The urinary system

The urinary system is the group of organs that forms, transports, stores and releases urine. Each kidney forms urine, which passes through a ureter to the urinary bladder for storage. The urethra carries urine from the bladder to the outside of the body.

Where are the kidneys located?

The kidneys lie against the posterior abdominal wall, the back wall of the abdomen, on either side of the vertebral column. They are retroperitoneal, meaning that they lie behind the peritoneum, the membrane lining the abdominal cavity. The right kidney usually lies slightly lower than the left kidney because of the liver above it.

Each kidney has an upper and lower pole, a convex lateral border facing away from the midline, and a concave medial border facing towards it. The medial border contains the renal hilum, a gateway through which vessels, nerves and the urine-collecting passage enter or leave. The hilum opens into the renal sinus, an internal space containing these structures and fat.

A fibrous capsule, a tough connective tissue covering, closely surrounds the kidney. Outside it are supporting fat and renal fascia, a connective tissue envelope. In the adjacent 3D model, rotate and zoom, then isolate a kidney to examine its orientation relative to neighbouring structures.

What organs and muscles are related to the kidneys?

The posterior relations are broadly similar on both sides. The diaphragm, the muscular partition between chest and abdomen, lies behind the upper part of each kidney. Lower down, the kidneys lie in front of psoas major and quadratus lumborum, muscles of the posterior abdominal wall, and the aponeurosis of transversus abdominis, its broad tendinous sheet, at the lateral side. Lower ribs also lie behind their upper portions.

The anterior relations, those in front of the kidneys, differ between the sides:

• Right kidney: the liver lies in front of much of its upper surface. The descending part of the duodenum, the initial part of the small intestine, lies near its medial border. The right colic flexure, the bend between the ascending and transverse colon, and small intestinal loops lie in front of lower regions.
• Left kidney: the stomach and spleen lie in front of its upper region. The pancreas crosses in front of the middle region, with the left colic flexure and small intestinal loops related to lower regions.

An adrenal gland lies above and towards the medial side of each kidney. Although closely related in position, it is a separate hormone-producing organ rather than part of the urinary system.

What are the cortex, medulla and renal pelvis?

A section through a kidney reveals an outer renal cortex and an inner renal medulla. The cortex extends between the subdivisions of the medulla as renal columns. The medulla forms renal pyramids, cone-shaped regions whose broad bases face the cortex and whose tips point towards the collecting spaces.

The tip of a pyramid is called a renal papilla. It projects into a minor calyx, a small cup-shaped collecting space. Minor calyces join to form major calyces, which open into the renal pelvis, the funnel-shaped collecting region that continues into the ureter.

Urine therefore passes from the papillae into the calyces, then through the renal pelvis and into the ureter. The calyces and part of the renal pelvis occupy the renal sinus rather than the cortex or medulla; the renal pelvis may extend beyond the hilum. The nephron is the functional unit of the kidney; its detailed microscopic structure is distinct from these gross anatomical regions.

How does blood enter and leave the kidney?

Each renal artery normally arises from the abdominal aorta, the main artery in the abdomen, and approaches the renal hilum. The right renal artery passes behind the inferior vena cava, the large vein returning blood from the lower body towards the heart.

The renal artery commonly divides into segmental branches before entering the kidney, and its branches follow a characteristic arrangement:

• Segmental arteries supply defined territories of renal tissue.
• Interlobar arteries run between the renal pyramids within the renal columns.
• Arcuate arteries curve along the boundary between cortex and medulla.
• Cortical radiate arteries extend into the cortex from the arcuate arteries.

Blood returns through small veins into larger veins and ultimately the renal vein, which drains into the inferior vena cava. The left renal vein crosses in front of the abdominal aorta and beneath the superior mesenteric artery, an artery supplying much of the intestine.

At the hilum, the usual order from front to back is renal vein, renal artery and renal pelvis. This arrangement helps distinguish the vascular structures from the urine-collecting passage.

Where do the ureters run, and where do they narrow?

Each ureter is a muscular tube that continues from the renal pelvis to the bladder. It descends behind the peritoneum on the front of psoas major, crosses the pelvic brim—the boundary between the abdominal and pelvic regions—and continues along the lateral pelvic wall. It then turns towards the bladder and passes obliquely through its wall.

The three classically described sites of narrowing are:

1. The pelviureteric junction: where the renal pelvis becomes the ureter.
2. The crossing of the pelvic brim: where the ureter passes over the iliac vessels, the major vessels serving the pelvis and lower limbs.
3. The ureterovesical junction: where the ureter enters and traverses the bladder wall.

In the female pelvis, the ureter passes beneath the uterine artery near the cervix, the lower part of the uterus. In the male pelvis, it passes beneath the ductus deferens, the tube carrying sperm towards the ejaculatory passage, before reaching the bladder.

Where is the urinary bladder, and what is the trigone?

The urinary bladder is a hollow muscular organ behind the pubic symphysis, the joint connecting the pubic bones at the front of the pelvis. When empty, it lies mainly within the pelvis; as it fills, its upper part rises into the abdomen. Its apex points forwards, its base faces backwards, and its neck continues into the urethra.

The bladder wall contains the detrusor muscle, interwoven smooth muscle that forms its muscular coat. Most of the inner lining forms folds when the bladder is empty. These folds flatten as the bladder expands.

The trigone is the smooth triangular area on the internal surface of the bladder base. Its upper corners are the openings of the ureters, and its lower corner is the internal urethral opening. Unlike the surrounding lining, it remains relatively smooth.

In males, the prostate lies directly below the bladder neck and surrounds the initial urethra. In females, the bladder lies in front of the vagina and below the uterus.

How does the urethra differ between females and males?

The urethra begins at the internal urethral opening and ends at the external urethral opening. Its course and relations differ substantially between the sexes.

The male intramural part lies within the bladder neck, and the prostatic part passes through the prostate. The membranous part passes through the deep perineal pouch, where it is surrounded by the external urethral sphincter, and pierces the perineal membrane below. The spongy part runs within the corpus spongiosum, erectile tissue surrounding the urethra in the penis.

The external urethral sphincter is skeletal muscle surrounding part of the urethra and contributing to voluntary control of its outlet. Its arrangement differs between females and males.