Learning

Endocrine System : Feedback Loops
  1. Feedback Loops
  2. Regulating Estrogen
  3. Fear Response

Feedback Loops

The endocrine system controls many bodily functions, including:
  • reproduction
  • sexual development
  • growth and maintenance
  • metabolism
  • responding to external stimuli
Some endocrine actions, such as reproductive cycles or growth, occur over long periods of time and can take months or years to complete. Other endocrine actions, such as your body's reaction to fear, are fast, sometimes occurring within seconds.

For every function it controls, the endocrine system follows this general pattern:
  • a signal is received
  • a hormonal response takes place
  • a reaction occurs
Most endocrine activities are regulated by a series of complex feedback loops. These feedback loops work like a thermostat that responds to temperature changes by telling a furnace to turn on and off. When it's cold, the thermostat signals the furnace to turn on and make heat. As the temperature rises above the thermostat's set point, the signal turns off and the furnace shuts down. When the temperature falls below the set point, the thermostat again signals the furnace to turn on and start another feedback cycle.

Endocrine glands react to hormonal changes in the blood in much the same way that a thermostat reacts to temperature changes. The glands, which do not constantly secrete hormones, rely on the presence or absence of hormones in the blood to turn their secretions on and off. If there is not enough hormone circulating in the blood, the endocrine glands make more, increasing blood hormone levels. If there is too much hormone, the glands stop producing it, leading to lower blood hormone levels. The liver also plays a role by removing hormones from the blood and breaking them apart.







Regulating Estrogen
Estradiol, the female estrogenic hormone, is regulated through a complex series of chemical indicators that involve at least three glands:
  • the hypothalamus
  • the pituitary
  • the ovary
The process starts when the hypothalamus notices a low level of estradiol in the blood and begins releasing a hormone known as gonadotropin releasing hormone (GnRH), which notifies the pituitary gland to make and release two more hormones:
  • LH or luteinizing hormone and
  • FSH or folicle-stimulating hormone

In females, LH and FSH tell the ovaries to secrete estradiol and progesterone, which stimulates the growth of the egg-producing ovarian follicle and prepares the uterus for pregnancy. In males, LH and FSH notify the testes to secrete testosterone, which stimulates sperm production.

The ovaries (and testes) make and release estradiol (and testosterone) until a certain level is reached in the bloodstream.

The hypothalamus and pituitary notice the increase and stop secreting GnRH, LH and FSH hormones. This causes the ovaries to stop releasing estradiol and progesterone (or the testes to stop releasing testosterone).

This feedback loop is also influenced by the liver. As part of its normal cleaning function, the liver degrades or takes apart some of the hormone molecules, removing them from the blood and lowering the amounts found in the blood.




Fear Response
How and why your heart pounds when you are scared or nervous or exercising is a good example of the endocrine system producing a quick response. Your heart rate increases because the endocrine system, in response to stimuli outside of your body, releases hormones that cause noticeable bodily changes. How does this happen?

  1. Your adrenal gland notices chemical signals put out by the nervous system in response to any bodily stress: hot, cold, life threatening danger or pleasure
  2. Once signaled, the adrenal gland releases the hormones adrenaline and noradrenaline into your bloodstream
  3. These hormones travel to a target cell, bind with the cell's receptor site and cause complex chemical chain reactions that mobilize glucose and fatty acids
  4. Cellular energy reserves are increased and your muscles contract
  5. These unconscious and almost instantaneous chemical responses bring about all the familiar feelings: the ball in the pit of your stomach, a tensing of muscles, goose bumps and a faster heart beat
All this happens in a matter of seconds as the body responds to an outside stimuli and decides whether to fight or take flight!

The endocrine system follows this general pattern - a signal is received, a hormonal response takes place and a reaction occurs - in every bodily function it controls, including:
  • reproduction
  • sexual development
  • growth and maintenance
  • metabolism, and
  • response to outside influences