renal 3 Flashcards
what is reabsorption?
-of the 180L/day that is filtered, less than 1% (1.5L) is actually secreted
-regulation reabsorption allows kidneys to selectively return ions and water to the plasma to maintain homeostasis
why bother filtering 180L if only 1% is excreted?
many foreign substances are filtered into the tubules but not reabsorbed, this is a rapid way to remove unwanted materials
-frequent filtration of ions and water into the tubules simplifies regulation and allows it to occur rapidly
what are the two types of transport used in reabsorption?
-transepithelial (transcellular) transport: substances cross the apical and basolateral membranes of the tubule cells to enter into the ECF
-paracellular transport: substances pass through the cell to cell junctions between two adjacent tubule cells (diffusion)
reabsorption may be ____________or ___________
active or passive
what is the active transport of Na+?
-basolateral Na transport is always active transport via Na-K-ATPase
-secondary active transport: symport with Na
what is the basolateral Na transport?
what is the secondary active transport symport of Na?
what are other reabsorption mechanisms?
-passive reabsorption
-endocytosis
what is passive reabsorption example?
urea
-nitrogenous waste product
what is endocytosis in reabsorption?
very small plasma proteins or peptides
-receptor mediated endocytosis
-receptor binding plasma proteins known as megalin
-once in the cell, the proteins are digested by lysosomes
the majority of substances transported in the nephron use ___________
membrane proteins
-only a set number
how does renal transport reach saturation?
-saturation: the maximum rate of transport that occurs when all available carriers are occupied
-the amount of glucose filtered depends on the plasma (glucose)
-in a healthy individual, all glucose is reabsorbed
what is the transport maximum?
the transport rate at saturation
-excess glucose filtered=more glucose than transporters and some glucose is excreted (diabetes)
what is glucose appearing in your urine known as?
glucosuria or glycosuria
-occurs with elevated blood glucose
-rare genetic disorder with reduced transporters
what is renal threshold?
-is the plasma concentration of a solute when it first begins to appear in the urine
-occurs at transport maximum
what is reabsorption in the peritubular capillaries?
-water and solutes are initially reabsorbed from tubule lumen to interstitial space and must reenter circulation
-hydrostatic pressure at glomerulus is 55mm Hg but after fluid is removed from the blood and resistance is encountered the hydrostatic pressure drops to 10mm Hg in the peritubular capillaries
-plasma proteins are not filtered, the oncotic pressure remains at 30 mmHg as blood travels into the peritubular capillaries
-the net pressure is now 20 mmHg driving fluid and solutes from the interstitial space back into the capillaries
what is secretion?
-the transfer of molecules from the extracellular fluid into the lumen of the nephron
-like reabsorption, it depends mainly on membrane transport proteins
what does secretion require?
-an active process requiring the movement of substances against their concentration gradients
-homeostatic regulation of K and H (distal) and organic compound removal (medications, food additives in proximal region) by secretion is important
Secretion enhances the __________ of a substance
EXCRETION
-if a substance is filtered and not reabsorbed, plus it is secreted then excretion of the substance will be very high
amount excreted= amount filtered- amount reabsorbed+ amount secreted
what is an example of the secretion of organic solutes?
-use a multi-step (tertiary) active transport process
-organic anion transporters: broad range of products transported
-aKG: a-ketoglutarate is a by-product of the citric acid cycle
-OAT transporter uses energy from transporting dicarboxylates down its concentration gradient to move OA-(organic anions) against its concentration gradient
-apical transporter responsible for anion diffusion recently identified
what are the competitors and penicillin excretion?
-prior to the 1930s, the leading cause of death was infection
-because penicillin was initially isolated from growth media/cultures it was difficult to keep up with the demand for this medication
-even worse, it was found that nearly all penicillin ingested would be excreted within 3-4 hours
-penicillin would then be given probenecid (competitor), which is preferentially secreted by the OAT transporter
what is excretion?
by the time fluid reaches the end of the nephron, it bears little resemblance to the filtrate
-glucose, amino acids and useful metabolites are reabsorbed
-although excretion tells us what the body is eliminating, it cannot on its own tell use details of renal function
excretion=filtration-reabsorption+secretion
renal handling of a substance and GFR are often of clinical interest:
-GFR as an indicator of overall kidney function
-renal handling/clearance information on new drugs is necessary
how do we directly look at filtration, reabsorption and secretion at the level of an individual nephron?
-experimentally (animals)
-indirect methods using only analysis of urine and blood
what is clearance?
-a noninvasive way to measure GFR
-the rate at which a solute disappears from the body by excretion or metabolism
-for a solute that is cleared only by the renal excretion, clearance is expressed as the volume of plasma passing through the kidneys that has been totally cleared of that solute in a given period of time. Looks at blood cleared of solute rather than amount secreted
what is the example calculation of clearance?
how to we most accurately calculate GFR?
we need to look at the clearance of a substance that is freely filtered and excreted (not reabsorbed or secreted)
-inulin, a polysaccharide found in a variety of plants, was found in isolated nephrons to be completely filtered and not reabsorbed
what do most clinicians use to measure GFR?
clinicians commonly use creatinine clearance as a measure of GFR and indicator of renal function
-product of phosphocreatine breakdown
-produced and broken down at relatively stable levels in the body
what is creatinine?
-freely filtered by the glomerulus, but it is secreted in very small amounts as well
-creatinine clearance slightly overestimates GFR
what is the example of creatinine clearance (example of test question)
what does clearance help determine?
NET RENAL HANDLING
what is net handling?
-once we know an individuals GFR, we can look at their kidneys net handling of any solute by measuring the solutes plasma concentration and excretion rate
-by comparing the filtered load (assuming the substance is filtered freely) with its excretion rate, we can tell how the nephron handled that substance. OR comparing GFR to clearance
-if less of a substance appears in the urine that was filtered, net reabsorption occurred (excreted=filtered-reabsorbed)
-if more appears in urine than was filtered: net secretion
