic arteries Systemicveins Heart Tissue capillaries O2 CO2 Blood enteringtissuecapillaries Blood leavingtissuecapillaries O2 CO2 100 40 40 45 O2 O2 CO2 CO2 Tissue cells <40 >45 O2 CO2 Figure 42.27

  4. 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

  5. Heme group Iron atom O2 loaded in lungs O2 O2 unloaded In tissues O2 Polypeptide chain • Hemoglobin must reversibly bind O2, loading O2 in the lungs and unloading it in other parts of the body Figure 42.28

  6. 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

  7. (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 O2 saturation of hemoglobin (%) 40 20 0 60 100 40 80 0 20 Tissues at rest Lungs Tissues during exercise PO2 (mm Hg) (b) pH and Hemoglobin Dissociation 100 pH 7.4 80 Bohr shift:Additional O2released from hemoglobin at lower pH(higher CO2concentration) 60 O2 saturation of hemoglobin (%) pH 7.2 40 20 0 Figure 42.29a, b 60 100 40 80 0 20 PO2 (mm Hg)

  8. 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

  9. Tissue cell Carbon dioxide produced bybody tissues diffuses into the interstitial fluid and the plasma. Most of the HCO3– diffuseinto the plasma where it is carried in the bloodstream to the lungs. CO2 transportfrom tissues 11 10 7 6 5 4 3 2 1 8 9 CO2 produced Interstitialfluid CO2 Over 90% of the CO2 diffuses into red blood cells, leaving only 7%in the plasma as dissolved CO2. Blood plasmawithin capillary CO2 Capillarywall In the HCO3– diffusefrom the plasma red blood cells, combining with H+ released from hemoglobin and forming H2CO3. CO2 H2O Some CO2 is picked up and transported by hemoglobin. Redbloodcell Hemoglobinpicks upCO2 and H+ H2CO3 Hb Carbonic acid Carbonic acid is converted back into CO2 and water. HCO3– + H+ Bicarbonate However, most CO2 reacts with water in red blood cells, forming carbonic acid (H2CO3), a reaction catalyzed bycarbonic anhydrase contained. Withinred blood cells. HCO3– To lungs CO2 formed from H2CO3 is unloadedfrom hemoglobin and diffuses into the interstitial fluid. CO2 transportto lungs HCO3– 9 6 2 7 5 4 3 1 8 + H+ HCO3– CO2 diffuses into the alveolarspace, from which it is expelledduring exhalation. The reductionof CO2 concentration in the plasmadrives the breakdown of H2CO3 Into CO2 and water in the red bloodcells (see step 9), a reversal of the reaction that occurs in the tissues (see step 4). Carbonic acid dissociates into a biocarbonate ion (HCO3–) and a hydrogen ion (H+). HemoglobinreleasesCO2 and H+ Hb H2CO3 H2O CO2 Hemoglobin binds most of the H+ from H2CO3 preventing the H+from acidifying the blood and thuspreventing the Bohr shift. CO2 11 10 CO2 CO2 Figure 42.30 Alveolar space in lung

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