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Chapter 22: All

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Gas Exchange

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Respiration Systems

A reminder about membranes

As with the digestive system, the primary function of the respiratory system is to transfer materials across the boundary between the body of the organism and its environment. In this case, the materials are gases. Molecular oxygen (O2) crosses into the body from the atmosphere (land animals), or an aquatic medium (fish with gills), to be used in cellular respiration as the electron receptor. Carbon dioxide (CO2) wastes from the same cellular respiration process pass out of the body and back into the atmosphere.

The transmission of both gases across the cell membrane is driven by diffusion, which occurs when there is a difference in concentration exists on two sides of a semi-permeable membrane. As we shall see, blood in the vessels that line the membrane tissues of the lungs is returning to the heart and lung systems, having completed a circuit of the body where it dropped its oxygen and nutrients and picked up carbon dioxide and other wastes. The oxygen concentration in the this blood much lower than the 20.9% oxygen concentration (determined as partial pressure) of the Earth's atmosphere, and in fish, it is lower than the 1% oxygen concentration in healthy aquatic environments. The carbon dioxide concentration is higher than atmospheric CO2 (less than 1% of the atmosphere), so it passes out of the blood through the membrane of the lunges and back into the atmosphere.

Humans are adapted to live at near sea-level atmospheric pressure, with its O2 and CO2 concentrations. At high elevations, the partial pressure of oxygen decreases, so humans may suffer hypoxia, or the inability to absorb oxygen across the membranes of the lungs at a high enough rate to replace that used in cellular respiration. The body may turn to anaerobic respiration, with painful results.

In closed conditions, where air cannot circulate, exhaling will increase the concentration of CO2 in the atmosphere, and slow down the rate at which CO2 is flushed from the system. This was a major problem for astronauts about Apollo 13 when emergency use of the Lunar Lander by three people (instead of the two it was built for) overloaded the "flushers" that pulled CO2 from the air. Engineers had to figure out how to rig a CO2 extraction system that would work from materials the astronauts had abord their spacecraft in order to prevent CO2 poisoning.

The type of structures involved in O2 and CO2 gas exchange depend largely on whether the animal gets its oxygen from the air or from the surrounding water. Extracting oxygen from water, where it is dissolved as a gas in low concentration, is very energy-expensive, and a fish will typically spend 20% of its total energy just to keep water moving over its gills at a rate sufficient to supply its oxygen needs. In contrast, land animals spend only 1-2% of their energy inhaling and exhaling, since the concentration of oxygen in the atmosphere is so much greater than it is in water..

Comparison of Respiration Systems

There are four major kinds of gas exchange systems found in animals. Because gas exchange is so important, many animals will have more than one gas exchange system; humans use both lungs and body surfaces to collect and expel gases.

Animal type Respiration Structures Description
Mollusks, annelids, small arthropods, some vertebrates Body surfaces Moist surfaces with mucus secretions allow gas to pass through surface to blood vessels immediately below/
Insects, mites, spiders, millipedes, centipedes Tracheal Tubes Air enters through surface holes (spiracles) and flows down tracheal tubes until it reaches sacks (tracheoles) filled with fluids that allow gas exchange to body cells.
Aquatic animals (some mollusks, crustaceans, amphibians, and fish) Gills Thin structures extending from the body surface. Water flowing over gills exchanges oxygen directly with gill blood vessels.
Arachnids, some mollusks, most vertebrates Lungs Structure developing inside the body cavity; air reaches structure through series of passageways.


In fish, the flow of oxygen and the flow of blood through the blood vessels work together to maximize the amount of oxygen the fish can absorb from one pass through the water. Water passes over the gill surface from the front to the back of the gill, and blood flows in the vessels just under the surface from the back to the front. The concentration of oxygen in the blood is lower than that in the water, so oxygen flows into the blood vessels. As the blood flows from back to front, it picks up more and more oxygen, so the difference in oxygen concentration between the blood and the water drops. But water toward the front of the gill has not yet passed over as much gill surface, so its oxygen concentration is higher than the "expended" water near the back of the gill. Thus while the concentration of oxygen in the blood rises as the blood flows forward in the gill, the concentration of oxygen in the water the blood encounters also increases, and remains high enough that oxygen continually flows into the blood.

The same diffusion laws work to expel concentrated carbon dioxide from the fish's blood into the water of its environment.

Blood Flow in Gills

For more details of this process, check out the How Fish Breath pages.

Lungs and Breathing

While many animals have lungs, the type of lungs and structure can differ. Reptile and amphibian lungs are simple sacks with a ridged and folded inner surface. In comparison with bird lungs which have air sacs like those of most mammals, reptile lungs have a small surface area, so their gas exchange is less efficient. As a result, most reptiles and amphibians are far less active than birds are.

Once the oxygen has crossed the membrane of the alveoli and entered the blood stream, it is captured by a red pigment in the blood cells called hemoglobin. Without hemoglobin, the plasma in blood could only cary about .25 milliters of oxgen per 100 milliters--not nearly enough to keep you alive. Because hemoglobin can latch onto oxygen molecules and hold them, normal blood can carry 20 milliters of oxygen per 100 milliters of blood--almost 100 times as much oxygen as plain plasma.

Blood also carries carbon dioxide, the product of cellular respiration, from the body cells back to the lungs for expulsion. While some of the CO2 actually hitches a ride on hemoglobin, most of it combines with hydroxide ions (OH-) in the blood to form bicarbonate molecules or carbonic acid. The concentration of CO2 in the blood is also what apparently triggers your breathing reflex. Chemoreceptors in your brain monitor the level of carbonic acid in the blood and force changes in the breathing rate to compensate for changes in the acid level.

As we saw earlier, anything that changes the concentration levels of oxygen and carbon dioxide in the blood or affects the passage of theses gases through the membrane walls of the alveoli can disrupt gas exchange and destroy the health of the organism. Changes in the concentration of oxygen or carbon dioxide concentrations aren't the only way gas exchange across the membrane is affected. Rapid decreases in pressure on the body (when a diver comes up too fast) can cause the carbon dioxide in the blood to bubble out as a gas, and give the victim "the bends"--painful cramps that can even result in death. And of course, damage to the alveoli from disease or chemical pollutants (such as cigarette smoke) can seriously decrease the body's ability to absorb oxygen.