When the body's chemical reactions are in balance, every chemical is maintained within its normal range; for example, the normal range for potassium in the blood is 15-20 mg per 100 ml blood. Levels of a chemical may continually rise and fall in the blood and other organs, but the result is the equilibrium condition called homeostasis.
Most of the body's chemical processes affect a variety of substances through interactions with other processes. When the level of phosphorus in the blood builds up above its normal range, for instance, it combines with calcium to form calcium phosphate. That may lower the blood level of calcium so far that too little calcium enters the skeleton, which needs it for making new bone. If this condition continues, as it may when a patient has kidney failure and can't excrete phosphorus, osteoporosis may result. Some other conditions may lead to too much calcium in the blood (hypercalcemia), a cause of kidney stones and irregular heartbeat.
Stress can affect body chemistry as well as emotional responses. During the well-known "fight or flight" reaction to threat, the adrenal glands respond by producing the hormone adrenaline. Adrenaline in turn causes the breakdown of glycogen (the form of glucose that is stored in the liver) to glucose, which can be used for producing ATP in muscle cells. Energized by the ATP, the arm and leg muscles can contract to strike an enemy or beat a hasty retreat. Adrenaline also affects the digestive system, inhibiting the digestion and absorption that can await a quieter time.
Mental illness in many cases results not from purely emotional causes but from homeostasis gone awry. If a neurotrans-mitter substance is produced in too great or too small an amount, the effect on the rest of the nervous system may bring about the symptoms of clinical depression, bipolar disorder, or other mental illness.
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