tubular function

Question 1

What are the transport processes in the nephron?

Answer 1

1. Glomerulus to the bowman’s capsule- ultrafiltration (plasma is filtered through the endothelium lining, basement membrane and podocytes) to create filtration
   ○ Non selective
   ○ Cells, proteins and large molecules cannot enter the bowman’s capsule
   ○ Produces ultrafiltrate that is like plasma but lacks proteins
   
   2. Tubular reabsorption- in the proximal convoluted tubule
      ○ Selective
      ○ Re absorption as already was absorbed in the small intestine
      ○ Reabsorbs useful solutes e.g. glucose and amino acids. Water is also reabsorbed
      ○ Most filtered solutes are too valuable to loose are reabsorbed
      ○ Glucose is too valuable to be wasted to is stated to be reabsorbed at the start of the PCT
   3. Tubular secretion- in the distal convoluted tubule
   • Peritubular blood vessels surround the tubules- diffusion and active transport can entered into the filtrate via this

Question 2

What is the role and the importance of the proximal convoluted tubule?

Answer 2

• Cells of the proximal convoluted tubule are folded / have a brush border on the filtrate side to increase the surface area for reabsorption so more reabsorption can take place
  
  • proximal convoluted tubule cells have a high organelle content/mitochondria which provide energy for Absorbtion.
  • Glucose is too valuable to be wasted to is stated to be reabsorbed at the start of the proximal convoluted tubule
  • proximal convoluted tubule is a 1/3 of the length of the nephron
  • Reabsorbs
    ○ Ions- 65-67% reabsorbed
    ○ Water- 65-67% reabsorbed
    ○ All glucose is reabsorbed
    ○ All amino acids are reabsorbed
    ○ All vitamins are reabsorbed

Question 3

What is the importance of Na+/K+ ATPase?

Answer 3

• All cells on the basolateral side have Na+/K+ ATPase to remove Na+ from cell (lower Na+ concentration in the cell) and put in the K+
  
  • The Na+ can be used to co-transport glucose into the cell
  • Na+ is most abundant ion in the blood
  • Move Na+ to increase Na+ in blood further to create a higher concentration gradient than in another would be moved (e.g. k+)
  • Na+/k+ ATPase creates an Na+ concertation gradient this provides a driving forces for Na+ to force into the cell by facilitated diffusion
  • Na+ is the most abundant ion in the body so had the largest effect when the diffusion gradient is changed
  • Pair Na+ reabsorption with the valuable nutrients e.g. glucose and animo acids

Question 4

How does reabsorption occur in the proximal convoluted tubule and what is the importance of Na+ for this?

Answer 4

• Transport of molecules or ions can be transcellular (through the cell)
  ○ Diffusion through tubular cell cytosol
  ○ From apical side to the basolateral side
  
  • paracellular (in-between cells)
    ○ Movement through the tight junctions which are leaky
    ○ Once a substance has moved into the interstitial fluid its concentration is higher than in the peritubular capillaries so the substance can diffuse into the blood by simple diffusion
  • reabsorption of Glucose/amino acids
    ○ Na+/K+ ATPase on the basolateral side of the proximal convoluted tubule cell will actively pump Na+ out of the cell and punk K+ into the cell
    ○ This will create a concentration gradient of Na+
    ○ Na will move down its concentration gradient by facilitated diffusion using a co-transported
    ○ Na+ is co-transported with glucose/animo acids into the cell against the glucose concentration gradient
    ○ Glucose/amino acids diffuses into the capillary by facilitated diffusion
    ○
  • reabsorption of HCO3-
    ○
    ○ Na+/K+ ATPase on the basolateral side of the proximal convoluted tubule cell will actively pump Na+ out of the cell and punk K+ into the cell
    ○ This will create a concentration gradient of Na+
    ○ Na+ will move down its concentration gradient by facilitated diffusion using a co-transported
    ○ Na+ will move into the cell in exchange for H+ which will move out of the cell
    ○ H+ + HCO3- = CO2 + H2O
    ○ CO2 and H2O dissolve into cell and from H+ and HCO3- which will then be used to transport more Na+ into the cell
    ○
  • reabsorption of Water
    ○ Na+/K+ ATPase on the basolateral side of the proximal convoluted tubule cell will actively pump Na+ out of the cell and punk K+ into the cell
    ○ This will create a concentration gradient of Na+
    ○ Na will move down its concentration gradient by facilitated diffusion using a co-transported
    ○ Na+ is co-transported with something else acids into the cell down the Na+ concertation gradient
    ○ This lowers the water potential inside the cell causing water to move into the blood transcellular using aquaporins or paracellular in between cells into the blood
    ○ Water will carry the dissolved substances in as well○
    please look at lecture notes for the diagrams