jimtrue.com : school : BSC2011 : CH 42: Animal Systems - Respiratory

Posted by Jim True on November 9, 2004 6:37 PM. Last Updated October 22, 2006 9:23 PM

Disclaimer for all material noted here is at the bottom of this web page.

CH 42: Animal Systems - Respiratory

Function

• Gas Exchange - Oxygen uptake with carbon dioxide released.

Environments

• Terrestrial and aquatic (freshwater, brackish & marine) produce very different conditions for gas exchange.
1. O₂ Saturation of Environment - (Repiratory Medium):
   • In terrestrial environment, atmospheric air is usually always saturated with O₂ at 210mg/l (~ 21%).
   • In aquatic environment, saturation is highly variable with changes in temperature, salinity and density.
   • Absolute maximum saturation of O₂ in H₂O is ~9mg/l (<1%).
   • As a result, aquatic animals must be extremely efficient at extracting O₂.
   • They are, but doing so is very costly:
   • ~20% (aquatic) vs. 2% (terrestrial) of all energy is used for respiration processes.
2. Rates of O₂ Diffusion (Respiratory Surface)
• Even in complex respiratory systems, gas exchange ULTIMATELY relies on the process of diffusion.
• Naturally slower in H₂O than in air because H₂O is so much denser than air.
• Rates of exchange are also affected by:
• Type of Surface - exchange is faster across a moist surface.
• Thickness of surface - the greater the thickness, the slower the rate.
• Concentration of O₂ - the sharper the gradient (more concentrated), the faster the diffusion.
• Diffusive Surface Area - the more area across which diffusion can take place, the faster it will happen.
• Thus the ideal respiratory surface is thin and moist with maximal surface area.
• Having a thin diffusive surface of relatively large area is not a problem for either terrestrial or aquatic animals.
• Maintaining a moist surface is definitely a problem for terrestrial animals.
• Because aquatic animals are completely immersed and can keep respiratory surfaces moist, they are constructed as evaginations of the body, ie., "gills".
• In order to keep respiratory surfaces moist, terrestrial animals possess pockets, infoldings or invaginations of the body, ie, "lungs".

Respiratory Systems

• Three main ways for respiration to occur.
• the first way can operate on its own, but can also work in combination with system #2 or #3.
• I. Cutaneous Respiration - This is basically simple diffusion directly across the skin (or cells in P.Porifera).
• As a sole source of gas exchange, this is not very effective unless:
• The metabolic rate is very low.
• The body surface area relative to the body volume is very high.
• Overall size or thickness of the organism is small.
• Some combination of these factors allows cutaneous respiration to be the primary or sole source of respiration in the following phyla:
• Phyla Porifera, Cnidaria, Platyhelminthes, Nematoda, Annelida, Echinodermata and Chordata (S.p. Urochordata).
• Cutaneous respiration is a significant source of gas exchange also in P.Mollusca and various groups of the P.Chordata (S.p. Cephalochordata, S.p. Vertebrata, C.Agnatha, Osteichthyes and Amphibia).
• In aquatic environments, because of the low oxygen concentrations and density of the medium, animals with cutaneous respiration, as well as gills, increase gas exchange by ventilation (movement of fresh oxygenated water across the areas for respiratory exchange).
• This is the increased movement of the respiratory medium over the respiratory surface by some physical means.
• II. Gills - Evaginations of the body wall in aquatic animals.
• Associated with aquatic organisms that possess a circulatory system so that blood vessels can move oxygenated blood from the gills to the tissues and deoxygenated blood can release CO₂ at the gills.
• Found in aquatic members of P.Mollusca, Arthropoda, some Annelida (aquatic polychaetes), Arthropoda (except s.p. Uniramia), and Echinodermata (C.Holothuroidea).
• In the phylum Chordata, pharyngeal slits originally involved as feeding structures (S.p. Urochordata), then combined both feeding and respiratory functions (S.p. Cephalochordata), and ultimately evolved into primarily respiratory organ (S.p. Vertebrata, C.Agnatha, Chondrichthyes, Osteichthyes, some Amphibia)
• Counter-current Gill - Found in the most efficient O₂ extractors (fishes).
• Water flows over the gill diffusive surfaces in an opposite direction to the blood flow.
• Maximizes the diffusive extraction of O₂
• In Osteichthyes, which have the best developed system, there is also a 2 stage buccal pumping system, that ensures one-way flow of H₂O over gills.
• III. Lungs - In basic design, this is a thin walled sac enclosed within the body that is richly vascularized (well supplied with blood vessels).
• Some variation of this is found in P.Mollusca (C.Gastropoda), P. Arthropoda (S.p. Chelicerata and Uniramia), and P.Chordata, S.p. Vertebrata (C.Amphibia, Reptilia, Aves, Mammalia).
• In terrestrial Mollusca, the lung is a simple vascularized sac.
• In Arthropoda, a number of variations:
• In chelicerates, "book lungs" present. Small openings in abdomen (spiracles) lead to a highly folded chamber around which circulatory fluids flow).
• In uniramians, spiracles lead to a unique tracheal system. Chitin bound tubes lead from spiracles directly to tissues.
• In smaller uniramians, air movement is by diffusion. In larger and in flying insects, pulsing of the abdomen help to move air.
• The vertebrate lung developed from the gas or swim bladder or as an outpouching of the Digestive system.
• In many bony fishes, a vascularized sac is connected by a duct to the pharynx (anabantoid fishes), or the intestine ("lung" fishes).
• In bony fishes, the intestinal wall is vascularized and exchanges atmospheric gas directly (e.g. many "cat" fishes).
• In these fishes, gulped air from the surface will be used to augment gas exchange at the gills.
• These adaptations are best developed in fishes whose habitats normally exhibit low oxygen (hypoxic) or even zero oxygen (anoxic) levels.
• In fishes, even the best developed "lung" is just a baglike sac. May be one or two present.
• This is essentially what we find in terrestrial amphibians. Pair of baglike vascularized sacs which are inflated by positive pressure.
• Air is forced into lungs by contraction of throat muscles.
• Air is exhaled by elastic rebound of lungs, meaning that the lungs collapse inward like a balloon, pushing the air back out.
• In reptiles, birds and mammals, the lungs exhibit elaborate branching (trachea, bronchi, bronchioles) into numerous vascularized sacs called alveoli and each alveolus has its own capillary network.
• Alveoli enormously increase the diffusive surface area.
• Human lungs have 50-90m2 of surface area, with ~1000km of capillaries.
• Lungs are NOT an efficient O₂ extraction system.
• Air passages are bidirectional, air enters and exits via same passageway.
• Also, because tubes are semi-rigid, there is "dead air" that moves back & forth.
• Therefore, it takes several breaths to completely turn over the air in the lungs and trachea (in humans, 6 breaths) , and some of the "new" air entering the lungs is mixed with "stale" air exiting lungs.
• Birds have the most efficient of the lung systems, because as described, there would not be enough oxygen exchanged fast enough to allow for the demands of flight.
• Birds have developed accessory air sacs off the anterior and posterior of the lungs.
• These allow for a modified air flow that is ONE WAY ACROSS THE LUNGS.