Lippincott Illustrated Reviews: Physiology (Lippincott Illustrated Reviews Series)

Osmosis and Body Fluids

3

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.

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 (π; measured in mm Hg) can be calculated from:

Equation 3.1

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).

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. MgCl₂dissociates 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.

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 (σ) to Equation 3.1 above.

π = σnCRT

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, σ 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.

2. 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.

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).

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.

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.

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.

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).

H+ is a common inorganic cation that is similar in many ways to Na⁺ and K+. It is attracted to and binds to anions, and it depolarizes cells when it crosses the plasma membrane. H⁺ deserves special consideration and cellular handling because its small atomic size allows it to form strong bonds with proteins. Such interactions alter a protein's internal charge distribution, weaken interactions between adjacent polypeptide chains, and cause conformational changes that may inhibit function such as hormone binding (Figure 3.8). High H⁺ concentrations denature proteins and cause cell degradation. Thus, the pH of fluid in which cells are bathed must be tightly controlled at all times.

Extracellular fluid (ECF) is Na⁺ rich, but it also contains a number of other charged solutes, or electrolytes, the bulk comprising common inorganic ions (K⁺, Ca²⁺, Mg²⁺, Cl⁻, phosphates, and HCO₃⁻). All cells are bathed in ECF. Because changes in the concentrations of any of these electrolytes can have significant effects on cell function, serum levels are maintained within a fairly narrow range, principally through modulation of kidney function (see Chapter 28). Blood tests typically include a standard electrolyte panel that measures serum Na⁺, K⁺, and Cl⁻ (Table 3.1). Serum Na⁺ and Cl⁻ levels are measured, in part, to assess kidney function, but also because they determine ECF osmolality and total body water. K⁺ is measured because normal cardiac function depends on stable serum K⁺ levels.

A. Acids

Blood has a pH of 7.4 and seldom varies by more than 0.05 pH units. This corresponds to a H⁺ concentration range of 35–45 nmol/L, which is impressive given that metabolism of carbohydrates, fats, and proteins pours ~22 moles of acid into the vasculature every day! Acid generated by metabolism comes in two forms: volatile and nonvolatile (Figure 3.9).

1. Volatile: The vast majority of daily acid output comes in the form of carbonic acid (H₂CO₃), which is created when CO₂ dissolves in water. CO₂ is generated from carbohydrates (such as glucose) during aerobic respiration (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O). Carbonic acid is known as a volatile acid because it is converted back to CO₂ and water in the lungs and then liberated to the atmosphere (see Figure 3.9).

2. Nonvolatile: Metabolism also generates smaller amounts (~70–100 mmol per day) of nonvolatile or fixed acid that cannot be disposed of via the lungs. Nonvolatile acids include sulphuric, nitric, and phosphoric acids, which are formed during catabolism of amino acids (e.g., cysteine and methionine) and phosphate compounds. Nonvolatile acids are excreted in urine (see Figure 3.9).

3. Range: Life can only exist within a relatively narrow pH range (pH 6.8–7.8, corresponding to a H⁺ concentration of 16–160 nmol/L), so excreting H⁺ in a timely manner is critical for survival. A decrease in plasma pH below 7.35 is called acidemia. Alkalemia is an increase in plasma pH above 7.45. Acidosis and alkalosis are more general terms referring to processes that result in acidemia and alkalemia, respectively.

B. Buffer systems

Cells produce acid continually. Their intracellular structures are protected from the deleterious effects of this acid by buffer systems, which immobilize H⁺ temporarily and limit its destructive effects until it can be disposed of. The body contains three primary buffer systems: the bicarbonate buffer system, phosphate buffer system, and proteins.

1. Bicarbonate: HCO₃⁻ is the body's primary defense against acid. HCO₃⁻ is a base that combines with H⁺ to form carbonic acid, H₂CO₃:

H₂CO₃ can then be broken down to form CO₂ and water, both of which are readily expelled from the body via the lungs and kidneys, respectively. Spontaneous conversion to CO₂ and H₂O occurs too slowly for the HCO₃⁻ buffer system to be of any practical use, but the reaction becomes essentially instantaneous when catalyzed by carbonic anhydrase (CA). CA is a ubiquitous enzyme expressed by all tissues, reflecting the central importance of the HCO₃⁻ buffer system.

There are at least 12 different functional CA isoforms, many of which are expressed in virtually all tissues. CA-II is a ubiquitous cytosolic isoform. CA-I is expressed at high levels in red blood cells, whereas CA-III is found primarily in muscle. CA-IV is a membrane-bound isoform that is expressed on the surface of pulmonary and renal epithelia, where it facilitates acid excretion.

2. Phosphate: The phosphate buffer system employs hydrogen phosphate to buffer acid, the end-product being dihydrogen phosphate:

HPO₄²− is used to buffer acid in the renal tubule during urinary excretion of nonvolatile acids.

3. Proteins: Proteins contain numerous H⁺-binding sites and, therefore, make a major contribution to net intracellular and extracellular buffering capacity. One of the most important of these is hemoglobin (Hb), a protein found in red blood cells (RBCs), that buffers acid during transit to the lungs and kidney.

C. Acid handling

Most acid is generated intracellularly at sites of active metabolism and then is transported in the vasculature to the lungs and kidneys for disposal. pH is carefully controlled by buffers and pumps at all stages of handling.

1. Cells: Intracellular structures are shielded from locally produced acid by buffers, the most important being intracellular proteins and HCO₃⁻. Cells also actively control their internal pH using transporters, although the pathways involved in cellular pH control have not been well delineated.

a. Acid: Most cells express a Na⁺-H⁺ exchanger to expel acid and can also take up HCO₃⁻ from the ECF if the need arises, via Na⁺-coupled Cl⁻-HCO₃⁻ exchange (Figure 3.10A).

b. Base: Most cells also express a Cl⁻-HCO₃⁻ exchanger to expel excess base. Alkalosis simultaneously suppresses Na⁺-H⁺ exchange to help lower intracellular pH (see Figure 3.10B).

2. Lungs: CO₂ produced by cells during aerobic respiration rapidly diffuses across the cell membrane and crosses through the interstitium to the vasculature. RBCs express high levels of CA-II, which facilitates conversion of CO₂ and H₂O to HCO₃⁻ and H⁺ (see 23·VII). H⁺ then binds to Hb for transit to the lungs. Pulmonary epithelia also contain high levels of CA, which facilitates conversion back to CO₂for transfer to the atmosphere (see Figure 3.9).

3. Kidneys: H⁺ that is formed from protein metabolism (nonvolatile acid) is pumped into the renal tubule lumen and excreted in urine as shown in Figure 3.9 (also see 28·V). The urinary epithelia are protected during excretion by buffers, primarily phosphate and ammonium, which the renal tubule secretes specifically for this purpose. Nonvolatile acid is generated at distant sites, however, and the cells responsible must be protected from this acid until transport to the kidney can be arranged. Thus, the renal epithelium also expresses high levels of CA-IV, which generates HCO₃⁻ and releases it to the vasculature for transport to the sites of acid generation (see Figure 3.9). H⁺ that is formed during HCO₃⁻ synthesis is pumped into the tubule lumen and excreted.

• The human body is composed largely of water that distributes between three principal compartments: intracellular fluid, interstitial fluid, and plasma. The latter two compartments together comprise extracellular fluid. Water movement between these compartments occurs principally by osmosis.

• Osmosis is driven by osmotic pressure gradients that are created by local differences in solute particle number. Water moves from regions containing low particle number toward regions with high particle numbers, generating osmotic pressure.

• Osmolarity and osmolality measure a solute's ability to generate osmotic pressure, whereas tonicity is governed by a solution's effect on cell volume.

• Most cells contain channels (aquaporins) that allow water to move easily between intracellular fluid and extracellular fluid (ECF) in response to transmembrane osmolality gradients. Increases in ECF osmolality cause water to leave the cell and its volume decreases. Cells respond by accumulating solutes (Na⁺, Cl⁻, and amino acids) to recruit water from the ECF by osmosis (a regulatory volume increase). Cell volume increases elicit a regulatory volume decrease, involving volume-activated K⁺ and Cl⁻ channel opening and secretion of small organic solutes (amino acids and polyalcohols).

• Regulatory volume changes allow cells to control intracellular water content. Kidney function is modulated to control total body Na⁺ content, which, in turn, determines how much water is retained by extracellular fluid (ECF). Plasma proteins determine how much of this ECF is retained by the vasculature.

• All cells rely on buffer systems to maintain the pH of intracellular and extracellular fluids within a narrow range. Acid is produced continually as a result of carbohydrate metabolism and amino acid catabolism. Carbohydrate metabolism yields CO₂, which dissolves in water to form carbonic acid (a volatile acid). Amino acid breakdown yields sulphuric and phosphoric acids (nonvolatile acids).

• The bicarbonate buffer system represents the body's primary defense against acid. The buffer system relies on the ubiquitous enzyme carbonic anhydrase to facilitate bicarbonate formation from CO₂ and water. Volatile acid is expelled as CO₂ from the lungs, whereas nonvolatile acid is excreted in urine by the kidneys.