Flashcards in Renal Chapter 6: Basic Renal Processes for Na+, Cl-, Water Deck (66)
Describe transport of water.
water follows osmoles
some regions low water permeability limits amount of water following osmoles
(no pumps for water)
Describe what Na, Cl, and water have in common.
all freely filterable at the renal corpsule
all undergo considerable reabsorption, usually more than 99% but normally no tubular secretion. (most renal ATP energy expended is used for this task
Describe the major tubular mechanisms for reabsorption of sodium.
mainly an active, transcellular process driven mostly by Na-K- adenosine triphosphatase (Na-K-ATPase)
Describe the major tubular mechanism for reabsorption of chloride.
reabsorption of chloride is both passive (paracellular diffusion) and active (transcellular), but it is directly or indirectly coupled with the reabsorption of sodium, thus explaining why the reabsorption of the 2 ions usually occurs in parallel.
(when describing the reabsorption of sodium, a parallel reabsorption of chloride is usually implied.
Describe reabsorption of water.
reabsorption of water is by osmosis and is secondary to reabsorption of solutes, particularly sodium and substances whose reabsorption is dependent on sodium reabsorption (mostly chloride)
Where is sodium mostly reabsorbed, in what percentages, how much is usually excreted?
in individual w average salt intake, PT reabsorbs 65% of filtered sodium, the thin and thick ascending limbs of Henle's loop 25% and distal convoluted tubule and collecting duct- system the remaining 10% so that final urine contains less than 1% of total filtered sodium
In all nephron segments, what is the essential event for active transcellular sodium reabsorption?
primary active transport of sodium from cell to interstitial fluid by the Na-K-ATPase pumps in the basolateral membrane
What is the importance of the Na/K pumps in basolateral membrane?
pumps keep the intracellular sodium concentration lower than in the surrounding media. Because the inside of the cell is negatively charged with respect to the lumen, luminal sodium ions enter the cell passively down their electrochemical gradient.
How is sodium and chloride percentages and locations of reabsorption related?
bc chloride reabsorption is dependent on sodium... tubular locations and percentages of filtered chloride reabsorbed are similar to those of sodium
What is absolute constraint of electroneutrality?
any finite volume of fluid reabsorbed must contain equal amounts of anion and cation equivalents
One L of normal filtrate contains 140mEq of sodium. Describe its anion concentration.
If 65% of the filtered sodium is reabsorbed in the proximal
tubule, how much chloride is reabsorbed?
about 140 mEq of anions, mainly chloride (110 mEq) and bicarbonate (24 mEq)
0.65 x 140=91mEq reabsorbed in PT. so some combination of 91mEq of chloride and bicarbonate must also be reabsorbed to accomnay sodium...
we know that about 90% of filtered bicarb is reabsorbed in PT
(.90x24= 22) 91-22 =69mEq of chloride that must be reabsorbed in the PT
What is the critical transport step for chloride? Why?
Describe the chloride transport process. Where are pumps necessary and why?
critical step is from lumen to cell.
the chloride transport process in the luminal membrane must achieve a high enough intracellular chloride conc. to cause downhill chloride movement out of the cell across the BL membrane...(of course, the movement of Cl across basolateral membrane is also prompted by negative potential within the cell)
so luminal membrane chloride transporters serve same purpose as BL membrane Na-K-ATPase pumps do for Na. (they move Cl uphill from lumen to cell against its electrochemical gradient)
Describe the major transporters on the apical lumen/basolateral sides of PT.
See Figure 6-1 p 90 or printout
According to the luminal membrane depicted in Figure 6–1, the major routes are
(1) paracellular absorption and (2) a complicated parallel set of Na-H and Cl-base
antiporters (described later). These mechanisms are dependent on sodium movement
across the membrane and are, therefore, linked to sodium reabsorption.
How does the kidney regulate how much water is excreted based upon hydration status? Where does this regulation of water reabsorption take place?
regardless of hydration state, collective actions of renal tubular segments before cortical collecting tubule reabsorb more solute than water...
this leaves large volume of dilute tubular fluid (approx 110 mOsm/kg H2O) entered the limited segment of the cortical collecting tubule.
if individual is overhydrated most of this water will be excreted w limited further reabsorption
if dehydrated, vast majority of the dilute water is reabsorbed.. leaving low volume of concentrated final urine
Where does water reabsorption occur and in what percentages?
always occurs in the proximal tubule (65% of filtered water), descending limb of Henle's loop (10%) and collecting duct system (where fractional reabsorption is the most variable)
Sodium and water reabsorption occur in the proximal tubule to the same extent; however, their reabsorption in Henle's loop differs. Explain how.
both are also reabsorbed in Henle’s loop, but not in equal proportions.
The part of the loop involved in water reabsorption is different from that for sodium
reabsorption, and the fraction of sodium reabsorbed by the loop as a whole is always greater than that of water (ie, the loop overall is a site where salt is reabsorbed and excess water is left in the lumen of the nephron: “separating salt from
sodium reabsorption, but not water reabsorption, occurs in the distal convoluted tubule.
both occur in the collecting-duct system, but
the percentages of sodium and water reabsorbed in the collecting-duct system vary
enormously depending on a number of factors
What are some routes by which water can move down an osmotic gradient?
simple net diffusion through the lipid bilayer
through aquaporins in plasma membrane of tubular cells
through tight junctions between cells
amt of water that moves for a given osmotic gradient and its route dep. on the water permeability of different cellular components
Which areas are more/less permeable to water?
basolateral membrane of all renal cells are quite permeable to water due to presence of protein aquaporins that act as water pores (so cystolic osmolality is always close to that of the surrounding interstitium. It is the luminal membrane and tight junctions where most variability lies
(1) The luminal
membranes of the proximal tubule and descending thin limb of Henle’s loop
always have a high water permeability
(2) the luminal membrane of the ascending limbs of Henle’s loop (both thin and thick; recall from Chapter 1 that only long
loops have ascending thin limbs) and the luminal membranes of distal convoluted tubule are always relatively water impermeable, as are the tight junctions;
(3) the water permeability of the luminal membrane of the collecting-duct system
is intrinsically low but can be regulated so that its water permeability increases
What is obligatory water loss?
Is it fixed?
the sum of urea, sulfate, phosphate, and other waste products, and a small number of nonwaste ions excreted each day normally averages approx 600mOsm/day. Therefore, the minimal volume of water in which this mass of solute can be dissolved is roughly 600mmol/1400mOsm/L =0.43L/day
(The human kidney can
produce a maximal urinary concentration of 1400 mOsm/kg in extreme dehydration)
-not fixed volume, changes. for ex: during increased tissue catabolism, as during fasting or trauma, releases excess solute and so increases obligatory water loss
What happens in the early portion (the proximal convoluted tubule)?
large fraction of filtered sodium enters the cell across the luminal membrane via an antiport with protons
(these protons which are supplied by CO2 and water cause the secondary active reabsorption of filtered bicarbonate)
so bicarb is a major anion reabsorbed w sodium, and the luminal bicarb conc. decreases markedly
How does most chloride reabsorption in the PT occur? paracellular or transcellular?
Describe its conc. along the early PT.
conc. of chloride in BC is basically same as plasma (about 110mEq/L)
along the early PT however, the reabsorption of water, driven by the osmotic gradient created by the reabsorption of sodium plus its co-transported solutes and bicarb, causes the chloride conc. in the tubular lumen to increase somewhat above that in the peritubular capillaries.
Describe chloride conc. as fluid flows through the middle and late proximal tubule.
conc. gradient, maintained by continued reabsorption of water, provides the driving force for paracellular chloride reabsorption by diffusion
in the late PT, it uses parallel Na-H and Cl-base antiporters. Chloride transport into the cell is powered by the downhill antiport of organic bases which are continuously generated in the cell by dissociation of their respective acids into a proton and base
Explain diagram 6-2 on p 94.
Describe what happens to protons generated within the cell by dissociation of acids.
Will the pH of lumen change?
They are actively transported into the lumen by Na-H antiporters. In the lumen the protons and organic bases recombine to form the acid, which is a neutral molecule. This nonpolar neutral acid then diffuses across the luminal membrane back into the cell where the entire process is repeated
most of the protons are not acidifying the lumen but are simply combining with the base and moving back into the cells
Describe result of parallel Na-H and Cl-base antiporters.
same as if Cl and Na were co-transported into cell together
Na in, Cl in.
What powers the luminal Na-H transporter?
the Na-K-ATPase in the basolateral membrane
Describe water reabsorption in the proximal tubule.
PT has high permeability to water
this mean that v small differences in osmolality (less than 1mOsm/L) will suffice to drive the reabsorption of v large quantities of water (normally about 65% of filtered water)
the osmolality difference is created by reabsorption of solute
The osmolality of the freshly filtered tubular
fluid at the very beginning of the proximal tubule is, of course, essentially the
same as that of plasma and interstitial fluid. Then, as solute is reabsorbed from
the proximal tubule, the movement of this solute out of the lumen lowers luminal osmolality (ie, raises water concentration) compared with interstitial fluid. It also raises interstitial fluid osmolality (only a bit bc high perfusion through peritubular capillaries keeps it close to plasma value)
How does water diffuse from lumen to interstitial fluid? Then what?
Osmotic gradient from lumen to interstitial fluid causes osmosis of water from the lumen across the plasma membranes via aquaporins and tight junctions into the interstitial fluid. The Starling forces across the peritubular capillaries in the interstitium favor reabsorption and so water and solutes then move into peritubular capillaries and are returned to general circulation
Given the tremendous amount of sodium reabsorbed, how can the luminal sodium concentration and osmolality not progressively decrease along the proximal tubule?
iso-osmotic volume reabsorption
of the mass of filtered sodium and total solute has been reabsorbed by the end of the proximal tubule, so has virtually the same percentage of filtered water. This is because the water permeability of the proximal tubule is so great that passive water
reabsorption always keeps pace with total solute reabsorption. Therefore, the
concentrations of sodium and total solute (osmolality), as opposed to their masses, remain virtually unchanged during fluid passage through the proximal tubule