Pharm 107 - Electrolytes Pt. 1

Coverage: Body Fluids, Extra- and Intracellular Ions, Physiological Acid-Base Balance

Total Body Water (TBW)

• Represents ~60% (avg of 45%-75%) of adult body weight.

• Divided into Intracellular and Extracellular fluid compartments.

Intracellular Fluid (ICF)

• ~2/3 of TBW (~40% of body weight).

• Fluid inside cells; primary cations: K⁺ and Mg²⁺; primary anion: phosphate (PO₄³⁻).

Extracellular Fluid (ECF)

• ~1/3 of TBW (~20% of body weight).

• Divided into:

  • Interstitial Fluid

  • Vascular/Intravascular Fluid

Interstitial Fluid

• Fluid found in the spaces around the cells.

• ~75% of ECF; 12-15% TBW

Vascular Fluid

• Also called plasma, refers to the liquid component of your blood.

• ~25% of ECF; 4-5% TBW

Solid Body Mass

• Around 40% of the body weight (proteins, lipids, minerals).

Liquid Body Mass

• Around 60% of the body weight (TBW).

Water Content by Demographics

• Men: ~60% TBW.

• Women: ~50–55% (due to higher fat content).

• Infants: Up to 75% TBW.

• Elderly: Lower TBW due to reduced muscle mass.

Water Gain (Input)

• Metabolic Water: ~200 mL/day from cellular respiration.

• Ingested Liquids: ~1,600 mL/day.

• Moist Food: ~700 mL/day.

Water Loss (Output)

• Urine: ~1,500 mL/day (main source).

• Skin (sweat/evaporation): ~600 mL/day.

• Lungs (exhalation): ~300 mL/day.

• Feces: ~100 mL/day.

Fluid Balance

• Homeostasis between water gain and water loss.

• Maintained via thirst, kidney function, hormonal regulation (ADH, aldosterone, ANP).

Membranes

• Separates compartments from each other.

• Permeable to H₂O and many other organic and inorganic solutes.

• Selectively permeable to Na⁺, K⁺, Mg²⁺ but impermeable to macromolecules.

Osmosis

• Passive water movement through a semipermeable membrane from low to high solute concentration.

Osmolality

• Osmoles/kg of water (more clinically relevant).

Osmolarity

• Osmoles/L of solution.

Tonicity

Effect of a solution on cell volume:

• Isotonic: No net water movement.

• Hypotonic: Water moves into cells (cell swells).

• Hypertonic: Water moves out (cell shrinks).

Isotonic Cell

Hypotonic Cell

Hypertonic Cell

Hypothalamus

• Thirst Center of the Body.

• Activity is triggered by increased plasma osmolality or decreased blood volume and flow of saliva.

RAA System

• Stands for the Renin-Angiotensin-Aldosterone system.

• Hormone system that plays a crucial role in regulating blood pressure and fluid balance in the body.

Hormonal Regulation of Fluids

• Angiotensin II

• Aldosterone

• Atrial Natriuretic Peptide (ANP)

• Antidiuretic Hormone (ADH or Vasopressin)

Angiotensin II

• Created by the synthesis of Angiotensin I through the Angiotensin-Converting Enzyme.

• Tightens the blood vessels and stimulates adrenal glands, the latter of which signals the production of aldosterone.

Effect of Angiotensin II

• Reduces loss of water in urine.

• Increases blood pressure.

Aldosterone

• Promotes urinary Na⁺ and Cl^- reabsorption and increases water reabsorption via osmosis.

• Promotes the excretion of K⁺.

Effect of Aldosterone

• Reduces loss of water in urine.

Atrial Natriuretic Peptide (ANP)

• Promotes natriuresis, which is the increase in excretion of Na⁺ and Cl^-, accompanied by water.

• Leads to a decrease of blood volume.

Effect of ANP

• Increases loss of water in urine.

• Decreased blood volume.

Antidiuretic Hormone (ADH)

• Also called vasopressin.

• Promotes insertion of water-channel proteins (aquaporin-2) into apical ducts of kidneys.

• Results to increased water permeability of cells and more water reabsorption.

Effect of Vasopressin

• Reduces loss of water in urine via renal water reabsorption.

Goal if Dehydrated

• Decrease urine output.

• Increase angiotensin II, aldosterone, ADH, and thirst center activity.

Goal if Increased NaCl

• Natriuresis

• Increase atrial natriuretic peptide and more natriuretic activity.

Hypernatremia

• Happens if the body has too little water and too much sodium.

• Common when dehydrated.

Hyponatremia

• Happens when sodium content in blood is abnormally low.

• Common when overhydrated.