Excretory System

Excretory System

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

The excretory system removes nitrogenous wastes (urea, uric acid, creatinine), maintains osmoregulation, and regulates acid-base balance. The paired kidneys sit retroperitoneally at T12–L3; each contains ~1 million nephrons, the structural and functional units. Urine formation proceeds in three steps: glomerular ultrafiltration at the renal corpuscle, selective reabsorption along the tubule, and tubular secretion into the lumen.

Must-know figures: GFR ≈ 125 mL/min (180 L/day); Net Filtration Pressure ≈ 10 mmHg (P_GC 55 − [P_BS 15 + π_GC 30]); renal threshold for glucose ≈ 180 mg/dL; normal urine output 1–2 L/day with an obligatory minimum of 500 mL. The counter-current multiplier in the loop of Henle and ADH-regulated water reabsorption in the collecting duct concentrate urine to ~1200 mOsm/L. MDCAT weight: ~3% under Biology (Zoology block); expect 1–2 MCQs on nephron structure or hormonal control.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Nephron Anatomy

Each nephron begins with a renal corpuscle (glomerulus + Bowman’s capsule) and continues as PCT → Loop of Henle → DCT → collecting duct. Two populations exist: cortical nephrons (~85%) with short loops in the cortex, and juxtamedullary nephrons (~15%) whose long loops descend into the medulla and generate the osmotic gradient. The vasa recta (peritubular capillaries around the loop) act as counter-current exchangers, preserving the medullary gradient established by the loop’s multiplier.

Three Processes of Urine Formation

1. Ultrafiltration — Blood flows through the glomerular capillary under high hydrostatic pressure. Podocytes with filtration slits and a basement membrane produce a filtrate identical to plasma except for plasma proteins. Net filtration pressure = P_GC − (P_BS + π_GC) ≈ 10 mmHg.

2. Selective reabsorption — The PCT recovers ~65–70% of Na⁺, water, and all glucose and amino acids via Na⁺-coupled symporters. The thick ascending limb of the loop actively pumps out Na⁺/K⁺/2Cl⁻ (impermeable to water), while the descending limb is water-permeable — this asymmetry is the counter-current multiplier. The DCT fine-tunes Na⁺ via aldosterone and Ca²⁺ via PTH.

3. Tubular secretion — H⁺, K⁺, NH₃, urea, creatinine, and organic acids/drugs are secreted from peritubular capillaries into the tubule, enabling precise acid-base regulation.

Hormonal Regulation

• ADH (vasopressin) — opens aquaporin-2 channels in the collecting duct → water reabsorption → concentrated urine.
• Aldosterone — increases Na⁺ reabsorption and K⁺ secretion in DCT/collecting duct.
• ANP (from atria) — opposes aldosterone; promotes Na⁺ and water excretion.
• Renin-angiotensin-aldosterone system (RAAS) — juxtaglomerular cells release renin when renal perfusion drops, raising blood pressure and aldosterone release.

Micturition and Clearance

The micturition reflex is a spinal reflex (centres at S2–S4) stretch-activated by a full bladder, modulated by cortical inhibition. Clearance of a substance C_x = (U_x × V) / P_x; inulin clearance equals GFR, while creatinine clearance ≈ GFR (clinical kidney marker). About 99% of the 180 L filtered daily is reabsorbed.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Counter-current Mechanism in Detail

The descending limb is permeable to water but not solutes, so filtrate concentrates as it descends into the hypertonic medulla (osmolality rises from ~300 to ~1200 mOsm/kg at the papilla). The thick ascending limb actively extrudes NaCl but is impermeable to water, diluting tubular fluid back to ~100 mOsm/kg by the cortex. This single effect, multiplied by the flow loop, builds the cortico-medullary osmotic gradient. Vasa recta — hairpin capillaries with descending and ascending limbs — permit passive exchange that supplies oxygen without washing out the gradient.

Acid-Base and Nitrogenous Waste Handling

The PCT generates NH₄⁺ from glutamine deamination to buffer H⁺ secretion; the collecting duct’s intercalated cells secrete H⁺ (α-cells) or HCO₃⁻ (β-cells), giving the kidney final authority over plasma pH (~7.35–7.45). Mammals primarily excrete urea (highly soluble, low toxicity); birds and reptiles convert it to uric acid (paste-like, water-conserving). Creatinine from muscle creatine breakdown is filtered and minimally secreted — its plasma concentration inversely tracks GFR, making serum creatinine a clinical kidney-function index.

Common MDCAT Traps

• Confusing filtration fraction (GFR/RPF ≈ 20%) with GFR itself.
• Forgetting that glucose is fully reabsorbed below the renal threshold (180 mg/dL) — glycosuria appears only when Tm is exceeded.
• Assuming ADH changes Na⁺ reabsorption (it does not; it alters water permeability).
• Mixing up Bowman’s capsule hydrostatic pressure (opposes filtration) with glomerular hydrostatic pressure (drives filtration).

Adjacent Connections

The kidney interfaces with the circulatory system (RAAS, ANP, erythropoietin from peritubular fibroblasts for RBC production) and the endocrine system (calcitriol activation via 1α-hydroxylase). Failure of counter-current multiplication explains why loop diuretics (furosemide) abolish the medullary gradient and produce hypotonic urine.

Practice Prompts

1. If plasma creatinine doubles while urine creatinine concentration stays the same, what happens to GFR and clearance, assuming urine flow is unchanged?
2. Predict urine osmolality and output in a patient with cranial diabetes insipidus versus primary hyperaldosteronism — explain using aquaporin and Na⁺/K⁺ handling.

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