Control of water and electrolytes Flashcards
How is water gained by the body?
ingested (food and drink) and formed in metabolism
How is water lost by the body?
Excretion (urine and faeces) and evaporation (sweat, expired air)
What is the average volume of water gained and lost per day?
gain ~2400ml/day and lose ~2400ml/day (balances)
What is the only aspect of water balance under homeostatic control?
Urinary excretion
What are the key components of a nephron?
glomerulus, proximal convoluted tubule, loop of Henle, distal convoluted tubule, collecting duct
Why is it important to understand renal physiology?
- salivary secretion is affected by water control
- some drugs are excreted in kidneys
- some drugs affect kidney function
- certain drugs are based on a kidney hormone (e.g. ADH in place of adrenaline)
What is the average renal blood flow?
1200ml/min
Function of glomerulus
filtration of plasma
What is the average glomerular filtration rate (GFR)?
120ml/min (10% of renal blood flow)
What drives the GFR?
glomerular hydrostatic pressure
What counteracts GFR?
Glomerular oncotic pressure and capsule pressure (HP in Bowman’s capsule)
What is the net glomerular filtration pressure?
10-15mmHg
What systemic factor may affect GFR?
Systemic blood pressure
Function of proximal convoluted tubule
resorption of 60-70% of the glomerular filtrate
What is resorbed and secreted at the proximal convoluted tubule?
ions and small organic molecules (e.g. glucose) are resorbed while H+ is secreted (acid-base balance)
How are substances resorbed/secreted in the PCT?
active transport or facilitated diffusion
Why is controlled H+ secretion in the PCT and DCT important?
to prevent metabolic acidosis or alkalosis
Function of loop of Henle
concentrating urine (and creating low water potential for water resorption in collecting ducts)
What is the name of the process by which urine is concentrated in the loop of Henle?
Counter current exchange mechanism
What does the ability of the loop of Henle to concentrate urine depend on?
active transport pumps in the thick, ascending limb and the length of the loop
How much fluid is reabsorbed in the loop of Henle?
10% GFR
What is resorbed in the distal convoluted tubule?
water, Na+, Cl-, Ca2+
What is secreted in the DCT?
H+ and K+
How is the activity of the DCT controlled?
hormones: aldosterone, atrial natriuretic hormone, ADH (distal parts of DCT), parathyroid hormone
Where is the greatest level of control over resorption and secretion in the nephron?
DCT (due to hormonal action)
Function of collecting ducts
water reabsorption
Which hormone stimulates water reabsorption in the collecting ducts?
Antidiuretic hormone (ADH)
How is water reabsorbed in the collecting ducts?
ADH induces the insertion of aquaporins in the luminal membrane of the collecting duct. Water then moves out of the collecting duct along the osmotic gradient created by the counter current exchange mechanism.
Which hormones regulate water and electrolytes?
ADH, aldosterone (renin-angiotensin-aldosterone mechanism) and atrial natriuretic hormone (ANH / Factor / Protein)
Which is the main hormone that regulates water?
Antidiuretic hormone (ADH)
What is ADH also known as?
vasopressin
When may vasopressin be used as a drug in dentistry?
In local anaesthetics in place of adrenaline
Where is ADH produced and released from?
Produced in hypothalamus and released from posterior pituitary gland (neurohypophysis)
Where does ADH act on in the nephron?
distal end of DCT and collecting ducts
Function of ADH
increases water permeability in DCT and collecting ducts
How does ADH increase water permeability of the DCT and collecting duct?
by inserting aquaporins which allows water to move passively down the osmotic gradient between the tubule lumen and interstitial fluid
What are the stimuli that trigger ADH secretion?
decreased plasma volume (ECF) / blood pressure or increased plasma (ECF) osmolarity
Which receptor detects decreased plasma volume and sends a signal to the hypothalamus?
baroreceptor (decreased distension)
Which receptor detects increased plasma osmolarity?
osmoreceptors in hypothalamus (increased activation)
Apart from increasing water permeability of collecting ducts, what is another effect of ADH?
Vasoconstriction (vaso-press-in)
By how much (%) can blood pressure decrease before ADH secretion is increased?
10% (small changes does not change anything)
What type of fluid does not have a great impact on urine flow?
isotonic NaCl (different response to 1L water)
What are electrolytes?
Na+, K+, H+, Ca2+ ions present in ICF and ECF
How is electrolyte concentration controlled?
homeostatic control operated by how much is excreted by the kidneys
What is the name of the system that controls Na+ concentration?
Renin-angiotensin-aldosterone system
Effect of the renin-angiotensin-aldosterone system
promotes reabsorption of Na+ and water in the DCT (in exchange for K+ and H+)
When Na+ is reabsorbed in the DCT, what is it exchanged for?
H+ and K+ (excreted)
What impact does angiotensin have on blood vessels?
Angiotensin is a potent vasoconstrictor
Which hormone has the opposite effect to aldosterone?
atrial natriuretic hormone (ANH)
Function of atrial natriuretic hormone (ANH)
Increases excretion of Na+ (and water)
Where is atrial natriuretic hormone released from?
stretched atria (in response to increased BP)
What is the stimulus that triggers the renin-angiotensin-aldosterone system?
decreased Na+ and decreased BP/plasma volume
Which element of the nephron detects the stimulus and releases renin?
Juxta-glomerular apparatus
Function of renin
cleaves angiotensinogen (precursor protein) to form angiotensin I
How is angiotensin I converted into angiotensin II?
Angiotensin converting enzyme (ACE)
What is the effect of angiotensin II in the renin-angiotensin-aldosterone system?
angiotensin II binds to the adrenal cortex and stimulates the release of aldosterone
What is atrial natriuretic hormone released in response to?
increased plasma volume which increases atrial distension
What are the effects of Atrial Natriuretic Hormone?
Increase GFR and decrease Na+ reabsorption. This increases Na+ excretion.
Why is K+ regulation important?
small changes in ECF K+ concentration can have significant effects on nerve and muscle function
Which hormone regulates K+?
Aldosterone (when Na+ is reabsorbed it is swapped for K+ and H+)
Which disease is associated with hyposecretion of ADH?
Diabetes insipidus (large volume of tasteless urine)
What is the name of the disorder associated with hypersecretion of ADH?
Syndrome of Inappropriate ADH (SIADH)
What is the effect of hypersecretion of ADH (e.g. SIADH)?
water retention
What is the effect of increased [K+] in the ECF?
Will lead to depolarisation and axons will fire AP. MP does not return to resting and instead there is an extended refractory period.
What can be used to desensitise ‘hypersensitive dentine’?
K+ salts in toothpaste
How does K+ reduce hypersensitive dentine?
K+ ions diffuse along tubules and raise concentration at inner end of tubule. Causes nerve depolarisation, inactivating Na+ channels, prolonged refractory period.
What is the issue with K+ in toothpaste?
Desensitising effect (K+ build up) does not persist, only lasts for brushing period
What is the benefit of using K+ toothpaste?
Reduction in sensitivity allows for brushing which stimulates production of intratubular dentine, closing the tubules. This reduces sensitivity
What is the effect of reduced plasma volume on sympathetic nerve activity?
Increases sympathetic nerve activity
What is the effect of increased sympathetic nerve activity on the renal system?
Decreases renal blood flow which therefore decreases GFR. This reduces the stretch of juxta-glomerular baroreceptors and reduces Na+ concentration in tubular fluid. This triggers the renin secretion from the juxta-glomerular apparatus. (Na+ reabsorption increases, leading to increased plasma volume)
