Tubular Reabsorption and Secretion Flashcards
summarize the transcellular path transport mechanism
- channels are holes and the rate of diffusion if proportional to the concentration of the substance
- transporters bind solutes and then move them by changing their conformation
- transporters have a lower rate of transport/facilitated diffusion because a maximum rate can be reached due to the need for a conformation change to function
list the main functions of the proximal tubule (3)
- resorbs or degrades most of the (little) protein present in the filtrate
- reabsorbs 2/3 of filtered water, electrolytes, and minerals plus 100% of glucose and amino acids
- secretes many pharmacological agents, organic substances, and toxic products of metabolism
explain the basic process for proximal tubular reabsorption of water, electrolytes, minerals, glucose, amino acids, and proteins (5 steps)
plus what happens with secretion
- active extrusion of Na+ from the tubular cell to the interstitium (on basolateral side)
- Na+ passively enters the cell (from the apical/luminal side) to replace the Na+ removed in step 1
- parallel movement of anions (Cl-, HCO3-) into the cell occurs to preserve electroneutrality
- osmotic flow of water from the tubular lumen to the interstitium occurs (through/around tight junctions between cells)
- bulk flow of water and solutes from the interstitium to the peritubular capillary
glucose and amino acids are co-transported passively into the cell with sodium
H+ ions (or other secretory substance) are exchanged with sodium when secretion occurs
list and describe the 2 categories (4 total transporters) of glucose transporters
sodium-glucose likes transporter (SGLT): co-transporter
SGLT2: 1 sodium for one glucose; early in the proximal tubule; is low affinity and high capacity (90%) glucose reabsorption)
SGLT1: 2 sodium for one glucose; later in the proximal tubule; high affinity, low capacity that absorbs the remaining 10%
glucose transporter: in basolateral membrane: facilitated diffusion
GLUT2: early segments, low affinity
GLUT1: later segments, high affinity
predict how disrupted proximal tubular transport of glucose, electrolytes, amino acids, or proteins impacts urine composition and the whole animal
proximal tubule is responsible for the bulk (2/3) of reabsorption of these molecules so if disrupted, could change urine composition and potentially lead to stones/sediment
describe the 2 types of limits on tubular transport (gradient-limited and tubular transport maximum-limited systems) and provide examples of substances for which these limits apply
gradient-limited: rate of absorption of the substance is limited by paracellular backleak; if tight junctions are very leaky to a given substance, like sodium, the steepness of the gradient increases; if the concentration of sodium in the interstitium is equal to or greater than that of the lumen, it will be difficult for sodium to move down its concentration gradient into the cell for other molecules to be cotransported along with it
tubular transport maximum-limited system: transporters have a maximum saturation limit because they have to undergo a conformational change in order to function; once this limit is reached, rate of transport slows and not everything can be absorbed; an example of this is glucose, which cannot be filtered past the renal threshold
identify causes and consequences of glucosuria and aminoaciduria
glucosuria: can be either hyperglycemia due to high blood glucose, meaning that transporters are saturated and no more glucose can be absorbed, or normoglycemic/renal glucosuria, meaning that blood glucose is normal but there is proximal tubular disease or pharmacologic inhibition resulting in an issue with glucose transporters and not all glucose is absorbed
aminoaciduria: example is cysteine; if the transporter for cysteine is unable to function, cysteine ends up in the urine and if the urine is acidic, crystals can form and aggregate to urinary stones
both glucosuria and aminoaciduria can result in osmotic diuresis, which is increased urine production due to an abnormally high amount of any solute left in the tubular lumen (holds water in lumen, causes Na+ to reach its gradient maximum, reducing Na+ reabsorption, loss of Na+, and even more water in the urine)
what are the 2 fates of filtered proteins?
- low molecular weight pathway: proteins smaller than 30kDa are degraded and the degradation products are returned to the blood supply
- albumin/high molecular weight pathway: for proteins between 30-150kDa, a small proportion of the protein is degraded, but the mainly intact protein is returned to the blood supply
when can proteinuria occur? (2)
- failure of the glomerular filtration barrier in retaining proteins (severe)
- failure of proximal tubule to resorb proteins
list the main functions of the loop of Henle; then list functions of specifically thin descending limb, thin ascending limb, and thick ascending limb
crucial for salt and water reabsorption; reabsorbing 25% of filtered Na+ and Cl- and 15% of filtered water; the separation between solute and H2O reabsorption promotes a hyperosmolar medullary interstitium
thin descending limb: passive reabsorption of water drive by hyperosmolarity of interstitium
thin ascending limb: passive reabsorption of NaCl driven by concentration gradient (high [Na+] and [Cl-] in tubular fluid thanks to water removed in descending limb)
thick ascending limb: active reabsorption of NaCl drive by sodium gradient established by Na+/K+ ATPase and crucially requiring Na+/K+/2Cl cotransporter (creates hyperosmolar environment for water reabsorption in thin descending limb)
list the main function of the distal tubule (early distal convoluted tubule and late distal tubule)
early distal convoluted tubule: similar to thick ascending limb of loop of Henle, reabsorbs Na+ and Cl-, but very little water, considered a diluting segment
late distal tubule: Na+, K+, H+ homeostasis similar to collecting ducts
list the main function of the collecting ducts
Na+, K+, and H+ homeostasis and urine concentration based on diuretic hormone (reabsorb water based on the body’s needs)
detail the main process for tubular transport of electrolytes, minerals, and water by the loop of Henle (6 steps)
- Na+/K+ ATPase moves Na+ into the interstitial space, creating an electrochemical gradient for Na+
- Na+/K+/2Cl cotransporter moves Na+ down is electrochemical gradient from the lumen into tubular cells; driving the secondary active transport of Cl- and K+ into the cell (an electroneutral transport)
- Cl- channel allows Cl- to follow the sodium that left via Na+/K+ ATPase passively out of the cell due to electrical and chemical gradients
- K+ channel allows most of the reabsorbed K+ to leak back into the tubular fluid through the channel in the apical membrane to keep this process going
- K+ that leaks back creates a slight positive charge in the tubular lumen that drives
- paracellular reabsorption of cations (Mg2+, Ca2+, Na+, and K+)
detail the main process for tubular transport of electrolytes, minerals, and water by the distal tubule (salt reabsorption in the early distal convoluted tubule, 4 steps)
salt reabsorption in early distal convoluted tubule:
1. Na+/K+ ATPase moves Na+ into the interstitial space, creating an electrochemical gradient for Na+
2. Na+/Cl- cotransporter allows Na+ and Cl- back into the cell
3. Na+ also passively flows into the cell from lumen, and is exchanged with Ca2+ via the Na+/Ca2+ exchanger, creating an electrochemical gradient for Ca2+
4. Ca2+ passively flows down its electrochemical gradient into the cell from the lumen
detail the main process for tubular transport of electrolytes, minerals, and water by the collecting ducts
- Na+/K+ ATPase pump establishes electrovhemical gradient, forcing Na+ out of cell to interstitium
- ENaC (epithelial sodium channel): allows Na+ to passively flow back into the cell from the lumen
- to replace the loss of positive charge (Na+) in the lumen, ROMK (renal outer medullary K+ channel) allows K+ to flow out of the cell into the interstitium
