renal 3

Question 1

what is reabsorption?

Answer 1

-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

Question 2

why bother filtering 180L if only 1% is excreted?

Answer 2

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

Question 3

what are the two types of transport used in reabsorption?

Answer 3

-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)

Question 4

reabsorption may be ____________or ___________

Answer 4

active or passive

Question 5

what is the active transport of Na+?

Answer 5

-basolateral Na transport is always active transport via Na-K-ATPase
-secondary active transport: symport with Na

Question 6

what is the basolateral Na transport?

Answer 6

Question 7

what is the secondary active transport symport of Na?

Answer 7

Question 8

what are other reabsorption mechanisms?

Answer 8

-passive reabsorption
-endocytosis

Question 9

what is passive reabsorption example?

Answer 9

urea
-nitrogenous waste product

Question 10

what is endocytosis in reabsorption?

Answer 10

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

Question 11

the majority of substances transported in the nephron use ___________

Answer 11

membrane proteins
-only a set number

Question 12

how does renal transport reach saturation?

Answer 12

-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

Question 13

what is the transport maximum?

Answer 13

the transport rate at saturation
-excess glucose filtered=more glucose than transporters and some glucose is excreted (diabetes)

Question 14

what is glucose appearing in your urine known as?

Answer 14

glucosuria or glycosuria
-occurs with elevated blood glucose
-rare genetic disorder with reduced transporters

Question 15

what is renal threshold?

Answer 15

-is the plasma concentration of a solute when it first begins to appear in the urine
-occurs at transport maximum

Question 16

what is reabsorption in the peritubular capillaries?

Answer 16

-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

Question 17

what is secretion?

Answer 17

-the transfer of molecules from the extracellular fluid into the lumen of the nephron
-like reabsorption, it depends mainly on membrane transport proteins

Question 18

what does secretion require?

Answer 18

-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

Question 19

Secretion enhances the __________ of a substance

Answer 19

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

Question 20

what is an example of the secretion of organic solutes?

Answer 20

-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

Question 21

what are the competitors and penicillin excretion?

Answer 21

-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

Question 22

what is excretion?

Answer 22

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

Question 23

renal handling of a substance and GFR are often of clinical interest:

Answer 23

-GFR as an indicator of overall kidney function
-renal handling/clearance information on new drugs is necessary

Question 24

how do we directly look at filtration, reabsorption and secretion at the level of an individual nephron?

Answer 24

-experimentally (animals)
-indirect methods using only analysis of urine and blood

Question 25

what is clearance?

Answer 25

-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

Question 26

what is the example calculation of clearance?

Answer 26

Question 27

how to we most accurately calculate GFR?

Answer 27

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

Question 28

what do most clinicians use to measure GFR?

Answer 28

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

Question 29

what is creatinine?

Answer 29

-freely filtered by the glomerulus, but it is secreted in very small amounts as well -creatinine clearance slightly overestimates GFR

Question 30

what is the example of creatinine clearance (example of test question)

Answer 30

Question 31

what does clearance help determine?

Answer 31

NET RENAL HANDLING

Question 32

what is net handling?

Answer 32

-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