20.0.2 20.8 Animals regulate their internal environment through homeostasis.

All of the functions and activities of a single-celled organism are straightforward, thanks to the advantages of living close to the external environment. Food and other materials necessary for life are nearby, just across a plasma membrane. Getting rid of waste products from the cell is similarly simple. But direct contact with the external environment has its costs, too. A single-celled organism is, to a large extent, at the mercy of its external environment. Changes in the environment—such as in temperature or pH—can have a great impact on the cell itself.

Like single-celled organisms, multicellular animals also must acquire food and other materials, as well as get rid of waste. These tasks become increasingly complex when cells are not in direct contact with the external environment. On the other hand, a cell can be protected from harsh or changing environmental conditions if it’s not in direct contact with the environment.

With multicellularity and increasing size, then, an animal’s internal environment takes on greater importance than the external environment in influencing cell functioning. In vertebrates, this internal environment consists of the extracellular fluid (also called the interstitial fluid) that fills the space between cells, bathing the cells. This fluid is primarily water, but it also contains nutrients and raw materials for growth and development, as well as waste products that have diffused out of or been removed from cells. The volume of interstitial fluid is not insignificant. In fact, one-third of the water in the human body is in interstitial fluid, with more than two and a half gallons (10 liters—picture five 2-liter soda bottles!) surrounding and bathing the cells.

One of the hallmarks of animal physiology, however, is that organisms generally maintain homeostasis, or the ability to remain within a narrow range of physical and chemical conditions, even in the face of changing external environmental forces. In this relatively constant, steady, internal environment, variables such as temperature, water-solute balances, pH, blood sugar levels, and O₂ and CO₂ concentrations in blood and other tissues are maintained within narrow ranges by the activities of cells within the organism’s tissues, organs, and organ systems (FIGURE 20-14).

Through homeostasis, organisms can maintain optimum metabolic functioning. For example, the work of cells, from DNA replication to protein production to intercellular communication, depends in large part on the activities of enzymes. In turn, an enzyme’s activity depends critically on the temperature and other characteristics of its immediate environment. Even slight changes in these variables can disrupt, reduce, or even stop enzyme functioning. And this can have catastrophic consequences, such as in the case of heat stroke described in Section 20-7.

Enzyme activity is just one of many facets of cell function influenced by temperature and other physical features of the cell’s internal environment; membrane permeability and the rates at which materials diffuse across membranes also respond to changes in the environment. In the following sections, we explore how homeostasis is maintained.