The Urinary System

The urinary system, also known as the renal system, is a vital organ system responsible for filtering waste products from the blood, maintaining fluid and electrolyte balance, regulating blood pressure, and producing hormones.

It is composed of the following main organs:

1. Kidneys: The primary organs of filtration and urine formation.
2. Ureters: Tubes that transport urine from the kidneys to the urinary bladder.
3. Urinary Bladder: A muscular sac that stores urine.
4. Urethra: A tube that excretes urine from the bladder to the outside of the body.

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Parts of the Urinary System: Upper and Lower Tracts

For anatomical and clinical convenience, the urinary system is often divided into two main parts:

1. Upper Urinary Tract

• Location: Primarily located in the abdominal cavity.
• Components:
  • Two Kidneys
  • Upper two-thirds of the Ureters

2. Lower Urinary Tract

• Location: Primarily located in the pelvis and perineum.
• Components:
  • Lower one-third of the Ureters
  • Urinary Bladder
  • Urethra

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Upper Urinary Tract Details

Kidneys

• Description: Two reddish-brown, bean-shaped organs, each about the size of a fist. They are the primary blood-filtering organs of the urinary system.
• Location: Located on the posterior abdominal wall, one on each side of the vertebral column, retroperitoneally (behind the peritoneum).
• Specific Positioning:
  • Typically extend from the level of the T12 vertebra to the L3 vertebra.
  • The right kidney is usually positioned slightly lower than the left kidney (approximately half an inch) due to the large size and position of the liver on the right side of the abdominal cavity.
• Mobility: Kidneys are not rigidly fixed. They exhibit some mobility and move vertically with respiration (descending during inspiration and ascending during expiration) and with changes in body position.
• Hilum: The medial concave border of each kidney features a vertical cleft called the renal hilum. This is the entry and exit point for the renal artery and vein, nerves, and the renal pelvis (which narrows to become the ureter). The hilum is typically located around the level of the L1 vertebra.
• Function:
  • Urine Formation: Filters blood to produce urine, removing metabolic wastes and excess water/electrolytes.
  • Erythropoietin Production: Produces the hormone erythropoietin, essential for red blood cell synthesis.
  • Renin Production: Produces renin, vital for blood pressure regulation.
  • Vitamin D Activation: Activates vitamin D.

Coverings of the Kidney (from innermost to outermost)

The kidneys are protected and held in place by four layers of tissue:

1. Renal Capsule (Fibrous Capsule):
   • Description: The innermost layer. A thin, tough, transparent, fibrous connective tissue capsule that directly surrounds the kidney parenchyma.
   • Attachment: It is closely and firmly attached to the kidney surface.
   • Function: Provides a barrier against infection and trauma.
2. Perirenal Fat (Adipose Capsule):
   • Description: A layer of adipose tissue (fat) that surrounds the renal capsule.
   • Function: Cushions the kidney against physical trauma and helps hold it in position. Its presence provides insulation.
3. Renal Fascia (Gerota's Fascia):
   • Description: A condensation of connective tissue that encloses both the kidney and the adrenal gland (which sits atop the kidney). It has an anterior and a posterior layer.
   • Function: Anchors the kidney to the posterior abdominal wall and surrounding structures, helping to maintain its position. It also separates the kidney from the surrounding pararenal fat.
4. Pararenal Fat (Paranephric Fat):
   • Description: The outermost layer, consisting of a variable amount of fat located external to the renal fascia. It is part of the broader retroperitoneal fat.
   • Function: Provides additional cushioning and support for the kidney.

Internal Structure of Kidneys

When sectioned, the kidney reveals two distinct regions: an outer cortex and an inner medulla.

Renal Lobes: Each renal pyramid, along with its overlying cap of cortical tissue and half of each adjacent renal column, constitutes a renal lobe. There are typically 8-18 renal lobes per kidney.

Collecting System within the Kidney

• Hilum: As mentioned, the hilum is the indentation on the medial aspect of the kidney. It serves as the point of entry for the renal artery, nerves, and lymphatic vessels, and the point of exit for the renal vein and the ureter. The arrangement of these structures from anterior to posterior at the hilum is typically Renal Vein (V), Renal Artery (A), Ureter (U), and sometimes a second artery (A), often remembered by the mnemonic "V.A.U.A.".
• Renal Sinus: This is the fat-filled space within the hilum, extending into the kidney. It contains the renal pelvis, calyces, renal vessels, nerves, and fat.
• Renal Pelvis: The upper, expanded, funnel-shaped part of the ureter, located within the renal sinus. It collects urine from the major calyces.
• Major Calyces: The renal pelvis typically divides into 2-3 large collecting tubes called major calyces.
• Minor Calyces: Each major calyx, in turn, divides into several smaller collecting cups called minor calyces. Each minor calyx receives urine directly from one or more renal papillae.
• Connection to Collecting Tubules: The renal papillae are the tips of the renal pyramids, where the collecting ducts open, allowing urine to drain into the minor calyces. The collecting tubules (part of the nephron unit) ultimately drain into these collecting ducts.

Relations of the Kidneys

The kidneys are retroperitoneal organs, meaning they lie behind the peritoneum that lines the abdominal cavity. Their relations differ slightly between the right and left sides due to the asymmetry of abdominal organs.

Blood Supply of Kidneys

The kidneys receive a rich blood supply, vital for their filtration function.

Arterial Supply:

• Renal Artery: Each kidney receives blood from a large renal artery, which is a direct branch of the abdominal aorta. These branches typically arise at the level of the L1 vertebra.
• Branching Pattern: The renal artery undergoes extensive branching within the kidney:
  1. Segmental Arteries: Within the renal sinus, the renal artery typically divides into 5 segmental arteries, supplying different vascular segments of the kidney.
  2. Lobar Arteries: Each segmental artery branches into lobar arteries, one for each renal lobe.
  3. Interlobar Arteries: Lobar arteries pass between the renal pyramids as interlobar arteries.
  4. Arcuate Arteries: At the junction of the cortex and medulla (base of the pyramids), interlobar arteries arch over the bases of the pyramids to become arcuate arteries.
  5. Cortical Radiate Arteries (Interlobular Arteries): Arcuate arteries give off numerous cortical radiate arteries that extend into the cortex.
  6. Afferent Glomerular Arterioles: These arise from the cortical radiate arteries and supply blood to the glomeruli, where filtration begins.
  7. Efferent Glomerular Arterioles: Blood leaves the glomerulus via efferent arterioles, which then form the peritubular capillaries (around convoluted tubules) and vasa recta (around loops of Henle in the medulla).

Venous Drainage:

• Renal Vein: Blood drains from the kidney via veins that generally follow the arterial pathway in reverse (e.g., cortical radiate veins, arcuate veins, interlobar veins). These converge to form the renal vein.
• Superior Vena Cava Connection: The right and left renal veins drain directly into the inferior vena cava (IVC). The left renal vein is typically longer than the right, as it crosses the aorta anteriorly to reach the IVC.

Lymphatic Drainage:

• The lymphatic vessels from the kidneys generally follow the renal arteries and veins.
• They drain into the paraaortic lymph nodes (or lumbar lymph nodes), which are located along the abdominal aorta.

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Ureters

• Description: The ureters are two slender, muscular tubes that transport urine from the renal pelvis of each kidney to the urinary bladder.
• Length: Each ureter is approximately 25-30 cm (10-12 inches) long in adults.
• Diameter: The diameter is about 3 mm, but it can vary and has several physiological narrowings.
• Propulsion of Urine: Urine is not transported by gravity alone. The muscular walls of the ureters undergo rhythmic, wave-like contractions called peristalsis, which actively propel urine downwards, regardless of body position. This peristaltic action typically occurs 1 to 5 times per minute.

Narrowings of the Ureters (Sites of Potential Obstruction)

These three anatomical narrowings are clinically important as they are common sites for the lodgement of urinary calculi (kidney stones), which can cause significant pain and obstruction.

1. Ureteropelvic Junction (UPJ): At the beginning of the ureter, where the renal pelvis narrows to become the ureter.
2. Pelvic Brim (Iliac Vessels Crossing): Where the ureter crosses the brim of the lesser pelvis, anterior to the bifurcation of the common iliac artery (or just distal to it, where it crosses the external iliac artery).
3. Ureterovesical Junction (UVJ): At the end of the ureter, where it passes obliquely through the muscular wall of the urinary bladder. This oblique entry acts as a physiological valve, preventing reflux of urine from the bladder back into the ureter.

Relations of the Ureter

The relations of the ureters are extensive due to their long course through the retroperitoneal space.

• POSTERIORLY:
  • Throughout its course in the abdomen, the ureter descends anterior to the psoas major muscle. This muscle separates the ureter from the lumbar transverse processes.
  • It crosses anterior to the common iliac vessels or their bifurcations at the pelvic brim.
  • In the pelvis, it runs anterior to the internal iliac artery branches.
• ANTERIORLY:
  • Right Ureter: Crossed anteriorly by the duodenum (descending part), right colic vessels, ileocolic vessels, right testicular/ovarian vessels, and may be crossed by coils of ileum.
  • Left Ureter: Crossed by the sigmoid colon and its mesocolon, left colic vessels, left testicular/ovarian vessels, and may be crossed by coils of jejunum.
  • In Females (Pelvic Part): The ureter passes inferior to the uterine artery ("water under the bridge"). This relationship is critically important during hysterectomy (surgical removal of the uterus) to avoid accidental ligation or injury to the ureter.

Blood Supply of the Ureters

The ureters have an extensive and variable arterial supply from several sources along their length, forming an anastomotic network.

• Upper Part: Supplied by branches from the renal artery.
• Middle Part: Supplied by branches from the gonadal arteries (testicular artery in males, ovarian artery in females), and also sometimes from the aorta, common iliac, or internal iliac arteries.
• Lower Part: Supplied by branches from the superior vesical artery (a branch of the internal iliac artery), middle rectal artery, and uterine artery (in females).

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Lower Urinary Tract

The lower urinary tract is responsible for the storage and elimination of urine. It is composed of the lower one-third of the ureters, the urinary bladder, and the urethra.

Urinary Bladder

• Description: A muscular, distensible reservoir located in the pelvic cavity that temporarily stores urine until it is excreted from the body. Its shape and position vary depending on the amount of urine it contains.
• Shape: When empty, it is roughly pyramidal in shape. As it fills, it becomes more ovoid or spherical.
• Main Function: Storage of urine.
  • Average Capacity: The average functional capacity is around 300-500 ml (0.5 liters), though it can distend to hold up to 1 liter or more before reflex emptying occurs or discomfort becomes significant.
  • Urge to Urinate: The first urge to urinate typically occurs when the bladder contains about 150-200 ml of urine.

Position and Relations (Varies with Filling)

• Empty Bladder:
  • It is a pelvic organ, lying mostly within the lesser pelvis.
  • It is located directly posterior to the symphysis pubis.
  • In males, it is superior to the prostate gland.
  • In females, it is anterior to the vagina and uterus.
• Filled Bladder:
  • As the bladder fills, it expands superiorly and anteriorly, lifting out of the pelvis and becoming an abdominal organ. This expansion occurs between the peritoneum and the anterior abdominal wall.
  • Clinical Significance: This superior expansion into the abdominal cavity, especially when distended, makes it more susceptible to injury from blunt abdominal trauma. It is cited as the second most commonly injured organ in blunt abdominal trauma (after the spleen).

Anatomical Parts and Surfaces

The pyramidal shape of an empty bladder helps describe its surfaces and relations. It has:

• Apex: The anterior-superior part, pointing towards the symphysis pubis. It is connected to the umbilicus by the median umbilical ligament (a remnant of the urachus).
• Neck: The most inferior and fixed part of the bladder, funnel-shaped, continuous with the urethra. It is supported by ligaments and, in males, is surrounded by the prostate gland.
• Posterior Surface (Base or Fundus): Faces posteriorly and slightly inferiorly.
  • In Males: Related to the rectum, seminal vesicles, and vasa deferentia.
  • In Females: Related to the vagina (anterior wall) and the uterus (cervix and lower body).
• Superior Surface: The uppermost surface.
  • In Males: Covered by peritoneum and related to coils of small intestine and the sigmoid colon.
  • In Females: Covered by peritoneum and related to the body of the uterus.
• Two Inferolateral Surfaces: These are the largest surfaces when the bladder is empty, facing anterolaterally. They are separated from the anterior abdominal wall by the retropubic space (of Retzius), which contains loose areolar tissue and fat (extraperitoneal fat).

Internal Structure

• Detrusor Muscle: The muscular wall of the bladder is composed of three layers of smooth muscle collectively known as the detrusor muscle. This muscle contracts during micturition (urination) to expel urine.
• Trigone: A smooth, triangular area on the internal posterior floor of the bladder. It is bounded by the openings of the two ureters (superiorly) and the internal urethral orifice (inferiorly). The mucosa of the trigone is smooth and firmly attached, unlike the rest of the bladder which forms folds (rugae) when empty. The trigone is sensitive to stretch, signaling the urge to urinate.
• Internal Urethral Sphincter: Involuntary smooth muscle fibers around the neck of the bladder, particularly well-developed in males, preventing semen reflux during ejaculation. Its role in urinary continence is debated in females.

Blood Supply of the Urinary Bladder

• Arterial Supply: Primarily supplied by branches of the internal iliac arteries:
  • Superior Vesical Arteries: Usually arise from the umbilical artery (a remnant of the fetal umbilical artery).
  • Inferior Vesical Arteries: In males, these typically supply the fundus of the bladder, prostate, and seminal vesicles.
  • Vaginal Arteries: In females, these branches from the internal iliac artery supply the inferior part of the bladder and the vagina.
  • (Additional supply from obturator and gluteal arteries may occur).
• Venous Drainage: Venous plexuses (vesical venous plexus) drain into the internal iliac veins.

Nerve Supply of the Urinary Bladder (Micturition Reflex)

The innervation of the bladder is complex, involving both sympathetic and parasympathetic divisions of the autonomic nervous system, as well as somatic innervation for the external urethral sphincter.

• Sympathetic Innervation (Hypogastric Nerves, T11-L2):
  • Effect on Detrusor Muscle: Causes relaxation of the detrusor muscle, allowing the bladder to fill and store urine.
  • Effect on Internal Urethral Sphincter (in males): Causes contraction of the internal urethral sphincter, preventing urine leakage.
  • Overall Role: Promotes urine storage.
• Parasympathetic Innervation (Pelvic Splanchnic Nerves, S2-S4):
  • Effect on Detrusor Muscle: Causes contraction of the detrusor muscle, leading to expulsion of urine.
  • Effect on Internal Urethral Sphincter (in males): Causes relaxation of the internal urethral sphincter.
  • Overall Role: Mediates the emptying (micturition) reflex.
• Somatic Innervation (Pudendal Nerve, S2-S4):
  • Innervates the external urethral sphincter, which is composed of skeletal muscle and is under voluntary control. This allows for conscious control over urination.

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The Urethra

The urethra is a tube that carries urine from the urinary bladder to the outside of the body. There are significant anatomical differences between the male and female urethra.

Male Urethra

• Length: Considerably longer than the female urethra, approximately 20 cm (8 inches) long.
• Function: Serves a dual role: it is a passageway for both urine (during micturition) and semen (during ejaculation).
• Course and Parts: The male urethra is divided into four main parts:
  1. Preprostatic Urethra: Shortest part (0.5-1.5 cm), extending from the internal urethral orifice to the prostate gland. Surrounded by the internal urethral sphincter.
  2. Prostatic Urethra: Passes through the prostate gland (3-4 cm). Receives the ejaculatory ducts and prostatic ducts.
  3. Membranous Urethra: The shortest and narrowest part (1-1.5 cm), extending from the apex of the prostate to the bulb of the penis. It passes through the urogenital diaphragm and is surrounded by the external urethral sphincter (voluntary skeletal muscle). This is the least protected part and most vulnerable to injury.
  4. Spongy (Penile) Urethra: The longest part (about 15 cm), passing through the corpus spongiosum of the penis. It ends at the external urethral orifice (meatus) at the tip of the glans penis. It receives the ducts of the bulbourethral glands.
• External Urethral Orifice: Opens at the end of the penis.

Female Urethra

• Length: Much shorter than the male urethra, approximately 3-4 cm (1.5 inches) long.
• Function: Serves only as a passageway for urine.
• Course: Extends from the internal urethral orifice of the bladder, passing through the deep perineal pouch, and opening at the external urethral orifice (meatus) into the vestibule, anterior to the vaginal opening.
• External Urethral Sphincter: Surrounds the middle part of the urethra, providing voluntary control over urination.
• Clinical Significance of Short Length: The short length and close proximity of the female urethra to the anus make females more susceptible to urinary tract infections (UTIs), as bacteria can more easily ascend into the bladder.