Hormone
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Hormones are biological chemicals that are found in plants and animals. They are made by cells. They are used to send messages to other cells. These messages can be sent to nearby cells or to far away cells. If a cell wants to send a message to a nearby cell, it puts the hormone into the tissue around it. If a cell wants to send a message to a far away cell, it puts the hormone into the blood. When a hormone is put in the blood it goes to all parts of the animal's body. Sometimes the cell that hears the message can even be the same cell that made the hormone (and sent the message.)
The cell or tissue that gets the message is called the target cell.
Many different kinds of cells can send a message. There are some kinds of cells whose main job is to make hormones. When many of these cells are together in one place, it is called a gland. Glands are a group of cells that make something and release it (put it outside the cell.) Some glands make hormones.
Endocrine means something that is made by cells and released into the blood or tissue. So endocrine glands make something and release it into the blood or tissue: hormones! The opposite word is exocrine and means released outside of the body. An example of exocrine is sweat glands or saliva glands. When people say endocrine they usually mean glands that make hormones.
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[edit] Actions
Hormones do many things. They regulate metabolism. Metabolism is all of the chemical and energy reactions that go on in a living thing. Hormones cause the growth and death of cells and of whole organisms. (Another word for a living thing like a plant or an animal is an organism.) Hormones also start and control sexual development. For example, the hormones estrogen and progesterone make girls start puberty. Hormones help keep homeostasis in an organism. Homeostasis means to keep a constant state inside the body like temperature, amount of water and salts, and amount of sugar. Hormones released by one gland can also tell other glands to make different hormones.
[edit] Types of hormones
There are four types of hormones in most animals. They are grouped by the chemicals from which they are made. When scientists say hormones are derived from it means they are made from something by changing it. These changes are chemical changes.
- Steroid hormones – these are derived from cholesterol. The three big groups of steroid hormones are: sex hormones, glucocorticoids, and mineralocorticoids.
- Lipid hormones – these are hormones derived from lipids, which are kinds of fats. These are mostly hormones that send messages nearby the cell that makes the hormones.
- Amino acid – these are derived from amino acids which are the molecules from which proteins are made. These are important in the brain. Many of these hormones are neurotransmitters. These are the hormones that one nerve cell sends to another nerve cell.
- Protein – these are hormones that are proteins which are made from amino acids that are connected together. Many of these hormones tell other glands to make other hormones. These are also important hormones that regulate metabolism.
[edit] Regulation of hormones
In biology regulation means to control something. So regulating hormones means controlling how much hormones are made and released from cells.
[edit] Negative feedback
Hormone regulation is mostly done by negative feedback.
In negative feedback a hormone makes an effect. The cells that make the hormone see that effect happen. When they see it happen, they stop making more hormone.
A good example of negative feedback is the hormone insulin. Insulin is a hormone that is made by the pancreas. Insulin is released by the pancreas when you eat glucose (a kind of sugar). The glucose goes from your stomach to the blood. The level of glucose in the blood goes up. The pancreas sees this high glucose level. It makes insulin and releases it into the blood. Then the insulin goes through the whole body and tells cells to take glucose out of the blood. Cells use some of this for energy. But some extra is also saved in the cells to use later. When cells take up glucose from the blood this makes the glucose level go down. The pancreas sees this and stops making insulin. When the pancreas stops sending this message (insulin), the cells in the body stop taking extra glucose out of the blood.
So the negative feedback works to keep the blood glucose level normal. If glucose is high, the pancreas makes insulin. The insulin causes the glucose to fall. Then this lower level of glucose tells the pancreas to stop making insulin.
[edit] Counter regulatory hormones
Sometimes more than one hormone controls the same thing. For example, blood glucose is very important to an organism. So it is not controlled by just one hormone. Other hormones also make the glucose level go up or down. If the glucose level gets too low, the body releases hormones that do the opposite of insulin. They do not tell the cells in the body to take up glucose from the blood. They tell the cells to put glucose back into the blood. These kind of hormones that work opposite of other hormones are called counter-regulatory hormones. Counter-regulatory hormones for insulin are glucagon and epinephrine.
Counter-regulatory hormones are important. An organism needs to keep certain things closely controlled. It is important that glucose not be too high. Insulin makes sure glucose is not too high. But it is also important that glucose not be too low. Glucagon and epinephrine make sure it is not too low. Counter-regulatory hormones help maintain homeostasis. They make sure the body does not have too much or to little of important things like salts, energy, water, and glucose.
[edit] Positive feedback
Most important things in an organism are kept in homeostasis by negative feedback and counter-regulatory hormones. However a few things are controlled in different ways. One rare way is positive feedback. In negative feedback, the hormone's effect makes a gland stop making hormones. In positive feedback the opposite happens. The effect of the hormone tells the gland to make even more hormones.
An example of positive feedback is the hormone that causes childbirth (when babies are born.) The hormone that causes this is oxytocin. This hormone is made by the pituitary gland. When the baby starts coming out, it stretches the muscle in the cervix (the bottom of the womb.) Nerves in the cervix send a message to the pituitary. This message makes the pituitary release more oxytocin. The oxytocin then causes the muscles of the womb to contract, or squeeze. This causes more stretching in the cervix. This stretching then tells the pituitary to make even more oxytocin. So levels of oxytocin keep rising until the squeezing or contractions of the womb. (The womb is also called the uterus.)
| Endocrine system |
| Adrenal gland - Corpus luteum - Hypothalamus - Ovaries - Pancreas - Parathyroid gland - Pineal gland - Pituitary gland - Testes - Thyroid gland - Hormone |
