The Urinary System

Female reproductive system | Main Anatomy Index | Male reproductive system

Last updated 30 March 2006

This page was contributed by David Boshell.

The Urinary System

• The urinary system consists of the kidneys, ureters, bladder and the urethra.

The Kidney

• This is a highly vascular organ receiving about 25% of cardiac output.
• It produces ultrafiltrate that is converted to urine by selective resorption and secretion by kidney cells.

Endocrine Functions of the Kidney

• These include:

1. Production of erythropoietin, a growth factor regulating red blood cell formation, secreted by interstitial cells;
2. Production of renin, a hormone controlling blood pressure and volume, secreted by juxtaglomerular cells in response to low blood pressure, blood volume or sodium ion concentration;
3. Hydroxylation of vitamin D to its active form: 1,25-dihydroxycholecalciferol.

Kidney Structure

• This consists of:

1. A thin, lining capsule, consisting of an outer layer of fibroblasts and collagen, and an inner layer of myofibroblasts;

2. An outer, reddish-brown cortex. This contains spherical, renal corpuscles, each bounding a glomerulus; tubules of nephrons, collecting tubules, and an extensive vascular supply.

• Straight collecting tubules and straight tubule components of the nephrons make up medullary rays that project into the cortex from the pyramid.
• Between the medullary rays, convoluted tubules of the nephrons and renal corpuscles form the cortical labyrinths.

3. An inner, paler medulla. This contains straight tubules of nephrons and collecting ducts, accompanied by a capillary network called the vasa recta.

• The tubules form pyramids, with their apices or papilla projecting into a minor calyx.
• These minor calyces form major calyces and, eventually, the renal pelvis.
• Cortical tissue surrounding the pyramids form renal columns.
• Each pyramid constitutes a lobe of the kidney, and each lobe divides into lobules that consist of a medullary ray and the nephrons it drains.

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The Nephron

• This is the functional unit of the kidney, producing urine.
• The parts of the nephron include:

1. The renal corpuscle;
2. The renal tubule, including the proximal segment, the loop of Henle, the distal segment.

• There are 3 types of nephron:

3. Juxtamedullary nephrons, which have their renal corpuscles close to the base of the medullary pyramid.

• These have long loops of Henle and long thin segments that extend well into the inner region of the pyramid.

4. Cortical nephrons, which have their renal corpuscles in the outer part of the cortex.

• These have short loops of Henle and the hairpin turn occurs in the distal thick segment.

5. Intermediate nephrons in the mid-region, which have intermediate length loops of Henle.

The Renal Corpuscle

• The renal (Malpighian) corpuscle consists of a tuft of capillary loops, the glomerulus, surrounded by a Bowman's (renal) capsule.
• The glomerular capillaries are supplied by an afferent arteriole and drained by an efferent arteriole.
• These penetrate the Bowman's capsule at its vascular pole.

• At the opposite end, the proximal convoluted tubule begins at the urinary pole.

• The Bowman's capsule has an outer parietal layer of simple squamous epithelium, continuous with the cuboidal epithelium of the proximal convoluted tubule.
• The inner, visceral epithelial layer, enclosing the filtration apparatus of the kidney, is formed by podocytes.

• Between these layers, the Bowman's (renal) space collects filtrate that has perfused from the high-pressure capillary lumen.
• This space is continuous with the lumen of the proximal convoluted tubule.

• Mesangial cells are phagocytic, modified smooth muscle cells that produce mesangium.

• Intraglomerular mesangial cells are found at the stalk of the glomerulus and the interstices of adjoining glomerular capillaries, enclosed by their basal lamina.
• Extraglomerular mesangial cells of the juxtaglomerular apparatus (lacis cells) lie along the vascular pole.

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The Filtration Apparatus

• This is a semi-permeable barrier with 3 parts:

1. The discontinuous endothelium of the glomerular capillaries;
2. The podocytes forming the discontinuous visceral layer of Bowman's capsule;
3. The continuous basal lamina between them.

• Podocytes extend around the fenestrated glomerular capillaries, with foot processes that interdigitate with their neighbours.
• Narrow filtration slits between the foot processes with a filtration slit membrane allow blood to enter the Bowman's space.

• The glomerular basement membrane (GBM, basal lamina) is the principal filtration barrier and an ion selective filter.
• It has 3 parts:

1. The lamina rara interna, on the capillary side;
2. The lamina rara externa, next to the podocytes;
3. The lamina densa, a thick, fused portion between the other 2 layers.

• The GBM has an overall negative charge.
• This restricts the movement of large proteins in the blood.
• Proteins that do pass through are reabsorbed in the proximal convoluted tubule.

The Proximal Segment

• This begins as the proximal convoluted tubule of cuboidal cells with a brush border invading the lumen.
• It enters a medullary ray, continuing in its thick segment as the descending straight tubule, and in its thin segment as the descending limb of the loop of Henle.

• The proximal convoluted tubule reabsorbs about 80% of the primary filtrate.

The Loop of Henle

• This U-shaped part consists of squamous cells.
• It includes the loop and the descending and ascending limbs of the proximal and distal segments respectively.

• Cortical nephrons have short loops of Henle and juxtamedullary nephrons have very long loops of Henle.

The Distal Segment

• This part of the tubule has fewer microvilli than the proximal segment.

• It ascends through the pyramid and medullary ray in thin, then thick segments towards the vascular pole of the renal corpuscle.
• Modified epithelial cells next to the afferent arteriole form the macula densa.

• The tubule then becomes the distal convoluted tubule, emptying into a connecting tubule, then straight collecting tubule in the medullary ray.
• The straight collecting tubules form cortical collecting ducts that continue into the apex of the pyramid, emptying into ducts of Bellini that open into the calyx.

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The Juxtaglomerular Apparatus

• This structure next to the vascular pole regulates blood pressure.
• It includes the macula densa, juxtaglomerular cells and extraglomerular mesangial cells.

• Juxtaglomerular cells are modified smooth muscle cells of the afferent arteriole.
• They have spherical nuclei that secrete renin from secretory granules.

• Renin catalyses the conversion of angiotensinogen to angiotensin I, which is then converted to angiotensin II in the lung.
• Angiotensin II, acting via the central nervous system, is a potent vasoconstrictor, but also causes the release of aldosterone from the zona glomerulosa of the adrenal gland.
• This causes increased distal tubal resorption of sodium and water.
• This in turn increases blood volume and thus blood pressure.

Interstitial cells

• The connective tissue parenchyma of the kidney cortex has macrophages and cells resembling fibroblasts.
• Cells resembling myofibroblasts are present in the medulla.

Vasa Recta

• The efferent arterioles form capillary networks around the renal tubules, known as peritubular capillaries.
• In cortical nephrons, the peritubular capillaries anastomose with those of other nephrons.

• In juxtamedullary nephrons, they descend with the loop of Henle as arterioles.
• They then loop and ascend as venules back to the cortex.
• These are the vasa recta, forming fenestrated capillary plexuses at various levels in the pyramid.

Countercurrent Exchange

• This occurs between the collecting system and the vasa recta.
• In the cortex, the interstitium is hypotonic, along with the distal convoluted tubule.
• In the medulla, the interstitium becomes hypertonic.

• Thus, as the vasa rectae descend, water is drawn from them and salts enter, making them hypertonic at the loop.
• As the vasa rectae ascend, however, the process is reversed, with blood losing salt and gaining water.

• This countercurrent exchange system occurs passively along ion gradients.

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Blood Supply

• This begins with the renal artery dividing anteriorly and posteriorly, sending interlobar branches between the pyramids.

• At the cortex they divide into arcuate arteries that follow the base of the pyramid.
• These arcuate arteries give interlobular arteries that ascend into the cortex.

• The interlobular arteries branch into afferent arterioles that form the glomerular capillaries.
• The glomerular capillary bed reunites as efferent arterioles.

• These give rise to the peritubular capillaries.
• Peritubular cortical capillaries and the vasa rectae drain into arcuate veins, interlobar veins, and the renal vein.

Lymphatics

• Lymphatic vessels drain into 2 major networks:

1. The larger capsular lymphatic vessels;
2. And the larger lymphatic vessels in the renal sinus.

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Excretory Passages

• These include the calyces, renal pelvis, ureter, bladder and urethra.
• Except for the urethra, their structure consists of:

1. A mucosa;
2. A muscularis;
3. An adventitia (or serosa in some regions);

• All these passages are lined with transitional epithelium, allowing distension.

• Plaques are present on the luminal surface of the plasma membrane.
• When the bladder is undistended, the plaques fold inward, but in the distended bladder, they unfold as the transitional epithelium flattens.

• The lamina propria is dense and collagenous.
• There is no muscularis mucosae or submucosa.

• In the ureters and urethra, bundles of smooth muscle below the lamina propria spiral downward with differing curvature.
• This gives the appearance of 3 layers of smooth muscle:

1. An outer longitudinal layer;
2. A middle circular layer;
3. And an inner longitudinal layer.

• Peristaltic contractions move the urine toward the bladder.

The Ureters

• The ureters follow an oblique path into the bladder wall.
• As the bladder distends, the openings of the ureters are thus compressed, preventing reflux of urine into the ureters.
• This protects the kidney from the spread of infection from the bladder.

The Urinary Bladder

• The smooth muscle within the bladder wall forms an internal sphincter around the opening of the urethra.
• The trigone, formed from the openings of the ureters and urethra, remains smooth and constant in thickness, regardless of the state of filling of the bladder.

Innervation of the Bladder

• Sympathetic fibres form a plexus in the adventitia of the bladder wall.

• Parasympathetic fibres in ganglia in muscle bundles and the adventitia form the efferent fibres of the micturition reflex.

• Sensory fibres going to the sacral spinal segments form the afferent fibres of the micturition reflex.

The Urethra

• In the male, the urethra is about 20 cm long.
• It has 3 segments:

1. The prostatic urethra (4cm), from the neck of the bladder through the prostate gland. Seminal vesicles and prostatic ducts open into this segment. The epithelium here is transitional.
2. The membranous urethra (1cm), from the apex of the prostate gland through the pelvic and urogenital diaphragms. The surrounding skeletal muscles form the external sphincter of the urethra. The epithelium becomes pseudostratified columnar here.
3. The penile urethra (15cm), through the penis to open at the glans penis. Here, the epithelium becomes stratified squamous, continuous with the skin of the penis. Ducts of the bulbourethral glands and mucous-secreting glands of Littre open into this portion.

• In the female, the urethra is about 4 cm long.
• The epithelium also changes from transitional to stratified squamous.

• Many small paraurethral and periurethral glands, producing an alkaline secretion, open into the female urethra.

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