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2. The Respiratory System Chapter 49

3. Gas Exchange One of the major physiological challenges facing all multicellular animals is obtaining sufficient oxygen and disposing of excess carbon dioxide In vertebrates, the gases diffuse into the aqueous layer covering the epithelial cells that line the respiratory organs Diffusion is passive, driven only by the difference in O 2 and CO 2 concentrations on the two sides of the membranes and their relative solubilities in the plasma membrane 3

4. Gas Exchange Rate of diffusion between two regions is governed by Fick’s Law of Diffusion R = Rate of diffusion D = Diffusion constant A = Area over which diffusion takes place  p = Pressure difference between two sides d = Distance over which diffusion occurs 4 R = DA  p d

5. Gas Exchange Evolutionary changes have occurred to optimize the rate of diffusion R –Increase surface area A –Decrease distance d –Increase concentration difference  p 5

6. Gas Exchange Gases diffuse directly into unicellular organisms However, most multicellular animals require system adaptations to enhance gas exchange Amphibians respire across their skin Echinoderms have protruding papulae Insects have an extensive tracheal system Fish use gills Mammals have a large network of alveoli 6

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10. Gills Specialized extensions of tissue that project into water Increase surface area for diffusion External gills are not enclosed within body structures –Found in immature fish and amphibians –Two main disadvantages Must be constantly moved to ensure contact with oxygen-rich fresh water Are easily damaged 10

11. Gills Branchial chambers –Provide a means of pumping water past stationary gills –Internal mantle cavity of mollusks opens to the outside and contains the gills Draw water in and pass it over gills –In crustaceans, the branchial chamber lies between the bulk of the body and the hard exoskeleton of the animal Limb movements draw water over gills 11

12. Gills Gills of bony fishes are located between the oral (buccal or mouth) cavity and the opercular cavities These two sets of cavities function as pumps that alternately expand Move water into the mouth, through the gills, and out of the fish through the open operculum or gill cover 12

13. 13 Gills

14. Some bony fish have immobile opercula –Swim constantly to force water over gills –Ram ventilation Most bony fish have flexible gill covers Remora switch between ram ventilation and pumping action 14

15. Gills 3–7 gill arches on each side of a fish’s head Each is composed of two rows of gill filaments Each gill filament consist of lamellae –Thin membranous plates that project into water flow –Water flows past lamellae in 1 direction only 15

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17. Gills Within each lamella, blood flows opposite to direction of water movement –Countercurrent flow –Maximizes oxygenation of blood –Increases  p Fish gills are the most efficient of all respiratory organs 17

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19. Gills Many amphibians use cutaneous respiration for gas exchange In terrestrial arthropods, the respiratory system consists of air ducts called trachea, which branch into very small tracheoles –Tracheoles are in direct contact with individual cells –Spiracles (openings in the exoskeleton) can be opened or closed by valves 19

20. Lungs Gills were replaced in terrestrial animals because –Air is less supportive than water –Water evaporates The lung minimizes evaporation by moving air through a branched tubular passage A two-way flow system –Except birds 20

21. Lungs Air exerts a pressure downward, due to gravity A pressure of 760 mm Hg is defined as one atmosphere (1.0 atm) of pressure Partial pressure is the pressure contributed by a gas to the total atmospheric pressure 21

22. Lungs Partial pressures are based on the % of the gas in dry air At sea level or 1.0 atm –P N 2 = 760 x 79.02% = 600.6 mm Hg –P O 2 = 760 x 20.95% = 159.2 mm Hg –P CO 2 = 760 x 0.03% = 0.2 mm Hg At 6000 m the atmospheric pressure is 380 mm Hg –P O 2 = 380 x 20.95% = 80 mm Hg 22

23. Lungs Lungs of amphibians are formed as saclike outpouchings of the gut Frogs have positive pressure breathing –Force air into their lungs by creating a positive pressure in the buccal cavity Reptiles have negative pressure breathing –Expand rib cages by muscular contractions, creating lower pressure inside the lungs 23

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25. Lungs Lungs of mammals are packed with millions of alveoli (sites of gas exchange) Inhaled air passes through the larynx, glottis, and trachea Bifurcates into the right and left bronchi, which enter each lung and further subdivide into bronchioles Alveoli are surrounded by an extensive capillary network 25

26. 26 Lungs

27. Lungs of birds channel air through very tiny air vessels called parabronchi Unidirectional flow Achieved through the action of anterior and posterior sacs (unique to birds) When expanded during inhalation, they take in air When compressed during exhalation, they push air in and through lungs 27

28. Lungs Respiration in birds occurs in two cycles –Cycle 1 = Inhaled air is drawn from the trachea into posterior air sacs, and exhaled into the lungs –Cycle 2 = Air is drawn from the lungs into anterior air sacs, and exhaled through the trachea Blood flow runs 90 o to the air flow –Crosscurrent flow –Not as efficient as countercurrent flow 28

29. 29 Lungs

30. Gas Exchange Gas exchange is driven by differences in partial pressures Blood returning from the systemic circulation, depleted in oxygen, has a partial oxygen pressure (P O 2 ) of about 40 mm Hg By contrast, the P O 2 in the alveoli is about 105 mm Hg The blood leaving the lungs, as a result of this gas exchange, normally contains a P O 2 of about 100 mm 30

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32. Lung Structure and Function Outside of each lung is covered by the visceral pleural membrane Inner wall of the thoracic cavity is lined by the parietal pleural membrane Space between the two membranes is called the pleural cavity –Normally very small and filled with fluid –Causes 2 membranes to adhere –Lungs move with thoracic cavity 32

33. Lung Structure and Function During inhalation, thoracic volume increases through contraction of two muscle sets –Contraction of the external intercostal muscles expands the rib cage –Contraction of the diaphragm expands the volume of thorax and lungs Produces negative pressure which draws air into the lungs 33

34. Lung Structure and Function Thorax and lungs have a degree of elasticity Expansion during inhalation puts these structures under elastic tension Tension is released by the relaxation of the external intercostal muscles and diaphragm This produces unforced exhalation, allowing thorax and lungs to recoil 34

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37. Lung Structure and Function Tidal volume –Volume of air moving in and out of lungs in a person at rest Vital capacity –Maximum amount of air that can be expired after a forceful inspiration Hypoventilation –Insufficient breathing –Blood has abnormally high P CO 2 Hyperventilation –Excessive breathing –Blood has abnormally low P CO 2 37

38. Lung Structure and Function Each breath is initiated by neurons in a respiratory control center in the medulla oblongata Stimulate external intercostal muscles and diaphragm to contract, causing inhalation When neurons stop producing impulses, respiratory muscles relax, and exhalation occurs Muscles of breathing usually controlled automatically –Can be voluntarily overridden – hold your breath 38

39. Lung Structure and Function Neurons are sensitive to blood P CO 2 changes A rise in P CO 2 causes increased production of carbonic acid (H 2 CO 3 ), lowering the blood pH Stimulates chemosensitive neurons in the aortic and carotid bodies Send impulses to respiratory control center to increase rate of breathing Brain also contains central chemoreceptors that are sensitive to changes in the pH of cerebrospinal fluid (CSF) 39

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41. Respiratory Diseases Chronic obstructive pulmonary disease (COPD) –Refers to any disorder that obstructs airflow on a long-term basis –Asthma Allergen triggers the release of histamine, causing intense constriction of the bronchi and sometimes suffocation 41

42. Respiratory Diseases Chronic obstructive pulmonary disease (COPD) (cont.) –Emphysema Alveolar walls break down and the lung exhibits larger but fewer alveoli Lungs become less elastic People with emphysema become exhausted because they expend three to four times the normal amount of energy just to breathe Eighty to 90% of emphysema deaths are caused by cigarette smoking 42

43. Respiratory Diseases Lung cancer accounts for more deaths than any other form of cancer Caused mainly by cigarette smoking Follows or accompanies COPD Lung cancer metastasizes (spreads) so rapidly that it has usually invaded other organs by the time it is diagnosed Chance of recovery from metastasized lung cancer is poor, with only 3% of patients surviving for 5 years after diagnosis 43

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45. Hemoglobin Consists of four polypeptide chains: two  and two  Each chain is associated with a heme group Each heme group has a central iron atom that can bind a molecule of O 2 Hemoglobin loads up with oxygen in the lungs, forming oxyhemoglobin Some molecules lose O 2 as blood passes through capillaries, forming deoxyhemoglobin 45

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47. Hemoglobin At a blood P O 2 of 100 mm Hg, hemoglobin is 97% saturated In a person at rest, the blood that returns to the lungs has a P O 2 about 40 mm Hg less Leaves four-fifths of the oxygen in the blood as a reserve This reserve enables the blood to supply body’s oxygen needs during exertion Oxyhemoglobin dissociation curve is a graphic representation of these changes 47

48. 48 Hemoglobin

49. Hemoglobin’s affinity for O 2 is affected by pH and temperature The pH effect is known as the Bohr shift –Increased CO 2 in blood increases H + –Lower pH reduces hemoglobin’s affinity for O 2 –Results in a shift of oxyhemoglobin dissociation curve to the right –Facilitates oxygen unloading Increasing temperature has a similar effect 49

50. 50 Hemoglobin

51. Transportation of Carbon Dioxide About 8% of the CO 2 in blood is dissolved in plasma 20% of the CO 2 in blood is bound to hemoglobin Remaining 72% diffuses into red blood cells –Enzyme carbonic anhydrase combines CO 2 with H 2 O to form H 2 CO 3 –H 2 CO 3 dissociates into H + and HCO 3 – –H + binds to deoxyhemoglobin –HCO 3 – moves out of the blood and into plasma –One Cl – exchanged for one HCO 3 – – “chloride shift” 51

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53. Transportation of Carbon Dioxide When the blood passes through pulmonary capillaries, these reactions are reversed The result is the production of CO 2 gas, which is exhaled Other dissolved gases are also transported by hemoglobin –Nitric oxide (NO) and carbon monoxide (CO) 53