Overview
This lecture covers the anatomy, functions, and physiology of the urinary system, focusing on how kidneys filter blood, form urine, and maintain homeostasis.
Components of the Urinary System
- The urinary system includes kidneys, ureters, bladder, and urethra.
- Kidneys produce urine, which flows via ureters to the bladder for storage, then exits via the urethra.
- Ureters use peristalsis to move urine to the bladder.
Functions of the Kidneys
- Kidneys filter blood to remove ions, metabolic wastes (e.g., urea, uric acid), and excess water.
- They regulate plasma ion concentration, stabilize blood pH, and maintain blood pressure by controlling blood volume.
- Kidneys conserve needed nutrients and synthesize calcitriol (vitamin D) and erythropoietin (stimulates red blood cell production).
Kidney Anatomy and Blood Flow
- Kidneys are retroperitoneal, located near the posterior abdominal wall, protected by ribs and fat layers.
- The right kidney is lower than the left due to the liver.
- Blood supply: Renal artery → segmental arteries → interlobar arteries → arcuate arteries → cortical radiate arteries → afferent arteriole → glomerulus → efferent arteriole → peritubular capillaries/Vasa recta → veins → renal vein → inferior vena cava.
Nephron Structure and Types
- Nephron = renal corpuscle (glomerulus + Bowman's capsule) + renal tubule (proximal convoluted tubule, nephron loop, distal convoluted tubule).
- Cortical nephrons (85%) are mostly in the cortex; juxtamedullary nephrons (15%) have longer loops in the medulla and concentrate urine more efficiently.
Urine Formation Mechanisms
- Filtration (at glomerulus): small molecules cross the filtration membrane; large proteins and blood cells do not.
- Tubular reabsorption: recovers nutrients, water, ions from filtrate back to blood, mainly in the proximal convoluted tubule.
- Tubular secretion: moves toxins/ions from blood to nephron for excretion.
Tubule Functions and Hormonal Regulation
- Descending nephron loop reabsorbs water; ascending loop reabsorbs sodium/chloride.
- Distal convoluted tubule and collecting duct respond to ADH (promotes water reabsorption) and aldosterone (promotes sodium reabsorption, potassium secretion).
- Urine is only officially urine once it enters the minor calyx.
Juxtaglomerular Complex
- Macula densa senses sodium/chloride in filtrate to assess nephron function.
- Juxtaglomerular cells monitor blood pressure, releasing renin to raise blood pressure if needed.
- Extra glomerular cells facilitate communication within the complex.
Ureters, Bladder, and Urethra
- Ureters have transitional epithelium and move urine via peristalsis.
- The bladder stores urine; its wall is lined with transitional epithelium and detrusor muscle for contraction.
- The male urethra is longer, reducing infection risk; the female urethra is shorter and not shared with the reproductive system.
- Internal urethral sphincter (smooth muscle, involuntary) and external urethral sphincter (skeletal muscle, voluntary) control urine release.
Urination (Micturition) Reflex
- Full bladder stretches baroreceptors, triggering detrusor muscle contraction and internal sphincter relaxation.
- Voluntary control is maintained by the external urethral sphincter.
Key Terms & Definitions
- Nephron — functional unit of the kidney responsible for filtration and urine formation.
- Glomerulus — tuft of capillaries involved in the filtration of blood.
- Bowman's capsule — surrounds the glomerulus and collects filtrate.
- Juxtaglomerular apparatus — regulates nephron function and blood pressure.
- Filtration — movement of fluid/solutes across the glomerular membrane.
- Reabsorption — process of recovering substances from filtrate back to blood.
- Secretion — movement of substances from blood into the nephron tubule.
- ADH (Antidiuretic hormone) — hormone that increases water reabsorption in kidneys.
- Aldosterone — hormone that increases sodium reabsorption and potassium secretion.
- Peristalsis — wave-like muscle contractions moving urine through ureters.
Action Items / Next Steps
- Review diagrams of nephron structure and urine flow.
- Study the effects of ADH and aldosterone on kidney function.
- Complete assigned readings on the urinary system in your textbook.