1. Water Homeostasis
• The body maintains a balance of water intake and output by a series of negative feedback loops involving the endocrine system and autonomic nervous system.
maintaining water homeostasis is a balancing act. The amount of water taken in must equal the amount of water lost.
insensible loss
• Every day we take in about 2300 milliliters of water in the form of food and beverages.
• Approximately 200 milliliters of body water is generated through cell metabolism for an approximate 2500 milliliters of total intake.
C6H12O6 + 6O2 → 6 H2O + CO2
• At the same time, we lose water, mostly through the kidneys, but also through the lungs, skin, and GI tract.
• We lose approximately 1500 milliliters per day from the kidneys in the form of urine.
• We also lose about 600 milliliters of water per day through the skin and 300 milliliters from the lungs in the form of water vapor in exhaled air. These two forms of water loss are called insensible loss because we are unaware of the process.
• We can lose much more than this insensible loss under conditions of extreme physical exertion. Under such conditions we can lose up to 5000 milliliters per day, through sweating.
• Under normal circumstances we also lose 100 milliliters of water per day though the GI tract.
• As you can see, maintaining water homeostasis is a balancing act. The amount of water taken in must equal the amount of water lost.

2. Disturbances of Water Homeostasis
Hypervolemia
• Hypervolemia occurs when too much water and solute are taken in at the same time. Although extracellular fluid volume increases, plasma osmolarity may remain normal.
Overhydration
• Overhydration occurs when too much water is taken in without solute. Volume increases, but because solute is not present, plasma osmolarity decreases.
Hypovolemia
• Hypovolemia occurs when water and solutes are lost at the same time. This condition primarily involves a loss of plasma volume. Plasma osmolarity usually remains normal even though volume is low.
Dehydration
• When water, but not solute, is lost, dehydration occurs.
• Dehydration involves a loss of volume but, because solutes are not lost in the same proportion, plasma osmolarity increases.
i.v., infussion of
isotonic solution
Drinking too
much water
Blood loss
sweating

3. Mechanisms of Fluid Balance
• Four primary mechanisms regulate fluid homeostasis:
1)Antidiuretic hormone or ADH
2)Thirst mechanism
3)Aldosterone
4)Sympathetic nervous system
Thirst Mechanism
• The thirst mechanism is the primary regulator of water intake and involves hormonal and neural input as well as voluntary behaviors.
• Stimulation of the thirst center in the hypothalamus gives you the desire to drink.
• There are three major reasons why dehydration leads to thirst:
1. When saliva production decreases, the mouth and throat become dry. Impulses go from the dry mouth and throat to the thirst center in the hypothalamus, stimulating that area.
2. When you are dehydrated, blood osmotic pressure increases, stimulating osmoreceptors in the hypothalamus and the thirst center in the hypothalamus is now further activated.
3. Decreased blood volume causes a decrease in blood pressure that is signaled by baroreceptors and stimulates the release of renin from the kidney. This causes the production of angiotensin II which stimulates the thirst center in the hypothalamus.

4. Sympathetic Stimulation in the Nephron
• Release of neurotransmitters from the sympathetic nerves in the kidney stimulates smooth muscle cells in the afferent arteriole to constrict.
• This process causes a decrease in blood flow into the glomerulus and a drop in glomerular filtration rate and results in less urine formation. Less water leaves the body.
• Sympathetic stimulation also causes the release of renin which, by stimulating aldosterone secretion, will increase the reabsorption of sodium.
• As a result, blood volume will stop decreasing and blood pressure may stabilize. However because the blood pressure and blood volume have not yet returned to normal, the baroreceptors will continue to be stimulated to prevent further loss of blood volume.
• In order to bring this person back into to homeostasis, we need to increase the blood volume by drinking fluids.

5. the thick ascending limb and the early distal tubule.
FREE-WATER CLEARANCE
Free water is defined as distilled water that is free of solutes (or solute-free water). In the nephron, free water is generated in the diluting segments, where solute is reabsorbed without water. The diluting segments of the nephron are the water-impermeable segments:
the thick ascending limb and the early distal tubule.
Measurement of free-water clearance (CH2O)
provides a method for assessing the ability of the kidneys to dilute or concentrate the urine. The principles underlying this measurement are as follows:
When ADH levels are low, all of the free water generated in the thick ascending limb and early distal tubule is excreted (since it cannot be reabsorbed by the collecting ducts). The urine is hyposmotic, and free-water clearance is positive.
When ADH levels are high, all of the free water generated in the thick ascending limb and the early distal tubule is reabsorbed by the late distal tubule and collecting duct. The urine is hyperosmotic, and free-water clearance is negative.

6. Measurement of CH2O calculated by the following equation:
CH2O can be zero, it can be positive, or it can be negative.
CH2O is Zero– Isosthenuria
where
CH2O
Free-water clearance (mL/min) V
Urine flow rate (mL/min)
Cosm
Clearance of osmoles (mL/min)
[U]osm
Urine osmolarity (mOsm/L)
[P]osm
Plasma osmolarity (mOsm/L)
CH2O is positive- Diabetes Insipidus.
CH2O is negative- SIADH

7. Electrolyte Homeostasis
• The fluid surrounding the cells in the body must maintain a specific concentration of electrolytes for the cells to function properly.
• Electrolytes are a major component of body fluids. They enter the body in the food we eat and the beverages we drink.
• While electrolytes leave the body mainly through the kidneys by way of the urine, they also leave through the skin and feces.
• Severe vomiting and diarrhea can cause a loss of both water and electrolytes from the body, resulting in both water and electrolyte imbalances.
• The concentrations of electrolytes in body fluids must be maintained within specific limits, and even a small deviation outside these limits can have serious or life-threatening consequences.
• In this topic we will concentrate on the three most clinically significant electrolytes sodium ions, potassium ions, and calcium ions.

8. Sodium Homeostasis
• The normal concentration range of sodium in the plasma is milliequivalents per liter, making sodium the ion with the most significant osmotic effect in the extracellular fluid.
145
136
Hypernatremia
• what will happen if the sodium concentration of the blood plasma increases, as in hypernatremia.
• What effect would this increase in sodium concentration have on the cells that are bathed by the interstitial fluid?
___ Cells swell
___ Cells shrink
• The high concentration of sodium in the extracellular fluid exerts osmotic pressure and helps determine the fluid levels in the intracellular space.
Hyponatremia
• What effect would this decrease in sodium concentration have on the cells that are bathed by the interstitial fluid?
___ Cells swell
___ Cells shrink
• The water moves into the cell, and the cell expands slightly.

9. Roles of Sodium in the Body
• nerve impulse conduction and muscle contraction,
primary regulator of water movement in the body because water follows sodium by osmosis.
• If sodium levels in the plasma change, those changes determine fluid levels in the other compartments.
Causes and Symptoms of Hypernatremia
Which of these reasons would most likely cause hypernatremia in the marathon runner?
____ Too much sodium added
____ Too much water lost
Symptoms of hypernatremia include non-specific signs of central nervous system dysfunction such as confusion and lethargy, and in severe cases, seizures and death.
• What do you think causes these symptoms?
___ Neurons shrink
___ Neurons swell
What will happen to urine output? 
Decreases
• When plasma osmolarity increases, antidiuretic hormone is released, resulting in reabsorption of water and decreased urine output.

10. Sodium Balance This diagram shows how sodium is
distributed in the body. Note the important role of the kidney in filtering and
subsequently reabsorbing that sodium. (Note: Although not listed in the diagram above
there can also be significant losses of sodium from the gastrointestinal system (vomiting
and diarrhea) and from the skin following burns and from the cardiovascular system
following hemorrhage. Please note that you don’t need to know the numbers just the
relative inputs and outputs.
Understanding the mechanisms of renal sodium transport is important for several reasons. First,
NaCl and water are filtered at the highest rates. Therefore there is an enormous amount of
water and sodium reabsorption. You will come to see that the reabsorption of water and sodium
are related. Second, transport of sodium (down its electrochemical gradient) is linked to essential
tubular reabsorptive and secretory processes for many other substances. Third, mechanisms of
NaCl reabsorption are modified clinically by diuretics. Treatment with diuretics aims to reduce
ECF volume by increasing renal excretion of NaCl and water. Diuretics are commonly used
medications in the treatment of pulmonary edema and hypertension. An easy way to remember
the importance of sodium and water movement is that ‘water will follow sodium’ in the nephron.
Increase sodium reabosrption will lead to increased water reabsorption while inhibition of
sodium reabsorption will lead to a diuresis.

11. Where Does Na+ Reabsorption Occur?
FE = 10%
[Na+] =145
FE = 3%
FE = 35%
The filtered load of sodium is far greater than the daily requirement for sodium excretion.
Typically the fractional excretion for sodium is about 1-2%. Filtration and reabsorption are the
only significant processes affecting NaCl and water excretion. Thus, regulation of excretion is
achieved largely by varying the amount that escapes tubular reabsorption.
The slide illustrates
segmental handling of
sodium (FE values for
chloride are similar). The
slide shows the proportion of
filtered sodium remaining in
the nephron at the point
indicated (fractional
excretion). For example, the
proximal tubule extracts 65%
of the filtered load of
sodium, so that the fraction
remaining at the end of this
segment is 35%. The loop of
Henle typically reabsorbs
another 20-25% of the
filtered load, so that around
10% of filtered NaCl enters
the distal tubule. The % of filtered NaCl delivered to the distal nephron does not change much
across a wide range of NaCl intakes. This design allows for fine regulation of NaCl excretion in
the distal tubule and collecting duct, particularly by aldosterone.
FE = %
[Na+] units = mmole/L
FE= Fractional excretion

12. The Na+/K+-ATPase Drives Na+ Reabsorption All Along The Renal Tubule
Lumen
Blood
Na+
3Na+
ATP
ADP
2K+
Active sodium reabsorption is the key driving force behind NaCl and water reabsorption along
the nephron. Chloride absorption is passive or occurs via secondary active transport coupled to
the movement of sodium. Water reabsorption is coupled to the reabsorption of NaCl and occurs
by osmosis.
The slide describes the basic process of transcellular sodium absorption, which is a two-step
process involving passive uptake across the apical membrane (through co-transporters,
antiporters or channels) and active extrusion across the basolateral membrane. The latter is
driven by the sodium/potassium-ATPase, which in renal epithelial cells keeps intracellular
sodium at a concentration of around 30 mEq/L. The energy in the large inward sodium gradient
at the luminal membrane is coupled to the uptake of many other substrates. The sodium entry
step is the target of inhibition by the diuretics in most common clinical use.
diuretics
Na+

13. In Early Proximal Tubule Na+ Absorption Is Linked To Nutrient Transport…..
Lumen
Blood
3Na+
Na+
ATP
ADP
2K+
nutrient
Nutrient
(a.a & gluc.)
The mechanism for nutrient uptake in
early proximal tubule is shown. Its key
feature is the presence of sodium linked
cotransport at the apical membrane. This
concentrates the nutrient molecule in the
cell and allows it to diffuse out of the cell
into the blood via facilitated diffusion.
Depending on the nutrient molecule
there are different cotransporters
involved. For glucose the main
transporter is called SGLT2 (sodiumglucose
transporter 2) – See the Figure
on the next page. This is the saturable
transporter that is overwhelmed in
hyperglycemic patients when the filtered
glucose load exceeds reabsorptive capacity, resulting in glucose in the urine. Mutations in SGLT2
also account for an inherited condition in which there is large amounts of glucose in the urine
unrelated to diabetes mellitus. In the case of amino acids, there are several sodium linked
cotransporters involved, each with different substrate specificity for particular classes of amino
acid (e.g. acidic amino acids). In some cases genetic disorders of amino acid wasting can be traced
to a failure of amino acid uptake in the kidney and GI tract due to defective cotransporters.
Familial renal glycosuria – SGLT2 mutations
Cystinuria – dibasic amino acid carrier
Hartnup’s disease – neutral amino acids

14. Absorption Mechanisms Change Along The Proximal Tubule
inulin
2.0
1.5
Cl-
[TFx/Px]
Na+
1.0
osm
The transport processes that result in
NaCl and water reabsorption in the
proximal tubule cause the concentrations
of several solutes to change along the
proximal tubule:
Q. Why does the concentration of inulin
increase?
Q. Why does osmolality stay the same?
A striking feature of the graph is the fall
in concentration of nutrient molecules
such as glucose and amino acids along
the proximal tubule. This shows that these solutes are preferentially reabsorbed from the filtrate
at a very early site. In physiological states, glucose and amino acid reabsorption is almost
complete one quarter of the way along the proximal tubule. The above slide also shows a marked
fall in bicarbonate concentration in the early part of the proximal tubule. This recovery of filtered
bicarbonate is an important feature of the renal contribution to acid-base homeostasis. In early
proximal tubule sodium absorption is mostly coupled to the uptake of nutrients and to
bicarbonate reabsorption, rather than to chloride recovery. This explains the rise in chloride
concentration in the lumen in early proximal tubule. In the mid- to late proximal tubule sodium
absorption is coupled to chloride absorption.
There is a very important point to be seen in this graph that is not entirely obvious at first
glance. We stated previously that the proximal tubule reabsorbs approximately 65% of the
filtered sodium load. You will see that although there is an enormous amount of sodium
reabsorption taking place there is little change in the sodium concentration and osmolarity.
How does this happen ? In the proximal tubule of the kidney there is ‘isosmotic’ reabsorption
of sodium and water. That is 65% of sodium and 65% of water reabsorption take place in the
proximal tubule. This is an important point to remember as we will later deal with the
mechanisms which alter sodium and water reabsorption.
HCO3-
0.5
Glucose/amino acids
0.0
Length of proximal tubule

15. + Na+ Uptake In The TALH Is Via A Cotransport Mechanism Blood Lumen
NKCC2
2Cl-
2K+ K+
Cl-
ROMK
CLC-Kb +
cations
Na, Ca and Mg
Loop of Henle
In the thin limbs of Henle’s loop there is no active transport of NaCl. The descending and
ascending thin limbs have differences in passive permeability, which are important to the urine
concentration mechanism. The thick ascending limb is metabolically very active.
The sodium and chloride uptake into
the cell are coupled via a cotransport
mechanism, which also includes uptake
of potassium. The process is electrically
neutral since it involves 1 sodium, 2
chloride and 1 potassium ions. At the
basolateral membrane sodium is
pumped out via the Na/K-ATPase,
whereas chloride leaves via an ion
channel. At the apical membrane the
presence of a potassium channel to
recycle potassium that enters during
NaCl uptake is important. First, the
concentrations of sodium chloride in
tubular fluid entering the thick
ascending limb are much higher than potassium concentration. This means that there would not
be sufficient potassium available to load the cotransporter for sodium chloride uptake, if
potassium was not recycled. The second reason the apical potassium conductance is important is
that, in conjunction with the basolateral chloride conductance a lumen positive tranepithelial
electrical potential difference is generated. This potential difference is important because the tight
junctions in this region are cation permeable, particularly to magnesium ions. The lumen positive
potential allows for a significant paracellular cation flux.
The molecular identity of all the transporters in the thick ascending limb cell model are known
(see slide). If a loss of function mutation arises in the cotransporter, the apical potassium channel
or the basolateral chloride channel, a rare inherited condition called Bartter’s Syndrome occurs.
This disease is associated with urinary wasting of NaCl due to the loss of reabsorption in the
thick ascending limb. Bartter’s Syndrome patients also lose large amounts of calcium and
magnesium, reflecting the importance of this segment in divalent cation handling.
Mutations in any of NKCC2, ROMK or CLC-Kb
= BARTTER’S SYNDROME
ADH targets NKCC2 & ROMK
‘loop diuretics’ Bumetanide & Furosemide block NKCC2

16. Early Distal Tubule Uses Na/Cl Cotransport For Na+ Absorption
Lumen
Blood
3Na+
Na+
NCCT
2K+
Cl-
Cl-
Distal Tubule
The distal tubule is a heterogeneous segment. Early distal tubule (“distal convoluted tubule”) is
lined by epithelial cells with the transport proteins shown below. The late distal tubule merges
with a connecting tubule, which in turn merges with a cortical collecting duct. During this
transition the epithelial lining becomes a mixture of cell types, including those seen in the early
distal tubule and the renal principal cells, which are described in the section on collecting duct
below.
In early distal tubule sodium and
chloride are coupled directly via
a cotransporter. Uphill sodium
exit is via the usual pathway of
the Na/K-ATPase, while that
for chloride is via an ion
channel. The molecular identity
of the cotransporter is known
and mutations in this protein are
the basis for a rare salt wasting
disease called Gittelman’s
Syndrome.
NCCT mutations cause
GITTELMAN’S SYNDROME
Thiazide Diuretics block NCCT

17. Na+ Entry In The Collecting Duct Is Via An Ion Channel
Lumen
Blood
Principal cell
Na+
ENaC
3Na+ K+
ROMK
2K+
Cortical Collecting Duct
This segment has two distinct cell types. Principal cells are the most abundant cells and are
associated with NaCl reabsorption. Intercalated cells are the other cell type present and are
involved with acid-base balance.
Principal cells are the only site
in the nephron where sodium
uptake from the lumen occurs
via simple diffusion through an
ion channel. The presence of
potassium channels in the
apical membrane is important
to the overall function of this
cell. Aldosterone signals to the
kidney to conserve sodium and
excrete more potassium.
Principal cells are the major
target for aldosterone, which
increases the activity of all the
transporters shown.
The sodium channel in the apical membrane of renal principal cells is ENaC. In Liddle’s
syndrome ENaC is mutated so that it becomes more active than it should be. Liddle’s Syndrome
is associated with salt-sensitive hypertension because patients reabsorb too much salt in the late
distal tubule and cortical collecting duct. This leads to expansion of the ECF volume and to
increased blood pressure. ENaC channels can also be mutated so that function is lost. This
causes a syndrome that appears to be the same as having no aldosterone production
ENaC gain of function = LIDDLE’S SYNDROME
ENaC loss of function =
PSEUDOHYPOALDOSTERONISM (PHA)
Aldosterone activates ENaC & ROMK while Amiloride like diuretics block ENaC