Urinary System

Urinary System Overview

  • Discussion on Glomerular Filtration Rate (GFR)

    • Filtration occurs at a high level in kidneys.

    • Massive amounts of fluid filtered daily; entire liquid portion of blood filters approximately every 20 minutes.

Tubular Reabsorption and Secretion

  • Essential for reclaiming large amounts of fluid post-filtration.

  • Proximal Convoluted Tubule (PCT)

    • Primary site for reabsorption due to high pressure and large surface area.

    • Microvilli on PCT cells increase surface area.

    • Reabsorption processes include:

    • Passive diffusion and osmosis.

    • Active transport mechanisms:

      • Sodium-potassium pump (Na+/K+ ATPase) critical for powering reabsorption.

      • Secondary active transport for glucose, amino acids, etc.

      • Pinocytosis for small proteins.

  • Reabsorption occurs throughout nephron but predominantly in PCT.

  • Tubular secretion also occurs along nephron length:

    • Major sites include Distal Convoluted Tubule (DCT) and Collecting Duct.

    • Key secretions:

    • Hydrogen ions for pH regulation.

    • Potassium ions.

    • Other substances: creatinine, ammonia, toxins, drug metabolites.

  • Regulation of Potassium: Rigid control necessary to maintain balance and prevent toxicity.

Mechanisms of Reabsorption

  • Routes of Reabsorption

    • Paracellular: passive movement between cells; ~50% reabsorption in PCT.

    • Transcellular: through cells; facilitated by various transport systems.

  • Nutrient Reabsorption: linked to sodium transport.

    • Sodium coupled transport: e.g., Na+-glucose symporter.

    • Reabsorption of water is osmotically driven (follows solutes).

  • Two types of water reabsorption:

    • Obligatory: through always-open aquaporin-1 channels, occurring automatically.

    • Facultative: through aquaporin-2 channels, regulated by Antidiuretic Hormone (ADH).

Glucosuria and Transport Maximum

  • Glucosuria: presence of glucose in urine (also known as glycosuria).

    • Transports glucose until plasma concentration exceeds transport maximum (Tmax) of 180 mg/dL, leading to glucose in urine.

    • Commonly associated with diabetes mellitus (lack of insulin or resistance).

    • Other causes: high-carb meals can temporarily exceed Tmax.

    • Genetic variations may lead to inefficient glucose transport.

Bicarbonate Reabsorption

  • Bicarbonate reclamation prevents acidosis.

  • Process involves:

    • Bicarbonate freely filtered at glomerulus; important to reclaim.

    • Deamination of glutamine in PCT helps produce ammonia, aiding pH regulation.

    • Bicarbonate enters blood, hydrogen ion enters tubule, contributing to acid-base balance without altering pH directly.

Distal Convoluted Tubule and Collecting Duct Functions

  • DCT and Collecting Duct: final adjustments of filtrate.

    • Further Na+ and Cl- reabsorption; reabsorbs Ca2+ regulated by Parathyroid Hormone.

    • Final urine concentration adjustments influenced by ADH presence:

    • Dilute urine formation when ADH is absent.

    • Concentrated urine formation when ADH is present (inserting more aquaporin-2 channels).

  • Principal Cells: sensitive to aldosterone; manage sodium reabsorption and potassium secretion.

  • Intercalated Cells: primarily pH regulation; manage bicarbonate and hydrogen ion levels.

Hormonal Regulation and Kidney Function

  • Renin-Angiotensin-Aldosterone System (RAAS):

    • Triggered by low blood flow, leading to renal reabsorption.

    • Aldosterone increases Na+ reabsorption, affecting blood pressure and volume.

  • Atrial Natriuretic Peptide (ANP): regulates blood pressure; can counteract effects of aldosterone when fluid volume increases.

  • Antidiuretic Hormone (ADH): maintains water balance by regulating water reabsorption in the kidneys.

Countercurrent Multiplication and Concentration of Urine

  • Two mechanisms: Countercurrent Multiplier and Countercurrent Exchange.

  • Countercurrent Multiplier

    • Establishes osmolarity gradient in the medulla through Na+ pumps in the ascending limb of Loop of Henle.

    • Descending limb permeable to water; ascension concentrates tubular fluid.

    • Resulting high osmolarity (up to 1200 mOsm/L) ensures maximal concentration of urine when necessary.

  • Countercurrent Exchange (via vasa recta):

    • Maintains osmotic gradient; prevents dilution by balancing movement of water and solutes.

Diuretics: Effects and Use

  • Diuretics promote urine formation; common for managing hypertension and congestive heart failure.

    • Types include osmotic (e.g., mannitol), loop diuretics (e.g., furosemide/Lasix), thiazides, and aldosterone antagonists.

  • Osmotic Diuretics: retain water in tubules by preventing reabsorption.

  • Loop Diuretics: impair the concentration gradient, leading to increased urine output.

  • Thiazides: calcium-sparing effects useful in particular patient populations.

Evaluating Kidney Function

  • Tests for BUN and creatinine levels to assess kidney function.

  • Urinalysis to detect abnormalities (e.g., glucose, proteins).

  • Oliguria (<400 mL/day) and anuria (<50 mL/day): assess for potential kidney dysfunction.

Kidney Injuries and Renal Failure

  • Acute Kidney Injury (AKI) recognized through decreased GFR.

  • Types of kidney injury:

    • Prerenal: due to insufficient blood distribution to kidneys (dehydration, hemorrhage).

    • Renal: direct damage to kidney tissue (inflammation, nephritis).

    • Postrenal: obstruction hindering urine flow from kidneys (stones, tumors).

  • Chronic Kidney Disease (CKD): persistent damage requiring assessment and management plans.

Diabetes mellitus as Cause for Urinary Symptoms

  • Diabetes leads to osmotic diuresis, common in type 1 and type 2 diabetes due to blood glucose levels exceeding Tmax.

  • Gestational diabetes parallels type 2 behavior. Renal diabetes linked to ineffective glucose transporter function.

  • Diabetes insipidus: absence of ADH, leading to excessive urination.

Dialysis: Life-sustaining Treatment

  • Patients undergoing dialysis experience extensive management due to end-stage renal disease.

  • Different forms of dialysis: hemodialysis and peritoneal dialysis with varying patient implications.

  • HD: typically requires vascular access via AV fistula for sustained treatments.

Structure and Function of the Ureters and Bladder

  • Ureters: Deliver urine from kidneys; retroperitoneal orientation.

  • Bladder: Elastic, muscular sac, sits below uterus in females; undergoes contraction during voiding (urination).

    • Micturition Reflex: Stretch receptors activate the detrusor muscle.

    • Sphincters: Internal (involuntary) and external (voluntary, skeletal muscle).

  • Urethra: Shorter in females, longer in males; differentiated structure with various regions as per anatomy.

Urinary Incontinence and Control

  • Types include stress incontinence (sudden pressure events) and urge incontinence (sudden detrusor muscle contractions).

  • Kegel exercises encouraged for women to strengthen pelvic floor to improve symptom management.