(uro) sodium potassium balance Flashcards
define osmolarity
a measure of the number of particles of a solute per litre of solution
= measured in osmoles/litre (osm/L)
what is one osmole?
used to describe when there is one mole of dissolved particles per litre of solution
(depends on the number of dissolved particles)
what is the osmolarity of a 1M NaCl solution?
(explain why)
1M = 1 mol/L
and 1 mol/L of NaCl = 2 Osm/L
as 1 mole of NaCl, will in solution, dissociate fully and become two separate particles and give 2 osmoles (1 mole of Na+, 1 mole of Cl-)
what is the expected molarity of 1 Osm/L NaCl solution?
NaCl
= in 1 mole of NaCl, there are 2 dissociated particles so 2 Osm/L
so if the osmolarity is 1 Osm/L, molarity must be 0.5 mol/L
differentiate between molarity and osmolarity
molarity = number of moles of solute per litre of solution (mol/L)
osmolarity = number of dissolved particles per litre of solution (osm/L)
what is the relationship between number of dissolved particles and osmolarity?
the greater the number of dissolved particles, the greater the osmolarity of the solution
our blood has a ‘constant osmolarity’
what is the significance of this statement?
water is the major component of our body fluids
= when the amount of salt changes, the amount of water also changes appropriately
(increase in salt = increase in water & vice versa)
what does an ECF osmolarity of 290mOsm/L mean?
for every change of 290 mosmols, the volume will changes by 1L as well
what is the normal plasma osmolarity?
285-295 mosmol/L
what is the most important solute in determining ECF volume?
sodium
(most prevalent, and most
what is the concentration of sodium in the plasma?
approx 140 mmol/L
= the more sodium you have in the plasme, the greater the ECF volume will be
how does increased dietary sodium affect body weight and why?
increased dietary sodium
= increased plasma sodium concentration
= increased subsequent water retention + more thirst (so more water drank)
= increased body weight
explain how an increase in dietary sodium affects blood pressure
increased dietary sodium
= increased plasma sodium concentration
= increased osmolarity (but this cannot happen as osmolarity is constant)
= compensatory increased water retention and increased water intake
= increased ECF volume
= increased blood pressure
explain how an decrease in dietary sodium affects blood pressure
decrease in dietary sodium
= decrease in plasma sodium concentration
= decreased osmolarity (but this cannot happen as osmolarity is constant)
= compensatory reduction in water intake and retention
= reduced ECF volume
= reduced blood pressure
when sodium levels are altered, why is the osmolarity not appropriately altered too?
the osmolarity remains constant always
= to ensure this, the only alterable feature, the water content, is changed accordingly
= so when sodium levels change, to keep osmolarity constant, water levels change
when sodium levels are altered, what happens to blood volume and blood pressure?
sodium levels increase/decrease
= blood volume increases/decreases subsequently to maintain osmolarity
= however, as there is a relatively FIXED blood volume, volume cannot change significantly
= so blood pressure increases/decreases accordingly
what are the two mechanisms by which sodium is regulated?
central = lateral parabrachial nucleus
peripheral = taste
describe the CENTRAL mechanism of sodium regulation
controlled by the lateral parabrachial nucleus
1) in euvolemic state
= cells that respond to glutamate and serotonin act to inhibit Na+ uptake
2) in the sodium deprived state = cells that respond to GABA and opioids act to increase Na+ uptake
describe the PERIPHERAL mechanism of sodium regulation
based on taste (bimodal)
1) in small amounts, salt enhances the taste of food = appetitive
2) at high concentrations, make food taste bad = aversive
how does the lateral parabrachial nucleus respond to euvolemia?
in the euvolemic state, inhibition of Na+ uptake is promoted by the neurotransmitters, glutamate & serotonin
how does the lateral parabrachial nucleus respond to sodium deprivation?
in the sodium deprived state, stimulation of Na+ uptake is promoted by the neurotransmitters, GABA and opioids
why is taste described as ‘bimodal’ when it comes to salt?
in low concentrations = enhances the taste of food (appetitive)
in high concentrations = makes food taste bad (aversive)
which neurotransmitter control the inhibition of sodium uptake?
serotonin and glutamate
which neurotransmitter control the stimulation of sodium uptake?
GABA and opioids
why do we want to maximise sodium retention?
we want to retain as much sodium as possible to maintain our ECF osmolarity
= as sodium is the most prevalent ion that contributes to ECF
where is sodium reabsorbed in the nephron and how much?
1) PCT = approx 67%
2) thick ascending limb = approx 25%
3) DCT = approx 5%
4) collecting ducts = approx 3%
so overall, less than 1% of sodium is excreted int he urine
how is sodium reabsorbed in the PCT?
approx 67% of filtered sodium is reabsorbed in the PCT
(same as the amount of water reabsorbed in this region)
= usually via a co/counter-transported ion mechanism, facilitating the reabsorption of other things such as glucose, amino acids or bicarbonate
how is sodium reabsorbed in the thick ascending limb?
approx 25% is reabsorbed
= via counter-current mechanisms OR the triple transporter (Na+/K+/Cl-)
how is sodium reabsorbed in the DCT?
approx 5%
= via the Na+/Cl- transporter of this region
how is sodium reabsorbed in the collecting ducts?
approx 3%
= via the Na+ channel ENACs
how much of the filtered sodium is actually excreted in the urine?
< 1% of the filtered sodium
= as the body wants to keep as much sodium within the blood as possible to maintain ECF osmolarity
how will changes to GFR affect sodium excretion?
if GFR goes up, more sodium is filtered into the filtrate and so total sodium excreted would go up (= 1% of a larger amount of sodium filtered)
= increased sodium excretion
= increased water loss
= reduce blood volume
what is the relationship between RPF and GFR?
approx 20% of the RPF is filtered to make up the GFR
so GFR = 0.2 x RPF
how much of the RPF is filtered?
approx 20% of the renal plasma enters the tubular system
how does RPF affect the GFR?
RPF is proportional to GFR so if the renal plasma flow increases, the GFR will also increase
(kidneys will filter 20% of a greater starting amount so, greater finishing - tubular - amount)
what is the RPF proportional to?
besides GFR, RPF is also proportional to blood pressure
so as blood pressure increases, so does RPF (and therefore so does GFR)
explain the relationship between blood pressure, RPF and GFR
over a significant range of blood pressures (up to 100 mmHg), RPF is proportional to blood pressure
= so as the blood pressure increases, the RFP (and therefore the GFR) also increase
how does the RPF-GFR relationship change during exercise and WHY?
during exercise, blood pressure increases
if the proportionality bw RPF and blood pressure was maintained beyond 100 mmHg, then during the high blood pressures of exercise
= the RPF would be so high that
= significant amounts of fluid + sodium would be lost
SO beyond 100mmHg, the proportionality is no longer maintained between RPF and blood pressure
why is RPF not proportional to blood pressure beyond 100 mmHg?
to prevent large amounts of fluid and sodium loss
(especially during exercise)
what impact does an increase in GFR have on the tubular filtrate?
increased GFR
= more RPF has been filtered so
= increased sodium + chlorine delivery to the distal nephrone
which renal structure is closely associated with the DCT and why is this important?
a specific part of the DCT (macula densa)
= is closely associated with the the renal glomerulus
= essential for regulating sodium excretion