Renal, GI, Senses

• The junctions found in epithelial cells are tight junctions, gap junctions, and desmosomes
• Tight junctions


  • Surround the cell and provide a solid barrier which can be either leaky or tight
  • Define the properties of the paracellular pathway
• Transcellular pathway


  Transport through a cell
• Paracellular pathway


  Transport between cells
• Tight junction in PCT

  Leaky, allows paracellular transport of Na+ and water, but solutes must move through transcellular pathway
• Gap junctions


  Connect cytoplasmic compartments via connexins and allow the passage of small ions and molecules
• Desmosomes


  Tightly connect cells via cadherins which are connected to dense plaques which organise intracellular filaments
• Epithelial cells

  • Have polarity, with an apical surface and basolateral surface
  • Apical surface contains microvilli for increased absorption or secretion
• In PCT

  Epithelium is leaky so has a high permeability to water through the transcellular (AQP1) and paracellular pathways
• In CCT

  Epithelium is tight so has a low permeability to water, so only travels through the transcellular pathway (AQP2)
• Basolateral membrane

  • Has a high permeability to water and K+
  • Has membrane infoldings
  • Only has Na+/K+-ATPase
• Primary active transport


  Moves solutes in opposing directions
• Secondary active transport


  Moves solutes in the same direction
• Diffusion occurs at a constant rate
• Na+ absorption

  1. Na+ enters epithelial cell via Na+ channel

  2. Na+ moves into interstitium via Na+/K+-ATPase
• Na+/K+-ATPase releases 3 Na+ into the interstitium and 2K+ into the cell, and requires the use of 1 ATP molecule which is hydrolysed to ADP
• Na+ gradient

  Provides a driving force for other substances such as glucose to be absorbed if there is a sodium-dependent transporter present (SGLT1 or SGLT2)
• Glucose release into interstitium

  1. Cell concentration reaches a certain level

  2. Released via facilitated diffusion through GLUT1 or GLUT2
• In leaky epithelium, Na+ absorption

  Causes the lumen to become negatively charged and the interstitium to become positively charged, driving paracellular pathway absorption of Cl- ions
• In leaky epithelium, Na+ absorption

  Generates an osmotic gradient, driving paracellular absorption and transcellular absorption (AQP1 on apical surface, AQP3 and AQP4 on basolateral surface) of water
• Only AQP2 and ENaC is present in tight epithelium on the apical surface
• In tight epithelium

  1. Na+ absorbed into cell by ENaC

  2. Water absorbed by AQP2

  3. 3Na+ leave cell and 2K+ enter cell via Na+/K+-ATPase

  4. Water leaves cell via AQP3 and AQP4
• Driving forces for ion/solute/water movement

  Chemical, electrical, and osmotic driving forces
• Acinar cells in salivary glands

  1. Produce an isotonic primary fluid containing ions and water

  2. Duct cells then modify the solution to make it slightly acidic
• Na+/K+-ATPase in salivary glands

  Generates a low Na+ concentration inside the cell, causing more to diffuse into the cell via a channel on both sides of the cell
• Secretion in salivary glands

  1. Na+, K+, and 2Cl- into the cell via NKCC1 cotransporter

  2. Cl- into the lumen via a chloride channel

  3. Na+ and K+ absorbed via the Na+/K+-ATPase pump
• Apical chloride secretion in salivary glands

  Results in negatively charged lumen and positively charged interstitium, driving paracellular sodium secretion in leaky epithelium
• Secretion of Na+, K+, and Cl- in salivary glands

  Makes the interstitium hypotonic and lumen hypertonic, so water is secreted through both transcellular and paracellular pathways
• If the chloride channel is mutated or defective, water secretion is disturbed and results in cystic fibrosis
• Leaky epithelia in PCT

  Uses Na+ coupled transport and has high rates of Na+ reabsorption (bulk absorption) and high water permeability
• Tight epithelia in CCT

  Uses Na+ channels and has low rates of Na+ reabsorption (fine tuning) and low water permeability (transcellular only)
• Water permeable segments of nephron

  • PCT, proximal straight tubule, thin descending limb
• Water impermeable segments of nephron

  • Thin ascending limb, thick ascending limb, DCT
• CCT is made of tight epithelia, so water permeability is facilitated by vasopressin
• Normal urine components

  • 95-98% water, creatinine, urea, H+, NH3, Na+, K+, drugs
• Pathological urine components

  • Glucose, protein, blood, haemoglobin, leucocytes, bacteria
• Normal urine pH

  Acidic, 5-6
• Pathological urine can be golden, red, brown, or blue in colour, taste sweet, or smell fruity or rotten
• Normal urine output is 1.5L/day
• Filtration

  Occurs in the glomerulus