Urinary system


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Control of water balance

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Osmolarity- Measure of the solute conc in a solution (osmoles/litre) 1osmole=



A variable osmolarity system means that we would be able to change our osmolarity. However we have constant osmolarity so increase in salt= increase in water

We consume around

more salt and water than we need to replace that lost

Must get rid of excess:

1. Volume- or else you will become oedematous and BP will

2. Water- or else you will dilute the salt in your body which will cause cells to swell

3. Salt- or you will have too high a salt level and cells will shrink

Sodium is most prevalent solute in plasma and ECF which means it's the one that has to be regulated

Water is used to regulate

Level of salt determines

Methods of water removal:

1. Skin and sweat- variable but uncontrollable- 450ml a day

2.Faeces- Uncontrollable - 100ml a day

3.Respiration- Uncontrollable 350ml/day

4. Urine- variable and controllable 1500ml/day

The minimum osmolarity of urine in humans is

The maximum osmolarity is

Water is reabsorbed throughout the kidney so all components are involved. 180L/day and 125ml/min

is absorbed before the loop of Henle


is absorbed in the loop of Henle before collecting duct

Then very variable how much is absorbed in collecting duct

Loop of Henle and concentration establishment:

It can't pump water so must have gradient.

Establishing of gradient-

countercurrent multiplication system- High salt at bottom so we can have high water higher up

Descending loop of Henle is

to water

Thick ascending limb of loop of Henle is

to water but can actively pump salt outside

Urea permeability in the

of LOH and collecting duct

At first, we have plasma osmolarity fluid of

all throughout the LOH. Then salt is pumped out of the

limb to generate a 200osmolar osmotic gradient

Then fluid leaves the descending limb of LOH to establish an osmolar equilibrium between interstitial fluid and descending LOH. As a result the descending limb then becomes

After the first cycle, descending is hypertonic and ascending is hypotonic

Next important event in the cycle is that fluid enters and pushes rest of fluid around the LOH. Fluid that enters is at

and therefore more hypotonic than the fluid that is just below it- there is a gradient in the LOH and this is used to generate a gradient in the interstitial fluid

Top of ascending limb- Fluid pumps out

to generate an osmolar gradient of 200 osmoses. However it is already more

than higher up, more sodium has to be exported to generate the 200osm gradient which means we have a discrepancy between osmolarity at top and bottom of interstitial fluid

Overall osmotic gradient is established and fluid becomes

as it moves around the loop of Henle

6. Water from descending limb then equilibrates with interstitial fluid. As a result fluid at top of ascending limb is now even more hypotonic as salt has been

Permeability to urea:

Permeable to urea at

of loop of Henle and bottom of cortical

Urea concentration very high at bottom of collecting duct because we are pumping water out of collecting duct as we gradually enter more hypertonic inner medulla

Urea then leaves and enters interstitial fluid and then into the bottom of thin


Then urea will cycle around the system until you reach an equilibrium which will

the osmolarity at the bottom of the medullary system

Effect of Vasa Recta:

Regulating permeability of tubular system to water is very important in controlling urine conc

If we pumped blood through the tubular system, we would eradicate the gradient we had built up as the salt would go into the

We get around this by having a blood supply completely

to salt and water that loops with the loop of Henle

Water diffuses out of the descending limb and solutes diffuse into descending limb, In the ascending limb the reverse happens

Because there is high osmolarity in the

it means that water would tend to leave the collecting duct if it was 100% permeable

Conversely if there was a really low osmolarity, it would likely mean that water would enter the collecting duct

Therefore conc of fluid that passes out of collecting duct- conc of urine- is very dependent upon there being a high medullary interstitium osmolarity

We can regulate urine production by altering permeability of


Aquaporin 2 molecules allow water passage

the collecting duct

Aquaporin 3/4 allow water passage

the collecting duct

These molecules synthesis are influenced by

If plasma osmolarity goes above

it triggers the release of ADH. This would be detected by

Also stimulated by a marked fall in blood volume or pressure. ADH release in inhibited by

which leads to dehydration as urine volume increases

Action of Vasopressin:

1. Blood-borne vasopressin binds with its receptor sites on

membrane of a principal cell in distal or collecting tubule

2. This binding activates

secondary messenger system within the cell

3. Cyclic AMP increases the opposite luminal membrane's permeability to H2O by promoting insertion of

water channels into the membrane. In the the absence of Vasopressin, this membrane is

4. Water enter the tubular cell from the tubular lumen through the inserted water channels

5. Water exits cell through a different water channel permanently positioned at basolateral border and then enters the blood and is reabsorbed

Overconsumption of water:

Decreased plasma osmolarity is detected by

Causes decrease in ADH secretion

Collecting duct permeability

and urne flow rate increase will cause

in plasma osmolarity


Increased plasma osmolarity detected by hypothalamic osmoreceptors. Causes

in ADH secretion and thirst receptors blah de blah

Disorders of water balance:

No/insufficient production of ADH

No detection of ADH

No response to ADH


- Unremitting thirst, excretion of large amounts of watery urine