The Posterior Pituatory Gland Flashcards
Two main nuclei in the posterior pituitary
Paraventricular nucleus
Supraoptic nucleus
Two hormones produced in the hypothalamus
Vasopressin and oxytocin
Travel to terminals and are released from here
Vasopressin
Antidiuretic hormone – controls water secretion into urine
Primarily from supraoptic nuclei
Peptide hormone
Plasma binding protein
Life is 5-10 mins
Neurophysin II (nicotine stimulated)
Oxytocin
expression of milk from the glands of the breasts to the nipples; promotes onset of labour
Primarily from paraventricular nuclei
Peptide hormone
5-10 mins lifetime
Neurophysin I- oestrogen stimulated
Production, transport and secretion of vasopressin
On slide
Regulating water balance
Water is ingested so fluids in body are diluted, reducing plasma osmolality
This causes increase in cellular hydration
So body feels that it doesn’t need to drink anymore so thirst decreases
No need for vasopressin so this decreases
So water intake decreases
As vasopressin is not being secreted anymore, more fluids are gonna be secreted in the kidneys (increase in renal water excretion)
This is the action of ADH, vasopressin
Opposite in low water levels
Graph
As plasma osmolality increases, AVP goes up
As AVP goes up, more water is reabsorbed from kidneys and the urine osmolality is going to go up As all the water from urine is being removed, so concentrated urine produced
Where does vasopressin act?
Collecting tubule of the nephron
Vasopressin: mechanism of action
Vasopressin is secreted and travels to the collecting tubule where it binds to the membrane receptor
Receptor (V2) activates cAMP second messenger system
CAMP causes ACP2 water receptors to be produced
Cell inserts ACP2 water pores into apical membrane
Water is reabsorbed by osmosis into the blood
On slide
Vasopressin receptors
Table
Osmoreceptors and Baroreceptors regulate ADH release
Vasopressin reaches the kidney in two ways: osmoreceptors and baroreceptors
Osmoreceptors
Detect the change in osmolality
Found in the hypothalamus
Baroreceptors
These detect changes in pressure in the vascular system- eg when in shock
This also stimulates ADH release
Found in the carotid sinus or aortic arch
When you’re in shock AVP (vasopressin) is released so you retain as much fluid as possible, as don’t wanna lose anymore fluids when in shock
Slide 20
Interplay between AVP and salt and water physiology
If blood volume goes down, central venous pressure goes down
Atrial detects this which then influences how much Na and fluids are being excreted
ADH will then not be secreted
When blood vol goes down, ADH will be secreted
Dehydration
Slide 22
What is osmolality?
Concentration of particles per kilo of fluid
size of particle not important, number is important - i.e one molecule of larger protein albumin same effect as Na+
sodium, potassium, chloride, bicarbonate, urea and glucose present at high enough concentrations to affect osmolality
alcohol, methanol, polyethylene glycol or mannitol-
exogenous solutes that may affect osmolality
Plasma osmolality can be approx calculated
Slide 23
Osmotic control of AVP release
AVP is released at a threshold
As osmolality increases, AVP is released
Relationship of plasma AVP concentration and urine concentration
The more vasopressin is secreted, the more concentration of the urine osmolality because water is being absorbed from the kidneys
Same thing happens in correlation with thirst
Loss of relationship between plasma osmolality and vasopressin
Drinking rapidly suppresses vasopressin release and thirst.
In pregnancy osmotic threshold for VP release and thirst is decreased.
Plasma VP concentrations increase with age (also thirst blunting, decreased renal concentrating ability, decreased fluid intake).
