Body Fluid Compartments, and Electrolyte Regulation

Fundamentals of Homeostasis and Physiological Balance

• Definition of Homeostasis: Homeostasis is defined as a dynamic and relatively stable state in the internal environment of the body.

• Internal Environment and Survival: Body cells can only live and function when the extracellular fluid (ECF) is compatible with their survival. The chemical composition and physical state of this environment must be maintained within narrow limits.

• Interdependence Circuit:

  • Cells: Make up body systems.

  • Body Systems: Maintain homeostasis.

  • Homeostasis: Is essential for the survival of cells.

• Homeostatically Regulated Factors: The following factors must be maintained for physiological stability:

  • Concentration of nutrients.

  • Concentration of $O_2$ and $CO_2$.

  • Concentration of waste products.

  • Changes in pH.

  • Concentrations of water, salt, and other electrolytes.

  • Volume and pressure.

  • Temperature.

• The Internal Environment Challenge: The body constant faces a changing external environment and internal physiological events/activities that can alter the balance of important variables.

• Categorization of Diseases: Pathological conditions or diseased states alter the internal environment and may require pharmacological intervention. Diseases are divided into two main categories:

  • Internal Failure: The pathophysiology involves the internal failure of a normal physiological process.

  • External Source: Originates from an external source, such as bacterial or viral infections.

• Role of Medication: Many medications are designed to assist the body in maintaining homeostasis when the body's own regulatory mechanisms fail.

Homeostatic Control Systems and Feedback Mechanisms

• Homeostatic Control System: Defined as an interconnected network of body components that operates to maintain a given factor in the internal environment at a relatively constant, optimal level.

• Functional Requirements for Maintenance:

  • Detect: Detect deviations from normal states.

  • Integrate: Integrate this information with other existing information.

  • Adjust: Make adjustments to restore the factor to its normal level.

• Classes of Homeostatic Control Systems:

  • Intrinsic (Local) Controls: These are built into an organ.

  • Extrinsic (Systemic) Controls: These are initiated outside an organ to alter that specific organ's activity.

• Negative Feedback Mechanisms:

  • Function: Opposes an initial change and is the most widely used method to maintain homeostasis.

  • Process: A change in a controlled variable triggers a response that drives the variable in the opposite direction of the initial change.

  • Components of Negative Feedback Control:

    • Sensor: Detects the deviation in the controlled variable (e.g., temperature-monitoring nerve cells).

    • Integrator: Sends instructions to effectors (e.g., temperature control center in the brain).

    • Effector: Brings about the compensatory response (e.g., skeletal muscle shivering/heat production).

• Positive Feedback Mechanisms:

  • Function: The output enhances or amplifies a change so the controlled variable continues to move in the direction of the initial change.

  • Usage: Less common than negative feedback; an essential example is the process of childbirth.

• Feedforward Mechanisms:

  • Function: Initiates responses in anticipation of a change.

  • Usage: Infrequently used.

  • Example: Insulin levels increase while a meal is still in the digestive tract as an anticipatory response before blood glucose actually rises.

• Pathophysiology and Death: Pathophysiology refers to the abnormal functioning of the body associated with disease. When homeostatic disruption is so severe that it is no longer compatible with survival, death results.

Body Fluid Compartments and Volume Distribution

• Total Body Water (TBW): Comprises approximately $60\%$ of total body weight in adults (higher in children, lower in the elderly).

• Main Compartments:

  • Intracellular Fluid (ICF): Fluid inside the cells. It accounts for approximately $2/3$ of total body water ($40\%$ of total body weight).

  • Extracellular Fluid (ECF): Fluid outside the cells. It accounts for approximately $1/3$ of total body water ($20\%$ of total body weight).

• Subdivisions of Extracellular Fluid (ECF):

  • Interstitial Fluid: Fluid between the cells. It makes up approximately $80\%$ of the ECF ($16\%$ of total body weight).

  • Plasma: The liquid portion of the blood. It makes up approximately $20\%$ of the ECF ($4\%$ of total body weight).

  • Transcellular Fluid: A specialized, small compartment ($\approx 1\%$ of total body water, or $1$ to $2$ Liters). It includes lymphatic fluid, pleural fluid, synovial fluid, peritoneal fluid, cerebrospinal fluid (CSF), pericardial fluid, intraocular fluid, bladder fluid, and gastrointestinal (GI) fluids.

• Relative Percentages Summary (Table 15-1):

  • Total Body Fluid: $60\%$ of Body Weight.

  • Intracellular Fluid (ICF): $40\%$ of Body Weight.

  • Extracellular Fluid (ECF): $20\%$ of Body Weight.

  • Plasma: $4\%$ of Body Weight.

  • Interstitial Fluid: $16\%$ of Body Weight.

• Composition of Whole Blood:

  • Plasma: Approximately $55\%$.

  • White Blood Cells and Platelets: Approximately $4\%$.

  • Red Blood Cells: Approximately $41\%$.

Body Fluid Composition and Electrolyte Concentrations

• Chemical Differences between ECF and ICF:

  • Extracellular Fluid (ECF): Contains large amounts of $Na^+$, $Cl^-$, and $HCO_3^-$ (bicarbonate). Includes nutrients like $O_2$, glucose, fatty acids, amino acids, and waste products like $CO_2$ (transported to lungs) and urea (transported to kidneys).

  • Intracellular Fluid (ICF): Contains large amounts of $K^+$, $Mg^{2+}$, and $PO_4^{3-}$ (phosphate) rather than $Na^+$ and $Cl^-$.

• Normal Serum/Plasma Concentrations ($mEq\,L^{-1}$):

  • Sodium ($Na^+$): $135$ - $145\,mEq\,L^{-1}$. Determines water retention/movement; imbalances cause neurological problems.

  • Potassium ($K^+$): $3.5$ - $5.0\,mEq\,L^{-1}$. Imbalances cause cardiac problems.

  • Chloride ($Cl^-$): $96$ - $106\,mEq\,L^{-1}$. Works with sodium to maintain osmotic pressure.

  • Calcium ($Ca^{2+}$)/Phosphate ($PO_4^{3-}$):

    • Calcium: $9.0$ - $10.5\,mg\,dL^{-1}$. Required for nerve impulses, muscle contraction, blood clotting; requires Vitamin D for absorption.

    • Phosphate: $3.0$ - $4.5\,mg\,dL^{-1}$.

• Other Lab Values:

  • BUN (Blood Urea Nitrogen): $6$ - $20\,mg\,dL^{-1}$.

  • Creatinine: $0.6$ - $1.3\,mg\,dL^{-1}$.

  • Hematocrit: Males ($42\%$ - $52\%$), Females ($37\%$ - $47\%$).

• Detailed Osmolar Substances (Table 25-2):

  • Plasma ($mOsm/L\,H_2O$): $Na^+$ (142), $K^+$ (4.2), $Cl^-$ (108), $HCO_3^-$ (24), Glucose (5.6), Protein (1.2), Urea (4).

  • Intracellular ($mOsm/L\,H_2O$): $Na^+$ (14), $K^+$ (140), $Cl^-$ (4), $Mg^{2+}$ (20), Phosphocreatine (45), ATP (5), Protein (4).

  • Total mOsm/L: Plasma (301.8), Interstitial (300.8), Intracellular (301.2).

  • Corrected Osmolar Activity: $281.0$ - $282.0\,mOsm/L$.

  • Total Osmotic Pressure at $37^\circ C$: Approximately $5423$ - $5443\,mmHg$.

Fluid Balance Concept and Regulation

• Balance Concept: To maintain a stable balance of an ECF constituent, its input must equal its output.

  • Positive Balance: Gains exceed losses.

  • Negative Balance: Losses exceed gains.

• Internal Pool: The amount of a substance in the ECF. This pool is increased by external transfer or metabolic production.

• Water Distribution: Water follows electrolytes ($H_2O$ follows salt).

• Importance of Regulation:

  • ECF Volume: Must be regulated to maintain blood pressure. Salt balance is the primary factor.

  • ECF Osmolarity: Must be regulated to prevent swelling (hypotonicity) or shrinking (hypertonicity) of cells. Water balance is the primary factor.

• Osmolarity and Osmolality:

  • Serum Osmolality: Range of $285$ - $295\,mOsm/kg$. High values indicate water deficit (concentrated); low values indicate water excess (dilute).

  • Urine Osmolality: Range of $50$ - $1200\,mOsm/kg$ (Average: $500$ - $800\,mOsm/kg$).

• Movement Mechanisms:

  • Passive Transport: Diffusion (osmosis), facilitated diffusion.

  • Active Transport: Primary (pumps), Secondary (symport, antiport).

  • Vesicular Transport: Phagocytosis, endocytosis (pinocytosis), exocytosis, transcytosis.

  • Barriers: Plasma membranes separate ECF/ICF (selective); blood vessel walls separate plasma/interstitial fluid (leaky).

Osmotic Equilibrium and Tonicity

• Osmosis: The movement of water across a membrane down its concentration gradient.

• Tonicity: How a cell reacts in a solution based on the relative concentration of non-penetrating solutes.

  • Isotonic: No net water movement; no change in cell volume (e.g., hemorrhage, isotonic IV administration).

  • Hypertonic (Hypertonicity/Dehydration): ECF concentration is higher than ICF. Water leaves the cell, causing the cell to shrink. Causes: insufficient intake, excessive loss, Diabetes Insipidus. Symptoms: neural dysfunction.

  • Hypotonic (Hypotonicity/Overhydration): ECF concentration is lower than ICF. Water enters the cell, causing the cell to swell. Causes: renal failure, rapid water ingestion, SIADH (Syndrome of Inappropriate Vasopressin Secretion). Symptoms: neural and non-neural.

• Electrical Disequilibrium: Separation of charged ions across the membrane creates electrical potential and electrochemical gradients.

Systemic Regulation of Fluid and Electrolyte Balance

• Renin-Angiotensin-Aldosterone System (RAAS):

  • Trigger: Low $Na^+$, low volume, or low arterial blood pressure detected by Juxtaglomerular (JG) cells in the kidneys.

  • Process: Renin is released -> Angiotensinogen converts to Angiotensin I -> Angiotensin II -> Adrenal cortex releases Aldosterone.

  • Result: Aldosterone increases $Na^+$ reabsorption (which pulls water back into the blood) and increases $K^+$ excretion.

• Antidiuretic Hormone (ADH) / Vasopressin:

  • Trigger: High serum osmolarity or high $Na^+$ detected by hypothalamic osmoreceptors.

  • Source: Released from the posterior pituitary.

  • Result: Increases water permeability of distal/collecting tubules for water reabsorption, concentrates urine, and mildly constricts blood vessels.

• Atrial Natriuretic Peptide (ANP) and BNP:

  • Trigger: High volume detected by stretch receptors in the right atrium of the heart.

  • Result: Inhibits ADH, stops the RAAS, increases $Na^+$ excretion in urine, and dilates blood vessels to decrease serum osmolarity.

• Thirst and Water Input:

  • Controlled by hypothalamic neurons; triggered by increased ECF osmolarity or decreased ECF volume (via left atrial volume receptors during large changes).

Fluid Spacing and Intake/Loss Data

• Fluid Spacing Categories:

  • First Spacing: Normal distribution of fluid in ICF and ECF.

  • Second Spacing: Edema (abnormal accumulation of interstitial fluid).

  • Third Spacing: Fluid accumulation in areas where it is not easily exchanged (e.g., Ascites, burn edema).

• Fluid Intake Sources (Approximate):

  • Liquids: $1500\,mL/day$.

  • Solid foods: $800\,mL/day$.

  • Metabolism: $300\,mL/day$.

  • Total Daily Need: Approximately $2000\,mL$.

• Fluid Loss Routes (Approximate):

  • Kidneys (Urine): $1200$ - $1500\,mL/day$.

  • Skin (Sweat/Insensible): $500$ - $600\,mL/day$.

  • Lungs (Vaporization): $400\,mL/day$.

  • GI Tract (Feces): $100$ - $200\,mL/day$.

• Note on Weight: $1\,Liter$ of water weighs $1\,kg$.