Active Reabsorption and transport maximum Flashcards
Reabsorption segments
-proximal tubule
-loop of henle (descending and ascending limbs)
-distal tubule
-connecting tubule
-collecting tubule
-collecting duct
How much glucose needs to be absorbed daily?
-180g of glucose/day that needs to be absorbed
% absorption of water, Na, glucose/amino acids
99%-100%
% absorption of urea
50%
% absorption of waste
0%
Reabsorption pathways
1.through tubular epithelium into the interstitial fluid (can be both active and passive)
>Transcellular path
> Paracellular path
2. through peritubular capillary membrane into the blood (bulk flow derived by hydrostatic and colloid osmotic forces)
Reabsorption of water
-passive transport
Active transport in the kidneys
1.Na/K ATPase
2.Hydrogen ATPase
3.Hydrogen-potassium ATPase
4. Calcium ATPase
Na/K pump
-pumps at basolateral membrane
-transport 3 Na out, 2 K in
-create 2 main forces for Na diffusion from the lumen into epithelial cells
>concentration gradient
>negative charge
Where does Na reabsorption occur?
-happens in most parts of the tubules
Na reabsorption at luminal (apical) membrane
-Na diffuses into the cells because of electrochemical gradient (created when Na pumped out, and K increased resulting in greater gradient.
Na reabsorption at basolateral membrane
-active transport against electrochemical gradient
Movement of Na to the peritubular capillaries
-occurs via passive ultrafiltration (hydrostatic and colloid osmotic pressure)
>Na also brings water with it
Na facilitated diffusion
-also occur at luminal membrane and it is important for secondary active transport of glucose and amino acids
>Glucose and amino acids are transferred to interstitial fluid because of concentration gradient
»Glucose secondary active transport is very efficient in proximal tubules (SGLT2 90%, SGLT1 10%). At basolateral membrane, facilitated diffusion through GLUT 2 and GLUT 1 occurs to get glucose into interstitial fluid.
Secondary active secretion of H+ & sodium reabsorption
-Active secretion of H+ into proximal tubules via a counter transport mechanism through Na/H exchanger (NHE)
Pinocytosis
-active reabsorption of proteins (occurs in proximal tubules)
-proteins are trapped in invaginations of cell membrane and a vesicle forms inside the cytoplasm. Then protein is digested into amino acids (lysosomes) and are reabsorbed
Fanconi’s syndrome
-occurs when the reabsorption process in renal tubules (especially proximal tubules) is compromised due to damage
>electrolytes and nutrients (Na, K, glucose, AA, HCO3-) are excreted and lost in urine
Fanconi’s syndrome causes
-genetic reasons (primary)
-toxins, drug reaction (secondary)
Fanconi’s syndrome symptoms
-excessive thirst
-excessive urination
-weight loss
-weakness
- lack of appetite
How to manage Fanconi’s syndrome?
-supply nutrients and electrolytes supporting acid-base balance
**response varies depending on degree of damage
Transport maximum
-indicates the limit of reabsorption or secretion of a substance
-related to the saturation of the transport proteins and enzymatic activities
Transport max for glucose
375mg/min
-Means that if more than 375 mg of glucose is filtered every minute, then all the excess glucose appears in urine
Transport max threshold level
-threshold level excretion is below transport max
**glucose threshold =250mg/min
-means that some traces of glucose appears in the urine. Due to variation in transport maximum of nephrons. Therefore transport maximum is assuming that all nephrons have reached their maximum reabsorption capacity
