Anatomy Of The Urinary System Review Sheet Exercise 40

The urinary system, a vital component of the human body, plays a crucial role in maintaining homeostasis by filtering blood, regulating fluid and electrolyte balance, and excreting waste products. Understanding its intricate anatomy is essential for healthcare professionals and anyone interested in learning more about the human body. This detailed review sheet exercise will cover the key structures of the urinary system, including the kidneys, ureters, urinary bladder, and urethra, along with their respective functions.

The Kidneys: The Filtration Powerhouse

The kidneys, bean-shaped organs located in the retroperitoneal space of the abdominal cavity, are the primary functional units of the urinary system. They are responsible for filtering blood, removing waste products, and producing urine. Each kidney is approximately 12 cm long, 6 cm wide, and 3 cm thick, weighing around 150 grams.

Gross Anatomy of the Kidneys

• Renal Capsule: A tough, fibrous outer layer that protects the kidney from injury and infection.
• Renal Cortex: The outer region of the kidney, containing the glomeruli and convoluted tubules of the nephrons.
• Renal Medulla: The inner region of the kidney, consisting of renal pyramids and columns.
• Renal Pyramids: Cone-shaped structures within the medulla, containing collecting ducts that drain urine.
• Renal Columns: 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 collects urine from the renal papilla.
• Major Calyx: Formed by the fusion of several minor calyces.
• Renal Pelvis: A funnel-shaped structure that collects urine from the major calyces and funnels it into the ureter.
• Renal Hilum: A medial indentation where the renal artery, renal vein, and ureter enter and exit the kidney.

Microscopic Anatomy of the Kidneys: The Nephron

The nephron is the functional unit of the kidney, responsible for filtering blood and producing urine. Each kidney contains approximately one million nephrons. The nephron consists of two main parts:

• Renal Corpuscle: The initial filtering component of the nephron, consisting of the glomerulus and Bowman's capsule.
  • Glomerulus: A network of capillaries where blood is filtered.
  • Bowman's Capsule: A cup-shaped structure that surrounds the glomerulus and collects the filtrate.
• Renal Tubule: A long, winding tube that processes the filtrate to form urine.
  • Proximal Convoluted Tubule (PCT): The first part of the renal tubule, responsible for reabsorbing water, glucose, amino acids, and electrolytes from the filtrate.
  • Loop of Henle: A hairpin-shaped section of the renal tubule that descends into the medulla and then ascends back to the cortex. It plays a crucial role in concentrating urine.
    • Descending Limb: Permeable to water but not to solutes.
    • Ascending Limb: Permeable to solutes but not to water.
  • Distal Convoluted Tubule (DCT): The last part of the renal tubule, responsible for further reabsorption of water and electrolytes under hormonal control (ADH and aldosterone).
  • Collecting Duct: A long tube that collects urine from multiple nephrons and transports it to the renal pelvis.

Blood Supply to the Kidneys

The kidneys receive a rich blood supply from the renal arteries, which branch directly from the abdominal aorta. The renal arteries divide into smaller and smaller arteries within the kidney, eventually forming the afferent arterioles that supply the glomeruli. After filtration, blood exits the glomeruli through the efferent arterioles, which then branch into peritubular capillaries that surround the renal tubules. Blood then flows into the renal veins, which drain into the inferior vena cava.

The Ureters: Transporting Urine

The ureters are two muscular tubes that transport urine from the kidneys to the urinary bladder. Each ureter is approximately 25-30 cm long and 3-4 mm in diameter. They originate at the renal pelvis and descend through the retroperitoneal space to enter the bladder wall obliquely.

Structure of the Ureters

The ureter wall consists of three layers:

• Inner Mucosa: A transitional epithelium that is impermeable to urine.
• Middle Muscularis: Two layers of smooth muscle (inner longitudinal and outer circular) that contract to propel urine towards the bladder via peristaltic waves.
• Outer Adventitia: A fibrous connective tissue layer that anchors the ureter to surrounding structures.

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 propel urine downwards, even against gravity. The oblique entry of the ureters into the bladder wall helps prevent backflow of urine from the bladder into the ureters.

The Urinary Bladder: Urine Storage

The urinary bladder is a hollow, distensible muscular organ that stores urine until it is released from the body. It is located in the pelvic cavity, posterior to the pubic symphysis. The bladder's capacity varies, but it can typically hold up to 500-800 mL of urine.

Structure of the Urinary Bladder

The bladder wall consists of four layers:

• Inner Mucosa: A transitional epithelium that is impermeable to urine. The mucosa is folded when the bladder is empty, allowing it to expand as it fills.
• Submucosa: A layer of connective tissue that supports the mucosa.
• Muscularis (Detrusor Muscle): A thick layer of smooth muscle that contracts to expel urine from the bladder.
• Outer Serosa (or Adventitia): A layer of connective tissue that covers the superior surface of the bladder.

Key Features of the Urinary Bladder

• Trigone: A triangular area on the posterior wall of the bladder, defined by the openings of the two ureters and the urethra. The trigone is a common site for bladder infections.
• Internal Urethral Sphincter: A smooth muscle sphincter located at the junction of the bladder and urethra. It is under involuntary control and prevents urine from leaking out of the bladder.
• Rugae: Folds in the bladder lining that allow for expansion.

Function of the Urinary Bladder

The urinary bladder serves as a temporary storage reservoir for urine. As urine accumulates in the bladder, the detrusor muscle relaxes, allowing the bladder to expand. When the bladder reaches a certain level of fullness, stretch receptors in the bladder wall send signals to the brain, triggering the urge to urinate.

The Urethra: Urine Elimination

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

Female Urethra

The female urethra is approximately 4 cm long and extends from the bladder to the external urethral orifice, located anterior to the vaginal opening. It is relatively short and straight, making females more susceptible to urinary tract infections (UTIs).

Male Urethra

The male urethra is approximately 20 cm long and serves as a passageway for both urine and semen. It is divided into three regions:

• Prostatic Urethra: Passes through the prostate gland.
• Membranous Urethra: A short segment that passes through the urogenital diaphragm.
• Spongy (Penile) Urethra: Runs through the length of the penis and opens at the external urethral orifice.

Sphincters of the Urethra

• Internal Urethral Sphincter: As mentioned earlier, this smooth muscle sphincter is located at the bladder-urethra junction and is under involuntary control.
• External Urethral Sphincter: A skeletal muscle sphincter located in the urogenital diaphragm. It is under voluntary control, allowing for conscious control of urination.

Function of the Urethra

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

The Process of Urine Formation: A Detailed Look

Urine formation is a complex process that involves three main steps: glomerular filtration, tubular reabsorption, and tubular secretion.

1. Glomerular Filtration

• Location: Glomerulus and Bowman's capsule.
• Process: Blood pressure forces water and small solutes (such as glucose, amino acids, electrolytes, and waste products) from the glomerular capillaries into Bowman's capsule, forming the filtrate.
• Filtration Membrane: The filtration membrane consists of three layers:
  • Capillary Endothelium: Contains fenestrations (pores) that allow for the passage of small molecules.
  • Basement Membrane: A glycoprotein layer that restricts the passage of large proteins.
  • Podocytes: Specialized cells that surround the glomerular capillaries and have filtration slits that further restrict the passage of large molecules.
• Filtration Rate: The glomerular filtration rate (GFR) is the volume of filtrate formed per minute by all the nephrons in both kidneys. A normal GFR is approximately 125 mL/min.

2. Tubular Reabsorption

• Location: Primarily in the proximal convoluted tubule (PCT), but also occurs in the loop of Henle, distal convoluted tubule (DCT), and collecting duct.
• Process: The movement of water and solutes from the filtrate back into the blood in the peritubular capillaries.
• Substances Reabsorbed: Water, glucose, amino acids, sodium, potassium, chloride, bicarbonate, and other electrolytes.
• Mechanisms of Reabsorption:
  • Active Transport: Requires energy to move substances against their concentration gradients (e.g., glucose, amino acids).
  • Passive Transport: Does not require energy and moves substances down their concentration gradients (e.g., water, chloride).
  • Osmosis: The movement of water from an area of high water concentration to an area of low water concentration.

3. Tubular Secretion

• Location: Primarily in the distal convoluted tubule (DCT), but also occurs in the proximal convoluted tubule (PCT).
• Process: The movement of substances from the blood in the peritubular capillaries into the filtrate in the renal tubule.
• Substances Secreted: Waste products (e.g., urea, uric acid, creatinine), drugs, toxins, and excess ions (e.g., hydrogen, potassium).
• Purpose of Secretion: To further eliminate waste products from the body and to regulate blood pH.

Hormonal Control of Kidney Function

Several hormones play a crucial role in regulating kidney function, including:

• Antidiuretic Hormone (ADH): Released by the posterior pituitary gland in response to dehydration or increased blood osmolarity. ADH increases water reabsorption in the collecting ducts, resulting in more concentrated urine and increased blood volume.
• Aldosterone: Released by the adrenal cortex in response to low blood pressure or low sodium levels. Aldosterone increases sodium reabsorption and potassium secretion in the distal convoluted tubule (DCT), leading to increased blood volume and blood pressure.
• Atrial Natriuretic Peptide (ANP): Released by the heart in response to increased blood volume or blood pressure. ANP inhibits sodium reabsorption in the distal convoluted tubule (DCT) and collecting ducts, leading to decreased blood volume and blood pressure.
• Parathyroid Hormone (PTH): Released by the parathyroid glands in response to low blood calcium levels. PTH increases calcium reabsorption in the distal convoluted tubule (DCT), leading to increased blood calcium levels.

Common Disorders of the Urinary System

Understanding the anatomy of the urinary system is essential for diagnosing and treating various disorders, including:

• Urinary Tract Infections (UTIs): Infections of the urinary system, typically caused by bacteria. Common symptoms include frequent urination, painful urination, and blood in the urine.
• Kidney Stones (Renal Calculi): Hard deposits that form in the kidneys from minerals and salts. They can cause severe pain as they pass through the urinary tract.
• Kidney Failure (Renal Failure): A condition in which the kidneys lose their ability to filter waste products from the blood. It can be acute or chronic.
• Glomerulonephritis: Inflammation of the glomeruli, the filtering units of the kidneys.
• Urinary Incontinence: Loss of bladder control.
• Overactive Bladder: A condition characterized by frequent and urgent urination.
• Prostate Enlargement (Benign Prostatic Hyperplasia - BPH): An enlargement of the prostate gland that can obstruct the urethra and cause urinary problems in men.
• Bladder Cancer: Cancer that forms in the lining of the bladder.

Review Questions

1. Describe the gross anatomy of the kidney, including the renal cortex, renal medulla, renal pyramids, renal columns, renal pelvis, and renal hilum.
2. Explain the structure and function of the nephron, including the renal corpuscle (glomerulus and Bowman's capsule) and the renal tubule (PCT, loop of Henle, DCT, and collecting duct).
3. Trace the path of blood flow through the kidney, from the renal artery to the renal vein.
4. Describe the structure and function of the ureters.
5. Describe the structure and function of the urinary bladder, including the trigone and the detrusor muscle.
6. Compare and contrast the male and female urethra.
7. Explain the three main steps of urine formation: glomerular filtration, tubular reabsorption, and tubular secretion.
8. Describe the role of ADH, aldosterone, ANP, and PTH in regulating kidney function.
9. List common disorders of the urinary system and their symptoms.

Conclusion

A thorough understanding of the anatomy of the urinary system is fundamental for anyone working in healthcare. From the filtration prowess of the kidneys to the storage capabilities of the bladder and the transport function of the ureters and urethra, each component plays a vital role in maintaining the body's internal environment. By studying the gross and microscopic structures, along with their functions, one can gain a deeper appreciation for the complexity and importance of this essential system. This review sheet exercise has provided a comprehensive overview of the anatomy of the urinary system, serving as a valuable resource for students, healthcare professionals, and anyone interested in learning more about the human body.