Urinary System: Anatomy and Nephron Structure

Detailed Notes

21.1 Overview of the Urinary System

The urinary system is the body's primary filtration and waste-elimination system. It consists of four principal organs arranged in series: two kidneys (where blood is filtered and urine is formed), two ureters (muscular tubes that transport urine from each kidney to the bladder), a single urinary bladder (a hollow, distensible reservoir for urine storage), and the urethra (the conduit through which urine exits the body). Beyond waste removal, the kidneys regulate blood volume, blood pressure, electrolyte balance, and acid-base homeostasis; they also produce the hormones erythropoietin (stimulates red blood cell production) and renin (regulates blood pressure via the renin-angiotensin-aldosterone system), and they activate vitamin D to its biologically active form, calcitriol.

The kidneys receive an astonishing 20–25% of cardiac output at rest — approximately 1.2 liters of blood per minute — despite weighing only about 0.5% of total body mass. This massive perfusion reflects the kidneys' continuous, energy-intensive task of filtering the entire plasma volume roughly 60 times per day.

21.2 Kidney Gross Anatomy

The kidneys are bean-shaped, reddish-brown organs located in the retroperitoneal space — behind the peritoneum against the posterior abdominal wall. They extend from approximately T12 to L3, with the right kidney positioned slightly lower than the left because the liver occupies the space above it. Each kidney measures roughly 11–14 cm in length, 6 cm in width, and 3 cm in thickness in an average adult.

External Anatomy

A tough, fibrous renal capsule (the true capsule) directly encases each kidney, providing a barrier against trauma and infection. Outside the renal capsule lies a cushioning layer of perirenal fat (adipose capsule), and beyond that, a dense renal fascia (Gerota's fascia) anchors the kidney to surrounding structures. The medial border of each kidney features a concave notch called the hilum — the entry and exit point for the renal artery, renal vein, ureter, lymphatics, and nerves. The order of structures at the hilum, from anterior to posterior, is: renal vein, renal artery, renal pelvis (mnemonic: V-A-P).

Internal Anatomy

A frontal section through the kidney reveals three distinct regions:

1. Renal Cortex — the outer, granular-looking zone. It appears granular because it contains the renal corpuscles (glomeruli encased in Bowman's capsules) and the convoluted portions of the tubules. Extensions of cortical tissue called renal columns dip inward between the pyramids.

1. Renal Medulla — the inner zone composed of 8–18 cone-shaped tissue masses called renal pyramids (medullary pyramids). The pyramids appear striated because they contain the parallel bundles of collecting ducts and loops of Henle running toward the papilla. The base of each pyramid faces the cortex; the apex, called the renal papilla, points toward the hilum and drains urine into a minor calyx.

1. Renal Sinus — the central cavity that houses the urine-collecting structures: the minor calyces (cup-shaped structures that envelop each renal papilla), the major calyces (formed by the convergence of several minor calyces), and the funnel-shaped renal pelvis (the expanded upper end of the ureter that collects urine from the major calyces and channels it into the ureter proper).

Table 21.1: Internal Kidney Structures

21.3 Renal Blood Supply

The kidneys have one of the most intricately organized vascular trees in the body, featuring a unique portal-like arrangement: an arteriole feeds a capillary bed, which drains into another arteriole rather than a venule. This architecture is essential for creating the high pressure needed for filtration in the glomerulus and the low pressure needed for reabsorption in the peritubular capillaries.

The vascular pathway, in sequence:

1. Renal Artery — branches directly off the abdominal aorta and enters the kidney at the hilum.
2. Segmental Arteries — the renal artery divides into five segmental branches shortly after entering the hilum.
3. Interlobar Arteries — segmental arteries ascend through the renal columns between the pyramids.
4. Arcuate Arteries — at the corticomedullary junction, the interlobar arteries arch over the bases of the pyramids, forming arcuate arteries.
5. Interlobular (Cortical Radiate) Arteries — arcuate arteries send smaller branches radially outward into the cortex.
6. Afferent Arterioles — each interlobular artery gives off numerous afferent arterioles, each supplying a single glomerulus. The afferent arteriole has a wider diameter than the efferent arteriole, helping maintain high glomerular capillary pressure.
7. Glomerulus — a tuft of fenestrated capillaries where filtration occurs. Glomerular capillary hydrostatic pressure is approximately 55 mmHg — significantly higher than in other capillary beds (~17–25 mmHg) — driving filtration.
8. Efferent Arterioles — blood exits the glomerulus through an efferent arteriole (not a venule). This is a unique vascular arrangement (two arterioles in series separated by a capillary bed).
9. Peritubular Capillaries (cortical nephrons) or Vasa Recta (juxtamedullary nephrons) — the efferent arteriole branches into a second capillary network that intimately surrounds the renal tubules. Peritubular capillaries reclaim reabsorbed water and solutes; the vasa recta are long, hairpin-shaped capillaries that descend into the medulla and are critical for the countercurrent mechanism that concentrates urine.
10. Venous Return — peritubular capillaries and vasa recta drain into interlobular veins → arcuate veins → interlobar veins → renal vein → inferior vena cava.

Table 21.2: Renal Vasculature — Key Vessels and Their Roles

Clinical note: The segmental arteries are functional end-arteries — they do not anastomose significantly with one another. Occlusion of a segmental artery results in infarction of that kidney segment.

21.4 The Nephron — The Functional Unit

Each kidney contains approximately 1 million nephrons — the microscopic structural and functional units responsible for filtering blood and forming urine. A nephron consists of two principal parts: the renal corpuscle (where blood plasma is filtered) and the renal tubule (where the filtrate is modified by reabsorption and secretion to produce urine).

There are two structurally and functionally distinct populations of nephrons:

Cortical Nephrons (~85%)

• Renal corpuscles are located in the outer cortex
• Short loops of Henle that barely penetrate the medulla
• Efferent arterioles feed peritubular capillaries that wrap around cortical tubules
• Primarily involved in bulk reabsorption of solutes and water under normal conditions

Juxtamedullary Nephrons (~15%)

• Renal corpuscles are located deep in the cortex, near the corticomedullary junction
• Long loops of Henle that plunge deep into the medulla, sometimes reaching the tips of the renal papillae
• Efferent arterioles give rise to the vasa recta — long, straight capillaries paralleling the loops of Henle
• Essential for producing concentrated urine through the countercurrent multiplier mechanism; their long loops establish the medullary osmotic gradient

Table 21.3: Cortical vs. Juxtamedullary Nephrons

21.5 The Renal Corpuscle — Site of Filtration

The renal corpuscle is a spherical structure, approximately 200 µm in diameter, that performs the first step of urine formation: glomerular filtration. It consists of two components working in intimate structural and functional partnership — the glomerulus (the capillary tuft) and Bowman's capsule (the epithelial envelope).

21.5.1 The Glomerulus

The glomerulus is a tuft of fenestrated capillaries supplied by the afferent arteriole and drained by the efferent arteriole. The endothelial cells of these capillaries are perforated by numerous fenestrations (pores 70–100 nm in diameter) that make them highly permeable to water and small solutes but not to blood cells or platelets. The glomerular capillaries are unique in the body in that they maintain a hydrostatic pressure of approximately 55 mmHg throughout their length — almost entirely dedicated to driving filtration — unlike typical capillary beds where pressure drops significantly from the arteriolar to venular end.

Between the capillary loops reside mesangial cells, contractile cells that provide structural support, regulate glomerular filtration surface area by contracting or relaxing, and phagocytose trapped debris from the filtration membrane.

21.5.2 Bowman's Capsule

Bowman's capsule is a double-walled epithelial cup that surrounds the glomerulus. It has two layers:

• Parietal Layer — the outer wall, composed of simple squamous epithelium. This layer is structural; it does not participate in filtration.
• Visceral Layer — the inner wall, intimately applied to the glomerular capillaries. It is composed of highly specialized epithelial cells called podocytes.

Podocytes have a complex octopus-like morphology: a cell body from which numerous foot-like extensions (pedicels or foot processes) wrap around the glomerular capillaries. The pedicels of adjacent podocytes interdigitate, leaving narrow gaps between them called filtration slits (25–30 nm wide). These slits are bridged by a thin protein meshwork called the slit diaphragm, composed largely of the transmembrane protein nephrin. Mutations in the nephrin gene cause congenital nephrotic syndrome (Finnish type), underscoring the slit diaphragm's essential role as a size-selective filter for proteins.

The space between the parietal and visceral layers is the capsular (Bowman's) space, which receives the glomerular filtrate and channels it into the proximal convoluted tubule at the urinary pole of the corpuscle.

21.5.3 The Filtration Membrane

The structure separating blood in the glomerular capillaries from the filtrate in Bowman's space is the filtration membrane, a three-layered barrier:

1. Fenestrated Endothelium (innermost layer) — the glomerular capillary endothelial cells with their large fenestrations. This layer restricts passage based on size (> 70 nm particles and blood cells are excluded) but is highly permeable to water, ions, and small molecules.

1. Glomerular Basement Membrane (GBM) — a shared, fused basement membrane between the endothelium and podocytes. The GBM is a meshwork of collagen type IV, laminin, and proteoglycans. Crucially, it carries a strong net negative charge due to heparan sulfate proteoglycans, which electrostatically repels negatively charged plasma proteins (particularly albumin). Damage to this charge barrier (e.g., in certain forms of glomerulonephritis) causes proteinuria — the leakage of protein into the urine.

1. Podocyte Filtration Slits (outermost layer) — the slit diaphragms between pedicels. The narrow width (25–30 nm) and the nephrin-based protein lattice constitute the finest size-selective filter, preventing molecules larger than approximately 70 kDa (such as most plasma proteins) from passing under normal conditions.

Together, these three layers permit the free passage of water, electrolytes, glucose, amino acids, urea, and small molecules while retaining blood cells, platelets, and virtually all plasma proteins in the circulation. A healthy adult produces approximately 180 liters of glomerular filtrate per day (glomerular filtration rate, GFR ≈ 125 mL/min), over 99% of which is subsequently reabsorbed by the tubules.

Table 21.4: The Three Layers of the Filtration Membrane

21.6 The Renal Tubule

After filtration at the renal corpuscle, the filtrate travels through a continuous epithelial tube with four sequential segments, each with distinct histological features tailored to its reabsorptive or secretory functions.

21.6.1 Proximal Convoluted Tubule (PCT)

The proximal convoluted tubule is the first and most metabolically active segment of the renal tubule. It is lined by simple cuboidal epithelium with an apical surface covered in a dense brush border (microvilli) that massively amplifies the luminal surface area for reabsorption. The cells are packed with mitochondria, reflecting their enormous demand for ATP to fuel active transport. The PCT reabsorbs approximately 65–70% of filtered water and sodium, along with nearly 100% of filtered glucose, amino acids, and bicarbonate, and most filtered potassium, phosphate, and calcium. It also actively secretes organic acids, bases, and drugs (e.g., penicillin, uric acid) from the peritubular capillaries into the tubular lumen.

Histological hallmarks: tall cuboidal cells with eosinophilic (pink) cytoplasm, prominent brush border, basal striations (infoldings of the basolateral membrane packed with mitochondria), and a fuzzy luminal surface that often obscures the lumen in cross-section.

21.6.2 Loop of Henle

The loop of Henle is a U-shaped tubule that dips into the medulla and returns to the cortex. It creates the hyperosmotic medullary interstitium essential for water conservation. The loop has two limbs with contrasting permeability properties:

• Descending Limb — the thin segment. Lined by simple squamous epithelium with few mitochondria and no brush border. It is highly permeable to water (via aquaporin-1 water channels) but impermeable to solutes (NaCl cannot cross). As the filtrate descends into the increasingly hyperosmotic medullary interstitium, water is drawn out by osmosis, concentrating the tubular fluid.

• Ascending Limb — predominantly the thick ascending limb. Lined by simple cuboidal epithelium with abundant mitochondria. It is impermeable to water (no aquaporins) but actively transports Na⁺, K⁺, and Cl⁻ out of the tubule via the apical NKCC2 co-transporter (Na⁺/K⁺/2Cl⁻ symporter) on the luminal membrane. This active solute transport, combined with water impermeability, dilutes the tubular fluid while building up the medullary interstitial osmotic gradient — the engine of the countercurrent multiplier system.

The term "countercurrent" refers to the opposite directions of flow in the descending and ascending limbs, which amplifies the medullary concentration gradient from ~300 mOsm at the corticomedullary junction to ~1200–1400 mOsm at the papillary tip.

21.6.3 Distal Convoluted Tubule (DCT)

The distal convoluted tubule is shorter and less convoluted than the PCT and is lined by simple cuboidal epithelium with fewer, shorter microvilli (no distinct brush border). The cells are smaller than PCT cells, so more nuclei are visible per cross-section, and the lumen appears wider and cleaner. The DCT is involved in fine-tuning Na⁺, K⁺, Ca²⁺, and acid-base balance, largely under hormonal control: aldosterone stimulates Na⁺ reabsorption and K⁺ secretion; parathyroid hormone (PTH) stimulates Ca²⁺ reabsorption.

A critical specialized region of the DCT is the macula densa — a cluster of tall, closely packed epithelial cells where the DCT makes intimate contact with the afferent arteriole of its own renal corpuscle. The macula densa cells function as chemoreceptors, sensing the NaCl concentration of the tubular fluid flowing past. When NaCl delivery is high (indicating high GFR), the macula densa signals the afferent arteriole to constrict, reducing GFR — this is tubuloglomerular feedback, a vital autoregulatory mechanism that stabilizes GFR.

21.6.4 Collecting Duct

The collecting duct is technically not part of the nephron — it is a separate distal segment into which several DCTs drain — but functionally it is the final arbiter of urine composition. It descends from the cortex through the medulla to empty urine into the minor calyces at the renal papillae.

The collecting duct is lined by two major cell types:

• Principal Cells — the predominant cell type, responsible for Na⁺ reabsorption and K⁺ secretion under aldosterone control. They also express aquaporin-2 (AQP2) water channels, which are inserted into the apical membrane only when antidiuretic hormone (ADH / vasopressin) is present. When ADH is high, the collecting duct becomes permeable to water, and concentrated urine is produced; when ADH is absent, water is not reabsorbed, and dilute urine is produced.

• Intercalated Cells — less numerous, with a critical role in acid-base balance. Type A intercalated cells secrete H⁺ (acid) and reabsorb HCO₃⁻ during acidosis; Type B intercalated cells secrete HCO₃⁻ and reabsorb H⁺ during alkalosis.

21.7 The Juxtaglomerular Apparatus (JGA)

The juxtaglomerular apparatus is a specialized endocrine structure formed at the point where the distal convoluted tubule contacts the afferent and efferent arterioles of its own glomerulus. It is the anatomical basis for renal autoregulation and systemic blood pressure control. The JGA has three components:

1. Macula Densa — a patch of tall, crowded epithelial cells in the wall of the DCT at its point of contact with the afferent arteriole. These cells are NaCl sensors: they detect the NaCl concentration in tubular fluid and signal the afferent arteriole. Low NaCl (indicating low GFR) triggers afferent arteriole dilation and renin release; high NaCl triggers constriction.

1. Juxtaglomerular (Granular) Cells — modified smooth muscle cells in the wall of the afferent arteriole (and to a lesser extent the efferent arteriole). These cells synthesize, store, and secrete the enzyme renin. Renin release is stimulated by three signals: (a) decreased stretch of the afferent arteriole (baroreceptor mechanism — detects low blood pressure), (b) low NaCl sensed by the macula densa (tubuloglomerular feedback), and (c) sympathetic nerve stimulation via β1-adrenergic receptors. Renin catalyzes the first and rate-limiting step of the renin-angiotensin-aldosterone system (RAAS), cleaving angiotensinogen into angiotensin I.

1. Extraglomerular Mesangial Cells (Lacis cells) — located in the triangular space between the afferent and efferent arterioles and the macula densa. Their function is not fully understood, but they are believed to communicate signals between the macula densa and the granular cells, possibly via gap junctions, and may have a contractile/phagocytic role.

Table 21.5: Components of the Juxtaglomerular Apparatus

21.8 Ureters

The ureters are paired muscular tubes, approximately 25–30 cm in length, that propel urine from the renal pelvis to the urinary bladder by peristalsis — rhythmic, wave-like contractions of smooth muscle. The ureter wall has three layers:

• Mucosa — lined by transitional epithelium (urothelium) , a stratified epithelium uniquely adapted to stretch and withstand the toxicity of urine. The luminal surface is thrown into longitudinal folds that allow expansion.
• Muscularis — two layers of smooth muscle in the upper two-thirds (inner longitudinal, outer circular) and a third outer longitudinal layer in the lower third. Peristaltic contractions, initiated by pacemaker cells in the renal pelvis, occur 1–5 times per minute.
• Adventitia — an outer connective tissue layer that anchors the ureter.

The ureters enter the bladder obliquely through the posterior bladder wall, creating a physiological valve: as the bladder fills and distends, the intramural portion of the ureter is compressed, preventing backflow (vesicoureteral reflux). Failure of this valve mechanism predisposes to reflux nephropathy and recurrent pyelonephritis.

21.9 Urinary Bladder

The urinary bladder is a hollow, highly distensible muscular organ that serves as a temporary reservoir for urine. When empty, it sits within the pelvis, posterior to the pubic symphysis; when full, it can expand superiorly into the abdominal cavity.

The bladder wall is composed of these layers:

• Mucosa — lined by transitional epithelium (urothelium) , the same stretch-accommodating tissue as the ureter. The luminal cells (umbrella cells) have specialized plaques of uroplakin proteins that create an impermeable barrier protecting the underlying tissue from urine's acidity and hypertonicity. In the empty bladder, the mucosa is thrown into folds called rugae.
• Submucosa — a connective tissue layer rich in elastic fibers.
• Detrusor Muscle — a thick, three-layered interlacing meshwork of smooth muscle (inner longitudinal, middle circular, outer longitudinal). Contraction of the detrusor is under parasympathetic control (pelvic splanchnic nerves, S2–S4) and is responsible for voiding.
• Serosa/Adventitia — the outer covering.

The trigone is a smooth, triangular region on the posterior bladder floor, defined by three points: the two ureteric orifices superiorly and the internal urethral orifice inferiorly. Unlike the rest of the bladder mucosa, the trigone is always smooth (no rugae) and its epithelium is firmly attached to the underlying muscle. Embryologically, the trigone is derived from the mesonephric ducts, which is why its mucosa is more tightly bound.

At the bladder neck, the detrusor muscle forms the internal urethral sphincter (smooth muscle, under involuntary autonomic control). This sphincter is functional in males (preventing retrograde ejaculation) but less well-defined in females.

21.10 Urethra

The urethra is a thin-walled muscular tube that drains urine from the bladder to the outside. Its anatomy differs significantly between sexes:

Male Urethra (~20 cm)

The male urethra has three anatomically and clinically distinct segments:

1. Prostatic Urethra (3–4 cm) — passes through the prostate gland. The ejaculatory ducts and prostatic ducts empty into this segment, making it a shared passage for urine and semen.
2. Membranous Urethra (1–2 cm) — the shortest segment, passing through the urogenital diaphragm (external urethral sphincter). Surrounded by skeletal muscle of the external sphincter, it provides voluntary control of urination. This segment is the least distensible and most vulnerable to injury during catheterization.
3. Spongy (Penile) Urethra (~15 cm) — the longest segment, running through the corpus spongiosum of the penis. Bulbourethral glands (Cowper's glands) drain into the proximal portion.

Female Urethra (~3–4 cm)

The female urethra is significantly shorter and runs a direct course from the bladder neck to the external urethral orifice, located anterior to the vaginal opening. Its short length and proximity to the anus and vagina are the primary anatomical reasons females have a higher incidence of urinary tract infections (UTIs) — bacteria have a shorter distance to travel to reach the bladder.

In both sexes, the external urethral sphincter is composed of skeletal muscle in the urogenital diaphragm and is under voluntary (somatic) control via the pudendal nerve (S2–S4). This is the sphincter that is consciously relaxed to initiate urination.

The epithelium transitions from transitional epithelium near the bladder to stratified columnar, then to stratified squamous epithelium near the external orifice.

21.11 Clinical Correlations

Nephrolithiasis (Kidney Stones)

Kidney stones are solid crystalline masses formed from urinary solutes that precipitate when their concentration exceeds solubility. The most common type is calcium oxalate (~75%), followed by struvite (magnesium ammonium phosphate, associated with urease-producing bacterial infections), uric acid, and cystine. Stones can lodge at any point along the urinary tract; the most common sites of obstruction are the ureteropelvic junction, the pelvic brim (where the ureter crosses the iliac vessels), and the ureterovesical junction. Classic presentation includes severe, colicky flank pain radiating to the groin, hematuria, and nausea. Risk factors include dehydration, hypercalciuria, hyperoxaluria, and low urine volume.

Urinary Tract Infection (UTI)

A UTI is a bacterial infection, most commonly by Escherichia coli (which ascends from the perineum). In females, the short urethra facilitates ascending infection. Cystitis (bladder infection) presents with dysuria, frequency, urgency, and suprapubic pain. Uncomplicated cystitis is a mucosal infection; when left untreated, bacteria can ascend to the kidneys and cause pyelonephritis — infection of the renal parenchyma — presenting with fever, chills, flank pain, and systemic illness. Pyelonephritis can lead to permanent renal scarring.

Hydronephrosis

Hydronephrosis is the dilation of the renal pelvis and calyces caused by obstruction of urine outflow at any level (stone, tumor, stricture, congenital anomaly, pregnancy, or benign prostatic hyperplasia). The backup of urine increases hydrostatic pressure in the collecting system, which is transmitted retrogradely to the nephrons, compressing the renal parenchyma and causing progressive atrophy. If prolonged and bilateral, hydronephrosis leads to renal failure.

Polycystic Kidney Disease (PKD)

Autosomal dominant polycystic kidney disease (ADPKD) is the most common inherited kidney disorder, caused by mutations in the PKD1 or PKD2 genes encoding polycystin proteins. It is characterized by the progressive development and enlargement of fluid-filled cysts throughout both kidneys, which compress and destroy normal renal parenchyma. Patients typically develop hypertension and progress to end-stage renal disease by middle age. Extrarenal manifestations include hepatic cysts, cerebral berry aneurysms, and cardiac valve abnormalities.

Glomerulonephritis

Glomerulonephritis refers to a group of immune-mediated disorders characterized by inflammation of the glomeruli, often presenting with proteinuria and hematuria. The clinical presentation can be classified:

• Nephrotic syndrome — characterized by massive proteinuria (> 3.5 g/day), hypoalbuminemia, generalized edema, hyperlipidemia, and lipiduria. The underlying defect is damage to the podocyte filtration barrier (e.g., minimal change disease, focal segmental glomerulosclerosis, membranous nephropathy), permitting large amounts of protein (especially albumin) to leak into the filtrate.
• Nephritic syndrome — characterized by hematuria (often with RBC casts), oliguria, azotemia (elevated BUN and creatinine), hypertension, and milder proteinuria (< 3.5 g/day). The underlying defect is immune-complex deposition and/or complement activation causing glomerular inflammation and rupture of the capillary wall (e.g., post-streptococcal glomerulonephritis, IgA nephropathy, lupus nephritis).

In glomerulonephritis, damage to the filtration membrane — whether the charge barrier (GBM heparan sulfate), the podocyte slit diaphragm (nephrin), or both — results in the hallmark finding of protein and/or blood in the urine.