Reabsorption

• substances that are removed during filtration are often needed for the body's functions
• occurs within the proximal convoluted tubule
• glucose, amino acids, vitamins and hormones are moved back into the blood by active transport
• 85% of the NaCl and water is reabsorbed
• Na+ ions are moved by active transport
• Cl- ions and water move down concentration gradients
• what are the adaptations of the PCT


  • microvilli - increases surface area over which substances can be reabsorbed
  • many mitochondria - to provide the ATP needed in active transport systems
• steep concentrations gradients are maintained by the constant blood flow in the capillaries
• filtrate at the end of PCT is isotonic with the tissue fluid surrounding the tubule, and with the blood
• over 80% of the glomerular filtrate has reabsorbed back into the blood, regardless of conditions
• the loop of Henle allows mammals to produce more concentrated urine than their blood
• loop of Henle acts as a countercurrent multiplier, using energy to produce concentration gradients resulting in the movement of substances
• descending loop


  • leads from PCT
  • water moves out of the filtrate
  • ion concentration in tissue fluid of medulla increases moving through cortex to pyramids
  • as it travels down, water moves out, so water potential decreases, into the surrounding tissue fluid
  • impermeable to ions so no active transport occurs
• the fluid reaching the hairpin bend is very concentrated and hypertonic to the blood in capillaries
• ascending loop


  • permeable to the ions
  • move out by diffusion, down concentration gradient
  • then actively pumped out into medulla tissue fluid, against concentration gradient
  • high ion concentration in medulla tissue
  • impermeable to water
• remaining fluid in ascending is increasingly dilute, while tissue fluid of medulla develops high concentration of ions
• the high concentration of ions is essential for the kidney to produce more concentrated urine
• blood that reached the top of ascending loop is hypotonic to blood, and enters DCT
• balancing of water occurs in DCT and collecting duct
• walls of tubules varies with the levels of ADH
• walls of DCT have many mitochondria

  • to allow for active transport
• DCT plays a role in balancing blood pH
• the body will return substances to their ideal levels

  • if salt conc in body is low, Na+ and Cl- ions will move out of DCT
• what is the role of the collecting duct

  • passes through concentrated tissue fluid of renal medulla
  • where concentration and volume of urine produced is determined
  • water moves out by diffusion as it passes through medulla
  • Na+ in surrounding fluid increases from medulla to cortex to pelvis
  • water can be removed all the way down the length, producing very hypertonic urine when body needs to conserve water
  • permeability of the duct is controlled by level of ADH