Urinary & Endocrine Physiology: Kidney, Hormonal Regulation & Homeostasis

Functions of the Urinary System

• Regulate ionic composition of plasma ($Na^+$, $K^+$, $Ca^{2+}$, Cl⁻, $HCO_3^-$, etc.)
• Long-term control of arterial blood pressure via blood (plasma) volume
• Maintain plasma osmolarity by adjusting water balance
• Acid–base balance (pH ≈ $7.4\pm0.05$)
• Excrete metabolic waste, toxins, drugs (“garbage”)
• Endocrine roles (secondary endocrine organ)
  • Erythropoietin (EPO) → ↑ red-blood-cell production when renal $O_2$ is low
  • Renin → initiates renin–angiotensin–aldosterone system (RAAS)
  • Calcitriol (active vitamin D) → Ca²⁺ balance
  • Gluconeogenesis during prolonged starvation (liver is primary, kidney assists)

Gross Anatomy & Blood Flow

• Kidneys: bean-shaped, high metabolic rate (≈16 % of resting ATP use)
• Receive ≈20 % of cardiac output at rest (rest-and-digest state favours filtration)
• Major vessels: renal artery → segmental → interlobar → arcuate → interlobular → afferent arteriole → glomerulus → efferent arteriole → peritubular/vasa recta → renal vein

Nephron – Structural Overview

• Functional unit; ≈1 million/kidney; arranged in renal pyramid
• Key parts (in flow order)
  1. Bowman's capsule + glomerulus = renal corpuscle (site of filtration)
  2. Proximal convoluted tubule (PCT) – “giant sponge”
  3. Loop of Henle (descending ↘, thin ascending ↗, thick ascending ↗)
  4. Distal convoluted tubule (DCT)
  5. Collecting duct (CD) → minor calyx → ureter
• Two nephron types
  • Cortical (≈85 %) – short LOH, mostly in cortex
  • Juxtamedullary (≈15 %) – long LOH deep into medulla → establish medullary osmotic gradient (up to $\approx1400\;\text{mOsm}$)
• Juxtaglomerular apparatus (JGA)
  • Macula densa (DCT) senses tubular flow & salt
  • Granular (JG) cells on afferent arteriole secrete renin; act as stretch/pressure sensors

Four Basic Renal Processes

• Filtration: plasma → nephron at renal corpuscle
• Reabsorption: tubule → blood (peritubular caps/vasa recta)
• Secretion: blood → tubule (post-glomerular)
• Excretion: fluid that remains in tubule and exits as urine
• Relationship: $\text{Excretion}=\text{Filtration}-\text{Reabsorption}+\text{Secretion}$

Glomerular Filtration

• Normal GFR ≈ $125\;\text{mL·min}^{-1} = 180\;\text{L·day}^{-1}$
• Pressures (Starling forces):
  • $P<em>{GC}$ (glomerular capillary hydrostatic) pushes out (≈55 mmHg) • $\pi</em>{GC}$ (oncotic) pulls in (≈30 mmHg)
  • $P<em>{BC}$ (Bowman’s capsule hydrostatic) pushes in (≈15 mmHg) • $\pi</em>{BC}$ ≈0 (no protein in filtrate)
  • Net: $P<em>{NF} = P</em>{GC} - P<em>{BC} - \pi</em>{GC}$ (≈16 mmHg favouring filtration)
• Clinical examples
  • Kidney stone blocking ureter ↑$P<em>{BC}$ → ↓GFR • Glomerulonephritis allows proteins into filtrate ↑$\pi</em>{BC}$ → ↑GFR

Regulation of GFR

Intrinsic (autoregulation)

• Myogenic: afferent arteriole constricts when BP ↑; dilates when BP ↓ (maintains steady flow between MAP ≈ 80–180 mmHg)
• Tubuloglomerular feedback: macula densa senses ↑NaCl/flow → constricts afferent arteriole via paracrine signals
  Extrinsic
• Sympathetic activation (severe stress/haemorrhage) → afferent constriction ↓GFR
• Systemic arterial pressure: ↑BP → ↑GFR (unless counter-regulated)

Proximal Tubule – Massive Reabsorption

• ~70 % filtered water & Na⁺ reabsorbed here; also glucose, amino acids, bicarbonate, etc.
• Transport mechanisms: Na⁺-K⁺ ATPase (basolateral), cotransporters (e.g., Na⁺-glucose SGLT)
• Transport maximum (Tₘ)
  • Each carrier-mediated solute has a finite Tₘ
  • Glucose Tₘ ≈ $375\;\text{mg·min}^{-1}$ → renal threshold ≈ 180–200 mg·dL⁻¹ (glycosuria when exceeded ⇒ polyuria in diabetes mellitus)

Loop of Henle & Counter-Current Multiplier

Descending limb

• Permeable to water, impermeable to solute → water leaves → tubular $\text{Osm}↑$
  Ascending limb
• Impermeable to water
• Thick segment actively pumps $Na^+, K^+, Cl^-$ out → tubular $\text{Osm}↓$
  Result: generates corticomedullary gradient (300 → 1400 mOsm) essential for water reabsorption in CD
  Blood supply (vasa recta) acts as counter-current exchanger to avoid washout

Distal Tubule & Collecting Duct – Sites of Regulation

• Baseline: relatively impermeable to water → large-volume, dilute urine
• Hormonal controls “fine-tune” final composition
  • Antidiuretic hormone (ADH/vasopressin)
  • Aldosterone
  • Atrial natriuretic peptide (ANP/ANF)

Hormonal Control of Water & Electrolytes

ADH (posterior pituitary)

• Stimuli: ↑plasma osmolarity (>~$300\;\text{mOsm}$), ↓blood volume/pressure, Ang II
• Inserts aquaporin-2 channels in CD → H₂O reabsorption → ↓urine volume, ↑urine osmolarity
• Clinical: drinking seawater (>1400 mOsm) exceeds concentrating ability ⇒ net dehydration

Aldosterone (adrenal cortex, zona glomerulosa)

• Direct stimulus: ↑plasma $K^+$
• Indirect: RAAS (↓renal perfusion → renin → Ang I → Ang II → aldosterone)
• Effects on DCT/CD: ↑Na⁺ reabsorption (ENaC & Na⁺/K⁺-ATPase), ↑K⁺ secretion → water follows salt → ↑ECF volume/BP, ↓urine volume, ↑urine $K^+$

Renin–Angiotensin–Aldosterone System (RAAS)

• JG cells release renin when afferent arteriole stretch ↓
• $\text{Angiotensinogen} \xrightarrow{\text{renin}} \text{Ang I} \xrightarrow[\text{lungs}]{\text{ACE}} \text{Ang II}$
• Ang II actions: vasoconstriction, ↑ADH, ↑aldosterone, ↑thirst

Atrial Natriuretic Peptide (ANP)

• Secreted by atrial myocytes when atrial stretch ↑ (hypervolemia)
• Inhibits renin, aldosterone, ADH; dilates afferent, constricts efferent arterioles → ↑GFR
• Net: ↑Na⁺ & H₂O excretion (natriuresis, diuresis) → ↓blood volume/BP

Osmolarity & Water Balance Examples

• Dehydration: ↑plasma osmolarity → ADH release → small-volume, dark, concentrated urine
• Water excess: ↓osmolarity → ↓ADH → large-volume, dilute (pale) urine
• Desert rodents: very long juxtamedullary nephrons → medullary gradient >1400 mOsm → extreme urine concentration, minimal water loss

Electrolyte Homeostasis

Sodium (Na⁺)

• Hypernatremia (high ECF Na⁺) → ↑ECF osmolarity → water retention/HTN; managed by ↓salt intake, ↑ANP
• Hyponatremia → ↓osmolarity → cerebral oedema risk

Potassium (K⁺)

• Hyperkalemia: cardiac arrhythmias, neuromuscular issues → triggers aldosterone release → ↑K⁺ secretion
• Hypokalemia: muscle weakness, arrhythmias → ↓aldosterone, or dietary replacement

Calcium (Ca²⁺)

• Essential for neurotransmission, muscle contraction, coagulation, bone matrix
• Hormonal triad
  • Parathyroid hormone (PTH): released when $[Ca^{2+}]<em>{plasma}↓$ → ↑bone resorption, ↑renal reabsorption, ↑calcitriol synthesis • Calcitriol (vitamin D₃): activated in skin → liver → kidney; ↑intestinal & renal Ca²⁺ uptake, works with PTH • Calcitonin (thyroid C-cells): released when $[Ca^{2+}]</em>{plasma}↑$; promotes bone deposition, minor role in adults
• Clinical note: inadvertent removal of parathyroids during thyroidectomy → fatal hypocalcaemia unless treated

Acid–Base Balance (Brief)

• Normal arterial pH ≈ $7.40$ (range $7.35–7.45$)
• Acidosis (
• Kidneys manage long-term by secreting $H^+$, reabsorbing/generating $HCO_3^-$ (mostly PCT & collecting duct intercalated cells)

Micturition (Urination)

• Urine flow: kidney → ureter → bladder (storage) → urethra (voiding)
• Components
  • Detrusor (bladder wall) – smooth muscle, involuntary (parasympathetic)
  • Internal urethral sphincter – smooth, involuntary
  • External urethral sphincter – skeletal, voluntary (learned control; Kegel exercises strengthen)
• Stretch reflex initiates urge; higher centres can delay via external sphincter
• Pregnancy, ageing, pelvic floor injury weaken control → incontinence; therapies: pelvic floor strengthening, surgical sling

Endocrine Overview – Primary vs Secondary Organs

• Primary: chief job is hormone secretion (e.g., pituitary, thyroid, adrenals)
• Secondary: primary job elsewhere but also secrete hormones (kidney, heart, GI tract, skin, liver)

Hypothalamus–Pituitary Axis

Posterior Pituitary (neurohypophysis)

• Hormones synthesised in hypothalamic nuclei; stored/released terminally
  • ADH (water retention)
  • Oxytocin (uterine & vas deferens contractions, milk ejection)

Anterior Pituitary (adenohypophysis) – tropic cascade

• Hypothalamic releasing/inhibiting hormones via portal vessels → anterior pituitary hormones
• Key pathways (memorise source → target → final hormone/effect):
  • $PRH/PIH$ → Prolactin → breast development & milk production
  • $TRH$ → TSH → thyroid gland → T₃/T₄ (metabolism)
  • $CRH$ → ACTH → adrenal cortex → cortisol, aldosterone, DHEA
  • $GHRH/GHIH$ → Growth hormone → body tissues & liver (somatomedins)
  • $GnRH$ → FSH/LH → gonads → gametogenesis & sex steroids (estrogen, progesterone, testosterone)

Adrenal Gland Summary

• Cortex (outer): zona glomerulosa (aldosterone), fasciculata (cortisol), reticularis (androgens)
• Medulla (inner): chromaffin cells act as post-ganglionic sympathetic neurons → secrete epinephrine & norepinephrine

Useful Numerical & Formula Recap

• $GFR = 125\;\text{mL·min}^{-1} = 180\;\text{L·day}^{-1}$
• Net Filtration Pressure: $P<em>{NF}=P</em>{GC}-P<em>{BC}-\pi</em>{GC}$
• Maximum urine concentrating ability ≈ $1400\;\text{mOsm}$ (humans)
• Transport maximum concept: spill-over into urine when \text{Filtered
  t} > T_m (e.g., glucose threshold ≈180–200 mg/dL)
• Excretion equation: $E = F - R + S$

Clinical & Real-World Connections

• Diabetics: hyperglycaemia → glycosuria → osmotic diuresis → polyuria & polydipsia
• High-salt diet → hypernatremia → water retention → hypertension; managed by salt restriction & diuretics
• Seawater ingestion: osmolarity > concentrating ability → obligatory water loss → dehydration
• Desert kangaroo rat: exceptionally long juxtamedullary nephrons → minimal obligatory water loss
• Kidney stones: obstruct outflow ↑$P_{BC}$ → ↓GFR, flank pain
• ACE inhibitors: block Ang II formation → ↓aldosterone/ADH, vasodilation → antihypertensive

Study Tips

• Know “where & what”: each segment of nephron + its primary function(s)
• Memorise hormonal triggers & effects (ADH, Aldosterone, ANP, PTH, etc.)
• Relate pressures to direction of filtration/reabsorption
• Practise applying formulas to clinical scenarios (e.g., how obstruction alters GFR)
• Use circadian rhythm concept: many anterior pituitary hormones peak early morning (cortisol) – relevant for lab values/sample timing