Tubular Reabsorption & Secretion(A)- Exam 3 Flashcards
What does clearance tell us?
Tells us how many mls of plasma is completely cleared of a substance.
Ex. 10mls/min- tells us in every minute 10 mls of plasma is completely cleared of that given substance
Excretion Equation
Excretion = Filtration - Reabsorption + Secretion
Where does filtration occur?
Glomerulus
Where does reabsorption and secretion occur?
Proximal tubule
Loop of Henle
Distal Tubule
Collecting Tubule
Filtration Rate Equation
GFR x Plasma concentration
Glucose concentration
1 g/L
Describe the filtration of glucose
Freely filtered
Normal GFR
125 mls/min; 180 L/day
% Filtered Load Reabsorbed: Glucose
100%
% Filtered Load Reabsorbed: Bicarb
> 99.9%
% Filtered Load Reabsorbed: Sodium
99.4%
% Filtered Load Reabsorbed: Chloride
99.1%
% Filtered Load Reabsorbed: Potassium
87.8%
% Filtered Load Reabsorbed: Urea
50%
% Filtered Load Reabsorbed: Creatinine
0%
How do the kidneys have independent control over exertion rate?
By changing appropriate reabsorption rate
What are the 4 distinct movements for water and solutes?
- Tubular lumen to tubular cells
- Tubular lumen to tubular interstitial space
- Interior of cells into tubular interstitial space
- Interstitial space into peritubular capillary
What drives fluid movement from interstitial space into peritubular capillary?
Capillary filtration forces (Bulk flow)
What is bulk flow?
Net movement into the capillaries
What type of gradient does water follow?
Osmotic gradient
What is the major driving force moving substances from lumen of tubule to interstitial spaces?
Diffusion
What is the transcellular path?
From tubular lumen into tubular cells
What is the Paracellular path?
from tubular lumen into tubular interstitial spaces
How does permeability at junctions compare to permeability of substances directly into tubular cells?
Permeability at the junction is lower ( see size of the arrow in slide 6 diagram)
Rank the permeabilities of bulk flow, paracellular path, and transcellular path.
Bulk flow (most permeable) > Transcellular Path > Paracellular path (least permeable)
What types of mechanisms does tubular reabsorption involve?
Both active and passive mechanisms
What are the 5 types of tubular reabsorption movements?
- Primary active transport
- Secondary active transport: Co-transport
- Secondary active transport: Counter-transport
- Pinocytosis (requires energy)
- Passive
What ATPase pumps use primary active transport?
Na/K
H/K
Hydrogen (secretion only)
Calcium
What pumps use secondary active transport: co-transport?
Sodium-glucose
Sodium-amino acid
What pumps use secondary active transport: counter-transport?
Sodium-hydrogen
What uses pinocytosis? Describe.
proteins; once in cell broken down to component amino acids and amino acids reabsorbed
What does pinocytosis require?
Energy
What is pinocytosis?
the ingestion of liquid into a cell by the budding of small vesicles from the cell membrane
What uses passive methods for reabsoprtion?
Water (osmotic gradient)
Bulk flow into peritubular capillaries
What is the membrane potential within the tubular epithelial cells (created by Na/K pump)?
-70 mV
What is the membrane potential within the tubular lumen?
-3mV
Where is the brush border found?
Luminal membrane of the proximal tubule
What is sodium actively transported?
Sodium is pumped out of tubular cells into the interstitial spaces.
Where is potassium actively pumped?
Pumped into tubular cells (Na/K pump; countertransport)
Where are the Na/K ATPase pumps found?
Basolateral sides of the tubular epithelial cells
Where does sodium follow the concentration gradient?
- Tubular lumen into tubular cells
2. From interstitial space into capillary
What type of gradient does sodium follow when passively diffusing from lumen to cells?
Diffusion down concentration AND electrical gradient
Brush border of proximal tubule luminal membrane creates what surface area increase for diffusion?
20x increase
What also enhances sodium reabsorption?
Carrier proteins through luminal membrane
What type of carrier proteins enhance sodium reabsorption?
Co-transport and counter-transport proteins
What moves along with sodium from interstitial fluid into peritubular capillary?
Water
How fast does sodium move from interstitial fluid into peritubular capillary?
Very Quickly!
How is glucose reabsorbed? (What mechanism)
Co-transport mechanism
What is glucose reabsorption tied to?
Sodium gradient from tubular lumen to interior of tubular cells
How efficient glucose reabsorption?
So efficient that is usually removes all filtered glucose
What are the two luminal transporters for glucose reabsorption?
SGLT2 and SGLT1
What percent of glucose is reabsorbed by SGLT2?
90%
What percent of glucose is reabsorbed by SGLT1?
10%
Where is glucose reabsorbed by SGLT2?
Early part of the proximal tubule
Where is glucose reabsorbed by SGLT1?
Late part of proximal tubule
What are the 4 transporters of glucose reabsorption?
SGLT1 (luminal)
SGLT2 (luminal)
GLUT1 (basolateral)
GLUT2 (basolateral)
What are the two basolateral transporters by glucose reabsorption?
GLUT1
GLUT2
What type of transport do the two basolateral glucose transporters use?
Passive facilitated transport down glucose concentration gradient
Where is GLUT2 vs GLUT1 used a a glucose transporter?
GLUT1 in the later stages
GLUT2 in the early stages of the proximal tubules
Where does bulk flow move glucose?
from interstitial spaces into the peritubular capillaries
How is glucose reabsorbed?
- Co-transport (sodium)
- Two luminal transporters (SGLT1, SGLT2)
- Two basolateral transporters (GLUT1, GLUT2)
- Bulk flow
How are amino acids reabsorbed?
- Co-transport (sodium)
- Luminal co-transporter pumps
- Diffusion
- Bulk flow
Amino acid co-transport mechanism is tied to what?
Sodium gradient from tubular lumen to interior of tubular cells
How efficient is the co-transport mechanism of amino acid reabsorption?
So efficient that it usually removes all filtered amino acids
Where does the luminal co-transporter system pump amino acids?
Into the cells
Where do amino acids undergo diffusion?
Diffuse out of the cells into the interstitial spaces
Where do amino acids undergo bulk flow?
Bulk flow moves the amino acids form interstitial space into peritubular capillaries
How does hydrogen secretion occur?
Counter-transport (sodium gradient)
Sodium-hydrogen exchanger
What is the hydrogen counter-transport mechanism tied to?
Sodium gradient from tubular lumen to the interior of the cell
Where is the sodium-hydrogen exchanger located?
Brush border of the luminal membrane
What is a transport maximum?
Max amt of solute that can be reabsorbed (transport max transport)
When does the transport maximum occur?
Occurs when tubular load exceed transport capacity of carrier proteins
Tubular load
amount of solute delivered to tubule
Glucose Tmax
375 mg/min
Glucose filtered load
GFR x [GFR]= 125 mls/min x 1 ml/mg = 125 mg/min
What is the threshold concentration of glucose?
250 mg/dl; concentration where glucose first appears in the urine
Why is the threshold concentration less than Tmax?
each individual nephron is different; chart represents action of both kidneys so Tmax reached when ALL nephrons have reached their max
Amino Acid Transport Max
1.5 mM/min
Plasma protein Transport Max
30mg/min
Creatinine Transport Max
16 mg/min
Para-aminohipuuric acid Transport Max
80 mg/min
What are the two excretion rates?
- Before secretion Tmax is reached; the amount excreted is the sum of amt filtered and amt secreted (steepest slope of excretion curve)
- After secretion Tmax is reached rate of excretion parallels filtration rate (slow of excretion curve matches slop of filtration curve)
Which solute may not show maximum rate of transport?
Solute reabsorbed passively and some actively reabsorbed solute
What does the rate of gradient-time transport depend on?
Electrochemical gradient
Membrane permeability for solute
Time fluid containing solute remains in tubule
How does transport rate relate to tubular flow rate?
Inversely
What is the transport max for sodium in the proximal tubules?
Does not show a transport max in the proximal tubule, even though it is actively reabsorbed
How does the capacity of the Na-K ATPase compare to net sodium reabsorption in the proximal tubule?
Capacity > Rate of net sodium reabsorption
In the proximal tubule
Does sodium leak back into the tubular lumen in the proximal tubule?
Yes, a significant amt of transported sodium leaks back
What does sodium “leak back” in the proximal tubule depend on?
Permeability of tight junctions between cells
Forces controlling bulk flow of water and solute into peritubular capillaries
As plasma concentration of sodium increases, what happens to sodium concentration in the proximal tubule and sodium reabsorption in proximal tubule?
Sodium concentration in prox tubule increases
Sodium reabsorption increases
A decrease in tubular flow rate will do what to sodium reabsorption in proximal tubule?
increase sodium reabsorption
What is the sodium tubular max transport in the distal tubule?
Classic tubular max transport
How does the capacity of Na-K ATPase compare to net sodium reabsorption in the distal tubule?
Capacity does not exceed rate of net sodium reabsorption.
How much “leak back” of sodium is there into the tubular lumen in the distal tubule? Why?
Minimal leak back; tighter, less permeable junctions couple transport of much smaller amt of sodium
What does aldosterone do?
Increases Tmax level in distal tubule
What drives the passive reabsorption of water?
Osmotic differences created by the movement of solute (mainly sodium) from tubular lumen to the tubular interstitial space
What affects passive reabsorption of water?
Cellular permeability (cell membranes and tight junctions)
Increased permeability does what to reabsorption and water excretion?
Increased reabsorption
Decreased water excretion
What is the permeability of the proximal tubule?
Highly permeable
Why is the solute gradient in the proximal tubule minimal?
Rapid movement
Solvent drag
water carries significant amount of sodium, chloride, potassium, calcium, magnesium because of high permeability
What is the permeability in the loop of henle (ascending loop)?
Low permeability
Describe the movement of water in the loop of henle.
Little movement of water even though there is a large osmotic gradient
What is the permeability in the distal tubule/collecting tubules/collecting ducts?
Variable permeability
What does cellular permeability in the distal tubule/collecting tubule/ and collecting ducts dpeend on?
Presence of ADH
Permeability is ___________related to ADH.
directly
What does changing water permeabiliy in the distal tubule/collecting tubules/ collecting ducts affect?
Only affects amount of water reabsorbed, not the amount of solute due to low solute permeability
What pulls chloride ions into the cell?
Sodium diffusion into cells creates electrical gradient that pulls negative chloride ions into the cell
What concentrates chloride, creating a concentration gradient into the cell?
Movement of water into cells
How is chloride reabsorbed?
Diffusion (electrical gradient - Na; concentration gradient - water)
Co-transport mechanism with sodium across the luminal membrane
What membrane is the co-transport mechanism of Cl and sodium?
Luminal membrane
What concentrates urea?
Movement of water into the cell
What is more permeable: water or urea?
Water
What contains specific passive urea transports which facilitates reabsorption?
Inner medullary collecting duct
What percent of filtered urea is reabsorbed?
50%
