selective reabsoprtion

• • Occurs in the proximal convoluted tubule 
  • Useful molecules are taken back from filtrate and returned to the blood as the filtrate flows along nephron  
• Selective reabsorption

  
• After the necessary reabsorption of amino acids, water, glucose and inorganic ions is complete, the filtrate eventually leaves the nephron and is now referred to as urine
• Urine flow


  1. Flows out of kidneys

  2. Along the ureters

  3. Into the bladder

  4. Temporarily stored
• Selective reabsorption


  • Many substances in glomerular filtrate need to be kept in the body
  • Reabsorbed into the blood as filtrate passes along the nephron
• Glucose reabsorption


  Occurs in the proximal convoluted tubule
• Lining of the proximal convoluted tubule


  • Single layer of epithelial cells
  • Adapted to carry out reabsorption
  • Microvilli
  • Co-transporter proteins
  • High number of mitochondria
  • Tightly packed cells
• Water and salts reabsorption

  • Occurs via the loop of Henle
  • Occurs via the collecting duct
• Adaptions of selective reabsorption

  
• Active reabsorption of solutes

  1. Blood capillaries close to proximal convoluted tubule

  2. Sodium-potassium pumps move sodium ions

  3. Sodium ions diffuse down concentration gradient

  4. Sodium ions pass through co-transporter proteins

  5. Solutes diffuse into blood
• Blood capillaries are located very close to the outer surface of the proximal convoluted tubule
• Blood in these capillaries comes straight from the glomerulus, it has very little plasma and has lost much of its water, inorganic ions and other small solutes
• Basic membranes


  Sections of the cell membrane that are closest to blood capillaries
• Sodium-potassium pumps in the basal membranes move sodium ions out of the epithelial cells and into the blood
• This movement of sodium ions lowers the concentration of sodium ions inside epithelial cells
• Lower concentration of sodium ions inside epithelial cells


  Causes sodium ions in the filtrate to diffuse down their concentration gradient through the luminal membranes
• Sodium ions do not diffuse freely through the luminal membranes
• Sodium ions must pass through co-transporter proteins in the membrane
• Types of solutes transported by co-transporter proteins

  • Glucose
  • Amino acids
• Once inside the epithelial cells, solutes diffuse down their concentration gradient passing through transport proteins in the basal membranes into the blood
• Molecules reabsorbed from the proximal convoluted tubule

  • Glucose
  • Amino acids
  • Vitamins
  • Inorganic ions
• All glucose in the glomerular filtrate is reabsorbed into blood
• No glucose should be present in urine
• Movement of solutes from the proximal convoluted tubule into the capillaries increases the water potential of the filtrate and decreases the water potential of the blood in capillaries
• Movement of solutes


  Creates a steep water potential gradient and causes water to move into the blood by osmosis
• A significant amount of urea is reabsorbed too
• The concentration of urea in the filtrate is higher in the capillaries causing urea to diffuse from the filtrate back into the blood
• Selective Reabsorption Diagram

  
• Reabsorption of water and salts 

  • As filtrate drips through the loop of Henle, necessary salts are reabsorbed back into blood by diffusion 
  • As salts are reabsorbed back into blood, water follows by osmosis 
  • Water is also reabsorbed from the collecting duct in different amounts depending on how much water the body needs 

  Selective reabsorption uses the same method of membrane transport that move sucrose into companion cell into phloem