Water Homeostasis

So wrote the great physiologist Robert F. Pitts describing the evolution of organisms from the ocean to land.  Marine animals survive in the high tonicity of seawater (500-1000 mOsm/kg) through a variety of mechanisms; the shark maintains a high tonicity in its body fluids whereas dolphins absorb water from foodstuffs while producing a highly concentrated urine through complex multilobed reniculate kidneys.  For those of us on land however, the challenge is not only water conservation, but in our world of coffee shops, bottled water, and “hydration for health” philosophies, water elimination.

In the following sections, we will use animations to help explain how our body regulates water intake and excretion, tightly orchestrating water balance.  The animations will help provide an overview of the physiology, and will be accompanied by more detailed written explanations and images that highlight the key principles. To play the animations, simply click on the arrow.  They can be paused at any time.  (The animations can be made larger by clicking on the YouTube link in the bottom right hand corner, which will open up a separate YouTube page.)  To replay, click the circular arrow in the lower left corner of the screen.    Please make sure to scroll down to the bottom of each page in order to get a link and arrow (bottom right-hand corner of screen) to the next section.  Please note that the table of contents can also be found in the upper left-hand corner by clicking on the button with the three lines.

We will cover the following key concepts:

1. Distribution of water in our body

2. Sensing changes in body concentration

3. Vasopressin and its effect on the collecting duct

4. Generating and maintaining a concentrated medullary interstitium

5. Clinical correlations

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Water is the most abundant component of the human body, constituting approximately 50-60% of body weight.  Since the intracellular space constitutes the largest body compartment, holding approximately 2/3rds of body fluid, changes in water homeostasis predominantly affect cells; water excess leads to cellular swelling, and water deficit to cellular shrinkage.  Although cells have an innate capacity to respond to changes in cell volume when extracellular osmolality changes, the body protects cells primarily by tightly regulating extracellular osmolality.  Despite a huge range of water intake, and a multitude of routes for water loss, including the respiratory and gastrointestinal tract, skin, and the kidneys, the amount of body water remains remarkably stable.

Water is distributed amongst three major body compartments; the intracellular, interstitial, and intravascular spaces.  The intracellcular compartment is the largest compartment, holding two thirds of body fluid, and is defined by the cell membrane (Figure 1).

Figure 1. The major fluid compartments in the body.  The intracellular space is defined by the cell membrane. The intravascular space is defined by the endothelial layer of the blood vessels. The interstitial space surrounds these two compartments.

The intravascular space is separated from the interstitial space by the capillary vascular wall.  These two “barriers” - the cell wall and the vessel wall, are similarly permeable to water, but differ in permeability to sodium. Na+/K+ ATPases on the cell membrane actively extrude sodium ions, thereby making the cell functionally impermeable to sodium ions. Thus, ingestion of water alone, compared to sodium and water, has vastly different consequences on the individual compartment sizes.  In the following two animations, we will explore how drinking water versus eating sodium and drinking water ultimately affect body fluid compartment sizes.  Both begin from the moments after these molecules have been absorbed and are circulating within the blood vessels, ready to distribute into the extravascular space.  In the first animation, focus on the water molecule.  In the second animation, focus on the sodium molecule. 

Important points...
1. Water molecules freely leave the vessel wall.  This is due to a lack of tight junctions between neighboring endothelial cells and water permeability across endothelial cell membranes.
2. Water molecules cross the cell membranes freely as well.
3. Of one liter of ingested fluid, two thirds (666 ml) end up in the cellular space, while only 111 ml remain within the blood vessel.  
4. The primary consequence therefore of water retention is an increase of cell size.

          Now lets focus on the fate of ingested sodium and water...

Important points...
1. Sodium molecules also pass easily across the vessel wall.
2. However, due to Na+/K+ ATPases (not shown) which actively extrude sodium ions, the cell wall is functionally impermeable to sodium.
3. Therefore, eating salty pizza with a cola leads to expansion of the interstitial and intravascular spaces, with no change of the intracellular compartment size.  
4. The primary consequence of sodium and water retention is an increase of the extracellular compartments.

In summary, disorders that lead to excess water cause cellular expansion.  The brain, due to a combination of its highly complex cellular function and space limitation within a tight calvarium, is most sensitive to changes in cell size, and water excess, otherwise known as hyponatremia, can results in a range of neurologic conditions, including weakness, confusion, seizures, and death.  Even retention of small amounts of water (one or two liters) can produce neurologic symptoms.

Disorders that lead to sodium retention, primarily through the dysregulation of the renin-angiotensin-aldosterone (RAS) axis, lead to expansion of the interstitial and intravascular compartments with no change in cell size.  This can manifest as peripheral edema and pulmonary edema, respectively.  One should note that it is exceedingly rare to eat sodium alone.  Almost always, sodium ingestion prompts thirst, leading to the need to wash the salty pretzels down with a beverage of choice.  Thus, the ultimate consequence of RAS activation is sodium and water retention.  Since the intersitial and intravascular space are relatively tolerant of changes in compartment size, and since sodium and water retention do not affect cell size, many liters of salty water can be retained with minimal to no symptoms.

As discussed in the previous section, water ingestion leads to cell expansion.  This process of cell expansion is tightly monitored by a specialized cell within the brain, which in turn, regulates the two primary mechanisms of body water intake, namely, thirst and the ability to concentrate urine.  This important cell is called the “osmoreceptor”, and is described in the following animation.

Important points...
1. When the concentration of sodium particles surrounding the cell increase, typically due to a lack of water, water exits the osmoreceptor causing its cell membrane to shrink.  In rare circumstances, ingestion of highly concentrated fluids, such as sea water, can produce a similar effect.  Importantly, note that it is the concentration of fluid surrounding the osmoreceptor that causes cell size change, not the total number of sodium particles.  Extra-cellular concentrations higher than within the osmoreceptor cause cell shrinkage, and lower concentrations cause cell expansion.
2. Focus on seconds 11-12 of the animation.  In this view of the shrinking osmoreceptor, we can see how the membrane shrivels. Embedded within the cell membrane of the osmoreceptor is a specialized mechanical-stretch receptor, known as Transient Receptor Potential Vanilloid (TRPV) 1.  Cell membrane shrinkage causes a conformational change of these receptors, stimulating the activation of inward cation flow (seconds 13-15), which increase the intracellular charge and spark an action potential that travels down the neuron to two targets, namely the higher parts of the brain (which are not shown) and the posterior pituitary. This is vizualized in the following static image.

3. The osmoreceptor is exquisitely sensitive to even small changes in plasma osmolality, activated with as little as 1 or 2 % change. The osmoreceptor generally activates the vasopressin pathway at a serum osmolality about 5 to 10 mOsm/kg lower than thirst pathway.  In other words, concentrating the urine is the first line of defense,  which is important, as it allows us to go about our daily activity, conserving water through concentrating the urine rather than constantly searching for a water fountain.

As discussed in the previous section, activation of the osmoreceptor stimulates thirst and vasopressin release.  Thirst obviously increases water intake.  In this section, we will focus on the role of vasopressin in facilitating the concentration of urine, thereby increasing water retention.

Important points...
1. Vasopressin is synthesized as a prohormone in the magnocellular cell bodies of the paraventricular and supraoptic nuclei of the posterior hypothalamus, and by binding to the carrier protein neurohypophysin, is transported to the posterior pituitary. Vasopressin is synthesized and stored in approximately 2 hours, with a half life of 20–30 minutes, metabolized by vasopressinases in the liver and kidney. Vasopressin acts on V1, V2, V3, and oxytocin-type receptors. V1 receptors are located on the vasculature, myometrium, and platelets. V3 receptors are mainly found in the pituitary. V2 receptors are located along the distal tubule and collecting duct.

2. Although plasma osmolality is the most sensitive stimulus for vasopressin release, there are other concentration independent stimuli, including nausea, pain, hypoxia, acidosis, sympathetic nervous system activation and certain medications. In addition, circulatory hemodynamics is a particularly important osmolality-independent stimuli.  Sensed body volume, as determined by pressure receptors within the heart and along the carotid arteries, and flow receptors within the juxtaglomerular apparatus of the kidney, can modify vasopressin release.

This phenomenon is illustrated to the left. Sensed volume depletion, in the setting of true volume depletion (e.g., diarrhea or vomiting) or in the setting of true volume overload (e.g., heart failure and cirrhosis), both amplify vasopressin release per given plasma osmolality.   

3. Water permeability along the length of the renal tubule is determined by the presence of tight junctions that “water-seal” the space between neighboring epithelial cells, and the presence of aquaporin (AQP) water channels.  AQP channels are specific channels that allow water passage, but not other molecules, through a cell membrane.  AQPs therefore confer water permeability, and are particularly important to cells that need to move large volumes of water or need to provide a fast mechanism of water adjustment, such as the collecting duct of the kidney.  AQP3 and AQP4 are constitutively expressed in the proximal tubule.  AQP2 is unique however.  Its presence, or absence, along the apical membrane of the collecting duct is controlled by vasopressin, as seen below.

4. Vasopressin can alter the permeability of the collecting duct. Binding to the V2 receptor on the basolateral membranes of the collecting duct, vasopressin stimulates a cascade of intracellular steps that earmark and stabilize pre-stored AQP2 to the apical membrane, as illustrated below, and as seen in seconds 12-20 in the animation above.

In the presence of vasopressin, increased production of cAMP activates protein kinase A (PKA), which in turn phosphorylates stored AQP-containing vesicles, and targets them to the apical membrane, increasing its water permeability, and facilitating water reclamation from the lumen. AQP3 and AQP4, constituitively expressed on the basolateral membrane, allow water egress from the cell. In the absence of vasopressin, AQP2 is endocytosed and internally degraded, conferring water impermeability to the apical membrane, thereby maximizing water excretion.

Thus, in times of water conservation, vasopressin binds to V2 receptors, inducing AQP2 channel expression and consequent water retention, and in times of water excess, AQP2 channels retreat from the apical membrane due to vasopressin’s absence.

We previously described how the body senses and responds
to changes in plasma osmolality.  Next we turn to the
final steps of osmotic homeostasis: renal water retention or
excretion.  Having a highly concentrated medullary interstitium
is essential for water conservation, providing the osmotic
force for water egress from filtered renal tubular fluid.  The
medulla, reaching up to four times the concentration of the
surrounding interstitial fluid, is like a concentration oasis, a
pocket of hypertonic fluid within a deeply vascular organ
unprotected by a barrier epithelium.  The generation and
maintenance of the medullary interstitial gradient is one of the
fundamental teachings of renal physiology, and is discussed in the followed section.

GENERATION OF THE MEDULLARY INTERSTITIUM GRADIENT

Lets start off with a simple example. Imagine that you have a pipe embedded in a tank of fluid with a concentration of 300 particles/L (Figure A). The pipe is solid, impermeable to either particles or water. You purchase a special “particle pump” that extrudes particles from the pipeline. The pump has an intrinsic and limited power capacity. It can maintain a maximum concentration difference between inside and outside of the pipe of only 50 particles/L, at which point the pump goes into standby mode. The pump does not care what the starting concentration of fluid within the pipe and within the tank is.  If it starts pumping fluid with a concentration of 200 particles/L, it will stop once it reduces the concentration in the tube’s fluid to 150 particles/L; if it starts at 400 particles/L, it will stop when the concentration of the fluid is 350 particles/L.  It is the difference between the two fluid concentrations (inside versus outside the pipe) that is important.  In this example, since the fluid in the beginning of the pipe has a concentration of 300 particles/L, the pump will reduce the concentration in the fluid exiting the pipe to 250 particles/ L.

Is there a way to improve the efficiency of the system?  In other words, can we use the same pump to build a larger concentration gradient?  In Figure B, we place a bend in the pipeline, to form a hairpin loop with the particle pump in the ascending portion or limb of the loop.

The simple change in shape allows particles to accumulate around the tip of the loop, but does little to change the concentration within the pipe. Fluid with the same concentration (300 particles/L) will be delivered to the particle pump, generating the same gradient as before (50 particles/L).  Next, however, in addition to adding a hairpin loop, you place small holes in the descending limb of the loop, yet make no changes to the ascending limb, as seen in Figure C.

Initially, the pump will extrude particles, generating the same gradient of 50 particles/L. Since the particles accumulate around the hairpin loop, the concentration of the fluid in the tank will increase. Since the descending limb is now permeable, particles will move down its concentration gradient into the descending limb, and at the same time, water will move out of the descending limb toward the more concentrated area. This movement of particles in and water out intensifies the concentration of fluid within the descending limb.  Remember, fluid is constantly flowing within the pipeline.  Thus, the “more concentrated” fluid within the descending limb is then pushed forward into the ascending limb.  The pump will continue to generate its same gradient of 50 particles/L. It does not care, however , what the starting concentration is; the only thing that matters is the difference between the inside and outside of the tube.  Thus, whereas the particle pump initially received fluid with a concentration of 300 particles/L, it is now the recipient of “more concentrated” fluid.  Therefore, it can generate a higher concentration with the same amount of work.  The key to this hairpin loop is the following concept: by allowing particles to move in and water to move out of the descending limb, increasingly more concentrated fluid is delivered to the pump, which can then continue to build a more concentrated gradient.  This cycle continues and, over time, the concentration of the fluid in the pipe rises and substantially exceeds that of the initial fluid entering this system.  By placing a loop into the system and by altering the permeability of certain sections of the tube, the net effect of the particle pump is multiplied.  This process, known as “countercurrent multiplication”, occurs in the Loop of Henle, and is animated below.

Notice the following...
1. Seconds 3-8. Sodium ions moving from the thick ascending limb      across into the thin descending limb.
2. Seconds 10-11 Sodium ions entering the thin descending limb. As the filtrate moves down the permeable thin descending limb, sodium ions from the interstitium enter (and perhaps water molecules exit, not shown).  This functions to increase the concentration of the tubular fluid that is then delivered into the ascending limb.
3. Seconds 17-25. The thick ascending limb. As the filtrate moves into the impermeable thick ascending limb, Na+/K+/2Cl- (NK2Cl) cotransporters actively pump ions from the lumen into the intersitium.  This is illustrated below in figure 2.  In the animation, sodium ions are orange, chloride are blue, and potassium are purple.

4. Seconds 25-28 Notice that except for the ion passage through the NK2Cl cotransporter, neither ions nor water can cross the ascending limb due to its impermeability.
5. Seconds 28-34.  The sodium ions that are pumped out of the thick ascending limb recycle back to the descending limb, completing the countercurrent mechanism.  In these steps, the countercurrent gradient is built.

MAINTAINING THE MEDULLARY CONCENTRATION GRADIENT

In the previous section, we have discussed building a concentration gradient within the medullary interstitium.  Given the tremendous amount of blood flow to the kidney, which receives almost one quarter of the cardiac output, preventing the “wash-out” of this concentration maintaining is key.  In the next section, we will discuss the mechanisms that maintain the concentration gradient.

The unique structure of the nephron allows sufficient perfusion to support filtration, thereby ensuring adequate removal of potential toxins from circulating plasma, while at the same time, minimizing washout of the medullary gradient.  As seen in Figure 3, there are three major levels of glomeruli - superficial, midcortical, and deeper juxtamedullary. The blood supply for each level differs.  For the superficial and midcortical glomeruli, the efferent arteriole and its associated vasa recta, which supplies the corresponding renal tubule, flow into the arcuate veins at the corticomedullary junction, and exit the kidney without descending into the deeper medulla.  However, the vascular supply for the juxtamedullary glomeruli, along with the associated Loops of Henle, descend deep into the medulla.

Among these three different pathways, the vasa recta that supply the superficial and midcortical glomeruli are the most abundant in the kidney; it is in this region that we find most of the proximal tubules from which much of the fluid and electrolytes filtered in the glomerulus are reabsorbed.  Many fewer vasa recta descend into the medulla, as reflected by the distribution of blood flow within the kidney.  Of the total renal blood flow, almost 90% circulates through the superficial or midcortical glomeruli vasa recta, with only 10% reaching the medulla.  Only 1% to 2% of the total renal blood flow reaches the deepest parts of the medulla; this organization of the vasculature is important in protecting the medullary concentration gradient and also results in relative hypoxia in this region of the kidney.  

However, even with this important structural design, 2% of renal blood flow represents almost 30 liters of blood flow per day for most individuals, and thus an additional mechanism to prevent medullary washout is needed.  This next mechanism, termed the countercurrent exchange (different than the countercurrent multiplication discussed in the previous section) is perhaps most simply illustrated by the following example.

Have you ever wondered how penguins can walk on the ice barefooted?  Don’t they get cold?  What prevents them from losing all their body heat across their bare webbed feet? Let’s take a closer look at the penguin. As illustrated in Figure 4, the blood supply within a penguin’s webbed foot is in the shape of a hairpin loop. Unlike the hairpin loops in the collecting duct, which have an ascending impermeable section, the blood vessel is permeable. Heat moves passively and freely from warm areas to cold areas.  At the distal aspect of the web, exposed to the ice, cold meets the warm arterial blood as it is pumped from the systemic circulation, and heat is lost to the ice on which the penguin is standing. At the hairpin turn, the temperature of the animal’s blood has dropped significantly.  Yet, as the cold blood travels back up the venous limb, it is once again exposed to warmer arterial blood by the countercurrent flow set up in the hairpin design. Warmth flows from the warmer descending limb into the colder ascending limb, bypassing the most distal aspect of the loop.  As cold is kept contained in a distal circuit; heat loss to the ice is minimized.  A circular temperature flow loop arises proximal to the penguin’s foot, thereby allowing the cold to remain within the distal aspect of the hairpin loop, and ultimately protecting the animal’s body temperature.  The unique countercurrent design of this hairpin blood supply maintains a temperature gradient in the web foot, with the cold kept distally and warmth proximally, while at the same time perfusing the animal’s foot with blood.

In a similar manner, a countercurrent blood supply in the medulla maintains the concentration gradient while simultaneously perfusing important cells within the medullary interstitium.  Analogous to the flow of heat in the penguin, in whom the arterial blood becomes colder as it approaches the loop’s hairpin, water moves from the descending to the ascending limb of the vasa recta and the fluid becomes more concentrated as it descends into the inner medulla.

It should be noted that there are important differences between the countercurrent multiplier in the Loop of Henle (discussed in the previous section) and the countercurrent exchange of the vasa recta. Since the Loop of Henle is made of epithelial cells, depending on the presence of water channels and sodium transporters, there are significant permeability differences between the descending and ascending limbs.  This allows the “multiplication” of the concentration gradient generated by the thick ascending limb.  In contrast, the vasa recta is made up of thin endothelial cells, with equal permeability across all sections.  Thus, no gradient is built; instead an “exchange” provides a circular flow of water in the upper limbs of the Loop, which prevents the water from washing out the particles that are in high concentration at the capillary loop tip.

We have now described how a gradient of increasing tissue osmolarity is generated and maintained in the renal medulla.  Although it provides a force for water movement out of the tubule, this only occurs if vasopressin converts the collecting duct to a water permeable structure through the insertion of AQP2 channels.  Of course, vasopressin, and thirst, are orchestrated by the osmoreceptor, which responds to changes in osmolality.  You now understand the fundamental of water homeostasis. In the next section, we explore some clinical examples.

When applying the physiology of dysnatremias clinically, take a step back and focus on the concepts of osmolality and cell size.  Remember that every dysnatremia that results in an appreciable change in serum osmolality results in a change in cell size.

Neuron size is tightly guarded and even small changes can result in clinical neurologic deficits.  These signs and symptoms are similar, whether the neuron swells or shrinks, but the results can be catastrophic nonetheless.  

As a reminder, cell size is challenged by increases or decreases in serum osmolality.  Water moves toward the environment with the higher osmolality, causing cell swelling in hypoosmolality (hyponatremia) or cell shrinking in hyperosmolality (hypernatremia).  Cells must either take on or release active osmoles to achieve equilibrium with the osmolality of the extracellular environment, thereby returning to its normal size.

This video is another demonstration of how an individual neuron body swells in response to infusion of an active osmole. When an active osmole such as KCl is added to the intracellular space, you can see the whole body of the neuron swell as the water follows the solute into the cell body.

While this example demonstrates manipulation of the intracellular environment, the clinical presentations of acute hyponatremia and hypernatremia result from altering the extracellular environment.  The serum sodium help us determine serum osmolality, which in turn, will change cell size.

Hyponatremia

Let’s focus on the acute changes that occur in hyponatremia.  The following post-operative CT scan demonstrates massive cerebral edema in a young girl resulting from intra-operative leakage of 2 liters of sterile water from the bladder into the circulation.  

She became increasingly lethargic, irritable, disoriented, and ultimately stopped breathing.

The rapid absorption of hypotonic fluids into the circulating volume created a severely hypo-osmotic extracellular environment for the astrocytes (serum sodium measured at 120 mEq/L).  As a result, water moved across the osmotic gradient to the intracellular space, causing a rapid increase in cell size and ultimately fatal cerebral edema.

Let’s look at another example of hyponatremia. This case report shows how the severity of brain swelling improves with resolution of the hyponatremia.  At a serum concentration of 109, there is marked loss of border between the gray and white matter and effacement of the sulci due to  brain swelling.

As the serum sodium increases, the normal architecture of the brain returns.  Along the “Sylvian fissure” cross-section, you will notice the ventricles gradually increasing in size as the serum sodium improves.  On the “Frontal lobe” cross-section, you will also notice that the sulci become increasing visible as the sodium improves to normal and the gray matter begins to predominate again.  

Hypernatremia

In an acutely hyperosmolar extracellular environment, water leaves the intracellular space across the osmotic gradient to achieve osmolar equilibrium.  The resulting decreased neuron size within the brain causes rupture of the bridging veins and the small veins of the dural sinuses.  In the CT imaging below of a patient with a serum sodium of 214 mEq/L, intracerebral bleeding (white) is seen.

The two arrowheads in each panel show the bright white areas indicative of hemorrhage on a CT scan.  Panel A shows a subdural hemorrhage and Panel B shows an intraventricular hemorrhage, likely arising from shearing of the venous networks described above.

These MRI studies of a newborn with a serum sodium of 185 mEq/L (due to significant insensible losses and poor fluid intake/feeding) demonstrates once again the bright white attenuation consistent with hemorrhage.  

Panel 1 is a sagittal T1-weighted image showing a parieto-occipital hemorrhagic infarct. Panel 2 shows additional hemorrhages in coronal section.   

“New” symptoms of dysnatremias

Increasing attention is being paid to other symptoms associated with dysnatremia that may be rooted in neurologic dysfunction.  As we have seen above, the classical teaching is that the neurologic symptoms include nausea, vomiting, altered mental status, with seizures and coma complicating the most severe cases.  These newer studies mostly address hyponatremia, given its higher prevalance and show that even mild hyponatremia can result in neurologic consequences.

Given the association of falls in the elderly with hyponatremia (and subsequent long bone fractures), this study demonstrated abnormal walking patterns in three patients with “asymptomatic” mild-to-moderate hyponatremia, based on a 3-step, eyes-open, tandem walk. The arrows represent deviations along the path, which seemed to improve with resolution of the hyponatremia.

Summary

Acute changes in the serum osmolality require rapid adjustment of the intracellular environment to prevent sudden changes in cell size. The consequences of the cerebral edema from acute hyponatremia and intracranial hemorrhage of hypernatremia can be catastrophic, as described above.  

Thank you for completing the module.  We hope this review of renal physiology was helpful in your understanding of how our bodies achieve water homeostasis.  Please click on the “Review Questions” button above to launch a quiz that will help you review the key concepts addressed in the module.  

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