Regulation of Osmolarity Flashcards
Why is maintaining osmolarity so important to the body?
Osmolarity is the main determinant of cell volume
Water passes freely across most cell membranes so osmolarity gradients are what drives it in or out of cells
Ignoring homeostasis and all that stuff - what is the effect on cells…
a) as a result of excess water intake
b) as a result excess Sodium intake
a) Excess water would cause ECF osmolarity to DECREASE –> water moves into cells and they swell
b) Excess salt would cause ECF osmolarity to INCREASE –> such that cells would shrink as water moves out
What hormone controls water regulation?
Anti-diuretic hormone (ADH)
Otherwise known as - Vasopressin
Describe the structure of Vasopressin
Vasopressin made up of 9 amino acids - including Arginine (hence AVP)
Where is Vasopressin synthesized and released?
Synthesized in Supraoptic (SO) and Paraventricular (PVN) nuclei of the hypothalamus
Released in posterior pituitary into blood
What does Vasopressin secretion respond to changes in?
ie what controls it
Plasma osmolarity
Increase in plasma osmolarity causes increase in ADH release
Describe how a decrease in plasma osmolarity would affect ADH release
Decrease in plasma osmolarity causes
Decrease in rate of discharge of ADH secreting neurones in the SO, PVN in the Hypothalamus
Therefore decreased release of ADH in the posterior pituitary
What mediates the changes in neuronal rate of discharge in response to osmolarity changes?
Changes in osmolarity are detected by Osmoreceptors in the anterior hypothalamus
Other receptors in the Lateral hypothalamus mediate thirst
Summarise the response to increased and decreased osmolarity
heres the slide
What is the normal value for plasma osmolarity?
280-290mOsm/kg H2O
When talking about the response of ADH to osmolarity - they say ‘effective OP’ instead of plasma osmolarity
Why is this?
An increase in osmolarity that does not cause an increase in tonicity is ineffective in causing an INCREASE in [ADH]
Ie it responds to tonicity and not osmolarity
Solutes that can penetrate membranes move together with water and don’t produce any “osmotic drag” or tonicity
Explain why drinking sea water is highly beneficial for your health
Ingestion of Hypertonic solutions such as seawater increases the Solute load to be excreted
The larger the solute load - the more H2O is required to excrete it
Thus it stimulates Urine flow and actually dehydrates you
Hypertonic solutions require more water to excrete than is ingested with them –> dehydration
Where is the site of water collection?
What is the effect of ADH on it?
Site of water collection is the Collecting duct funnily enough
It’s permeability is under the control of Vasopressin (ADH)
Whether or not the dilute urine delivered to the distal tubule is concentrated and to what extent depends on the presence or absence of the posterior pituitary hormone, ADH
At a cellular level - how does ADH affect the permeability of the collecting duct to water?
ADH increases their permeability to H2O
It does this by stimulating them to incorporate aquaporins into their cell membranes on the lumenal surface of the duct
If ADH is present - what happens between the cortical collecting duct and cortical interstitium?
ADH present –> allows water to be reabsorbed from the CD
This means the cortical CD and cortical interstitium equilibrate
if MAXIMAL ADH is present - what happens between the Medullary Collecting duct and Medullary interstitium?
If Maximum ADH is present then the collecting duct contents equilibrate with that of the medullary interstitium via osmotic efflux of H2O
this means it becomes highly concentrated at the tip of the medulla
What is the effect of maximal ADH on produced urine?
What happens to the water extracted?
Maximal ADH produces highly concentrated Urine
This contains relatively less filtered H2O than under normal circumstances
Maximal ADH just extracts pure H2O from the collecting duct
H2O is reabsorbed by the oncotic P of vasa recta, which will be even greater then usual in the presence of the H2O deficit
What happens to the osmolarity of the collecting duct contents in the absence of ADH?
No H2O extracted from collecting duct
Lots of dilute urine (low osmolarity) is produced to get rid of the excess water (which accounts for the lack of ADH)
What is the role of urea in water conservation?
Urea is basically an aid to the effects of ADH
CD membrane is relatively permeable to Urea and ADH causes urea permeability to increase
In times of anti-diuresis –> ADH causes increased permeability so more Urea moves out of the Collecting ducts
This increases the interstitial gradients near the loops of Henle so more water is Reabsorbed
What is meant by Urea recycling?
In times where theres lots and lots of ADH - lots of Urea moves out of the collecting ducts into the interstitium (and thus increases reabsorption)
Urea is retained in order to save water and reinforce medullary gradient in region of thin ascending limb of LoH.
Uraemia occurs
Aside from plasma osmolarity - what else affects the levels of vasopressin release?
How does this alter its release?
ECF volume
Increased ECF volume ==> Decreased [ADH]
Decreased ECF volume ==> Increased [ADH]
Where are the receptors that measure ECF volume?
(ie that cause changes in vasopressin secretion in response to changes in ECF volume)
What do these actually measure?
There are stretch receptor afferents that exist in 2 types of area…
Low P areas (often called volume receptors):
- Left and right atria
- Great veins
High P areas:
- Carotid and Aortic arch baroreceptors
What is the effect of decreasing ECF on ADH secretion?
Decreased ECF causes increased ADH secretion
Moderate changes picked up by atrial receptors - which exert tonic inhibitory responses on ADH secretion through the Vagus nerve
A decrease causes decrease in inhibitory signals thus causing increased ADH secretion
Aside from plasma osmolarity and ECF volume - what minor factors increase ADH secretion?
Pain, emotion, Stress, exercise, nicotine, trauma/surgery
basically more sympathetic-ey stuff causes ADH increase
Aside from osmolarity and ecf - what minor factors contribute to lower ADH secretion?
Alcohol
Just think alcohol relaxes you - more parasympathetic-ey state
Heres a nice summary for you
xo
What is Diabetes Insipidus?
ADH deficiency
Caused by damage/disease to hypothalamic areas synthesizing ADH - ie due to tumours, or in meningitis(Central DI)
Or due to the collecting duct being insensitive to ADH which can be secondary to Hypercalcaemia or Hypokalaemia (peripheral DI)
What are the symptoms of Diabetes insipidus?
Polyuria
Polydipsia
