Each person ingests a large number of molecules per day with meals and snacks (Figure 9.1). A similarly large number of molecules enters the body through respiration (Figure 9.2). Body processes—such as building proteins, producing energy, and replenishing lost nutritional stores—use many of these molecules (recall Table 7.1). But a sizeable number of ingested chemicals are either not usable or not needed by the body, and therefore must be eliminated. In addition, metabolic processes generate waste products that are toxic if they accumulate in body tissues. These molecules must also be eliminated.
The problem of elimination of molecules is staggeringly complex, because the diversity of molecules that can be ingested is enormous (recall Table 7.3). In addition to molecules gained in our diet, each person—in the course of sleeping, walking, or bicycling through his or her normal day—will inhale diverse molecules that float freely in the atmosphere. The systems of elimination in the body must be capable of responding to this onslaught; they must safely manage both the number of molecules and the diversity of molecules to ensure the chemical balance that is necessary for life.
Life processes require a rather stringent set of chemical conditions. As examples, Chapter 2 described the importance of pH, which must be maintained within narrow limits for proper function of cells and proteins, and Chapter 5 described the importance of sodium and potassium concentrations in the normal function of neurons. The function of communication systems within the body, such as the nervous system and the endocrine system, depends on the release, movement, and recognition of specific chemical messengers. These systems cannot function well if stray molecules compete with normal recognition processes. This is one of the reasons that chemical composition within the brain is tightly regulated by the blood–brain barrier. Most organs and tissues do not have individual protective mechanisms like that of the brain; instead they rely on organs that are specialized for molecular exchange and excretion to maintain an overall homeostasis in the body.