Regulation of Osmolarity

Question 1

Why is maintaining osmolarity so important to the body?

Answer 1

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

Question 2

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

Answer 2

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

Question 3

What hormone controls water regulation?

Answer 3

Anti-diuretic hormone (ADH)

Otherwise known as - Vasopressin

Question 4

Describe the structure of Vasopressin

Answer 4

Vasopressin made up of 9 amino acids - including Arginine (hence AVP)

Question 5

Where is Vasopressin synthesized and released?

Answer 5

Synthesized in Supraoptic (SO) and Paraventricular (PVN) nuclei of the hypothalamus

Released in posterior pituitary into blood

Question 6

What does Vasopressin secretion respond to changes in?

ie what controls it

Answer 6

Plasma osmolarity

Increase in plasma osmolarity causes increase in ADH release

Question 7

Describe how a decrease in plasma osmolarity would affect ADH release

Answer 7

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

Question 8

What mediates the changes in neuronal rate of discharge in response to osmolarity changes?

Answer 8

Changes in osmolarity are detected by Osmoreceptors in the anterior hypothalamus

Other receptors in the Lateral hypothalamus mediate thirst

Question 9

Summarise the response to increased and decreased osmolarity

Answer 9

heres the slide

Question 10

What is the normal value for plasma osmolarity?

Answer 10

280-290mOsm/kg H2O

Question 11

When talking about the response of ADH to osmolarity - they say ‘effective OP’ instead of plasma osmolarity

Why is this?

Answer 11

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

Question 12

Explain why drinking sea water is highly beneficial for your health

Answer 12

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

Question 13

Where is the site of water collection?

What is the effect of ADH on it?

Answer 13

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

Question 14

At a cellular level - how does ADH affect the permeability of the collecting duct to water?

Answer 14

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

Question 15

If ADH is present - what happens between the cortical collecting duct and cortical interstitium?

Answer 15

ADH present –> allows water to be reabsorbed from the CD

This means the cortical CD and cortical interstitium equilibrate

Question 16

if MAXIMAL ADH is present - what happens between the Medullary Collecting duct and Medullary interstitium?

Answer 16

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

Question 17

What is the effect of maximal ADH on produced urine?

What happens to the water extracted?

Answer 17

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

Question 18

What happens to the osmolarity of the collecting duct contents in the absence of ADH?

Answer 18

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)

Question 19

What is the role of urea in water conservation?

Answer 19

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

Question 20

What is meant by Urea recycling?

Answer 20

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

Question 21

Aside from plasma osmolarity - what else affects the levels of vasopressin release?

How does this alter its release?

Answer 21

ECF volume

Increased ECF volume ==> Decreased [ADH]

Decreased ECF volume ==> Increased [ADH]

Question 22

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?

Answer 22

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

Question 23

What is the effect of decreasing ECF on ADH secretion?

Answer 23

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

Question 24

Aside from plasma osmolarity and ECF volume - what minor factors increase ADH secretion?

Answer 24

Pain, emotion, Stress, exercise, nicotine, trauma/surgery

basically more sympathetic-ey stuff causes ADH increase

Question 25

Aside from osmolarity and ecf - what minor factors contribute to lower ADH secretion?

Answer 25

Alcohol

Just think alcohol relaxes you - more parasympathetic-ey state

Question 26

Heres a nice summary for you

Answer 26

xo

Question 27

What is Diabetes Insipidus?

Answer 27

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)

Question 28

What are the symptoms of Diabetes insipidus?

Answer 28

Polyuria

Polydipsia