RCP 100 Ch. 13

• Chapter 13 focuses on solutions, body fluids, and electrolytes.

Definition of a Solution

• A solution is a stable mixture of two or more substances in a single phase and 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.

Osmotic Pressure of Solutions

• Solutions have the ability to exert pressure.
• Semipermeable membranes allow solvent molecules to pass and an example of this is a capillary wall.

Osmotic Pressure

• Osmotic (oncotic) pressure redistributes solvent molecules for the equal 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 similar tonicity to body solutions, like a 0.9% NaCl solution (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, drawing water into cells.
• Most cells exist in a hypotonic environment.
• Various tonicities facilitate the movement of water, nutrients, and other substances through the body.

Quantifying Solute Content & Activity

• Solute content and activity can be quantified through actual weight in grams (g) or milligrams (mg).
• Equivalent weights define amounts of substances with equal chemical combining forces.
• Equivalent weights are stated in gram equivalent weight (gEq) or milliequivalent (mEq).
• Concentrations of most chemicals in the body are small, hence mEq is used.

Electrolytic Activity and Acid-Base Balance

• The pH scale describes the concentration of H+ in a solution.
• A pH >7 has less acid or more base than water (alkaline).
• A pH <7 has more acid or less base than water (acidic).

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 hydrostatic pressure gradient at the arterial end.
• Proteins (albumin) exert osmotic pressure, tending to draw fluid back into the capillary (~25 mm Hg).

Pulmonary Fluid Exchange

• Alveoli must be "dry” for efficient gas exchange because fluid thickens the interstitial space.
• Pulmonary hydrostatic pressures are low.
• Osmotic pressures are higher than hydrostatic along the entire length of the capillary bed.
• In pathology, hydrostatic pressures may exceed osmotic pressures, leading to interstitial or alveolar edema.
• Left ventricular failure (CHF) is cited as the most common cause.

Electrolytes

• Electrolytes maintain the internal environment, facilitating chemical and physiological events.
• Seven major electrolytes include:
• Sodium
• Chloride
• Bicarbonate
• Potassium

Sodium

• Normal sodium levels are 135-145 mEq/L.
• Low sodium is hyponatremia; caused by fluid loss from diuretics, vomiting, diarrhea, or fluid gain from CHF and IV therapy, Decreased skin turgor & blood pressure, may also be present.
• High sodium is hypernatremia, caused by dehydration.

Sodium Imbalances

• Hyponatremia is the most common electrolyte imbalance in hospitalized patients.
• Defined as serum Na+ levels <135 mEq/L.
• Mild cases can impact cognitive function and gait stability.
• Hyponatremia can lead to cerebral edema due to changes in osmotic pressure.
• The two most common causes of acute hyponatremia include postoperative iatrogenic causes and self-induced water intoxication.
• Treatment of hypovolemic hyponatremia can have substantial consequences.
• If fluid is administered too quickly, damage to the central nervous system can occur.
  • Significant Na+ fluid shifts and rapid changes in cellular volume can lead to cell damage and cell death (apoptosis).

Serum Electrolytes

• Chloride is the most common anion in the extracellular space, and changes follow changes in serum sodium levels.
• Normal chloride levels are 80–100 mEq/L.
• Hypochloremia is associated with metabolic alkalosis.
• Hyperchloremia is associated with metabolic acidosis.

Bicarbonate

• Normal range is 22–26 mEq/L.
• Bicarbonate (HCO3–) is the next most important anion after chloride.
• It's evenly distributed between intracellular and extracellular spaces.
• Plays essential role in acid-base balance.
• Act as strong base bicarbonate/carbonic acid buffer pair.
• It's the primary method of CO2 transport.
• Kidneys reabsorb almost all HCO3–.
• In respiratory acidosis, kidneys retain HCO3– to buffer extra acid due to CO2 retention, a slow process taking hours or days.
• In respiratory alkalosis, the kidneys will eliminate HCO3– in the urine.

Potassium

• About 90% of total body potassium (K+) is intracellular.
• Normal potassium levels are 3.5–4.5 mEq/L.
• Decreased potassium is hypokalemia.
• May be due to metabolic alkalosis, renal loss, or vomiting.
• Can cause flattened T waves on an EKG.
• Increased potassium is hyperkalemia.
• Usually due to kidney failure.
• Can cause spiked T waves on an EKG and may cause paralysis.

Total Serum Carbon Dioxide

• Total Serum Carbon Dioxide contains carbon dioxide in the form of dissolved carbon dioxide (CO2).
• It's a major buffering system controlling the body's acid-base milieu, 23-30 normal value.