Flashcards in Physiological Aspects of the Endocrine System Deck (46):
What is a hormone?
A chemical signal that enables an event in one part of the
body to have an effect elsewhere in the body.
What produces hormones?
May be produced from specialist organs – endocrine
glands – e.g. thyroid gland
May be produced from cells distributed around the body
– e.g. endothelial cells
How many hormones can an endocrine gland secrete?
Two or more different hormones
E.g. Thyroid Gland
T3 and T4- follicular cells
Calcitonin- parafollicular celks
What types of tissues secrete hormones
Nearly all tissues
Give an example of a hormone that is secreted at multiple sites
What are the 3 types of chemical signalling?
What does exocrine mean?
Outside the body
What is endocrine signalling?
Within the body, directly into the blood stream, transported to target cells
What is autocrine signalling?
Signalling molecule that actually has an effect on itself
Give an example of autocrine signalling
E.g. T cells, produce growth signals which have effect on themselves
What is paracrine signalling?
Signalling molecules that have an effect on cels around them
Give and example of paracrine signalling
Synapses affect other synapses
Some cells can have multiple modes of signalling, give an example of this
Has both endocrine and exocrine functions
Beta cells- produce insulin
Endocrine- has effect on multiple cells e.g. adipose, liver and skeletal muscle to enable uptake of glucose
Autocrine- when insulin is released it has some sort of subsequent release from its cells
What are target cells?
A particular hormone usually affects only a limited number of cells, these limited cells are target cells
Why is a cell a target?
Because it has a specific receptor for the hormone
What are the two types of hormone?
Proteins, peptides, amines
Steroids & Thyroid hormones
What is the structure of proteins peptides and amine hormones?
Hydrophilic (can't cross membrane)
Cell surface receptors, act via second messenger system
Circulate primarily dissolved in plasma
What is the structure of steroids & thyroid hormones
Hydrophobic (Can easily pass across membrane)
Intracellular receptors, promote or suppress gene transcription
Circulate primarily bound to plasma proteins
What are the sizes of polypeptides and protein hormones?
Size 3AA to 200AA
How are hormones initially assembled?
By pre pro ribosomes creating pro hormones
Where do pro hormones then go?
Golgi aparatus where it is packaged and put into vesicles where it can be stored
What happens to pro hormones in vesicles?
Can be cleaved to form active hormone
What are the steps of hormone synthesis?
What type of hormone is insulin?
What are the steps of the synthesis of insulin?
Insulin- released as hormone
C peptide also released
How are steroid hormones synthesised?
Synthesis of precursor
Synthesis of enzyme
These two join and are released
What two ways can steroid hormones be released?
Free binding proteins
Why can't steroid hormones be stored in vesicles?
As they are lipid soluble and can easily cross membranes
Why does it take longer for a steroid hormone to have effect?
It has to be synthesised first
What are steroid hormones based on?
Shape of ring and side chains vary
What is the mechanism of action of peptide hormones?
Peptide hormone binds to a receptor on cell surface membrane
Once in the cell a inactive secondary messenger is activated which iniates a protein kinase cascade
The peptide hormone then enters the nucleus where it acts as a transcription factor which switches on gene for a specific protein
What is the mechanism of action of steroids?
The steroid hormone enters the cytoplasm
Steroid hormone binds with steroid receptor to form a hormone-receptor complex
This then enters the nucleus
In the nucleus, the complex binds to receptor sites on chromatin activating mRNA transcription
What factors determine blood concentration of hormones?
The Rate of Secretion
Rates of Inactivation and Excretion
Whats are examples of the rate of secretion?
What are examples of rates of inactivation and excretion?
Peptide hormones – degraded in target tissues, have short
half-life in the blood (minutes to hours)
Steroid hormones – usually metabolised in the liver and
eliminated in the urine.
Longer half-life (hours to days)
What is the Feedback in Endocrine Systems
Stimulus – Fall in Blood
Controlled condition –
Blood Calcium Level
Receptors – Calcium
receptor on parathyroid cell
Control Centre –
Effectors – Bones, Kidneys
Response – increased
calcium release from bones,
and increased calcium
re-uptake in kidneys
What are multiple feedback loops?
More than one feedback loop
I.e. Long loop, short loop, ultra short loop
What is a short loop?
Less sensitive than long loop
What is an ultrashort loop?
E.g. hypothalamus feedbacks on itself
How can cells change their sensitivity to a hormone?
By changing the number of receptors
What is receptor down-regulation?
When a high extracellular concentration of the hormone is maintained for
some time, the total number of target cell receptors for that hormone may
What does receptor down-regulation reduce?
Target cell's responsiveness
What is an example of a receptor down-regulation?
Prolonged high concentration of insulin down regulates its receptors
What is receptor up-regulation?
When cells are exposed for a prolonged period to very low concentrations
of a hormone, they may develop more receptors and have increased
What is an example of a receptor up-regulation?
Prolactin increases the number of its receptors in the breast for