Renal Physiology (Part 2) Flashcards
Which ion is the major cation of the ECF?
Na+
The reabsorption of Na+ into the ECF after filtration is critically important in maintaining what?
- normal ECF volume
- normal blood volume
- normal blood pressure
The kidneys ensure that Na+ _____ exactly equals Na+ ____, a matching process called what?
intake
excretion
Na+ balance
If Na+ excretion is less than Na+ intake, then the person is in _____ Na+ balance.
positive
What happens if the person is in positive Na+ balance?
Extra Na+ is retained, therefore there is ECF volume expansion which causes blood volume and arterial pressure to increase. This may result in edema.
If Na+ excretion is greater than Na+ intake, then the person is in _____ Na+ balance.
negative
What happens if the person is in negative Na+ balance?
Excess Na+ is lost from the body, therefore is ECF volume contraction which causes blood volume and arterial pressure to decrease.
How can a person have increased Na+ content but normal Na+ concentration?
If water content is increased proportionally
In nearly all cases, changes in Na+ concentration are caused by what?
Changes in body water content
*not Na+ content
Excretion of Na+ is __% of the filtered load.
Net reabsorption is __% of the filtered load.
1
99
Where does the bulk of Na+ reabsorption occur?
In the proximal convoluted tubule
__% of the filtered load is reabsorbed in the proximal convoluted tubule.
67
What is water reabsorption linked to in the proximal convoluted tubule?
Na+ reabsorption
__% of the filtered load is reabsorbed in the thick ascending loop of Henle.
25
Is the thick ascending loop of Henle permeable to water?
no
__% of the filtered load is reabsorbed in the terminal portions of the nephron (the distal tubule and the collecting ducts).
8
__% of the filtered load is reabsorbed in the early distal convoluted tubule.
5
__% of the filtered load is reabsorbed in the late distal convoluted tubule and collecting ducts. These structures are ultimately responsible for what?
3
the fine-tuning of Na+ reabsorption which ensures Na+ balance
In which part of the nephron are sites of action of the Na+ regulating hormone aldosterone?
the late distal convoluted tubule and collecting ducts
As already stated, the entire proximal convoluted tubule reabsorbs 67% of the filtered Na+, it also reabsorbs 67% of the filtered water. This tight coupling between Na+ and water reabsorption is called what?
isosmotic reabsorption
The proximal tubule is also the site of what?
glomerulotubular balance
What is the first half of the proximal convoluted tubule called?
the early proximal convoluted tubule
The early proximal convoluted tubule performs the ______ priority reabsorptive work
highest
In the early proximal convoluted tubule, what are the most essential solutes that are reabsorbed?
- glucose
- amino acids
- bicarbonate
What are the 2 mechanisms by which solutes are reabsorbed into the proximal convoluted tubule?
- cotransport
- countertransport
What are the 5 cotransport mechanisms in the proximal convoluted tubule?
1) Na+ - glucose (SGLT)
2) Na+ - amino acid
3) Na+ - phosphate
4) Na+ - lactate
5) Na+ - citrate
Once Na+, glucose, amino acid, phosphate, lactate, and citrate are in the cell how are they extruded from the cell into the blood?
Na+ by way of the Na+ - K+ ATPase
Glucose and the other solutes are extruded by facilitated diffusion
What is the only contertransport mechanism in the proximal convoluted tubule?
Na+ - H+ exchange
Via the Na+ - H+ exchange system __ is transported into the lumen in exchange for __.
H+
Na+
In the early proximal convoluted tubule what anion is reabsorbed with Na+?
bicarbonate
Describe how the H+ that is transported into the lumen is converted to HCO3-
H+ combines with filtered HCO3-, converting it to CO2 and water, which then move from the lumen into the cell. Inside the cell, CO2 and water are reconverted to H+ and HCO3-. The H+ is transported again by the Na+ - H+ exchanged and HCO3- is reabsorbed into the blood by facilitated diffusion.
What is the net result of the countertransport cycle in the early proximal convolutes tubule?
reabsorption of filtered HCO3-
There is a lumen-_____ potential difference across the cells of the early convoluted tubule, what is this created by?
negative
Na+ - glucose and Na+ - amino acid cotransport
What are the 4 modifications made to the glomerular filtrate by the time it reaches the midpoint of the proximal tubule?
1) 100% of the filtered glucose and amino acids have been reabsorbed
2) 85% of the filtered HCO3- has been reabsorbed
3) most of the filtered phosphate, lactate, and citrate have been reabsorbed
4) Na+ is extensively reabsorbed (because it is coupled to each of the above transport mechanisms)
Fluid entering the late proximal convoluted tubule has no ____ or _____, little _____, and high ____ concentration.
glucose or amino acids
bicarbonate
Cl-
Why is Cl- concentration high in the late proximal convoluted tubule?
because HCO3- has been preferentially reabsorbed in the early proximal convoluted tubule, leaving Cl- behind in the tubular fluid
What does the late proximal convoluted tubule primarily reabsorb?
NaCl
What is the driving force for NaCl reabsorption?
high tubular Cl- concentration
What are the 2 components of NaCl reabsorption?
- cellular
- paracellular
The cellular component of NaCl reabsorption contains 2 exchange mechanisms, what are they?
1) Na+ H+ exchanger
2) Cl- formate anion exchanger
The combined function of the Na+ H+ and Cl- formate anion exchangers is to do what?
transport NaCl from the lumen into the cell
After NaCl is transported from the lumen into the cell what happens?
Na+ is extruded into blood by the Na+ - K+ ATPase
Cl- moves into blood by diffusion
Are the junctions between the cells of the proximal tubule tight?
No, they are permeable to small solutes (such as NaCl) and to water
The diffusion of Cl- between cells, from the lumen to blood extablishes a Cl- _____ potential. This makes the lumen ____ with respect to blood.
diffusion
positive
Both the cellular and paracellular route what is the net result?
reabsorption of NaCl
What type of reabsorption is a hallmark of proximal tubular function? Why is this reabsorption consider equilibrium?
Isosmotic Reabsorption
Solute and water reabsorption are coupled and are proportional to each other
In the proximal convoluted tubule does solute follow water reabsorption or does water follow solute reabsorption?
Solute reabsorption is the primary event, and water follows passively
Describe the steps involved in Isosmotic Reabsorption
1) Na+ enters the cell across the luminal membrane by way of one of the previously mentioned mechanisms
2) water follows Na+ to maintain isosmolarity
3) Na+ is pumped out of the cell by the Na+ - K+ ATPase
4) water follows Na+ to maintain isosmolarity
5) water and Na+ fill the lateral intracellular space
6) high oncotic pressure of the peritubular blood drives reabsorption of the isosmotic fluid into the peribular capillary
What is the major regulatory mechanism of the proximal tubule?
Glomerulotubular balance
What does glomerulotubular balance describe?
the balance between filtration in the glomerular and reabsorption in the proximal tubule
What does glomerulotubular balance ensure?
that a constant fraction of the filtered load is reabsorbed by the proximal tubule, even if the filtered load increases or decreases
This constant fraction is normally maintained at __% of the filtered load
67
How does the glomerulus “communicate” with the proximal tubule to maintain constant fractional reabsorption?
Via changes in the oncotic pressure of the peritubular capillary blood
Increases in GFR produce _____ reabsorption in the proximal tubule.
increase
Increased reabsorption in the proximal tubule leads to _____ oncotic pressure in the peritubular capillary and _____ reabsorption in the proximal tubule
increased
increased
Decreases in GFR produces ______ in the filtration fraction.
decreases
Decreases in the filtration fraction lead to ______ oncotic pressure in the peritubular capillary and _____ reabsorption in the proximal tubule
decreased
decreased
How can glomerulotubular balance be altered?
by changes in ECF volume
ECF volume expansion produces a(n) _____ in fractional reabsorption in the proximal tubule
decrease
When ECF volume increases the capillary hydrostatic pressure (Pc) _____. How does this happen?
increases, because the plasma protein concentration is decreased by dilution
Increases in capillary hydrostatic pressure results in ______ oncotic pressure in the peritubular capillary
decreased
What is the end result of ECF volume expansion?
A portion of fluid that would have been reabsorbed instead leaks back into the lumen of the tubule and is excreted
What are the 2 mechanisms the contribute to the increased proximal tubule reabsorption that occurs in ECF volume contraction
- Starling forces
- Angiotensin II
ECF volume contraction produces a(n) _____ in fractional reabsorption in the proximal tubule
increase
When ECF volume decreases the capillary hydrostatic pressure (Pc) _____. How does this happen?
decreases, because the plasma protein concentration increases
Decreases in capillary hydrostatic pressure results in ______ oncotic pressure in the peritubular capillary
increased
The alteration of glomerulotubular balance that occurs with ECF volume contraction is a mechanism for what?
Protection, in which the kidneys try to restore the ECF volume by reabsorbing more solute and water than usual
Other than the Starling forces, what is another mechanism that contributes to the increased proximal tubule reabsorption that occurs in ECF volume contraction?
Decreased ECF volume leads to decreased blood volume and arterial pressure that activates the renin-angiotensin-aldosterone system which produces Angiontensin II