ch 3

Distribution of Body Fluids and Aging Changes

• Body Fluid Compartments

  • Body fluids are distributed among functional compartments that provide a transport medium for cellular and tissue function.

  • Intracellular Fluid (ICF): Fluid within cells, comprising approximately two-thirds ($\frac{2}{3}$) of Total Body Water (TBW).

  • Extracellular Fluid (ECF): Fluid outside cells, comprising approximately one-third ($\frac{1}{3}$) of TBW.

    • Interstitial fluid: Surrounds cells and occupies spaces between cells, outside of blood vessels.

    • Intravascular fluid: Located within blood vessels; commonly referred to as blood plasma.

    • Transcellular fluids: Smallest ECF component, contained within epithelial-lined cavities of the body.

• Transcellular Fluid Electrolyte Concentrations

  • Saliva: $\text{Na}^+ = 33\text{ mEq/L}$, $\text{K}^+ = 20\text{ mEq/L}$, $\text{Cl}^- = 34\text{ mEq/L}$, $\text{HCO}_3^- = 40\text{ mEq/L}$.

  • Gastric juice: $\text{Na}^+ = 60\text{ mEq/L}$, $\text{K}^+ = 9\text{ mEq/L}$, $\text{Cl}^- = 84\text{ mEq/L}$, $\text{HCO}_3^- = 0\text{ mEq/L}$. The $\text{Cl}^-$ concentration exceeds the combined $\text{Na}^+$ and $\text{K}^+$ concentration by $15\text{ mEq/L}$, reflecting hydrochloric acid secretion by parietal cells.

  • Bile: $\text{Na}^+ = 149\text{ mEq/L}$, $\text{K}^+ = 5\text{ mEq/L}$, $\text{Cl}^- = 101\text{ mEq/L}$, $\text{HCO}_3^- = 45\text{ mEq/L}$.

  • Pancreatic juice: $\text{Na}^+ = 141\text{ mEq/L}$, $\text{K}^+ = 5\text{ mEq/L}$, $\text{Cl}^- = 77\text{ mEq/L}$, $\text{HCO}_3^- = 92\text{ mEq/L}$.

  • Ileal fluid: $\text{Na}^+ = 129\text{ mEq/L}$, $\text{K}^+ = 11\text{ mEq/L}$, $\text{Cl}^- = 116\text{ mEq/L}$, $\text{HCO}_3^- = 29\text{ mEq/L}$.

  • Cecal fluid: $\text{Na}^+ = 80\text{ mEq/L}$, $\text{K}^+ = 21\text{ mEq/L}$, $\text{Cl}^- = 48\text{ mEq/L}$, $\text{HCO}_3^- = 22\text{ mEq/L}$.

  • Cerebrospinal fluid (CSF): $\text{Na}^+ = 141\text{ mEq/L}$, $\text{K}^+ = 3\text{ mEq/L}$, $\text{Cl}^- = 127\text{ mEq/L}$, $\text{HCO}_3^- = 23\text{ mEq/L}$.

  • Sweat: $\text{Na}^+ = 45\text{ mEq/L}$, $\text{K}^+ = 5\text{ mEq/L}$, $\text{Cl}^- = 58\text{ mEq/L}$, $\text{HCO}_3^- = 0\text{ mEq/L}$.

  • Other transcellular fluids include pleural, synovial, peritoneal, pericardial, and intraocular fluids.

• Total Body Water (TBW) Volumes and Proportions

  • TBW volume is expressed as a percentage of body weight in kilograms.

  • Medium Weight Young/Middle-Aged Adult Male: TBW is $58 \pm 8\%$ of body weight ($42\text{ L}$ total volume for standard weight); ICF comprises $28\text{ L}$ ($\frac{2}{3}$ of TBW); ECF comprises $14\text{ L}$ ($\frac{1}{3}$ of TBW), subdivided into Interstitial ($11\text{ L}$, $\frac{4}{5}$ of ECF) and Intravascular ($3\text{ L}$, $\frac{1}{5}$ of ECF).

  • Medium Weight Young/Middle-Aged Adult Female: TBW is $48 \pm 6\%$ of body weight ($35\text{ L}$ total volume for standard weight); ICF comprises $24\text{ L}$ ($\frac{2}{3}$ of TBW); ECF comprises $11\text{ L}$ ($\frac{1}{3}$ of TBW), subdivided into Interstitial ($9\text{ L}$, $\frac{4}{5}$ of ECF) and Intravascular ($2\text{ L}$, $\frac{1}{5}$ of ECF).

• Daily Normal Water Gains and Losses (70-kg Individual)

  • Daily Intake Total: $2400\text{ to }3200\text{ mL}$.

    • Drinking: $\approx 60\%$ ($1400\text{ to }1800\text{ mL}$).

    • Water in food: $\approx 30\%$ ($700\text{ to }1000\text{ mL}$).

    • Water of oxidation: $\approx 10\%$ ($300\text{ to }400\text{ mL}$).

  • Daily Output Total: $2400\text{ to }3200\text{ mL}$.

    • Urine: $\approx 60\%$ ($1400\text{ to }1800\text{ mL}$).

    • Stool: $\approx 2\%$ ($100\text{ mL}$).

    • Skin (visible sweat and insensible water loss): $\approx 10\%$ ($300\text{ to }500\text{ mL}$).

    • Lungs: $\approx 28\%$ ($600\text{ to }800\text{ mL}$).

• Influence of Body Fat and Age

  • Adipose tissue is hydrophobic (water-repelling) and contains very little water. Individuals with higher body fat percentage have proportionately less TBW and heightened susceptibility to dehydration.

  • Newborns (up to 1 month): TBW is $70\%$ to $80\%$ of body weight ($74\%$ in medium weight newborns) due to low fat storage. A physiologic loss of approximately $5\%$ of body weight occurs in the immediate postnatal period as the infant adapts to the external environment.

  • Infant vulnerabilities: High metabolic rate, high ratio of body surface area to total body size causing significant evaporative loss, and immature renal fluid/electrolyte conservation mechanisms make infants highly vulnerable to rapid dehydration (e.g., from diarrheal fluid losses).

  • Childhood (1 to 10 years): TBW decreases slowly to $60\%$ to $65\%$ ($60\%$ in medium weight children).

  • Adolescence to Adulthood: Sex differences emerge due to hormonal influences. Estrogens promote higher body fat accumulation in females (medium weight young/middle-aged female TBW: $34\%$ to $48\%$), whereas increased muscle mass in males yields higher water content (medium weight young/middle-aged male TBW: $48\%$ to $50\%$).

  • Older Adults: TBW declines further (medium weight older adult male: $46\%$ to $48\%$; older adult female: $32\%$ to $33\%$). Driven by increased fat mass, muscle loss, decreased renal efficiency in conserving sodium and concentrating urine, increased cutaneous insensible loss, and impaired thirst perception. Physiologic stress (e.g., fever, acute illness) can produce rapid, severe fluid loss.

Fluid Movement Dynamics Between Compartments

• ICF-ECF Water Shift

  • Water moves freely across cell membranes via simple diffusion through the lipid bilayer and via specialized water channel proteins called aquaporins.

  • Movement is governed primarily by osmotic forces. $\text{Na}^+$ establishes ECF osmotic balance; $\text{K}^+$ maintains ICF osmotic balance.

  • The osmotic force exerted by non-diffusible intracellular proteins and organic compounds is balanced by the active transport of ions out of the cell. Under normal conditions, ICF and ECF osmolalities exist in equilibrium.

• Plasma-Interstitial Fluid Exchange (Starling Forces)

  • Capillary fluid distribution and movement of nutrients/wastes are governed by four physical forces (Starling forces):

    1. Capillary Hydrostatic Pressure (blood pressure): Pushes water out of the capillary into the interstitial space.

    2. Capillary (plasma) Oncotic Pressure: Osmotically pulls water from the interstitial space into the capillary (generated primarily by plasma albumin).

    3. Interstitial Hydrostatic Pressure: Facilitates movement of water from the interstitial space into the capillary.

    4. Interstitial Oncotic Pressure: Osmotically pulls water from the capillary into the interstitial space.

  • Net Filtration Equations:

$\text{Net Filtration} = \text{Forces Favoring Filtration} - \text{Forces Opposing Filtration}$

$\text{Forces Favoring Filtration} = \text{Capillary Hydrostatic Pressure} + \text{Interstitial Oncotic Pressure}$

$\text{Forces Opposing Filtration} = \text{Capillary Oncotic Pressure} + \text{Interstitial Hydrostatic Pressure}$

• Pressures Along the Capillary Bed

  • Arterial Capillary End: Capillary hydrostatic pressure ($35\text{ mmHg}$) + Interstitial oncotic pressure ($0\text{ mmHg}$) = Net Hydrostatic Pressure ($33\text{ mmHg}$) when subtracting Interstitial hydrostatic pressure ($2\text{ mmHg}$). Capillary oncotic pressure ($24\text{ mmHg}$) - Interstitial oncotic pressure ($0\text{ mmHg}$) = Net Oncotic Pressure ($24\text{ mmHg}$). Net Filtration Pressure = $+9\text{ mmHg}$ (favors filtration into tissue).

  • Venous Capillary End: Capillary hydrostatic pressure ($18\text{ mmHg}$) - Interstitial hydrostatic pressure ($1\text{ mmHg}$) = Net Hydrostatic Pressure ($17\text{ mmHg}$). Capillary oncotic pressure ($25\text{ mmHg}$) - Interstitial oncotic pressure ($0\text{ mmHg}$) = Net Oncotic Pressure ($25\text{ mmHg}$). Net Filtration Pressure = $-8\text{ mmHg}$ (favors reabsorption into vascular bed).

  • Lymphatic Drainage: Approximately $10\%$ of filtered interstitial fluid and small amounts of protein do not reenter capillaries directly; they drain into lymphatic vessels. The lymphatic system connects to the venous system where the thoracic duct joins the left subclavian vein near the left internal jugular vein.

Mechanisms and Clinical Manifestations of Edema

• Pathophysiologic Mechanisms of Edema

  • Edema is excessive fluid accumulation within interstitial spaces caused by fluid shifts from intravascular/lymphatic spaces into tissues.

  • 1. Increased Capillary Hydrostatic Pressure:

    • Venous Obstruction: Increases pressure behind the blockage, forcing fluid into interstitium. Causes include thrombophlebitis, venous blood clots, hepatic venous outflow obstruction, right-sided heart failure, restrictive clothing around extremities, and prolonged standing.

    • Sodium and Water Retention: Plasma volume expansion elevates capillary pressure. Causes include heart failure, renal failure with oliguria, and liver cirrhosis.

  • 2. Decreased Capillary Oncotic Pressure:

    • Occurs when plasma protein (albumin) levels drop due to reduced synthesis or accelerated loss.

    • Decreased Synthesis: Severe liver disease, protein malnutrition.

    • Increased Loss: Glomerular diseases (nephrotic syndrome), hemorrhage, serous wound drainage, extensive burns, protein-losing enteropathies.

  • 3. Increased Capillary Membrane Permeability:

    • Direct vessel endothelial damage or inflammatory responses cause leakage of plasma proteins into the interstitial space. Interstitial oncotic pressure increases, pulling more water into tissues.

    • Triggers include trauma (burns, crushing injuries), neoplastic disease, allergic reactions, and severe infections.

  • 4. Lymphatic Channel Obstruction (Lymphedema):

    • Blockage or surgical removal of lymphatic channels prevents normal reabsorption of proteins and fluid. Accumulation in interstitium leads to severe swelling.

    • Commonly seen in limbs after surgical removal of axillary or femoral lymph nodes during cancer treatment.

• Clinical Manifestations and Types

  • Localized Edema: Confined to a single site or organ (e.g., sprained joint, vasogenic cerebral edema, pulmonary edema, laryngeal edema). Organ-specific edema can be rapidly fatal.

  • Generalized Edema: Uniform distribution of interstitial fluid throughout the entire body.

  • Anasarca: Severe, widespread generalized edema.

  • Dependent Edema: Fluid accumulates in gravity-dependent body areas (feet/legs when standing, sacrum/buttocks when supine). Manifests as pitting edema where digital pressure over bony prominences leaves a persistent depression.

  • Lymphedema vs Capillary Edema: Capillary-derived edema is compressible (pitting). Lymphedema tissue is firm, non-compressible, and can cause gross structural distortion.

  • Effects on Tissue Function: Fluid accumulation expands the physical distance required for oxygen, nutrients, and waste products to diffuse between capillaries and parenchymal cells. Increased interstitial pressure restricts capillary perfusion, precipitating tissue ischemia, delayed wound healing, and heightened risk of infection or pressure injury.

  • Effusion & Third Spacing: Accumulation of fluid within body cavities. Examples include pleural effusion (pleural cavity), pericardial effusion (pericardial sac), and ascites (peritoneal cavity). Third spacing refers to fluid trapping in spaces where it cannot participate in metabolic or circulatory perfusion processes (e.g., peritoneal cavity, intestinal lumen in acute bowel obstruction).

• Treatment Measures

  • Symptomatic management includes diuretic therapy, elevation of edematous extremities, application of compression stockings, avoidance of prolonged standing, and dietary sodium restriction.

Regulation of Sodium, Chloride, and Water Balance

• Electrolyte Distribution in Compartments

  • Extracellular Cations: $\text{Na}^+ = 142\text{ mEq/L}$, $\text{K}^+ = 5\text{ mEq/L}$, $\text{Ca}^{2+} = 5\text{ mEq/L}$, $\text{Mg}^{2+} = 2\text{ mEq/L}$ (Total Cations = $154\text{ mEq/L}$).

  • Extracellular Anions: $\text{HCO}_3^- = 24\text{ mEq/L}$, $\text{Cl}^- = 104\text{ mEq/L}$, $\text{HPO}_4^{2-} = 2\text{ mEq/L}$, $\text{Proteins} = 16\text{ mEq/L}$, Other anions = $8\text{ mEq/L}$ (Total Anions = $154\text{ mEq/L}$).

  • Intracellular Cations: $\text{Na}^+ = 10\text{ mEq/L}$, $\text{K}^+ = 156\text{ mEq/L}$, $\text{Ca}^{2+} = 4\text{ mEq/L}$, $\text{Mg}^{2+} = 26\text{ mEq/L}$ (Total Cations = $196\text{ mEq/L}$).

  • Intracellular Anions: $\text{HCO}_3^- = 12\text{ mEq/L}$, $\text{Cl}^- = 4\text{ mEq/L}$, $\text{HPO}_4^{2-} = 40\text{ to }95\text{ mEq/L}$, $\text{Proteins} = 54\text{ mEq/L}$, Other anions = $31\text{ to }86\text{ mEq/L}$ (Total Anions = $196\text{ mEq/L}$ average).

• Sodium Homeostasis

  • $\text{Na}^+$ constitutes $90\%$ of all ECF cations. ECF concentration is $142\text{ mEq/L}$; ICF concentration is $10\text{ mEq/L}$.

  • Dietary intake ranges from $5\text{ to }6\text{ g/day}$; minimal daily physiological requirement is $500\text{ mg}$.

  • Primary functions: Osmotic pressure regulation, nerve impulse conduction, neuromuscular excitability, acid-base balance, enzyme activation, and transmembrane transport mechanisms.

• Renin-Angiotensin-Aldosterone System (RAAS)

  • Trigger: Reduced renal blood flow/perfusion pressure, decreased circulating volume, low $\text{Na}^+$ levels in renal distal tubules, or increased renal sympathetic nerve activity.

  • Cascade: Juxtaglomerular cells of kidney release the enzyme Renin. Renin cleaves Angiotensinogen (secreted by liver) to form Angiotensin I. Angiotensin-Converting Enzyme (ACE) (located predominantly in pulmonary capillaries, vascular endothelium, and renal epithelial cells) converts Angiotensin I to Angiotensin II.

  • Actions of Angiotensin II: Potent systemic arterial vasoconstrictor (elevates blood pressure and restores renal perfusion). Stimulates adrenal cortex to secrete Aldosterone. Stimulates posterior pituitary to release Antidiuretic Hormone (ADH).

  • Actions of Aldosterone: Mineralocorticoid that acts on distal tubules and collecting ducts of kidneys to promote active reabsorption of $\text{Na}^+$ and water, alongside increased excretion of $\text{K}^+$.

  • Pharmacologic Inhibition: ACE inhibitors and Angiotensin Receptor Blockers (ARBs) disrupt RAAS to lower blood pressure in hypertension.

• Natriuretic Peptide System

  • Natural antagonists to RAAS ("third factor" in sodium regulation; 1st factor = GFR, 2nd factor = Aldosterone).

  • Atrial Natriuretic Peptide (ANP): Synthesized and released by atrial cardiac myocytes in response to elevated transmural atrial stretch/volume.

  • B-Type Natriuretic Peptide (BNP): Released by cardiac ventricular myocytes in response to increased wall stress/pressure (measured clinically to diagnose and evaluate heart failure).

  • Urodilatin: Synthesized by distal nephron epithelial cells in response to increased renal artery pressure and blood flow.

  • Physiologic Effects: Induce systemic vasodilation, inhibit proximal tubular $\text{Na}^+$ reabsorption, increase Glomerular Filtration Rate (GFR), suppress renin and aldosterone secretion, and promote urinary loss of sodium (natriuresis) and water (diuresis).

• Chloride Regulation

  • Primary ECF anion ($104\text{ mEq/L}$ ECF vs $4\text{ mEq/L}$ ICF). Provides electroneutrality in relation to $\text{Na}^+$.

  • Transport is largely passive and follows active $\text{Na}^+$ movements. $\text{Cl}^-$ concentration varies inversely with $\text{HCO}_3^-$ concentration.

• Antidiuretic Hormone (ADH) and Thirst Mechanism

  • Synthesized in cell bodies of hypothalamic neurons, transported down axons, and stored/released from the pars nervosa of the posterior pituitary gland.

  • Stimuli for Release: High ECF osmolality (detected by hypothalamic osmoreceptors); low arterial blood volume or blood pressure (detected by volume receptors in left/right atria and thoracic vessels, and baroreceptors in carotid sinus, aorta, and pulmonary arteries).

  • Physiologic Actions: Increases water permeability in distal renal tubules and collecting ducts via aquaporins, increasing renal water reabsorption, decreasing urine output, and concentrating urine. High ADH concentrations trigger systemic arterial vasoconstriction.

Alterations in Sodium, Chloride, and Water Balance

• Classification by Tonicity

  • Normal plasma osmolality range: $275\text{ to }295\text{ mOsm/kg}$.

  • Serum Osmolarity Formula:

$\text{Serum Osmolarity} = 2 \times [\text{Na}^+] + \frac{[\text{Glu}]}{18} + \frac{\text{BUN}}{2.8}$

    (where $[\text{Na}^+]$ is in $\text{mEq/L}$, $[\text{Glu}]$ and $\text{BUN}$ are in $\text{mg/dL}$)

• Isotonic Alterations

  • Tonicity remains equal to $0.9\%$ $\text{NaCl}$ solution; cellular volume remains unchanged.

  • Isotonic Fluid Loss (Hypovolemia):

    • Causes: Hemorrhage, severe wound drainage, excessive diaphoresis, gastrointestinal losses.

    • Clinical Findings: Weight loss, skin and mucous membrane dryness, oliguria, elevated hematocrit, tachycardia, flattened neck veins, normal or low blood pressure. Hypovolemic shock in severe cases.

    • Treatment: Isotonic fluids (oral fluids, IV $0.9\%$ normal saline, or Ringer's lactate [RL]).

  • Isotonic Fluid Excess (Hypervolemia):

    • Causes: Excessive IV normal saline administration, hyperaldosteronism, corticosteroid therapy (e.g., prednisone), oliguric renal failure.

    • Clinical Findings: Sudden weight gain ($1\text{ L}$ fluid weight = $1\text{ kg}$), neck vein distention, elevated blood pressure, decreased hematocrit and plasma protein concentrations (dilutional effect), edema, pulmonary edema, heart failure.

    • Treatment: Diuretic therapy, salt restriction.

• Hypertonic Alterations

  • ECF osmolality $> 295\text{ mOsm/kg}$. Attracts water out of cells, causing intracellular dehydration and cell shrinkage.

  • Hypernatremia ($\text{Na}^+ > 145\text{ mEq/L}$):

    • Hypovolemic Hypernatremia: Concurrent loss of $\text{Na}^+$ and water, with water loss exceeding $\text{Na}^+$ loss. Causes: Loop diuretics, osmotic diuresis (uncontrolled diabetes mellitus, mannitol administration), kidney disease with concentration defects.

    • Isovolemic Hypernatremia: Loss of free water with normal body sodium. Causes: Inadequate intake (coma, confusion, immobilization, inability to communicate thirst in infants), fever with hyperventilation, cutaneous evaporative losses (burns), diabetes insipidus (central or nephrogenic).

    • Hypervolemic Hypernatremia: Increased TBW accompanied by greater $\text{Na}^+$ elevation. Causes: Hypertonic saline IV infusion, Cushing syndrome, adrenal hyperplasia, ingestion of excessive salt water or soy sauce.

  • Hyperchloremia: Serum $\text{Cl}^- > 105\text{ mEq/L}$. Secondary to hypernatremia or severe bicarbonate deficits (hyperchloremic metabolic acidosis).

  • Clinical Manifestations of Dehydration & Hypernatremia: Thirst, fever, dry mucous membranes, reduced skin turgor, oliguria, high urine specific gravity ($>1.030$). Central nervous system manifestations secondary to brain cell shrinkage: muscle twitching, hyperreflexia, weakness, lethargy, confusion, seizures, coma, cerebral hemorrhage.

  • Treatment: Hypovolemic/Isovolemic: Slow administration of oral free water or IV $5\%$ dextrose in water ($\text{D}_5\text{W}$) to prevent cerebral edema. Hypervolemic: Loop diuretics.

• Hypotonic Alterations

  • ECF osmolality $< 275\text{ mOsm/kg}$. Osmotic force drives water into cells, causing intracellular edema and cell swelling.

  • Hyponatremia ($\text{Na}^+ < 135\text{ mEq/L}$; severe $< 120\text{ mEq/L}$):

    • Hypovolemic Hyponatremia: Loss of total body water with greater loss of $\text{Na}^+$. Causes: Prolonged vomiting, severe diarrhea, adrenal insufficiency (hypoaldosteronism), diuretic use.

    • Isovolemic Hyponatremia: Loss of $\text{Na}^+$ without significant water loss. Causes: SIADH, hypothyroidism, glucocorticoid deficiency.

    • Dilutional Hyponatremia (Water Intoxication): Ingestion or infusion of massive free water quantities. Causes: Continuous $\text{D}_5\text{W}$ administration, tap water enemas, compulsive psychogenic polydipsia, fresh-water drowning, SSRI therapy.

    • Hypervolemic Hyponatremia: Increased total body $\text{Na}^+$ with excess TBW expansion. Causes: Congestive heart failure, liver cirrhosis, nephrotic syndrome.

    • Hypertonic Hyponatremia (Pseudohyponatremia): Osmotic shift of water from ICF to ECF caused by high levels of solutes like glucose (hyperglycemia), hyperlipidemia, or hyperproteinemia, diluting serum $\text{Na}^+$ concentration.

  • Clinical Manifestations: Impaired neuronal conduction, altered action potentials, cellular swelling. Nausea, vomiting (at $125\text{ to }130\text{ mEq/L}$). Neurologic symptoms ($<125\text{ mEq/L}$): headache, lethargy, confusion, apprehension, seizures, coma, cerebral edema, elevated intracranial pressure.

  • Treatment: Fluid restriction for dilutional states. Cautious IV hypertonic ($3\%$) saline for life-threatening symptoms (e.g., seizures); rapid correction must be avoided to prevent osmotic demyelination syndrome (axonal brain damage). Vasopressin receptor antagonists (vaptans) used for hypervolemic/isovolemic hyponatremia.

  • Hypochloremia: Serum $\text{Cl}^- < 97\text{ mEq/L}$. Accompanies hyponatremia or metabolic alkalosis. Pathognomonic hallmark in cystic fibrosis.

Potassium Homeostasis and Imbalances

• Physiologic Principles of Potassium Balance

  • Predominant intracellular cation ($150\text{ to }160\text{ mEq/L}$ ICF vs $3.5\text{ to }5.0\text{ mEq/L}$ ECF). Total body potassium content is $\approx 4000\text{ mEq}$.

  • Maintained by the active $\text{Na}^+/\text{K}^+$ ATPase pump.

  • The ratio of ICF $\text{K}^+$ to ECF $\text{K}^+$ determines resting membrane potential ($E_m$) of excitable tissues (nerve and muscle cells).

  • Dietary intake: $40\text{ to }150\text{ mEq/day}$ ($1.5\text{ mEq/kg}$ body weight). $\approx 90\%$ absorbed in GI tract.

  • Shift Factors: Insulin, epinephrine, and alkalosis shift $\text{K}^+$ into cells. Acidosis, insulin deficiency, aldosterone deficiency, cell lysis, and strenuous exercise shift $\text{K}^+$ out of cells. Glucagon blocks cellular entry.

  • Renal Handling: Glomerulus freely filters $\text{K}^+$; $90\%$ is reabsorbed in proximal tubule and loop of Henle. Principal cells of distal tubule secrete $\text{K}^+$ under aldosterone control; intercalated cells reabsorb $\text{K}^+$.

  • Normal Daily Losses: Stool $5\text{ to }10\text{ mEq/L}$; Sweat $0\text{ to }20\text{ mEq/L}$; Urine $40\text{ to }120\text{ mEq/L}$.

• Hypokalemia ($\text{K}^+ < 3.5\text{ mEq/L}$)

  • Causes: Reduced dietary intake (starvation, eating disorders, severe alcoholism); ECF-to-ICF shifts (alkalosis, insulin administration, treatment of diabetic ketoacidosis [DKA], treatment of pernicious anemia with B12/folate, familial hypokalemic periodic paralysis); GI loss (diarrhea, vomiting, nasogastric suction, laxatives, licorice overuse); Renal loss (loop and thiazide diuretics, hyperaldosteronism, increased tubular flow rate, hypomagnesemia, Bartter/Gitelman syndromes, nephrotoxic drugs like amphotericin B, gentamicin, nafcillin).

  • Electrophysiology: Resting membrane potential becomes hyperpolarized ($E_m$ becomes more negative, e.g., $-90\text{ mV}$ to $-100\text{ mV}$). Increased distance between $-100\text{ mV}$ and threshold potential ($E_t$) decreases cell excitability.

  • ECG Changes: Delayed ventricular repolarization. Decreased T-wave amplitude, flattened or inverted T waves, prominent U waves, ST-segment depression, peaked P waves, prolonged QT interval. Dysrhythmias: sinus bradycardia, AV block, paroxysmal atrial tachycardia. Potentiates digitalis toxicity.

  • Systemic Effects: Skeletal muscle weakness (progressing from legs/arms to diaphragm, causing respiratory arrest), smooth muscle atony (constipation, paralytic ileus, distention), impaired glycogen synthesis, impaired renal concentrating ability (polyuria, polydipsia).

  • Treatment: Oral replacement ($40\text{ to }80\text{ mEq/day}$ max); IV replacement (max $20\text{ mEq/h}$, concentration $\le 40\text{ mEq/L}$). Hypomagnesemia must be corrected simultaneously.

• Hyperkalemia ($\text{K}^+ > 5.5\text{ mEq/L}$)

  • Causes: Excessive intake (K+ salt substitutes, IV boluses of Penicillin G, rapid whole blood infusion); ICF-to-ECF shifts (hypoxia, acidosis, insulin deficiency, DKA, massive crush injuries, severe burns, digitalis toxicity); Decreased renal excretion (oliguric acute or end-stage kidney failure, Addison disease, hypoaldosteronism, ACE inhibitors, ARBs, K+-sparing diuretics, aldosterone antagonists).

  • Electrophysiology: Resting membrane potential becomes hypopolarized ($E_m$ becomes less negative, e.g., $-90\text{ mV}$ to $-70\text{ mV}$). Initially increases cell excitability. In severe hyperkalemia ($\ge 6.0\text{ mEq/L}$), $\text{Na}^+$ channels become inactivated, preventing repolarization and suppressing conduction velocity.

  • ECG Changes: Narrow, tall, peaked T waves; shortened QT interval; prolonged PR interval; depressed ST segment; loss of P waves; widening of QRS complex. Progresses to sine-wave pattern, ventricular fibrillation, and cardiac arrest.

  • Systemic Effects: Neuromuscular irritability, restlessness, intestinal cramping, diarrhea (early/mild state). Flaccid paralysis, severe weakness, loss of muscle tone (late/severe state).

  • Treatment: Emergency membrane stabilization with IV calcium gluconate; cellular $\text{K}^+$ redistribution via IV insulin and glucose or $\text{NaHCO}_3$; oral potassium binders; loop diuretics; hemodialysis.

Calcium and Phosphate Regulation and Imbalances

• Calcium ($\text{Ca}^{2+}$) Physiology

  • Total body content: $\approx 1200\text{ g}$ ($99\%$ in bone as hydroxyapatite crystals, $1\%$ in ECF/ICF).

  • Normal total serum calcium: $9.0\text{ to }10.5\text{ mg/dL}$ ($4.5\text{ to }5.5\text{ mEq/L}$).

    • Protein-bound fraction: $\approx 50\%$ (bound primarily to albumin).

    • Ionized/free fraction: $\approx 40\%$ ($5.5\text{ to }5.6\text{ mg/dL}$ or $2.2\text{ to }2.5\text{ mmol/L}$). Represents the biologically active form.

    • Complexed fraction: $\approx 10\%$ (bound to citrate or phosphate).

  • Functions: Structural rigidity of bone/teeth, enzymatic cofactor for blood clotting cascades, cell receptor signal transduction, hormone secretion, cell membrane stability/permeability, nerve impulse transmission, muscle contraction.

• Phosphate ($\text{HPO}_4^{2-}$) Physiology

  • Total body content: $85\%$ in bone, $15\%$ in ICF and ECF.

  • Normal serum inorganic phosphate: $2.5\text{ to }4.5\text{ mg/dL}$ ($6.0\text{ to }7.0\text{ mg/dL}$ in infants/children).

  • Functions: Intracellular anion buffer, component of nucleic acids, high-energy compounds (ATP, creatine phosphate), red blood cell 2,3-diphosphoglycerate (2,3-DPG).

  • Reciprocal Relationship: Calcium and phosphate concentrations are governed by a constant solubility product:

$[\text{Ca}^{2+}] \times [\text{HPO}_4^{2-}] = K$

• Hormonal Endocrine Control

  • Parathyroid Hormone (PTH): Secreted by parathyroid glands in response to hypocalcemia. Increases serum $\text{Ca}^{2+}$ by stimulating osteoclast-mediated bone resorption, enhancing distal renal tubular reabsorption of $\text{Ca}^{2+}$, inhibiting proximal renal tubular reabsorption of phosphate (increasing urinary phosphate excretion), and triggering renal conversion of Vitamin D to its active form.

  • Vitamin D (Calcitriol / $1,25(\text{OH})_2\text{D}_3$): Fat-soluble steroid activated sequentially in liver and kidney. Kidney activation stimulated by low serum calcium and elevated PTH. Calcitriol enhances intestinal absorption of $\text{Ca}^{2+}$ and phosphate, promotes bone calcification, and enhances renal tubular reabsorption of $\text{Ca}^{2+}$.

  • Calcitonin: Secreted by thyroid C cells in response to hypercalcemia. Inhibits osteoclastic bone resorption and increases renal excretion of $\text{Ca}^{2+}$ and phosphate.

  • pH Influence: Acidosis decreases protein binding of calcium, increasing ionized $\text{Ca}^{2+}$. Alkalosis increases calcium binding to albumin, decreasing biologically active ionized $\text{Ca}^{2+}$.

• Hypocalcemia ($\text{Ca}^{2+} < 9.0\text{ mg/dL}$; ionized $< 5.5\text{ mg/dL}$)

  • Causes: Hypoparathyroidism (surgical removal of glands), Vitamin D deficiency, malabsorption syndromes (celiac disease, short bowel syndrome), severe hypomagnesemia, pancreatitis (fat saponification), massive citrate-preserved blood transfusions, alkalosis, hypoalbuminemia.

  • Electrophysiology: Threshold potential ($E_t$) becomes more negative, moving closer to resting membrane potential ($E_m$). Hypopolarization increases neuromuscular excitability.

  • Clinical Manifestations: Paresthesias (perioral, fingers/toes), carpopedal spasm, hyperreflexia, laryngospasm, seizures, tetany, hyperactive bowel sounds, intestinal cramps.

    • Chvostek Sign: Facial twitching elicited by tapping over the facial nerve/zygomatic arch.

    • Trousseau Sign: Carpal spasm elicited by inflating a blood pressure cuff above systolic pressure for 3 to 5 minutes.

    • ECG: Prolonged QT interval (delayed ventricular repolarization).

  • Treatment: Emergency IV $10\%$ calcium gluconate; oral calcium replacement; Vitamin D supplementation.

• Hypercalcemia ($\text{Ca}^{2+} > 10.5\text{ mg/dL}$)

  • Causes: Primary hyperparathyroidism, thyrotoxicosis, malignancy (secretion of PTH-related protein [PTHrP] or bony metastases), sarcoidosis (excess active Vitamin D), Vitamin D toxicity, prolonged immobilization.

  • Electrophysiology: Threshold potential ($E_t$) becomes less negative (more positive), increasing distance from resting membrane potential ($E_m$). Reduces cell membrane excitability.

  • Clinical Manifestations: Muscle weakness, fatigue, lethargy, anorexia, nausea, constipation, confusion, nephrolithiasis (calcium kidney stones), polyuria.

    • ECG: Shortened QT segment, widened depressed T waves, bradycardia, heart blocks.

  • Treatment: IV normal saline hydration, loop diuretics, bisphosphonates, calcitonin, denosumab, cinacalcet.

• Hypophosphatemia ($\text{HPO}_4^{2-} < 2.0\text{ mg/dL}$)

  • Causes: Intestinal malabsorption, Vitamin D deficiency, excessive use of aluminum/magnesium antacids, severe alcoholism, refeeding syndrome, respiratory alkalosis, hyperparathyroidism, IV iron therapy, FGF23 gene mutations.

  • Clinical Manifestations: Depleted cellular ATP and red blood cell 2,3-DPG. Tissue hypoxia, bradycardia, heart block, muscle weakness, respiratory failure, leukocyte/platelet dysfunction (infection and bleeding risks), irritability, confusion, seizures, coma, rickets/osteomalacia.

  • Treatment: Oral or IV phosphate supplementation.

• Hyperphosphatemia ($\text{HPO}_4^{2-} > 4.7\text{ mg/dL}$)

  • Causes: Acute or chronic renal failure with loss of GFR, tumor lysis syndrome following chemotherapy, hypoparathyroidism, phosphate-containing laxatives or enemas.

  • Clinical Manifestations: Reciprocal hypocalcemia with neuromuscular tetany. Long-term: Metastatic soft tissue calcifications in blood vessels, joints, kidneys, and lungs.

  • Treatment: Phosphate binders (sevelamer, lanthanum carbonate, ferric citrate), dietary restriction, dialysis.

Magnesium Homeostasis and Imbalances

• Physiologic Principles

  • Second most abundant intracellular cation ($1.5\text{ to }3.0\text{ mg/dL}$ or $1.5\text{ to }3.0\text{ mEq/L}$ in plasma; $1/3$ protein-bound, $2/3$ ionized).

  • Distribution: $40\%$ to $60\%$ stored in bone and muscle; $30\%$ intracellular; $1\%$ extracellular.

  • Functions: Enzymatic cofactor in cellular reactions, nucleic acid stability, protein synthesis, ATP hydrolysis, cardiac electrophysiology, vascular smooth muscle tone regulation, and calcium channel antagonism.

• Hypomagnesemia ($\text{Mg}^{2+} < 1.5\text{ mEq/L}$)

  • Causes: Malnutrition, GI malabsorption, alcoholism, renal tubular dysfunction, loop/thiazide diuretics, proton pump inhibitors.

  • Pathophysiological Interplay: Loss of $\text{Mg}^{2+}$ removes inhibition on renal outer medullary potassium (ROMK) channels, precipitating renal $\text{K}^+$ wasting and refractory hypokalemia. Inhibits PTH secretion, causing secondary hypocalcemia.

  • Clinical Manifestations: Neuromuscular irritability, positive Chvostek and Trousseau signs, hyperreflexia, ataxia, nystagmus, tetany, seizures, tachycardia, hypotension, ventricular dysrhythmias.

  • Treatment: Intramuscular or IV magnesium sulfate.

• Hypermagnesemia ($\text{Mg}^{2+} > 3.0\text{ mEq/L}$)

  • Causes: Renal insufficiency/failure, excessive ingestion of magnesium antacids or cathartics, adrenal insufficiency.

  • Clinical Manifestations: Suppressed neuromuscular transmission. Loss of deep tendon reflexes, muscle weakness, lethargy, drowsiness, nausea, vomiting, hypotension, bradycardia, respiratory depression, cardiac arrest.

  • Treatment: Avoidance of magnesium sources, IV calcium gluconate (antagonizes magnesium toxicity), hemodialysis.

Fundamentals of Acid-Base Balance and Buffer Systems

• Hydrogen Ion Concentration and pH Scale

  • Normal arterial hydrogen ion concentration is $\approx 0.0000001\text{ mg/L}$ ($10^{-7}\text{ mg/L}$).

  • Standard pH definition: $\text{pH} = -\log_{10}[\text{H}^+]$.

  • Logarithmic structure: Each unit change on the pH scale represents a 10-fold change in $\text{H}^+$ concentration.

  • Normal Arterial Blood pH: $7.35\text{ to }7.45$ (midpoint $7.40$). Acidemia: $\text{pH} < 7.35$. Alkalemia: $\text{pH} > 7.45$.

  • Fluid pH Values: Gastric juice ($1.0\text{ to }3.0$); Urine ($5.0\text{ to }6.0$); Venous blood ($7.37$); CSF ($7.32$); Pancreatic fluid ($7.8\text{ to }8.0$); Bile ($7.0\text{ to }8.0$); Small intestine fluid ($6.5\text{ to }7.5$).

• Types of Body Acids

  • Daily metabolic acid production: $50\text{ to }100\text{ mEq/day}$ ($1\text{ mEq/kg/day}$).

  • Volatile Acid: Carbonic acid ($\text{H}_2\text{CO}_3$). Weak acid that dissociates into carbon dioxide ($\text{CO}_2$) gas and water ($\text{H}_2\text{O}$) in the presence of carbonic anhydrase. Excreted via pulmonary ventilation.

  • Nonvolatile Acids: Lactic acid, sulfuric acid, phosphoric acid, acetoacetic acid, beta-hydroxybutyric acid. Cannot be converted to gas; eliminated solely by renal tubules.

• Three Lines of Defense for Acid-Base Regulation

  1. Chemical Buffer Systems: Plasma bicarbonate, phosphate, protein, hemoglobin. Act instantaneously.

  2. Respiratory System Control: Regulates $\text{CO}_2$ elimination. Acts within seconds to minutes.

  3. Renal System Control: Excretes $\text{H}^+$, reabsorbs/regenerates $\text{HCO}_3^-$. Slowest onset (hours to days), but provides definitive long-term correction.

• Bicarbonate–Carbonic Acid Buffer System

  • System Ratio: Normal blood contains $24\text{ mEq/L}$ $\text{HCO}_3^-$ to $1.2\text{ mEq/L}$ $\text{H}_2\text{CO}_3$ (derived from arterial $\text{PaCO}_2$ of $40\text{ mmHg}$).

  • Target Buffer Ratio: $20:1$

$\text{pH} = \frac{\text{Base}}{\text{Acid}} = \frac{\text{Renal Regulation (slow)}}{\text{Pulmonary Regulation (fast)}} = \frac{[\text{HCO}_3^-]}{[\text{H}_2\text{CO}_3]} = \frac{20}{1} = 7.40$

• Protein and Cellular Buffers

  • Intracellular proteins absorb or release $\text{H}^+$ ions. Hemoglobin in red blood cells buffers $\text{H}^+$ and binds $\text{CO}_2$ directly to form carbaminohemoglobin ($\text{HHbCO}_2$).

  • Transcellular Ion Exchange: $\text{K}^+$ shifts out of cells in exchange for $\text{H}^+$ entering cells during acidosis. Conversely, $\text{K}^+$ shifts into cells during alkalosis.

• Renal Mechanisms of Acid Elimination

  1. Bicarbonate Filtration Reabsorption: Filtered $\text{HCO}_3^-$ combines with secreted $\text{H}^+$ to form $\text{H}_2\text{CO}_3$, converted by carbonic anhydrase to $\text{CO}_2$ and $\text{H}_2\text{O}$, which diffuse into tubular cells and are reconstituted back to $\text{HCO}_3^-$ for reabsorption into plasma.

  2. Formation of Titratable Acid (Phosphate Buffer): Secreted $\text{H}^+$ combines with monobasic dibasic phosphate ($\text{HPO}_4^{2-}$) to form $\text{H}_2\text{PO}_4^-$, which is lipid insoluble and excreted in urine.

  3. Ammonium Buffering: Glutamine breakdown in renal tubular cells yields ammonia ($\text{NH}_3$). Secreted $\text{NH}_3$ binds $\text{H}^+$ to form ammonium ($\text{NH}_4^+$), trapping it in urine for excretion.

Pathophysiology of Primary and Mixed Acid-Base Imbalances

• Compensation vs Correction

  • Compensation: The non-offending organ system adjusts its response (pulmonary ventilation or renal excretion) to return the $\text{HCO}_3^- : \text{H}_2\text{CO}_3$ ratio back to $20:1$, normalizing pH even though absolute electrolyte parameters remain abnormal.

  • Correction: Occurs when the primary cause is resolved and both components of the buffer pair return to absolute normal values.

• Metabolic Acidosis

  • Diagnostic Criteria: $\text{pH} < 7.35$, $\text{HCO}_3^- < 22\text{ mEq/L}$.

  • Anion Gap Calculation:

$\text{Anion Gap} = [\text{Na}^+] + [\text{K}^+] - ([\text{HCO}_3^-] + [\text{Cl}^-])$

$\text{Anion Gap} = 140 + 4 - (24 + 110) = 10\text{ to }12\text{ mEq/L}$

*   *Elevated Anion Gap Causes:* Accumulation of unmeasured non-carbonic acids. Ketoacidosis (DKA, alcoholic ketoacidosis, starvation), lactic acidosis (hypoxemia, shock), renal failure, toxic ingestions (salicylates, ethylene glycol, methanol, paraldehyde).
*   *Normal Anion Gap (Hyperchloremic) Causes:* Direct loss of $\text{HCO}_3^-$ with compensatory $\text{Cl}^-$ retention. Severe diarrhea, ureterosigmoidoscopy, early CKD, renal tubular acidosis.
*   *Clinical Findings:* Kussmaul respirations (deep, rapid breathing to blow off $\text{CO}_2$), headache, lethargy, confusion, coma, anorexia, nausea, vomiting, abdominal discomfort, dysrhythmias, hypotension. Shifts oxyhemoglobin dissociation curve to the right.
*   *Emerging Pharmacotherapy:* **Veverimer**, a non-absorbed polymer that binds $\text{H}^+$ and $\text{Cl}^-$ in the gastrointestinal tract, removing them via feces to treat chronic metabolic acidosis in CKD without adding sodium load.

• Metabolic Alkalosis

  • Diagnostic Criteria: $\text{pH} > 7.45$, $\text{HCO}_3^- > 26\text{ mEq/L}$.

  • Causes: Gastric fluid loss (vomiting, nasogastric suction), hyperaldosteronism, diuretic therapy, excess sodium bicarbonate ingestion (baking soda antacids), massive citrate blood transfusions.

  • Pathophysiology: Loss of $\text{Cl}^-$ in gastric juice causes renal paradoxical preservation of $\text{HCO}_3^-$ to retain electroneutrality (hypochloremic metabolic alkalosis).

  • Clinical Findings: Slow, shallow hypoventilation (to retain $\text{CO}_2$), paresthesias, carpopedal spasm, hyperreflexia, tetany, seizures, atrial tachycardia. Shifts oxyhemoglobin dissociation curve to the left.

  • Treatment: Isotonic $\text{NaCl}$ solution infusion (allows renal $\text{HCO}_3^-$ excretion); potassium replacement.

• Respiratory Acidosis

  • Diagnostic Criteria: $\text{pH} < 7.35$, $\text{PaCO}_2 > 45\text{ mmHg}$.

  • Causes: Alveolar hypoventilation (hypercapnia). Respiratory center depression (opioid overdose, brainstem trauma), neuromuscular paralysis, chest wall restriction (flail chest, severe kyphoscoliosis, morbid obesity), or intrinsic lung pathology (COPD, severe asthma, pulmonary edema).

  • Acute vs Chronic: Acute states lack immediate renal compensation (normal $\text{HCO}_3^-$). Chronic states demonstrate full renal compensation over several days (elevated $\text{HCO}_3^-$ with near-normal pH).

  • Clinical Findings: Headache, blurred vision, restlessness, lethargy, tremors, seizures, coma, cerebral vasodilation (flushed/pink skin). Myocardial depression and dysrhythmias in severe chronic cases.

  • Treatment: Restoration of effective alveolar ventilation; mechanical ventilation if required. Cautious oxygen therapy in chronic hypercapnia to avoid suppressing hypoxic respiratory drive.

• Respiratory Alkalosis

  • Diagnostic Criteria: $\text{pH} > 7.45$, $\text{PaCO}_2 < 38\text{ mmHg}$.

  • Causes: Alveolar hyperventilation (hypocapnia). Hypoxemia (high altitude), hypermetabolic conditions (fever, thyrotoxicosis, severe anemia), early salicylate toxicity, acute anxiety/panic disorders, improper mechanical ventilator settings.

  • Clinical Findings: Dizziness, confusion, paresthesias of fingertips and lips, carpopedal spasm, seizures, coma, deep rapid breathing (tachypnea). Cerebral vasoconstriction decreases cerebral perfusion.

  • Treatment: Address underlying etiology; manage anxiety or pain; rebreathing into a paper bag (with caution) or using rebreathing circuits to re-elevate inspired $\text{CO}_2$

• Mixed Acid-Base Disorders

  • Simultaneous presence of two or more primary acid-base disturbances (e.g., severe COPD with diuretic-induced metabolic alkalosis; acute renal failure with severe hyperventilation).

  • Blood pH may appear deceptively normal despite marked alterations in both $$\text{PaCO