lecture 33 Flashcards
what is total body water? (TBW)
male - 60%
female - 55%
How much is extracellular fluid?
1/3 of TBW
how much is intracellular fluid?
2/3 of TBW
how much is plasma?
1/5 of ECF
how much is interstitial fluid?
4/5 of ECF
how is body water balanced?
- total body water remains relatively constant
- intake and loss of water must balance
- urine output is adjusted to maintain balance
what is osmolarity?
the total number of solute molecule in a solution
e.g. there is 145mmol/L of NaCl. After being put in a solution there will be 145mosmoll/L of Na+ and 145mosmol/L of Cl-. Total osmolarity will be 290mosmol/L
what happens when the amount of water in the ECF changes?
A change in the amount of water in the ECF changes the osmolarity
what is normal osmolarity? what type of solution is it?
normal osmolarity in the ICF and ECF is 275-300mosmol/L
this is an isosmotic solution
- same amount of solute molecules per L
what causes increased osmolarity? what type of solution is it?
a decrease in water leads to an increase in plasma osmolarity
this causes a hyperosmotic solution
- more solute molecules per L e.g. dehydration
waht causes decreased osmolarity? what type of solution is it?
Increase in water leads to decrease in osmolarity
this is a hypoosmotic solution
- less solute molecules per L e.g. hyperhydration
what happens when there is a loss of water?
Starts off as normal ICF and ECF volumes and osmolarity
then there is a loss of water
the water loss creates an osmotic gradient between the ICF and ECF. Water from the ICF will go to the ECF because water wants to go where osmolarity is higher
This results in loss of water in both the ECF and ICF so the cells shrink, but they are balanced
what happens where there is a gain of water?
starts off as normal ICF and ECF volumes and osmolarity
then there is a gain of water which results in decreased osmolarity
this water gain creates an osmotic gradient between the ECF and ICF.
this results in gain of water in both the ECF and ICF so the cells swell
what happens when there is a loss of isosmotic fluid?
Loss of isosmotic fluid (water and NaCl) results in the osmolarity of ICF and ECF being the same
this results in no osmotic gradient, no net water movement and a decrease in ECF volume only
what happens when there is a gain of isosmotic fluid?
gain of isosmotic fluid (water and NaCl) results in the osmolarity of ICF and ECF being the same
this results in no osmotic gradient, no net water movement and an increase in ECF volume only
where does water reabsorption occur in the nephron? how much absorption occurs in these places?
Proximal convulated tubule
- 67%
Descending limb
- 25%
Collecting duct
- 2-8%
what type of absorption occurs in the PCT and descending limb?
bulk water reabsorption
what are the characteristics of the bulk reabsorption that occurs in the PCT and descending limb?
- accounts for 92% of water reabsorption (67+25)
- not regulated (automatic)
- leaky epithelia
- trans and paracellular water reabsorption
what type of absorption occurs in the collecting duct?
regulated water reabsorption
what are the characteristics of water reabsorption in the collecting duct
- accounts for 2-8% of total water reabsorption
- regulated by ADH
- tight epithelia
- only transcellular reabsorption (no paracellular)
what places does sodium reabsorption occur in the nephron? how much sodium reabsorption occurs in each place?
proximal convoluted tubule
- 67%
ascending limb
- 25%
distal convoluted tubule
- 5%
collecting duct
- 2-3%
what type of sodium reabsorption occurs in the PCT and ascending limb?
bulk sodium reabsorption
- accounts for 92% of sodium reabsorption
what type of sodium reabsorption occurs in the DCT and collecting duct?
regulated sodium reabsorption
- accounts for 7-8% of total sodium reabsorption
- regulated by aldosterone (RAAS)
what is water reabsorption driven by? what is the process of reabsorption in the proximal tubule? explain all solutes involved.
- Water reabsorption in the proximal tubule is driven by Na+ reabsorption
- glucose and sodium are transported through the proximal tubule cells through a sodium-glucose cotransporter
- glucose is reabsorbed from the cell by diffusing through a glucose transporter
- sodium in then pumped out of the cell via the Na+/K+ ATPase in exchange for K+. The K+ then leaks out through the K+ channel
- because of the leaky epithelia, chloride follows potassium via the paracellular pathway
- water follows sodium and chloride by the paracellular pathway and through the transcellular pathway through aquaporins
what do the descending loop and ascending loop do in reabsorption?
- The descending loop has leaky epithelium for water absorption via aquaporins and the paracellular pathway
- the ascending loop reabsorbs Na+ into the interstitial fluid which generates a hyper-osmotic medullary gradient (HOMG)
- this means that the loop creates a higher osmolarity deeper into the medulla
how does the collecting duct carry out regualted water reabsorption?
- it has tight epithelia
- only has transcellular reabsorption
- regulated by ADH (more ADH = less urine)
how do the distal convoluted tubule and collecting duct carry out regulated sodium reabsorption?
- regulated by aldosterone (RAAS)
what happens when total body water decreases?
- total body water decreases
- this results in an increase in ECF osmolarity/sodium concentration
- this is detected by the osmoreceptors in the hypothalamus
- this increases the release of ADH from the posterior pituitary
- insertion of aquaporins in apical membrane of collecting duct cells leads to an increase of water permeability
- this results in an increase in water reabsorption and decrease in urine volume
- the ECF osmolarity returns to normal
what happens when total body water increases?
- total body water increases
- leads to decreases in ECF osmolarity/sodium concentration
- detected by osmoreceptors in the hypothalamus
- decreases the release of ADH by posterior pituitary
- no aquaporins which decreases water permeability
- decrease is water reabsorption and increase in urine volume
- ECF osmolarity returns to normal
how does HOMG relate to ADH?
ADH allows us to have a small volume of concentrated urine. this is because if we had no ADH, no water would be reabsorbed as we would produce large volume of dilute urine
what happens when there is a decrease in blood volume/loss of isosmotic fluid
- the decrease in blood volume/loss of isosmotic fluid is detected by pressure receptors in the kidney
- this leads to activation of RAAS which increases the release of aldosterone from the adrenal gland
- this leads to increase sodium channels in the apical membrane of collecting duct and DCT
- this leads to increased sodium and water reabsorption
- blood volume returns to normal
what happens when there is an increase in blood volume/isosmotic fluid?
- the increase in blood volume is detected by cardiac muscle cells
- this leads to release of ANP
- this leads to increase Na+ lost in the urine and increased water loss in the urine
- blood volume/isosmotic fluid volume is returned to normal
