Proximal Tubule and Loop of Henle

Question 1

how much of the fluid passing through the glomerulus is filtered through into the lumen of the tubules via bowmans capsule?

Answer 1

20%

Question 2

where does most reabsorption occur in the nephron?

Answer 2

proximal tubule
(however does occur to some extent along the whole nephron)

Question 3

composition of initial tubular fluid?

Answer 3

similar composition to plasma

but doesn’t contain any plasma proteins

Question 4

why is reabsorption needed?

Answer 4

entire circulation is filtered around 65 times per day so important substances must be retained

Question 5

how much of each substance is reabsorbed in the tubules?

Answer 5

99% of fluid
99% of salt
100% of glucose
100% of amino acids
50% of urea
0% of creatinine

Question 6

reabsorption is specific/non-specific and filtration is specific/non-specific?

Answer 6

reabsorption = specific (each substance needs specific transporters etc)
filtration = non-specific (anything small enough to fit through the pores will pass through)

Question 7

what is glomerular filtrate and how much of this is reabsorbed in the proximal tubule?

Answer 7

modified filtrate of blood containing ions and solute

around 80ml/min reabsorbed

Question 8

how does flow rate change between proximal tubule and loop of henle?

Answer 8

GFR at proximal tubule = 125ml/min
GFR at loop of henle = 45ml/min
due to lower volume of fluid as some is reabsorbed

Question 9

how is osmolality of glomerular filtrate change alone length of nephron?

Answer 9

stays the same

fluid reabsorbed in proximal tubule has same osmolality as filtrate

Question 10

what 5 substances are reabsorbed in PT?

Answer 10

sugars
amino acids
phosphate
sulphate
lactate

Question 11

what 7 substances are secreted by cells into the PT into the tubular fluid?

Answer 11

H+
hippurates
Neurotransmitters
bile pigments
uric acid
drugs
toxins

Question 12

what lines tubules?

Answer 12

single cell epithelium

Question 13

what must a substance pass through in movement from tubular fluid to blood via transcellular route?

Answer 13

across apical membrane of epithelial cells
across cytoplasm
across basolateral membrane of epithelial cell
through interstitial fluid
across capillary endothelial wall

Question 14

how does paracellular reabsorption occur?

Answer 14

passes between tight junctions of epithelial cells of tubule epithelim

Question 15

what determined how much paracellualr reabsorption can take place?

Answer 15

how tightly packed the epithelial cells are
some are very tight
some are leaky

Question 16

what is often needed for transcellular reabsorption?

Answer 16

membrane transport proteins to hep substance to cross apical/basolateral membrane of epithelial cells

Question 17

what is a primary active transporter and give an example?

Answer 17

e.g sodium potassium pump (3 Na out and 2K out for every ATP hydrolysed)
energy is directly required to operate the carrier and move the substrate against its concentration gradient
(i.e needs energy as Na concentration is higher outside the cell to takes energy to pump sodium out of cell against conc gradient)

Question 18

what is a secondary active transporter?

Answer 18

carrier molecule is transported coupled to the concentration gradient of an ion (usually Na)
can have symporters (moving substrate in same direction as Na)
antiporters = moving substrate in opposite direction to sodium

Question 19

what is a facilitated diffusion transport protein?

Answer 19

passive carrier-mediated transport of a substance down its concentration gradient, bypassing the lipid bilayer (e.g glucose)

Question 20

what drives salt reabsorption?

Answer 20

Na K+ pump at only expressed at basolateral membrane of epithelial cells
help to maintain sodium concentration gradient which is used to drive movement of Na ions through ion channels or 2ndary active transporters

Question 21

what is transport maximum?

Answer 21

rate at which you can reabsorb a substance

applies to any substance which depends on transport proteins

Question 22

rate of filtration is equal to what?

Answer 22

plasma conc X GFR (usually constant)

therefore increased plasma conc = increased filtration

Question 23

how are rates of filtration, reabsorption and excretion related?

Answer 23

filtration increases linearly with increase in plasma glucose
rate of reabsorption plateaus at the point where transport proteins become saturated and cant filter any more
at point where reabsorption plateaus, if filtration keeps increasing, rate of excretion increases as the substance cant be resorbed and just spills out into the urine

Question 24

transport proteins are used and can be saturated in both reabsorption and secretion, give an example of secretion?

Answer 24

PAH (para-aminohippurate) is completely secreted from the plasma into the tubuar fluid at glomerulus and then excreted into the urine
secretion into tubular fluid is dependant on transport proteins which can become saturated

Question 25

describe how Na+ resorption occurs in the proximal tubule?

Answer 25

transport mechanisms at apical membrane of epithelial cell bring Na into the cell from tubular fluid
Na/K pump at basolateral membrane pumps sodium out of the cell into interstitial fluid (transcellular)
Na+ +ve charge sets up electrical gradient between tubular and interstitial fluid which attracts -vely charged Cl- from tubular into interstitial fluid (paracellular) = resorption of salt

Question 26

water reabsorption from the proximal tubule is dependant on what 2 forces?

Answer 26

water follows salt
therefore passive water absorption down NaCl gradient occurs down osmotic gradient
oncotic drag of peritubular plasma
- blood in capillary has less fluid content as 20% was filtered out at the glomerulus, therefore there is a relative increased concentration of plasma proteins in the blood which increases the oncotic pressure gradient, pulling water out of the tubular fluid and out into the interstitial fluid, then the blood

Question 27

how is glucose reabsorbed in the proximal tubule from tubular fluid?

Answer 27

glucose brought into cell coupled to Na transport (co-transporter) creating glucose concentration gradient
facilitated diffusion uses this concentration gradient to pump glucose out of cell at basolateral membrane
this sets up an osmotic gradient between tubular and interstitial fluid which water follows via paracellular route
100% glucose is resorbed

Question 28

what is the function of the loop of henle?

Answer 28

generates a cortico-medullary solute concentration gradient between the interstitial fluid and tubular fluid (increases osmolarity of interstitial fluid)
this enables the formation of hypertonic urine

Question 29

describe flow of fluid in the loop of henle

Answer 29

opposing flow in the descending/ascending limbs - termed the countercurrent flow
the entire loop functions as a countercurrent multiplier

Question 30

which type of nephron creates a more hyper-osmotic interstitial fluid and why?

Answer 30

juxtamedullary

has a longer loop of henle and vasa recta

Question 31

what is absorbed in the ascending limb?

Answer 31

active salt (NaCl) reabsorption
no water reabsorption
little to no water follows salt reabsorption as thigh junctions are very tight between epithelial cells (impermeable to water reabsorption)

Question 32

what is reabsorbed in the descending limb?

Answer 32

highly permeable to water

no salt is reabsorbed

Question 33

how does transport occur in the ascending limb?

Answer 33

triple co-transporter expressed at apical membrane of epithelial cell which brings Na, Cl and K into cell (K+ pumped straight back out)
net movement of Na and Cl across cell
Na leaves basolateral membrane via Na/K pump, Cl leaves via Cl/K co-transporter
(osmotic gradient is created but tight junctions to tight for water to follow the gradient)

Question 34

how do loop diuretics affect transport in the ascending limb?

Answer 34

block the triple transporter

Question 35

what allows the movement of water from the tubular fluid of the descending limb into the interstitial fluid?

Answer 35

salt is removed from ascending limb and pumped into interstitial fluid, increasing its osmolarity
this creates an osmotic gradient between interstitial fluid and tubular fluid in descending limb
this allows water to leave via osmotis
fluid in the descending limb is therefore concentrated

Question 36

what is countercurrent multiplication?

Answer 36

process of using energy to create an energy gradient

Question 37

how does countercurrent multiplication occur?

Answer 37

tubular fluid becomes more concentrated as it goes down ascending limb due to loss of water into increasingly hyperosmotic interstitial fluid
then becomes less concentrated as it goes up the ascending limb due to loss of salts

Question 38

what is the osmolarity of the tubular fluid entering and leaving the loop of henle?

Answer 38

entering = 300 (iso-osmotic to interstitial fluid)
leaving = 100 (hypoosmotic to interstitial fluid)

Question 39

in the steady state, how does concentration change in different areas of the kidney?

Answer 39

increases in osmolarity down from cortex to medulla from 300-1200

Question 40

what 2 solutes generate the corticomedullary concentration gradient?

Answer 40

salt and urea

Question 41

why is countercurrent multiplication needed?

Answer 41

to concentrate the medullary interstitial fluid
to enable the kidney to function to produce urine of different volume and concentration according to the amounts of circulating ADH

Question 42

normal urine production?

Answer 42

usually around 1ml/min
depends on fluid intake
- can be between 0.3-25ml/min

Question 43

vasa recta acts as a countercurrent exchanger, what does this mean?

Answer 43

capillary blood equilibrates with interstitial fluid across leaky endothelium
blood osmolarity increases and decreases as it passes down and up into the cortex

Question 44

purpose of vasa recta?

Answer 44

ensures corticomedullary concentration gradient stays in place by ensuring solute isn’t washed away

Question 45

how does the vasa recta minimise the problem of NaCl and urea being washed away as blood passes through medulla?

Answer 45

vasa recta capillaries follow hairpin loops, are freely permeable to NaCl and water
blood flow to vasa recta is low (few juxtamedullary nephrons)