Parts of the Nephron

Why This Matters

The nephron is the structural and functional unit of the kidney. It transforms blood into urine through a sequence of filtration, reabsorption, and secretion. Understanding how each segment contributes to homeostasis connects directly to cardiovascular physiology, acid-base balance, and hormonal regulation.

Don't just memorize the parts in order. Focus on what each structure does and why its unique features make that function possible. Ask yourself: Is this segment filtering, reabsorbing, secreting, or regulating? What makes it permeable or impermeable? What hormones act here? When you can answer those questions, you'll handle both multiple-choice and free-response questions confidently.

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Filtration: Where It All Begins

The nephron's first job is to create filtrate from blood plasma. This happens in the renal corpuscle, where high pressure forces water and small solutes out of the blood while retaining cells and large proteins. The key principle is pressure-driven bulk filtration across a selectively permeable barrier.

Renal Corpuscle (Bowman's Capsule and Glomerulus)

• Glomerulus: a specialized capillary tuft where glomerular filtration occurs. Its fenestrated (porous) endothelium allows passage of water, ions, glucose, and small molecules while blocking blood cells and most proteins.
• Bowman's capsule: a double-walled epithelial cup surrounding the glomerulus that collects filtrate. The visceral (inner) layer contains podocytes, cells with foot processes that wrap around capillaries and leave narrow filtration slits between them.
• Filtration membrane: a three-layer barrier consisting of the fenestrated endothelium, the basement membrane, and the podocyte filtration slits. Together these layers determine what enters the tubular system. Damage to any layer causes proteinuria (protein in the urine).

Afferent Arteriole

• Delivers blood to the glomerulus: its diameter directly controls how much blood enters for filtration.
• Regulates glomerular filtration rate (GFR): vasodilation increases GFR; vasoconstriction decreases it. Responds to sympathetic nervous input and local signals.
• Autoregulation site: the myogenic response (smooth muscle contracts when stretched) and tubuloglomerular feedback maintain stable GFR despite blood pressure fluctuations.

Efferent Arteriole

• Carries filtered blood away from the glomerulus: its smaller diameter compared to the afferent arteriole helps maintain high glomerular capillary pressure.
• Critical for GFR regulation: constriction increases glomerular pressure and filtration. Angiotensin II preferentially constricts the efferent arteriole during low blood pressure to preserve GFR.
• Supplies downstream capillaries: branches into peritubular capillaries (cortical nephrons) or vasa recta (juxtamedullary nephrons).

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Bulk Reabsorption: Reclaiming the Essentials

After filtration, the nephron must recover valuable substances before they're lost in urine. The proximal convoluted tubule handles the heavy lifting. This segment prioritizes quantity over fine-tuning.

Proximal Convoluted Tubule (PCT)

• Reabsorbs ~65% of filtered water and sodium: the $Na^+/K^+$-ATPase pump on the basolateral membrane creates a sodium gradient that drives secondary active transport of glucose, amino acids, and other solutes. Water follows by osmosis.
• Brush border (microvilli): dramatically increases surface area for absorption. Loss of this border, as in acute tubular necrosis, severely impairs reabsorption.
• Secretes organic acids and bases: eliminates drugs, toxins, and metabolic waste like creatinine. This secretory function is why creatinine clearance is used clinically to estimate GFR.

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Concentration Gradient: Creating the Osmotic Engine

The kidney's ability to produce concentrated urine depends entirely on the osmotic gradient in the renal medulla. The Loop of Henle and its parallel blood supply (vasa recta) work together through countercurrent mechanisms to establish and maintain this gradient. Without this system, you couldn't conserve water during dehydration.

Loop of Henle

• Descending limb: permeable to water but not to solutes. Water leaves by osmosis as tubular fluid descends into the increasingly hypertonic medulla, concentrating the filtrate.
• Ascending limb: impermeable to water. The thick ascending limb actively transports $Na^+$, $K^+$, and $Cl^-$ out via NKCC2 transporters, diluting the tubular fluid while adding solutes to the medullary interstitium. (Loop diuretics like furosemide block NKCC2 here.)
• Countercurrent multiplier: the opposing flow directions and different permeabilities create a progressively concentrated medulla, reaching up to 1200 mOsm/L at the papilla. This gradient is what allows the collecting duct to concentrate urine later.

Vasa Recta

• Specialized capillaries paralleling the Loop of Henle: found only in juxtamedullary nephrons. They supply oxygen and nutrients to the medulla without washing away the osmotic gradient.
• Countercurrent exchanger: blood flowing down into the medulla gains solutes and loses water; blood flowing up loses solutes and gains water. This is a passive process that preserves medullary hypertonicity.
• Clinical relevance: damage to the vasa recta impairs urine concentrating ability, which is why medullary ischemia causes concentrating defects.

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Fine-Tuning: Hormonal Control of Final Composition

The distal nephron segments (DCT and collecting duct) make final adjustments to urine composition based on the body's current needs. These segments respond to hormones like aldosterone, ADH, and parathyroid hormone to precisely regulate electrolyte and water balance. This is where homeostatic feedback loops meet nephron anatomy.

Distal Convoluted Tubule (DCT)

• Hormone-regulated reabsorption: aldosterone increases $Na^+$ reabsorption (and $K^+$ secretion); parathyroid hormone (PTH) increases $Ca^{2+}$ reabsorption.
• Secretes $K^+$ and $H^+$: principal cells handle potassium balance; intercalated cells manage acid-base balance by secreting $H^+$ or reabsorbing $HCO_3^-$ (bicarbonate).
• Contains macula densa cells: specialized epithelial cells that sense tubular $NaCl$ concentration and signal the juxtaglomerular apparatus. This is the sensory arm of tubuloglomerular feedback.

Collecting Duct

• Final water reabsorption site: ADH (antidiuretic hormone) triggers insertion of aquaporin-2 channels into the apical membrane, making the duct permeable to water. Without ADH, the duct stays impermeable and dilute urine is produced.
• Urea recycling: the inner medullary collecting duct reabsorbs urea, which contributes roughly 50% of the medullary osmotic gradient. This process is also regulated by ADH.
• Convergence point: multiple nephrons drain into each collecting duct, which eventually merges into the renal pelvis. This is the final opportunity to modify urine before excretion.

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Vascular Support: Reabsorption and Secretion Partners

The peritubular capillaries form a low-pressure network that receives everything the tubules reabsorb and delivers substances for secretion. Without them, reabsorbed materials would have nowhere to go.

Peritubular Capillaries

• Surround cortical nephron tubules: arise from efferent arterioles. Their low hydrostatic pressure and high oncotic pressure (due to proteins concentrated by filtration) favor reabsorption from the interstitial fluid.
• Reabsorption highway: water, glucose, amino acids, $Na^+$, and other reabsorbed substances enter here to return to systemic circulation.
• Secretion source: deliver waste products ($H^+$, $K^+$, drugs, creatinine) to tubular cells for secretion into the filtrate.

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Regulation: The Nephron's Control Center

The juxtaglomerular apparatus (JGA) acts as a sensor-effector unit that monitors filtrate composition and blood pressure, then adjusts nephron function accordingly. This structure integrates local feedback with systemic hormonal control.

Juxtaglomerular Apparatus (JGA)

• Location: where the DCT contacts the afferent arteriole. It includes three components: juxtaglomerular (JG) cells, the macula densa, and extraglomerular mesangial cells.
• Renin release: JG cells are modified smooth muscle cells in the afferent arteriole wall. They secrete renin when blood pressure drops or the macula densa senses low $NaCl$. Renin initiates the renin-angiotensin-aldosterone system (RAAS), which ultimately raises blood pressure and increases $Na^+$ reabsorption.
• Tubuloglomerular feedback: when macula densa cells detect high $NaCl$ in the filtrate (indicating GFR is too high), they signal the afferent arteriole to constrict, reducing GFR. This is a local autoregulatory mechanism independent of systemic hormones.

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Quick Reference Table

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Self-Check Questions

1. Which two nephron segments are impermeable to water, and how does this impermeability serve a different function in each?

2. A patient has low ADH levels. Which nephron structure will be most affected, and what will happen to urine concentration?

3. Compare the roles of the afferent and efferent arterioles in regulating GFR. How would constricting each one differently affect filtration?

4. The Loop of Henle and vasa recta both use countercurrent arrangements. Explain how one creates the medullary gradient while the other preserves it.

5. Trace the path of a glucose molecule from the glomerulus back to the bloodstream. Which structures are involved, and what transport mechanisms move glucose at each step?