Have you ever wondered why you don't faint every time you stand up? Does it surprise you that even if you skip lunch you still can walk and talk? Explanations of those occurrences are quite complex.  For instance, the cells in your brain all are exceedingly sensitive to tiny changes in the levels of oxygen and sugar. Your blood pressure automatically rises when you stand up in order to maintain adequate oxygen flow to your brain.  Likewise, you can skip lunch because a declining level of sugar in your bloodstream triggers your liver to release sugar held in storage.  Your body must continuously make adjustments to create and maintain an environment for your brain to function.

These adjustments are made automatically and assure that conditions within your body remain within rather narrowly defined limits, a condition of balance called homeostasis (see Figure E5.1). Homeostasis is a fundamental characteristic of all living systems.  In animals, internal organs that are similar in function to those in humans help to maintain homeostasis.  Maintaining balance means life, and losing homeostatic balance for an extended period of time means death.  To maintain homeostasis, two things are required.  First, an organism must be able to sense when changes have occurred in the external and internal environment.  Second, it must be able to respond with appropriate adjustments.

For example, humans can monitor stimuli such as cold because we have sensory neurons in our skin that allow us to feel the outside temperature.  Once the messaging “cold” is received in the brain, our body can respond by changing blood flow.  Our heart rate may increase, and certain blood vessels may constrict. We do not consciously control this physiological process, it is involuntary. In other words, we do not decide what the body should do.  The body attempts to keep the brain, heart, and liver at a nearly constant temperature even if that means sacrificing fingers and toes.  The human body’s response to change is quite specific as well as involuntary.  For example, the body responds to cold temperate by diverting circulation to keep the most important internal organs warm.  This type of response is appropriate for the external conditions.  If the body becomes too hot, however, the circulatory system diverts blood flow away from the internal organs to protect them from damage caused by excess heat. These examples are rather dramatic, but the human body routinely senses and responds to thousands of small changes each day.  It is through many small, specific, automatic changes that living organisms sense and react to an environment that is ever changing and sometimes hostile.

This reading adapted from BSCS: A Human Approach