With the exception of an organism's reproductive system, which perpetuates the species, all body structures and functions work in ways that maintain life.
Requirements of Organisms
Life depends upon the following environmental factors:
1. Water is the most abundant substance in the body. It is required for a variety of metabolic processes, and it provides the environment in which most of them take place. Water also transports substances within organisms and is important in regulating body temperature.
2. Food refers to substances that provide organisms with necessary chemicals (nutrients) in addition to water. Nutrients supply energy and raw materials for building new living matter.
3. Oxygen is a gas that makes up about one-fifth of the air. It is used in the process of releasing energy from nutrients. The energy, in turn, is used to drive metabolic processes.
4. Heat is a form of energy. It is a product of metabolic reactions, and it partly controls the rate at which these reactions occur. Generally, the more heat, the more rapidly chemical reactions take place. Temperature is a measure of the amount of heat present.
5. Pressure is an application of force on an object or substance. For example, the force acting on the outside of a land organism due to the weight of air above it is called atmospheric pressure. In humans, this pressure plays an important role in breathing. Similarly, organisms living under water are subjected to hydrostatic pressure—a pressure exerted by a liquid—due to the weight of water above them. In complex organisms, such as humans, heart action produces blood pressure (another form of hydrostatic pressure), which keeps blood flowing through blood vessels.
Although the human organism requires water, food, oxygen, heat, and pressure, these factors alone are not enough to ensure survival. Both the quantities and the qualities of such factors are also important. Table 1.2 summarizes the major requirements of organisms.
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