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Gas Exchange in Animals

49. Gas Exchange in Animals. Gas exchange systems are made up of surfaces and the mechanisms that ventilate and perfuse those surfaces Ventilation is ambient fluid (water or air) flowing past exterior of exchange surface

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Gas Exchange in Animals

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  1. 49 Gas Exchange in Animals

  2. Gas exchange systems are made up of surfaces and the mechanisms that ventilate and perfuse those surfaces • Ventilation is ambient fluid (water or air) flowing past exterior of exchange surface • Perfusion is circulatory fluid (blood, hemolymph) flowing inside exchange surface

  3. O2 and CO2 are respiratory gasesexchanged by diffusion along their concentration gradients • Partial pressure is concentration of a gas in a mixture • Barometric pressure – atmospheric pressure at sea level is 760 mm Hg • Partial pressure of O2 (PO2) is159 mm Hg or 20.9% of atmosphere www.allstar.fiu.edu/aero/images

  4. Oxygen is easier to obtain from air than from water: • O2 content of air is higher than water • O2 diffuses much faster through air • Air and water must be moved by animal over its gas exchange surfaces to obtain O2 – itrequires more energy to move denser/more viscous water across membranes than air

  5. O2 diffusion is slow • even cells with low metabolism must be only 1–2 mm from O2 source • limits size and shape of species without internal systems for gas exchange • these species have evolved • larger surface areas • central cavities, or • specialized respiratory systems

  6. An aquatic animal’s body temperature and metabolic rate rise with an increase in water temperature • Dilemma: need for O2 increases while available O2 decreases in warmer water • Cold water holds more oxygen Increase in altitude reduces available O2 for air breathers due to lower partial pressure of O2 at high altitudes www.as.ua.edu/ant/bindon/ant475/altitude

  7. Respiratory systems have adaptations to maximize exchange of O2 and CO2 • Increase surface area • Example: Gill filaments, aveoli in lungs • Maximize partial pressure gradients • Example: Blood flow – always low oxygen levels in blood returning to lungs • Minimize diffusion path length • Example: Capillaries surrounding aveoli in lungs create short diffusion length for the oxygen • Minimize diffusion taking place in aqueous medium http://www.biologyreference.com/images/biol_02_img0187.jpg http://www.sbs.auckland.ac.nz/services/micrographics/images/Gills.jpg

  8. Adaptations - Insects Insects have a tracheal system: • Spiracles in abdomen open to allow gas exchange and close to limit water loss • Spiracles open into tracheae, that branch to tracheoles, that end in air capillaries • Limits insect size

  9. Adaptations - Fish Fish gills use countercurrent flow to maximize gas exchange • Water flows unidirectionally into mouth, over gills, and out from under opercular flaps • Constant water flow maximizes PO2 on external gill surfaces and blood circulation minimizes PO2 on internal surfaces  maximizing O2 diffusion Basking shark Goliath grouper www.flmnh.ufl.edu/fish/education/questions www.florida-scuba.com/images/special/

  10. Adaptations - fish Countercurrent flow optimizes PO2 gradient • Afferent blood vessels bring blood to gills and efferent vessels take blood away • Blood flows through lamellae in direction opposite to water flow

  11. Adaptations - Birds • Birds – can sustain high levels of activity (migration) at very high altitudes Differences from mammals: Unidirectional Bird lungs are compressed during inhalation and expand during exhalation Bird lungs have unidirectional air flow (as opposed to bidirectional in mammals) to maintain high PO2 gradient  eliminates dead space • air sacs receive inhaled air – not sites of gas exchange • Air enters through trachea, divides into bronchi, then into parabronchi, and then into air capillaries

  12. Adaptations - Birds

  13. Adaptations - Mammals Gas exchange in mammals • Ventilation in lungs is tidal – air flows in and out along same path Tidal volume (TV) • amount of air that moves in and out per breath at rest • measured by a spirometer

  14. Adaptations - Mammals • Inspiratory (IRV) – breathe in as much as possible • expiratory reserve (ERV) – forcefully exhale • Vital capacity (VC) = tidal volume + inspiratory reserve volume + expiratory reserve volume • Residual volume (RV) - is air that cannot be expelled from lungs • Total lung capacity - is sum of vital capacity and residual volume

  15. Adaptations – Mammals • Two features offset inefficiency of tidal breathing in mammals: • Enormous surface area • Very short path length for diffusion

  16. Adaptations - Mammals Air pathway in humans • Air enters human lung through oral cavity or nasal passage  pharynx (area for both food and air) • Below pharynx, the trachea leads to the lungs • At beginning of larynx is larynx, or voicebox • Trachea branches into two bronchi (both have cartilagenous rings for support) • bronchi branch repeatedly into bronchioles • bronchioles terminate in alveoli

  17. Capillaries surround and lie between alveoli • Diffusion path between blood and air is less than 2 mm • Less than diameter of a red blood cell

  18. Adaptations - Mammals Mammalian lungs produce two secretions that affect ventilation: mucus and surfactant • Mucus lines airways and captures dirt and microorganisms • Mucus escalator is group of cells with cilia that sweep mucus and particles out of airways • Surfactant reduces surface tension of liquid • fluid covering alveoli has surface tension that makes lungs elastic • Lung surfactant is released by alveolar cells when stretched – less force is needed to inflate lungs http://themodulator.org/archives/Trachea-thumb.jpg

  19. Adaptations - Mammals • Premature babies may not have developed the ability to make lung surfactant. • Without it, they have great difficulty breathing, known as respiratory distress syndrome • Treatments include respirators, hormones to speed lung development, and aerosol surfactants.

  20. Adaptations - Mammals Human lungs are suspended inside right and leftthoracic cavity • Diaphragm is a sheet of muscle at bottom of these cavities • Pleural membrane lines each cavity and covers each lung • No real space but there is thin film of fluid allowing movement during breathing • The pleural space contains fluid to help membranes slide past each other during breathing • Negative pressure is created in pleural space • Slight negative pressure present in between breaths keeps alveoli inflated • Injury that effects this pressure will result in difficulty inflating lung – “collapsed lung” www.tcnj.edu/~mckinney

  21. Inhalation begins when diaphragm contracts • Diaphragm pulls down on thoracic cavity and on pleural membranes • Pleural membranes pull on lungs, air enters through trachea, and lungs expand • Exhalation begins when diaphragm relaxes • Intercostal muscles, located between ribs, can also change volume of thoracic cavity • External intercostal muscles lift ribs up and outward, expanding cavity • Internal intercostal muscles decrease volume by pulling ribs down and inward

  22. Transport of gases in blood Hemoglobin is a protein with four polypeptide subunits • Each polypeptide surrounds heme group that contains iron (Fe) that can bind one O2 molecule • One molecule of hemoglobin can bind up to four molecules of O2 http://fig.cox.miami.edu/~cmallery/150/chemistry

  23. Transport of gases in blood Hemoglobin will pick up or release O2 depending on PO2 of environment • If PO2 of plasma is high (lungs), hemoglobin will pick up its maximum of four O2 molecules • As blood circulates through tissues with lower PO2, hemoglobin will release some O2

  24. Transport of gases in blood • Carbon monoxide (CO) • Binds to Hb with much higher affinity than does O2 • Deadly poison  reduces ability of Hb to transport O2 http://yalenewhavenhealth.org/library/healthguide/en-us/images/media/medical/hw http://www.thrombosis-consult.com/articles/images/

  25. Transport of gases in blood Myoglobin is single polypeptide molecule in muscles and can bind one molecule of O2 • has higher affinity for O2, binds it at lower PO2 values when hemoglobin molecules would release their O2 • provides reserve for high metabolic demand for O2 • Very high in diving mammals www.brooklyn.cuny.edu/bc/ahp/SDPS/graphics/

  26. Transport of gases in blood – affinity of Hemoglobin for oxygen Bohr effect – pH effect on hemoglobin affinity • When blood pH falls, H+ ions bind to Hb decreases its affinity for O2 releases O2 • Blood pH decreases as it picks up CO2, lactic acid, fatty acids at tissues • O2-binding curve shifts to right •  Hb releases more O2 to tissues where pH is low http://members.aol.com/Bio50/LecNotes/LNPics

  27. Transport of gases in blood CO2 in the blood • Very little CO2 is transported by blood, most of it is transported to lungs in form of bicarbonate ions • CO2 is highly soluble, moves easily thru cell membranes into blood, where PCO2 is lower • CO2 reacts with H2O to form carbonic acid CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3– • Most (~68%) CO2 is transported as bicarbonate ion • ~22% transported bound to Hb • ~10% transported dissolved in blood

  28. Transport of gases in blood

  29. Brain stem generates breathing rhythm via autonomic nervous system • Neurons within medulla increase firing rate just prior to inhalation • If spinal cord is cut below pons, breathing continues but is irratic • Medulla is still able to get signals to lungs • If spinal cord is cut below medulla, breathing stops

  30. In humans and mammals, breathing rate is more sensitive to changes in PCO2 than to PO2 • PCO2 of blood is primary metabolic feedback for breathing • The major site of sensitivity to PCO2 is on medulla’s ventral surface • CO2 in blood lowers pH, actually more sensitive to pH • Sensitivity to PO2 is monitored by carotid and aortic bodies in blood vessels leaving heart • If PO2 falls, chemo-receptors in these bodies send nerve impulses to brain stem to stimulate breathing • This works at high altitudes or when blood pressure is low CO2 O2

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