Quiz 3: Renal System

• What are the functions of the kidney?

  excretion, regulation, and secretion
• Excretion

  • metabolic end-products
  • drugs and exogenous compounds (xenobiotics)
• Regulation

  • essential substances (Na, K, H, Ca, Mg, Cl, HCO3, HPO4, H2O)
  • osmolarity
  • pH
  • blood pressure
• Secretion

  • renin secretions
  • erythropoietin secretion
  • vitamin D activation
• 3 major parts of the kidney

  1. cortex
  2. medulla
  3. calyces
• Cortex

  outer area/cortical region; inner juxtamedullary region
• Medulla

  inner area of renal pyramids
• Calyces

  minor: papillae extend

  major: converge to form pelvis
• A nephron is a functional unit of the kidney composed of a corpuscle (filters blood plasma using glomerular capillaries and the Bowman's glomerular capsule) and a 50mm renal tube (composed of the proximal and distal tubules and the loop of Henle), as well as a collecting duct
• What are the two nephron types?

  Cortical (85%) - reabsorption and peritubular capillaries

  Juxtamedullary (15%) - concentrates urine and has vasa recta and peritubular capillaries; contains a long loop of Henle with deep penetration
• Urinary secretion is dependent on its filtration, reabsorption, and secretion
• Filtration (blood to lumen) is seen in the glomerulus as it reabsorbs nutrients to avoid excretion
• Reabsorption (lumen to blood) is seen in the proximal tubule, descending and ascending loops of Henle, distal tubule, and collecting duct
• Secretion (blood to lumen) is seen in the proximal and distal tubule and collecting duct
• Excretion = Filtration - Reabsorption + Secretion
• The glomerular filtration rate (GFR) is the amount of plasma filtered from the glomeruli into the Bowman's space per unit time (avg human GFR = 125 mL/min or 180 L/day)
• GFR processing:

  1. plasma volume enters afferent arteriole (100%)
  2. 80% reabsorbed and 20% of volume filters
  3. greater than 19% of fluid is reabsorbed within the nephron
  4. greater than 99% of plasma entering kidney reenters the blood
  5. less than 1% of volume is excreted to external environment
• The renal corpsucle is composed of the glomerulus (capillary network) and the Bowman's capsule (beginning of tubule with a parietal and visceral epithelium)
• Filtration barrier results in a filtrate (protein-free dialysate of plasma) and is composed of:

  1. fenestrated capillary epithelium
  2. basement membrane (basal lamina)
  3. pores formed by podocytes
• Factors that affect GFR:

  stimule: P_H (glomerular capillary hydrostatic pressure)

  inhibit: P_fluid (Bowman's hydrostatic pressure) and pi (coiloidosmotic force)
• P_H is the blood pressure and is the primary GFR regulating mechanism
• P_fluid is the fluid pressure created by the Bowman's capsule
• pi is due to proteins in plasma but not the Bowman's capsule
• Net GFR = k_f (P_H - P_BS - pi) where k_f is the filtration coefficient and is determined by the space the glomerulus has and how leaky the membrane is
• Increase in the hydrostatic pressure (P_H) causes GFR to increase because of the increase in blood pressure and volume
• P_H is influenced by:

  • arterial pressure (could be buffered)
  • afferent arteriole resistance
  • efferent arteriole resistance
• If the afferent arteriole of the glomerulus dilates, then there is will be an increase in blood pressure, an increase in GFR, and an increase in renal blood flow
• Autoregulation of GFR maintains a constant GFR with an average blood pressure between 80 and 180 mmHg independent of P_H influence, which is due to three mechanisms:

  1. myogenic response
  2. tubuloglomerular feedback
  3. hormones and autonomic neurons
• Myogenic response

  • similar to autoregulation in other systemic arterioles
  • increases renal blood pressure by constricting (allowing for limiting and control) the afferent arteriole
  • caused by stretch receptors in smooth muscle
• Tubuloglomerular feedback

  • paracrine control (local control)
  • increase in GFR causes increase in NaCl sensed by the macula densa to cause constriction of afferent arteriole
  • composed of three cells: macula densa cell, granular (juxtaglomerular) cell, and mesangial cell
• Steps of tubuloglomerular feedback

  1. GFR increases
  2. flow through tubule increases
  3. flow past macula densa increases
  4. paracrine from macula densa to afferent arterioles
  5. afferent arteriole constricts to increase resistance and decrease P_H in glomerulus
• Macula densa cell

  osmoreceptors (modified epithelial cells) in distal tubule (sodium sensor)
• Granular (juxtaglomerular) cell

  modified smooth muscle of afferent arteriole and secretes renin
• Mesangial cell

  contractile to regulate glomerular filtration
• Hormones and Angiotensin system

  • converts angiopepsinogen peptide to angiotensin I
  • angiotensin I becomes angiotensin II in blood vessel using enzyme
  • angiotensin II increases blood pressure by constricting the heart and causing salt and water retention in the kidney
• Renin-angiotensin is stimulated by:

  • decrease in blood pressure
  • sympathetic innervation
  • low osmolarity of tubular fluid (through macula densa)
• Angiotensin II restores blood pressure by constricting arterioles, secreting aldosterone, and stimulating the CNS to increase thirst and release ADH (to retain more water)
• Renal clearance is a measurement of renal excretion ability; volume of plasma from which "x" is completely cleared per unit time; the renal ability to remove a substance from the placemat

  C = ([U]*V)/[P]

  where [U] is the urinary concentration of the substance, v is the urine flow rate, and [P] is the plasma concentration of the substance
• The renal clearance is equal to GFR when substance "x" is:

  1. freely filterable at the glomerulus
  2. not reabsorbed by tubules
  3. not secreted by tubules
  4. not synthesized by tubules
  5. not broken-down by tubules