Option D5 Hormones and Metabolism Flashcards
Endocrine system
Comprised of ductless glands that release chemicals into the blood to regulate body functions.
- Slower to initiate than the nervous system, but has a more prolonged response
Hormone
A chemical messenger that is transported indiscriminately via the bloodstream to act on distant target cells.
- Specific (will only activate cells/tissues that possess an appropriate target receptor)
Endocrine glands
Secrete their hormones directly into the bloodstream rather than through a duct.
- Major glands: pancreas, adrenal gland, thyroid gland, pineal gland, and gonads
- Some organs (ex. adipose tissue) can secrete hormones despite not being an endocrine gland
Neuroendocrine glands
The hypothalamus and pituitary gland, which function to link nervous and endocrine systems.
Pineal gland
- Secretes: melatonin
- Targets: many organs
- Function of hormone: a biological clock
Pituitary gland
- Secretes: FSH/LH, ADH, growth hormone, oxytocin, and prolactin
- Targets: ovaries, kidneys, many organs, uterus, and breast tissue respectively
- Function of hormone: menstrual cycle, osmoregulation, growth/division, birth contractions, and milk production respectively
Thyroid gland
- Secretes: thyroxin
- Targets: liver
- Function of hormone: metabolic rate
Adrenal glands
- Secretes: adrenaline and cortisol
- Targets: many organs (for both)
- Function of hormone: fight or flight and anti-stress respectively
Pancreas
- Secretes: insulin/glucagon
- Targets: liver
- Function of hormone: regulation of blood glucose levels
Ovaries
- Secretes: estrogen/progesterone
- Targets: uterus
- Function of hormone: menstrual cycle
Testes
- Secretes: testosterone
- Targets: many organs
- Function of hormone: male characteristics
Steroid hormones
- Lipophilic (fat-loving), meaning they can freely diffuse across plasma membranes
- Bind to receptors in the cytoplasm/nucleus of the target cell to form an active receptor-hormone complex
- Activated complex will move into the nucleus and bind directly to DNA, acting as a transcription factor for gene expression
- Examples: those produced by the gonads (ex. estrogen, progesterone, and testosterone)
Peptide hormones
- Hydrophilic and lipophobic (fat-hating), meaning they cannot freely cross plasma membranes
- Bind to receptors on the surface of the cell, which are typically coupled to internally-anchored proteins
- Receptor complex activates a series of second messengers
- Examples: insulin, glucagon, leptin, ADH, and oxytocin
Second messengers
A series of intracellular molecules that initiate cell activity when activated by the receptor complex.
- Enables the amplification of initial signal
- Examples: cyclic AMP, calcium ions, nitric oxide, and protein kinases
Hypothalamus
The section of the brain that links the nervous and endocrine systems in order to maintain homeostasis.
- Receives information from nerves and initiates endocrine responses
- Secretes neurochemicals (releasing factors) into a portal system which target the anterior pituitary gland
- Secretes hormones directly into blood via neurosecretory cells that extend into the posterior pituitary lobe
Pituitary gland
Receives instructions from the hypothalamus and consists of two lobes (anterior and posterior).
- Lies adjacent to the hypothalamus and is in direct contact due to a portal blood system
Anterior pituitary gland (adenohypophysis)
Releasing factors from the hypothalamus are released into portal vessels by neurosecretory cells, stimulating endocrine cells in the anterior pituitary to release specific hormones into the bloodstream.
- Example: releasing factor GnRH triggers the release of LH/FSH from the anterior pituitary
Posterior pituitary gland (neurohypophysis)
Releases hormones produced by the hypothalamus itself via neurosecretory cells.
- Neurosecretory cells extend into the posterior lobe from the hypothalamus and release hormones into the blood
Targets of pituitary hormones
Mnemonic device: MARGE
- Metabolism (ex. thyroxin）
- Adult development (ex. LH/FSH in triggering puberty)
- Reproduction (ex. LH/FSH control menstruation)
- Growth (ex. growth hormone)
- Equilibrium/homeostasis (ex. maintaining water balance through ADH)
Growth hormone (somatotropin)
An anabolic peptide hormone that stimulates growth.
- Acts directly to reduce the formation of adipose cells (less nutrients stored as fat)
- Acts indirectly via insulin growth factors (IGF) produced by the liver to increase muscle mass and bone size
Somatotropin as performance enhancers
- The use of human growth hormone is banned in sports, with proven cases of doping strictly punished
- Traditional urine testing could not detect doping, which historically made bans difficult to enforce
- Recent blood tests can now identify between natural and artificial variants of growth hormone
Lactation
The production and secretion of milk by maternal mammary glands following birth.
- Predominantly controlled and regulated by oxytocin and prolactin
Prolactin
Responsible for the development of mammary glands and the production of milk.
- Secreted by the anterior pituitary in response to the release of PRH (prolactin releasing hormone) from the hypothalamus
- Effects are inhibited by progesterone, which prevents milk production from occurring prior to birth
Oxytocin
Responsible for the release of milk from the mammary glands (milk ejection reflex).
- Produced in the hypothalamus and secreted by neurosecretory cells that extend into the posterior pituitary
- Triggered by the stimulation of sensory receptors in the breast tissue by the suckling infant
- Creation of a positive feedback look will result in continuous oxytocin secretion until infant stops feeding
Types of cell signalling
- Autocrine: when chemicals released by the cell stimulate the cell itself.
- Paracrine: when chemicals released stimulate a neighbor cell
- Endocrine: when chemicals released travel in the bloodstream to activate distant cells
Types of secretion
- Constitutive: hormones released as they are produced (with vesicles fused with the cell membrane)
- Regulated: hormones are stored in vesicles and released following an external signal
Peptide hormones
- Synthesized to require further processing to activate
- Stored in vesicles for regulatory secretion
- Mostly polar and water-soluble (can travel freely in blood)
- Binds to receptors on cell membrane and transduce signal via the use of second messenger systems
- Gene expressions can change
- Examples: insulin, glucagon, and prolactin
Steroid hormones
- Synthesized in a series of reactions from cholesterol
- Released immediately (constitutive secretion)
- Generally non-polar and require carrier proteins to travel in blood
- Binds to intracellular receptors to change gene expression directly
- Slower onset, longer duration in changes of gene expression
- Examples: cortisol, estrogen, progesterone, and testosterone
Amino acid derivatives (hormone)
- Synthesized from amino acid tyrosine
- Stored before release (but mechanism varies)
- Some (ex. adrenaline) are polar, others are protein-bound
- Acts on membrane or nuclear receptors
- Function like peptides or similar to steroids
- Examples: adrenaline and thyroxin
Negative feedback
Involves a response that is the reverse of the change detected (it functions to reduce the change).
- Examples: thermoregulation, blood sugar regulation, osmoregulation
Positive feedback
Involves a response that reinforces the change detected (it functions to amplify the change).
- Examples: childbirth, lactation, ovulation, blood clotting
Goiter
A condition whereby the individual develops an enlarged thyroid gland because of an iodine deficiency.
- Thyroxin contains iodine and cannot be produced if iodine is deficient in the diet
- Iodine is not a component of most diets (common in seafood but not others)
