Chapter 3: Osmosis and Body Fluids



I. Overview

One of the more memorable quotes from the popular television series Star Trek: The Next Generation came from a silicon-based alien life form that referred to the intrepid Captain Picard as an “ugly bag of mostly water.” The average human body comprises 50%–60% water by weight, depending on body composition, gender, and age of the individual. The proportion of water in cells is even greater (~80%) as shown in Figure 3.1, the remainder largely comprising proteins. Water is the universal solvent, facilitating molecular interactions, biochemical reactions, and providing a medium that supports molecular movement between different cellular and subcellular compartments.

The biochemistry of life is highly sensitive to solute concentration, which, in turn, is determined by how much water is contained within a cell. Thus, the autonomic nervous system (ANS) closely monitors total body water (TBW) and adjusts intake and output pathways (drinking and urine formation, respectively) to maintain water balance (see 28?II). Although TBW is tightly regulated, water moves freely across cell membranes and between the body’s different fluid compartments.

Loss of water from the cell raises intracellular solute concentrations and, thereby, interferes with normal cell function. The body does not contain a transporter capable of redistributing water between compartments, so its approach to water management at the cellular and tissue level is to manipulate solute concentrations within intracellular fluid (ICF), extracellular fluid (ECF), and plasma. This approach is effective because water is enslaved to solute concentration by osmosis.

Figure 3.1 Cellular composition.

II. Osmosis

Osmosis describes a process by which water moves passively across a semipermeable membrane, driven by a difference in water concentration between the two sides of the membrane. Pure water has a molarity of ;55 moles/L. Although cells do not contain pure water, it is nevertheless a superabundant chemical. The concentration difference required to generate physiologically significant water flow across membranes is very small, so, in practice, it is far more convenient to discuss osmosis in terms of the amount of pressure that water is capable of generating as it moves down its concentration gradient. Thus, a chemical concentration gradient becomes an osmotic pressure gradient.

A. Osmotic pressure

Osmotic pressure gradients are created when solute molecules displace water, thereby decreasing water concentration. An apparent peculiarity of the process is that pressure is determined entirely by solute particle number and is largely independent of the size, mass, chemical nature of the solute, or even its electrical valence. Therefore, two small ions such as Na+ generate a higher osmotic pressure than a single complex glucose polymer such as starch (MW ;40,000) as shown in Figure 3.2. The osmotic pressure of a solution (?; measured in mm Hg) can be calculated from:

where n is the number of particles that a given solute dissociates into when in solution, C is solute concentration (in mmol/L), and R and T are the universal gas constant and absolute temperature, respectively. Osmotic pressure can be measured physically as the amount of pressure required to precisely counter water movement between two solutions with dissimilar solute concentrations (Figure 3.3).

Figure 3.2 Osmosis.

Figure 3.3 Osmotic pressure.

B. Osmolarity and osmolality

Osmolarity is a measure of a solute’s ability to generate osmotic pressure that takes into account how many particles a solute dissociates into when dissolved in water. Glucose does not dissociate in solution, so a 1-mmol/L glucose solution has an osmolarity of 1 milliOsmole (mOsm). NaCl dissociates into two osmotically active particles in solution (Na+ and Cl?) and, thus, a 1 mmol/L–NaCl solution has an osmolarity of ~2 mOsm. MgCl2 dissociates into three particles (Mg2+ + 2Cl?) and, thus, a 1 mmol/L–MgCl2 solution has an osmolarity of 3 mOsm.

Osmolality is an almost identical measure to osmolarity but uses water mass in place of volume (i.e., Osm/kg H2O). A liter of water has a mass of 1 kg at 4°C, but water volume increases with temperature, which causes osmolarity to fall slightly. Because mass is invariant, Osm/kg H2O is the preferred unit for use in discussions of human physiology.

C. Tonicity

Tonicity measures a solute’s effect on cell volume, the term recognizing that membrane-permeant solutes cause cells to shrink or swell through effects on ICF osmolality.

1. Nonpermeant solutes

Sucrose cannot cross the plasma membrane of most cells. Therefore, if a cell is placed in a sucrose solution whose osmolality matches that of the ICF (300 mOsm/kg H2O), cell volume will remain unchanged because the solution is isotonic (Figure 3.4A). Volume changes only occur when there is an osmotic gradient across the plasma membrane that forces water to enter or leave the cell.

Figure 3.4 Tonicity.

All osmolality values are in mOsm/kg H2O.

Note that ICF typically has an osmolality of 290 mOsm/kg H2O in vivo. The value of 300 mOsm/kg H2O used in this and the following examples is for ease of illustration only.

A 100–mOsm/kg H2O sucrose solution is hypotonic compared with the ICF. Water molecules will migrate across the membrane from ECF to ICF following the osmotic gradient, and the cell will swell (see Figure 3.4B). Conversely, a 500–mOsm/kg H2O sucrose solution is hypertonic: Water will be drawn out of the cell by osmosis, causing the cell to shrink (see Figure 3.4C).

2. Permeant solutes

Urea is a small (60 MW) organic molecule that, unlike sucrose, readily permeates the membranes of most cells via a urea transporter (UT). Thus, although 300–mOsm/kg H2O urea and 300–mOsm/kg H2O sucrose have identical osmolalities (i.e., they are isosmotic), they are not isotonic. When a cell is placed in a 300–mOsm/kg H2O urea solution, urea crosses the membrane via UT and raises ICF osmolality. Water then follows urea by osmosis, and the cell swells. A 300–mOsm/kg H2O urea solution is, thus, considered to be hypotonic.

3. Mixed solutions

A solution containing 300 mOsm/kg H2O urea plus 300 mOsm/kg H2O sucrose has an osmolarity of 600 mOsm/kg H2O and is, thus, hyperosmotic relative to the ICF. It is also functionally isotonic, however, because urea rapidly crosses the membrane until the intracellular and extracellular urea concentrations equilibrate at 150 mOsm/kg H2O. With solution osmolality on both sides of the membrane now standing at 450 mOsm/kg H2O, the driving force for osmosis is zero, and cell volume remains unchanged.

4. Reflection coefficient

When calculating the osmotic potential of a solution that bathes a cell, it is necessary to add a reflection coefficient (?) to Equation 3.1 above.

The reflection coefficient is a measure of the ease with which a solute can traverse the plasma membrane. For highly permeant solutes such as urea, ? approaches 0. The reflection coefficient for nonpermeant solutes (such as sucrose and plasma proteins) approaches 1.0.

D. Water movement between intracellular and extracellular fluids

The plasma membrane’s lipid core is hydrophobic, but water enters and exits the cell with relative ease. Some water molecules slip between adjacent membrane phospholipid molecules, whereas others are swept along with solutes in ion channels and transporters. Most cells also express aquaporins (AQPs) in their surface membrane, large tetrameric proteins that form water-specific channels across the lipid bilayer. AQPs, unlike most ion channels, are always open and water permeable (see 1?V?A).

E. Cell volume regulation

ECF solute composition is maintained within fairly narrow limits by the pathways involved in TBW homeostasis (see 28?II), but ICF osmolality changes constantly with changing activity levels. When cell metabolism increases, for example, nutrients are absorbed, metabolic waste products accumulate, and water moves into the cell by osmosis, causing it to swell.

Cells that exist on the boundary between the internal and external environment (e.g., intestinal and renal epithelial cells) are also subject to acute changes in extracellular osmolality, causing frequent changes in cell volume. The mechanisms by which cells sense and transduce volume changes are still not well defined, but they respond to osmotic shrinkage and swelling by enacting a regulatory volume increase (RVI) or a regulatory volume decrease (RVD), respectively.

1. Regulatory volume increase

When ECF osmolality rises, water is drawn out of the cell by osmosis, and it shrinks. The cell responds with an RVI, which, in the short term, involves accumulation of Na+ and Cl? through increased Na+-H+ exchanger and Na+-K+-2Cl? cotransporter activity (Figure 3.5). Na+ and Cl? uptake raises ICF osmolality and restores cell volume by osmosis. In the longer term, cells may accumulate small organic molecules, such as betaine (an amino acid), sorbitol, and inositol (polyalcohols) to maintain increased ICF osmolality and retain volume.

Figure 3.5 Regulatory volume increase.

Regulatory volume decrease

Cell swelling initiates an RVD, which principally involves K+ and Cl? efflux via swelling-activated K+ channels and Cl? channels. The resulting fall in ICF osmolality causes water loss by osmosis, and cell volume renormalizes. Cells may also release amino acids (principally glutamate, glutamine, and taurine) as a way of reducing their osmolality and volume.

III. Body Fluid Compartments

A 70-kg male contains 42 L of water, or around 60% of total body weight. Females generally have less muscle and more adipose tissue as a percentage of total body mass than do males. Because fat contains less water than muscle, their total water content is correspondingly lower (55%). TBW usually decreases with age in both sexes due to loss of muscle mass (sarcopenia) associated with aging.

A. Distribution

Two thirds of TBW is contained within cells (ICF = ~28 L of the 42 L cited above). The remainder (14 L) is divided between the interstitium and blood plasma (Figure 3.6).

Figure 3.6 Total body water distribution.

1. Plasma

The cardiovascular system comprises the heart and an extensive network of blood vessels that together hold ~5 L of blood, a fluid composed of cells and protein-rich plasma. Approximately 1.5 L of total blood volume is contained within blood cells and is included in the value given for ICF above. Plasma accounts for 3.5 L of ECF volume.

2. Interstitium

The remaining 10.5 L of water resides outside the vasculature and occupies spaces between cells (the interstitium). Interstitial fluid and plasma have very similar solute compositions because water and small molecules move freely between the two compartments. The main difference between plasma and interstitial fluid is that plasma contains large amounts of proteins, whereas interstitial fluid is relatively protein free.

Clinical Application 3.1: Hyponatremia and Osmotic Demyelination Syndrome

Hyponatremia is defined as a serum Na+ concentration of 135 mmol/L or less. Patients who develop hyponatremia usually have an impaired ability to excrete water, often due to an inability to suppress antidiuretic hormone (ADH) secretion. Hyponatremia with appropriate ADH suppression is also seen with advanced renal failure and low dietary sodium intake. Normally, the kidneys can excrete 10–15 L of dilute urine per day and maintain normal serum electrolyte levels, but higher flow rates may exceed their solute resorptive capabilities, and hyponatremia ensues.

Because Na+ is the primary determinant of extracellular fluid (ECF) osmolality, hyponatremia creates an osmotic shift across the plasma membrane of all cells and causes them to swell. Hyponatremic patients may develop severe neurologic symptoms (i.e., lethargy, seizures, coma), which typically only occur with acute and severe hyponatremia (serum sodium concentration <120 mmol/L), and rapid correction with hypertonic saline is necessary in this clinical scenario. Hyponatremia that develops slowly and chronically (more commonly the case) allows time for a regulatory volume decrease, and severe symptoms may be delayed until serum Na+ levels fall even further.

When hyponatremia has developed slowly, and a patient has no neurologic symptoms, correction to normal serum sodium levels must also be undertaken slowly to avoid a treatment complication known as the osmotic demyelination syndrome ([ODS] formerly called central pontine myelinolysis). ODS occurs when a too-rapid rise in ECF Na+ concentration creates an osmotic gradient that draws water from neurons before they have a chance to adapt, causing cell shrinkage and demyelination (myelin is a lipid-rich layered membrane that electrically insulates axons to enhance their conduction velocity; see 5?V?A).

ODS may manifest as confusion, behavioral changes, quadriplegia, difficulties with speech or swallowing (dysarthria and dysphagia, respectively), or coma. Because these devastating changes may not be reversible, the maximum rate of correction in stable patients with chronic hyponatremia should not exceed ~10 mmol/L in the first 24 hours.

Osmotic demyelination in the pons region of the brain.

Photograph from Eisenberg, R.L. An Atlas of Differential Diagnosis. Fourth Edition. Lippincott Williams & Wilkins, 2003.

A variable amount of fluid is held behind cellular barriers that separate it from plasma and interstitial fluid (transcellular fluid). This includes cerebrospinal fluid, fluid within the eye (aqueous humor), joints (synovial fluid), bladder (urine) and intestine. Transcellular fluid volume averages between 1–2 L and is not considered in calculations of TBW.

B. Restricting water movement

Water moves freely and rapidly across membranes and capillary walls, which creates the possibility of one fluid compartment (the ICF, for example) becoming hypohydrated or hyperhydrated relative to the other compartments to the detriment of body function (Figure 3.7). Thus, the body puts mechanisms in place that independently control the water content and that limit net water movement between the ICF, ECF, and plasma.

Figure 3.7 Movement between fluid compartments during dehydration.

1. Intracellular fluid

ICF osmolality typically averages ~275–295 mOsm/kg H2O, due primarily to K+ and its associated anions (Cl?, phosphates, and proteins). The ICF’s K+-rich composition is due to the plasma membrane Na+-K+ ATPase, which concentrates K+ within the ICF and expels Na+. Net water loss or accumulation from the interstitium is prevented by regulatory volume increases and decreases, respectively, as discussed above.

2. Extracellular fluid

Plasma and interstitial fluid also have an osmolality of ~275–295 mOsm/kg H2O, but principal solutes here are Na+ and its associated anions (Cl? and HCO3 ?). ECF water content is tightly controlled by centrally located osmoreceptors acting through antidiuretic hormone (ADH). When TBW falls as a result of excessive sweating, for example (see Figure 3.7, panel 1), ECF osmolality rises because its solutes have concentrated.

The rise in osmolality draws water from ICF by osmosis (see Figure 3.7, panel 2) and triggers a RVI in all cells, but not before the central osmoreceptors have initiated ADH release from the posterior pituitary as shown in Figure 3.7, panel 3 (also see 28?II?B). ADH stimulates thirst and enhances AQP expression by the renal tubule epithelium, permitting increased water recovery from urine. TBW and ECF osmolality are restored to normal as a result (see Figure 3.7, panel 4). When TBW is too high, AQP expression is suppressed, and the excess water is expelled from the body.

3. Plasma

Plasma is the smallest but also the most vital of the three internal fluid compartments. The heart absolutely depends on blood volume to generate pressure and flow through the vasculature (see 18?III). Plasma volume must be preserved even if ECF volume is falling due to prolonged sweating or reduced water ingestion, for example. The body cannot regulate plasma volume directly because most small blood vessels (capillaries and venules) are inherently leaky and, thus, plasma and interstitial fluid (the two ECF components) are always in equilibrium with each other.

The solution to maintaining adequate plasma volume lies with plasma proteins, such as albumin, which are synthesized by the liver and remain trapped in the vasculature by virtue of their large size. Here, they exert an osmotic potential (plasma colloid osmotic pressure) that draws fluid from the interstitium, regardless of changes in bulk ECF osmolality or ECF volume depletion as shown in Figure 3.7, panel 2 (also see 19?VII?A).