F L U I D S a n d E L E C T R O L Y T E S

Major Fluid Compartments of the Body

• The body is divided into two primary fluid compartments:     * Intracellular Fluid (ICF): This fluid resides inside all different types of cells. It accounts for approximately $40\%$ of total body weight.     * Extracellular Fluid (ECF): This fluid resides outside of the cells. It accounts for approximately $20\%$ of total body weight.

• Sub-compartments of Extracellular Fluid:     * Intravascular Fluid: Fluid found inside the blood vessels, such as blood and plasma.     * Interstitial Fluid: Fluid that surrounds the cells within the tissues. It is not inside the cells but occupies the space between them. When fluid builds up here, it results in swelling or edema.     * Transcellular Fluid: A third sub-compartment, though it is discussed less frequently in this foundational context.

• Third Spacing:     * In clinical practice, the interstitial space is often referred to as the "third space."     * Third Spacing occurs when fluid accumulates in the interstitial space, presenting clinically as edema or swelling.

Movement of Fluids and Particles

• Fundamental Goal: The movement of fluids and particles is designed to maintain Homeostasis, a state of balance within the body.

• Regulation:     * Body fluid is regulated primarily in the extracellular compartment and indirectly in the intracellular compartment, largely by the kidneys.     * Fluids and particles move across cell membranes and capillary membranes located throughout the body and within renal tubules.

• Mechanisms of Transport:     * Osmosis: The movement of water from an area of low particle concentration to an area of high particle concentration to dilute it and achieve balance. In osmosis, water is the active mover.     * Diffusion: The movement of particles (solutes) from an area of high concentration to an area of low concentration.     * Filtration and Reabsorption: Mechanisms facilitated primarily by the kidneys to move fluids, though these are not the primary focus of this specific unit.

Pressures and Concentration Measurement

• Osmotic Pressure:     * Generated as water moves through a membrane.     * Termed a "pulling pressure" because it pulls water across the membrane toward higher particle concentrations.

• Hydrostatic Pressure:     * Termed a "pushing pressure."     * In the arterial system, hydrostatic pressure is greater due to the contraction of the heart, pushing fluid out of vessels into surrounding tissues or cells.     * In the venous system, osmotic pressure becomes greater than hydrostatic pressure, facilitating the "pulling" of fluid back into the venules and veins for return to the heart.

• Serum Osmolarity:     * Refers to the concentration of solutes in the blood ("serum" means blood).     * High Serum Osmolarity: Indicates the blood is highly concentrated with solutes and lacks sufficient water.     * Low Serum Osmolarity: Indicates the blood is dilute with a high water-to-solute ratio.     * A normal serum osmolarity value is approximately $300\,mOsm/L$.

Nervous and Hormonal Regulation of Fluid

• Neural Regulation (Thirst):     * When blood volume decreases and serum osmolarity increases (dehydration), the brain triggers the thirst mechanism to promote water intake to restore homeostasis.

• Renin-Angiotensin-Aldosterone System (RAAS):     * Triggered when the kidneys sense decreased perfusion (decreased blood flow/pressure).     * The Cascade:         1. Release of Renin from the kidneys.         2. Conversion of Angiotensin I to Angiotensin II.         3. Release of Aldosterone.     * Effect of Aldosterone: Causes the body to retain sodium. Because water follows sodium, water is also retained, ultimately increasing blood volume and blood pressure.     * Clinical Relevance: Dysfunction in the kidneys or liver can impair blood pressure regulation via the RAAS. Pharmacological treatments often target this system to manage hypertension.

Intravenous (IV) Fluid Tonicity

• Tonicity: Refers to a solution's concentration in comparison to the body's normal serum osmolarity ($300\,mOsm/L$).

• Crystalloids: Thin, aqueous solutions containing water-soluble molecules like electrolytes, minerals, or glucose.

• Colloids: Thick solutions containing large molecules that do not easily cross cell membranes. They remain in the intravascular space longer because the body takes more time to metabolize them.

• Fluid Shifts by Tonicity:     * Isotonic Solutions: Have a concentration equal to the blood (e.g., Normal Saline $0.9\%\,NaCl$). No fluid shift occurs because the balance between the intracellular and intravascular spaces is already equal. These fluids "stay where you put them."     * Hypotonic Solutions: Have a lower concentration than serum osmolarity (e.g., Half Normal Saline $0.45\%\,NaCl$). Water moves from the intravascular space into the cells toward the higher concentration, causing the cells to swell.     * Hypertonic Solutions: Have a higher concentration than serum osmolarity. Water is pulled from the cells into the intravascular space to dilute the concentrated solution, causing the cells to shrink. These patients are at risk for fluid volume overload, particularly if they have heart or kidney conditions.

Electrolytes: Functions and Distribution

• Definition: Chemicals dissolved in water that carry an electrical charge. Most enter the body through diet (meat, vegetables, fluids) and must be consumed daily as many are not stored.

• Primary Roles: Regulate fluid balance, maintain acid-base balance, facilitate enzyme reactions, and promote neuromuscular transmission and muscle contraction.

• Key Electrolytes and Symbols:     * Potassium ($K$): Primary focus is cardiac function. Normal range: $3.5\text{ to }5.0\,mEq/L$.     * Magnesium ($Mg$): Acts as a muscle relaxant. Normal range: $1.3\text{ to }2.1\,mEq/L$.     * Calcium ($Ca$): Essential for bone density, nerve impulses, and muscle contraction. Regulated by calcitonin and parathyroid hormone. Normal range: $9.0\text{ to }10.5\,mg/dL$.     * Sodium ($Na$): Essential for neural function (the brain) and fluid balance. Normal range: $136\text{ to }145\,mEq/L$.     * Phosphorus ($P$): Important for bone density and muscle contraction; has a stimulating effect on muscles. Normal range: $3.0\text{ to }4.5\,mg/dL$.     * Chloride ($Cl$): Involved in acid-base balance; usually tied closely with sodium. Normal range: $98\text{ to }106\,mEq/L$.

Clinical Electrolyte Imbalances

• Magnesium Imbalances:     * Hypermagnesemia: High magnesium leads to excessive muscle relaxation (depressant effect).     * Hypomagnesemia: Low magnesium leads to "excited" cells, hyperactive reflexes, twitching, and tremors.

• Calcium Imbalances:     * Hypocalcemia: Results in hyperactive reflexes and numbness/tingling.     * Trousseau Sign: Hand twitching triggered by inflating a blood pressure cuff.     * Spastic Sign (Chvostek): Facial twitching of the eye or upper facial muscles triggered by tapping the facial nerve.

• Potassium Imbalances:     * Hypokalemia: Associated with confusion, weakened muscles, slow/thready pulse, and a flattened or depressed T-wave on an EKG.     * Hyperkalemia: Associated with numbness/tingling (paresthesia) and tall, peaked T-waves on an EKG. This condition can be deadly if untreated due to cardiac rhythm changes.

• Sodium Imbalances:     * Hypernatremia: Often tied to dehydration. Symptoms include thirst, dry mucous membranes, low-grade fever, and decreased urine output as the kidneys attempt to conserve water.     * Hyponatremia: Primarily manifests as neural abnormalities, including confusion, seizures (especially when levels are in the low $120s$ or $110s$), and coma.

Types of Fluid Volume Imbalances

• Hypovolemia (Fluid Volume Deficit/Dehydration):     * Primary Dehydration: Results from inadequate intake of water.     * Secondary Dehydration: Results from fluid loss (e.g., vomiting, diarrhea, hemorrhage).     * Electrolyte levels vary; loss via GI tract may result in low sodium, whereas concentrated blood from poor intake may show high sodium.

• Hypervolemia (Fluid Volume Overload):     * Occurs when the body cannot properly regulate fluid volume, often due to heart or kidney failure.     * Water Intoxication: A less common form of hypervolemia.

Mechanisms and Assessment of Edema

• Edema: Swelling in the interstitial space caused by fluid leaking from blood vessels due to pressure imbalances or low protein levels.

• Oncotic Pressure and Proteins: Proteins (measured as Albumin in the blood) strengthen blood vessel walls. Low protein makes vessels "leaky," contributing to edema.

• Clinical Types of Edema:     * Peripheral Edema: Affected by gravity. Assessed in the feet in ambulatory patients or the sacral area in bedbound patients. Pitting edema is characterized by indentations left after palpation.     * Pulmonary Edema: Fluid accumulation in the alveoli (lung air sacs), leading to difficulty breathing and impaired oxygenation.     * Ascites: Fluid accumulation in the abdominal cavity (third spacing) specifically due to liver failure.

Hemoconcentration and Urine Specific Gravity

• Hemodilution: Occurs when there is an excess of plasma volume relative to solutes. Lab values (such as Hemoglobin and Hematocrit) may look falsely low because they are diluted by the high water volume.

• Hemoconcentration: Occurs when plasma volume is low, making solutes and cells look more concentrated. Lab values appear falsely elevated.

• Urine Specific Gravity:     * Measures the concentration of urine relative to distilled water (which is $1.0$).     * Normal Range: Approximately $1.0$ to $1.025$.     * High specific gravity indicates dark, concentrated urine (common in dehydration as the kidneys conserve water). Low specific gravity indicates dilute urine.