LEC 28: URINE

• Countercurrent Mechanism


  1. Ascending Limb of the Loop: Impermeable to water, Allows solutes like Na+ to pass

  2. Descending Limb of the Loop: Impermeable to solutes, Allows water to pass

  3. Active transport of Na+ in the Thick Segment of the Ascending Limb

  4. Establishment of the osmotic gradient to concentrate urine

  5. Magnification of effects by countercurrent mechanism
• Countercurrent Multiplier


  Utilizes gradient from ion movement and water diffusion, Positive feedback cycle
• Vasa Recta


  • Slow movement of blood, Allows absorption and release of water and solutes along the nephron loop, Prevents circulatory system interference
• How Dilute Urine is Formed

  ADH released from pituitary gland, ADH opens aquaporin H2O channels in the collecting duct, Water is reabsorbed through aquaporins by osmosis, As dilute urine passes down the collecting duct, water is reabsorbed, making urine more concentrated
• How Concentrated Urine is Formed

  Decreases ADH levels, Leads to closure of aquaporins in the collecting duct, Water isn't reabsorbed from urine in the collecting duct, A larger volume of dilute urine is produced
• Renal Clearance


  Volume of plasma cleared of a specific substance in a given time, Utilizes inulin as a standard measure for glomerular filtration rate (GFR), Lower than inulin: reabsorption occurs, Higher than inulin: secretion by kidneys occurs, Zero clearance value = complete reabsorption by renal tubules, Assesses kidney efficiency and aids in diagnosing renal disorders
• As urine sits, bacterial action can lead to changes in odour, Particularly if it becomes more alkaline
• Freshly voided urine is aromatic, Over time, bacterial metabolism of urea in urine leads to ammonia odour
• Urine is slightly acidic (pH 6), Can vary from 4.5 to 8.0 based on metabolic or dietary factors
• Urine appearance ranges from pale yellow to deep amber depending on concentration, Clear urine = proper hydration, Cloudy urine = infection or other issues
• Ureters


  • Connect the kidneys to the urinary bladder, Smooth muscle lined with transitional epithelium, Convey urine from the kidneys to the bladder via peristalsis
• Urinary Bladder


  • Located in the pelvis, Smooth muscle (detrusor muscle) lined with transitional epithelium that allows it to stretch, Detrusor muscle is highly folded to allow distention of bladder without large increase in internal pressure, Stores urine until it is convenient to empty, Bladder expands -> wall stretches thin -> rugae disappears
• Urethra


  • Extends from the urinary bladder to the external urethral orifice, Mucous membrane-lined tube surrounded by smooth muscle, Allows urine to exit the body, In males, it serves as a passageway for semen
• Micturition


  Stimulated by parasympathetic nerves, Inhibited by sympathetic nerves, Distention of bladder = activates stretch receptors = triggers spinal reflexes, Reflexes can be overridden by prefrontal cortex (matures between ages 2 & 3), High levels of activity from the limbic system can interfere with this control
• Diuretics


  Increase urine output, Acts as an osmotic gradient or inhibiting Na+ = water reabsorption
• Breakdown of Old Damaged RBC

  Heme from hemoglobin turns into greenish biliverdin -> reddish bilirubin -> colourless urobilinogen -> yellowish urobilin, Yellowish urobilin leaves the body in yellow urine or concentrated brownish stercobilin in large intestine (brown feces)
• what happens when GFR is higher than inulin?

  secretion by the kidneys occurs