Tubular Function Flashcards
1
Q
What happens to filtrate?
A
- completely reabsorbed: glucose (162g in filtrate, none in urine), amino acids, bicarbonate ion
- regulated and thus is partially reabsorbed: water (180L in filtrate, 1-2L in urine), sodium (570g in filtrate, 4g in urine), potassium, chloride
- excreted as waste: urea, creatinine (1.6g in filtrate, 1.6g in urine), drugs and drug metabolites, uric acid (8.5g in filtrate, 0.8g in urine)
2
Q
What is a difference between the ascending and descending limbs of the loop of Henle?
A
- descending limb is water permeable
- ascending limb is not water permeable (can move ions out, but can’t move water out)
3
Q
What is the collecting duct’s permeability to water?
A
-variably permeable to water
4
Q
What are the arcuate arteries? What other vessels branch from them?
A
- arteries run along the border of the medulla and cortex
- put off straight vessels called interlobular arteries
- interlobular arteries have afferent arterioles to each nephron
5
Q
Describe capillaries beds in renal tubules
A
- branches into a capillary bed through the tubules to collect things back into blood stream
- spans across the proximal/distal convoluted tubule and loop of Henle
- then goes to venous drainage
- vasa recta runs parallel to loop of Henle
- blood collects into arcuate vein
6
Q
How can reabsorption occur?
A
Active: requires energy
Passive: chemicals following their electrochemical gradients
7
Q
How does water move?
A
- osmosis
- passive mechanism by which water follows its concentration gradient through a semipermeable membrane
- osmosis is driven by the movement of the solutes from the tubules to the interstitial fluid which often requires energy
- about 90% of water reabsorption is obligatory (water is dragged along by the solutes being moved from tubules)
- much of the obligatory reabsorption of water will occur in the proximal convoluted tubule and descending loop of Henle because these areas have tubules which are permeable to water
- the last 10% is facultative which means it can increase or decrease depending on the amount required by the body
- facultative reabsorption of water is under control by ADH which makes the cells in the collecting duct permeable to water and thus water can leave the ducts
8
Q
Describe osmosis in the proximal convoluted tubule
A
- movement of water will be by osmosis
- this osmosis is linked to passive reabsorption (diffusion) of a number of ions as well as the waste product urea
- sodium transport is active
- most active area for reabsorption is the proximal convoluted tubule
- by the end of the PCT, almost 100% of organic solutes have been reabsorbed
- about 65% of water has been reabsorbed
9
Q
How does passive transport occur?
A
- the movement of solutes often does not involve additional energy being added
- some solutes can slip between the tight junctions of the cells (paracellular) or into or out of the cells of the tubules (transcellular) strictly by following their electrochemical gradients
- diffusion may be facilitated by transport proteins as in the movement of glucose from inside of the tubular cells to the interstitial fluid
- leakage channels also exist for some ions to facilitate their walk down the concentration gradient
10
Q
Describe passive sodium reabsorption
A
- fluid in tubule lumen
- apical membrane (part of brush border) to increase surface area for absorption
- tubule cells lining tubule with tight junctions
- basolateral membrane
- to move Na passively, concentration must be higher in the lumen than interstitial fluid
- can go paracellularly into the interstitial fluid and capillary
11
Q
What are the types of active transport?
A
- primary active transport functions strictly with the use of ATP
- secondary active transport uses the energy of the movement of ions down their concentration gradients to transport other solutes like ions and larger uncharged molecules like glucose or amino acids (symporter if secondary active transport protein moves the ion and solute molecule in same direction, antiporter if they move in opposite direction)
- both active transport mechanisms require a carrier protein to assist the movement of the ions and other solutes
12
Q
What happens when transport mechanism of transport proteins is saturated?
A
- transporters are proteins that can only bind and then move solutes at a fixed rate so they have a maximum rate at which they can function
- the saturation of the transport mechanism limits reabsorption of substances like glucose
- conditions like glucosuria occur strictly because the Tm of the active transporter has been reached
- the sign of glucosuria indicates that the pathology diabetes is present and the Tm has been reached for the Na-glucose symporter
13
Q
Are active and passive transport mechanisms happening at the same time?
A
- yes
- active process consumes about 6% of total ATP at rest
14
Q
Describe active sodium transport
A
- sodium moves down its concentration gradient into the cell
- can’t leave Na in the cell to maintain the membrane potential (and it would also cause water to come in)
- sodium potassium pump takes ATP and moves sodium out into interstitial fluid and potassium in
15
Q
Describe glucose movement in the proximal convoluted tubule
A
- sodium glucose symporter at apical membrane
- 2 sodiums going down concentration gradient is enough to pull 1 glucose into tubule cell
- sodium potassium pump moves Na into interstitial fluid
- glucose facilitated diffusion transporter on basolateral membrane allows glucose to go down its concentration gradient to go into interstitial fluid
16
Q
Describe the sodium/hydrogen antiporter in the proximal convoluted tubule
A
- CO2 being made and combines with H2O to form carbonic acid which rapidly becomes H+ and bicarbonate
- sodium hydrogen antiporter on basolateral membrane uses one Na to move a H+ into the lumen
- get rid of H+ from inside the body so you don’t become acidic
- sodium potassium ATP pump moves Na out of cell
- bicarb is saved and moves through bicarb facilitated diffusion transporter down its concentration gradient and be taken back into blood stream
- in kidney, you can kick out an H+ while also saving the bicarb (whereas through breathing you lose the bicarb)
17
Q
Describe the symporter system at the thick limb of the LOH
A
- Na+ K+ 2CL- symporter
- pulls in Na, K, and 2Cl (Na is the one moving down its concentration gradient, other ones come along with it)
- sodium and potassium pump moves sodium out
- Cl leaves through leak channel (if there is too much chloride there are lots of leak channels into filtrate, too little chloride there are leak channels back into interstitial fluid)
- potassium leak channels that let potassium back into filtrate usually (can go the other way also)
- set up a charge gradient by moving negative ions out so that it pulls positive ions from the lumen into interstitial fluid paracellulary
21
Q
Patient comes into ER. Generalized weakness, nausea, and overall sense of illness. Vital signs are normal but there is pitting edema. K+ levels are 7.5mmol/L (normal is 3.5-5)
A
-kidney failure
24
Q
A
