M21

• Urinary System


  Major excretory system; controls the composition & volume of body fluids

  
• Functions of Urinary System


  • Excretion of waste products
  • Regulation of blood volume and pressure
  • Regulation of the concentration of solutes in the blood
  • Regulation of extracellular fluid pH
  • Regulation of red blood cell synthesis
  • Regulation of vitamin D synthesis

  
• Kidneys


  • Bean-shaped organs located retroperitoneally & on each side of the vertebral column; the major excretory organ of urinary system
  • Divided into two major regions, outer cortex & inner medulla, which surrounds renal sinus
  • formed from joining of all the calyces
  • Ureter - a small tube from narrowed renal pelvis which exits the kidney & connects to urinary bladder

  
• Urine Flow

  1. Filtration

  2. Tubular Reabsorption

  3. Tubular Secretion

  
• Nephron


  • A functional unit of the kidney and each consists of: Renal Corpuscle, Proximal Convoluted Tubule, Loop of Henle, Distal Convoluted Tubule

  
• Renal Corpuscle


  • Housed the filtration portion of nephrons
  • Bowman Capsule - enlarged end of nephron which is intended to form a double-walled chamber; cavities open into proximal convoluted tubule which carries fluid away from the capsule
  • Glomerulus - a tuft of capillaries that lies within the indentation of Bowman capsule
  • Juxtaglomerular Apparatus - a vital regulatory structure located next to glomerulus; responsible for the secretion of enzyme renin

  
• Proximal Convoluted Tubule


  • Primary site for reabsorption of solutes & water
• Loop of Henle


  • Descending Limb - extends toward the renal sinus; critical site for water reabsorption by osmosis
  • Ascending Limb - extends toward the cortex where the filtered fluid flows going to distal convoluted tubule

  
• Distal Convoluted Tubule


  • Empties fluid into a single collecting duct, in which multiple ducts empty into a single papillary duct
• Types of Nephrons

  • Juxtamedullary Nephrons
  • Cortical Nephrons
• Blood Supply through Kidneys


  Renal Arteries ► Interlobar Arteries ► Arcuate Arteries ► Interlobular Arteries ► Afferent Arteriole ► Glomerulus ► Efferent Arteriole ► Peritubular Capillaries, including Vasa Recta ► Interlobular Veins ► Arcuate Veins ► Interlobar Veins ► Renal Vein

  
• Filtration


  1. Movement of materials across the filtration membrane into the Bowman capsule to form filtrate

  2. Driving force is the blood pressure

  3. Filtration Pressure - a pressure gradient which forces the fluid from glomerular capillary across filtration membrane into the Bowman capsule to form filtrate; influenced by three factors: glomerular capillary blood pressure, blood protein concentration, pressure in Bowman capsule
• Tubular Reabsorption


  1. Solutes are reabsorbed across the wall of nephrons into the interstitial fluid

  2. Solutes & water reabsorbed from proximal convoluted tubule enter the peritubular capillaries (65% of filtrate volume)

  3. To descending limb, reabsorbed water & solutes then to the vasa recta (15% of filtrate volume)

  4. To ascending limb which dilutes the filtrate by removing solutes

  5. Solutes diffuse out of the nephron (Na+ via active transport; K+ & Cl- via cotransport)

  6. Highly concentrated filtrate is converted to dilute solution as it travels

  7. Enters the interstitial fluid of medulla & help concentration of solutes high

  8. To distal convoluted tubule & collecting duct which remove water & additional solutes (19% of filtrate volume) (Na+ & Cl- are the principal reabsorbed solutes)

  9. Leaving only 1% of original filtrate volume as urine
• Tubular Secretion


  Solutes are secreted across the wall of the nephron into the filtrate; can either be through: Active Mechanism, Passive Mechanism
• Regulation of Urine Concentration and Volume


  1. Increase body fluid = increase water reabsorption = concentrated urine (eliminates 5!solute & conserves water)

  2. Decrease body fluid concentration = decrease water absorption = diluted urine (eliminates 5!water & conserves solutes)

  3. Increase blood volume = Increase blood pressure = large volume of urine

  4. Decrease blood volume = decrease blood pressure = small volume of urine
• Hormonal Mechanisms


  • Renin-Angiotensin-Aldosterone (RAA) Mechanism
  • Antidiuretic Hormone (ADH) Mechanism
  • Atrial Natriuretic Hormone (ANH) Mechanism
• Urinary Bladder


  • Hollow, muscular container that stores urine; can hold to a maximum of about 1000mL
  • Trigone - a triangle-shaped portion of urinary bladder located between the opening of ureters & urethra
  • Internal Urethral Sphincter - smooth muscles at the junction of urinary bladder & urethra that prevents urine leakage from the urinary bladder
  • External Urethral Sphincter - skeletal muscle; allows a person to voluntarily start or stop the flow of urine out

  
• Micturition Reflex


  Activated by stretch of urinary bladder wall; an automatic reflex but can be inhibited or stimulated by higher centers in brain

  
• Body Fluid Compartments

  • Intracellular Fluid Compartment
  • Extracellular Fluid Compartment
• pH


  Concentration of H+ in body fluids (normal value: 7.35 – 7.45); controlled by three factors: Buffers, Respiratory System, Kidneys
• Diseases and Disorders of Urinary System

  • Kidney Stone / Renal Calculus
  • Glomerulonephritis
  • Acute Renal Failure