Transcript Chapter 26

Chapter 26
Fluid, Electrolyte, and Acid Base Homeostasis
James F. Thompson, Ph.D.
Fluid Compartments
• Body Fluids are separated by semipermeable 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
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
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
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
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
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
Regulating Fluid Intake - Thirst
• Recall the role of the Renin-Angiotensin System
in regulating thirst along with the Autonomic NS
reflexes diagramed below
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
Regulation of Fluid Output
• Hormonal control
– AntiDiuretic Hormone (ADH) [neurohypophysis]
– Aldosterone [adrenal cortex]
– Atrial Natriuretic Peptide (ANP) [heart atrial walls]
• Physiologic fluid imbalances
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–
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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
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)
Distribution of H2O & Electrolytes
• Recall Starling’s Law of the Capillaries which
explains fluid and solute movements from Ch. 19
Distribution of Electrolytes
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
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
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]
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
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
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
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
Electrolytes
• Phosphate (H2PO4-, HPO42-, PO43-)
– 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
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
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
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)
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+]
Acid-Base Imbalances
• Acidosis
– High blood [H+]
– Low blood pH, <7.35
• Alkalosis
– Low blood [H+]
– High blood pH, >7.45
Acid-Base Imbalances
• Acid-Base imbalances may be due to
problems with ventilation or due to a
variety of metabolic problems
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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
Causes of Acid-Base Imbalances
• Respiratory Acidosis
– Chronic Obstructive Pulmonary Diseases e.g.,
emphysema, pulmonary fibrosis
– Pneumonia
• Respiratory Alkalosis
– Hysteria
– Fever
– Asthma
Causes of Acid-Base Imbalances
• Metabolic Acidosis
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Diabetic ketoacidosis, Lactic acidosis
Salicylate poisoning (children)
Methanol, ethylene glycol poisoning
Renal failure
Diarrhea
• Metabolic Alkalosis
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Prolonged vomiting
Diuretic therapy
Hyperadrenocortical disease
Exogenous base (antacids, bicarbonate IV, citrate
toxicity after massive blood transfusions)
End Chapter 26