1. LIMITS TO AEROBIC METABOLISM (Factorial Scope @ 10 X)

• Possible Causes?

1. Ventilation — limit to how fast ambient air/water can be supplied
2. Diffusion — from respiratory surface ® blood, blood ® tissues
3. Oxygen Transport — blood oxygen affinity and carrying capacity, blood flow rates
4. Oxidative Metabolism — limit to how fast aerobic ATP production can operate

• Which system is THE limiting step is not known with great certainty (although it doesn’t seem to be lung diffusing capacity for mammals). It is likely that rates at any one step in the pathway do not markedly exceed others. This concept is known as Symmorphosis.

1. DIFFUSION

• Rate of diffusion is determined by Fick’s Law:

• From Fick’s Law, we see that gas (O₂ or CO₂) diffusion is dependent on:

1. Total surface area of respiratory structures (increased S.A. ® increased diffusion)
2. Permeability of the surface (D_x [a constant], thickness)
3. Gradient of O₂ or CO₂

1. RESPIRATION IN WATER AND AIR

1. Water and air present two very different sets of problems and limitations.

• Life originated in the water.
• Large-scale (high % of organisms within a particular taxon) adaptations for air breathing have occurred only in Arthropods (includes insects) and Vertebrates. This is due to the difficulty of dealing with the problem of dehydration.

1. Comparison of Water and Air Breathing

1. Partial Pressure = pressure exerted by an individual gas in a gas mixture. PP of gas in water = PP of gas in air with which water is in equilibrium.
2. Amount of gas dissolved in water depends on:

1. Solubility
2. PP in gas phase
3. Temperature (increased temperature decreases solubility)
4. Presence of other solutes (SW < FW)

1. Oxygen Absorbency Coefficient (OAC) = measure of O₂ concentration in water, expressed as volume % (ml O₂/100 ml water) after equilibration with a gas sample where pb = 760 mm Hg.

• How much O₂ available to a water-breather at 15° C?

1. X 21% (O₂ in air) = 0.7 volume %

• How much O₂ available to an air-breather at 15° C?

• So, about 30-times more oxygen is present in air than in water (at 15° C)
• If calculate with O₂ at lung surface (about 13 volume %), air has about 19-times more O₂ available than water.
• Bottom Line = Oxygen availability is much greater for air-breathers than for water-breathers.

1. Viscosity and Density — water is much denser and more resistant to flow than air, so much more energy is necessary to mover water over the respiratory surfaces.

• This is why lungs don’t work well in water ® we can’t move water over the respiratory surfaces fast enough.
• Coupling the decreased availability of O₂ in water with its increased viscosity and density, we see that much more energy is required for water breathing than for air breathing.
• Water-breathers use about 20% of RMR for ventilation
• Air-breathers use about 1-2% of RMR for ventilation

1. Diffusion is much faster in air than in water (by about 8000-times), so diffusion requires much less time for equilibration in air than in water.

• This means that air-breathers experience few problems with depleting the O₂ supply, but this can be a problem for water-breathers (that is why they have mechanisms to achieve water flow over the gills).

1. VENTILATION

• 4 Mechanisms:

1. Skin = most primitive; simple diffusion from environment (no circulatory system) suffices for small invertebrates.
2. Gills = larger aquatic invertebrates and aquatic vertebrates (fish and aquatic amphibians).
3. Lungs = used by vertebrates (including some fish) and some terrestrial invertebrates
4. Tracheae = found in insects.

• Consist of small openings on the body that lead to a system of tubes that branch and lead to all parts of the body. (Overhead — Willmer et al., p. 162)
• Allow direct delivery of O₂ to the tissues, which is very efficient and allows the extremely high factorial scopes (100-200 X) found in some insects.

• Measurement of Ventilation:
• Minute Volume = breaths/min X volume medium/breath = volume medium/min
• Breaths/min is the respiratory frequency or breathing rate
• Volume/breath is the tidal volume
• For Humans: Minute Volume at rest @ 6 liters/min, during activity @ 100 liters/min
• Variation exists among animals in how minute volume increases during activity. Breathing Rate and/or Tidal Volume may increase. Birds and mammals increase both, reptiles tend to increase only tidal volume.
• Usually, there is a conservative relationship such that Minute Volume is tightly correlated with metabolic rate in Tetrapods. This is not so for fish (MR doesn’t increase 1:1 with Minute Volume), probably due to the resistance to flow of water.

1. GILLS = involved in aquatic respiration

• Must provide a flow of water over the gill surface, otherwise water at the gill surface would be rapidly depleted of O₂ because diffusion in water is relatively slow.
• This can be accomplished in two ways:

1. Moving the gill through the water — used by small aquatic invertebrates (e.g., aquatic insect larvae) and amphibians with external gills (when in still water). Limiting factor = resistance to movement of gill through water.
2. Moving water over the gills — much more common method of ventilating gills. This can be accomplished by living in moving water (e.g., streams) of by one of three methods in still water.

1. Ciliary Flow = cilia create water currents. Used in mollusc gills.
2. Pump-like Action = pump water over gills. Used by fish and crabs.
3. Ram Ventilation = swimming thorough water with mouth open. Used by tunas, mackerels, sharks, and paddlefish.

• Gill Surface Area is associated with the animal’s way of life. For example, active fishes maintain a higher gill surface area than sluggish fishes. (Overhead — Willmer et al., p. 270)
• For adequate gas exchange in water (recall low O₂ availability relative to air) a high rate of water flow and close contact between water and gill surface are necessary.

1. Gills lie under a bony plate (operculum), which opens to the outside. This provides protection and allows sufficient flow of water over the gills.
2. Countercurrent Exchange = water and blood flow in opposite directions, which allows very efficient O₂ extraction at the gill lamellae (Overhead — Willmer et al., p. 269).

• Tropical still waters often provide hypoxic conditions (= low O₂ availability; recall that warm water holds less O₂ than cold water).
• How do fish in these waters obtain sufficient oxygen?

1. Expensive to maintain high surface area of gills (osmotic considerations, etc.), so they don’t have markedly increased gill surface area.
2. Many fish will gulp air and use a vascularized mouth or esophagus for gas exchange; some fishes actually possess lungs. Supplementary air breathing allows them to survive periods of hypoxia, although they usually aren’t capable of normal levels of activity during these periods.

• Lungfish are the most adept at supplementary air breathing (some are obligate air-breathers), but they still can’t support high levels of activity this way because of a low surface area of the lungs.
• During extreme drought periods, when the ponds they are living in dry up, lungfish will aestivate (dormant state) within a mucous case. They may survive in this state for five years or more, until the ponds fill up again.

1. LUNGS

1. Fishes = lungs are simple sacs with only minor subdivision at best. Thus, they have a low surface area and this doesn’t support high levels of aerobic activity.
2. Amphibians/Reptiles = amphibians with only minor subdivision, reptiles with modest subdivision — still not highly aerobic organisms.

• Amphibians use a buccal force pump (like fish) to ventilate the lungs
• Reptiles use a suction pump system (similar to mammals, but less efficient since no diaphragm is present)

1. Mammals = highly branched subdivision within the lung. Ventilate respiratory surface using a negative pressure system with a diaphragm. Bi-directional air flow over the respiratory surface.

• This lung is capable of supporting higher levels of aerobic metabolism

1. Birds = complicated air sac respiratory system allows unidirectional airflow over finely subdivided lungs.

• Gas exchange occurs in air capillaries, which open to a parabronchus, through which air passes in only one direction. (Overhead — Willmer et al., p. 166)
• Blood flow in the avian lung is via a crosscurrent mechanism relative to airflow. This functions similar to a countercurrent system and is highly efficient for O₂ extraction. (Overhead — Willmer et al., p. 166)
• This system is capable of supporting the very high levels of aerobic metabolism associated with flight in birds.
• Also, it allows normal activity at high altitudes (e.g., some geese migrate over the Himalayas)
• Tucker (1968) exposed mice and house sparrows to an atmospheric pressure of 350 mm Hg (corresponds to 20,000 ft. in altitude).
• Mice were barely able to crawl, but sparrows were able to fly about normally
• Mice and sparrows had similar mass, MR, and blood oxygen affinity, so differences were likely explained by the increased oxygen extraction efficiency of the bird lung resulting from the specialized respiratory system.

CUTANEOUS RESPIRATION = gas exchange across the body surfaces. This works in both air and water, but requires a thin, moist, highly vascularized skin to be effective.

• Diffusion is limited by the thickness of the diffusion barrier (recall Fick’s Law). Maximum thickness for sufficient O₂ diffusion is about 1 mm, so cells must be within about 1 mm of the body surface for this to work.
• The diffusion barrier consists of both the thickness of the respiratory surface plus the thickness of the unstirred boundary layer of the respiratory medium (O₂ depleted) immediately adjacent to the respiratory surface.

1. Boundary Layers may account for 80-90% of the total resistance to diffusion in aquatic situations, so they may limit gas exchange in water. The Boundary Layer is not important in air due to the much faster rates of diffusion compared to water.
2. The thickness of the Boundary Layer is dependent on:

1. the velocity of the respiratory medium (faster velocity = smaller boundary layer)
2. the diffusion coefficient of the gas (boundary layer not a problem for CO₂, even in water due to the higher solubility of CO₂ than O₂ in water)
3. the linear dimensions of the animal (the larger the animal, the higher the velocity required to dissipate the boundary layer)

1. Primitive Invertebrates (large) — Includes sponges, nematodes, coelenterates, etc.

• These organisms may also depend on simple diffusion across the integument, in association with internal channels though which water may pass so that diffusion distances are greatly reduced.

1. Complex Invertebrates — Some lack specialized respiratory structures. In these organisms, cutaneous respiration is accompanied by the appearance of a circulatory system that functions to transport gas from the skin to the tissues. Blood vessels close to the skin surface pick up the O₂ and carry it to the tissues.

• Thus, body size no longer limits O₂ diffusion to the tissues.
• In invertebrate organisms, cutaneous gas exchange (CGE) remains important, even in those groups with specialized respiratory structures. CGE may account for 20-50% of total gas exchange in these organisms.

1. Vertebrates — many fish and amphibians and a few reptiles rely to a variable extent on CGE, although a small degree of CGE is present in essentially all vertebrates (See Handout — p. 336, Prosser)

• CGE is trivial in most birds and mammals, accounting for less than 1-2% or total gas exchange, except in bats, which may lose up to 12% of their CO₂ through their wing membranes under certain conditions.
• CGE may account for 50% of O₂ uptake in some fish
• Virtually all amphibians rely on CGE to some extent. Some salamanders (Plethodontidae) lack lungs entirely and depend to a great degree on CGE (supplemented with vascularized mouth regions).
• Some amphibians show specializations that increase the S.A. of the skin for gas exchange (See Handout — p. 588-589, Withers).

1. extra skin folds (e.g., Lake Titicaca frog)
2. vascularized skin papillae (African hairy frog)

• Relative importance of skin and lungs for respiration changes with temperature. Skin is more important at low temperatures, lungs at high temperatures ® results in seasonal differences in the relative importance of respiratory structures. (See Handout — Schmidt-Nielsen, p. 27).