Hormones are chemicals that carry messages from glands to cells within tissues or organs in the body. They also maintain chemical levels in the bloodstream to help achieve homeostasis, which is a state of stability or balance within the body. There are two types of these chemicals, known as steroids and peptides. The word "hormone" comes from a Greek word that means "to spur on." This reflects how hormones act as catalysts for chemical changes at the cellular level that are necessary for growth, development and energy.
How They Work
As members of the endocrine system, glands manufacture hormones. These chemicals circulate freely in the bloodstream, waiting to be recognized by a target cell, which is their intended destination. The target cell has a receptor that can be activated only by a specific type of hormone, after which the cell knows to start a certain function within its walls. Genes might get activated, for example, or energy production resumed. An autocrine hormone acts on the cells of the secreting gland, and a paracrine hormone acts on nearby — but unrelated — cells.
In general, steroids are sex hormones that are related to sexual maturation and fertility. Steroids are made from cholesterol, either by the placenta when the body is still inside the mother's womb or by the body's adrenal gland or gonads — the testes or ovaries — after birth.
Cortisol, an example of a steroid hormone, breaks down damaged tissue so that it can be replaced. Steroids determine physical development and fertility cycles from puberty through old age. If a person's body is not synthesizing the correct steroidal hormones, he or she can sometimes supplement them pharmaceutically, as with estrogen and progesterone.
Peptides regulate other functions, such as sleep and blood sugar concentration. They are made from long strings of amino acids, so they sometimes they are referred to as protein hormones. Human growth hormone, for example, helps the body burn fat and build muscles. Another peptide hormone, insulin, starts the process to convert sugar into cellular energy.
Hormones so perfectly and efficiently manage homeostasis because of negative feedback cycles. The body's goal is to keep the concentration of a certain chemical, such as testosterone, at a constant level for a certain period of time, similar to how a thermostat works. Using negative feedback, a change in conditions causes a response that returns the conditions to their original state. For example, when a room's temperature drops, the thermostat responds by turning on the heat. The room then returns to the ideal temperature, and the heater turns off, keeping the conditions relatively constant.