Endocrine System
The endocrine system is another control system of the body. The nervous system is the fast acting system while the endocrine system is slow to respond. While the nervous system depends on electrical and chemical (neurotransmitters) transmission of signals the endocrine system communicates only by chemical messengers, called hormones.
The endocrine system uses hormones to control several body functions:
The endocrine system uses hormones to control several body functions:
- reproduction
- growth and development
- mobilization of immune response
- maintenance of much of homeostasis
- regulation of metabolism
How does it work?
- hormones are produced by specialized cells
- cells secrete hormones into extracellular fluids
- blood transfers hormones to target sites
- these hormones regulate the activity of other cells
Hormones
Hormones affect only certain tissues or organs (target cells or organs). Target cells must have specific protein receptors. When these hormones bind to the receptors, the cellular activity of the target cells is altered. This change can be seen as:
Hormones are classified chemically:
Hormones either act by direct gene activation or by the second-messenger system.
Hormone levels in the blood are mostly controlled by negative feedback loops. A stimulus or low hormone levels in the blood triggers the release of more hormone. Hormone release stops once an appropriate level in the blood is reached.
The majority of the time, endocrine glands are activated by other hormones.
Blood levels of certain ions (ie. calcium) and nutrients (sucrose) stimulate hormone release. For example, parathyroid hormone and calcitonin are produced in response to changing levels of blood calcium. Another example is the production of insulin by the pancreas in response to changing levels of blood glucose.
Nerve impulses can also stimulate hormone release as in the sympathetic nervous system. Dangerous conditions perceived by the nervous system will trigger the release of norepinephrine (noradrenaline) and epinephrine (adrenaline) by the adrenal medulla (like in the game "Code Fred").
Hormones affect only certain tissues or organs (target cells or organs). Target cells must have specific protein receptors. When these hormones bind to the receptors, the cellular activity of the target cells is altered. This change can be seen as:
- a change in plasma membrane permeability or electrical state
- synthesis of proteins (ie. enzymes)
- activation or inactivation of enzymes
- stimulation of mitosis
- promotion of sensory activity
Hormones are classified chemically:
- amino acid-based (proteins, peptides & amines)
- steroids (made from cholesterol)
- prostaglandins (made from highly active lipids that act as local hormones)
Hormones either act by direct gene activation or by the second-messenger system.
Hormone levels in the blood are mostly controlled by negative feedback loops. A stimulus or low hormone levels in the blood triggers the release of more hormone. Hormone release stops once an appropriate level in the blood is reached.
The majority of the time, endocrine glands are activated by other hormones.
Blood levels of certain ions (ie. calcium) and nutrients (sucrose) stimulate hormone release. For example, parathyroid hormone and calcitonin are produced in response to changing levels of blood calcium. Another example is the production of insulin by the pancreas in response to changing levels of blood glucose.
Nerve impulses can also stimulate hormone release as in the sympathetic nervous system. Dangerous conditions perceived by the nervous system will trigger the release of norepinephrine (noradrenaline) and epinephrine (adrenaline) by the adrenal medulla (like in the game "Code Fred").
Major Endocrine Organs
Some glands are purely endocrine glands (anterior pituitary, thyroid, adrenals, parathyroids) while others are mixed. Others are mixed, with both endocrine and exocrine functions (pancreas & gonads). Endocrine glands secrete their product directly into the blood. Exocrine glands secrete their products onto an epithelial surface by means of a duct.
Endocrine System part 1
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Endocrine System part 2
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