Concise Summary of the Urinary System and Related Clinical Cases

Clinical Case of Hidden Bleeding

• Mike, a 14-year-old, was in a motorcycle accident and arrives at the trauma center unconscious with no measurable blood pressure despite 4 liters of fluids.
• Emergency whole blood transfusions cannot stabilize his blood pressure.
• Suspected intra-abdominal injuries due to his bloated abdomen and abrasions lead to an exploratory surgery.
• Findings during surgery include:
  • Ruptured spleen requiring a splenectomy.
  • Continued blood loss with no visible bleeding after clearing blood from abdominal structures.

Introduction to the Urinary System

• Function: Removes metabolic waste and regulates body fluids.
• Major components include:
  • Kidneys: Produce urine (water, ions, solutes).
  • Ureters: Transport urine to bladder.
  • Bladder: Stores urine.
  • Urethra: Conducts urine out of the body.

Major Functions of the Urinary System

• Excretion: Removal of bodily wastes (e.g., urea, uric acid).
• Elimination: Discharge of wastes via urine.
• Homeostasis: Regulates blood volume, pressure, and electrolytes; stabilizes blood pH; conserves nutrients; assists in detoxification of poisons.

Organs and Structures

• Kidneys:
  • Location: Retroperitoneal, between T12 and L3, with the left kidney slightly higher.
  • Structure includes a fibrous capsule, renal cortex, renal medulla with pyramids, and nephron units (about 1.25 million per kidney).
• Blood Flow:
  • Renal artery to segmental arteries to interlobar arteries.
  • Blood flows through nephrons where filtration occurs.

Nephron Structure and Function

• Components:
  • Renal corpuscle: Glomerulus and glomerular capsule for filtration.
  • Renal tubule: Includes proximal convoluted tubule (PCT), nephron loop, distal convoluted tubule (DCT), and collecting ducts.
• Filtration Process:
  • Blood pressure forces water and solutes from glomerulus into the capsular space, creating filtrate.
  • Relevant pressures: Glomerular hydrostatic pressure (GHP), capsular hydrostatic pressure (CSHP), blood colloid osmotic pressure (BCOP).

Urine Formation Processes

1. Filtration: Initial filtration happens in the renal corpuscle, leading to formation of filrate.
2. Reabsorption: Tubular reabsorption of nutrients, ions, and water occurs primarily in the PCT and nephron loop.
3. Secretion: Further modification via secretion of ions and wastes occurs in the DCT and collecting system.

• Significant solutes in urine include urea, creatinine, and uric acid.

Countercurrent Mechanisms in the Kidneys

• Countercurrent multiplication: This process establishes a concentration gradient for urine concentration in the nephron loop.
• Role of ADH: Increases water permeability in collecting ducts, enhancing water reabsorption and concentrating urine.

Urinary Tract Anatomy

• Ureters: Muscular tubes transporting urine from kidneys to bladder.
• Bladder: Stores urine temporarily; shape varies based on fullness.
• Urethra: Conducts urine from bladder to outside the body; differs in length and structure between genders.

Aging and Kidney Function

• Age-related effects include:
  • Decrease in nephron numbers.
  • Reduced GFR and renal blood flow.
  • Changes in responsiveness to hormones, urinary reflexes, increased risk of incontinence, and urinary retention issues.

Kidney Function Testing

• Urinalysis: Evaluates urine composition, detects diseases.
• Creatinine clearance: Assesses kidney function by comparing urine and plasma creatinine levels.
• Blood urea nitrogen (BUN) test: Measures urea levels to evaluate kidney health.