renal system physiology

• 5 Homeostatic Functions of Kidneys


  • Regulates plasma ion concentrations
  • Regulates BV and BP
  • Regulation of blood osmolarity
  • Helps stabilize blood pH
  • Removal of waste
• Kidneys


  • All about absorption and removal
  • Also production of hormones and enzymes
• Location of Kidneys


  • Located on either side of vertebral column
  • Left kidney superior to right kidney
  • Superior surface capped by suprarenal (adrenal) gland

  
• Typical Adult Kidney


  • ~4 in. long
  • ~2.2 in. wide
  • ~1.2 in. thick
  • Weighs ~150 g
• Structures of the Urinary System


  • Inferior vena cava
  • Abdominal aorta
  • Adrenal gland
  • Kidney
  • Renal hilus
  • Renal artery
  • Renal vein
  • Ureter
  • Bladder
  • Neck of bladder
  • Urethra

  
• Internal Kidney Regions


  • Renal cortex: granular-appearing superficial region
  • Renal medulla: deep to cortex, composed of cone-shaped medullary (renal) pyramids
  • Renal pelvis: Funnel-shaped tube continuous with ureter
• Urine Flow


  Renal pyramid → minor calyx → major calyx → renal pelvis → ureter
• Nephrons


  • Structural and functional units that form urine
  • More than 1 million per kidney
• Parts of a Nephron


  • Renal corpuscle
  • Renal tubule
• Renal Corpuscle


  Comprised of Bowman's Capsule and glomerulus
• Bowman's Capsule


  Hollow structure containing the glomerulus
• Glomerulus


  "Ball" of specialized capillaries (fenestrated)
• Renal Tubule


  • Proximal tubule
  • Nephron Loop or Loop of Henle
  • Distal tubule
  • Collecting ducts
• Proximal Tubule


  • Most of reabsorption occurs here
  • Cuboidal cells with dense microvilli that form brush border, increase surface area
• Nephron Loop or Loop of Henle


  Creates dilute urine, and regulates concentration gradient to concentrate urine
• Distal Tubule


  Fine regulation of salt and water balance
• Collecting Ducts


  Fine regulation of salt and water balance, collect filtrate from many nephrons
• Classes of Nephrons


  • Cortical Nephrons
  • Juxtamedullary Nephrons
• Cortical Nephrons


  • 85% of nephrons
  • Almost entirely in the cortex
• Juxtamedullary Nephrons


  • Originate close to the junction between the cortex and medulla
  • Important for the production of concentrated urine
  • Have very long nephron loops
• Nephron Capillary Beds


  • Glomerulus
  • Peritubular capillaries
  • Vasa recta
• Glomerulus


  • Capillaries are specialized for filtration
  • Different from other capillary beds because they are fed and drained by arteriole
  • Afferent arteriole enters glomerulus and leaves via efferent arteriole
  • Blood pressure in glomerulus high
• Peritubular Capillaries


  • Low-pressure, porous capillaries adapted for absorption of water and solutes
  • Arise from efferent arterioles
  • Cling to adjacent renal tubules in cortex
  • Empty into venules
• Vasa Recta


  • Long, thin-walled vessels parallel to long nephron loops of juxtamedullary nephrons
  • Arise from efferent arterioles serving juxtamedullary nephrons
  • Function in formation of concentrated urine
• Juxtaglomerular Complex


  • Each nephron has one juxtaglomerular complex (JGC)
  • Involves modified portions of distal portion of ascending limb of nephron loop and afferent (sometimes efferent) arteriole
  • Important in regulating rate of filtrate formation and blood pressure
• Cell Populations in Juxtaglomerular Complex


  • Macula densa
  • Granular cells (juxtaglomerular, or JG cells)
  • Extraglomerular mesangial cells
• Macula Densa


  • Tall, closely packed cells of ascending limb
  • Contain chemoreceptors that sense NaCl content of filtrate
• Granular Cells (Juxtaglomerular, or JG Cells)


  • Enlarged, smooth muscle cells of arteriole
  • Act as mechanoreceptors to sense blood pressure in afferent arteriole
  • Contain secretory granules that contain enzyme renin
• Extraglomerular Mesangial Cells


  • Located between arteriole and tubule cells
  • Interconnected with gap junctions
  • May pass signals between macula densa and granular cells
• Processes Involved in Urine Formation and Adjustment of Blood Composition


  • Glomerular filtration
  • Tubular reabsorption
  • Tubular secretion
• Glomerular Filtration

  • 1. Hydrostatic pressure forces fluids and solutes through filtration membrane into glomerular capsule
  • 2. No reabsorption into capillaries of glomerulus occurs
  • Produces cell- and protein-free filtrate
  • Movement of blood into lumen of nephron, takes place only at renal corpuscle
  • Walls of glomerular capillaries and Bowman's capsule are modified to allow bulk flow
  • A passive process
  • No metabolic energy required for glomerular filtration
• Tubular Reabsorption


  Selectively returns 99% of substances from filtrate to blood in renal tubules and collecting ducts
• Tubular Secretion


  Selectively moves substances from blood to filtrate in renal tubules and collecting ducts
• Excreted


  Filtered - Reabsorbed + Secreted
• Movement of blood into lumen of nephron


  Takes place only at renal corpuscle
• Walls of glomerular capillaries and Bowman's capsule


  • Modified to allow bulk flow
• Filtration membrane


  • Porous membrane between blood and interior of glomerular capsule
  • Allows water and solutes smaller than plasma proteins to pass
  • Normally no cells can pass
  • Macromolecules "stuck" in filtration membrane are engulfed by glomerular mesangial cells
  • Proteins in filtrate indicate membrane problem
• Layers of filtration membrane


  • Fenestrated endothelium of glomerular capillaries
  • Basement membrane: fused basal laminae of two other layers
  • Foot processes of podocytes with filtration slits; slit diaphragms repel macromolecules
  • Filtration membrane allows molecules smaller than 3 nm to pass, e.g. water, glucose, amino acids, nitrogenous wastes
• Plasma proteins remain in blood to maintain colloid osmotic pressure
• Starling forces favoring filtration


  • Glomerular capillary hydrostatic pressure (60 mm Hg)
  • Bowman's capsule oncotic pressure (0 mm Hg)