🤾🏻‍♂️Human Physiology Engineering Unit 8 – Urinary System

Urinary System Overview

• Consists of the kidneys, ureters, bladder, and urethra which work together to filter blood, remove waste products, and regulate fluid and electrolyte balance
• Filters about 180 liters of blood per day, producing 1-2 liters of urine
• Maintains homeostasis by controlling the composition and volume of body fluids
• Regulates blood pressure by secreting hormones (renin) and controlling salt and water balance
• Produces erythropoietin (EPO) to stimulate red blood cell production in the bone marrow
• Activates vitamin D for calcium absorption and bone health
• Eliminates toxins, drugs, and metabolic waste products (urea, creatinine, uric acid) from the body

Kidney Structure and Function

• Bean-shaped organs located in the retroperitoneal space, one on each side of the vertebral column
• Composed of an outer cortex and inner medulla, with the medulla divided into pyramids
• Contains approximately 1 million nephrons, the functional units of the kidney
• Receives about 20% of the cardiac output through the renal arteries
• Regulates acid-base balance by secreting or reabsorbing hydrogen ions and bicarbonate
• Maintains electrolyte balance by adjusting the reabsorption of sodium, potassium, calcium, and magnesium
• Concentrates urine to conserve water and dilutes urine to excrete excess water

Nephron Anatomy and Physiology

• Nephrons are the microscopic functional units of the kidney, responsible for filtering blood and forming urine
• Consists of a renal corpuscle (glomerulus and Bowman's capsule) and a renal tubule (proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct)
• Glomerulus is a network of capillaries where blood filtration occurs, creating an ultrafiltrate
• Bowman's capsule surrounds the glomerulus and collects the ultrafiltrate
• Proximal convoluted tubule reabsorbs essential nutrients, glucose, and amino acids, and secretes organic acids and bases
  • Reabsorbs about 65% of the ultrafiltrate
• Loop of Henle creates a concentration gradient in the medulla for water reabsorption
  • Descending limb is permeable to water, while the ascending limb is impermeable to water but actively transports sodium, potassium, and chloride
• Distal convoluted tubule fine-tunes the composition of the filtrate by reabsorbing or secreting specific ions and responds to hormones (aldosterone, parathyroid hormone)
• Collecting duct is responsible for final water reabsorption under the influence of antidiuretic hormone (ADH) and contributes to acid-base balance

Urine Formation Process

• Urine formation involves three main processes: glomerular filtration, tubular reabsorption, and tubular secretion
• Glomerular filtration is the passive process of blood filtration in the glomerulus, creating an ultrafiltrate containing small molecules (water, glucose, amino acids, electrolytes, and waste products)
  • Filtration is driven by Starling forces (hydrostatic and oncotic pressures) across the glomerular capillary wall
  • Glomerular filtration rate (GFR) is about 125 mL/min or 180 L/day in a healthy adult
• Tubular reabsorption is the selective process of reclaiming essential nutrients, electrolytes, and water from the ultrafiltrate back into the bloodstream
  • Occurs throughout the nephron tubule segments via passive and active transport mechanisms
  • Regulated by hormones (aldosterone, parathyroid hormone, antidiuretic hormone) and local factors
• Tubular secretion is the active process of removing specific substances (hydrogen ions, potassium, organic acids, and drugs) from the peritubular capillaries into the tubular lumen
  • Helps maintain acid-base balance and eliminates toxins and medications
• The final urine composition is determined by the net effects of filtration, reabsorption, and secretion along the nephron

Regulation of Fluid Balance

• The kidneys play a crucial role in maintaining fluid balance by adjusting urine concentration and volume
• Antidiuretic hormone (ADH), also known as vasopressin, is released from the posterior pituitary gland in response to increased blood osmolarity or decreased blood volume
  • ADH increases water permeability in the collecting ducts, promoting water reabsorption and producing concentrated urine
  • Absence of ADH results in dilute urine and increased urine output (diabetes insipidus)
• Aldosterone, a mineralocorticoid hormone secreted by the adrenal cortex, regulates sodium and potassium balance
  • Aldosterone stimulates sodium reabsorption and potassium secretion in the distal convoluted tubule and collecting duct
  • Increased sodium reabsorption leads to increased water retention and blood volume
• Atrial natriuretic peptide (ANP), released by the atria of the heart in response to increased blood volume, promotes sodium excretion and dilates afferent arterioles
  • ANP counteracts the effects of aldosterone and ADH, leading to increased urine output and decreased blood volume
• Renin-angiotensin-aldosterone system (RAAS) is a hormonal cascade that regulates blood pressure and fluid balance
  • Renin, released by the juxtaglomerular cells in response to decreased renal perfusion, converts angiotensinogen to angiotensin I
  • Angiotensin-converting enzyme (ACE) converts angiotensin I to angiotensin II, a potent vasoconstrictor that stimulates aldosterone secretion
  • Aldosterone promotes sodium and water retention, increasing blood volume and pressure

Urinary Tract Anatomy

• The urinary tract consists of the ureters, bladder, and urethra, which transport, store, and eliminate urine from the body
• Ureters are muscular tubes that propel urine from the renal pelvis to the bladder via peristaltic contractions
  • About 25-30 cm long and 3-4 mm in diameter
  • Contain smooth muscle layers and a mucosal lining
• Bladder is a hollow, muscular organ that stores urine until voluntary micturition occurs
  • Can hold up to 500-600 mL of urine in adults
  • Composed of the detrusor muscle (smooth muscle), which contracts during micturition, and the trigone, a triangular area between the ureteral orifices and the internal urethral orifice
• Urethra is the final passage for urine excretion from the bladder to the external environment
  • Shorter in females (3-4 cm) compared to males (18-20 cm)
  • In males, the urethra is divided into prostatic, membranous, and penile (spongy) portions
  • Contains internal and external urethral sphincters for voluntary and involuntary control of micturition

Common Urinary System Disorders

• Urinary tract infections (UTIs) are bacterial infections affecting the bladder (cystitis), urethra (urethritis), or kidneys (pyelonephritis)
  • More common in females due to shorter urethra and proximity to the anal opening
  • Symptoms include frequent, painful urination, cloudy or blood-tinged urine, and abdominal discomfort
• Kidney stones (nephrolithiasis) are hard deposits of minerals and salts that form in the kidneys and can cause severe pain when passing through the urinary tract
  • Risk factors include dehydration, high-protein diets, and certain metabolic disorders
  • Types of stones include calcium oxalate, calcium phosphate, uric acid, and struvite (magnesium ammonium phosphate)
• Renal failure is the inability of the kidneys to adequately filter waste products from the blood, leading to the accumulation of toxins and fluid imbalances
  • Acute renal failure is sudden and often reversible, caused by decreased renal perfusion, nephrotoxins, or obstruction
  • Chronic kidney disease (CKD) is a progressive, irreversible decline in kidney function over months to years, often due to diabetes, hypertension, or glomerulonephritis
• Urinary incontinence is the involuntary leakage of urine, which can be caused by stress (physical activity), urgency (overactive bladder), or overflow (neurogenic bladder)
  • Treatment options include pelvic floor exercises, medications, and surgical interventions (slings, bladder neck suspension)
• Benign prostatic hyperplasia (BPH) is a non-cancerous enlargement of the prostate gland in older men, which can cause urinary symptoms (frequency, urgency, weak stream, and incomplete emptying)
  • Managed with alpha-blockers, 5-alpha reductase inhibitors, or surgical procedures (transurethral resection of the prostate, TURP)

Engineering Applications in Urology

• Urodynamic studies involve the use of pressure and flow sensors to assess bladder and urethral function during filling and voiding phases
  • Help diagnose conditions such as overactive bladder, urinary incontinence, and bladder outlet obstruction
  • Utilize catheters with pressure transducers and uroflowmeters to measure urine flow rate and volume
• Extracorporeal shock wave lithotripsy (ESWL) is a non-invasive technique that uses focused shock waves to break kidney stones into smaller fragments that can pass more easily through the urinary tract
  • Employs a lithotripter machine with an electromagnetic or piezoelectric generator to create high-energy shock waves
  • Requires precise targeting of the stones using fluoroscopy or ultrasound guidance
• Urinary diversion procedures involve surgically redirecting urine flow from the bladder to an alternative storage and elimination system, often in cases of bladder cancer, neurogenic bladder, or congenital abnormalities
  • Ileal conduit is a common type of incontinent diversion, where a segment of the ileum is used to create a stoma on the abdominal wall, and urine is collected in an external pouch
  • Continent diversions (Indiana pouch, Kock pouch) create an internal reservoir from intestinal segments, which can be catheterized through an abdominal stoma
• Artificial urinary sphincters (AUS) are implantable devices designed to treat severe stress urinary incontinence, particularly in men after prostate surgery
  • Consist of an inflatable cuff around the urethra, a pressure-regulating balloon in the abdomen, and a control pump in the scrotum
  • The cuff remains inflated to maintain urethral closure and is manually deflated by squeezing the pump to allow voiding
• Sacral neuromodulation (SNM) is a treatment for refractory overactive bladder, urinary retention, and fecal incontinence that involves electrical stimulation of the sacral nerve roots
  • A small, implantable pulse generator (IPG) delivers controlled electrical pulses to the S3 sacral nerve via a lead placed through the sacral foramen
  • Modulates the neural pathways responsible for bladder and bowel control, improving symptoms and quality of life