JW

Kidney Function & Osmoregulation – Vocabulary Review

Core Concept: Homeostasis

  • Goal: maintain constant internal conditions (temperature, pH, osmotic pressure, ion balance, nutrient levels, waste removal).
  • Benefits
    • Enzymes work at optimum conditions → maximal metabolic efficiency.
    • Protects proteins/membranes from denaturation.
    • Allows organisms to function across wider external environments without performance loss.
    • Reduces energy spent on corrective processes (e.g.
      shivering, panting).

Excretion vs Osmoregulation

  • Excretion: removal of toxic metabolic wastes (mainly nitrogenous) from body fluids.
    • Mammalian main waste = urea.
  • Osmoregulation: regulation of osmotic concentration (water : solute ratio) of body fluids.
    • Expressed in \text{osmol\;L}^{-1} (or \text{mOsmol\;kg}^{-1} in physiology).
    • Primarily performed by the kidneys.
  • Distinction
    • Excretion eliminates molecules; osmoregulation may conserve or discard water/ions without necessarily removing toxins.

Nitrogenous Wastes Across Taxa

  • Ammonia (NH₃) produced by deamination is highly toxic, requires abundant water to dilute.
  • Conversion pathways (energy trade-off vs. water saving):
    • Aquatic animals: excrete NH_3 directly into water.
    • Mammals: convert to urea (moderate toxicity, moderate energy cost, water soluble).
    • Birds & many reptiles: convert to uric acid (non-water-soluble, high energy cost, minimal water loss → lighter for flight).

Kidney Gross Anatomy

  • Each kidney supplied by renal artery, drained by renal vein.
  • Ureter conveys urine → bladder → urethra → exterior.
  • Regions
    • Cortex: outer; site of ultrafiltration & fine adjustment.
    • Medulla: inner; site of osmotic gradient & bulk water reabsorption.
    • Renal pelvis: collects urine before ureter.

Nephron – Functional Unit

  • Millions per kidney; all drain into ureter.
  • Major parts (cortex → medulla → cortex route):
    • Bowman's capsule & glomerulus (ultrafiltration).
    • Proximal convoluted tubule (PCT) (selective reabsorption).
    • Loop of Henle – descending & ascending limbs (counter-current multiplier for water conservation).
    • Distal convoluted tubule (DCT) (fine ion & pH regulation).
    • Collecting duct (final water reabsorption; site of ADH action).
  • Associated vessels
    • Afferent arteriole → glomerulus → efferent arteriole.
    • Peritubular capillaries surround PCT & DCT.
    • Vasa recta (counter-current blood flow paralleling loop of Henle).

Ultrafiltration (Glomerulus & Bowman's Capsule)

  • Blood arrives via wide afferent arteriole, leaves via narrow efferent arteriole → high hydrostatic pressure.
  • Structures enabling filtration (size cut-off <65\,000\ \text{amu}):
    • Fenestrated endothelium (≈100\ \text{nm} pores) holds back cells.
    • Basement membrane (negatively-charged glycoprotein mesh) excludes plasma proteins.
    • Podocytes with foot processes & filtration slits further limit passage.
  • Produces glomerular filtrate (water, ions, glucose, amino acids, urea, small peptides, HCO₃⁻, etc.)
  • Daily formation: 1{-}1.5\ \text{L} urine from ~180\ \text{L} filtrate → ∼45\ \text{g} solute (≈30\ \text{g} urea + 15\ \text{g} NaCl).

Selective Reabsorption (Proximal Convoluted Tubule)

  • Filtrate initially contains glucose, amino acids, vitamins, hormones, Na^+, Cl^-, urea, water.
  • PCT epithelial adaptations:
    • Microvilli → huge surface area.
    • Many mitochondria → ATP for active transport.
    • Basal membrane invaginations → shorten diffusion distance.
  • Transport mechanisms
    • Na^+: pumped into interstitial fluid by Na^+/K^+-ATPase; drives secondary transport.
    • Glucose & amino acids: co-transported with Na^+, then diffuse into blood.
    • Cl^-: follows electrochemical gradient (facilitated & active).
    • Water: osmosis following solute removal.
    • Urea: some passive diffusion.
  • Quantities
    • ~1.5\ \text{kg} salt & 5.5\ \text{kg} glucose filtered daily; ~100 % reclaimed here in healthy person.

Loop of Henle – Countercurrent Multiplier

  • Purpose: generate high osmotic gradient in medulla to conserve water.
  • Descending limb
    • Permeable to water, impermeable to ions.
    • Water exits → filtrate becomes hypertonic.
  • Ascending limb
    • Thin lower section: passive Na^+, Cl^- diffusion out.
    • Thick upper section: active transport of Na^+, Cl^- to medulla; impermeable to water.
  • Result: medullary interstitial fluid reaches >1200\ \text{mOsm}, creating gradient for collecting duct.
  • Countercurrent nature: filtrate flows opposite directions; at each horizontal level, descending limb fluid slightly more concentrated than ascending limb fluid → multiplier effect.

Distal Convoluted Tubule (Fine Adjustment)

  • Regulates:
    • pH: secretes H^+ / NH3, reabsorbs HCO3^-.
    • Ions: variable reabsorption of Na^+/Cl^-; secretion of K^+ under aldosterone control.
    • Water: permeable; amount reabsorbed depends partly on ADH.

Collecting Duct – ADH-Mediated Osmoregulation

  • Passes through high-osmolar medulla → water potential gradient favors water exit.
  • Antidiuretic hormone (ADH)
    • Synthesised in hypothalamus; stored/released by posterior pituitary.
    • Osmoreceptors detect plasma osmolarity.
    • When dehydrated (↑ plasma osmolality), ADH release increases.
    • ADH triggers insertion of aquaporins into collecting-duct membrane → ↑ water permeability.
    • Outcome: more water reabsorbed, urine volume ↓, concentration ↑ (anti-diuresis).
  • If hydrated → ↓ ADH → fewer aquaporins → dilute urine.
  • Typical human plasma range influencing thirst & ADH: 280{-}320\ \text{mOsmol\ kg}^{-1}.

Quantitative Relationships & Example Questions

  • Percentage change formula: \frac{\text{new} - \text{old}}{\text{old}} \times 100\%.
  • Predicted ADH at 300\ \text{mOsmol\ kg}^{-1} (from trend): ~9{-}10\ \text{pmol\ dm}^{-3}.
  • Drinking to satiety → plasma osmolarity ↓ toward 280\ \text{mOsmol\ kg}^{-1}; ADH falls; thirst quenched.
  • Causes of ↑ plasma osmolarity: sweating, high salt intake, diarrhea, inadequate water intake.

Relationship: Medulla Thickness vs. Urine Concentration

  • Relative medullary thickness (RMT) strongly correlates with maximum solute concentration (MSC) of urine.
    • Desert rodents (e.g.
      Eligmodontia typus, RMT 11.4) achieve MSC >8{,}600\ \text{mOsm}.
  • Thicker medulla → longer loops of Henle → larger gradient → more water conservation.

Filtration, Reabsorption & Excretion Fractions

  • Daily filtration vs. excretion (approx.):
    • Water: >99\% reabsorbed (only \approx1\% excreted).
    • Glucose: \approx100\% reabsorbed; 0\% excreted in healthy state.
    • Urea: ~50\% reabsorbed, 50\% excreted.
    • Na^+ / Cl^-: ~99.5\% reabsorbed; 0.5\% excreted.
    • Proteins/peptides: normally \approx100\% retained in blood (should not appear in filtrate).

Regulation of Renal Blood Flow

  • Vasoconstriction / vasodilation of afferent & efferent arterioles adjust glomerular filtration rate.
  • Systemic activity changes (sleep, exercise):
    • During vigorous activity: ↑ skeletal muscle flow, ↓ gut & kidney flow (but kidney must remain fairly constant for waste removal → autoregulation mechanisms).
    • Piglet study: asleep vs. awake shows organ-specific redistribution; in piglets kidney flow unchanged whereas in adult humans it often decreases during sleep (species difference in developmental needs & thermoregulation).

Clinical & Real-World Connections

  • Glucosuria indicates PCT saturation → diagnostic for diabetes mellitus.
  • Loop diuretics (e.g.
    furosemide) inhibit Na^+ reabsorption in ascending limb → ↑ urine volume.
  • ADH analogues treat diabetes insipidus; ADH antagonists manage hyponatremia.
  • Desert mammal kidney morphology inspires water-saving technologies (biomimicry).
  • Ethical consideration: water availability & renal disease management in low-resource settings.

Key Equations & Values Recap

  • Osmotic concentration unit: 1\ \text{osmol\ L}^{-1} = 1\ \text{mol\ of\ osmotically\ active\ particles\ per\ litre}.
  • Pressure driving ultrafiltration: glomerular capillary pressure ≈55\ \text{mmHg} minus opposing forces.
  • Daily filtrate: \approx180\ \text{L}; daily urine: 1{-}1.5\ \text{L}.
  • Threshold MW for filtration: <65{,}000\ \text{amu}.
  • Counter-current gradient: medullary interstitium from cortex (~300\ \text{mOsm}) to inner medulla (>1200\ \text{mOsm}).

Study Checklist

  • [ ] Define and contrast excretion & osmoregulation.
  • [ ] Identify anatomical parts of kidney & nephron.
  • [ ] Explain mechanisms of ultrafiltration & selective reabsorption with structural adaptations.
  • [ ] Describe countercurrent multiplier in loop of Henle.
  • [ ] Explain ADH control of collecting duct permeability.
  • [ ] Interpret data linking medulla thickness, urine concentration, ADH vs. plasma osmolarity.
  • [ ] Calculate percentage change & solute balances.
  • [ ] Relate blood flow redistribution to physiological states.