Water Balance Flashcards
On an average day we consume 20-25% more water and salt than we need.
This results in 3 reasons for urine production:
- Must get rid of the excess volume
- Must get rid of any excess water
–To keep osmolarity up
•Must get rid of any excess salt
–To stop osmolarity going too high
We operate in a constant osmolarity environment, thus the regulation of water and salt balance are inter-related in order to maintain osmotic homeostasis, i.e. if we increase the salt concentration, osmolarity increases but then water follows the salt so the osmolarity returns to the original level.
DO questions at end of lecture
Water balance is used to regulate …………….. osmolarity
The level of salt determines the ……… volume
Water balance is used to regulate plasma osmolarity
The level of salt determines the ECF volume
How do we get rid of water?
Control of water excretion
Within the tubular system, water removal occurs in all regions except the
Name the region
Approx. 125ml/min (180L/day) of water passes through the fenestrae of the glomerular capillaries
The bulk of the water is reabsorbed in the …….. (70%) – this is because most of the solute pumping occurs here, and water follows to maintain osmotic balance
The rest of the water reabsorption is variable, with approx. 30% occurring in the descending limb of the loop of Henle + 20% in the DCT – water output from the renal collecting duct is thus very variable (0.7-1.4ml/min or 1-2L/day)
Water reabsorption in the in the convoluted tubules is driven by ion pumping, therefore to account for the variation in water output through the kidney nephron, consider the ………………………… + ……………………
t In order to maintain plasma osmolarity, urine must be concentrated above the normal plasma osmolarity – this is done by producing a region of hyperosmotic interstitial fluid (from the cortex to the inner medulla, the osmolarity of the interstitial fluid increases from 290-1200mosmol/l)
Control of water excretion
Within the tubular system, water removal occurs in all regions except the ascending limb of the loop of Henle
Approx. 125ml/min (180L/day) of water passes through the fenestrae of the glomerular capillaries
The bulk of the water is reabsorbed in the PCT (70%) – this is because most of the solute pumping occurs here, and water follows to maintain osmotic balance
The rest of the water reabsorption is variable, with approx. 30% occurring in the descending limb of the loop of Henle + 20% in the DCT – water output from the renal collecting duct is thus very variable (0.7-1.4ml/min or 1-2L/day)
Water reabsorption in the in the convoluted tubules is driven by ion pumping, therefore to account for the variation in water output through the kidney nephron, consider the loop of Henle + renal collecting duct
t In order to maintain plasma osmolarity, urine must be concentrated above the normal plasma osmolarity – this is done by producing a region of hyperosmotic interstitial fluid (from the cortex to the inner medulla, the osmolarity of the interstitial fluid increases from 290-1200mosmol/l)
The loop of Henle
The thin descending limb is permeable to water, and the cells are involved in a lot of water reabsorption (large brush border with few mitochondria)
The thick ascending limb is permeable to salt but not water, and the cells are involved in a lot of ion pumping (tight junctions with many mitochondria)
As tubular fluid flows through the ascending limb, it generates a salt gradient of approx. 200mosmol/l and interstitial fluid of ~400mosmol/l
To equilibriate that, water is reabsorbed from the descending limb, decreasing the interstitial fluid osmolarity restoring the equilibrium
However at the bottom of the loop + the beginning of the ascending limb, more salt is pumped out generating a further difference of ~200mosmol/l – thus the tubular fluid leaving the loop of Henle is hypoosmolar (the water reabsorption in the descending limb is not sufficient to fully equilibriate the IF conc)
The collecting duct is then permeable to water under presence of ADH, therefore as the collecting duct transverses the renal medulla urine is concentrated as water moves out of the tubular fluid to maintain the IF osmolarity
ONLY PUMP 200MMOL IN THE IMAGE BELOW
What parts of a nephron are permeable to urea
All the tubular cells require oxygen + nutrients in order to maintain function.
However an ordinary blood supply would result in a loss of the hyperosmotic gradient as excess salt would move into the capillaries
how is this prevented
Renal medullary blood flow
All the tubular cells require oxygen + nutrients in order to maintain function.
However an ordinary blood supply would result in a loss of the hyperosmotic gradient as excess salt would move into the capillaries
This gives a need for a specialised blood supply – the vasa recta
The vasa recta is a looped blood system which follows the tubules, but is permeable to water and solutes
This means that in the descending limb, water diffuses out/solutes diffuse into the limb from the vasa recta. But in the ascending limb the reverse happens; therefore the concentration gradient is maintained on leaving the loop of Henle
Vasopressin
Also known as antidiuretic hormone (ADH)
Peptide hormone (9 aa long) – synthesis in hypothalamus, packaged into granules then secreted from neurohypophysis (PPG)
Binds to specific receptors on basolateral membrane of the ……………. cells
This causes insertion of aquaporins into the cells luminal membrane, Aquaporins are an active water uptake system which are stored in vesicles unless under the influence of ADH (therefore water uptake in the tubule is regulated). This increases the permeability of the duct to water.
ADH also stimulates …….. transport from the inner medullary collecting duct into the thin ascending limb of loop of Henle
Vasopressin
Also known as antidiuretic hormone (ADH)
Peptide hormone (9 aa long) – synthesis in hypothalamus, packaged into granules then secreted from neurohypophysis (PPG)
Binds to specific receptors on basolateral membrane of the principal cells
This causes insertion of aquaporins into the cells luminal membrane, Aquaporins are an active water uptake system which are stored in vesicles unless under the influence of ADH (therefore water uptake in the tubule is regulated). This increases the permeability of the duct to water.
ADH also stimulates urea transport from the inner medullary collecting duct into the thin ascending limb of loop of Henle
What triggers ADH release?
What triggers ADH release?
Release is regulated by hypothalamic osmoreceptors which respond to an increase in plasma osmolarity >300Mos
Baroreceptors also act as a secondary signal, stimulated by a fall in BP/blood volume
Ethanol inhibits ADH increase in urine volume + dehydration
Summary
- Loop of Henle establishes concentration gradient of Na and Cl in medulla.
- ADH controls permeability of Collecting duct to water (due to high osmolarity in medulla, water will in presence of ADH, by osmosis, move into medulla and into peritubular veins).
- ADH released by posterior pituitary.
- ADH keeps ECF osmolarity in tight range by controlling water reabsorption (but does not determine ECF volume).
Diabetes insipidus
Disorder of water balance, caused by: o No/insufficient ADH production o Mutant ADH receptor no detection o Mutant aquaporin no response to ADH binding Results in polyuria (>30l/day) + polydipsia (unremitting thirst)
