Difficulty: Intermediate–Advanced | Prerequisites: Blood vessel anatomy (Ch. 20, especially capillary types and arterioles), tissue types, peritoneal relationships (Ch. 23)
Tags: urinary system, kidney anatomy, nephron, glomerulus, Bowman's capsule, loop of Henle, renal corpuscle, cortical nephron, juxtamedullary nephron, juxtaglomerular apparatus, ureter, urinary bladder, urethra, micturition, APK2100c
The urinary system filters blood, produces urine, and maintains fluid and electrolyte balance. Structurally, this chapter centres on the kidney and its functional unit, the nephron. You need to know the gross anatomy of the kidney (external coverings, internal regions), the microscopic anatomy of the nephron (which parts are corpuscle, which are tubule), the blood supply running alongside the nephron, and how these structures relate to urine formation. You will also cover the ureters, bladder, and urethra, including the differences between male and female urethras. This builds on your understanding of capillary types and arterioles from Ch. 20.
The kidneys sit retroperitoneally in the upper abdomen, each containing roughly a million nephrons. Each nephron has a renal corpuscle (glomerulus + Bowman's capsule) for filtration and a renal tubule (PCT, loop of Henle, DCT) for reabsorption and secretion. Blood arrives via afferent arterioles, gets filtered in the glomerulus, and the filtrate is processed as it travels through the tubule. Urine drains through collecting ducts into the renal pelvis, down the ureters, into the bladder, and out via the urethra. Micturition is controlled by both autonomic and voluntary neural pathways.
Renal corpuscle
The filtration unit of the nephron, consisting of the glomerulus (capillary tuft) and its surrounding Bowman's capsule (glomerular capsule). Located exclusively in the renal cortex.
Think of it as the filter head of the nephron.
Glomerulus
A ball-like cluster of fenestrated capillaries within Bowman's capsule. Blood pressure forces water and small solutes through the capillary walls into the capsular space.
Bowman's capsule (glomerular capsule)
A double-walled cup that encloses the glomerulus. The visceral layer is made of podocytes (specialised cells with foot processes that wrap around the capillaries, forming filtration slits). The parietal layer is simple squamous epithelium.
Proximal convoluted tubule (PCT)
The first section of the renal tubule after Bowman's capsule. Lined with cuboidal cells bearing a prominent brush border (microvilli) for reabsorption. Handles most reabsorption of water, glucose, amino acids, and ions.
Loop of Henle (nephron loop)
A U-shaped section that dips from the cortex into the medulla and back. It has a descending limb (thin, permeable to water) and an ascending limb (thick portion actively transports ions). Establishes the concentration gradient in the medulla.
Distal convoluted tubule (DCT)
The section after the loop of Henle, located in the cortex. Involved in selective reabsorption and secretion under hormonal control (aldosterone, ADH). Part of the DCT contacts the afferent arteriole to form the juxtaglomerular apparatus.
Collecting duct
Receives filtrate from multiple nephrons. Passes through the medulla to the renal papilla. Under ADH influence, it becomes permeable to water, concentrating the urine.
In simple terms, this is the final concentrating tube before urine exits the kidney.
Cortical nephron
A nephron whose renal corpuscle lies in the outer cortex and whose loop of Henle is relatively short, dipping only into the outer medulla. These make up about 85% of nephrons.
Juxtamedullary nephron
A nephron whose renal corpuscle lies near the cortex-medulla junction and whose loop of Henle is long, plunging deep into the medulla. These are critical for producing concentrated urine.
Juxtaglomerular apparatus (JGA)
A specialised structure where the DCT contacts the afferent arteriole of its own nephron. It consists of juxtaglomerular (JG/granular) cells in the afferent arteriole (secrete renin) and the macula densa cells in the DCT wall (sense NaCl concentration). It plays a key role in regulating blood pressure and filtration rate.
Trigone
A smooth, triangular area on the internal floor of the urinary bladder, defined by three openings: two ureteral orifices (posterolateral) and the internal urethral orifice (inferior/anterior). Clinically important because infections tend to persist here.
Micturition
The process of urination. It involves relaxation of the internal urethral sphincter (smooth muscle, autonomic/parasympathetic control) and the external urethral sphincter (skeletal muscle, voluntary/somatic control via the pudendal nerve).
The urinary system consists of two kidneys, two ureters, one urinary bladder, and one urethra
Kidneys: retroperitoneal, in the superior posterior abdominal wall, flanking the vertebral column at approximately T12–L3
The right kidney sits slightly lower than the left because the liver occupies space above it
Ureters: retroperitoneal muscular tubes running from each kidney to the bladder
Urinary bladder: in the pelvic cavity, posterior to the pubic symphysis
Urethra: from the bladder to the external urethral orifice
External coverings (superficial to deep):
Renal fascia: anchors the kidney to surrounding structures
Perirenal fat capsule (adipose): cushioning
Fibrous (renal) capsule: tough, transparent membrane directly on the kidney surface
Internal anatomy:
Renal cortex: outer region; contains renal corpuscles, convoluted tubules, and the cortical portions of the loops of Henle and collecting ducts
Renal medulla: inner region; contains renal (medullary) pyramids, which are cone-shaped structures made up of loops of Henle and collecting ducts
Renal pyramids: their tips (renal papillae) point toward the renal pelvis; the bases face the cortex
Renal columns: extensions of cortical tissue between the pyramids
Minor calyces: cup-shaped structures that collect urine dripping from the renal papillae
Major calyces: formed by the merging of several minor calyces
Renal pelvis: a funnel-shaped structure formed by the merging of the major calyces; narrows to become the ureter at the hilum
Renal corpuscle (filtration component):
Glomerulus (fenestrated capillary tuft)
Bowman's capsule (visceral layer with podocytes, parietal layer of simple squamous epithelium, capsular space between them)
Renal tubule (processing component):
Proximal convoluted tubule (PCT): cuboidal epithelium with microvilli; reabsorption
Loop of Henle: descending limb (thin, simple squamous, permeable to water) and ascending limb (thick portion, cuboidal, actively pumps ions out)
Distal convoluted tubule (DCT): cuboidal epithelium without prominent microvilli; secretion and selective reabsorption
Collecting duct: not technically part of the nephron but receives filtrate from multiple nephrons; cuboidal to columnar epithelium
Cortical nephrons (~85%):
Corpuscle in the outer cortex
Short loop of Henle, penetrating only into the outer medulla
Peritubular capillaries surround the tubules
Juxtamedullary nephrons (~15%):
Corpuscle near the cortex-medulla boundary
Long loop of Henle, extending deep into the medulla
Peritubular capillaries plus vasa recta (long, straight capillary loops that run alongside the loop of Henle into the medulla)
Critical for concentrating urine via the countercurrent mechanism
Blood takes an unusual path through the kidney, passing through two capillary beds in series:
Renal artery → segmental arteries → interlobar arteries → arcuate arteries (at the cortex-medulla junction) → cortical radiate (interlobular) arteries
Afferent arteriole → enters the glomerulus
Glomerulus (first capillary bed): fenestrated capillaries; site of filtration
Efferent arteriole → exits the glomerulus (note: arteriole to arteriole, not arteriole to venule)
Peritubular capillaries (second capillary bed): low-pressure capillaries surrounding the PCT and DCT; site of reabsorption and secretion
In juxtamedullary nephrons, the efferent arteriole also gives rise to vasa recta, which descend into the medulla alongside the loop of Henle
Peritubular capillaries/vasa recta → cortical radiate (interlobular) veins → arcuate veins → interlobar veins → renal vein
The key exam point: blood passes through two arterioles and two capillary beds (glomerulus, then peritubular capillaries) in the kidney.
The glomerular capillaries are fenestrated (porous), allowing water and small solutes to pass through
The filtration membrane has three layers:
Fenestrated endothelium of the glomerular capillary
Basement membrane (fused basal laminae of the endothelium and the podocytes)
Filtration slits between the podocyte foot processes (pedicels)
This membrane allows water, ions, glucose, amino acids, urea, and other small molecules to pass into the capsular space
It blocks blood cells and most proteins (too large)
The high blood pressure in the glomerulus (maintained by the difference in diameter between the afferent and efferent arterioles) drives filtration
Located where the DCT passes between and contacts the afferent and efferent arterioles of its own nephron
Macula densa: modified, tall epithelial cells in the DCT wall that sense NaCl concentration in the filtrate
Juxtaglomerular (JG/granular) cells: modified smooth muscle cells in the wall of the afferent arteriole that secrete renin
Extraglomerular mesangial cells: between the arterioles and the macula densa; role in signalling
Physiological significance: the JGA is a feedback sensor. When NaCl or blood pressure drops, the macula densa signals JG cells to release renin, triggering the renin-angiotensin-aldosterone system (RAAS), which raises blood pressure and increases sodium/water reabsorption
Ureters:
~25–30 cm long, retroperitoneal
Wall: mucosa (transitional epithelium), muscularis (smooth muscle: inner longitudinal, outer circular; lower third adds a third outer longitudinal layer), adventitia
Peristalsis propels urine from the renal pelvis to the bladder
Urinary bladder:
Muscular sac in the pelvic cavity
Wall: mucosa (transitional epithelium with rugae), submucosa, muscularis (detrusor muscle: three layers of smooth muscle), serosa/adventitia
Trigone: smooth triangular area on the posterior floor; its three corners are the two ureteral orifices and the internal urethral orifice
Structures contributing to the trigone: the two ureters (their openings define the superior two corners) and the urethra (its internal opening defines the inferior corner)
Urethra:
Carries urine from the bladder to the exterior
Female urethra: short (~3–4 cm), runs anterior to the vagina; carries only urine
Male urethra: long (~18–20 cm), passes through the prostate, urogenital diaphragm, and penis; carries both urine and semen; has three named regions: prostatic urethra, membranous urethra, spongy (penile) urethra
Internal urethral sphincter: smooth muscle at the bladder-urethra junction; involuntary control (autonomic)
External urethral sphincter: skeletal muscle in the urogenital diaphragm; voluntary control (somatic, pudendal nerve)
Both sexes have both sphincters
The pelvic floor (pelvic diaphragm) is formed primarily by the levator ani and coccygeus muscles
These are skeletal muscles
They support the pelvic organs (bladder, uterus in females, rectum) and help maintain urinary and faecal continence
Functional significance: weakness or damage (e.g., after childbirth) can lead to pelvic organ prolapse and urinary incontinence
Micturition (urination):
As the bladder fills, stretch receptors in the detrusor muscle send afferent signals via pelvic splanchnic nerves to the sacral spinal cord
The micturition reflex is a spinal (parasympathetic) reflex: parasympathetic efferents cause the detrusor to contract and the internal urethral sphincter to relax
Voluntary control comes from the brain: descending signals via the pudendal nerve keep the external urethral sphincter contracted until the individual chooses to urinate
When ready, the brain inhibits the pudendal nerve, the external sphincter relaxes, and urine is expelled
The fenestrated capillaries of the glomerulus and the filtration membrane are why kidney disease often shows up first as protein in the urine (proteinuria): if the filtration barrier is damaged, proteins that should be retained leak through. The JGA and the RAAS pathway are the targets of ACE inhibitors and ARBs, two of the most commonly prescribed classes of blood pressure medication. Understanding the structural difference between the male and female urethra explains why urinary tract infections are far more common in women: the shorter urethra provides less distance for bacteria to travel.
Students often think the efferent arteriole leads directly to a vein. It does not. It leads to a second capillary bed (peritubular capillaries or vasa recta) before the blood reaches the venous system. The kidney has two capillary beds in series.
The collecting duct is not technically part of the nephron, even though it is essential for urine concentration. The nephron ends at the DCT.
The loop of Henle is not entirely "thin." The descending limb is thin-walled, but the ascending limb has a thick segment with active ion transport.
Transitional epithelium is not the same as stratified squamous. It is a special epithelium that stretches, and it lines the ureters, bladder, and part of the urethra. Students sometimes write "stratified squamous" for the bladder lining on exams.
⚠️ Classify each nephron structure as corpuscle or tubule: glomerulus and Bowman's capsule = corpuscle; PCT, loop of Henle, DCT = tubule.
⚠️ Know the path of blood through the kidney: renal artery → afferent arteriole → glomerulus → efferent arteriole → peritubular capillaries/vasa recta → renal vein. Two arterioles, two capillary beds.
⚠️ Three layers of the filtration membrane: fenestrated endothelium, basement membrane, podocyte filtration slits.
⚠️ Components and significance of the JGA: macula densa (senses NaCl), JG cells (secrete renin).
⚠️ Structures forming the trigone of the bladder (two ureteral orifices + internal urethral orifice).
⚠️ Male vs. female urethra: length, regions, what it carries.
⚠️ Micturition reflex: parasympathetic contracts detrusor and relaxes internal sphincter; pudendal nerve (somatic) controls external sphincter voluntarily.
True or False: The renal corpuscle is located in the renal medulla.
Fill in the blank: The afferent arteriole leads into the __________, and the efferent arteriole leads out to the __________.
True or False: Cortical nephrons have long loops of Henle that extend deep into the medulla.
Fill in the blank: The JGA cells that secrete renin are called __________ cells.
True or False: The male urethra is about 4 cm long.
Answers: 1. False (it is in the cortex). 2. Glomerulus; peritubular capillaries (or vasa recta). 3. False (juxtamedullary nephrons have the long loops). 4. Juxtaglomerular (JG/granular). 5. False (approximately 18–20 cm).
Q: Name the structures of the nephron and classify each as part of the corpuscle or the tubule.
A: Corpuscle: glomerulus and Bowman's capsule. Tubule: proximal convoluted tubule (PCT), loop of Henle (descending and ascending limbs), and distal convoluted tubule (DCT).
Q: Trace the path of blood through the kidney from the renal artery to the renal vein.
A: Renal artery → segmental arteries → interlobar arteries → arcuate arteries → cortical radiate arteries → afferent arteriole → glomerulus (first capillary bed) → efferent arteriole → peritubular capillaries/vasa recta (second capillary bed) → cortical radiate veins → arcuate veins → interlobar veins → renal vein.
Q: What are the three layers of the glomerular filtration membrane?
A: Fenestrated endothelium of the glomerular capillary, the fused basement membrane, and the filtration slits formed by podocyte foot processes.
Q: What are the components of the juxtaglomerular apparatus and what is its physiological significance?
A: The JGA consists of the macula densa (modified DCT cells that monitor NaCl concentration), juxtaglomerular (granular) cells (modified smooth muscle cells in the afferent arteriole that secrete renin), and extraglomerular mesangial cells. It regulates blood pressure and glomerular filtration rate through the renin-angiotensin-aldosterone system.
Q: Compare and contrast the male and female urethras.
A: The female urethra is short (~3–4 cm) and carries only urine. The male urethra is much longer (~18–20 cm) and has three regions: prostatic, membranous, and spongy (penile); it carries both urine and semen. Both have an internal urethral sphincter (smooth muscle, involuntary) and an external urethral sphincter (skeletal muscle, voluntary).
Q: What structures define the trigone of the bladder?
A: The two ureteral orifices (where the ureters enter the bladder) and the internal urethral orifice (where the urethra exits), forming a smooth triangular area on the posterior bladder floor.
This connects to Blood Vessels (Ch. 20) through the fenestrated capillaries of the glomerulus and the concept of two capillary beds in series (analogous to the hepatic portal system, but with two arterioles rather than an artery and a vein). The retroperitoneal position of the kidneys builds on peritoneal relationships from Ch. 23. The male urethra connects to the Reproductive System (Ch. 25) because it passes through the prostate and carries semen as well as urine.
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