Elements and gases are part of every living organism on earth. Life on earth began with elements and gases. Plants and animals formed from cells using those materials when the earth was forming. You can survive without food or water far longer than you can survive without oxygen.
How living organisms deal with gases depends on the organism. First, what is the organism’s environment like? Does the organism live on land (and have oxygen available in air), or does it live in water? The size and shape of the organism also matter, as does its metabolic rate (how fast it respires) and whether it has a way of transporting gases inside itself.
Oxygen in air versus oxygen in water
Air has more than three times the oxygen that water does, and it is much less dense. There are no pockets of air that are lacking oxygen — except maybe deep within a cave — but there can be pockets of water that have little or no oxygen in them. As a result, it is easier to get oxygen from air than from water, if you are designed for the task.
A fish expends 25 percent of its total energy just moving water over its gills. Because water is more dense, and because more water needs to pass over the gills to get a good amount of oxygen out, it takes more energy. Humans, however, expend only 1 to 2 percent of their total energy breathing — which leaves humans energy to expend on other things, like jobs and studying science.
Gases are exchanged at every cell in your body, not just in your respiratory system. If you took off your skin, your body would be extremely moist. It wouldn’t stay that way for long, though, because water evaporates from the body very rapidly over moist surfaces. Your skin, and the skin on other animals, is a protective barrier. Skin prevents gas exchange from occurring too rapidly.
Why body size and shape matter
Oxygen goes into solution with water, but once in the water, it diffuses very slowly. So, oxygen must be moved through most organisms. Only the flattest, smallest organisms have no oxygen transport mechanisms. Flatworms are one example. These worms are so small and flat (hence their name) that all of their cells are near a gas exchange surface. Oxygen can come in, and carbon dioxide can go out as they please. This method of exchange also limits the size of the organism.
Most of the larger, rounder animals living here on earth have circulatory systems that shuttle oxygen around bodies. In humans, oxygen is carried by the red blood cells through blood vessels, and then it diffuses into regular cells through capillary exchange.
What does the metabolic rate have to do with it?
If you added up the results of everything you took into your body — all the food, all the oxygen, all the water — and you summed up every reaction that occurred in your body, you would get your metabolic rate.
Because most of the reactions that occur in the body are aerobic (they require oxygen), one way to determine the metabolic rate is to measure how much oxygen is taken in by the organism over a unit of time (for example, 1 liter of oxygen per hour).
The metabolic rate usually is measured when the organism is in its normal state (resting but not sleeping), just going through the normal metabolic processes that sustain life; it is called the standard metabolism.
Several factors are taken into consideration when determining the metabolic rate of an organism:
The size and weight of the organism have an effect, as do the species of the organism and the environment in which it lives.
Temperature plays a role in the metabolism of heterothermic animals. Heterothermic animals, such as fish and insects, experience changes in their body temperature when the temperature of the environment changes. Standard metabolic rates rise when the temperature rises, and they decrease when it is cold. They have very little insulation on their bodies. When their metabolic rate is very low — when temperatures are very low — they are extremely inactive.
Homeothermic animals, such as birds and mammals, maintain a stable body temperature, although it is high. Homeothermic mammals have higher metabolic rates because metabolic reactions are occurring more rapidly, and their bodies produce more energy. The energy is released in the form of heat.
Homeothermic animals such as yourself also have two ways of protecting against the loss of too much heat. These animals have insulation in the form of feathers or hair and body fat to keep them warm. The second protective measure is homeostasis.