Our body is always in flux. When we are cold, our body shivers. When we are hot, our body sweats. Our body engages in these activities to keep our internal temperature constant. This is an example of homeostasis, which is defined as the ability of the body to maintain a constant internal state. One major way the body maintains homeostasis is through the endocrine system. The endocrine system is a system of glands that produce hormones to regulate and control bodily activities. The endocrine system controls a number of important functions throughout the body, including growth and development, metabolism, and homeostasis. There are many different hormones in the body and each is responsible for different functions. Estradial, testosterone, insulin, growth hormone, and epinephrine are just a few examples of hormones. Although they are involved in different functions of the endocrine system, hormones work in a similar manner. First, a gland detects a change in the body's internal environment. This gland then increases the secretion of hormones, which enter the bloodstream. Hormones travel through the blood and can have potential effects throughout the body. However, hormones will not act on every cell. Hormones activate target cells by buying to specific receptors on the inside or outside of the target cells. When a hormone binds to receptor, the receptor sends a signal throughout the cell. Just like a megaphone amplifies the volume of a person's voice, a hormone's message is amplified inside the cell. This signal goes a long way. Just a few hormones can induce great change. Once hormones have succeeded in sending their message to their target cells, and in this case, homeostasis is achieved, the gland decreases the secretion of hormones. The mechanism by which the gland stops accruing hormones is called negative feedback. What exactly is negative feedback? Negative feedback allows a system to self-regulate a response or output. The following example may help explain. A house regulates temperature using negative feedback. When the thermostat detects a decrease in temperature, it sends a message to the furnace to produce heat. When the thermostat detects that the room temperature has returned to the correct level, it stops sending its message to the furnace. A furnace will turn odd and off several times a day to keep the temperature relatively constant. Now let's look at a specific example of homeostasis. Using a hormone many of you may have already heard of. Insulin. Let's say a person needs a meal. As the fruit is digested and glucose is absorbed into the bloodstream, blood glucose levels begin to rise. It is important for this glucose to be removed from the blood, otherwise it may cause damage to important organs such as the liver, brain, and kidneys. Thus, to maintain blood glucose levels, insulin is released from an endocrine gland, the pancreas. The pancreas senses increased blood glucose and releases insulin into the blood. Insulin travels through the blood and binds to receptors in muscles, fat tissue, and is shown here the liver. Upon binding to receptors, insulin causes an increase in glucose transporters, allowing excess glucose to be taken from the blood and stored in the liver for future use. This uptake of glucose by the liver and other organs for storage restores blood glucose levels to a balanced state. Homeostasis has been achieved. Because blood glucose levels have reached homeostasis, the pancreas will stop secreting insulin. This is just one type of hormone involved in the endocrine system. Remember, glands of the endocrine system send messages to the body in a similar manner. A gland senses a change in the body and secretes a hormone. Hormones travel through the blood. Hormones are receptor specific. A hormone signal is amplified. And finally, hormones are regulated by feedback, many times negative feedback.