BiologyNEB 2076 (old course)
Describe the structure and function of uriniferous tubule with labeled diagram. OR Describe the structure of female reproductive organs with menstrual cycle in human being.
8Answer
Structure of the Uriniferous Tubule
The uriniferous tubule is the tubular part of a nephron that processes the glomerular filtrate into urine.
It consists of four distinct segments, each with specialized epithelial cells and transport mechanisms:
| Segment | Location in nephron | Key epithelial cells | Major transporters / channels | Primary function |
|---|---|---|---|---|
| Proximal Convoluted Tubule (PCT) | Immediately after the glomerulus | Cuboidal epithelium with microvilli (brush border) | Na⁺/H⁺ exchanger (NHE3), Na⁺/K⁺‑ATPase, aquaporin‑1 | Reabsorption of ~65 % of filtered Na⁺, water, glucose, amino acids, and bicarbonate |
| Loop of Henle | Descending limb → ascending limb | Descending: simple squamous; ascending: simple cuboidal | Water channels (AQP1) in descending limb; Na⁺/K⁺/2Cl⁻ cotransporter (NKCC2) in thick ascending limb | Establishes medullary osmotic gradient; reabsorption of NaCl in ascending limb |
| Distal Convoluted Tubule (DCT) | After ascending limb | Simple cuboidal | Na⁺/Cl⁻ cotransporter (NCC), ENaC, H⁺‑ATPase | Fine‑tuning of Na⁺, K⁺, Ca²⁺, and H⁺ balance; regulated by aldosterone and ADH |
| Collecting Duct | Final segment, converges from many nephrons | Principal cells & intercalated cells | AQP2 (ADH‑regulated), ENaC, H⁺‑ATPase | Final concentration of urine; secretion of K⁺ and H⁺ |
The entire tubular system is surrounded by a network of peritubular capillaries that provide the blood supply for reabsorption and secretion.
Function of the Uriniferous Tubule
The primary role of the uriniferous tubule is to transform the glomerular filtrate into urine by selectively reabsorbing useful solutes and secreting waste products. The process can be summarised in three stages:
Reabsorption – Movement of solutes and water from the tubular lumen back into the peritubular capillaries.
- PCT: Reabsorbs ~65 % of Na⁺, water, glucose, amino acids, and bicarbonate.
- Loop of Henle: Thick ascending limb reabsorbs NaCl (counter‑current multiplication).
- DCT & Collecting Duct: Fine regulation of Na⁺, K⁺, Ca²⁺, and H⁺ under hormonal control.
Secretion – Transfer of substances from blood into the tubular lumen.
- PCT: Secretes organic acids, drugs, and excess H⁺.
- DCT: Secretes K⁺ and H⁺ (intercalated cells).
- Collecting Duct: Secretes K⁺ and H⁺ under aldosterone influence.
Excretion – The remaining filtrate, now urine, is transported to the renal pelvis and excreted.
- The final urine volume and composition depend on the balance of reabsorption and secretion, which is modulated by hormones such as aldosterone, antidiuretic hormone (ADH), and atrial natriuretic peptide (ANP).
Transport Mechanisms in Detail
- Na⁺/K⁺‑ATPase: Located on the basolateral membrane of PCT, DCT, and collecting duct cells; pumps 3 Na⁺ out and 2 K⁺ in, creating a Na⁺ gradient that drives secondary active transport.
- Na⁺/H⁺ Exchanger (NHE3): In PCT, exchanges Na⁺ for H⁺, facilitating Na⁺ reabsorption and acid secretion.
- Na⁺/Cl⁻ Cotransporter (NCC): In DCT, reabsorbs NaCl; inhibited by thiazide diuretics.
- Na⁺/K⁺/2Cl⁻ Cotransporter (NKCC2): In thick ascending limb, reabsorbs NaCl; inhibited by loop diuretics.
- Aquaporins (AQP1, AQP2): Water channels; AQP1 is constitutively expressed in PCT and descending limb; AQP2 is ADH‑regulated in collecting duct, controlling water reabsorption.
- H⁺‑ATPase & H⁺/K⁺‑ATPase: In intercalated cells, mediate acid–base balance.
Clinical Relevance
- Diuretics:
- Loop diuretics (e.g., furosemide) inhibit NKCC2 → decreased NaCl reabsorption → increased urine output.
- Thiazide diuretics inhibit NCC → similar effect but less potent.
- Potassium‑sparing diuretics (e.g., spironolactone) block ENaC → reduce K⁺ secretion.
- Acid–Base Disorders:
- Metabolic acidosis stimulates H⁺ secretion in intercalated cells.
- Metabolic alkalosis reduces H⁺ secretion.
- Hypertension:
- Aldosterone excess → increased Na⁺ reabsorption in DCT and collecting duct → volume expansion.
- ANP reduces Na⁺ reabsorption → natriuresis.
Summary
The uriniferous tubule is a highly specialized tubular system that orchestrates the selective reabsorption and secretion of ions, water, and metabolites. Its segmented architecture—PCT, loop of Henle, DCT, and collecting duct—allows for precise regulation of body fluid composition and volume. Understanding its structure and function is essential for diagnosing and managing renal disorders, interpreting laboratory values, and applying pharmacologic interventions such as diuretics.
Discussion
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