30 Regulation of Urine Concentration and Volume

Flashcards covering the regulation of urine concentration and volume, medullary osmotic gradient, countercurrent mechanisms, urine composition, urinalysis, micturition, and acid-base homeostasis.

Facultative H2O reabsorption

Determines if the urine is dilute or concentrated.

Renal Corpuscle Osmolarity

Filtrate entering and leaving is iso-osmotic to plasma (300 mOsm).

Thin descending limb permeability

Permeable to H2O but not solutes.

Thick ascending limb permeability

Impermeable to H2O.

Filtrate of Descending Limb

Concentrated as it travels down loop, becoming 900 mOsm at the bottom.

Filtrate of Ascending Limb

Becomes less concentrated to 100 mOsm at ascending limb and distal tubule.

Na+ and Cl- Reabsorption Location

Continues in distal tubule and collecting system.

Effect of Excess H2O on Urine Volume

Larger urine volume produced (1.8 liters/day).

Medullary Osmotic Gradient

Produces a concentrated (1200 mOsm) urine.

Interstitial fluid in renal cortex osmolarity

300 mOsm

Counter Current Mechanism

Produces Concentrated Urine; gradient disappears when filtrate enters medullary collecting duct

Medullary Osmotic Gradient

Starts at 300 mOsm at cortex/medulla border and increases deeper into medulla, reaching 1200 mOsm in deepest medullary regions.

Medullary Osmotic Gradient Factors

1. Counter current multiplier. 2. Recycling of urea in medulla. 3. Vasa recta countercurrent exchanger.

Countercurrent Multiplier

Involves long loops of juxtamedullary nephrons; creates medullary osmotic gradient. Relies on filtrate of 2 limbs flowing in opposite directions.

Urea Recycling

Urea becomes concentrated as H2O reabsorbed from filtrate; diffuses out of filtrate and into interstitial fluid. Some urea enters thin descending limb of nephron loop to rejoin filtrate.

Vasa Recta and Countercurrent Exchanger

Exchange of substances by opposite flow of filtrate and blood maintains a steep medullary osmotic gradient

Urine Color

Yellow due to urochrome (hemoglobin breakdown product). May be altered by foods, vitamins, drugs, dyes and blood.

Urine Translucency

Should always be translucent. Cloudy urine may indicate infection or large amounts of protein.

Urine Odor

Fresh urine should have a mild odor. May be altered by diseased states such as diabetes mellitus, infection and foods.

Urine pH

Urine should be about pH 6.0 but can range from 4.5 to 8.0.

Urine Specific Gravity

Compares amount of solutes in a solution to amount in deionized water (1.0). Urine ranges from 1.001 to 1.035.

Micturition

Urination or voiding; discharge of urine from urinary bladder, activated by micturition reflex by parasympathetic nervous system.

Micturition Reflex Stimulus

Stimulated by stretching of urinary bladder.

Micturition Reflex Steps

1. Urine fills the bladder and stretches its wall. 2. Stretch receptors send sensory afferent signals to sacral spinal cord. 3. Parasympathetic efferent fibers stimulate detrusor muscle to contract and internal urethral sphincter to relax. 4. Interneurons in spinal cord communicate to micturition center in pons. 5. If micturition is appropriate cerebral cortex allows external urethral sphincter to relax and urine is voided.

Regulation of Blood pH

Kidneys and lungs determine number of hydrogen ions and basic ions in blood. Kidneys maintain acid-base balance by controlling HCO3- in blood.

H+ Secretion Location

In Distal Tubule and Collecting System; Intercalated cells have primary active transporters that pump H+ into filtrate