The internal environment in the body is stable and any changes are short-lived. Homeostatic mechanisms restore the normal state after any disturbance. Homeostasis ensures that the human body is in harmony with its internal and external milieu.

Homeostatic mechanisms work through negative feedback. In negative feed-back operating in homeostasis, deviations from the norm are detected by specialized sensors and compensatory changes are triggered to restore the status quo.

Positive feed-back is not found in normal physiology because it is harmful. If a change occurs, positive feed-back would reinforce it in a cyclic way that causes more of the same until all homeostasis is lost.

Blood sugar levels are an example of homeostatic control using negative feedback. Following food ingestion, digestion, and absorption, the blood sugar level rises. This elicits a negative feedback mechanism consisting of insulin release that increases cell intake of sugar decreasing the blood level.

INTERACTION WITH THE ENVIRONMENT

The interaction between the internal and external environments is purposive and is not a product of chance. The nervous and endocrine systems ensure that the human body can adjust to rapidly changing circumstances of the external environment. The human body interacts with the physical environment, the biological environment, the microbiological environment, and the chemical environment. Humans unlike other creations are able to change their environment for good or for bad.

The biological environment consists of animals and plants. Plants are a source of food and are involved in the oxygen, carbon dioxide, and nitrogen cycles that are necessary for human comfort. Animals provide food, transport and materials for human use. Humans are at the top of the food chain and all food webs.

The microbial environment consists of viruses, prokaryotes, and eukaryotes. There are 3 basic types of host-parasite relations: symbiosis, parasitism, and commensalism. There is a balance sheet between the benefits and harm of microorganisms.

The chemical environment consists of water and chemical elements and chemical compounds. Water is 45-75% of body weight. The main elements of the body are H, C, I, P, S, Cl, K, and Ca. Humans take elements from the earth as food, drink, and by respiration. They detoxify any toxic products of metabolism before release into the external environment.

EQUILIBRIUM AND BALANCE