21-1 Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Chapter 21: Gas exchange

21-1Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Chapter 21: Gas exchange in animals


Air and water

• All animals exchange O2 and CO2 in the process of respiration

• Animals undertake exchange in air or water• Composition of air is stable under normal

conditions• Proportions of dissolved gases in water vary with

depth, salinity and temperature

(cont.)


Air and water (cont.)

• Concentration of gas in medium depends on– partial pressure of gas

proportion of total pressure of gas mixture (e.g. air) provided by nominated gas (e.g. O2 or CO2)

– solubility of gas different gases do not dissolve at the same rate in all media water has greater capacitance for CO2 than for O2


Exchanging gases

• O2 must pass from external environment to mitochondria

• CO2 must pass from mitochondria to external environment

– process of diffusion (passive, requires no energy)

• Dissolved gases diffuse across membranes provided that there is a partial pressure gradient

– example: mitochondria utilise O2, so the partial pressure of O2 inside a mitochondrion (PO2) is zero

(cont.)


Exchanging gases (cont.)• Rate of diffusion across a surface depends on

– difference in partial pressures of gas on either side of membrane

– properties of membrane permeability surface area thickness

• Gases pass most rapidly across large, highly-permeable thin membranes


Surface area and volume

• Surface area is important in gas exchange• Diffusion is only effective in small organisms or

over small distances• As organisms increase in volume, the surface area

does not increase at the same rate– diffusion becomes ineffective– alternative mechanisms for transporting gases


Ventilation

• Passive transfer of gas across a surface depends on difference in partial pressure of gas on either side of that surface

• If medium (air, water) is stagnant, then partial pressure may drop (O2) or increase (CO2)

– reduces difference in partial pressure– decreases diffusion rate

(cont.)


Ventilation (cont.)

• Animals may take advantage of the natural flow (convection) of a medium

• Animals may set up currents to circulate air or water (ventilation)

• Internal convection (perfusion) transports gases through body fluids

• Animals have internal circulatory system to transport gases


Fig. 21.3: Pathways of O2 and CO2


Respiratory systems

• Respiratory systems for gas exchange– may involve ventilation of respiratory surfaces

• Circulatory system for gas transport– distributes O2 to cells and removes CO2 from cells

• Dissolved gases pass across membranes by diffusion


Water breathers

• Low O2 content of water means that animals require constant movement of water across respiratory surfaces

• Boundary layer of water around an aquatic organism forms a layer that is quickly depleted of O2

– animals must disturb or dispel the boundary layer

• Example: sponges circulate water using flagellated cells (choanocytes)


Cutaneous exchange

• Gas exchange across the body surface• Ventilation is achieved by moving the surface

through the water• Area for gas exchange may be reduced if body

surface has protective covering– development of specialised respiratory structures– cutaneous respiration becomes secondary method of gas

exchange


Gas-exchange structures

• Specialised structures for gas exchange vary from tiny outgrowths on body wall to elaborate gills

• Despite differences in form, all structures have a large surface area and are thin to maximise diffusion of gas

• Gills are usually ventilated by muscular action


Fish gills

• Fish gills are large and elaborate structures• Gills supported by gill arches

– filaments projecting from gill arches are folded into lamellae

– increases surface area

• Gills are ventilated by forcing water from the buccal cavity into the operculum cavity

• Many fast-swimming fish use ram ventilation


Countercurrent mechanisms

• Countercurrent flow increases efficiency of gas exchange

• Water passes over gill in opposite direction to blood (or haemolymph) flow within gill

• Maximises difference in partial pressure of O2 in water and blood

– blood entering gill has low PO2

– water leaving gill also has low PO2, but there is a gradient,

so O2 passes across


Fig. 21.8a: Countercurrent flow


Fig. 21.8b: Co-current flow


Transition to land

• Gills are ineffective in air – they are likely to collapse under their own weight– surface tension causes surfaces to adhere– reduce area for gas exchange

• Intertidal and semi-terrestrial invertebrates have strengthened gills to prevent collapse

– reduced surface area is offset by higher O2 concentration of air

– gills enclosed to reduce water loss through evaporation


Air-breathing fish

• Many fish gulp air from the surface• Use a buccal-force pump to push air down into

gas-bladder– outgrowth from alimentary tract

• Some fish use internal surface of gut for gas exchange


Lungs

• Specialised for respiration in air– internal, paired sac-like structures – in most vertebrates, air moved over surface in tidal

ventilation

• Amphibians– ventilate lungs using buccal-force pump– also use cutaneous respiration

• Reptiles– ventilate lungs using aspirating pump


Mammal lungs

• Gas exchange takes place in alveoli– alveolar walls are thin and highly vascularised– coated with phospholipid surfactant, which prevents

collapse of alveoli

• Air in dead space between the trachea and alveolar ducts is not involved in gas exchange

• Lungs are ventilated when muscular diaphragm contracts to increase space around lung

– creates negative pressure allowing air to enter lungs


Bird lungs

• Air flow in birds is unidirectional not tidal • Parabronchi (gas exchange structures) are

associated with air sacs– air sacs have no respiratory function but act as bellows

• Air enters through parabronchi and passes into vascularised air capillaries

• Air is shunted through the air sacs and parabronchi during inspiration and expiration


Fig. 21.20a: Flow of air through bird lung


Fig. 21.20b: Flow of air through bird lung


Tracheae

• Insects possess tracheae that carry gases to and from tissues

• Air enters through spiracles on the side of an insect’s thorax and abdomen

• Tracheae branch into tracheoles– tracheole have chitinous walls to prevent collapse

– deliver O2 to with a few μm of cells


Transporting oxygen

• Respiratory pigments increase amount of O2 carried in a fluid

– examples: haemoglobin, haemocyanin

• In many vertebrates, haemoglobin is found in red blood cells (erythrocytes)

– presence of respiratory pigment in cells prevents osmotic problems


Oxygen-carrying capacity

• Respiratory pigments reversibly bind to O2

• Amount of O2 bound to pigment depends on partial pressure

• Oxygen equilibrium curve– also known as oxygen dissociation curve

– relationship between PO2 and total O2 content


Fig. 21.25: Oxygen equilibrium curve


Oxygen affinity

• Different pigments have different affinities for O2

– pigments with high affinity for O2 become saturated at low partial pressures

• Example: fetal haemoglobin has a higher affinity for O2 than maternal haemoglobin

– fetal haemoglobin has to bind O2 at a lower partial pressure because maternal tissues have already depleted some of the available O2

(cont.)


Oxygen affinity (cont.)

• Bohr effect– O2 affinity of haemoglobin decreases with pH

– binds O2 in regions of high pH (lungs or gills)

– releases O2 in regions of low pH (areas high in CO2)

• Root effect– O2 affinity of haemoglobin decreases if CO2 is also bound

to the pigment molecule


Transport of carbon dioxide

• CO2 is carried in solution in plasma or combined with haemoglobin in erythrocytes

• CO2 hydrated to form carbonic acid (H2CO3)– dissociates to bicarbonate (HCO3

–) and hydrogen (H+) ions

– reaction rate is increased by carbonic anhydrase

• HCO3– diffuses from erythrocytes and is replaced

by Cl–

– at lungs, Cl– is replaced by HCO3–

– chloride shift


Control of ventilation

• Convection requirement of water-breathing animals is substantially higher than that of air-breathing animals

– reflects low availability of O2 in water

– ventilation rate depends on O2 concentration

• Air-breathing animals face problem of higher level of internal CO2

– smaller air volume required in respiration means that CO2 may not be removed at sufficient rate

– ventilation rate depends on CO2 concentration

(cont.)


Control of ventilation (cont.)

• Chemoreceptors detect changes in blood chemistry

• Chemoreceptive tissue in medulla– Responds to changes in CO2, pH

• Carotid bodies in carotid arteries contain glomus cells

– Respond to changes in O2 (also CO2 and pH)

• Result in increased ventilation

Documents

21-1 Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Chapter 21: Gas exchange