Q3 LONG ANSWER 10M ★ High-yield

Describe the histology of the kidney. Explain the filtration barrier and how the proximal and distal tubules are distinguished.

Anatomy · 1st Professional — Pre-Clinical

Cortex — contains the renal corpuscles, proximal and distal convoluted tubules, and the medullary rays
Medulla — contains the loops of Henle, collecting ducts and vasa recta, arranged in pyramids
The nephron is the functional unit — about 1 to 1.3 million per kidney
Cortical nephrons (85%) have short loops; juxtamedullary nephrons (15%) have long loops and are responsible for concentrating the urine

Component	Structure
Glomerulus	A tuft of fenestrated capillaries between an afferent and an efferent arteriole
Bowman capsule	Parietal layer — simple squamous; visceral layer — podocytes
Podocytes	Send pedicels (foot processes) that interdigitate, leaving filtration slits bridged by slit diaphragms
Mesangial cells	Between the capillaries; phagocytic and contractile; regulate filtration
Vascular pole	Where the arterioles enter and leave
Urinary pole	Where the proximal tubule begins

Feature	Proximal convoluted tubule	Distal convoluted tubule
Number in a section	More numerous (it is longer)	Fewer
Lumen	Narrow and indistinct, filled with brush border	Wide and clear
Brush border	Present	Absent
Cytoplasm	Deeply eosinophilic (many mitochondria)	Paler
Nuclei per cross-section	Fewer (3–5), widely spaced	More (6–8), closely spaced
Cell borders	Indistinct	More distinct

Loop of Henle — thin segment: simple squamous, resembling a capillary but with no blood cells in the lumen and a thicker wall
Collecting duct: pale simple cuboidal to columnar with very distinct cell boundaries — the most easily recognised tubule of the medulla

Component	Derived from	Function
Macula densa	Distal convoluted tubule where it touches its own glomerulus	Senses the sodium chloride concentration of the tubular fluid
Juxtaglomerular (granular) cells	Modified smooth muscle of the afferent arteriole	Secrete renin
Extraglomerular mesangial (lacis) cells	Mesangium	Signalling between the two

It is the sensor and the effector of blood pressure control in one place — a fall in perfusion or in sodium delivery triggers renin release and the renin–angiotensin–aldosterone cascade

Lined throughout by transitional epithelium (urothelium) — from the minor calyces to the upper urethra
Ureter — a characteristic star-shaped lumen; muscle in inner longitudinal and outer circular layers (the reverse of the gut), with a third outer longitudinal layer in the lower third
Bladder — thick three-layered muscle (the detrusor); the urothelium is thick when empty and thin when full
Umbrella cells at the surface have rigid plaques and are impermeable to urine

The kidney receives about 20–25% of the cardiac output for only 0.5% of body weight
Two capillary beds in series — the glomerular (high pressure, for filtration) and the peritubular (low pressure, for reabsorption), connected by the efferent arteriole
Vasa recta — long straight vessels accompanying the loops of Henle of juxtamedullary nephrons; they run down and back in a hairpin
This countercurrent arrangement preserves the medullary osmotic gradient — a straight capillary would wash it away
The medulla therefore works at the edge of hypoxia, since flow must be kept low; this is why the thick ascending limb and the outer medulla are the first to die in ischaemia

Segment	Epithelium	Chief function
Proximal convoluted tubule	Cuboidal, brush border, eosinophilic	Reabsorbs 65–70% of filtrate; all glucose and amino acids
Thin descending limb	Simple squamous	Permeable to water, not to solute
Thin ascending limb	Simple squamous	Permeable to solute, not to water
Thick ascending limb	Cuboidal, no brush border, many mitochondria	Active NaCl reabsorption; the site of action of loop diuretics
Distal convoluted tubule	Cuboidal, clear wide lumen, many nuclei	Sodium reabsorption; site of thiazides
Collecting duct	Pale cuboidal with distinct cell borders; principal and intercalated cells	Water reabsorption under ADH; acid–base handling

Intercalated cells of the collecting duct are darker and handle hydrogen and bicarbonate; principal cells are paler and respond to ADH and aldosterone

Nephrotic syndrome — damage to the filtration barrier; in minimal change disease the light microscope is normal and only electron microscopy shows fusion of the podocyte foot processes
Loss of the negative charge on the basement membrane allows albumin through even when the pores are intact — selective proteinuria
Acute tubular necrosis affects the proximal tubule and the thick ascending limb first, being the most metabolically active segments
Renal biopsy is examined by light microscopy, immunofluorescence and electron microscopy together — each answers a different question
Transitional cell carcinoma can arise anywhere the urothelium extends, and is often multifocal — which is why the whole tract must be examined
Renal cell carcinoma arises from the proximal tubule and has a clear cytoplasm from the glycogen and lipid dissolved out during processing

All questions in Systemic Histology