GAS EXCHANGE

• Because the respiratory surface must supply O2 and expel CO2 for the entire body, the structure of a respiratory surface depends mainly on the size of the organism, whether it lives in water or on land, and by its metabolic demands.

• An endotherm has a larger area of respiratory surface than a similar-sized ectotherm.

• Some animals, such as earthworms and some amphibians, use the entire outer skin as a respiratory organ.

• Just below the moist skin is a dense net of capillaries.

• However, because the respiratory surface must be moist, their possible habitats are limited to water or damp places.

• Animals that use their moist skin as their only respiratory organ are usually small and are either long and thin or flat in shape, with a high ratio of surface area to volume.

• For most other animals, the general body surface lacks sufficient area to exchange gases for the entire body.

• The solution is a respiratory organ that is extensively folded or branched, enlarging the surface area for gas exchange.

• Gills, tracheae, and lungs are the three most common respiratory organs.

2. Gills are respiratory adaptation of most aquatic animals

• Gills are outfoldings of the body surface that are suspended in water.

• The total surface area of gills is often much greater than that of the rest of the body.

• Water has both advantages and disadvantages as a respiratory medium.

• There is no problem keeping the cell membranes of the respiratory surface moist, since the gills are surrounded by the aqueous environment.

• However, O2 concentrations in water are low, especially in warmer and saltier environments.

• Thus, gills must be very effective to obtain enough oxygen.

• This flow pattern is countercurrent exchange.

Tracheal systems and lungs are respiratory adaptations of terrestrial animals

• As a respiratory medium, air has many advantages over water.

• Air has a much higher concentration of oxygen.

• Also, since O2 and CO2 diffuse much faster in air than in water, respiratory surfaces exposed to air do not have to be ventilated as thoroughly as gills.

• When a terrestrial animal does ventilate, less energy is needed because air is far lighter and much easier to pump than water and much less volume needs to be breathed to obtain an equal amount of O2.

• Air does have problems as a respiratory medium.

• The respiratory surface, which must be large and moist, continuously loses water to the air by evaporation.

• This problem is greater reduced by a respiratory surface folded into the body.

• The tracheal system of insects is composed of air tubes that branch throughout the body.

• The largest tubes, called tracheae, open to the outside, and the finest branches extend to the surface of nearly every cell where gas is exchanged by diffusion across the moist epithelium that lines the terminal ends.

• The open circulatory system does not transport oxygen and carbon dioxide.

• In contrast, mammals ventilate their lungs by negative pressure breathing.

• The volume of air an animal inhales and exhales with each breath is called tidal volume.

• It averages about 500 mL in resting humans.

• The maximum tidal volume during forced breathing is the vital capacity, which is about 3.4 L and 4.8 L for college-age females and males, respectively.

• The lungs hold more air than the vital capacity, but some air remains in the lungs, the residual volume, because the alveoli do not completely collapse.

• Since the lungs do not completely empty and refill with each breath cycle, newly inhaled air is mixed with oxygen-depleted residual air.

• Therefore, the maximum oxygen concentration in the alveoli is considerably less than in the atmosphere.

• This limits the effectiveness of gas exchange.

• Ventilation is much more complex in birds than in mammals.

• The entire system - lungs and air sacs - is ventilated when the bird breathes.

• Air flows through the interconnected system in a circuit that passes through the lungs in one direction only, regardless of whether the bird is inhaling or exhaling.

• Instead of alveoli, which are dead ends, the sites of gas exchange in bird lungs are tiny channels called parabronchi, through which air flows in one direction.

• This system completely exchanges the air in the lungs with every breath.

• Therefore, the maximum lung oxygen concentrations are higher in birds than in mammals.

• Partly because of this efficiency advantage, birds perform much better than mammals at high altitude.

• For example, while human mountaineers experience tremendous difficulty obtaining oxygen when climbing the Earth’s highest peaks, several species of birds easily fly over the same mountains during migration.