Cellular Environment: Fluids, Electrolytes, Acids, and Bases

 # Distribution of Body Fluids

• Total Body Water (TBW) Components:     - Intracellular Fluid (ICF): Fluid located inside the cell.     - Extracellular Fluid (ECF): Fluid located outside the cell. Sub-categories include:         - Interstitial fluid.         - Intravascular fluid.         - Cerebrospinal fluid (CSF).         - Lymphatic, synovial, intestinal, biliary, hepatic, pancreatic, pleural, peritoneal, pericardial, and intraocular fluids.         - Sweat.         - Urine.

Aging and the Distribution of Body Fluids

• Total Body Water Variations Across Lifespan:     - Newborn: Total body water accounts for $75\%$ to $90\%$ of body weight.     - Childhood: Total body water accounts for $60\%$ to $65\%$ of body weight.     - Adults: Total body water accounts for $60\%$ of body weight.     - Older Adults: The percentage of total body water declines with increasing age.

• Gender Differences: Men have a greater percentage of body water when compared with women.

Water Movement Between the ICF and ECF

• Osmolality and Osmotic Forces:     - Water movement is driven by osmotic forces.     - Sodium ($Na^+$): The primary osmotic force for the ECF.     - Potassium ($K^+$): The primary osmotic force for the ICF.

• Aquaporins: This is a family of water channel proteins that provide permeability to water, allowing it to move through cell membranes.

• Osmosis: This is the specific mechanism by which water moves between the ICF and ECF compartments.

• Plasma to Interstitial Movement: Water moves between the plasma and interstitial fluid through the combined actions of osmosis and hydrostatic pressure across the capillary membrane.

• Net Filtration: Refers to the movement of fluid across the capillary wall. This is described by the Starling law (or Starling hypothesis).

The Starling Hypothesis

• Definition: Net filtration is equal to the forces favoring filtration minus the forces opposing filtration.

• Forces Favoring Filtration:     - Capillary Hydrostatic Pressure: Also known as blood pressure.     - Interstitial Oncotic Pressure: A force that pulls water into the interstitial space.

• Forces Opposing Filtration (Favoring Reabsorption):     - Plasma Oncotic Pressure: A force that pulls water into the plasma.     - Interstitial Hydrostatic Pressure: Pressure within the interstitial space.

Alterations in Water Movement: Edema

• Definition: The accumulation of fluid specifically in the interstitial spaces.

• Causes of Edema:     - Increased Capillary Hydrostatic Pressure: Often resulting from venous obstruction.     - Decreased Plasma Oncotic Pressure: Caused by losses or diminished production of albumin.     - Increased Capillary Permeability: Occurs during inflammation and immune responses.     - Lymph Obstruction: Known as lymphedema.     - Sodium Retention: Contributes to fluid volume expansion.

• Clinical Manifestations:     - Localized vs. Generalized: Edema can be limited to one area or spread throughout the body.     - Dependent Edema: Fluid accumulation in gravity-dependent areas.     - Pitting Edema: When pressure applied to the swollen area leaves an indentation.     - ‘Third space’: Fluid accumulation in areas where it is not easily exchangeable with the rest of the ECF.     - Physical Signs: Swelling and puffiness; tighter-fitting clothes and shoes; weight gain.

• Treatment Strategies:     - Elevate edematous limbs.     - Use compression stockings or specialized compression devices.     - Avoid standing for prolonged periods.     - Restrict salt intake.     - Administer diuretic agents.

Overview of Electrolytes

• Compartmentalization: Electrolytes are present in both ECF and ICF compartments but at significantly different concentrations.

• Migration: All electrolytes move across compartments, but they must remain in balance for optimal health.

• Intracellular (ICF) Primary Ions:     - Cation: Potassium ($K^+$).     - Anions: Phosphate and organic ions.

• Extracellular (ECF) Primary Ions:     - Cation: Sodium ($Na^+$).     - Anions: Chloride ($Cl^-$) and Bicarbonate ($HCO_3^-$).

Sodium ($Na^+$) and Chloride ($Cl^-$) Balance

• Sodium Characteristics:     - The primary cation of the ECF.     - Regulates osmotic forces.     - Roles include managing neuromuscular irritability, acid-base balance, cellular reactions, and the transport of various substances.

• Chloride Characteristics:     - The primary anion of the ECF.     - Provides electroneutrality.     - Primarily follows the movement of sodium.

• Hormonal Regulation of Balance:     - Renin-Angiotensin-Aldosterone System (RAAS):         - Aldosterone: Secreted to increase the reabsorption of sodium by the distal tubule of the kidney.     - Natriuretic Peptides: Act to decrease tubular resorption and promote the urinary excretion of sodium. Types include:         - Atrial Natriuretic Peptide (ANP).         - Brain Natriuretic Peptide (BNP).         - Urodilantin (produced in the kidney).

Water Balance Regulation

• Primary Mechanisms: Regulated by thirst perception and Antidiuretic Hormone (ADH), also known as arginine vasopressin.

• Thirst Perception:     - Osmoreceptors: Located in the hypothalamus; stimulated by hyperosmolality, dry mouth, or plasma-volume depletion to increase water intake.     - Baroreceptors: Stimulated by depleted plasma volume; triggers the release of ADH.

• Antidiuretic Hormone (ADH) Action:     - Released in response to increased plasma osmolality or decreased circulating blood volume.     - Directly increases water reabsorption in the kidneys.

Alterations in Sodium, Chloride, and Water Balance

• Isotonic Alterations: Occur when total body water changes proportionally with electrolyte changes.     - Isotonic Volume Depletion: Hypovolemia.     - Isotonic Volume Excess: Hypervolemia.

• Hypertonic Alterations:     - Hypernatremia: Serum sodium levels exceeding $147\,mEq/L$.         - Caused by sodium gain or water loss.         - Causes water to move from the ICF to the ECF, leading to intracellular dehydration.         - Manifestations: Intracellular dehydration, convulsions, pulmonary edema, hypotension, tachycardia.         - Treatment: Isotonic salt-free fluids.     - Hyperchloremia: Occurs in conjunction with hypernatremia or a bicarbonate deficit; treated by addressing underlying disorders.     - Water Deficit (Dehydration):         - Manifestations: Tachycardia, weak pulse, postural hypotension, elevated hematocrit, elevated serum sodium levels, headache, dry skin, and dry mucous membranes.         - Treatment: Administration of water; cessation of fluid loss using hypotonic saline solutions or $5\%$ dextrose in water.

• Hypotonic Alterations: Characterized by decreased osmolality.     - Hyponatremia: Serum sodium level below $135\,mEq/L$.         - Causes plasma hypoosmolality and cellular swelling (as water moves into the cell).         - Types include pure sodium deficits, low intake, and dilutional hyponatremia.         - Manifestations: Lethargy, headache, confusion, apprehension, seizures, and coma.         - Treatment: Restrict water intake and administer intravenous (IV) fluids as dictated by the underlying disorder.     - Hypochloremia: Usually results from hyponatremia or elevated bicarbonate concentration (e.g., from vomiting, metabolic alkalosis, or cystic fibrosis).     - Water Excess:         - Causes: Compulsive water drinking (water intoxication), decreased urine formation, and Syndrome of Inappropriate ADH (SIADH).         - Manifestations: Cerebral edema, muscle twitching, headache, and weight gain.         - Treatment: Fluid restriction; hypertonic sodium chloride IV solution may be required.

Potassium ($K^+$) Dynamics

• Overview: The major intracellular cation. Concentration is maintained by the sodium-potassium ($Na^+/K^+$) pump.

• Distribution Influences:     - Into Cells: Facilitated by Aldosterone, insulin, epinephrine, and alkalosis.     - Out of Cells: Facilitated by Insulin deficiency, aldosterone deficiency, acidosis, and strenuous exercise.

• Essential Functions: Transmission and conduction of nerve impulses, maintenance of normal cardiac rhythms, skeletal and smooth muscle contraction, regulation of ICF osmolality, and glycogen deposition in liver/skeletal muscle cells.

• Potassium Adaptation: A mechanism where the body slowly accommodates increased levels of potassium intake.

• Hypokalemia: Potassium levels below $3.5\,mEq/L$.     - Causes: Reduced intake, increased entry into cells, or increased loss.     - Manifestations: Membrane hyperpolarization leading to decreased neuromuscular excitability, skeletal muscle weakness, smooth muscle atony, cardiac dysrhythmias, and the presence of a U wave on an electrocardiogram (ECG).     - Treatment: Oral or intravenous potassium replacement.

• Hyperkalemia: Potassium levels exceeding $5.5\,mEq/L$.     - Causes: Increased intake, shift from ICF to ECF, decreased renal excretion, hypoxia, acidosis, insulin deficiency, and cell trauma.     - Mild Attacks: Tingling of lips and fingers, restlessness, intestinal cramping, diarrhea, and peaked T waves on ECG.     - Severe Attacks: Muscle weakness, loss of muscle tone, flaccid paralysis, and cardiac arrest.     - Treatment: Calcium gluconate, insulin and/or glucose, sodium bicarbonate, cation exchange resins, or dialysis.

Calcium and Phosphate Balance

• Calcium ($Ca^{2+}$):     - $99\%$ is located in the bone as hydroxyapatite; $1\%$ is in plasma and body cells.     - Functions: Bone/teeth structure, blood clotting, hormone secretion, cell receptor function, and muscle contractions.     - Hypocalcemia (<8.5\,mg/dl): Caused by low intake, low PTH/Vitamin D, or blood transfusions. Symptoms include increased neuromuscular excitability, muscle spasms, Chvostek sign, Trousseau sign, convulsions, and tetany. Treat with calcium gluconate or replacement.     - Hypercalcemia (>12\,mg/dl): Caused by hyperparathyroidism, bone metastasis, excess Vitamin D, immobilization, and acidosis. Symptoms include decreased neuromuscular excitability, muscle weakness, kidney stones, constipation, and heart block. Treat with oral phosphate, IV saline, bisphosphonates, calcitonin, corticosteroids, or mithramycin.

• Phosphate ($HPO_4^{2-}$):     - $85\%$ is located in the bone.     - Functions: High-energy bonds in creatine phosphate and ATP, anion buffering, and muscle contraction energy.     - The Calcium-Phosphate Constant: $Ca^{2+} \times HPO_4^{2-} = K$ (where $K$ is a constant). If one increases, the other decreases.     - Hypophosphatemia: Caused by malabsorption, renal excretion, Vitamin D deficiency, antacid use, or alcohol abuse. Symptoms include osteomalacia (soft bones), bleeding disorders, and muscle weakness.     - Hyperphosphatemia: Caused by renal failure or long-term phosphate laxatives. Manifestations are similar to hypocalcemia.

• Hormonal Regulators:     - Parathyroid Hormone (PTH): Increases plasma calcium via kidney reabsorption.     - Vitamin D: A fat-soluble steroid that increases calcium absorption from the GI tract.     - Calcitonin: Decreases plasma calcium levels.

Magnesium ($Mg^{2+}$)

• Overview: An intracellular cation stored mostly in muscles and bones.

• Plasma Concentration: Normal range is $1.8$ to $2.4\,mg/dl$.

• Functions: Co-factor in intracellular reactions, protein synthesis, nucleic acid stability, and neuromuscular excitability. Interacts with calcium.

• Hypomagnesemia: Results from malabsorption; associated with hypocalcemia and hypokalemia. Symptoms: neuromuscular irritability, tetany, convulsions, and increased reflexes. Treatment: magnesium sulfate.

• Hypermagnesemia: Results from renal failure. Symptoms: skeletal muscle depression, muscle weakness, hypotension, respiratory depression, and bradycardia. Treatment: avoid magnesium; dialysis.

Acid-Base Balance

• pH Scale: Defined as the negative logarithm of the hydrogen ($H^+$) concentration.     - Each number represents a factor of 10. Moving from pH 7 to pH 6 means $H^+$ ions increased tenfold.     - Low pH = high $H^+$ (Acidic).     - High pH = low $H^+$ (Alkaline/Basic).

• Vital Statistics:     - Normal body pH: $7.35$ to $7.45$.     - pH below $6.8$ = Death.     - pH above $7.8$ = Death.

• Acid Formation: Acids are end-products of the metabolism of proteins, carbohydrates, and fats.

• Major Organs of Regulation: Lungs, kidneys, and bones.

• Types of Acids:     - Volatile Acids: Carbonic acid ($H_2CO_3$), eliminated as $CO_2$ gas via the lungs.     - Nonvolatile Acids: Sulfuric, phosphoric, and other metabolic acids, eliminated by renal tubules.

• Chemical Formula of the Respiratory-Renal Link:     - $CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons HCO_3^- + H^+$     - The lungs regulate $CO_2$; the kidneys regulate $HCO_3^-$ and $H^+$.

Buffering Systems

• Buffer Definition: A chemical that binds excessive $H^+$ or $OH^-$ to prevent significant pH changes. Pairs consist of a weak acid and its conjugate base.

• Carbonic Acid–Bicarbonate System:     - Most important plasma buffering system.     - At pH $7.4$, the ratio of bicarbonate to carbonic acid must be maintained at $20:1$.

• Compensation:     - Respiratory Compensation: Increasing or decreasing ventilation to adjust carbonic acid levels (fast acting).     - Renal Compensation: Producing acidic or alkaline urine by reabsorbing or regenerating bicarbonate (slower acting).

• Other Systems:     - Protein Buffering: Negatively charged proteins (especially hemoglobin) bind $H^+$.     - Cellular Ion Exchange: Exchange of $K^+$ for $H^+$ during acid-base shifts.

Acid-Base Imbalances

• Metabolic Acidosis:     - Causes: Lactic acidosis, renal failure, diabetic ketoacidosis, diarrhea, starvation.     - Logic: Noncarbonic acids increase or $HCO_3^-$ is lost.     - Stats: pH < 7.35, HCO_3^- < 24\,mEq/L.     - Manifestations: Headache, lethargy, Kussmaul respirations.     - Anion Gap: Normally $10$ to $12\,mEq/L$; used to distinguish causes of metabolic acidosis.

• Metabolic Alkalosis:     - Causes: Prolonged vomiting, gastric suctioning, hyperaldosteronism, diuretic therapy.     - Stats: pH > 7.45, HCO_3^- > 26\,mEq/L.     - Manifestations: Weakness, muscle cramps, hyperactive reflexes.

• Respiratory Acidosis:     - Causes: Hypoventilation, respiratory center depression, chest wall disorders, lung diseases (COPD, edema).     - Stats: pH < 7.35, elevated $pCO_2$ (hypercapnia).     - Manifestations: Restlessness, blurred vision, muscle twitching, coma.

• Respiratory Alkalosis:     - Causes: Hyperventilation (anxiety, high altitude, fever, salicylate intoxication).     - Stats: pH > 7.45, pCO_2 < 38\,mm\,Hg (hypocapnia).

• Summary of Acid-Base States (Reference Charts):     - Acidemia: Normal $HCO_3^-$ but high $PaCO_2$ = Respiratory Acidosis.     - Acidemia: Normal $PaCO_2$ but low $HCO_3^-$ = Metabolic Acidosis.     - Alkalemia: Normal $PaCO_2$ but high $HCO_3^-$ = Metabolic Alkalosis.     - Alkalemia: Normal $HCO_3^-$ but low $PaCO_2$ = Respiratory Alkalosis.

Questions & Discussion

• Question 1: A person with heart failure has edema in the lower legs and sacral area. The nurse suspects this condition is due to a(n):     - Answer: Increase in capillary hydrostatic pressure.

• Question 2: A person reports severe diarrhea for 2 days. The nurse understands this stimulates a(n):     - Answer: Increase in antidiuretic hormone secretion.

• Question 3: A person arrives in the emergency department after a loss of consciousness and the development of Kussmaul respirations. The individual has a history of diabetes and 2 days of vomiting and diarrhea. The nurse suspects the person has:     - Answer: Metabolic acidosis.

• Question 4: A person with a history of chronic lung disease arrives in the clinic with a 1-week history of a productive cough, hypoventilation, headache, and muscle twitching. The nurse suspects the person is experiencing:     - Answer: Respiratory acidosis.

• Case Study 1: A 17-year-old boy in a persistent vegetative state (4 years post-TBI) requires debridement of a sacral wound.     - Tissue type of sacral area: Epithelial (and connective).     - Reason for tissue removal: Necrosis (resulting from ischemia and cell death).

• Case Study 2: The teen develops generalized and dependent swelling while recuperating.     - Pathophysiologic process of edema here: Venous obstruction (though other factors like decreased plasma oncotic pressure or increased capillary permeability can contribute to edema in different contexts).