Lesson 30: Topic 26 - Transepithelial Transport Flashcards
true or false: transepithelial transport is selective
true
about how much of filtrate is entering glomerulus and tubules?
125mL
what is the selective reabsorption of molecules and water in the tubule?
substances within the tubule (filtrate) are transferred to the peritubular capillaries = trans-epithelial transport
why is the process of transepithelial transport high selective?
because there are certain specific channels that help to move some contents through the tubule and into the peritubular capillary
is transepithelial transports selectivity constant throughout the nephron or variable?
variable throughout the length of the nephron and under physiological control
what is renal plasma flow?
volume of plasma that passes through the kidneys per unit of time
- this encompasses the glomerular filtration rate(625mL/min renal plasma flow) (how much enters the kidneys, not how much is filtered)
what is renal clearance?
- volume of plasma that is completely removed of a substance per unit of time
- selective reabsorption in the tubule
- we are talking about, how much of this volume of plasma is COMPLETELY removed of a substance per unit time
in what case will we see no reabsorption?
where we have soluble molecules like inulin and creatinine, it just gets filtered into the filtrate and will not get reabsorbed. therefore it just goes straight into urine
why is inulin important clinically?
we can determine what is going on with someone’s glomerular filtration rate because inulin determines how much plasma is entering through the glomeruli and into the urine
when would complete reabsorption of the tubules happen?
with glucose
- we like to keep glucose to use as an energy source - one of the major fuel sources within the body
what is renal clearance of no reabsorption?
125mL/min
what is renal clearance of complete reabsorption?
0mL/min
what are some substances that are partially reabsorbed in tubules?
sodium and urea
- if the body wants to retain sodium, it is going to be partially reabsorbed in different components of the tubule system
why would we want to retain urea?
while it is a waste product, urea also contains nitrogen
what does reabsorbing urea help with?
helps to concentrate our urine by concentrating urea within our renal medulla
what is the renal clearance of partial reabsorption?
anywhere between 1-124mL/min (in between complete and no reabsorption)
what molecules are going to not be able to reabsorb but will be able to secrete?
hydrogen ions, potassium ions and penicillin
- these are things that we will want to excrete but are not reabsorbed
an accumulation of hydrogen ions and potassium ions are due to overexerted exercise, what can it lead to?
fatigue
because penicillin is so efficiently cleared out of the body via urine, what does this mean clincially?
we need higher doses
what is the renal clearance of no reabsorption but secretion in tubules?
- 126-625mL/min
- so high because not only are we not reabsorbing into the venous system, we are also secreting the substance into the tubules to really concentrate the urine
what does the tubular reabsorption process start with?
sodium ions (positive charge) being absorbed by active transport from the tubule into the peritubular capillary
by allowing sodium ions to be absorbed via active transport from the tubule into the peritubular capillary, what does this create?
an electrochemical gradient to allow for anion reabsorption
after the electrochemical gradient is created and ions accumulate in the peritubular capillary, what happens?
accumulating ions in the interstitial fluid (peritubular capillary) creates an osmotic gradient
by creating an osmotic gradient in the peritubular capillaries, what is going to happen?
it is going to want to draw in water, so water moved by osmosis, following solute reabsorption
in summary, what is the basis concepts of the tubular reabsorption process?
- Na+ absorbed by active transport
- This creates an electrochemical gradient that allows anion reabsorption
- accumulating ions in interstitial fluid (peritubular capillaries) creates an osmotic gradient
- water moves by osmosis, following solute reabsorption
as water is reabsorbed, remaining _______ in tubule become concentrated and _____ are reabsorbed by diffusion
solutes, SOME
what is the osmotic gradient used for in the tubules?
to move water across
what is between two adjacent tubular epithelial cells?
a tight junction
there are two sides of the membrane on the tubular epithelial cells. what are they called?
- luminal membrane (facing towards the tubular lumen)
- basolateral membrane (facing towards the lateral space which is a specific space between the tubular epithelial cells that is part of the interstitial fluid)
what are the five distance barriers that a substance must traverse in order to be reabsorbed (moving from the filtrate to the plasma)?
- the luminal cell membrane
- the cytosol (of the tubular epithelial cell)
- the basolateral cell membrane
- the interstitial fluid
- the capillary wall
how does sodium enter the tubular cell?
through a sodium channel
- it is a dedicated channel that helps to move sodium into the tubular cell and that occurs passively(no energy required) by concentration and electrochemical gradients
how does sodium passively enter the tubular cell via the centration gradient?
we have a high amount of sodium in our tubular lumen, but there is a low concentration of sodium in the tubular cell
how is the concentration gradient of sodium in the tubule created?
with the basolateral sodium potassium ATPase carrier (sits in the basolateral membrane)
how does the sodium potassium pump ATPase carrier work?
it is going to take energy and undergoes hydrolysis to produce ADP and then will pump 3Na+ ions out of the cell for every 2 K+ ions pumped in the cell
how does sodium passively enter the tubular cell via the electrochemical gradient?
via sodium potassium pump
- responsible for repolarization
- negative electrochemical gradient because it pumps more positive ions out than it pumps in
what is the net change in ions in the tubular cell?
net positive OUT
what is the concentration gradient in the lumen?
usually -3mV
what is the concentration gradient inside the tubular cell?
usually - 70mV
the negative voltage inside the tubular cell does what?
attracts positive ions
in summary, what are the three steps of sodium reabsorption?
- Na+ passively enters through Na+ channels via concentration and electrochemical gradient
- Na+ is actively pumped to basolateral side of cell by Na+/K+ ATPase
- Na+ diffuses into peri-tubular capillary
what is the purpose of the Na+//K+ ATPase in the tubular cell?
- actively pump Na+ to aid in Na+ efflux from tubular cell
- maintain electrochemical gradient for passive Na+ influx into tubular cell
- maintain concentration gradient for passive Na+ influx into tubular cell
what % of filtered Na+ is reabsorbed?
99.5%
80% of the kidneys total ATP use is devoted to?
transporting Na+
why is it important to reabsorb Na+?
because the movement of Na+ drives the reabsorption of many other substances as well
each area of the tubule is responsible for?
reabsorbing sodium in different amounts
which part of the tubule reabsorbs the largest amount of sodium?
proximal tubule, reabsorbing 67% of sodium
why is so much of sodium reabsorption important at the proximal tubule?
at the proximal tubule, it helps to reabsorb glucose, amino acids, water, chloride ions, and urea
what tubule area is 25% of sodium being reabsorbed?
in the ascending limb of the loop of Henle
what is the role of Na+ reabsorption at the ascending limb of the loop of Henle?
plays critical role in kidney’s ability to produce urine of varying concentrations
in the distal and collecting tubules, what % of sodium reabsorption happens here?
8%
the sodium being absorbed at the distal and collecting tubules, what is its role?
- hormonal control and regulating ECF volume
summary of how water is reabsorbed (how water enters through and across into the plasma of the peritubular capillary/
- H20 moves through selective water channel (aquaporins) in the luminal membrane
- H20 moves out of tubular cell into the interstitial fluid via osmosis due to [Na+] gradient in lateral space
- H20 moves in peritubular capillary due to hydrostatic pressure + osmosis
can water enter the tubular cell through tight junctions?
yes
the interstitial fluid is confined, tight space. we bring water in there via osmosis so what happens to the interstitial space then?
we create a pressure gradient which will then move water into the peritubular capillary
is water reabsorption active or passive?
all passive, no energy is required
