Renal System 2

• Control of Glomerular Filtration Rate (GFR)


  • It is important for the animal to regulate the rate of plasma filtration in the kidney. Too much filtration results in loss of important substances and water

  
• Mechanisms that regulate filtration


  Modifications to the glomerulus

  
• Glomerular filtration


  Plasma filtration in the kidney

  
• Autoregulation of GFR


  • Prevents unintentional shifts in GFR due to fluctuations in Mean Arterial Pressure (MAP)
  • Achieved by myogenic activity and tubuloglomerular feedback

  
• Myogenic activity


  The more the JG cells stretch due to increased blood pressure, the more they constrict reducing blood flow into glomerulus

  
• Tubuloglomerular feedback (TGF)


  The distal tubule can sense changes in sodium concentrations and filtrate volume. Macula densa cells sense sodium and filtrate volume and release paracrine factors that effect JG cell contraction and afferent arterial vasoconstriction

  
• Juxtaglomerular apparatus (JGA)


  • Location where ascending distal tubule, after loop of Henle, contacts glomerulus
  • Allows paracrine communication between Macula densa and Juxtaglomerular cells
• Juxtaglomerular cells


  Smooth muscle cells around afferent arteriole
• Macula densa


  Distal tubule epithelial cells
• Extrinsic control of GFR


  • Sympathetic nervous system contracts JG cells causing afferent arterial vasoconstriction, decreasing GFR and reduced solute and water loss
  • Helps to avoid excessive filtration and maintain appropriate blood volume and pressure

  
• Arterial and low-pressure baroreceptors

  Sense low blood pressure, activate medulla oblongata and the sympathetic nervous system

  
• Renin-Angiotensin-Aldosterone system


  1. Triggered by low blood pressure sensed by baroreceptors (extrinsic)

  2. Low blood pressure in kidney sensed by JG cells (intrinsic)

  3. Low sodium and filtrate volume sensed by macula densa (intrinsic)

  
• Renin-Angiotensin-Aldosterone system


  1. Renin secreted by JG cells

  2. Renin converts angiotensinogen to angiotensin I

  3. Angiotensin I converted to angiotensin II

  4. Angiotensin II causes adrenal cortex to secrete aldosterone

  5. Aldosterone increases sodium reabsorption in tubules

  6. Water follows sodium by osmosis, increasing blood volume and pressure

  7. Angiotensin II and osmoreceptors stimulate vasopressin release
• Aldosterone


  • Increases sodium and water reabsorption along tubule and potassium secretion into tubule
  • Increases sodium-potassium ATPase pumps and sodium/potassium channels
• Atrial natriuretic peptides (ANPs)

  • Released by heart atria in response to increased blood volume/pressure
  • Reduce sympathetic output, inhibit Renin-Angiotensin-Aldosterone system, increase kidney sodium and water excretion

  
• Functions of parts of a nephron


  • Bowman's capsule - collects glomerular filtrate
  • Proximal tubule - uncontrolled reabsorption and secretion
  • Loop of Henle - establishes osmotic gradient
  • Distal tubule and collecting duct - controlled reabsorption and secretion
  • Afferent arteriole - carries blood to glomerulus
  • Glomerulus - filters plasma
  • Efferent arteriole - carries blood from glomerulus
  • Peritubular capillaries - supply oxygen and exchange with tubular fluid
  • Juxtaglomerular apparatus - produces renin

  
• Isotonic urine


  Urine with the same solute or osmotic pressure as normal tissues (around 300 mOsm)
• When a mammal is in ideal fluid balance, an isotonic urine is produced at a moderate rate of about 1 ml/min at approximately 300 mOsm
• Filtration at glomerulus


  1. Plasma filtered into tubular component (Bowman's capsule)

  2. Efferent arteriole carries blood from glomerulus

  3. Peritubular capillaries supply oxygen and exchange with tubular lumen

  
• Juxtaglomerular apparatus


  Produces hormones involved in control of kidney function - Renin

  
• Functions of kidney nephron


  • Secretion
  • Reabsorption
  • Blood flow
  • Filtrate

  
• When a mammal is in ideal fluid balance, an isotonic urine is produced at a moderate rate
• For humans this is about 1 ml/min of urine at approximately 300mOsm
• Isotonic urine


  Has the same solute or osmotic pressure as normal tissues (normally 300 mOsm in animals)
• Osmoles (Osm) = measure of solute concentration/L of solution
• When the animal is dehydrated

  The kidneys will produce a small amount of concentrated urine: low water volume and high concentration of solutes (hypertonic; > 300 mOsm)

  
• When the animal is hydrated


  The kidneys produce a large amount of dilute urine: large water volume and low concentration of solutes (hypotonic urine; < 300 mOsm)

  
• Anti-diuresis


  Small volume of urine
• Diuresis


  Large volume of urine
• The Loop of Henle

  1. Descending limb is permeable to water, filtrate becomes concentrated

  2. Hairpin and thin ascending limb is permeable to sodium, sodium leaves lumen

  3. Thick ascending limb actively transports sodium and chloride out of lumen, water remains inside
• Osmoconcentration


  Hypertonic or high solute concentration
• The system allows the body another chance to reabsorb water from the distal tubule and connecting duct before excretion and greatly affects urine volume
• Reabsorption of water is achieved by action of vasopressin on the distal tubule and collecting duct
• The Countercurrent Multiplier System

  1. Filtrate in proximal tubule is isotonic

  2. Descending limb of Loop of Henle is permeable to water, water exits lumen and lumen filtrate becomes concentrated

  3. Hairpin and thin ascending limb has low water permeability but is permeable to sodium, sodium starts to leave lumen

  4. Thick ascending limb actively transports sodium and chloride out of the lumen and is impermeable to water, lots of sodium pumped out but water remains inside
• Osmoconcentration is achieved because of the close proximity and countercurrent flow of tubular filtrate between the descending and ascending limbs of loop
• A vertical osmotic gradient is established as the countercurrent flow of tubular filtrate continues to deliver more sodium-chloride into the medulla interstitial fluid
• Vasopressin and Water Reabsorption


  1. Tubular filtrate exiting the loop and entering the distal tubule is hypotonic (100 mOsm) and then empties into the collecting duct

  2. The collecting duct continues through the medulla tissue with a high solute concentration, increases potential for water to be reabsorbed from the duct

  3. Vasopressin triggers insertion of aquaporins (AQP-2) into the apical membrane of collecting duct cells, water is reabsorbed and urine becomes hypertonic or concentrated with solutes

  
• During animal dehydration


  AVP secretion is increased, and water is reabsorbed from the collecting duct by osmosis through AQP-2 channels
• In absence of AVP


  The distal tubule and collecting duct are impermeable to water
• When hydrated, animal AVP secretion is low and water is not reabsorbed