Dibartola Chapter 1: Applied Physiology of Body Fluids in Dogs and Cats

What percent of an adult animal’s body weight is water in health?

60%

What percent of a neonate’s body weight is total body water?

80%

Calculating Fluid Needs Based on Weight vs Lean Body Mass

Fat has a lower water content than lean tissue so fluid needs should be estimated on the basis of lean body mass to avoid overhydration

Body weight is a reasonable estimate of lean body mass in thin patients

What % of body weight is due to fat in normal small animal patients?

20%

What % of body weight is fat in morbidly obese patients?

~30%

Lean Body Mass Equations

Normal body weight x 0.8 = lean body mass

Obese body weight x 0.7 = lean body mass

Thin body weight x 1.0 - lean body mass

What is the largest volume of fluid in the body?

Intracellular fluid

What percent of body weight is intracellular fluid?

~40%

What fraction of total body water is intracellular fluid?

~2/3

Composition of ICF vs ECF

What fraction of total body water is extracellular fluid?

~1/3

Where does loss of fluids occur initially from in most disease states?

ECF

What % of lean body weight is ECF?

Estimates vary widely (15-30%)

Most accurate estimate is ~27% of lean body weight

60:40:20 rule

60% of body weight is water

40$ of body weight is ICF

20% of body weight is ECF

Where is the interstitial fluid?

Spaces surrounding cells

What % of body weight and total body water is interstitial fluid?

~15% of body weight

~24% of total body water

Subcompartments of the Extracellular Fluid (ECF)

Interstitial fluid

Intravascular component (plasma)

Transcellular fluid

Where is the intravascular fluid located?

Within blood vessels

Most of the intravascular fluid is plasma

What % of body weight, body water, and lean body mass is intravascular fluid?

~5% of body weight

~8-10% total body weight

Blood volume is a function of lean body mass, ~8% body weight in a horse

What is transcellular fluid?

Fluids produced by specialized cells to form cerebrospinal fluid, gastrointestinal fluid, bile, glandular secretions, respiratory secretions, and synovial fluid

What % of body weight and total body water is transcellular fluid?

~1% of body weight

~2% total body water

Simplified Distribution of Total Body Water for Fluid Therapy

ICF ~2/3 of total body water

ECF ~1/3 of total body water

ISF ~3/4 of ECF

Intravascular fluid is ~1/4 of ECF

Distribution of Body Solutes

• Healthy vascular endothelium is relatively impermeable to the cellular components of blood and plasma proteins so the volume of distribution of cells and proteins is the plasma space

• The vascular endothelium is freely permeable to ionic solutes and the concentration of these ions is almost the same in ISF as in plasma

• Cell membranes maintain intracellular solutes at very different concentrations from those of the ECF

Plasma Na+

142 mEq/L

Plasma K+

4.3 mEq/L

Plasma Ca2+

2.5 mEq/L

Plasma Mg2+

1.1 mEq/L

Plasma Cl-

104 mEq/L

Plasma HCO3-

24 mEq/L

Plasma HPO4^2-, H2PO4-

2 mEq/L

Plasma Proteins

14 mEq/L

Plasma Other Anions

5.9 mEq/L

Plasma Water Na+

152.7 mEq/L

Plasma Water K+

4.6 mEq/L

Plasma Water Ca2+

2.7 mEq/L

Plasma Water Mg2+

1.2 mEq/L

Plasma Water Cl-

111.9 mEq/L

Plasma Water HCO3-

25.8 mEq/L

Plasma Water HPO4^2-, H2PO4-

2.2 mEq/L

Plasma Water Proteins

15 mEq/L

Plasma Water Other Anions

6.3 mEq/L

Interstitial Fluid Na+

145.1 mEq/L

Interstitial Fluid K+

4.4 mEq/L

Interstitial Fluid Ca2+

2.4 mEq/L

Interstitial Fluid Mg2+

1.1 mEq/L

Interstitial Fluid Cl-

117.4 mEq/L

Interstitial Fluid HCO3-

27.1 mEq/L

Interstitial Fluid HPO4^2-, H2PO4-

2.3 mEq/L

Interstitial Fluid Proteins

0 mEq/L

Interstitial Fluid Other Anions

6.2 mEq/L

Intracellular Fluid - Skeletal Muscle Cell Na+

12 mEq/L

Intracellular Fluid - Skeletal Muscle Cell K+

140 mEq/L

Intracellular Fluid - Skeletal Muscle Cell Ca2+

4.0 mEq/L

Intracellular Fluid - Skeletal Muscle Cell Mg2+

34 mEq/L

Intracellular Fluid - Skeletal Muscle Cell Cl-

4 mEq/L

Intracellular Fluid - Skeletal Muscle Cell HCO3-

12 mEq/L

Intracellular Fluid - Skeletal Muscle Cell HPO4^2-, H2PO4-

40 mEq/L

Intracellular Fluid - Skeletal Muscle Cell Proteins

50 mEq/L

Intracellular Fluid - Skeletal Muscle Cell Other Anions

84 mEq/L

Why is there a slightly increased concentration of cations and anions in interstitial fluid compared with plasma water?

Primarily because of the presence of negatively charged proteins in plasma

Gibbs-Donnan Equilibrium

• Determines the equilibrium concentrations of permeable anions and cations across the vascular endothelium

• Occurs because negatively charged, nondiffusible proteins affect the distribution of other small charged solutes

• In clinical practice, the difference in concentrations of anions and cations across the vascular endothelium is negligible and the effects of the Gibbs-Donnan equilibrium are usually ignored

  • In clinical practice, plasma concentrations of solutes are considered to reflect solute concentrations throughout the ECF

What is the most abundant cation in the ECF?

Na+

Where is most of the body Na+ located?

The extracellular space

How much of body Na+ is exchangable?

~70% of body Na+ in humans is exchangeable and 30% is fixed as insoluble salts in bone

• Only exchangeable solutes are osmotically active

Movement of Na+ In and Out of Cells

Cell membranes are permeable to Na+ which tends to diffuse into cells

In health, cell membrane Na+K+ ATPase actively removes Na+ from cells, maintaining a steep extracellular to intracellular concentration gradient for Na+

What are the most abundant anions in the ECF?

Cl- and HCO3-

Where is Cl- mostly located?

Volume of distribution of Cl- is primarily the ECF volume

Where is HCO3- located?

HCO3- is present in all body fluids and can be generated from CO2 and H2O in the presence of carbonic anhydrase

What are the primary cations in the ICF?

K+ and Mg2+

Where is most of the body K+ located?

In the ICF where K+ is the most abundant cation

Movement of K+ In and Out of Cells

Cell membranes are permeable to K+

Cell membrane Na+K+ ATPase maintains the concentration gradient between ICF and ECF by moving K+ into cells against a concentration gradient

The ratio of intracellular to extracellular K+ concentration is important in generating and maintaining the cell membrane potential at approximately -70 mV

How much of K+ is exchangeable?

Almost 100% of body K+ in humans is exchangeable

What are the predominant anions in the ICF?

Organic phosphates and proteins

Atomic Mass (Relative Atomic Mass or Atomic Weight)

• Most naturally occurring elements consist of one or more isotopes of that element, each of which has a different mass

• Atomic mass - an average mass based on the distribution of stable isotopes for an element, determined by the weight of that element relative to the weight of the ¹²C isotope of carbon, which is defined as 12.000

• Usually reported with no units are as atomic mass units

Molecular Mass (Molecular Weight)

The sum of the atomic masses of the atoms that form the compound

Formula Weight

Ionic compounds do not really form molecules so a more appropriate term for the mass of these substances is formula weight

Mole

• 6.023 x 10²³ particles

• Defined as the number of atoms in exactly 12 g of ¹²C

• One mole of a substance weighs its molecular weight in grams

Molar Mass

The mass in grams of 1 mole of a substance

Numerically equivalent to atomic or molecular weight but are reported in grams

Molality and Molarity

• Molality - the number of moles of solute per kilogram of solvent

• Molarity - the number of moles of solute per liter of solution

• Molarity and molality of most biologic solutions are approximately equal because the density of water is 1 kg/L

• Slight difference between molarity and molality of a substance in plasma is because of nonaqueous proteins and lipids

  • In body fluids this difference is relatively unimportant and the terms are  often used interchangeably

Concentration

• The amount of a substance that is present in a specified volume

• Amount of a substance can be expressed as mass (g or mg), moles (or millimoles), or equivalents (milliequivalents)

• Volume is usually expressed as L, dL, or mL

• Percent concentration refers to a number of parts in 100 parts of a solution

  • May be used to express concentration in terms of weight per unit weight, weight per unit volume, or volume per unit volume

Cation

Atom or molecule with a positive charge

Monovalent

One positive or negative charge

Divalent

Two positive or negative charges

Anion

An atom or molecule with a negative charge

Valence

• Ions in body fluids combine according to ionic charge (valence) rather than weight

• The number of cations in a solution always equals the number of anions to maintain electroneutrality

• Useful to express concentrations of solutes in body fluids in equivalents per liter (Eq/L) or milliequivalents per liter (mEq/L) to reflect the charge or valence of the solute

• The equivalent weight of a substance is the atomic, molecular, or formula weight of a substance divided by the valence

Electrochemical Equivalence

• One equivalent is defined as the weight in grams of an element that combines with or replaces 1 g of H+

• Because 1 g of H+ is equal to 1 mol of H+, 1 mol of any univalent anion will combine with this H+ and is equal to 1 equivalent (Eq)

Equivalent Weight

• The atomic, molecular, or formula weight divided by the valence

• Millimolecular weight/valence = milliequivalent weight

• Millimoles x valence = milliequivalents (mEq)

• mEq/L = mmol/L x valence

• mEq/L = (mg/dL x 10/molecular weight) x valence

How many particles does 1 mol of any substance contain?

Regardless of its weight, 1 mol of any substance contains the same number of particles (6.023 x 10²³; Avogadro's number)

What does the osmotic effect of solutes in a solution depend on?

Solutes exert an osmotic effect in solution that is dependent only on the number of particles in solution, not their chemical formula, weight, size, or valence

What is the definition of 1 osmole?

1 g molecular weight of any nondissociable substance so each osmole contains 6.023 x 10²³ molecules

If a substance does not dissociate in solution, how many osmoles are there?

1 mol equals 1 Osm

If a substance dissociates in solution, how many osmoles are there?

The number of osmoles equals the number of dissociated particles

Osmolality

The number of osmoles per kg of solvent

Osmolarity

The number of osmoles per liter of solution

Effective and Ineffective Osmoles

• In any fluid compartment, the osmotic effect of a solute is in part dependent on the permeability of the solute across the membranes separating the compartment

• Membrane dividing the two compartments is freely permeable to urea and water but is impermeable to glucose

• When urea is added to the left compartment, it moves down its concentration gradient from left to right and water moves down the concentration gradient from right to left until there are equal concentrations of urea and water on both sides of the membrane

  • No fluid rises in the column attached to the left fluid compartment because urea is an ineffective osmole and does not generate osmotic pressure

  • In biologic fluids, urea is a small molecule that freely diffuses across most cell membranes and therefore does not contribute to effective osmolality

• When glucose is added to the left compartment, water moves down its concentration gradient from right to left, but glucose cannot move across the membrane

  • Osmosis - movement of water from a solution of lesser solute concentration across a semipermeable membrane to a solution of greater solute concentration

  • The influx of water into the left compartment resulting from the osmotic effect of glucose causes the solution to rise in the column

    • The height of fluid in the column is proportional to the osmotic pressure generated by glucose

  • Glucose is an effective osmole because it generates oncotic pressure by causing a shift of water across the boundary membrane

  • Glucose is an effective osmole in this setting because the boundary membrane is impermeable to glucose, but permeable to water

Tonicity

Effective osmolality of a solution

Measured osmolality of a solution includes both effective and ineffective osmoles

Tonicity of a solution may be less than the measured osmolality if both effective and ineffective osmoles are present

Measured Osmolality

Osmolality determined with an osmometer

Typically not the same as the calculated osmolality estimated using formulas

Calculated Osmolality

Estimate of serum osmolality using various formulas

Often less than measured osmolality because the formulas either exclude some osmotically active particles or estimate their contribution

Osmolal Gap

Difference between measured osmolality and calculated osmolality

Colloids

Large molecular weight (MW = 30,000) particles present in a solution