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Chapter 26
Fluid, Electrolyte, and Acid - Base Homeostasis
James F. Thompson, Ph.D.
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Fluid Compartments• Body Fluids are separated by semi-
permeable membranes into various physiological (functional) compartments
• Two Compartment Model– Intracellular = Cytoplasmic (inside cells)– Extracellular (outside cells)
• The Two Compartment Model is useful clinically for understanding the distribution of many drugs in the body
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Fluid Compartments• Three Compartment Model
– [1] Intracellular = Cytoplasmic (inside cells)
– [Extracellular compartment is subdivided into:]
– [2] Interstitial = Intercellular = Lymph (between the cells in the tissues)
– [3] Plasma (fluid portion of the blood)• The Three Compartment Model is more
useful for understanding physiological processes
• Other models with more compartments can sometimes be useful, e.g., consider lymph in the lymph vessels, CSF, ocular fluids, synovial and serous fluids as separate compartments
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Fluid Compartments• Total Body Water
(TBW) - 42L, 60% of body weight– Intracellular Fluid (ICF) -
28L, 67% of TBW– Extracellular Fluid (ECF) -
14L, 33% of TBW• Interstitial Fluid - 11L, 80%
ECF• Plasma - 3L, 20% of ECF
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Fluid Balance• Fluid balance
– When in balance, adequate water is present and is distributed among the various compartments according to the body’s needs
– Many things are freely exchanged between fluid compartments, especially water
– Fluid movements by:• bulk flow (i.e.,
blood & lymph circulation)
• diffusion & osmosis – in most regions
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Water• General
– Largest single chemical component of the body: 45-75% of body mass
– Fat (adipose tissue) is essentially water free, so there is relatively more or less water in the body depending on % fat composition
– Water is the solvent for most biological molecules within the body
– Water also participates in a variety of biochemical reactions, both anabolic and catabolic
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Water• Water balance
– Sources for 2500 mL - average daily intake
• Metabolic Water• Preformed Water
– Ingested Foods– Ingested
Liquids
– Balance achieved if daily output also = 2500 mL
• GI tract• Lungs • Skin
– evaporation – perspiration
• Kidneys
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Regulating Fluid Intake - Thirst• Recall the role of the Renin-Angiotensin
System in regulating thirst along with the Autonomic NS reflexes diagramed below
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Regulating Fluid Intake - Thirst Quenching• Wetting the oral mucosa (temporary)• Stretching of the stomach• Decreased blood/body fluid osmolarity
= increased hydration (dilution) of the blood is the most important
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Regulation of Fluid Output• Hormonal control
– AntiDiuretic Hormone (ADH) [neurohypophysis]– Aldosterone [adrenal cortex]– Atrial Natriuretic Peptide (ANP) [heart atrial walls]
• Physiologic fluid imbalances– Dehydration: blood pressure, GFR– Overhydration: blood pressure, GFR– Hyperventilation - water loss through lungs– Vomiting & Diarrhea - excessive water loss– Fever - heavy perspiration– Burns - initial fluid loss; may persist in severe burns– Hemorrhage – if blood loss is severe
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Concentrations of Solutes• Non-electrolytes
– molecules formed by only covalent bonds– do not form charged ions in solution
• Electrolytes– Molecules formed with some ionic bonds; – Disassociate into cations (+) & anions (-) in
solutions (acids, bases, salts)– 4 important physiological functions in the
body• essential minerals in certain biochemical reactions• control osmosis = control the movement of water
between compartments• maintain acid-base balance• conduct electrical currents (depolarization events)
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Distribution of H2O & Electrolytes• Recall Starling’s Law of the Capillaries which
explains fluid and solute movements from Ch. 19
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Distribution of Electrolytes
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Distribution of Major Electrolytes• Na+ and CL- predominate in extracellular fluids
(interstitial fluid and plasma) but are very low in the intracellular fluid (cytoplasm)
• K+ and HPO42- predominate in intracellular fluid
(cytoplasm) but are in very low concentration in the extracellular fluids (interstitial fluid and plasma)
• At body fluid pH, proteins [P-] act as anions; total protein concentration [P-] is relatively high, the second most important “anion,” in the cytoplasm, [P-] is intermediate in blood plasma, but [P-] is very low in the interstitial fluid
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Distribution of Minor Electrolytes
• HCO3- is in intermediate concentrations in all
fluids, a bit lower in the intracellular fluid (cytoplasm); it is an important pH buffer in the extracellular comparments
• Ca++ is in low concentration in all fluid compartments, but it must be tightly regulated, as small shifts in Ca++ concentration in any compartment have serious effects
• Mg++ is in low concentration in all fluid compartments, but Mg++ is a bit higher in the intracellular fluid (cytoplasm), where it is a component of many cellular enzymes
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Electrolyte Balance• Aldosterone [Na+] [Cl-] [H2O] [K+]
• Atrial Natriuretic Peptide (opposite effect)
• Antidiuretic Hormone [H2O] ( [solutes])
• Parathyroid Hormone [Ca++] [HPO4-]
• Calcitonin (opposite effect)
• Female sex hormones [H2O]
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Electrolytes• Sodium (Na+) - 136-142 mEq/liter
– Most abundant cation• major ECF cation (90% of cations present)• determines osmolarity of ECF
– Regulation• Aldosterone• ADH• ANP
– Homeostatic imbalances• Hyponatremia - muscle weakness, coma• Hypernatremia - coma
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Electrolytes• Chloride (Cl-) - 95-103 mEq/liter
– Major ECF anion• helps balance osmotic potential and electrostatic
equilibrium between fluid compartments • plasma membranes tend to be leaky to Cl-
anions
– Regulation: aldosterone– Homeostatic imbalances
• Hypochloremia - results in muscle spasms, coma [usually occurs with hyponatremia] often due to prolonged vomiting
• elevated sweat chloride diagnostic of Cystic Fibrosis
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Electrolytes• Potassium (K+)
– Major ICF cation• intracellular 120-125 mEq/liter• plasma 3.8-5.0 mEq/liter
– Very important role in resting membrane potential (RMP) and in action potentials
– Regulation:• Direct Effect: excretion by kidney tubule • Aldosterone
– Homeostatic imbalances• Hypokalemia - vomiting, death• Hyperkalemia - irritability, cardiac fibrillation,
death
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Electrolytes• Calcium (Ca2+)
– Most abundant ion in body• plasma 4.6-5.5 mEq/liter • most stored in bone (98%)
– Regulation:• Parathyroid Hormone (PTH) - blood Ca2+
• Calcitonin (CT) - blood Ca2+
– Homeostatic imbalances:• Hypocalcemia - muscle cramps, convulsions• Hypercalcemia - vomiting, cardiovascular
symptoms, coma; prolonged abnormal calcium deposition, e.g., stone formation
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Electrolytes• Phosphate (H2PO4
-, HPO42-, PO4
3-)– Important ICF anions; plasma 1.7-2.6 mEq/liter
• most (85%) is stored in bone as calcium salts• also combined with lipids, proteins, carbohydrates,
nucleic acids (DNA and RNA), and high energy phosphate transport compound
• important acid-base buffer in body fluids
– Regulation - regulated in an inverse relationship with Ca2+ by PTH and Calcitonin
– Homeostatic imbalances• Phosphate concentrations shift oppositely from calcium
concentrations and symptoms are usually due to the related calcium excess or deficit
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Electrolytes• Magnesium (Mg2+)
– 2nd most abundant intracellular electrolyte, 1.3-2.1 mEq/liter in plasma• more than half is stored in bone, most of the rest
in ICF (cytoplasm)• important enzyme cofactor; involved in
neuromuscular activity, nerve transmission in CNS, and myocardial functioning
– Excretion of Mg2+ caused by hypercalcemia, hypermagnesemia
– Homeostatic imbalance• Hypomagnesemia - vomiting, cardiac
arrhythmias• Hypermagnesemia - nausea, vomiting
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Acid-Base Balance• Normal metabolism produces H+ (acidity)• Three Homeostatic mechanisms:
– Buffer systems - instantaneous; temporary– Exhalation of CO2 - operates within minutes;
cannot completely correct serious imbalances– Kidney excretion - can completely correct any
imbalance (eventually)• Buffer Systems
– Consists of a weak acid and the salt of that acid which functions as a weak base
– Strong acids dissociate more rapidly and easily than weak acids
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Acid-Base Balance• Carbonic Acid - Bicarbonate Buffer
– A weak base (recall carbonic anhydrase)
– H+ + HCO3- H2CO3 H2O + CO2
• Phosphate Buffer – NaOH + NaH2PO4 H2O + Na2HPO4
– HCl + Na2HPO4 NaCl + NaH2PO4
• Protein Buffer (esp. hemoglobin & albumin)– Most abundant buffer in body cells and plasma– Amino acids have amine group (proton acceptor
= weak base) and a carboxyl group (proton donor = weak acid)
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Acid-Base Balance• CNS and peripheral
chemoreceptors note changes in blood pH
• Increased [H+] causes immediate hyperventilation and later increased renal secretion of [H+] and [NH4
+]
• Decreased [H+] causes immediate hypoventilation and later decreased renal secretion of [H+] and [NH4
+]
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Acid-Base Imbalances• Acidosis
– High blood [H+]– Low blood pH, <7.35
• Alkalosis– Low blood [H+]– High blood pH, >7.45
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Acid-Base Imbalances
• Acid-Base imbalances may be due to problems with ventilation or due to a variety of metabolic problems– Respiratory Acidosis (pCO2 > 45 mm Hg)
– Respiratory Alkalosis (pCO2 < 35 mm Hg)
– Metabolic Acidosis (HCO3- < 23 mEq/l)
– Metabolic Alkalosis (HCO3- > 26 mEq/l)
• Compensation: the physiological response to an acid-base imbalance begins with adjustments by the system less involved
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Causes of Acid-Base Imbalances• Respiratory Acidosis
– Chronic Obstructive Pulmonary Diseases e.g., emphysema, pulmonary fibrosis
– Pneumonia
• Respiratory Alkalosis– Hysteria– Fever– Asthma
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Causes of Acid-Base Imbalances• Metabolic Acidosis
– Diabetic ketoacidosis, Lactic acidosis– Salicylate poisoning (children)– Methanol, ethylene glycol poisoning– Renal failure– Diarrhea
• Metabolic Alkalosis– Prolonged vomiting– Diuretic therapy– Hyperadrenocortical disease– Exogenous base (antacids, bicarbonate IV, citrate
toxicity after massive blood transfusions)
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End Chapter 26