Body Fluids and Fluid Compartments

Body Fluids and Fluid Compartments

Key Definitions

  • Fluid: A substance that can flow; encompasses both liquids and gases.

  • Electrolyte: Minerals in the body that have an electric charge, essential for bodily functions.

  • Acid-Base: Refers to the regulation of hydrogen ion concentration in the body to maintain pH balance.

Overview

  • Chemical Reactions in Body Fluids: Occur in aqueous solutions involving proteins, electrolytes such as sodium (Na ext{+}) and chloride (Cl ext{-}).

  • Solutes: Substances dissolved in a solution.

  • Osmosis: Water movement through semi-permeable membranes of cells, driven by concentration gradients of water and solutes.

Body Water Content

  • Total Body Water Percentage:

    • Adults: 50-60% of body weight.

    • Infants: Approximately 75% due to high water content.

  • Organ Water Content:

    • Brain and kidneys have the highest proportions of water.

  • Fluid Compartments: Body water must be organized into compartments due to variability across different body parts.

Fluid Compartments

  • Intracellular Fluid (ICF): Fluid within cells, accounting for about 66% of total body fluid.

    • Cytoplasm: This is a stable volume and should maintain consistency over time.

  • Extracellular Fluid (ECF): Fluid outside of cells, making up about 33% of total body fluid.

    • Plasma: The liquid component of blood, comprising 20% of body fluids.

    • Interstitial Fluid: Surrounds cells and is not found within blood vessels.

    • Other ECF examples include cerebrospinal fluid, synovial fluid, pleural fluid, pericardial fluid, peritoneal fluid, and aqueous humor.

Clinical Correlate: Edema

  • Definition: Accumulation of excess water in body tissues, particularly soft tissues like extremities.

  • Common Causes:

    • Underlying medical conditions.

    • Certain drugs and pregnancy.

    • Localized injuries or allergic reactions.

  • Detection: Pressing a finger into the area; if the depression persists, it indicates pitting edema.

  • Pulmonary Edema: Often a consequence of left-sided heart failure.

  • Treatment Options: Limb elevation, use of compression socks, and reduced salt intake.

Water Balance

  • Water Sources: Most water intake occurs through the gastrointestinal (GI) tract.

    • About 200 mL gained from metabolic processes.

    • Average daily intake totals approximately 2500 mL.

  • Water Loss: Equal to 2500 mL daily through urine, sweating (insensible), and lungs (insensible).

Regulation of Water Intake

  • Osmolality: Concentration of solute in a liquid measured as osmol/L.

  • Plasma Osmolality: Ratio of solutes to water, reflecting hydration state.

    • Higher osmolality indicates lower hydration levels, presenting as higher specific gravity in urine (denoting more solutes).

Clinical Correlate: Dehydration

  • Definition: A state of insufficient body water volume.

  • Physiological Response:

    • Low blood volume (BV) activates the Renin-Angiotensin-Aldosterone System (RAAS): increased heart rate and contraction.

    • Stimulates thirst prompting water intake and the release of Antidiuretic Hormone (ADH) promoting kidney water reabsorption offering a feedback mechanism.

    • Results in decreased salivary gland secretions leading to dry mouth.

  • Detection: Monitored by osmoreceptors that sense low water levels and affect the brain.

  • Risks: Dehydration can lead to significant health outcomes, including death, particularly in vulnerable populations like infants and children.

Regulation of Water Output

  • Majority of water excretion occurs through urine.

  • Minimum Necessary Output: Approximately 0.5L per day to effectively eliminate waste.

    • Proportionately right after intake of large volumes of fluid.

  • Terms:

    • Diuresis: Production of excess urine due to ADH effects, which may lead to vasoconstriction and an increase in aquaporin channels in collecting ducts.

Electrolyte Balance

  • Primary Functions:

    • Facilitate electrical impulses.

    • Stabilize protein structure.

    • Aid in hormone release.

  • Essential Electrolytes: Sodium (Na ext{+}), Potassium (K ext{+}), Chloride (Cl ext{-}), Bicarbonate (HCO3 ext{-}), Calcium (Ca2 ext{+}), and Phosphate (HPO4^{2-}).

  • Excretion: Most electrolytes are lost via urine.

Sodium

  • Role: Major extracellular fluid cation.

  • Daily Requirement: 1-2 mmol/day; however, the average U.S. intake is around 150 mmol/day.

  • Health Impact: High sodium consumption is linked to hypertension (HTN).

  • Homeostasis: Sodium is primarily reabsorbed in kidneys impacting blood osmotic pressure and blood pressure (BP).

Clinical Correlate: Hyponatremia

  • Definition: Low sodium levels in the body.

  • Common Causes:

    • Excessive sweating, vomiting, and diarrhea.

    • Use of diuretics.

    • High urine production due to diabetes mellitus (DM).

    • Acidosis conditions (metabolic or diabetic ketoacidosis).

  • Consequences:

    • Neuronal swelling, decreased oxygen delivery by RBCs, potential brain damage and death.

Clinical Correlate: Hypernatremia

  • Definition: Elevated sodium levels.

  • Causes:

    • Water loss, which may involve hormonal factors (specifically ADH and aldosterone impacts).

  • Symptoms:

    • Neuromuscular irritability, convulsions, and potentially coma.

Potassium

  • Role: Major intracellular cation significant for establishing resting membrane potential in neurons and muscle fibers.

  • Recommended Intake: 4700 mg per day, primarily excreted through urine.

Clinical Correlate: Hypokalemia

  • Definition: Low potassium levels in the body.

  • Causes:

    • Decreased dietary intake (starvation).

    • Vomiting, diarrhea, and alkalosis conditions.

    • Resulting in metabolic acidosis, CNS confusion, and cardiac arrhythmias.

Clinical Correlate: Hyperkalemia

  • Definition: Elevated potassium levels.

  • Causes:

    • Increased intake scenarios or poor excretion when the heart cannot relax.

    • Leads to acute symptoms including death in minutes, mental confusion, numbness, and respiratory muscle weakness.

Chloride

  • Role: Predominant extracellular anion critical for maintaining electrical neutrality in the body.

  • Normal Range: 96-106 mEq/L; low (<95 mEq/L) indicates hypochloremia, high (>106 mEq/L) suggests hyperchloremia.

  • Measurement: Levels assessed in blood, sweat, urine, and feces.

Clinical Correlate: Hypochloremia

  • Definition: Low chloride levels.

  • Causes:

    • Defective renal tubular absorption, vomiting, diarrhea, and metabolic acidosis.

    • Associated with conditions such as heart failure and lung diseases like emphysema.

Clinical Correlate: Hyperchloremia

  • Definition: High chloride levels.

  • Causes:

    • Dehydration, excessive salt intake, and ingestion of seawater, along with conditions like CHF and cystic fibrosis.

Bicarbonate (HCO3-)

  • Function: Maintains acid-base balance and serves as a buffering agent in the body.

  • Chemical Reaction:
    extCO2+extH2extO<br>ightleftharpoonsextH2extCO3<br>ightleftharpoonsextHCO3−+extH+ext{CO}_2 + ext{H}_2 ext{O} <br>ightleftharpoons ext{H}_2 ext{CO}_3 <br>ightleftharpoons ext{HCO}_3^{-} + ext{H}^{+}

Calcium

  • Role: Essential for bone formation, muscle contraction, enzyme activity, and blood coagulation.

  • Absorption Requirement: Needs Vitamin D for intestinal absorption.

Clinical Correlate: Hypocalcemia

  • Definition: Low calcium levels due to conditions such as hypoparathyroidism, poor dietary intake, and Vitamin D deficiency.

  • Effects:

    • Can result in cardiac depression, muscle cramps, and diseases like rickets or osteomalacia.

Clinical Correlate: Hypercalcemia

  • Definition: Elevated calcium levels often due to primary hyperparathyroidism or malignancies.

  • Symptoms:

    • Arrhythmias, muscle weakness, CNS confusion, and coma resulting from elevated levels.

Phosphate

  • Presence: Primarily in bones, teeth, phospholipids, and ATP.

  • Hypophosphatemia: Low phosphate, triggered by factors like renal dysfunction and heavy usage of antacids.

Retention Check

  • Match the following medical terms to their definitions:

    • Hypochloremia: Low chloride

    • Hypernatremia: High sodium

    • Hypercalcemia: High calcium

    • Hypokalemia: Low potassium

Acid-Base Balance

  • Definition: Critical for the function of proteins, which perform a wide range of tasks in the body.

  • pH Scale: Scales from 0 (most acidic) to 14 (most basic).

    • Examples of common substances and their placement on the scale:

    • 0: Battery acid

    • 7: Pure Water

    • 14: Liquid drain cleaner

Buffers in the Body

  • Buffers: Systems that help maintain pH within a narrow range by compensating for acid-base fluctuations.

  • Key Organs:

    • Kidneys: Manage excess H ext{+} ions and HCO3 ext{-} levels.

    • Lungs: Control CO2 to indirectly influence carbonic acid levels.

Clinical Correlate: Ketoacidosis

  • Occurrence: Frequently in uncontrolled diabetes mellitus (DKA).

  • Mechanism: When glucose utilization is impaired, the body resorts to ketones for energy, raising blood acidity.

  • Symptoms: Rapid, deep breathing, fruity breath, nausea, and vomiting; can lead to coma or death.

Disorders of Acid-Base Balance

  • Types:

    • Metabolic Acidosis & Alkalosis: Relate to bicarbonate levels affected by various dysfunctions.

    • Respiratory Acidosis & Alkalosis: Related to carbonic acid levels generally due to respiratory issues.

Compensation Mechanisms

  • Respiratory Compensation: Adjusts breathing rate to regulate CO2 levels, hence affecting acid-base balance more dramatically in acidosis.

  • Metabolic Compensation: Kinetics of kidney function to readjust HCO3 ext{-} production or H ext{+} excretion in response to blood pH changes.

Diagnosing Acidosis vs Alkalosis

  • Testing Parameters:

    • Measure pH, CO2, and HCO3 ext{-} concentrations to determine the type of acidosis or alkalosis.

    • Blood pH indicates overall acid or base state, while CO2 and HCO3 ext{-} levels clarify if it is metabolic or respiratory and help assess compensation mechanisms.