Egans+Ch.13

Chapter 13: Solutions, Body Fluids, and Electrolytes

Definition of a Solution

  • Solution: A stable mixture of two or more substances in a single phase that cannot be separated using a centrifuge.

  • Solute: A substance dissolved in a solution.

    • Example: Sugar in coffee.

  • Solvent: Any liquid in which another substance can be dissolved.

Osmotic Pressure of Solutions

  • Solutions have the ability to exert pressure, essential in various biological processes.

  • Semipermeable membrane: A barrier that allows solvent molecules to pass while restricting solute molecules.

    • Example: Capillary wall.

Oncotic and Hydrostatic Pressure

  • Oncotic Pressure:

    • The pressure exerted by proteins in the blood that draws fluid into the capillaries.

  • Hydrostatic Pressure:

    • The pressure that pushes fluid out of the capillaries.

  • Initial Conditions:

    • Balanced gradients where hydrostatic pressure in capillaries is countered by oncotic pressure.

    • The concentration of proteins in capillaries prevents the transcapillary escape of interstitial fluid, maintaining a net movement of fluid that is zero.

Osmotic Pressure and Tonicity

  • Osmotic (oncotic) pressure of solutions:

    • Redistributes solvent molecules to achieve the same concentration on both sides of a membrane.

    • Water distribution in the body occurs via osmosis.

  • Tonicity: Describes the osmotic force exerted by a solution.

    • Isotonic: Solutions with tonicity similar to body solutions (e.g., 0.9% NaCl solution—normal saline).

    • Hypertonic: Solutions with greater tonicity than 0.9% NaCl, which draw fluids out of the cells.

    • Hypotonic: Solutions with less tonicity than 0.9% NaCl, which draw water into the cell.

    • Most cells exist in a hypotonic environment to facilitate movement of water, nutrients, and other substances through the body.

Quantifying Solute Content & Activity

  • Two methods to quantify:

    • Actual Weight in g or mg.

    • Equivalent Weights: Defines amounts of substances with equal chemical combining forces, generally stated in gram equivalent weight (gEq) or milliequivalent (mEq).

Milligram Equivalent Weights

  • Concentrations of most chemicals in the body are typically small.

    • mEq is used to express these minute values.

Electrolytic Activity and Acid-Base Balance

  • pH Scale:

    • Used to describe the concentration of H+ in solution.

    • A pH > 7 indicates a more alkaline solution (less acid than water).

    • A pH < 7 indicates a more acidic solution (more acid than water).

Body Fluids and Electrolytes

  • Transport Between Compartments:

    • Homeostasis relies on body fluids and their transport.

    • Passive diffusion from capillaries to the interstitial space occurs due to capillary permeability and hydrostatic pressure gradients at the arterial end.

    • Proteins such as albumin exert osmotic pressure, tending to draw fluid back into capillary (~25 mm Hg).

Pulmonary Fluid Exchange

  • Alveoli must remain “dry” for efficient gas exchange.

    • Excess fluid thickens the interstitial space.

  • Pulmonary Hydrostatic Pressures: Typically low.

  • Osmotic Pressures: Higher than hydrostatic pressures along the entire length of the capillary bed.

  • In pathological conditions, hydrostatic pressures may exceed osmotic pressures, leading to interstitial or alveolar edema.

    • Most common cause: Left ventricular failure (congestive heart failure).

Major Electrolytes in the Body

  • Electrolytes maintain the internal environment and facilitate chemical and physiological events.

  • Seven Major Electrolytes:

    • Sodium (Na+)

    • Chloride (Cl-)

    • Bicarbonate (HCO3-)

    • Potassium (K+)

Sodium (Na+)

  • Normal Range: 135-145 mEq/L

  • Hyponatremia:

    • Low sodium; defined as serum Na+ levels < 135 mEq/L.

    • Causes include fluid loss from diuretics, vomiting, diarrhea, fluid gain from congestive heart failure (CHF), and IV therapy.

    • Symptoms include decreased skin turgor and decreased blood pressure.

  • Hypernatremia:

    • High sodium; typically due to dehydration.

Skin Turgor

  • Reduced skin turgor is indicated when skin remains elevated after being pulled up and released.

Sodium Imbalances: Hyponatremia

  • Most common electrolyte imbalance found in hospitalized patients.

  • Defined as serum Na+ levels < 135 mEq/L.

  • Even mild cases can significantly impact:

    • Cognitive function.

    • Gait stability (potential contribution to falls).

  • Can lead to cerebral edema due to changes in osmotic pressure.

  • **Common Causes of Acute Hyponatremia: **

    • Postoperative iatrogenic causes.

    • Self-induced due to water intoxication.

Serum Electrolytes: Chloride (Cl-)

  • The most common anion in the extracellular space.

  • Changes in serum Cl- levels follow changes in serum sodium levels.

  • Normal Range: 80 – 100 mEq/L

  • Hypochloremia: associated with metabolic alkalosis

  • Hyperchloremia: associated with metabolic acidosis


Bicarbonate normal range 22-26 mEq/L

Potassium- about 90% of total body potassium in intracelluar

Normal range: 3.5-4.5