Solutions, Solvents, and Tonicity

• A solution is a stable mixture of two or more substances in a single phase that cannot be separated using a centrifuge.
• A solute is a substance dissolved in a solution.
• A solvent is any liquid in which another substance can be dissolved.
• Solutions can exert pressure.
• Semipermeable membranes allow solvent molecules to pass, for example, capillary walls.
• Osmotic pressure of solutions redistributes solvent molecules for same concentration on both sides of a membrane.
• Water is distributed in the body via osmosis.
• Tonicity describes the osmotic force exerted by a solution.
• Isotonic solutions have a tonicity similar to that of body solutions, such as a 0.9% solution of NaCl (normal saline).
• Hypertonic solutions have greater tonicity than 0.9% NaCl, drawing fluids out of cells.
• Hypotonic solutions have less tonicity than 0.9% NaCl, causing water to be drawn into cells.
• Most cells exist in a hypotonic environment.
• Various tonicities facilitate the movement of water, nutrients, and other substances through the body.
• Solute content and activity are quantified by actual weight in grams or milligrams, or by equivalent weights.
• Equivalent weights define amounts of substances having equal chemical combining forces, generally stated in gram equivalent weight (gEq) or milliequivalent (mEq).
• Concentrations of most chemicals in the body are small, so mEq is used to express the minute values.

Transport Between Compartments

• Homeostasis depends on body fluids and their transport.
• Passive diffusion occurs from capillaries to the interstitial space due to capillary permeability and the hydrostatic pressure gradient at the arterial end.
• Proteins exert osmotic pressure, tending to draw fluid back into the capillary.
• Alveoli must be "dry" for efficient gas exchange.
• Fluid accumulation thickens the interstitial space.
• Osmotic pressures are typically higher than hydrostatic pressures along the entire length of the capillary bed.
• In pathology, if hydrostatic pressures exceed osmotic pressures, it can lead to interstitial or alveolar edema.
• Left ventricular failure is the most common cause of edema.
• Electrolytes maintain the internal environment and facilitate chemical and physiological events.
• Seven major electrolytes include sodium, chloride, bicarbonate, and potassium.

Sodium

• Normal sodium levels are between 135-145 mEq/L
• Low sodium is hyponatremia, which can be caused by fluid loss from diuretics, vomiting, diarrhea, or fluid gain from congestive heart failure and intravenous therapy
• High sodium is hypernatremia, often due to dehydration
• Symptoms of low sodium include decreased skin turgor and low blood pressure
• Skin turgor is when skin remains elevated after being pinched

Sodium

• Hyponatremia is the most common electrolyte imbalance found in hospitalized patients.
• Hyponatremia is defined as serum Na+ levels.