Lecture 35 - Gas Exchange I

Lecture 35 - Gas Exchange I - Tuesdays Class OXYGEN CARBON...

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Unformatted text preview: Tuesday’s Class OXYGEN & CARBON DIOXIDE PHYSIOLOGY I Chapter 11 pp. 442-­‐452 Chapter 9 pp. 410-­‐422 What you should be able to do by the end of today’s lecture GAS EXCHANGE • Explain how physical and chemical processes influence the design of respiratory systems. • Describe the evolutional diversity in respiratory tissues. • Discuss the fundamental differences in breathing air versus water. Antarctic Icefishes Henry’s Law • Gas molecules in the air must first dissolve in liquid in order to diffuse into a cell • The concentration of gas in a liquid is proportional to its partial pressure & solubility of the gas • Henry’s law • [G] = concentration of the gas [G] = Pgas ž Sgas • Pgas = partial pressure of the gas • Sgas = solubility of the gas equilibrium based on partial pressure not concentration Figure 9.2b Diffusion Rates • Graham’s law – Diffusion rate is proportional to solubility/ÖMW • Combining the Fick equation with Henry’s law and Graham’s law – Diffusion rate of a gas molecule is proportional to • D ´ A ´ DPgas ´ Sgas / X ´ ÖMW The Physics of Respiratory Systems • Diffusion of gases • Fick equation • When applied to gases, we typically think of the pressure gradient of gas, rather than the concentration gradient • To maximize diffusion respiratory surfaces are typically thin, with a large surface area Surface Area to Volume Ratio • As radius increases, volume increases faster than surface area • As organisms grow larger, the ratio of surface area to volume decreases • Larger size limits the surface area available for diffusion and increases the diffusion distance • Only very small organisms can rely solely on the diffusion oxygen to support metabolism – Larger animals must transport oxygen by bulk flow Figure 9.4 Overview: Cardiorespiratory Physiology • Unicellular and small multicellular organisms rely on diffusion for gas exchange • Larger organisms must rely on a combination of bulk flow and diffusion for gas exchange – Bulk flow • Ventilation – Moving medium (air or water) over respiratory surface (lung or gill) • Circulation – Transport of gases in the circulatory system Figure 9.2 Convection/Diffusion: O2 cascade Large differences in PO2 at gas transport boundaries drives diffusion of O2 into tissues from site of acquisition to site of usage Dissolved gas transport over longer distances: convection • Convection = flow that moves “stuff” – A convection oven is like a standard oven except it has a fan to flow heat around the food and cook it faster. – Flow is also used to increase the distance over which gases are exchanged • Flow can be achieved in many ways – Use abiotic environmental features to establish convective currents – Use biotic mechanisms to establish convective currents • Breathing (tidal flow -­‐ in and out) • Circulation (unidirectional flow) • The rate of flow (Q) determined by the difference in pressure (DP) and the resistance to flow (R) • Q = DP/R • Increase gas carrying capacity of fluid by binding it to gas transport proteins (e.g., Hemoglobin in red blood cells) Fluids flow from areas of high to low pressure Respiratory Strategies • Animals more than a few millimeters thick use one of three respiratory strategies – Circulating the external medium through the body • Sponges, cnidarians, and insects – Diffusion of gases across the body surface accompanied by circulatory transport • Cutaneous respiration • Skin must be thin and moist • Most aquatic invertebrates, some amphibians, eggs of birds – Diffusion of gases across a specialized respiratory surface accompanied by circulatory transport • Gills (evaginations) or lungs (invaginations) • Vertebrates Gas Exchange Surfaces • Areas of body where gas exchange occurs – Highly vascularized • Skin: surface of organisms – – – – – • Terrestrial and marine organisms Passive gas exchange Evolutionarily simple Relatively low surface area Desiccation risk Gills (Branchial): Evaginations of tissue – – – – Aquatic organisms (fishes, invertebrates) Active, unidirectional movement of water past gills High surface area Many other functions • • • Skin osmotic regulation in fishes and crustaceans (Na+/K+ ATPase) feeding in bivalve molluscs Lungs (Pulmonary): Invaginations of tissue – – – – Terrestrial organisms (birds, mammals) Aquatic air breathing organisms Active, tidal movement of air into lungs High surface area Titicaca frog Baggy Skin =High SA, highly vascularized Ventilation • Ventilation of respiratory surfaces reduces the formation of static boundary layers • Types of ventilation – Nondirectional • Medium flows past the respiratory surface in an unpredictable pattern – Tidal • Medium moves in and out of the chamber – Unidirectional • Medium enters the chamber at one point and exits at another • The pattern, but not the direction, of ventilation can change with environmental or metabolic conditions Orientation of Medium and Blood Flow Non-­‐directional (a & b) and Tidal Ventilation (c) Figure 9.6a–c Orientation of Medium and Blood Flow With unidirectional ventilation, the blood can flow in three ways relative to the flow of the medium Figure 9.6d–f Ventilation of Water and Air • Because of the different physical properties of air and water, animals use different strategies depending on the medium in which they live • Differences • [Oair] is 30 times greater than [Owater] • 30 times more water than air must be ventilated to get the same amount of oxygen • Water is more dense and viscous than air • It is more difficult to ventilate water • Tidal ventilation is too energetically costly in water • Water breathers rely on unidirectional ventilation • allows for countercurrent exchange Gas exchange through skin Fully aquatic organisms: No desiccation stress, but skin breathing inefficient because of low O2 content of water relative to air Amphibious organisms: Skin breathing in air efficient because of high O2 content of air relative to water and life in moist environment Adult bullfrogs use skin for CO2 exchange and lungs for O2 exchange Seasonal variation: underwater over-­‐ wintering Terrestrial organisms: Skin breathing in air inefficient because of high desiccation stress 1) Cold water (low body temp, low O2 requirements) 2) Low activity (low O2 requirements) 3) Cold water (high O2 content) 4) Fast running water (well oxygenated) 5) Flaps of skin (increase surface area) One of the largest species of salamanders relies mostly on cutaneous respiration Hellbenders (Cryptobranchus alleganiensis) are extremely large, completely aquatic salamanders native to the eastern United States. The largest hellbender ever recorded was nearly 2.5 ft. long Respiratory surface area increases with metabolic demands Flow of water over the gill surface necessary to prevent boundary layer of anoxic water a) The gills themselves can be moved through still water. (e.g. mudpuppy Necturus) b) Can have water passively passed them (ram ventilation), this can occur during swimming or just by sitting in a strong current (e.g. tuna) c) Can be actively ventilated by buccal movement (a metabolically expensive strategy used by most other fish) Gills also serve other functions in fish including acid-­‐base regulation & nitrogenous waste excretion Gas Exchange Organs: Gills Figure 7.10 Willmer et al. 2005 Gill Structure: High Surface Area Gill Function: Countercurrent Exchange water Gill surface blood Figure 11.15 Wednesday’s Class RESPIRATORY PHYSIOLOGY I Chapter 11 pp. 452, 456-­‐458, 462-­‐473 Chapter 9 pp. 422, 426-­‐428, 432-­‐443 ...
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