- 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.