The Endocrine System Flashcards
The endocrine system is under the control of which gland?
The pituitary gland (master gland)
It controls the activities of all the glands in the body through the hormones it secretes.
What is a hormone?
A hormone is a specific chemical substance produced by glands and is transported to a target organ to regulate physiological activities in the body.
What are the two types of glands and their definitions?
The exocrine glands; these convey their secretions to the site of action by special ducts
The endocrine glands; these lack ducts and transport their secretions instead by the blood.
What stimulates secretion of hormones from the endocrine glands?
Endocrine glands are stimulated to secrete hormones either by impulses from motor nerves or by hormones from other glands. (Chemical/electrical)
What links the endocrine system to the nervous system?
The hypothalamus (which exerts a major control over the pituitary gland of the endocrine system)
What is a metabolite?
Any molecule taking part in metabolism
What links the endocrine system to the nervous system?
The hypothalamus (which exerts a major control over the pituitary gland of the endocrine system)
Which mechanisms control the release of hormones?
1) The presence of a specific metabolite in the blood; for example glucose in the blood causes the release of insulin from the islets of the Langerhans of the pancreas which lowers the glucose level.
2) The presence of another hormone in blood. Such hormones are called stimulating hormones and most of them are produced by the anterior pituitary gland like thyroid stimulating hormone.
3) Stimulation by neurons from the autonomic nervous system like adrenaline and noradrenaline are released from the cells of the adrenal medulla by the arrival of the nerve impulse in situations of anxiety and danger.
What is the cascade effect?
This is the release of hormones by the presence of another circulating hormone usually under the control of the hypothalamus and pituitary gland.
Why is the cascade effect significant?
It enables the effect of the release of a small amount of initial hormone to become amplified (magnified) at each stage in the pathway.
An example of the cascade effect in the control of the conversion of glucose to glycogen as a result of the release of adrenocorticotrophic releasing factor. (Fig 17.47 BS page 601)
Explain the mechanisms by which hormones exert their effects on target cells?
1) The cell membrane:
Insulin exerts one of the effects by increasing the uptake of glucose into cells. It binds with a receptor site and alters the permeability of the membrane to glucose. Adrenaline works on smooth muscle cells by opening or closing ion channels for sodium or potassium ions or both, changing membrane potentials and either stimulating or inhibiting contraction as a result.
2) Second messenger mechanism):
Adrenaline and many peptide hormones bind to receptor sites on the cell membrane but cannot enter the cells themselves.
Instead they cause the release of a ‘second messenger’ which triggers a series of enzyme-controlled reactions.
These eventually bring about the hormonal response. In many cases this second messenger is the nucleotide cyclic AMP (cyclic adenosine monophosphate).
An example is how adrenaline causes the release of glucose from a liver cell.
The activation of membrane bound adenyl cyclase produces cyclic AMP which activates enzyme systems leading to the breakdown of glycogen to glucose.
Glucose then diffuses out of the cell into the bloodstream. At each stage in the process an amplification occurs because only a few molecules of adenyl cyclase are needed to activate many molecules of protein kinase, and so on. This is the cascade effect.
3) Genes:
Steroid hormones (sex hormones and other hormones secreted by the adrenal cortex) pass through the cell surface membrane and bond to a receptor protein in the cytoplasm.
The complex formed passes to the cell nucleus where the hormones exert a direct effect upon the chromosomes by switching on genes and stimulating transcription. The messenger RNA enters the cytoplasm and is translated into new proteins, such as enzymes, which carry out a particular function. For example, the hormone thyroxine passes through the surface membrane and binds directly to receptor proteins in the chromosomes, switching on certain genes,
What are target cells?
They are cells that respond to specific hormones.
Explain how a very small amount of hormone is able to exert a large effect on a target cell.
By use of a two messenger system where a very small amount of hormone can lead to the synthesis of a comparatively large amount of cyclic AMP which in turn evoke a correspondingly large response - the cascade effect.
What are the functions of the hypothalamus?
- It regulates homeostatic activities such as thirst, sleep and temperature control.
- It monitors the level of hormones and other chemicals in blood passing through it.
- It controls the functioning of the anterior pituitary gland.
- It produces antidiuretic hormone and oxytocin which are stored in the posterior pituitary gland.
The hypothalamus is the link between the nervous and endocrine systems;
Describe how it controls thyroxine production by the thyroid gland
i) The hypothalamus produces thyrotrophin releasing factors (TRF) which passes to the pituitary along blood vessels.
ii) TRF stimulates the anterior pituitary gland to produce thyroid stimulating hormone (TSH).
iii) TSH stimulates the thyroid gland to produce thyroxine.
iv) As the level of thyroxine builds up in the blood it suppresses TRF production from the hypothalamus and TSH production
by the anterior pituitary gland. By this form of negative feedback the levels of thyroxine in the blood is maintained at a constant level.
Why is the pituitary gland called a Master gland?
It produces a number of hormones many of which influence the activity of other endocrine glands.
What are trophic hormones?
These are hormones that stimulate the activity of their respective endocrine glands. Usually secreted by the anterior pituitary gland
State the hormones secreted by the anterior pituitary gland as well as their respective releasing factors from the hypothalamus
- Growth hormone releasing factor - growth hormones
- Thyrotrophin releasing factor- thyroid stimulating hormones (thyrotrophic hormones)
- Adrenocorticotrophic releasing factor - adrenocorticotrophic hormone
- Luteinizing hormone releasing factor- Luteinizing hormone
- Follicle stimulating factors- follicle stimulating hormones
- Prolactin releasing factor- Prolactin hormones
What is the function of growth hormones?
- Control protein synthesis and general body metabolism.
- Promotes growth of skeletal muscles.
What is the function of thyrotrophic hormones/ thyroid stimulating hormones?
- Stimulates growth of thyroid glands.
- Stimulates the Thyroid gland to secrete thyroxine hormone
