concentrating mechanisms Flashcards
urine appearance
almost colourless to deep amber, yellow colour due to urochrome, from the breakdown of haemoglobin
- drugs, food stuffs can change this
urine odour
urine starts to smell when the urea in urine is broken down by bacteria broken down to ammonia
pH of urine
range: 4.5 - 8.2 , usually 6.0
Urine tends to be acidic because it is the main mechanism for excretion of protons
chemical composition
95% water water, 5% solutes
- urea, Na+, K+, Cl-, creatinine
osmolality of urine
ranges from 50 mOsm/kg-1 to 1200 mOsmkg-1
osmolality of plasma
despite changes in water intake, the osmolality of plasma appears to stay relatively constant (290-300 mOsm kg-1)
- tells us that the kidney must be able to regulate water loss in urine by: producing large amounts of dilute urine when water is in abundance
- and by concentrating urine when water is limited
what can the kidneys do to regulate water loss
- produce a hypo-osmotic urine - low con dilute
- produce a hyper-osmotic urine - high conc
- maximum conc. 1400mOsmol
- 5 x more concentrated than plasma
how does the kidney regulate this
- water reabsorption as fluid flows through the medullary collecting ducts
- interstitial fluid surrounding these collecting ducts in the medulla is extremely hyper osmotic and varies dramatically
- urea transport
increase of medullary osmolality in the renal system
cortex - 300mOsmol
corticomedullar - 300-600 mOsmol
medulla - 600-900 mOsmol
inner medulla - 900mOsmol
papillar/ pelvis - 1200 mOsmol
where is this increase of medullary osmolality created within the renal system
the loop of Henle
LOH
- descending limb
- thin ascending limb
- thick ascending limb
- osmolality similar to plasma - 300mOsmol
filtrate in LOH: no glucose, Na+, K+, and H20, Cl-, AA
method of transport in LOH
in descending limb:
- passive reabsorption water - aquaporin 1
- UREA= passive secretion in the thin descending limb
- secretion via UT- facilitated diffusion
- no movement of Na+/Cl-
-tubular fluid (filtrate) becomes more concentrated (less volume) as descend into the medulla
what happens in the descending limb of the loop of Henle
- urea transport is by passive diffusion. freely permeable across apical membrane via UT-A2 in thin descending limb
- water moves by osmosis out of the loop into interstital fluid
- increased concentration in the filtrate
- urea accumulates in the filtrate then it returns to the interstitial fluid in the renal medulla by reabsorption in the collecting duct
- this is critical in the kidney being able to concentrate urine
ascending limb
impermeable to water
- no movement of water
- no aquaporins
- active reabsorption of Na+/k+/Cl-
why does osmolality increase as descend into the medulla
ion transport in the LOH determines the rise in osmolality
COUNTERCURRENT MULTIPLIER
- active reabsorption of NaCl in ascending limb with no parallel movement of water
ion transport in the LOH
reabsorption of Na+/K+/Cl- ascending limb
- no parallel reabsorb of water
fluid of the medulla hyperosmotic to the fluid in the ascending limb
counter current multiplier
active transport of NaCl with no passive release of water
- passive secretion of urea
vasa recta
a parallel countercurrent multiplier exists in the vasa recta
- due to increased osmolality of ECF water in plasma leaves descending limb of vasa recta
- re enters ascending limb of the vasa recta
- solutes leave ascending limb and re - enters descending limb of vasa recta
maintain osmotic gradient
loop of Henle function
reabsorbs: water (descending limb)
urea secretion
Na+ and Cl- and K+ (ascending limb)
distal tubule function
reabsorbs: Na+, Cl-, bicarbonate, K+. h20
secretes: H+, k+
COLLECTING DUCT FUNCTION
reabsorbs: Na+, Cl-, H20, urea (K+)
secretes: H+NH4+(K+)
factors that would increase the effectiveness of the LOH to concentrate urine
- an increase in the length of the loop of Henle
- a reduction in the flow rate of filtrate through the loop
- alter protein in diet? increase UT transport
- increase in the number or capacity of the pumps (Na+/K+/2Cl-) - increase conc gradient
consequences of the counter current multiplier
the fluid leaving the ascending limb is hypo-osmotic compared to that entering it
- there is a continuous gradient of tissue fluid from 300m Osmol to 1200 mOsmol
- this gradient allows water reabsorption in the collecting ducts where permeability is high under the influence of hormones
what alters water permeability in the collecting duct
anti diuretic hormone alters water permeability in the collecting duct
antidiuretic hormone
- also known as arginine vasopressin
- 9 amino acid peptide
- released from the posterior pituitary gland
- rapid-acting with a short t1/2
- released in response to a rise in plasma osmolality
what is an antidiuretic hormone receptor released in response to
released in response to plasma osmolality
- primarily detected by osmoreceptors
- volume and pressure sensors detect a fall in blood volume (5-10%)
- stimulated by angiotensin 2
ADH
ADH present at very low concentrations when plasma osmolality less than 280 mOsmol
- threshold 280mOsmol - normal plasmal small amount circulating - 2 pg/ml
- osmolality above 290 mOsmol kg sharp increase in ADH secretion
- decreased blood volume -> increased ADH -> increased permeability of DCT and CD (aquaporins
what is ADH release stimulated by
- osmolality ECF detected by osmoreceptors in hypothalamus
- circulating blood volume detected by cardiovascular volume receptors
- arterial BP detected by cardiovascular baroreceptors
what can ADH also be stimulated by
pain
stress
nausea
what is ADH inhibited by
alcohol
over hydration
increased blood volume - dilute tubular fluid
- decreased body fluid osmolality
- decreased firing hypothalamic osmoreceptors (which detect increasing and decreasing osmolality)
- posterior pituitary decrease ADH secretion
- decreased plasma ADH
- decreased permeability of CD to water
- increased water excretion (large amount of dilute urine)
factors affecting medullary osmolality
- ADH secretion
- dietary protein (increased urea)
- increased medullary blood flow (increase BP)
Wash out NaCl + urea
decrease osmotic gradient
less conc urine
aging and dehydration
- associated with an inability to conc urine so risk of dehydration
- thought to be decreased function of nephrons
- shown to be associated with reduced AQP-2 expression in IMCD under ADH control
