Sodium and Potassium Balance Flashcards
Define osmolarity and give its units
Measure of the solute concentration in a solution (osmoles/liter)
What does osmolarity depend on?
The number of dissolved particles
As the number of dissolved particles increases what happens to osmolarity?
It also increases
What is normal plasma osmolarity?
285-295 mosmol/L
What is the most prevalent solute in the ECF?
Sodium
What is the osmolarity of sodium in the ECF?
140 mmol/L
When sodium levels are normal what happens to desire to intake sodium?
Its supressed
What structure suppresses or stimulates sodium intake?
Lateral parabrachial nucleus in the brainstem
What receptors are involved in suppressing sodium intake?
Serotonin and glutamate
What receptors are involved in stimulating sodium intake?
GABA and opioids
What are the 2 mechanisms involved in sodium intake? Briefly describe them
Central mechanism- involves lateral parabrachial nucleus
Peripheral mechanism- involves taste
How does the peripheral mechanism for sodium intake work?
When we have low levels of salt in our body, foods with salt will taste more appetising
Where does most sodium reabsorption occur?
How much of sodium is reabsorbed around the rest of the nephron?
PCT (67%) Thick ascending limb (25%) DCT (5%) CCD (3%) Excretion (<1%)
What happens to GFR and renal blood flow as mean arterial pressure (MAP) increases
GFR and renal blood flow will increase proportionally to MAP until a certain point but then they will plateau (don’t need to excrete any more Na)
As the BP and amount of sodium entering the nephron increases, what happens in the DCT to reduce perfusion pressure and GFR?
DCT is in contact with JGA-
- High sodium levels in tubular fluid are detected by specialised macula densa cells in the JGA
- This increases sodium/chloride uptake via triple transporter
- Adenosine is released from macula densa cells
- Detected by extraglomerular mesangial cells
- This promotes afferent SMC contraction
- Reduces perfusion pressure and so GFR
- Adenosine also reduces renin production- this is short term however so won’t affect long term renin
What affect does increased sympathetic activity have on the nephron
INCREASES Na REABSORPTION
- Increased contraction of the afferent arteriole to decrease GFR
- Stimulates sodium uptake by PCT cells
- Stimulates cells of the juxtaglomerular apparatus to produce renin, renin leads to angiotensin II production, this stimulates cells of the PCT to take up sodium
- Angiotensin II stimulates adrenal glands to produce aldosterone -> aldosterone stimulates sodium uptake in the DCT and collecting duct
How does anti naturietic peptide affect sodium levels?
DECREASES Na REABSORPTION
Vasodilator so increases GFR
Reduces sodium movement into the PCT, DCT AND CT
Inhibits JGA production of renin
How is sympathetic activity affected when blood pressure and fluid volume falls?
Increases B1 sympathetic activity - vasoconstriction afferent arteriole (dec. GFR)
Increases Renin production
Renin converts angiotensinogen to angiotensin I
ACE converts this to angiotensin II
This stimulates production of aldosterone (zona glomerulosa) and vasoconstriction of afferent arteriole (decreases GFR)
All this increases Na/H2O reabsorption
How is sympathetic activity affected when blood pressure and fluid pressure rises?
Decreases beta 1 sympathetic activity
Increases ANP
Opposite of sympathetic activation
What type of hormone is aldosterone?
Steroid
Where and where is aldosterone synthesised and released?
From the adrenal cortex (zona glomerulosa)
Synthesized in response to dec. BP via baroreceptors
How does angiotensin II specifically increase aldosterone synthesis?
Promotes synthesis of aldosterone synthase which carries out the final 2 steps in aldosterone synthesis
What does aldosterone stimulate?
Sodium reabsorption
Potassium secretion
What does aldosterone excess lead to?
Hypokalaemic alkalosis- Aldosterone causes increased potassium excretion- this leads to negative charge in lumen, causing H+ to move in, leading to alkalosis
How does aldosterone work?
Aldosterone is water soluble so travels through membrane
Binds to mineralocorticoid receptor
Aldosterone then binds to HSP90, which causes HSP90 to be removed
The mineralocorticoid receptor now dimerises so it can translocate into the nucleus
It binds to DNA and stimulates production of mRNAs for genes under its control
What genes does aldosterone help upregulate?
The gene for the epithelium sodium channel in the collecting duct (ENaC)
The sodium potassium ATPase
What occurs physiologically in hypoaldosteronism?
Reabsorption of sodium in the distal nephron is reduced
Increased urinary loss of sodium
ECF volume falls
Increased renin, Ang II and ADH
What are some symptoms of hypoaldosteronism?
Dizziness, low BP, salt craving, palpitation
What occurs physiologically in hyperaldosteronism?
Reabsorption of sodium in the distal nephron is increased
Reduced urinary loss of sodium
ECF volume increases (hypertension)
Reduced renin, ang II and ADH
What are some symptoms of hyperaldosteronism?
High blood pressure, muscle weakness, polyuria, thirst
What is Liddle’s syndrome?
Looks like hyperaldosteronism but normal or low aldosterone
An inherited disease of high blood pressure.
What occurs physiologically in Liddle’s syndrome?
Mutation in the aldosterone activated sodium channel.
Channel is always ‘on’- aldosterone suppression treatment will have no effect
Results in sodium retention, leading to hypertension
Where are baroreceptors that detect low BP located?
Atria
Right ventricle
Pulmonary vasculature
Where are baroreceptors that detect high BP located?
Carotid sinus
Aortic arch
Juxtaglomerular apparatus
What is the response of the low pressure side baroreceptors to high pressure?
Atrial strech
ANP and BNP are released