Anatomy of the Kidneys, Bladder and Urethra

1. Introduction
2. The Kidneys
   • Position and Surface Anatomy
   • Coverings of the Kidney
   • Internal Structure
   • Blood Supply
   • Lymphatic Drainage and Innervation
3. The Ureters
4. The Bladder
   • Position and Peritoneal Relations
   • Internal Structure
   • Blood Supply, Lymphatics and Innervation
5. The Urethra
   • Male Urethra
   • Female Urethra
6. Clinical Correlations
7. Summary
8. Quiz

• The urinary tract is a continuous system from the renal cortex, where urine is formed, to the external urethral meatus, where it is voided. Each segment has a distinct anatomy designed for its role: filtration in the kidney, transport in the ureter, storage in the bladder and conduction in the urethra.
• The kidneys lie in the retroperitoneum (the space behind the peritoneal lining of the abdomen) at the level of T12-L3 (the 12th thoracic to the 3rd lumbar vertebra). They have an unusual blood supply; the renal artery divides into successively smaller branches (segmental, interlobar, arcuate and interlobular) before giving rise to the afferent arterioles, the small vessels that carry blood to each glomerulus.
• The ureters are 25-30 cm muscular tubes that propel urine by peristalsis. They have three anatomical constrictions (narrow points): the pelvi-ureteric junction (where the renal pelvis becomes the ureter), the pelvic brim (the rim of the true pelvis, where the ureter crosses the iliac vessels) and the vesico-ureteric junction (where the ureter joins the bladder). These are the sites where renal stones are classically said to impact.
• The bladder is a smooth-muscle reservoir lined by transitional epithelium (urothelium). The trigone, between the two ureteric orifices and the internal urethral orifice, is fixed and smooth, while the rest of the bladder wall is folded into rugae (folds of the lining that flatten as the bladder fills).
• The male urethra is about 20 cm long with four parts (pre-prostatic, prostatic, membranous, spongy); the female urethra is 4 cm and is not divided into named parts. The shorter female urethra is one reason why urinary tract infections are more common in women.

Introduction

The urinary tract has four functional zones: the kidneys (filtration), the ureters (transport), the bladder (storage) and the urethra (voiding). They share a single anatomical theme: a smooth-muscle wall lined by an epithelium specialised for urine. Knowing where each segment sits and how it is supplied is the foundation for understanding nearly every urological presentation. So is knowing how each segment relates to the neighbouring organs (viscera). Examples range from a missed pyelonephritis (kidney infection) to stress incontinence after childbirth (postpartum).

This article gives the gross anatomy of each zone in order. The microscopic structure of the filtering unit is covered in The Nephron, the physiology of filtration in Glomerular Filtration Rate and the Measurement of Kidney Function, and the neural control of voiding in Micturition.

The Kidneys

Position and Surface Anatomy

The kidneys are paired bean-shaped organs lying in the retroperitoneum on the posterior abdominal wall. Each is approximately 11 cm long, 6 cm wide and 3 cm thick and weighs around 150 g. They lie obliquely against the psoas muscle (a muscle running along the side of the lumbar spine), so their upper poles are closer to the midline than their lower poles.

The vertebral levels routinely tested are:

• Right kidney: upper pole at the level of T12, lower pole at L3.
• Left kidney: upper pole at T11, lower pole at L2-L3.

The right kidney sits about 1-2 cm lower than the left because the right lobe of the liver displaces it inferiorly. The hila (the notches on the inner border of each kidney where the vessels and ureter enter and leave) of both kidneys lie at approximately the level of L1, the transpyloric plane (a horizontal plane halfway between the top of the sternum and the pubic bones).

Anteriorly, the kidneys are related to a stack of structures that vary between sides:

• Right kidney: liver (large area), duodenum (descending part), right colic flexure (the bend of the colon beneath the liver), small bowel and the right adrenal gland.
• Left kidney: stomach, spleen, pancreas (body and tail), left colic flexure, jejunum and the left adrenal gland.

Posteriorly, both kidneys lie against the diaphragm superiorly and the psoas major, quadratus lumborum and transversus abdominis muscles (the muscles of the back wall of the abdomen) inferiorly. The 12th rib crosses the posterior surface of each kidney; the upper pole therefore lies behind the pleura (the lining of the lung cavity), which is clinically relevant for percutaneous (through the skin) renal procedures and for blunt trauma.

Coverings of the Kidney

Each kidney is wrapped in three concentric layers (listed below), with a fourth layer of fat, the pararenal fat, outside them. They are important for understanding both surgical access and pathology:

1. Renal capsule: a tough fibrous coat directly applied to the kidney. It cannot stretch (it is non-distensible), which is why swelling of the kidney tissue (parenchymal swelling, e.g. acute pyelonephritis, a kidney infection) produces flank pain.
2. Perirenal fat (perinephric fat): a continuous layer of adipose tissue surrounding the capsule. The renal artery, vein and ureter run within it at the hilum.
3. Renal fascia (Gerota's fascia): a fibrous envelope enclosing the perirenal fat and the adrenal gland. It is open inferiorly along the line of the ureter, so a perinephric collection (a pool of fluid or pus around the kidney) can spread down into the pelvic cavity.

External to the renal fascia is the pararenal fat, which fills the renal angle (the angle between the 12th rib and the muscles beside the spine) and is the layer surgeons enter through a posterior lumbar approach.

Internal Structure

Diagram: coronal section of the kidney showing the outer cortex, the medullary pyramids, the minor and major calyces converging on the renal pelvis, the renal artery and vein at the hilum, and the ureter.

Creative commons source by BruceBlaus, CC BY 3.0.

On longitudinal section, the kidney has two clearly distinguishable zones:

• The cortex, the outer, paler region containing the glomeruli, proximal and distal convoluted tubules (the twisting tubes of the nephron) and the cortical collecting ducts.
• The medulla, the inner, darker region organised into 8-12 cone-shaped renal pyramids. The base of each pyramid faces the cortex; the apex (the papilla) projects into a minor calyx.

Cortical tissue extends down between the pyramids as the renal columns (of Bertin). A lobe of the kidney is one pyramid plus its overlying cortex, so a kidney has one lobe for each pyramid.

The collecting system is a series of progressively larger funnels:

• Minor calyces (8-12) receive urine from the papillae.
• Major calyces (typically 2-3) drain groups of minor calyces.
• The major calyces converge into the renal pelvis, a flattened funnel that exits the hilum.
• The renal pelvis narrows abruptly at the pelvi-ureteric junction (PUJ) to become the ureter.

Blood Supply

The kidneys receive about 20-25% of the resting cardiac output, far in excess of their oxygen demand: this reflects their role as filtration organs rather than tissues with high metabolic need.

Each kidney is usually supplied by a single renal artery, arising from the abdominal aorta at L1-L2, just below the origin of the superior mesenteric artery (SMA), the artery to the gut. Two practical points:

• The right renal artery is longer than the left because it must cross the midline behind the inferior vena cava.
• An accessory renal artery is present in around 25% of people, usually arising from the aorta and entering the lower pole. It is significant in renal transplant donor surgery and in some causes of secondary hypertension (high blood pressure that has an underlying cause).

Inside the kidney the artery branches in a stereotyped sequence: renal → segmental → interlobar → arcuate → interlobular (cortical radiate) → afferent arteriole → glomerulus. The afferent arteriole (the vessel carrying blood towards the glomerulus) and the efferent arteriole (the vessel carrying blood away from it) are the two resistance vessels that together set glomerular capillary pressure. Angiotensin II is a major determinant of efferent arteriolar tone, so ACE inhibitors and ARBs (angiotensin-converting enzyme inhibitors and angiotensin receptor blockers) dilate the efferent arteriole and lower glomerular capillary pressure.

From the glomerulus, blood enters the efferent arteriole, a second resistance vessel. The cortical efferents form the peritubular capillaries surrounding the proximal and distal tubules. The juxtamedullary efferents (those next to the medulla) continue as the vasa recta, long hairpin loops that descend into the medulla. They are essential for the counter-current mechanism, the system that lets the kidney concentrate urine. The vasa recta are described in detail in Control of Plasma Osmolarity.

Venous drainage mirrors the arterial supply, with venules combining into interlobular, arcuate and interlobar veins, then a single renal vein draining into the inferior vena cava (IVC). The left renal vein is longer than the right because the IVC sits on the right of the midline. It passes anterior to the aorta and is crossed by the superior mesenteric artery (SMA); compression between the SMA and the aorta is the cause of the rare nutcracker syndrome. The left renal vein also receives the left adrenal, left gonadal (testicular or ovarian) and left phrenic veins, whereas on the right these drain directly into the IVC.

Lymphatic Drainage and Innervation

Lymph from the kidney drains to the lateral aortic (para-aortic) nodes at the level of the renal hilum.

The kidney is innervated by autonomic fibres from the renal plexus. The dominant input is sympathetic from T10-L1 (via the lesser, least and lumbar splanchnic nerves, which carry autonomic fibres to the abdominal organs), with a small contribution from the vagus. Sympathetic activity constricts the afferent arteriole, reducing renal blood flow and contributing to the renal response to haemodynamic shock. Sensory fibres (afferents) travel back along the same routes, so renal pain is referred to (felt in) the flank, in the T10-L1 dermatomes (the areas of skin supplied by those spinal nerves), and may radiate to the groin as the ureter is followed inferiorly.

The Ureters

The ureters are paired muscular tubes about 25-30 cm long that propel urine from the renal pelvis to the bladder by peristalsis. The wall has three layers: an inner mucosa lined by urothelium, a middle muscular layer and an outer adventitia (a connective tissue coat). The muscular layer has an inner longitudinal and an outer circular coat in the upper two-thirds, with an additional outer longitudinal coat in the lower third.

Each ureter has an abdominal and a pelvic course. The abdominal ureter descends in the retroperitoneum on the medial border of psoas major, anterior to the tips of the lumbar transverse processes. It crosses the genitofemoral nerve (a nerve that runs down the front of psoas major) and is itself crossed in front by the gonadal (testicular or ovarian) vessels. It then crosses the bifurcation of the common iliac artery at the pelvic brim (the rim of the true pelvis): this bifurcation is a useful surgical landmark. The pelvic ureter passes inferiorly and medially along the lateral pelvic wall, then turns anteromedially to enter the bladder at the vesico-ureteric junction.

The relationship of the ureter to other pelvic structures is sex-specific and clinically important:

• In the male, the ductus (vas) deferens (the tube carrying sperm from the testis) crosses the ureter superiorly and medially as the duct passes from the deep inguinal ring (the internal opening of the inguinal canal) towards the seminal vesicle (a gland that adds fluid to semen).
• In the female, the uterine artery passes over the ureter (in front of and above it) just lateral to the cervix: "water (the ureter) under the bridge (the uterine artery)". The ureter is at risk of injury during hysterectomy at this point.

The classical teaching point is the three anatomical constrictions where renal stones are said to impact (recent imaging studies suggest that stones lodge mostly in the upper ureter and at the vesico-ureteric junction):

1. The pelvi-ureteric junction (PUJ).
2. The pelvic brim, where the ureter crosses the iliac vessels.
3. The vesico-ureteric junction (VUJ), where the ureter passes through the bladder wall. This is one of the narrowest points of the ureter.

The blood supply is segmental and tortuous, with a different source for each third. The upper third is supplied by branches of the renal artery, the middle third by branches of the gonadal and common iliac arteries, and the lower third by branches of the internal iliac artery (vesical, uterine, vaginal, middle rectal). There are few connections (anastomoses) between these branches, so the supply is poor. This explains the sensitivity of the ureter to surgical devascularisation (loss of its blood supply). Pain from a ureteric stone follows the dermatomes T11-L2 (the areas of skin supplied by those spinal nerves) and is classically described as "loin to groin" colic, intense and intermittent.

The Bladder

Position and Peritoneal Relations

The bladder is a muscular reservoir for urine. In adults it lies in the true pelvis (the part of the pelvis below the pelvic brim), posterior to the pubic symphysis (the joint between the two pubic bones), when empty. As it fills it rises into the abdomen and can be palpated above the pubic symphysis when it contains roughly 300 mL or more. In infants the bladder lies higher than in adults. It is usually described as an abdominal organ, although only about half of it lies above the pelvic brim (the entrance to the true pelvis), and it descends gradually into the pelvis during childhood.

The bladder has four surfaces:

• The superior surface: covered by peritoneum (the lining of the abdominal cavity), in contact with sigmoid colon and ileum (and, in the female, the body of the uterus).
• Two inferolateral surfaces: against the levator ani and obturator internus muscles (muscles of the pelvic floor and the side wall of the pelvis).
• The posterior surface (base): in the male related to the rectum (separated by the rectovesical pouch (the peritoneal pocket between the rectum and the bladder) superiorly and the seminal vesicles (glands that add fluid to semen) and ductus deferens inferiorly), in the female related to the cervix and upper vagina.

The apex of the bladder lies anteriorly and is connected to the umbilicus by the median umbilical ligament, the obliterated (closed-off) remnant of the embryological urachus, an embryonic duct that joined the bladder to the umbilicus. The neck of the bladder is the most inferior point and is anchored by the puboprostatic ligaments (male) or pubovesical ligaments (female).

Internal Structure

The bladder wall has four layers:

• A mucosa lined by urothelium (transitional epithelium), which can stretch from several layers of cube-shaped (cuboidal) cells when empty to a flattened, squamous-like (flat-celled) layer when full, all without losing its barrier function.
• A submucosa (the connective tissue layer beneath the lining).
• A thick three-layered detrusor muscle, the smooth-muscle generator of voiding contraction. The detrusor is supplied by parasympathetic nerves acting on M3 muscarinic receptors (acetylcholine receptors on the muscle): the basis for antimuscarinic therapy in overactive bladder and urgency urinary incontinence.
• An outer adventitia (or peritoneum on the superior surface).

The internal surface of the bladder has two distinct regions:

• The trigone: a smooth, fixed triangular area at the base of the bladder bounded by the two ureteric orifices superolaterally and the internal urethral orifice inferiorly. It is classically taught to derive from the mesonephric ducts (embryonic ducts of mesodermal origin), whereas the rest of the bladder develops from the endoderm (the inner embryonic germ layer). This is now disputed: recent studies suggest that the trigone is also endodermal.
• The remainder of the bladder: folded into rugae when empty.

The ureters pierce the bladder wall obliquely, traversing about 1.5 cm of muscle before opening at the trigone. This oblique course creates a flap-valve mechanism that prevents reflux of urine up the ureter when the bladder contracts. Failure of this mechanism causes vesico-ureteric reflux, a major cause of recurrent pyelonephritis and chronic kidney disease in children.

At the bladder neck, the smooth muscle thickens to form the internal urethral sphincter (sympathetic, acting through α1-adrenoceptors, the sympathetic receptors that make this muscle contract). Below this lies the external urethral sphincter, formed by skeletal muscle of the pelvic floor and under voluntary control via the pudendal nerve (a nerve from S2-S4 that supplies the perineum, the area between the anus and the genitals). The two-sphincter arrangement is essential for continence and is described in detail in Micturition.

Blood Supply, Lymphatics and Innervation

Arterial supply is from the superior and inferior vesical arteries (vesical means relating to the bladder), branches of the internal iliac artery. Additional small branches come from the obturator and inferior gluteal arteries; in the female, vaginal and uterine arteries also contribute.

Venous drainage is through the vesical venous plexus, which drains to the internal iliac veins. In the male this plexus is continuous with the prostatic venous plexus, providing one of the routes by which prostate cancer spreads to the lumbar vertebrae through the valveless Batson plexus (a network of veins around the spine with no valves, so blood can flow in either direction).

Lymphatics drain to the external iliac, internal iliac and obturator nodes.

Innervation is autonomic and somatic:

• Parasympathetic fibres from S2-S4 (pelvic splanchnic nerves) cause detrusor contraction and internal urethral sphincter relaxation. This is the voiding response.
• Sympathetic fibres from T10-L2 (via the hypogastric plexus) inhibit the detrusor and contract the internal urethral sphincter. This promotes filling and continence.
• Somatic fibres from the pudendal nerve (S2-S4) supply the external urethral sphincter for voluntary continence.

Sensory fibres carrying bladder fullness travel with the parasympathetics to S2-S4. Pain fibres from the bladder dome (the top of the bladder) travel with the sympathetics to T10-L2 and refer pain to the suprapubic region (above the pubic bones) or lower abdomen. Pain from the trigone is referred to the perineum.

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