Flashcards in 1. Introduction To The Endocrine System Deck (45):
What is homeostasis?
-homeostatic mechanisms act to counteract changes in the internal environment
-variables are regulated so that internal conditions remain stable and relatively constant
-dynamic equilibrium and failure in homeostasis leads to disease
What are the characteristics of a control system?
2. Receptor - detect the stimuli
Communication. ---> afferents (nervous / endocrine)
3. Control centre - determine the set point, analyse input, respond
Communication ----> efferent (nervous/endocrine)
4. Effector - cause a change
What are biological rhythms?
- set point of control centre can vary
-circadian (or diurnal rhythm)
- biological clock, in brain, in a small group of neurones in suprachiasmatic nucleus
What cues from the environment keep the body on a 24hr cycle?
Cues from the environment (zeitgebers) keep the body on a 24 hr cycle. These include:
-eating and drinking patterns
Which hormone is responsible for setting the biological clock?
-the hormone melatonin
-released from the pineal gland
What is negative feedback?
Response in a way to reverse the direction of change. Most common form of feedback in physiological systems.
What is positive feedback?
Response in way so as to change the variable even more in the direction of change. Rare and there are few examples. It is used when rapid change is needed. Examples include blood clotting and ovulation.
What is the distribution of body water in humans?
A typical 70Kg man has 42 L of water:
- 2/3 (28L) intracellular
-1/3 (14L) extracellular - interstitial (11L) and plasma (3L)
-the 3L plasma combined with 2L RBC to give 5L blood
What is osmolarity and what is osmolality?
Osmolality: the number of osmoles per kg of solution (mass
Osmolarity: the number of osmoles per litre of solution (volume)
What is in a 1 Molar (1M) solution?
Contain 1 mole of substance in 1 Litre
What is an osmole?
The amount of substance that dissociates in solution to form one mole of osmotically active particles e.g. A 1mM solution of NaCl corresponds to an osmolarity of 2mOsmol/L ( 1 from Na+ ion and 1 from Cl - ion)
What is the reference range for hyponatraemia?
(Low Na+ in blood)
The reference range is 275-296 mOsmol/kg
What is meant by hypertonic?
- a solution that has a higher concentration of solutes than another solution is said to be hypertonic
-water molecules will tend to diffuse into a hyper tonic solution
-therefore cells which are in a hypertonic solution will shrink (water is going to leave the cell and go out into the hypertonic solution)
What is meant by hypotonic?
-a solution that has a lower concentration of solutes than another solution is said to be hypotonic
-water molecules will tend to diffuse out of a hypotonic solution
-cells in a hypotonic solution will tend to swell (water moving out of solution and into cell) with the risk of eventually bursting
What is meant by the term isotonic?
- 2 solutions that have the same concentration of solutes are said to be isotonic (equal tension)
What happens if there is high blood osmolality?
- this means that the body needs to conserve water
-detected by osmoreceptors in the hypothalamus
-stimulates thirst and drinking reduces osmolality
-posterior pituitary secrete ADH
-increased reasbsorbtion of water from the urine into blood in collecting ducts in the kidney
-return to normal
What happens when there is a low blood osmolality?
-body needs to excrete
-detected by osmoreceptors in hypothalamus
-posterior pituitary secretes less ADH
-decreased reabsorption of water from ruin einto blood in collecting ducts in kidneys
- large volume of dilute urine
What happens when plasma glucose falls?
- if not corrected patient can go into coma
-so when plasma glucose falls
-plasma releases glucagon (from the alpha cells)
-stimulates glycogenolysis in liver and glucose is released into the blood
-plasma glucose increases
What happens when plasma glucose increases?
- if too high can lead to problems with microvasculature
- so if plasma glucose increase then the beta cells in the pancreas release insulin
-insulin stimulates glycogenesis in liver (storing in liver)
-stimulates glucose uptake into tissues (GLUT4)
-plasma glucose declines
Where are GLUT 2 and GLUT 4 found?
GLUT 2: pancreatic beta cells, kidney, liver and small intestine
GLUT 4: insulin regulated adipose tissue and striated muscle
What are hormones?
Hormones are chemical signals produced in endocrine glands or tissues that travel in the bloodstream to cause an effect on other tissues.
What is autocrine communication?
When a hormone signal acts back on the cell of origin
What is meant by the term paracrine?
Hormone signal carried to adjacent cells over a short distance via interstitial fluid.
What is meant by endocrine?
Hormone signal is released into the bloodstream and carried to distant target cells.
What is meant by the term neurocrine?
What is meant by the term neurocrine?
Hormone originates in a neurone and after transport down axon is released into the bloodstream and carried to distant target cells.
What are peptide/polypeptide hormones?
-short chains of amino acids
-e.g. Insulin, glucagon, growth hormone
What are the amine hormones?
- synthesised from aromatic amino acids
-adrenal medulla hormones are water soluble
-thyroid hormones lipid soluble
-e.g. Adrenaline, thyroid hormone , melatonin
What are glycoproteins?
-large protein molecules
-carbohydrate side chains
-all water soluble
What are steroid hormones?
-derived from cholesterol
-steroidogenic tissues convert cholesterol to different hormones e.g. Cortisol, and testosterone
-all lipid soluble
In hormone transport which form is active?
Only the free form is active.
There is a dynamic equilibrium between bound and free forms of hormones in plasma.
What is the role of carrier proteins?
- increase solubility of hormone in plasma
-readily accessible reserve
What factors determine hormone levels in blood?
1. Rate of production
2. Rate of delivery
3. Rate of degradation
Where do hormones bind?
At a receptor! If there is no receptor for the hormone on the target cell there will be no response to the hormone
What happens when a hormone binds to a G protein coupled receptor?
- dissociation of the G protein alpha subunit
-activation of an effector protein e.g. Adenylyl cyclase
-formation of a second messenger e.g. CAMP
-activation of a protein kinase e.g. PKA
-phosphorylation of target proteins
What happens binding of a hormone to a tyrosine kinase receptor?
-dimerisation (except insulin receptor which is already dimerised)
-autophosphorylation of specific tyrosines
-recruitment of adaptor proteins and signalling complex
-activation of protein kinase e.g. PKB
-phosphorylation of target proteins
Where do lipid soluble hormones bind?
Lipid soluble hormones can diffuse across the plasma membrane.
1) the hormone binds to the cytoplasmic receptor and the hormone receptor complex enters the nucleus and binds to DNA
2) or hormone enters nucleus and binds to pre bound receptor on DNA e.g. Thyroid hormone. Binding relieves repression of gene repression.
Receptor binds to a specific DNA sequence called a hormone response element (HRE) in promoter region of specific genes. Expression of new protein mediates the effects of hormones.
What is obesity?
- results from a chronic imbalance between energy intake and energy expenditure. BMI > 30kg/m2
Where is appetite controlled from?
The appetite control (satiety) centre is found in the hypothalamus. The arcuate nucleus plays a central role in controlling appetite.
What are the 2 types of primary neurone in the arcuate nucleus?
- stimulatory neurones: contain NPY (neuropeptide Y) and AgRP (Agouti related peptide) which promote hunger
-inhibitory neurones: POMC (proopiomelanocortin) which yields several neurotransmitters including a-MSH and B-endorphin which promote satiety.
The primary neurones synapse with secondary neurones in other regions of the hypothalamus and the signals are integrated to alter feeding behaviour
What is ghrelin?
-released from the stomach wall when empty
-stimulates appetite (stimulates excitatory primary neurones in ARC nucleus)
-filling of stomach inhibits ghrelin release
What is PYY ?
-short peptide hormone released by cells in the ileum and colon in response to feeding
-suppress appetite (inhibiting the excitatory primary neurones of the ARC nucleus and stimulate the inhibitory neurones)
-blunted response following food intake in obese humans
What is leptin?
-leptin is a peptide hormone released into the blood by adipocytes
-suppression of appetite
-stimulates inhibitory neurones, and inhibits stim neurones in ARC
-leptin induces expression of uncoupling proteins in mitochondria and so energy is dissipated as heat
What is insulin?
-suppresses appetite by similar mechanisms as leptin
-but less important than leptin in this respect
What is amylin?
-secreted by beta cells in pancreas
-decreases glucagon secretion
-slows gastric emptying
-pramlintide is amylin analogue approved for type 2 diabetes