Urology & renal - Sodium & potassium balance Flashcards
What is the effect of an increased dietary sodium level?
- Increased dietary sodium leads to an increased total body sodium
- This increases osmolarity but this cant happen due to semi-permeable membranes
- This results in an increased water intake and retention which leads to an increase extra-cellular fluid volume
- This causes an increased blood pressure and volume
What sends signals to regulate sodium intake?
Centrally - through the lateral parabrachial nucleus
Peripherally - taste, too salty = not nice, lack of salt = bland
Where is sodium absorbed in the nephron?
- 67-70% is absorbed in the proximal convoluted tubule
- 25% is absorbed at the thick ascending limb of the loop of henle
- the last bit is absorbed through the distal convoluted tubule and collecting duct
- Excrete <1%
What is the function of the macula densa?
- Specialised cells in the distal convoluted tubule
- They respond to high sodium levels by releasing adenosine
- This causes the extraglomerular mesangial cells to interact with the smooth muscle cells of the afferent arteriole to constrict and therefore reduce glomerular filtration rate
What occurs in response to a requirement for less sodium excretion
- Less sodium is filtered by vasodilating the efferent arteriole
- Sympathetic activity also causes dilation of the afferent arteriole and more absorption in the proximal covoluted tubule
- Angiotensin II also increases absorption at the PCT but also stimulates the adrenal glands to produce aldosterone
- Aldosterone increases the amount of absorption at both the distal convoluted tubule and the collecting duct
What is the function of aldosterone?
- Increased sodium reabsorption
- Increased potassium secretion
- Increased hydrogen ion secretion
How does aldosterone work?
- It is a steroid hormone
- It enters the cell and binds to the mineralocorticosteroid receptors
- This causes a release of the protein HSP 90
- The mineralocorticosteroid can then enter the nucleus and produce mRNA and associated proteins
What is Liddle’s syndrome?
- Mutation in the aldosterone activated sodium channel
- Channel is always ‘on’
- This results in sodium retention, leading to hypertension
- Symptoms of hypoaldesteronism with normal aldosterone levels
What is atrial natriuretic peptide and how does it work?
- Made in the atria
- Released in response to atrial stretch
- Vasodilation of renal (and other systemic) blood vessels
- Inhibition of sodium reabsorption in proximal tubule and in the collecting ducts
- Inhibits the release of renin and aldosterone
- Reduces BP
How does ACE inhibitors work as a diuretic?
- They prevent the conversion of angiotensin I to angiotensin II
- This leads to vasodilation
- This leads to a lower blood pressure
- They also reduce sodium uptake so less water is reabsorbed
What are osmotic diuretics?
- They are retained and not absorbed in the proximal tubule
- This results in a higher osmolarity
- This means less water is reabsorbed
How do carbonic anhydrase inhibitors work as a diuretic?
They reduce the production of protons so sodium cannot be reabsorbed when exchanging the protons and sodium.
This means that the osmolarity of the tubular fluid remains higher and less water is secreted
How do thiazide diuretics work?
- Reduce sodium reuptake and therefore reduce water reabsorption
What occurs to dietary potassium?
- It is absorbed into the blood
- This increases plasma potassium concentration
- The excess is then absorbed into tissue with the aid of insulin
- Insulin increases the amount of sodium in the cells through the sodium proton pump
- The excess sodium in the cells is then excreted through the sodium potassium pump which get rid of sodium and brings in potassium
What can lead to hypokalaemia?
- Inadequate dietary intake
- Diuretics
- Surreptitous vomiting
- Diarrhoea
- Genetics
