Sodium & potassium balance Flashcards
(31 cards)
What is the normal osmolarity of plasma?
285-295 mosmol/L
Which part of the brain controls central regulation of sodium intake?
Which neurotransmitters increase appetite for Na+?
Which neurotransmitters decreases appetite for Na+?
Lateral parabrachial nucleus
Increase: GABA, opioids
Decrease: serotonin, glutamate
What does the peripheral regulation of sodium intake?
Taste buds - salty foods become more appetitive/aversive
Where is the main site of sodium reabsorption in the nephron? What %
Proximal convoluted tubule
67%
What is the equation for GFR, incorporating renal plasma flow rate?
GFR = RPF x 0.2
~20% of renal plasma enters tubular system
What % of sodium is reabsorbed in:
Thin descending limb
Thick ascending limb
DCT
Collecting duct
Desc: 0
Asc: 25%
DCT: 5%
CD: 3%
How does increased tubular sodium affect GFR (tubuloglomerular feedback)? (5 steps)
- high tubular sodium detected by macula densa cells
- Increased sodium/chloride uptake via triple transporter
- adenosine released from macula densa cells
- adenosine detected by extraglomerular mesangial cells & causes contraction of smooth muscle cells
- reduces renin production, renal blood flow and GFR
What 3 methods are used to increase sodium retention?
- increased sympathetic activity
- angiotensin II
- aldosterone
How does increased sympathetic activity affect sodium retention?
Increases sodium retention
1. constricts SMCs of afferent arteriole
2. stimulates reabsorption of Na+ at PCT
3. stimulates renin production from JGA cells
How does angiotensin II affect sodium retention?
Increases sodium retention
1. stimulates Na+ uptake in PCT
2. increases aldosterone production
3. causes SMC contraction
How does aldosterone affect sodium retention? Which parts of the nephron does it act on?
Increases sodium retention
1. increases synthesis of epithelial sodium channels (ENaCs)
2. increases synthesis of Na+/K+ ATPase channels
Mineralocorticoid receptors in DCT and collecting duct
How does atrial natriuretic peptide affect sodium retention?
Promotes sodium excretion via vasodilation
Where is aldosterone synthesised?
What is it released in response to?
Zona glomerulosa of adrenal glands
Angiotensin II and low blood pressure
What are the functions of aldosterone?
What can excess aldosterone lead to?
- increased Na+ reabsorption
- decreased K+ reabsorption
- decreased H+ reabsorption
Conn’s - hypokalaemic alkalosis
How does aldosterone act within the cell?
- steroid hormone; passes through CSM
- binds to mineralocorticoid receptor
- Hsp90 chaperone protein removed
- aldosterone-MR complex moves into nucleus
- increases transcription of ENaCs and Na/K+ ATPase
What is Liddle’s syndrome?
Genetic condition of high BP due to mutation in ENaCs
Channel always open to Na+ regardless of aldosterone levels
Increased sodium retention, leading to hypertension
Where are the baroreceptors found in the body on the low pressure side?
Atria
Right ventricle
Pulmonary vasculature
Where are the baroreceptors on the high pressure side found in the body?
Carotid sinus
Aortic arch
Juxtaglomerular apparatus
How do baroreceptors on the low pressure side respond to low pressure?
Reduced baroreceptor firing
Signal through afferent fibres to brainstem
Increase sympathetic activity
ADH released, greater H2O retention
How do baroreceptors on the low pressure side respond to high pressure?
Atrial stretch
ANP and BNP released
How do baroreceptors on the high pressure side respond to low pressure?
Reduced baroreceptor firing
Signals to afferent fibres to brainstem & JGA cells
Sympathetic activity + ADH
Renin released from JGA
Where is atrial natriuretic peptide made?
What is it released in response to?
What are its actions?
Atria
Atrial stretch
Actions:
Vasodilation
Inhibition of sodium reabsorption in PCT and CD
Inhibits release of renin and aldosterone
Reduces BP
How do osmotic diuretics work?
Where do they act?
Substance that does not get reabsorbed, increasing osmolarity of filtrate and reducing water reabsorption out of lumen
PCT, thin descending limb
How do carbonic anhydrase inhibitors work?
Where do they act?
Blocks carbonic anhydrase
Reduces H+ production
Reduces activity of Na+/H+ antiporter
Less Na+ reabsorbed
Less H2O reabsorbed
PCT
