Body Fluids

Introduction

Single-cell organisms obtain nutrients directly from their environment and expel waste directly into it.
Multicellular organisms face a more complex situation: each cell requires nutrients and produces waste, but most cells are not in direct contact with the external environment.
Body fluids act as a medium for transporting nutrients to and waste products from cells, and they also carry chemical messengers that coordinate activities between cells.

Composition of Body Fluids

Body fluids consist of:

Water: Makes up the majority of the fluid component in the body. It accounts for approximately 60% of the total body weight (TBW) in a 70kg man.
- $Water \approx 60\% \text{ of TBW}$
Solids:
- Organic:
  - Amino acids
  - Carbohydrates (CHO)
  - Proteins
  - Fatty acids
- Inorganic

Variations in Body Fluids

Variations occur due to:

Age:
- Neonates have a higher water content than adults, around 75-80%, with a proportionally greater amount of extracellular fluid (ECF).
- By 12 months, this decreases to about 60%, which is the adult level.
- Total body water (TBW) as a percentage of total body weight declines with increasing age.
- By age 60, TBW decreases to approximately 50% of total body weight in males, primarily due to an increase in adipose tissue.
Individuals:
- Significant individual variation exists in the ratio of TBW to total body weight, largely due to differences in adipose tissue.
- Obese adults generally have a lower ratio of TBW to body weight.
- For specific tissues, the variation is smaller but still mainly attributed to differences in adipose tissue.

Total Body Water (% of Body Weight in Relation to Age and Sex)

(Note: A table or graph illustrating the relationship between age, sex, and total body water percentage would be included here in a real set of notes.)

Fluid Compartments

Compartment	% of Body Weight	% of Body Water	Volume (L)
Total Body Water	60	100	42
Extracellular Fluid	20	33	14
Intracellular Fluid	40	67	28

Compartments of Body Fluid

Compartmentalization is maintained by barriers that dictate which substances can move between compartments.
Body water is distributed among numerous organs and tissues.
These fluids are categorized into larger collections, called compartments, for physiological discussion. The main division is between Intracellular Fluid (ICF) and Extracellular Fluid (ECF), based on their location relative to the cell membrane.

Intracellular Fluid (ICF)

Separated from ECF by the cell membrane.
Represents 2/3 of Total Body Water, equivalent to 40% of body weight in a 70kg man.
- $ICF = \frac{2}{3} \text{ TBW} = 40\% \text{ Body Weight}$
Water content is approximately 75-80%.
Major cations: $K^+$ and $Mg^{2+}$ .
Major anions: Proteins and organic phosphates (ATP, ADP, AMP).
pH ranges from 6.8 to 7.4, independent of metabolic processes.

Extracellular Fluid (ECF)

Composed of interstitial fluid and plasma.
Represents 1/3 of Total Body Water, or 20% of body weight (16% interstitial fluid and 4% plasma).
- $ECF = \frac{1}{3} \text{ TBW} = 20\% \text{ Body Weight}$
Interstitial fluid is the true environment of the body.
Major cation: $Na^+$ .
Major anions: $Cl^-$ and $HCO_3^-$ .
Ions in ECF determine osmotic pressure and pH of the internal environment.
pH is 7.4.
Interstitial fluid and plasma are separated by the capillary walls (smallest blood vessels).

Transcellular Fluids

Small compartment consisting of fluids formed by the transport activities of cells.
Contained within epithelial-lined spaces.
Includes cerebrospinal fluid (CSF), gastrointestinal tract fluids, bladder urine, aqueous humor, and joint fluid.

Plasma

Comprises 1/4 of the ECF.
- $Plasma = \frac{1}{4} \text{ ECF}$
Equals 1/12 of TBW.
- $Plasma = \frac{1}{12} \text{ TBW}$
Major plasma proteins are albumin and globulins.

Interstitial Fluid

Comprises 3/4 of the ECF.
- $Interstitial Fluid = \frac{3}{4} \text{ ECF}$
Equals 1/4 of TBW.
- $Interstitial Fluid = \frac{1}{4} \text{ TBW}$
Composition is similar to plasma, but with very little protein.
Considered an ultrafiltrate of plasma.

Measuring the Volumes of the Fluid Compartment

Indicator dilution method (or Dye dilution method).
Compartment volumes are determined by measuring the volume of distribution of a tracer substance.
A known amount of tracer is added to a compartment.
Tracer concentration is measured after sufficient time for uniform distribution.
The compartment volume is calculated as:
- $\text{Volume} = \frac{\text{Amount of tracer}}{\text{Concentration of tracer}}$

Formula to measure volume of fluid by indicator dilution method

$V = \frac{M - \text{amount of substance excreted}}{C}$
- V = Volume of the compartment
- M = Mass or total quantity of marker substance injected
- C = Concentration of marker substance in the sample fluid
This method is used to measure ECF volume, plasma volume, and total body water volume.

Properties of an Ideal Tracer

Nontoxic.
Rapidly and evenly distributed throughout the target compartment.
Does not enter other compartments.
Not metabolized or excreted (or excretion can be corrected for) during the equilibration period.
Easy to measure.
Does not interfere with body fluid distribution.

Volume Indicators

Volume	Indicator(s)
Total Body Water	$^3H<em>2O$ , $^2H</em>2O$ , antipyrine
Extracellular Fluid	$^{22}Na$ , $^{125}I$ -iothalamate, thiosulphate, inulin
Intracellular Fluid	Calculated: TBW – ECF
Plasma Volume	$^{125}I$ -albumin, Evans blue
Blood Volume	$^{51}Cr$ -labeled red blood cells
Interstitial Fluid	Calculated: ECF - PV

Total Body Water Measurement

Estimated by measuring the volume of distribution of water isotopes.
Tritium oxide (THO) is commonly used due to its ease of measurement via liquid scintillation counter (weak beta emitter).
Rapid mixing occurs during a 3-4 hour equilibration period.
Results are accurate and reproducible to within 2%.

Plasma Volume Measurement

Requires a tracer that remains mostly within the plasma compartment.
Tracers commonly bind to albumin.
Examples: Evan's blue dye (T1824) and radio-iodine labeled serum albumin (RISA).

Blood Volume

$\text{Blood volume} = \frac{\text{plasma volume} \times 100}{(100 - Hct)}$
- Hct = Hematocrit

Tonicity

Tonicity is the effective osmolality, representing the sum of solute concentrations that can exert osmotic force across a membrane.
Isotonic Fluid: Has the same tonicity as body fluids (e.g., 0.9% NaCl solution).
Hypertonic Fluid: Has greater tonicity than body fluids (e.g., 2% NaCl solution).
Hypotonic Fluid: Has less tonicity than body fluids (e.g., 0.3% NaCl solution).

Shifts of Water Between Compartments

Water shifts between ECF and ICF to equalize osmolarities.
Osmolarity of ICF and ECF is assumed to be equal after a brief equilibration.
Solutes like NaCl and mannitol do not cross cell membranes and remain in the ECF.

Examples of Shifts

Infusion of isotonic saline: Addition of isotonic fluid (isosmotic volume expansion).
- ECF volume increases.
- No change in osmolarity of ECF or ICF.
- No water shift.
- Plasma protein concentration and hematocrit decrease (dilution).
- RBCs do not shrink or swell.
- Arterial pressure increases (ECF volume increase).
Diarrhea: Loss of isotonic fluid (isosmotic volume contraction).
- ECF volume decreases.
- No change in osmolarity.
- No water shift.
- Plasma protein concentration and hematocrit increase from loss of ECF.
- RBCs do not shrink or swell.
- Arterial blood pressure decreases (ECF volume decrease).

Water Balance

Water balance occurs when water input equals water output.
Input > Output: Overhydration and edema.
Output > Input: Dehydration.
Intake controlled by thirst center in the hypothalamus.
Output controlled by osmoreceptor-ADH system.

Sources of Water Input

Drinks: ~1L/24 hrs
Food: ~1.2L/24 hrs
Metabolism: ~0.3L/24 hrs

Sources of Water Output

Urine: ~1.5L/24 hrs
Feces: ~0.1L/24 hrs
Insensible loss: ~0.9L/24 hrs (temperate climates; ~1.5L/24 hrs in the tropics).
- Insensible loss occurs through the skin and lungs.

Expressing Concentration of Body Fluids

Molality: Moles of solute per kg of solvent.
Molarity (M): Moles of solute per liter of solution (M = moles/liter).
Electrochemical Equivalence (Eq): The weight in grams of an ionic substance that replaces or combines with one gram (mole) of monovalent $H^+$ ions.
- For monovalent ions: 1 equivalent = 1 molar (GMW).
- For divalent ions: 1 equivalent = one-half of a molar (GMW).
Osmole: The amount of a substance that yields Avogadro's number of particles (in ideal solution), depressing the freezing point of the solvent by 1.86K.
Osmolality: Number of osmoles of solute per kilogram of solvent.
Osmolarity: Number of osmoles of solute per liter of solution.

Body Electrolytes

Electrolytes constitute approximately 7% of total body weight.
They perform many major functions in the body.

Distribution of Ions in the ECF and ICF (Values Are in mEq/L of H2O)

(Note: A table with ion concentrations in ECF and ICF would be included here in a real set of notes.)

Important Notes on Electrolytes

Essentially all body $K^+$ is in the exchangeable pool.
Only 65-70% of body $Na^+$ is exchangeable.
Almost all body $Ca^{2+}$ and $Mg^{2+}$ are non-exchangeable.
Only the exchangeable solutes are osmotically active.

Functions of Electrolytes

Maintain acid-base balance in body fluids.
Maintain proper osmolality and volume of body fluids.
Specific physiological functions are determined by the concentration of certain electrolytes (e.g., calcium ions on neuromuscular excitability).