LECTURE 11 (Urine formation by the kidneys II) Flashcards
What is the equation to calculate Urinary excretion?
Urinary excretion = Glomerular filtration - Tubular reabsorption + Tubular secretion
EXPLANATION: urine formation represents the sum of glomerular filtration, tubular reabsorption and tubular secretion
How do we calculate Filtration?
Filtration = Glomerular filtration rate X Plasma concentration
[calculation assumes that substance is freely filtered and not bound to plasma proteins]
What differentiates Tubular reabsorption to Glomerular filtration?
Tubular reabsorption is is highly selective whereas Glomerular filtration is relatively non-selective
EXPLANATION: Tubular reabsorption reabsorbs glucose + amino acids almost entirely, ions depending on needs of body and waste products in small amounts whereas Glomerular filtration filters everything but Proteins
Describe Tubular reabsorption transport
1) Across the tubular epithelial membranes into the renal interstitial fluid
[can be transported through cell membranes themselves “TRANSCELLULAR ROUTE” or through spaces between cell junctions “PARACELLULAR ROUTE”]
2) Through the peritubular capillary membrane back into blood
[transport by “ULTRAFILTRATION” (bulk flow) by hydrostatic and colloid osmotic forces]
Which two ways can solutes be transported across epithelial cells?
- Transcellular pathway = resorbed/secreted across the cells
- Paracellular pathway = between the cells by moving across tight junctions and intracellular spaces
How is Sodium transported from the tubular lumen back into the blood?
1) Na2+ diffuses across the luminal membrane (“apical membrane”) into the cell down an electrochemical gradient established by the Sodium-Potassium ATPase pump on the basolateral side of the membrane
2) Na2+ is transported across the basolateral membrane against an electrochemical gradient by the Sodium-Potassium pump
3) Sodium, water and other substances are reabsorbed into the peritubular capillaries by ULTRAFILTRATION (process driven by hydrostatic and colloid osmotic pressure gradients)
What are the properties of the Sodium-glucose co-transporters (SGLT2 and SGLT1)?
- located on brush border of proximal tubular cells + carry glucose into the cell cytoplasm against a concentration gradient
- 90% of glucose absorbed by SGLT2 in early part of proximal tubule “S1 segment”
- 10% transported by SGLT1 in lateral segments of proximal tubules
- glucose diffuses from cell into interstitial spaces by GLUT2 in S1 segment + GLUT1 in S3 segment of proximal tubule
What happens in glucose reabsorption?
Secondary active transport occurs at the luminal membrane -> passive facilitated diffusion occurs at the basolateral membrane -> passive uptake by bulk flow occurs at the peritubular capillaries
What happens in Pinocytosis?
Some parts of the proximal tubule reabsorb large molecules (e.g proteins) by PINOCYTOSIS
PROCESS:
1) Protein attaches to brush border of luminal membrane + portion of membrane invaginate to the interior of the cell + completely pinches off and forms vesicle
2) Once inside cell, protein is digested into amino acids which are reabsorbed through basolateral membrane into interstitial fluid
[A form of active transport since requires energy]
What is “Transport maximum”?
The limit to the rate at which the solute can be transported
EXPLANATION: The limit is due to saturation of the specific transport systems involved when the amount of solute delivered to the tubule exceeds the capacity of the carrier proteins and specific enzymes involved in the transport process
What is “Gradient-time transport”?
The rate of transport that depends on the electrochemical gradient and the time that the substance is in the tubules which depends on tubular flow rate
EXPLANATION: These substances are passively reabsorbed and do not depend on carrier proteins but on electrochemical gradient, permeability of membrane and time of fluid in tubule instead
Describe water reabsorption in the nephron
- When solutes are transported out of the tubule -> concentration decreases inside tubule but increases in renal interstitium -> concentration difference causes osmosis of water in the same direction
- In proximal tubules = high permeability for water + solutes so move across by tight junctions rapidly
- Ascending loop of hence = water permeability is low -> no water is reabsorbed despite a large osmotic gradient
- Distal tubules, collecting tubules, collecting ducts = can be high or low depending on presence or absence of ADH
[ADH greatly increases water permeability in distal and collecting tubules]
Describe reabsorption of Chloride, Urea and Creatinine
- Active reabsorption of sodium allows PASSIVE reabsorption of chloride by way of an ELECTRICAL POTENTIAL and a CHLORIDE CONCENTRATION GRADIENT
[transport of +ve Na2+ leaves lumen -ve charged compared to interstitial fluid -> ions diffuse passively through paracellular pathway; reabsorption of water increases concentration in lumen allowing for Cl- diffusion into interstitial fluid] - Urea is passively reabsorbed from tubule (in the same mechanism as Cl- with water) but to a lesser extent since it needs UREA TRANSPORTERS
- Almost NONE of creatinine is reabsorbed since is impermeable to tubular membrane
How is the proximal tubule adapted to reabsorb large amounts of water and solutes?
- Highly metabolic + have large numbers of mitochondria to support powerful active transport processes
- Extensive brush border on luminal side of membrane + labyrinth of intercellular and basal channels -> extensive surface area for rapid transport
- Protein carrier molecules -> transports large amount of Na2+
What is the difference between the first and second half of the proximal tubule?
- First half = sodium is reabsorbed by co-transport with glucose, amino acids and other solutes
- Second half = sodium is reabsorbed with CHLORIDE ions
[when glucose and other solutes are reabsorbed it leaves chloride to have a higher concentration -> favours diffusion from lumen into renal interstitial fluid]
