19 THE URINARY SYSTEM: FLUID AND ELECTROLYTE BALANCE
The Concept of Balance
To maintain homeostasis, what comes into the body and what is produced must equal the sum of what
is used by the body and what is eliminated:
Input + production = eliminated + output
FACTORS AFFECTING THE PLASMA COMPOSITION
The volume of plasma is determined almost entirely by its water content because solutes make only a
negligible direct contribution to the volume.
The amount of solute in plasma indirectly affects plasma volume because changes in plasma
osmolarity can cause water to shift between the plasma and other body fluid compartments.
The transport of materials across the wall of the gastrointestinal tract normally amounts to a net gain
of solutes and water by the body.
The transport of materials across the walls of the renal tubules amounts to a net loss of water and
solutes by the body.
SOLUTE AND WATER BALANCE
Changes in volume and/or composition occur when materials enter the plasma faster than they exit,
or vice versa.
For water balance, water intake plus any metabolically produced water must equal water output plus
any water used in chemical reactions.
Only kidneys regulate the rate of water loss for the purpose of water balance.
Plasma volume is directly related to blood pressure: increase in plasma volume increases MAP.
Can also affect osmolarity: an increase in plasma water with no increase in solute decreases
osmolarity; decrease in plasma water with no decrease in solute increases osmolarity.
OSMOLARITY AND THE MOVEMENT OF WATER
The kidneys vary the volume of water excreted by regulating water reabsorption in the late distal
tubules and collecting ducts.
Water can only move if the tubule membranes are permeable to water.
The total solute concentration of the plasma is approx. 300mOsm. It is also the normal osmolarity of
the interstitial fluid and intracellular fluid, therefore osmotic pressure is equal (osmotic equilibrium).
Kidneys excrete excess solutes that were ingested while minimizing water loss by excreting a small
volume of water.
Kidneys compensate for changes in plasma volume and osmolarity by adjusting the rate of water
excretion. Because water is reabsorbed but not secreted, these adjustments are achieved through
changes in the rate of water reabsorption.
In renal tubules, water reabsorption is passive and is coupled with active reabsorption of solutes.
Reabsorption of solute increases the osmoalrity of the peritubular fluid (drives reabsorption of
water), and that in the collecting duct the medullary osmotic gradient drives reabsorption of water).
WATER REABSORPTION IN THE PROXIMAL TUBULE
Primary solute is Na and most of it is reabsorbed in the proximal tubule, producing the osmotic
gradient that drives water reabsorption.
Uses active transport of Na across the basolateral membrane from the epithelial cell.
Na also crosses the apical side by secondary active transport with glucose. 19 THE URINARY SYSTEM: FLUID AND ELECTROLYTE BALANCE
ESTABLISHMENT OF THE MEDULLARY OSMOTIC GRADIENT
Medullary osmotic gradient: outer regions of the medulla have a lower osmolarity than the inner
At the edge of the medulla = 300 mOsm; innermost portion near the renal pelvis = 1200-1400 mOsm.
This gradient exists due to the countercurrent multiplier and the facilitated diffusion of urea from the
lumen of the collecting duct into the medullary interstitial fluid.
o The descending limb is permeable to water, so water diffuses when an osmotic gradient exists.
o The thick ascending limb is impermeable to water and has Na/K/Cl active transporters. They
pump the ions into the interstitial fluid, increasing osmolarity.
o Countercurrent refers to the fact that fluid flowing through the descending and ascending
limbs, which parallel one another, moves in opposite directions.
o How the countercurrent multiplier creates osmotic gradient:
1. Starts with no osmotic gradient. Fluid entering the descending limb is iso-osmotic with the
interstitial fluid, at 300mOsm. The water freely crosses the wall of the tubule and is
therefore reabsorbed along with solutes. As fluid travels up the ascending limb, Na, Cl, and
K are actively transported.
2. Increasing the osmolarity of the interstitial fluid from 300 to 400 mOsm and lowering the
osmolarity of fluid in the ascending limb to 200 mOsm. When osmolarity in the peritubular
fluid increases, water moves out of the descending limbd and into the peritubular fluid.
3. The descending limb and the peritubular fluid are iso-osmotic again at 400 mOsm. The
fluid in the ascending limb now has a lower osmolarity.
4. As more 300 mOsm fluid enters the loop of Henle from the proximal tubule, the fluid
pushes the 400 mOsm deeper into the medulla.
5. Active transport of Na, Cl, K in the ascending limb raises the osmolarity of deeper
medullary interstitial fluid from 400 mOsm to 50 mOsm, which causes water movement
into the interstitial fluid from the descending limb.
6. The descending limb is now iso-osmotic with the interstitial fluid. More 300 mOsm enters
the loop of Henle from the proximal tubule, pushing the higher osmolarity fluid toward the
tip of the loop of Henle.
7. The process continues until the osmotic gradient is created and the system is in a steady
o At the steady state: entering fluid is iso-osmotic with the interstitial fluid at 300 mOsm. But the
osmolarity in the tubular fluid in the limbs is greater in deeper portions of the renal medulla.
The tip of the loop of Henle is 1400 mOsm.
o Any given level in the medulla, the osmolarity of the fluid in the ascending ilmb is always lower
than the fluid in the descending limb, because the ascending limb has active transports but
prevents water from following them.
o As the fluid leaves it is hypoosmotic to the interstitial fluid at approx. 100-200 mOsm.
ROLE OF UREA IN THE MEDULLARY OSMOTIC GRADIENT
o Additional solute is needed to maintain the gradient. That is urea, a waste product generated
during catabolism of proteins.
ROLE OF THE VASA RECTA IN PREVENTING DISSIPATION OF THE MEDULLARY OSMOTIC GRADIENT
o The vasa recta capillaries prevents the diffusion of water and solutes from dissipating the
medullary osmotic gradient.
o As vasa recta enter the medulla with higher osmolarity, water leaves the capillaries by
osmosis, and solutes enter the plasma by diffusion.
o It continues to the tip of the vasa recta because of increasing osmolarity.
o However, as the blood flow goes back to the cortex, the direction of the osmotic gradient acros
the capillary walls reverses: water moves in, and solutes move out. 19 THE URINARY SYSTEM: FLUID AND ELECTROLYTE BALANCE
o This tends to rise the osmolarity of the interstitial fluid.
o Result: the osmolarity of the interstitial fluid stays constant, and the osmolarity in the plasma
leaving the medulla in the vasa recta is slightly hyperosmotic.
ROLE OF THE MEDULLARY OSMOTIC GRADIENT IN WATER REABSORPTION IN THE DISTAL TUBULE
AND COLLECTING DUCT
70% of the water filtered from plasma at the renal corpuscle is reabsorbed in the proximal tubule.
Approximately 20% of the filtered water is reabsorbed in the distal tubule.
10% is reabsorbed in the collecting ducts.
AQUAPORINS AND WATER PERMEABILITY
o The epithelial cells lining the late distal tubule and collecting duct are connected by tight
junctions such that water can’t pass between cells from peritubular fluid to tubular fluid.
o The lipid bilayers of these cells are impermeable to water. Water depends on aquaporins
(water channels or pores).
o Aquaporins are always present in the basolateral membrane and in the apical membrane (only
in the presence of ADH).
o The more permeable these tubules, the greater the water reabsorption.
o The walls of the late distal tubule and collecting duct are impermeable to water. Osmotic
gradient exists, but can’t move the water. Therefore the osmotic gradient gets larger as it
travels down the collecting duct. Final result: excretion of large volume of urine with low
o Kidneys can conserve water when the late distal tubule and the collecting duct are permeable
to water. Fluid is initially hypo-osmotic to the cortical interstitial fluid, and water is
reabsorbed. As the collecting duct leaves, the fluid is iso-osmotic. Water continues to be
reabsorbed from the collecting duct into the interstitial space such that the fluid is always
nearly iso-osmotic with the interstitial fluid. Tubular fluid osmolarity can never exce