Chapter 48 – Gas Exchanges in Animals
48.1 What Physical Factors Govern Respiratory Gas Exchange? (1025)
- CO2 and O2 are the respiratory gases that animals must exchange.
- Diffusion (random movement of molecules or other particles, resulting in even distribution of the particles when no
barriers are present) is the only means of gas exchange between the internal body fluids of an animal and the outside
medium (water or air). Diffusion is a physical process. Diffusion is faster in higher temperatures and faster in air than
Diffusion is Driven by Concentration Differences (1025)
- Net movement of molecules via diffusion is always down its concentration gradient.
- Partial pressure of the gases is one way biologists express the concentrations of different gases in a mixture.
- Solubility of a gas in liquid is a factor that makes it more difficult to describe of respiratory gases in a liquid such as
- Actual amount of a gas in a liquid depends on the partial pressure of that gas in the gas phase in contact with the liquid
as well as on the solubility of that gas in that liquid.
Fick’s Law Applies to All Systems of Gas Exchange (1026)
- Fick’s law of diffusion describes diffusion quantitatively with an equation (all environmental variables that limit
respiratory gas exchange and all adaptations that maximize respiratory has exchange are included):
Q = DA-
- Q is the rate at which a gas such as O2 diffuses between two locations.
- D is the diffusion coefficient (i.e. perfume has a higher D than motor oil vapour).
- A is the cross-sectional area through which the gas is diffusion.
- P1 and P2 are the partial pressures of the gas at the two locations.
- L is the path length, or distance, between the two locations.
- (P1-P2)/L is a partial pressure gradient.
Air is a Better Respiratory Medium than Water (1026)
- Oxygen can be obtained easier from air than from water because:
- O2 content of air is much higher than in water.
- O2 diffuses about 8000 times more rapidly in air than in water.
- More energy is done to move water than to move air, because water is 800x more dense.
- Eukaryotic cells carry out cellular respiration in the mitochondrion which is in the cytoplasm – an aqueous medium – as
well as they are bathed in extracellular fluids which is also an aqueous medium.
- Animals in liquid mediums (i.e. fish) have gills which are very efficient in gas exchange – provide large surface area for
High Temperatures Create Respiratory Problems for Aquatic Animals (1026)
- Because most water breathers are ectotherms, their body temperature and metabolic rate increases as the environment’s
temperature increases; they need more O2 as the water gets warmer and warm water hold less dissolved gas than cold
O2 Availability Decreases with Altitude (1026)
- Rise in altitude reduces O2 availability. The P02 (partial pressure of oxygen) decreases as well, and since diffusion (gas
exchange) relies on this, gas exchange is less efficient and O2 uptake is constrained.
C O2 is Lost by Diffusion (1027)
- CO2 diffuses out of the body as O2 diffuses in. Direction and rate of diffusion across the respiratory exchange surfaces
depend on the partial pressure gradients of the gases.
- Partial pressure of CO2 does not change with altitude.
- Getting rid of CO2 is not a problem for water breathers because if is more soluble in water than O2.
48.2 What Adaptations Maximize Gas Exchange? (1028)
Respiratory Organs Have Large Surface Areas (1028)
- Larger surface areas = greater rate of gas exchange/diffusion.
- External gills provide a large surface area for gas exchange with water. They minimize the path length (L) (i.e. larval
- Internal gills are similar to external gills but have protective body cavities (i.e. fish).
- Lungs are the internal cavities for respiratory has exchange with air. They have a large surface area because they are
highly divided, and they are elastic so that they can be inflated with air and deflated.
- Most abundant air-breathing invertebrate are insects which have a gas exchange system consisting of a network of air-
filled tubes called tracheae that branch through all tissues of the insect’s body.
*Look at Lecture notes for ventilation and perfusion*
- Minimizing the path length, higher surface area, and low volume is good for gas exchange/diffusion.
- An animal’s gas exchange system is made up of its gas exchange surfaces and the mechanisms it uses to ventilate and
perfuse those surfaces.
Insects Have Airways Throughout their Bodies (1028)
- Respiratory gases diffuse through air most of the way to and from every cell in insects.
- Spiracles are gated opening in which the insect repiratiory system communicates with the outside environment. Spiracles
can open to allow gas exchange and close to decrease water loss.
- Spiracles extend and become smaller and smaller – from spiracles to tracheae, tracheoles, and then air capillaries.
Fish Gills Use Countercurrent Flow to Maximize Gas Exchange (1029)
- In fish, water flow unidirectionally into the fish’s mouth, over the gills, and out forom under the opercular flaps. The
constant, one-way flow of water moving over the gills maximizes P02 on the external gills surfaces. Gills have a large
surface area for gas exchange because they are so highly divided.
- The lamellae which cover the gill filaments are the actual gas exchange surfaces, and they minimize L.
- Flow of blood perfusing the inner surfaces of the lamellae is unidirectional as well.
- Affarent blood vessels bring blood to the gills, and efferent blood vessels do the opposite.
- The blood flow of the lamellae is opposite to the flow of water over the gills – countercurrent flow – optimizes PO2
gradient between water and blood.
Birds Use Unidirectional Ventilation to maximize Gas Exchange (1030)
- Air flows through the lungs in the parabronchi and diffuse into the air capillaries, which are the gas exchange surfaces –
provide a large surface area because they are so numerous. Birds take two breaths instead of one and their non-ventilated
volume (dead space) is less; they do not as much extra air left after breathing unlike mammals.
Tidal Ventilation Produces Dead Space that Limits Gas Exchange Efficiency (1031)
- Lungs’ structures have evolved, but still remain dead-end sacs in all air-breathing vertebrates except birds. Ventilation
cannot be constant and unidirectional because lungs are dead-end sacs, but must be tidal: air flows in and exhaled gases
flow out by the same route.
- The residual air in the lungs after exhalation represents dead space.
- Spirometer is a device that measures the volumes of air that a person breathes in or breathes out.
- Tidal volume: Amount of air that moves in and out per breath when we are at rest.
- Inspiratory Reserve Volume: the additional volume of air we can take in above normal tidal volume.
- Expiratory Reserve Volume: The extra air that we can forcefully breathe out after normal exhalation.
- Vital Capacity: Tidal volume + inspiratory reserve volume + expiratory reserve volume. Vital capacity decreases with
age and is greater in an athlete than in a non-athlete.
- Even after the deepest exhalation, there is still some air in the dead space; vital capacity is not the entire lung volume.
The total lung capacity is the residual volume + vital capacity.
- Tidal breathing limits the partial pressure gradient available to drive the diffusion of oxygen from air into the blood.
- Fresh air is not moving into the lungs during part of the breathing cycle; therefore, the average PO2 of air in the lungs is
considerably less than it is outside the lungs.