The endocrine system Flashcards
What is the definition of a hormone?
A chemical messenger produced by a cell or organ (synthesised by endocrine cells), released into circulation (secreted into and transported in blood at very low concentrations) to produced an effect/physiological response on a distant target organ with an appropriate receptor.
What are hormones secreted by?
- Endocrine glands
- exocrine glands
- isolated endocrine glands (GI Tract and Skin)
- neuroendocrine organs ( hypothalamus and adrenal medulla -> neurohormones)
- cells of immune system (cytokines or the hormone)
what is the difference between endocrine and exocrine glands?
Endocrine glands:
- secreted inwards
- hormones secreted directly into the bloodstream, ductless
- eg, pituitary, thyroid, pancreas
Exocrine glands:
- secrete outwards
- proteins into duct to external environment
- eg, sweat glands, salivary glands, pancreas as well when it secretes into the duct to go to the stomach
What are the different types of endocrine cells in terms of distance of signal?
- autocrine -> acts on same cell
- paracrine -> para=beside, eg acts on nearby cells
- endocrine -> acts on distant cells via circulation in plasma
what happens in ADH secretion?
- the pituitary gland releases ADH which acts on the kidney
what happens in cortisol secretion?
Called the HPA axis: hypothalamic-pituitary-adrenal axis
- the hypothalamus secretes corticotropin-releasing hormone
- acts on the pituitary gland to secrete corticotropin
- corticotropin acts on the adrenal glands to secrete cortisol
- cortisol has effects on nearly all cells!
what are peptide hormones?
- synthesised from amino acids
- most common, many tissues, ADH and other pituitary hormones, insulin
- water soluble, hydrophilic
what are steroid hormones?
- synthesised from cholesterol
- only made in a few organs
- cortisol and sex hormones
- hydrophobic - water insoluble
what are amine hormones?
- contain nitrogen
- synthesised from tyrosine and tryptophan
- diverse range and various tissues
- thyroid hormones, melatonin
what does the solubility of hormones affect?
- solubility determines a hormones storage and secretion
- transport and half life
- receptor-binding and physiological effects
describe how the solubility of hormones affect their storage and secretion
water soluble:
- cannot diffuse out of the cell because hydrophilic, cannot pass hydrophobic plasma membrane
- stored in cell and secreted when needed via a protein channel in the membrane that is signalled
Lipid soluble:
- diffuses easily out of the cell as can move through lipid layer of membrane
- synthesised when needed rather than stored in cell
- have a reservoir of hormone in the plasma
describe transport and half life of hormones
water soluble:
- dissolves in plasma
- short half life (minutes)
lipid soluble:
- not soluble in plasma
- transported by a carrier
- longer half life because of this (hours)
describe the receptor binding and physiological effects of hormones
water soluble:
- cannot enter the cell
- binds to cell surface receptors
- second messenger system, affects cellular activity
- alters existing proteins -> fast and short response
lipid soluble:
- always some amount of unbound hormone in plasma. when one molecule of unbound hormone leaves the plasma and enters the cell, this prompts one of the carrier proteins to release the hormone to keep concentration of unbound the same
- diffuses easily into cell across the cell membrane
- binds to intracellular receptors: nucleus or cytoplasm, DNA binding
- formation of new proteins -> slower and longer response
what are the different specificities of hormones versus receptors?
> hormones are selective - can choose which receptor to bind to
- more selective - selective effects at different receptors
- less selective - same effect at different receptors
> receptors are specific - have only specific hormones that can bind to produce a response.
- can also be more specific - ie only one hormone,
- or less specific - a few different hormones can bind to it
can also have different subtypes for same hormone that causes different effects.
e.g., alpha receptors on blood vessels for epinephrine = constriction
beta receptors with epinephrine = dilations
what is a dose-response relationships in hormones and receptors?
- target cells respond to CHANGES in hormone concentrations
- only small hormone concentrations is usually needed for effects
- binding is usually saturable -> leaves spare receptors
- therefore a safety net as one or more receptors can get damaged and still produce a response.
