37- Urine Concentration and Dilution

urea is transported to all segments of a nephron except

• TAL

• DT

• cortical and outer medullary CD

how is urea absorbed and secreted

simple and facilitated diffusion

if conc diff b/t tubular fluid and blood is high…

reabsorption of urea is high

is urea freely filtered

yes

___% of urea is reabsorbed by PT

50%

what occurs in thin DL for urea

its secreted into tubular fluid from interstitial space

if ADH is low or absent what happens to urea

its reabsorption dec

describe urea in the inner medullary CD

ADH activates UT1 and urea reabsorbed into interstitial space 

urea recycling mechanism is enhanced by what

presence of ADH

describe the 4 steps in mechanism of urea recycling

1. ADH inc h20 reabsorption in late DT and early CD- no urea reabsorption in these locations

2. urea concentration in TF increases

3. inner medullary CD- ADH inc water reabsorption AND inc transporters for urea

4. large conc diff between TF and interstitial fluid so urea diffuses into interstitial fluid

what does ADH use to inc water reabsorption in late distal tubule 

aquaporins 

more urea is recycling making the…..

gradient more extreme and urine to more conc

what is the most important action of ADH

acts on principal cells in late DT and CD to inc water reabsorption in principal cells

• make concentrated urine

• ADH binds to V2 receptor that inserts aquaporins

what are the other 2 actions of ADH

1. acts on mTAL to inc activity of Na/K/2Cl co transporter which inc reabsorption of Na, K, CL (inc osmol in interstitium)

2. acts on inner medullary collecting ducts to inc urea transporter-1 (inc urea reabsorption)

whats hyperosmotic urine

concentrated = hyposthenuric >300 mOsm/L

whats SIADH

ADH too high and not released due to osmotic stimulus

no feedback inhibition

rare in domestic animals

describe the 5 step production process of hyperosmotic urine

1. osmol of filtrate identical to blood and remains at 300 along PCT due to isosmotic reabsorption

2. NaCl reabsorbed in TAL but NO WATER=dilution of TF (100)

3. NaCl reabsorbed in early DT but no water = dilution of TF (80)

4. principal cells in late DT can reabsorb water w ADH (TF = 300)

5. principal cells in CD reabsorb water w ADH. TF exposed to higher interstitial osmolarity as it flows down CD and drives more water reabsorption (1200 at the end)

bold = ADH action

whats hyposmotic urine

dilute = hyposthenuria <300 mOsm/L

describe the 4 step process of hyposmotic urine

1. reabsorption in PCT not affected

2. in TAL, TF more conc (120) due to lower activity of Na/K/2Cl cotransporter

3. Early DT - NaCl reabsorbed but no water (110)

4. late DT and CD now impermeable to water - less water reabsorbed in response to increasing interstitial gradient (final urine about 75)

what are conditions that can lead to abnorm production of hypoosmotic urine

central diabetes insipidus

nephrogenic diabetes insipidus

describe central diabetes insipidus

• hypothalamus /pituitary damage

• condition of passing large amt of urine

• refers to dilute urine

describe nephrogenic diabetes insipidus

ADH secreting normal but defect in receptor or signaling in principal cells 

characteristics of mammal urine

1. urine color

2. odor

3. consistency

4. nitrogen

5. osmolarity

6. pH

describe urine color

colorless to pale yellow due to urobilin (urochrome oxidized)

*bilirubin from RBC breakdown

describe urine odor

characteristic depending on species

slightly aromatic and becomes stronger as exposed to air

breakdown of urea = ammonia

describe urine consistency

watery in most species

horse thicker due to mucus secreted in kidneys and ureters

describe urine nitrogen

for mammals nitrogen excreted as urea (formed from ammonia)

for birds and reptiles, nitrogen excreted as uric acid

describe urine osmolarity

50-1200 mOsm depending on species and hydration

describe urine pH

range 4.5-8 typically 6 depending on diet

• meat more acidic

• plant more alkaline

why is mucus added in horse urine

bc horse urine is high in carbonate and phosphates which want to precipitate after urine is excreted so mucus helps carry these compounds

amount of urine varies on what

diet, work, external temp, water consumption, season, pathological conditions

what does specific gravity depend on

proportion of dissolved solutes and water

• can help evaluate renal function by assessing whether water is excreted or conserved appropriately

cat is best at

concentrating urine

cattle produces…

most dilute urine

cow SG

1.005-1.040

cat SG

1.035- 1.060

polyura

inc urine output

oliguria

dec urine output

anuria

no urine ouput

dysuria

difficult or painful urination

stranguria

slow/painful discharge or urine caused by spasms in urethra and bladder

clinical sign of urethral obstruction