Urea recycling and the medullary osmotic gradient

Urea -

helps form medullary gradient

urea works by -

1. Urea enters filtrate in ascending thin limb of nephron loop by facilitated diffusion

2. Cortical collecting duct reabsorbs water, leaving urea behind

3. In deep medullary region, now highly concentrated urea leaves collecting duct and enters interstitial fluid of medulla

• Urea then moves back into ascending thin limb

• Contributes to high osmolality in medulla

Diuretics:

Chemicals that enhance urinary output

• ADH inhibitors, such as alcohol

• Na⁺reabsorption inhibitors (and resultant H₂O reabsorption), such as caffeine or drugs for hypertension or edema

Urinalysis:

urine is examined for signs of disease

• Can also be used to test for illegal substances

• Assessing renal function requires both blood and urine examination

• Example: renal function can be assessed by measuring nitrogenous wastes in blood only

to determne renal clearenc, what are requeired?

-urine

-blood

Renal clearance:

volume of plasma kidneys can clear of a particular substance in a given time 

• Renal clearance tests are used to determine GFR

• To help detect glomerular damage

• To follow progress of renal disease

• Renal clearance rate is calculated as:
  

• C = UV/P

• C = renal clearance rate (ml/min)

• U = concentration (mg/ml) of substance in urine

• V = flow rate of urine formation (ml/min)

• P = concentration of same substance in plasma

Inulin, a plant polysaccharide, is standard used

• Freely filtered and neither reabsorbed nor secreted by kidneys

• Its renal clearance = GFR (~125 ml/min)

• If C < 125 ml/min, means substance reabsorbed

• If C = 0, substance was completely reabsorbed, or not filtered

• If C = 125 ml/min, no net reabsorption or secretion

• If C > 125 ml/min, substance was secreted (most drug metabolites)

Chronic renal disease:

defined as a GFR < 60 ml/min for 3 months

• Filtrate formation decreases, nitrogenous wastes accumulate in blood, pH becomes acidic

• Seen in diabetes mellitus and hypertension

Renal failure:

• defined as GFR < 15 ml/min

  • Causes uremia: ionic and hormonal imbalances, metabolic abnormalities, toxic molecule accumulation

  • Symptoms: fatigue, anorexia, nausea, mental changes, cramps

  • Treatment: hemodialysis or transplant