16: Water + Electrolyte Balance Pt. 2

• (2) The Proximal Tube:

  • Filtrate leaves Bowman's capsule and enters a convoluted structure called the proximal tubule
  • The fluid inside this tubule contains water and small solutes such as urea, glucose, amino acids, vitamins, and electrolytes
  • The epithelial cells have small projections called microvilli facing the lumen -> increases the epithelial surface area
• (2) The Proximal Tube

  • Functions in active transport of selected molecules out of the filtrate
  • Solutes leave the proximal tube and enter epithelial cells; water follows along the osmotic gradient
  • In this way, valuable solutes and water are reabsorbed and returned to the body
• Selective reabsorption in the proximal tubule

  1. Na+/K+ - ATPase in the basolateral membranes removes intracellular Na+ into the interstitial fluid

  2. Creates a gradient favouring Na+ entry from the lumen

  3. Na+ dependent cotransporters in the apical membrane use the gradient to remove ions and nutrients selectively from the filtrate

  4. Movement of na+ into the cell along its electrochemical gradient provides the means for moving other solutes against their gradients

  5. Solutes that move into the cell diffuse across the basolateral membrane into nearby blood vessels

  6. Water follows ions from the proximal tubule into the cell and then into the blood vessels
• (2) The proximal tubule

  • Almost all nutrients, along with 2/3 of the NaCl and water, are reabsorbed through the proximal tubule
  • The osmolarity of the tubular fluid is unchanged despite the huge change in volume because the water reabsorption is proportional to solute reabsorption
• (3) The loop of Henle

  • Fluid from the proximal tubule enters the loop of Henle
  • In most nephrons, the loop is short and barely enters the medulla (cortical nephrons)
  • But in ~20% of nephrons in a human kidney, the loop is long and punges from the cortex of the kidney deep into the medulla (juxtamedullary nephrons)
• (3) The loop of Henle

  1. As fluid flows down the descending limb, the fluid inside the loop loses water to the tissue surrounding the nephron
  2. This movement of water is passive, down its osmotic gradient
  3. The fluid inside the nephron loses Na+ and Cl- in the thin ascending limb
  4. The ions move passively along electrochemical gradients
  5. Near the cortex, the osmolarity of the surrounding interstitial fluid is low
  6. Additional Na+ and Cl- are actively transported out of the nephron in the thick ascending limb
• (3) The loop of Henle

  • The countercurrent flow of fluid is self-reinforcing - the presence of an osmotic gradient stimulates water and ion flows that in turn maintain the osmotic gradient
• The water and salt that move out of the loop of Henle diffuse into the vasa recta

  • The network of blood vessels that runs along the loop
  • As a result, water and electrolytes that are reabsorbed are returned to the bloodstream instead of being excreted in the urine
• Once filtrate has passed through the loop of Henle, it enters the distal tubule (4)

  • This fluid is slightly hyposmotic to blood, and the solutes it contains are mainly urea and other waste products
  • The fluid that enters the distal tubule is always dilute
  • In contrast, the urine that leaves the collecting duct is dilute when the individual is well hydrated, but concentrated when the individual is dehydrated
• The activity of the distal tubule and collecting duct is highly regulated and altered in response to osmotic stress

  • The amount of Na+, Cl-, and water that is reabsorbed in the distal tubule and in the collecting duct varies with the animal's hydration
  • Changes in the distal tubule and collecting duct are controlled by hormones - signaling molecules in the blood
• If Na+ levels in the blood are low, the adrenal glands release the hormone aldosterone, which leads to activation of sodium pumps and reabsorption of Na in the distal tubule

  • Aldosterone saves sodium and water
  • It stimulates the secretion of K+ and H+ from the blood into the distal tubule
• If an individual is dehydrated, the brain releases antidiuretic hormone (ADH)

  • ADH saves water
  • Has 2 important effects on epithelial cells in the collecting duct:
  • Triggers the insertion of aquaporins into the apical membrane
  • Cells become more permeable to water and large amounts of water are reabsorbed
  • Increases permeability to urea, which is reabsorbed into the surrounding fluid
  • This helps create a concentration gradient favouring water reabsorption from the filtrate
• With ADH present:

  • Water leaves the collecting duct passively - following the concentration gradient maintained by the loop of Henle
  • Water is conserved and urine is strongly hyperosmotic relative to blood
• When ADH is absent, few aquaporins are found in the collecting duct epithelium, leaving it relatively impermeable to water and resulting in hyposmotic urine
• The collecting duct is the final place where the composition of the filtrate can be altered
• Urine exiting the collecting ducts moves from the kidneys into ureters and then is stored in the bladder until urination