Fluid, Electrolyte, and Acid - Base Homeostasis

1
Chapter 26

• Fluid, Electrolyte, and Acid - Base Homeostasis

James F. Thompson, Ph.D.
2
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

3
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

4
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

5
Fluid Balance

• Fluid balance
• When in balance, adequate water is present and is
  distributed among the various compartments
  according to the bodys 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

6
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

7
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

8
Regulating Fluid Intake - Thirst

• Recall the role of the Renin-Angiotensin System
  in regulating thirst along with the Autonomic NS
  reflexes diagramed below

9
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

10
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

11
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)

12
Distribution of H2O Electrolytes

• Recall Starlings Law of the Capillaries which
  explains fluid and solute movements from Ch. 19

13
Distribution of Electrolytes
14
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

15
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

16
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

17
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

18
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

19
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

20
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

21
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

22
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

23
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

24
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)

25
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

26
Acid-Base Imbalances

• Acidosis
• High blood H
• Low blood pH, lt7.35
• Alkalosis
• Low blood H
• High blood pH, gt7.45

27
Acid-Base Imbalances

• Acid-Base imbalances may be due to problems with
  ventilation or due to a variety of metabolic
  problems
• Respiratory Acidosis (pCO2 gt 45 mm Hg)
• Respiratory Alkalosis (pCO2 lt 35 mm Hg)
• Metabolic Acidosis (HCO3- lt 23 mEq/l)
• Metabolic Alkalosis (HCO3- gt 26 mEq/l)
• Compensation the physiological response to an
  acid-base imbalance begins with adjustments by
  the system less involved

28
Causes of Acid-Base Imbalances

• Respiratory Acidosis
• Chronic Obstructive Pulmonary Diseases e.g.,
  emphysema, pulmonary fibrosis
• Pneumonia
• Respiratory Alkalosis
• Hysteria
• Fever
• Asthma

29
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)

30
End Chapter 26