L4 Tubular Transport Flashcards
Tubular reabsorption
From tubular lumen to PTC
Valuable molecules
Glucose, AAs
Tubular secretion
From PTC to tubular lumen
Waste products, metabolites, toxins, etc
Transport rate
Amount of material added to (secreted) or removed (reabsorbed) from glomerular filtrate
If pos, then some material was removed from filtrate by reabsorption
If neg, then some material was added to the filtrate by secretion
Tx= FL -ER
Tx=0, expect to observe in GFR marker
Routes of reabsorption
Paracellular (btw cells) Leaky epithelium ( in btw adjacent epithelial cells, through tight junctions)
Transcellular reabsorption must cross two barriers:
Luminal (apical) membrane
Basolateral membrane
Diffusion through interstitial fluid and capillary wall is fast
Transport mechanisms
Simple diffusion (usually via pores)
Facilitated diffusion (carrier mediated)
Primary active transport
Secondary active transport
Endocytosis
Proximal tubule
Always reabsorbs 2/3 Na and H2O even in dehydrated people
Na 67% active reabsorption
H2O 65% passive, solute-linked reabsorption
What drives fluid movement from the nephron lumen into peritubular capillaries?
Active transport of Na across basolateral membrane
Slight osmotic gradient pulls water from the nephron lumen into basolateral compartments
Anions (Cl,etc) follow Na
Oncotic peritubular capillary is primary force driving fluid movement from basolateral compartments into peritubular capillaries
Reabsorption of water and electrolytes is considered to isosmotic in the proximal tubule
Mechanism for reabsorbing Na, Cl, glucose, phosphate, citrate, lactate and amino acids in the proximal tubule
Na enters PT cells via cotransport w organica and by Na-H+ antiport (NHE3)
Cl enters PTC via paracellular routes
Na leaves the cell by Na-k-ATPase or via cotransport with HCO3
Hormonal and neural influences on Na reabsorption by the proximal tubule
AT II
Stimulates Na-H exchange across apical membrane (NHE family of transporters)
Increase Na reabsorption and H secretion
Sympathetic nerve activity
Stimulates Na reabsorption
Parathyroid hormone
Inhibits Na-phosphate cotransport
Increase urinary excretion of phosphate
Tubular fluid to plasma conc ratios (TF/P)
Proximal tubule
TF/P ratios for freely filtered solutes are 1 in bowmans space
Water and solute are reabsorbed along length of PT
Inulin ratio (not secreted or reabsorbed) shows that volume of tubule fluid is decreasing
Reabsorption can produce very small TF/P ratios
Note that osmolarity does not change
Tubular maxima
Many renal transport systems exhibit a maximal rate at which they can transport a particular solute
Due to saturation of membrane transport proteins
TM is the maximum rate a solute can be transported
Below TM all filtered load is reabsorbed
The portion of load above TM is excreted
TMs are found primarily in proximal tubule
TM glucose
Maximum rate at which glucose can be reabsorbed
Threshold: plasma conc where glucose first appears in urine. Depends on GFR
splay: due to heterogeneity of nephrons; not all nephrons have the same TM
Effect of changing GFR
Increase GFR, hit TM at lower plasma conc, decrease threshold
Decrease GFR, hit TM at higher plasma conc, increase threshold
Osmotic diuresis - proximal tubule
Excess unreabsorbed solute (ie mannitol) inhibits osmotic water flow from lumen to basolateral spaces
Causes Na back-diffusion into lumen of tubule w increased Na and water loss (diuresis)
Result in rapid loss of sodium and water (Polyuria)
Can accompany:
A high filtered load of urea
When the glucose exceeds TM in diabetic patients (excess glucose in filtrate/blood)
Mannitol
Non reabsorbed carbohydrate can be given IV to induce an osmotic diuresis
