Review Sheet Anatomy Of The Urinary System

The urinary system, a vital component of maintaining homeostasis, orchestrates the intricate processes of filtering blood, eliminating waste, and regulating fluid balance. Comprehending its anatomy is foundational to understanding its function and associated pathologies. This review sheet delves into the anatomical structures of the urinary system, providing a comprehensive overview for students and healthcare professionals.

I. Overview of the Urinary System

The urinary system primarily comprises the kidneys, ureters, urinary bladder, and urethra. Each organ plays a distinct role in the formation, transport, storage, and excretion of urine. The kidneys, the workhorses of this system, filter blood and produce urine. The ureters transport urine from the kidneys to the urinary bladder, a distensible reservoir for urine storage. Finally, the urethra conveys urine from the bladder to the outside of the body.

Functions of the Urinary System:

• Excretion of Metabolic Waste: The urinary system eliminates nitrogenous wastes, such as urea, creatinine, and uric acid, which are byproducts of protein metabolism.
• Regulation of Blood Volume and Blood Pressure: By controlling the amount of water and sodium reabsorbed into the bloodstream, the kidneys influence blood volume and, consequently, blood pressure.
• Regulation of Electrolyte Balance: The kidneys maintain the balance of electrolytes, including sodium, potassium, calcium, and phosphate, in the extracellular fluid.
• Regulation of Acid-Base Balance: The urinary system helps maintain blood pH by excreting or reabsorbing hydrogen ions (H+) and bicarbonate ions (HCO3-).
• Hormone Production: The kidneys produce erythropoietin, a hormone that stimulates red blood cell production, and renin, an enzyme involved in blood pressure regulation. They also activate vitamin D, which is essential for calcium absorption.

II. The Kidneys: Structure and Function

The kidneys, bean-shaped organs located in the retroperitoneal space, are the primary filtering units of the urinary system. Each kidney weighs approximately 150 grams and measures about 12 cm in length, 6 cm in width, and 3 cm in thickness.

A. External Anatomy:

• Renal Capsule: A fibrous capsule that surrounds each kidney, providing protection and maintaining its shape.
• Renal Hilum: A concave indentation on the medial side of the kidney where the renal artery, renal vein, and ureter enter and exit.
• Renal Sinus: A cavity within the kidney that contains the renal pelvis, calyces, and branches of the renal vessels and nerves.

B. Internal Anatomy:

• Renal Cortex: The outer region of the kidney, characterized by a granular appearance due to the presence of nephrons.
• Renal Medulla: The inner region of the kidney, consisting of cone-shaped structures called renal pyramids.
• Renal Pyramids: Triangular structures within the medulla, containing bundles of collecting ducts that transport urine.
• Renal Columns: Inward extensions of the renal cortex that separate the renal pyramids.
• Renal Papilla: The apex of each renal pyramid, where urine is discharged into the minor calyx.
• Minor Calyx: A cup-shaped structure that surrounds the renal papilla and collects urine.
• Major Calyx: Formed by the fusion of several minor calyces, it collects urine and drains into the renal pelvis.
• Renal Pelvis: A funnel-shaped structure that collects urine from the major calyces and connects to the ureter.

C. Nephron: The Functional Unit of the Kidney

The nephron is the structural and functional unit of the kidney, responsible for filtering blood and forming urine. Each kidney contains approximately one million nephrons.

Components of the Nephron:

1. Renal Corpuscle: Located in the renal cortex, it consists of the glomerulus and Bowman's capsule.
   • Glomerulus: A network of capillaries where filtration occurs.
   • Bowman's Capsule: A cup-shaped structure that surrounds the glomerulus and collects the filtrate.
2. Renal Tubule: A long, convoluted tubule that extends from Bowman's capsule and consists of the proximal convoluted tubule, loop of Henle, and distal convoluted tubule.
   • Proximal Convoluted Tubule (PCT): The first section of the renal tubule, responsible for reabsorbing most of the water, electrolytes, and nutrients from the filtrate.
   • Loop of Henle: A U-shaped structure that extends into the renal medulla, responsible for establishing a concentration gradient in the medulla. It has a descending limb and an ascending limb.
   • Distal Convoluted Tubule (DCT): The last section of the renal tubule, responsible for further reabsorption of water and electrolytes, as well as secretion of wastes.
3. Collecting Duct: Receives urine from several nephrons and transports it to the renal papilla.

Types of Nephrons:

• Cortical Nephrons: Located primarily in the renal cortex, with short loops of Henle. They account for about 85% of nephrons.
• Juxtamedullary Nephrons: Located near the corticomedullary junction, with long loops of Henle that extend deep into the renal medulla. They play a crucial role in concentrating urine.

D. Blood Supply to the Kidneys:

The kidneys receive a rich blood supply, accounting for approximately 20-25% of the cardiac output at rest.

• Renal Artery: A branch of the abdominal aorta that delivers blood to the kidney.
• Segmental Arteries: Branches of the renal artery that enter the renal hilum.
• Interlobar Arteries: Branches of the segmental arteries that pass through the renal columns.
• Arcuate Arteries: Branches of the interlobar arteries that arch over the base of the renal pyramids.
• Cortical Radiate Arteries (Interlobular Arteries): Branches of the arcuate arteries that radiate outward into the renal cortex.
• Afferent Arterioles: Branches of the cortical radiate arteries that supply blood to the glomerulus.
• Glomerular Capillaries: Capillaries within the glomerulus where filtration occurs.
• Efferent Arterioles: Vessels that carry blood away from the glomerulus.
• Peritubular Capillaries: Capillaries that surround the renal tubules and are involved in reabsorption and secretion. In juxtamedullary nephrons, the efferent arterioles give rise to the vasa recta.
• Vasa Recta: Long, straight capillaries that parallel the loop of Henle in the renal medulla, playing a crucial role in concentrating urine.
• Cortical Radiate Veins (Interlobular Veins): Veins that drain blood from the peritubular capillaries and vasa recta.
• Arcuate Veins: Veins that receive blood from the cortical radiate veins.
• Interlobar Veins: Veins that drain blood from the arcuate veins.
• Renal Vein: A vein that carries blood away from the kidney and drains into the inferior vena cava.

E. Juxtaglomerular Apparatus (JGA):

The juxtaglomerular apparatus is a specialized structure located near the glomerulus, playing a crucial role in regulating blood pressure and glomerular filtration rate.

Components of the JGA:

• Juxtaglomerular Cells (Granular Cells): Modified smooth muscle cells in the wall of the afferent arteriole that secrete renin in response to low blood pressure or decreased sodium delivery to the distal tubule.
• Macula Densa: Specialized epithelial cells in the wall of the distal tubule that monitor sodium chloride concentration in the filtrate and signal the juxtaglomerular cells to release renin.
• Extraglomerular Mesangial Cells: Cells located outside the glomerulus that provide support and may play a role in regulating glomerular filtration.

III. The Ureters: Structure and Function

The ureters are paired tubes that transport urine from the renal pelvis of each kidney to the urinary bladder. Each ureter is approximately 25-30 cm long and has a diameter of about 3-4 mm.

A. Structure of the Ureters:

The ureter wall consists of three layers:

1. Mucosa: The innermost layer, lined with transitional epithelium, which allows for stretching and recoil.
2. Muscularis: The middle layer, consisting of two layers of smooth muscle: an inner longitudinal layer and an outer circular layer. Peristaltic contractions of the muscularis propel urine toward the bladder.
3. Adventitia: The outermost layer, composed of fibrous connective tissue that supports and protects the ureter.

B. Function of the Ureters:

The primary function of the ureters is to transport urine from the kidneys to the urinary bladder. Peristaltic contractions of the smooth muscle in the ureter wall, along with gravity, facilitate urine flow. Valves at the ureterovesical junction prevent backflow of urine into the ureters.

IV. The Urinary Bladder: Structure and Function

The urinary bladder is a hollow, distensible muscular organ that stores urine until it is excreted from the body. Its capacity is approximately 700-800 ml in adults.

A. Structure of the Urinary Bladder:

1. Location: Located in the pelvic cavity, posterior to the pubic symphysis.
2. Shape: When empty, the bladder is collapsed and wrinkled. As it fills with urine, it becomes more spherical.
3. Layers of the Bladder Wall:
   • Mucosa: The innermost layer, lined with transitional epithelium. The mucosa contains folds called rugae, which allow for expansion of the bladder.
   • Submucosa: A layer of connective tissue that supports the mucosa.
   • Muscularis (Detrusor Muscle): The middle layer, consisting of three layers of smooth muscle: an inner longitudinal layer, a middle circular layer, and an outer longitudinal layer. Contraction of the detrusor muscle expels urine from the bladder.
   • Adventitia: The outermost layer, composed of fibrous connective tissue that supports and protects the bladder. On the superior surface, the adventitia is replaced by the serosa (peritoneum).
4. Trigone: A triangular region on the posterior wall of the bladder, defined by the openings of the two ureters and the urethra. The trigone is a smooth area, lacking rugae.

B. Function of the Urinary Bladder:

The primary function of the urinary bladder is to store urine temporarily. The bladder can expand to accommodate increasing volumes of urine without a significant increase in pressure, thanks to the transitional epithelium and the rugae in the mucosa. When the bladder is full, stretch receptors in the bladder wall trigger the micturition reflex, leading to urination.

V. The Urethra: Structure and Function

The urethra is a tube that conveys urine from the urinary bladder to the outside of the body. Its structure and length differ between males and females.

A. Female Urethra:

• Length: Approximately 4 cm long.
• Location: Extends from the urinary bladder to the external urethral orifice, located anterior to the vaginal opening.
• Structure: Lined with transitional epithelium near the bladder and stratified squamous epithelium near the external opening.
• External Urethral Sphincter: A voluntary sphincter located near the external urethral orifice, allowing for conscious control of urination.

B. Male Urethra:

• Length: Approximately 20 cm long.
• Location: Extends from the urinary bladder to the external urethral orifice, located at the tip of the penis.
• Structure: The male urethra is divided into three regions:
  • Prostatic Urethra: Passes through the prostate gland.
  • Membranous Urethra: The shortest and narrowest portion, passing through the urogenital diaphragm.
  • Spongy (Penile) Urethra: Passes through the penis and opens at the external urethral orifice.
• Sphincters:
  • Internal Urethral Sphincter: An involuntary sphincter located at the junction of the bladder and the urethra.
  • External Urethral Sphincter: A voluntary sphincter located in the urogenital diaphragm, allowing for conscious control of urination.

C. Function of the Urethra:

The primary function of the urethra is to transport urine from the urinary bladder to the outside of the body. In males, the urethra also serves as a passageway for semen during ejaculation.

VI. Innervation of the Urinary System

The urinary system is innervated by both the sympathetic and parasympathetic divisions of the autonomic nervous system.

A. Kidneys:

• Sympathetic Innervation: Primarily from the renal plexus, derived from the celiac plexus and aorticorenal ganglia. Sympathetic stimulation causes vasoconstriction of the renal arterioles and stimulates renin release.
• Parasympathetic Innervation: Vagal nerve fibers have limited direct influence on the kidneys.

B. Ureters:

• Sympathetic Innervation: From the renal, aortic, and hypogastric plexuses.
• Parasympathetic Innervation: From the pelvic splanchnic nerves.

C. Urinary Bladder:

• Sympathetic Innervation: From the hypogastric plexus. Sympathetic stimulation causes relaxation of the detrusor muscle and contraction of the internal urethral sphincter, promoting urine retention.
• Parasympathetic Innervation: From the pelvic splanchnic nerves. Parasympathetic stimulation causes contraction of the detrusor muscle and relaxation of the internal urethral sphincter, promoting urination.
• Somatic Innervation: The external urethral sphincter is controlled voluntarily by the pudendal nerve.

VII. Clinical Significance

Understanding the anatomy of the urinary system is crucial for diagnosing and treating various clinical conditions, including:

• Kidney Stones (Nephrolithiasis): Formation of mineral deposits in the kidneys, which can obstruct urine flow and cause pain.
• Urinary Tract Infections (UTIs): Infections of the urinary system, commonly caused by bacteria.
• Renal Failure: Loss of kidney function, leading to the accumulation of waste products in the blood.
• Bladder Cancer: Cancer of the urinary bladder, often associated with smoking and exposure to certain chemicals.
• Urinary Incontinence: Involuntary leakage of urine, often due to weakened pelvic floor muscles or nerve damage.
• Benign Prostatic Hyperplasia (BPH): Enlargement of the prostate gland in males, which can compress the urethra and cause urinary problems.

VIII. Review Questions

1. Describe the major organs of the urinary system and their functions.
2. Draw and label a nephron, including its major components.
3. Explain the roles of the afferent and efferent arterioles in glomerular filtration.
4. What are the three layers of the ureter wall, and what is the function of each layer?
5. Compare and contrast the male and female urethra.
6. Describe the innervation of the urinary bladder and explain how the sympathetic and parasympathetic nervous systems regulate urination.
7. What is the juxtaglomerular apparatus, and what is its role in regulating blood pressure?
8. Explain the clinical significance of understanding the anatomy of the urinary system.

IX. Conclusion

The urinary system, with its complex anatomical structures and intricate physiological functions, plays a pivotal role in maintaining homeostasis. A thorough understanding of the anatomy of the kidneys, ureters, urinary bladder, and urethra is essential for healthcare professionals to diagnose and treat a wide range of urinary disorders effectively. This review sheet provides a comprehensive overview of the urinary system's anatomy, serving as a valuable resource for students and practitioners alike. By mastering this knowledge, individuals can better appreciate the remarkable efficiency and importance of this vital system.