Gas Exchange in Animals

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

13. Adaptations - Birds

14. 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

15. 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

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

17. 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

18. 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

19. 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

20. 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.

21. 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

22. 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

23. 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

24. 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

25. 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/

26. 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/

27. 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

28. 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

29. Transport of gases in blood

30. 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

31. 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