Urinary System – Renal Physiology, Filtration, and Urine Transport

Transport Maximum (Tm) & Renal Thresholds

  • Concept

    • Each solute has a finite number of carrier proteins in tubular epithelium.

    • Once all carriers are occupied → saturation → transport maximum (Tm).

  • Significance

    • Tm sets the renal threshold: plasma concentration at which the solute first appears in urine.

    • Exceeding Tm → portion of solute remains in tubular fluid → detectable in final urine; clinically useful as an early sign of metabolic disorders.

  • Glucose

    • Renal threshold ≈ (180\text{–}220\ \text{mg·dL}^{-1}).

    • >180\ \text{mg·dL}^{-1} ⇒ carriers saturated ⇒ glycosuria (hallmark of diabetes mellitus, stress hyper-glycemia, IV dextrose overload).

  • Amino acids

    • Lower threshold ≈ 65\ \text{mg·dL}^{-1}.

    • Physiologic aminoaciduria after high-protein meal; persistent aminoaciduria can indicate tubular disorders.

Glomerular Filtration (GF)

Filtration Membrane Anatomy

  • Three serial barriers

    • Fenestrated capillary endothelium (size barrier).

    • Dense layer (basement membrane; charge barrier).

    • Filtration slits between podocyte pedicels (final size barrier).

  • Outcome: ultrafiltrate nearly protein-free, isotonic to plasma.

Filtration Pressures

  • Glomerular Hydrostatic Pressure (GHP)

    • Blood pressure inside glomerular capillaries; elevated by smaller efferent arteriole.

    • Average 50\ \text{mmHg} → pushes fluid/solutes into capsular space.

  • Capsular Hydrostatic Pressure (CsHP)

    • Back-pressure of filtrate within capsule + tubules.

    • Average 15\ \text{mmHg} → opposes filtration.

  • Net Hydrostatic Pressure (NHP)

    • \text{NHP}=\text{GHP}-\text{CsHP}=50-15=35\ \text{mmHg}.

  • Blood Colloid Osmotic Pressure (BCOP)

    • Osmotic pull of retained plasma proteins.

    • Average 25\ \text{mmHg} → opposes filtration.

  • Net Filtration Pressure (NFP)

    • \text{NFP}=\text{NHP}-\text{BCOP}=35-25=10\ \text{mmHg}.

    • Positive value ensures continuous filtration; even modest changes alter GFR.

Glomerular Filtration Rate (GFR)

  • Definition: volume of filtrate formed per minute by both kidneys.

  • Normal adult ≈ 125\ \text{mL·min}^{-1} ≈ 180\ \text{L·day}^{-1} (≈10 % of renal plasma flow).

  • Clinical relevance

    • Gold standard marker of renal function (e.g., creatinine clearance, inulin clearance).

    • Falls in hemorrhage, dehydration, hypotension; rises in pregnancy, high-protein meal.

Regulation of GFR

  1. Autoregulation (intrinsic)

    • Myogenic & tubuloglomerular feedback keep GFR constant despite MAP ≈80\text{–}180\ \text{mmHg}.

    • ↓ GFR triggers

      • Afferent arteriole dilation

      • Glomerular capillary dilation

      • Efferent arteriole constriction.

    • ↑ GFR produces stretch → afferent constriction (myogenic reflex).

  2. Hormonal Regulation

    • Renin–Angiotensin–Aldosterone System (RAAS)

      • Juxtaglomerular complex releases renin when renal perfusion ↓.

      • Cascade: angiotensinogen → \text{Ang I}\xrightarrow{ACE}\text{Ang II}.

      • Ang II actions

      • Constricts efferent arteriole → raises glomerular pressure.

      • Stimulates Na⁺/water reabsorption, thirst, systemic vasoconstriction.

      • Promotes aldosterone & ADH release → further water retention.

    • Natriuretic peptides (ANP, BNP)

      • Secreted by cardiac muscle when atria/ventricles stretch (↑ blood volume).

      • Dilate afferent + constrict efferent arterioles → ↑ GFR.

      • Inhibit NaCl reabsorption; net effect = diuresis & natriuresis.

  3. Autonomic Regulation (sympathetic)

    • Severe sympathetic activation (shock, stress) → afferent constriction → ↓ GFR to conserve volume.

    • Also stimulates renin release.

Reabsorption & Secretion

  • Urine composition results from

    • Filtration at corpuscle.

    • Reabsorption: nutrients, ions, water returned to blood; nearly complete for glucose/AA (unless Tm exceeded).

    • Secretion: additional wastes (e.g., creatinine, drugs) actively added to tubular fluid.

  • Diuresis: production of large urine volumes.

    • Diuretics (loop, thiazide, K⁺-sparing) intentionally increase diuresis to lower BP, reduce edema, treat CHF; ethical consideration: monitor electrolytes to prevent hypokalemia/arrhythmias.

  • Urine characteristics

    • Clear, sterile, yellow (urobilin pigment from heme breakdown).

    • Urinalysis: cost-effective diagnostic for diabetes (glycosuria), renal disease (proteinuria), infections (nitrites, WBCs), bilirubin disorders.

Urine Transport, Storage & Elimination

Ureters

  • Pair of retroperitoneal muscular tubes from renal pelvis to posterolateral bladder wall.

  • Enter bladder obliquely; slit-like orifices act as functional valves preventing backflow during micturition.

  • Peristaltic waves every ≈30 s propel urine.

Urinary Bladder

  • Hollow detrusor muscle; temporary reservoir (capacity ≈1\ \text{L}).

  • Mucosa with rugae flattens as bladder fills.

  • Trigone

    • Smooth triangular region between ureteric orifices & internal urethral orifice.

    • Funnels urine toward urethra; common site of infections (cystitis).

  • Neck + Internal sphincter

    • Thickened smooth muscle; involuntary control.

Urethra

  • Conveys urine from bladder exterior.

  • Male (≈7–8 in.)

    • Segments: prostatic → membranous (passes urogenital diaphragm) → spongy (penile) → external orifice.

  • Female (≈1–2 in.)

    • Short path from bladder to vestibule; proximity to vagina/rectum predisposes to UTIs.

External Urethral Sphincter

  • Skeletal muscle in urogenital diaphragm; voluntary (somatic) control.

  • Normally tonically contracted; relaxation coordinated with detrusor contraction (micturition reflex).

  • Behavioral/clinical note: delayed toilet training, neurogenic bladder, ethical aspect of catheterization techniques to preserve continence.

Micturition Reflex

  • Stretch receptors fire when bladder volume ≈200\text{–}250\ \text{mL}.

  • Parasympathetic efferents contract detrusor + relax internal sphincter.

  • In adults, pontine storage/voiding centers override reflex until appropriate; loss of cortical control (spinal cord injury, dementia) → incontinence.

Integrated Clinical Connections

  • Estimating GFR crucial before prescribing nephrotoxic drugs (aminoglycosides, contrast dyes).

  • RAAS inhibitors (ACE-I, ARB) lower efferent constriction → may precipitate renal failure in bilateral renal artery stenosis.

  • Natriuretic peptide analogs (nesiritide) used experimentally for acute HF but limited due to hypotension.

  • Ethical/philosophical: equitable access to dialysis/transplant when GFR <15 mL/min; balancing cost vs quality of life.

  • Public health: patient education on hydration, BP control, diabetes management to preserve filtration function.