🫀Anatomy and Physiology II Unit 8 – Urinary System: Kidney Anatomy & Physiology

Key Structures of the Urinary System

• Kidneys filter blood, remove waste products, and maintain fluid and electrolyte balance
• Ureters transport urine from the kidneys to the urinary bladder
• Urinary bladder stores urine until it is ready to be excreted from the body
• Urethra carries urine from the bladder to the outside of the body during urination
• Adrenal glands sit atop the kidneys and secrete hormones (aldosterone, cortisol)
• Prostate gland surrounds the urethra in males and secretes fluid that nourishes sperm
• Female urethra is shorter than the male urethra, increasing susceptibility to urinary tract infections (UTIs)

Kidney Anatomy: From Macro to Micro

• Kidneys are bean-shaped organs located in the retroperitoneal space of the abdominal cavity
• Renal cortex is the outer portion of the kidney containing glomeruli and convoluted tubules
  • Cortex receives most of the kidney's blood supply
• Renal medulla is the inner portion consisting of renal pyramids and collecting ducts
  • Pyramids contain loops of Henle and collecting ducts
• Renal pelvis is a funnel-shaped structure that collects urine from the collecting ducts
• Renal artery supplies oxygenated blood to the kidney while the renal vein carries deoxygenated blood away
• Nephrons are the functional units of the kidney, each containing a glomerulus and renal tubule
• Juxtaglomerular apparatus (JGA) regulates blood pressure and glomerular filtration rate (GFR)

Nephron: The Functional Unit

• Each kidney contains around 1 million nephrons responsible for blood filtration and urine production
• Nephrons consist of a renal corpuscle (glomerulus and Bowman's capsule) and a renal tubule
• Glomerulus is a network of capillaries where blood filtration occurs
  • Podocytes wrap around glomerular capillaries and create filtration slits
• Bowman's capsule surrounds the glomerulus and collects the filtrate
• Proximal convoluted tubule (PCT) reabsorbs nutrients, water, and ions from the filtrate
• 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
• Distal convoluted tubule (DCT) fine-tunes urine composition and responds to hormones
• Collecting duct system collects urine from multiple nephrons and conducts it to the renal pelvis

Urine Formation Process

• Glomerular filtration is the first step, where blood is filtered in the glomerulus
  • Filtration is driven by Starling forces (hydrostatic and oncotic pressures)
• Tubular reabsorption occurs in the PCT, loop of Henle, and DCT, reclaiming essential substances
  • Glucose and amino acids are actively transported out of the tubule
  • Sodium is reabsorbed via secondary active transport, creating an osmotic gradient for water reabsorption
• Tubular secretion actively removes additional waste products (hydrogen ions, potassium, drugs) from the blood
• Countercurrent multiplication in the loop of Henle creates a concentration gradient in the medulla
  • Allows for the production of concentrated urine to conserve water
• Collecting ducts respond to antidiuretic hormone (ADH) to fine-tune water reabsorption
• Urine composition is a balance of filtration, reabsorption, and secretion processes

Hormonal Regulation of Kidney Function

• Antidiuretic hormone (ADH) from the posterior pituitary increases water reabsorption in collecting ducts
  • Released in response to increased blood osmolarity or decreased blood volume
• Aldosterone from the adrenal cortex increases sodium reabsorption and potassium secretion in the DCT
  • Helps maintain blood volume and pressure
• Atrial natriuretic peptide (ANP) from the heart atria decreases sodium reabsorption and increases GFR
  • Released in response to increased blood volume and atrial stretch
• Parathyroid hormone (PTH) increases calcium reabsorption and phosphate excretion in the DCT
• Renin-angiotensin-aldosterone system (RAAS) regulates blood pressure and fluid balance
  • Renin from the JGA converts angiotensinogen to angiotensin I
  • Angiotensin-converting enzyme (ACE) converts angiotensin I to angiotensin II, a potent vasoconstrictor
  • Angiotensin II stimulates aldosterone release and increases blood pressure

Common Kidney Disorders

• Acute kidney injury (AKI) is a rapid decline in kidney function due to various causes (ischemia, toxins, obstruction)
• Chronic kidney disease (CKD) is a gradual, progressive loss of kidney function over months or years
  • Often caused by diabetes, hypertension, or glomerulonephritis
• Urinary tract infections (UTIs) occur when bacteria invade the urinary system, causing inflammation
  • More common in females due to shorter urethra
• Nephrolithiasis (kidney stones) are hard deposits of minerals that can obstruct urine flow and cause pain
  • Risk factors include dehydration, high-protein diets, and certain medications
• Glomerulonephritis is inflammation of the glomeruli, often caused by autoimmune disorders or infections
• Polycystic kidney disease (PKD) is an inherited disorder characterized by multiple cysts in the kidneys
• Renal cell carcinoma (RCC) is the most common type of kidney cancer, often presenting with hematuria

Clinical Applications and Diagnostics

• Urinalysis examines urine for abnormalities (blood, protein, glucose, pH, specific gravity)
  • Dipstick tests provide quick screening for various substances
• Blood tests assess kidney function by measuring creatinine, blood urea nitrogen (BUN), and electrolytes
  • Estimated glomerular filtration rate (eGFR) is calculated from serum creatinine, age, sex, and race
• Imaging studies (ultrasound, CT, MRI) visualize kidney structure and detect abnormalities (stones, tumors, cysts)
• Renal biopsy involves removing a small sample of kidney tissue for microscopic examination
  • Helps diagnose glomerular diseases and other kidney disorders
• Dialysis (hemodialysis or peritoneal dialysis) is used to replace kidney function in end-stage renal disease (ESRD)
• Kidney transplantation is the definitive treatment for ESRD, using a donor kidney to restore function

Key Takeaways and Study Tips

• Understand the anatomy and function of the urinary system, from the kidneys to the urethra
• Focus on the structure and function of the nephron, the basic functional unit of the kidney
• Learn the steps of urine formation: glomerular filtration, tubular reabsorption, and tubular secretion
  • Recognize the role of each nephron segment in these processes
• Familiarize yourself with the hormones that regulate kidney function and their effects on urine composition
  • ADH, aldosterone, ANP, PTH, and the RAAS are essential to understand
• Review common kidney disorders, their causes, and their clinical presentations
  • Be able to differentiate between AKI and CKD, as well as other conditions like UTIs and kidney stones
• Understand the clinical applications and diagnostic tests used to assess kidney function and detect abnormalities
  • Urinalysis, blood tests, imaging studies, and renal biopsy are important tools
• Use diagrams and flowcharts to visualize the relationships between structures and processes in the urinary system
• Create flashcards or mnemonics to memorize key terms, hormones, and their functions