Ultrafiltration and selective reabsorption

• What is the difference between small and large molecules during filtration in the kidneys?

  • Small molecules pass out of the capillaries (out of the glomerulus), while large molecules like proteins and RBCs remain in the vessel
  • basement membrane acts as a sieve, so this is possible
• Which small molecules are filtered out of the capillaries in the kidneys?

  water, glucose, and amino acids
• What layers do molecules pass through in the kidneys?


  • Epithelium of capillaries
  • the basement membrane of the glomerulus
  • and the epithelium of the bowman’s capsule
• What can happen to tissue fluid components in the absence of reabsorption?

  Tissue fluid components may enter surrounding cells or form lymph
• Explain how selective reabsorption happens in the proximal convulated tube

  • Sodium ions are actively transported out of the cell into tissue fluid
  • glucose/AAs and sodium ions enter cells with Na+ ions by facilitated diffusion
  • glucose and AAs then diffuse back into blood capillary, this is selective re absorption
  • reduces WP of PCT cells but increases WP of tubule lumen outside of cells. Water is reabsorbed into blood by osmosis
• How are cells of the proximal convulated tubule adapted?

  • Cell surface folded to form microvilli to increase SA for reabsorption
  • membrane has co-transporter proteins for facilitated diffusion of sodium glucose and amino acids
  • oppositie side of membrane contains sodium potassium pump to maintain/create a concentration gradient
  • contains mitochondria for active transport of sodium and potassium against their conc grad at basement membrane
  • close to blood caps- good blood supply- maintains conc grad
• What substances are re-absorbed back into the blood in the PCT?

  • all glucose
  • all AAs
  • some salts
  • all vitamins
  • some urea
  • some water
• What is ultrafiltration?

  filtering small molecules from the blood under high pressure
• What is selective reabsorption?

  allows kidneys to reabsorb useful small molecules back into blood
• How does urea move out of the proximal convoluted tubule?

  By diffusion from a high to a low concentration
• Summarise the process of selective reabsorption

  Selective reabsorption:

  
• What happens to blood as it leaves the kidney?

  Blood leaves with reduced levels of urea and almost unchanged levels of glucose and amino acids.
• What is the significance of selective reabsorption in the nephron?

  Selective reabsorption ensures that essential substances like glucose are retained in the blood.
• Describe the role of podocytes

  • Extensions called pedicels
  • Wrap around capillaries and form slits to make sure cells platelets or plasma proteins that have made it through epithelial cells and basement membrane don’t get through to the PCT
• Outline the similarities between the processes of ultrafiltration in the kidneys and tissue fluid formation. (4 marks) (PPQ)

  Similarities:

  •Process: Both involve the movement of fluid and small solutes from a high-pressure area to a low-pressure area across a selectively permeable membrane.

  •Forces involved: Both are driven by hydrostatic pressure differences, primarily due to blood pressure.

  •Selective permeability: Both membranes allow the passage of water and small solutes but restrict larger molecules and cells.
• Outline the differences between the processes of ultrafiltration in the kidneys and tissue fluid formation. (4 marks) (PPQ)

  • Ultrafiltration - in the glomerulus, nephron, tissue fluid formation - at the arterial end of capillaries
  • The glomerular membrane- more complex, with three layers (endothelium, basement membrane, podocytes), capillary walls = single layer of endothelial cells, thin.
  • Glomerular filtrate contains a wider range of solutes.
  • Glomerular filtrate - renal tubule for further processing, forming urine. Tissue fluid- reabsorbed into the venous end of capillaries.
• how is blood forced out of the capillaries in the kidney?

  • comes in though a wide afferent arteriole
  • and leaves through a narrow efferent arteriole
  • creates high pressures in the nephron

  
• true or false? the filtrate entering the renal capsule contains same concentration of substances found in blood plasma

  TRUE
• What is the volume of blood filtered through the kidneys at a given time known as?

  glomerular filtration rate
• Explain the adaptations of the PCT cells


  Adaptations:

  A) microvili

  B) surface area

  C) co-transporter

  D) mitochondria

  E) sodium potassium

  F) tightly

  
• Explain how selective reabsorption happens in the proximal convoluted tube

  • Sodium ions are actively transported out of the cell into tissue fluid
  • glucose/AAs and sodium ions enter cells with Na+ ions by facilitated diffusion
  • glucose and AAs then diffuse back into blood capillary of glomerulus, this is selective re absorption
  • Water follows and is reabsorbed into blood by osmosis