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Respiratory Systems Overview Respiration – sequence of events that result in the exchange of oxygen and carbon dioxide between the external environment and the mitochondria Mitochondrial (cellular) respiration – production of ATP via oxidation of fuels; oxygen is consumed and carbon dioxide is produced To meet metabolic needs animals must obtain O2 and dispose of CO2 External respiration – gas exchange at the respiratory surface (atmosphere and blood) Internal respiration – gas exchange at the tissue (blood and tissue) Gas molecules move down gradients Overview 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 The Physics of Respiratory Systems Fick Equation dQ/dt = D x A x dC dx dQ/dt = Rate of diffusion D = diffusion coefficient (D) A = area of the membrane (A) dC = energy gradient Difference in pressure (not concentration) dx = Diffusion distance Rate of diffusion will be greatest when the diffusion coefficient (D), area of the membrane (A), and energy gradients (dC) are large, but the diffusion distance (dx) is small Consequently, gas exchange surfaces are typically thin, with a large surface area Dalton’s Law of Partial Pressure The pressure exerted by a gas is related to the number of moles of the gas and the volume of the chamber Ideal gas law PV = nRT Air is a mixture of gases Nitrogen (78%), oxygen (21%), argon (0.9%), and carbon dioxide (0.03%) In a gas mixture each gas exerts its own partial pressure The sum of all partial pressures is equal to the total pressure of the mixture n/V = P/RT ? temp, ? volume, ? [gas], while pressure is constant Henry’s Law partial pressures are equal, [ ] is higher in air due to solubility so animals using water as respiratory medium must move 30X more medium Solubility also ? as temperature ?, and as [ions] ? Gas molecules in air must first dissolve in liquid to diffuse into a cell The concentration of gas in a liquid is proportional to its partial pressure Henry’s law: [G] = Pgas ? Sgas where, [G] = concentration of the gas, Pgas = partial pressure of the gas, Sgas = solubility of the gas Graham’s law Diffusion rate is proportional to solubility/ MW Combining the Fick equation with Henry’s law and Graham’s law Diffusion rate of a gas molecule is proportional to: D x A x ?Pgas x Sgas / X x MW At a constant temp, rate of diffusion is proportional to: Partial pressure gradient (?pgas), Cross-sectional area (A), and Solubility of the gas in the fluid (Sgas) And Inversely proportional to: Diffusion distance (X) and Molecular weight of the gas (MW) Diffusion Rates Compare O2 and CO2 Bulk Flow of Gases Fluids flow from areas of high pressure to areas of low pressure Boyle’s law P1V1 = P2V2 P1V1 = initial pressure and volume of the gas P2V2 = final pressure and volume of the gas For example, if you increase the volume of a chamber of gas, the pressure of the gas will decrease Temperature is constant The rate of flow (Q) determined by the difference in pressure (?P) and the resistance to flow (R) Q = ?P/R Figure 9.3 Bulk Flow of Gases 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 Max metabolic rate per gram of tissue must ? BUT Diffusion rate can be ? by movement of medium 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) 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 rate or pattern, but not the direction, of ventilation can change with environmental or metabolic conditions Orientation of Medium and Blood Flow Gases enter the blood at the respiratory surface Movement of blood through the respiratory surface can affect efficiency of gas exchange Comparison of Po2 in medium and blood as they enter and leave the respiratory surface Figure 9.6d–f Orientation of Medium and Blood Flow With unidirectional ventilation, the blood can flow in three ways relative to the flow of the medium Velocity of flows is particularly important 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 30X 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 Ventilation strategies Unidirectional Most water breathers Allows for countercurrent exchange Tidal Air-breathers Air flows easily; it would require too much work for tidal ventilation of water Air-filled tubes Insects High diffusion rates of gases in air Figure 9.7 Sponges and Cnidarians Circulate external medium through an internal cavity Sponges Flagella move water in through ostia and out through the osculum Cnidarians Muscle contractions move water in and out through the mouth Gases diffuse directly in and out of cells Molluscs Two strategies for ventilating gills (ctenidia) Snails and clams Cilia on gills move water across the gills unidirectionally Flow is countercurrent Cephalopods Muscular contractions of mantle propel water unidirectionally past the gills in the mantle cavity Flow is countercurrent Figure 9.9 Crustaceans Filter feeding (barnacles) or small species (copepods) lack gills and rely on diffusion Shrimp, crabs, and lobsters have gills derived from modified appendages within a branchial cavity Movements of gill bailer propels water out of branchial chamber; negative pressure sucks water across gills Figure 9.11a Jawless Fishes Lamprey and hagfish have multiple pairs of gill sacs Hagfish Muscular pump (velum) propels water through respiratory cavity Water enters the mouth and leaves through the gill opening Flow is unidirectional Blood flow is countercurrent Figure 9.11b Jawless Fishes – cont’d Lamprey When not feeding, ventilation is similar to hagfish When feeding, the mouth is attached to a prey Ventilation is tidal through gill openings Figure 9.12 Elasmobranchs Steps in ventilation Expand buccal cavity Increased volume sucks water into buccal cavity via mouth and spiracles Mouth and spiracles close Muscles around the buccal cavity contract, forcing water past gills and out the gill slits Unidirectional and pulsatile Blood flow is countercurrent Teleost Fishes Gills are located in the opercular cavity protected by the flaplike operculum Steps in ventilation With the mouth open and the opercular valve closed, the buccal and opercular cavities expand Pressure decreases and sucks water in through mouth Mouth closes Floor of buccal cavity raises and operculum expands Skeletal muscle pumps Pressure pushes water into opercular cavity Opercular valve opens and water leaves through the opercular slit Active fish can also use ram ventilation Swimming with mouth and opercular valve open Figure 9.13 Teleost Fishes Think Boyle’s Law: P1V1 = P2V2 Figure 9.14 Countercurrent Flow in Fish Gills Fish gills are arranged for countercurrent flow Complex with large SA Very efficient when flow of medium and blood are matched Ventilation and Gas Exchange in Air Two major animal lineages have colonized terrestrial habitats Vertebrates Amphibians Reptiles Birds Mammals Arthropods Crustaceans Chelicerates Insects Figure 9.15 Chelicerates (Spiders and Scorpions) Have four book lungs 10–100 lamellae project into air-filled cavity Cavity opens to outside via spiracle Gases diffuse in and out Some spiders also have a tracheal system Series of air-filled tubes Figure 9.16 Insects Have an extensive tracheal system Air-filled tubes called tracheae Open to outside via spiracle Tracheae branch to form tracheoles Ends of tracheoles are filled with hemolymph Cells seldom < 100 ?m from tracheole Gases diffuse in and out Moist and high SA Insect Ventilation Mechanisms Contraction of abdominal muscles or movements of the thorax Can be tidal or unidirectional (enter anterior spiracles and exit abdominal spiracles) Ram ventilation (draft ventilation) in some flying insects Expansion and contraction of tracheae Discontinuous gas exchange Phase 1 (closed phase): no gas exchange with environment; O2 used and CO2 converted to HCO3-; ? in total P Phase 2 (flutter phase): air is pulled in Phase 3: total P ? as CO2 can no longer be stored as HCO3-; spiracles open and CO2 is released Insects – cont’d Do Insects breathe by lung ventilation like humans? X-ray synchrotron at Argonne Retrieved from http://www.aps.anl.gov/News/APS_News/2003/20030127a.htm, 06/03/06 Fish Air breathing has evolved multiple times in fishes Types of respiratory structures – most highly vascularized Reinforced gills that do not collapse in air Mouth or pharyngeal cavity Vascularized stomach Specialized pockets of the gut Lungs Ventilation is tidal (unidirectional in water breathing fish) using buccal force similar to other fish Essentially swallow air! Figure 9.22a,b Birds Lungs are stiff and change little in volume Lungs are between a series of air sacs that act as bellows Posterior and anterior air sacs Gas exchange occurs as air flows through parabronchi in lungs Air flow through parabronchi is unidirectional Richly vascularized Large SA, thin walls Blood flow is crosscurrent Figure 9.23 Bird Ventilation Requires two cycles of inhalation and exhalation air flow is unidirectional PO2 of blood leaving lung is higher than PO2 of exhaled air Mammals Two main parts to respiratory system Upper respiratory tract Mouth, nasal cavity, pharynx, trachea Lower respiratory tract Bronchi and gas exchange surfaces (alveoli) Alveoli are the site of gas exchange Thin wall of type I alveolar cells Type II surfactant cells secrete fluid Outer surface of alveoli are covered in capillaries 80-90 % of surface Figure 9.25 Pleural Sac Each lung is surrounded by a pleural sac Two membranes with small space between them Pleural cavity Has cohesive forces Pleural cavity contains a small volume of pleural fluid Transpulmonary pressure = Palv – Pip Intrapleural pressure is subatmospheric Keeps lung expanded Mammalian Tidal Ventilation Steps: Inhalation Somatic motor neuron firing stimulates inspiratory muscles Contraction of the external intercostals and the diaphragm Ribs move outwards and the diaphragm moves down Volume of thorax ?, intrathoracic pressure ?, Lungs expand, Air is pulled in Exhalation Nerve stimulation of inspiratory muscles stops Muscle relax, Ribs and diaphragm return to their original positions Volume of the thorax ?, intrathoracic pressure ? Passive recoil of the lungs pushes air out During rapid and heavy breathing, exhalation is active via contraction of the internal intercostal muscles Energy required will depend on elastic properties and resistance Compliance – how easy to stretch Elastance – how readily returns to original shape; if low, active Emphysema – elastin destroyed Figure 9.26 Mammalian Ventilation Work required for ventilation depends on: Lung compliance (?V/?P) How easily the lungs stretch during inhalation Surface tension in alveolar fluid lowers compliance Surfactants – reduce surface tension by disrupting the cohesive forces between water molecules lining alveoli Surface tension is the primary force resisting inflation Results in an increase in lung compliance and a decrease in the force needed to inflate the lungs Stretch of type 2 cells stimulates surfactant secretion In humans, surfactant synthesis does not begin until late gestation Resistance Recall Poiseuille’s equation As diameter ?, resistance ? Higher resistance requires a large transpulmonary pressure gradient Parasympathetic nerve stimulation causes bronchoconstriction Sympathetic nerve stimulation causes bronchodilation
