Renal System – Comprehensive Study Notes

Renal System Components

• Major organs & structures
  • Kidneys (2)
  • Ureters (2)
  • Urinary bladder
  • Urethra
• Primary mission: maintain homeostasis by regulating water, electrolytes (Na⁺, K⁺, Cl⁻, Ca²⁺), acid–base balance, nitrogenous wastes (urea, creatinine, uric acid), blood volume & pressure, erythrocyte production, drug clearance, vitamin D activation, & glucose levels.

Kidney: Location & Gross Anatomy

• Retroperitoneal position (posterior to parietal peritoneum).
• Adrenal (suprarenal) glands sit like "little hats" atop each kidney.
• Hilum (medial indentation) = entry/exit for renal artery, vein & ureter, leading to the renal sinus (fat-filled cavity for protection).
• External/internal divisions
  • Renal capsule (fibrous covering)
  • Renal cortex (outer region)
  • Renal medulla (inner) → composed of renal pyramids separated by renal columns (cortical in-foldings).
  • Minor calyces → major calyces → renal pelvis → ureter.
• Microscopic units: \approx 1,000,000 nephrons / kidney (site of urine formation).

Core Physiological Functions of the Kidneys

• Filtration & Detoxification: remove metabolic wastes & xenobiotics.
• Selective Reabsorption & Secretion: recover needed solutes/water; fine-tune plasma composition.
• Acid–base regulation → maintain arterial pH near neutral; failure ⇒ metabolic acidosis; excessive fluid loss ⇒ metabolic alkalosis.
• Blood volume / BP control via renin–angiotensin–aldosterone system (RAAS).
• Erythropoietin release → stimulates marrow RBC production.
• Vitamin D (calcitriol) activation → Ca²⁺ & bone homeostasis.
• Glucose homeostasis: renal gluconeogenesis & glucose reabsorption; significance in diabetes (↓ insulin clearance when GFR ↓).
• Drug & hormone clearance (e.g., insulin, many antibiotics, creatine supplements).
• Receives \approx 20\% of cardiac output.

Vascular Supply (High-Yield Branching Sequence)

• Arterial tree: Renal a. → segmental a. → interlobar a. → arcuate a. → cortical radiate a. → afferent arteriole → glomerulus → efferent arteriole.
• Venous drainage mirrors arteries except no segmental veins.

Nephron Anatomy & Specialized Segments

• Glomerulus (fenestrated capillaries inside Bowman’s capsule) → filtration.
• Proximal convoluted tubule (PCT): ~65 % of Na⁺ & H₂O, plus K⁺, HCO₃⁻, glucose, amino acids reabsorbed; secretion of H⁺ (acid–base role).
• Loop of Henle
  • Descending limb: H₂O reabsorption.
  • Ascending limb: NaCl & K⁺ reabsorption; creates osmotic gradient; target of loop diuretics (e.g., furosemide).
• Distal convoluted tubule (DCT)
  • Early DCT: parathyroid hormone ⇒ ↑ Ca²⁺ reabsorption.
  • Late DCT: aldosterone ⇒ ↑ Na⁺ reabsorption → water follows osmotically.
• Collecting duct (not part of nephron proper)
  • ADH (vasopressin) inserts aquaporins ⇒ free-water reabsorption; concentrates urine.

Key Processes in Urine Formation

1. Filtration (glomerulus) — passive, pressure-driven.
2. Reabsorption (mainly PCT) — selective reclamation of useful solutes/H₂O.
3. Secretion — additional solutes (creatinine, drugs, H⁺, K⁺, uric acid) moved from peritubular capillaries into tubule (PCT & DCT).
4. Excretion — final urine leaves body.

Counter-Current Mechanism

• Loop of Henle & vasa recta create medullary osmotic gradient; allows kidney to make either dilute or concentrated urine as needed so we don’t “drink water all day.”

Hormonal & Peptide Regulators

• ADH (posterior pituitary): ↑ H₂O reabsorption in collecting duct.
• Aldosterone (adrenal cortex): ↑ Na⁺ (and thus H₂O) reabsorption in late DCT.
• Renin (juxtaglomerular cells): initiates RAAS when BP ↓.
• Natriuretic peptides
  • ANP (atria), BNP (ventricles/brain), CNP (endothelium)
  • Promote natriuresis & diuresis by ↑ GFR & inhibiting Na⁺ reabsorption → rapid ↓ blood volume.

Glomerular Filtration Rate (GFR)

• Quantifies kidney function; normally high.
• \text{↓ GFR} ⇒ impaired renal function; declines naturally with age; also influenced by BP, volume status, disease, drugs.

Nitrogenous Waste Products

• Urea: from amino-acid catabolism; 95 % recycled/handled by kidneys; excess blood urea nitrogen (BUN) ⇒ uremia (N/V, lethargy, anorexia).
• Creatinine: from creatine phosphate; used clinically to estimate GFR (Cr clearance).
• Uric acid: from nucleic-acid breakdown; accumulation ⇒ gout.
• Historical note: Wöhler’s synthesis of urea launched the field of biochemistry.

Urine Composition & Volumes

• \approx 95\% water.
• Solutes: urea, uric acid, creatinine, electrolytes, hormones, drugs.
• Volume/Concentration governed by ADH level & hydration.
• Bladder capacity: 600\;\text{mL}; urge to void at \sim 150\;\text{mL}.

Urinalysis (Dipstick & Microscopy)

• Parameters tested: pH, specific gravity, glucose, ketones, blood, leukocyte esterase, nitrites, protein, bilirubin / urobilinogen, crystals/casts (microscopy).
• Sample ideally a clean-catch mid-stream to minimize contamination.
• Uses: detect UTI (nitrite, LE), kidney disease (protein, blood, casts), diabetes (glucose, ketones), liver disease (bilirubin).

Ureters

• Muscular tubes (transitional epithelium + smooth muscle) using peristalsis to carry urine □ kidney → bladder.
• Can reverse flow (ureterorenal reflex) if obstructed.
• Kidney stones lodged here cause severe colic pain.

Urinary Bladder

• Hollow pelvic organ; detrusor smooth muscle (symp/parasymp innervation).
• Transitional epithelium allows stretching.
• Trigone: triangular area between ureteric & urethral openings.
• Inflammation = cystitis (commonly bacterial).
• Anatomical relations: anterior to uterus (♀); anterior to rectum (♂).

Urethra & Micturition

• Length: much shorter in women ⇒ higher UTI risk; men’s urethra passes through prostate (BPH → obstruction).
• Structure: mucosa with mucus glands; smooth & skeletal muscle for involuntary + voluntary control.
• Micturition reflex parallels defecation reflex: detrusor contraction + internal/external sphincter relaxation; aging/childbirth/pathology may cause incontinence.

Clinical Correlations & Pearls

• Renal failure ⇒ metabolic acidosis, fluid overload, toxin/drug accumulation, ↓ insulin clearance (diabetics need ↓ dose), anemia (↓ EPO), bone disease (↓ vit D).
• Diuretics:
  • Loop (e.g., furosemide) act on ascending limb.
  • Thiazides act on early DCT ("hydrochlorothiazide" name origin from loop of Henle discussion).
• Gout due to uric acid crystals; assessed via serum uric acid & urinalysis.
• Stones → hematuria, obstruction, hydronephrosis, severe flank pain.
• Prostatic hypertrophy causes male outflow obstruction; may need TURP surgery.

Imaging Orientation Note

• CT/MRI axial views: patient supine, feet toward viewer, right side on left of screen.
• Kidneys & great vessels (IVC, aorta) are retroperitoneal; liver dominates RUQ; GI tract lies anterior to kidneys; ureters track down to bladder; descending colon becomes sigmoid/rectum.

Quick-Review Question Answers (from lecture)

• Urine composition 95\% water.
• ADH reabsorbs water at collecting duct.
• Ureters carry urine kidney → bladder.
• Glomerulus site of nephron filtration.
• Kidney does not produce bile (liver function).