Body Fluids

Introduction

• Body fluids form the internal environment essential for survival, transport, and cellular function. Their distribution changes with age, sex, and body composition, while precise regulation of water and electrolytes, especially sodium and potassium, is vital for maintaining normal physiology and clinical stability.

Body Fluids and Distribution

• Cellular functions depend on fluids present in both intracellular and extracellular compartments.
• Unicellular organisms exchange gases and nutrients directly across the cell membrane.
• Multicellular organisms rely on interstitial fluid and transport systems for exchange with the environment.

Body Composition

• An average adult male body contains about 60% water, 15% fat, 18% proteins, and 7% minerals.
• Total body water is lower in females due to higher adipose tissue content.
• Infants have a higher percentage of body water, but lower absolute volume.
• Elderly individuals show a reduced total body water compared to adults.

Fluid Compartments

• Approximately two-thirds of total body water is present in the intracellular fluid.
• About one-third is present in the extracellular fluid, including interstitial and plasma components.

Body Fluid Compartments

Extracellular Fluid Compartment

• Extracellular fluid refers to fluid located outside the cells.
• It includes plasma, interstitial fluid, and small amounts of transcellular fluid.

Plasma

• Plasma is the liquid component of blood. It forms about one-fourth of the extracellular fluid and nearly 5% of body weight.
• Plasma volume can be estimated from blood volume and hematocrit value.

Blood Volume and Red Cell Volume

• Blood volume consists of plasma and cellular elements. It is approximately 80 milliliters per kilogram body weight.
• Red cell volume represents the volume occupied by circulating erythrocytes.
• It can be calculated by subtracting plasma volume from total blood volume.
• It may also be measured using labeled red blood cells for accuracy.

Table 8.1: TBW as % of body weight

Age Group	Males (% of body weight)	Females (% of body weight)
Infants	65–70%	65–70%
Children (up to 9 years)	60–65%	59–62%
Adolescents (10–17 years)	58–60%	55–58%
Young adults (18–39 years)	58–64%	48–55%
Middle-aged adults (40–59 years)	52–58%	45–50%
Older adults (≥60 years)	50–55%	42–48%

• Note: Total body water decreases with age in both sexes. Females generally have lower values due to higher adipose tissue content.

Table 8.2: Distribution of total body water (TBW) in ECF and ICF compartments in a 70 kg adult male

Fluid Compartment	Percentage of Body Weight	Percentage of Total Body Water	Approximate Volume
Total Body Water (TBW)	60%	100%	42 liters
Extracellular Fluid (ECF)	20%	33%	14 liters
Intracellular Fluid (ICF)	40%	67%	28 liters

• Notes:
  • Total body water is distributed mainly between intracellular and extracellular compartments.
  • The intracellular fluid forms the largest proportion of body water.
  • The extracellular fluid includes plasma and interstitial fluid

Table 8.3: Distribution of ECF in a 70 kg adult male

ECF Component	Proportion of ECF	Percentage of Body Weight	Approximate Volume	Percentage of Total Body Water
Plasma	~25%	4–5%	~3.5 liters	~8%
Interstitial and Transcellular Fluid	~75%	~15%	~10.5 liters	~25%

• Notes:
  • Extracellular fluid (ECF) is divided mainly into plasma and interstitial compartments.
  • Plasma is confined within the vascular system.
  • Interstitial fluid surrounds cells and facilitates exchange of nutrients and wastes.

Interstitial Fluid

• Interstitial fluid is the extracellular fluid present between tissue cells, outside blood vessels.
• It surrounds most cells and includes lymph, which contributes a small fraction of body weight.
• It is continuously exchanged with plasma, enabling transport of nutrients and wastes.
• Its volume is approximately 15% of body weight.
• It cannot be measured directly and is estimated by subtracting plasma volume from extracellular fluid volume.

Transcellular Fluid

• Transcellular fluid is located within epithelial-lined spaces of the body.
• It includes cerebrospinal fluid, pleural, peritoneal, synovial, and ocular fluids, as well as digestive secretions.
• It also includes fluid within renal tubules and the gastrointestinal lumen.
• Its total volume is about 1–1.5% of body weight, roughly equal to 1 liter.

Intracellular Fluid Compartment

• Intracellular fluid is the fluid present within body cells.
• It constitutes about two-thirds of total body water.
• It is not measured directly and is calculated by subtracting extracellular fluid volume from total body water.

Measurement Of Body Fluid Volumes

General Principle

• Body fluid volume is determined using the indicator dilution principle.
• A known amount of substance is introduced intravenously and allowed to distribute uniformly.
• The final concentration is measured after equilibrium.
• Volume is calculated from the relationship:
• $V = \frac{A}{C}$
• Here, A denotes amount administered, C denotes concentration, and V denotes volume.

Characteristics of an Indicator

• An indicator should be easily measurable using reliable and precise analytical methods.
• It must remain restricted to the specific fluid compartment being evaluated.
• It should not disturb normal fluid distribution or physiological equilibrium.
• The substance must be non-toxic and safe for intravenous administration.
• It should distribute uniformly throughout the compartment.
• It must remain chemically stable, or any loss should be accurately corrected during calculation.
• The size of a fluid compartment is determined by introducing a known quantity of an indicator substance.
• The substance distributes uniformly within the target fluid compartment after adequate mixing.
• The measured concentration allows calculation of volume of distribution.
• Volume equals the amount present divided by final concentration.
• $V = \frac{\text{Amount injected} – \text{Amount removed}}{\text{Final concentration}}$

• Any loss due to excretion or metabolism must be subtracted for accurate estimation.

Measurement of ECF Volume

• Measurement of extracellular fluid volume is challenging because its boundaries are not sharply defined.
• Suitable indicators must distribute rapidly and remain confined to the extracellular space.
• Very few substances fulfill these criteria completely, limiting accuracy.

Methods of Measurement

• The extracellular fluid volume is estimated using the indicator dilution principle with substances that remain mainly outside cells.
• Inulin, a polysaccharide with molecular weight about 5200, is widely used because it distributes only in extracellular space.
• Radioactive forms of inulin allow accurate measurement using radiation detection techniques.
• Isotopes of chloride ions are also used, as chloride is predominantly extracellular. However, small intracellular penetration of chloride causes slight overestimation of volume.
• Other indicators include bromide, sulphate, thiosulphate, thiocyanate, and ferrocyanide. These ions may exchange with chloride, leading to higher calculated values.
• Mannitol and sucrose are alternative indicators that remain largely extracellular.
• Selection of indicator depends on accuracy, distribution, and minimal cellular entry.

Measurement of Plasma Volume

Plasma volume is measured using dilution techniques based on indicator distribution within the vascular compartment.

First Method:

• In the first method, indicators remain confined to plasma and do not enter red cells.
• Evans blue dye binds strongly to plasma proteins and remains intravascular.
• Radioiodinated albumin is commonly used and is quantified using radiation detection methods.
• Some labeled proteins may slowly leave circulation, so correction factors are applied.

Second Method:

• In the second method, labeled substances bind to red blood cells, allowing measurement of red cell volume.
• Common labels include radioactive chromium and iron isotopes.
• After mixing, the proportion of labeled cells is measured to estimate total red cell volume.
• Packed cell volume represents the fraction of blood occupied by red cells.
• Plasma volume is then calculated from blood volume and packed cell volume.

• $\text{Plasma volume} = \frac{100 – \text{PCV}}{100} \times \text{Blood volume}$

• This approach provides reliable estimation when proper mixing and corrections are ensured.

Measurement of Interstitial Fluid Volume

• Interstitial fluid volume cannot be measured directly because sampling from this compartment is difficult.
• No indicator distributes exclusively within interstitial fluid without entering plasma.
• Substances equilibrate in both compartments, preventing direct estimation. Therefore, interstitial fluid volume is calculated as the difference between extracellular fluid volume and plasma volume.

Measurement of ICF Volume

• Intracellular fluid volume cannot be measured directly because no indicator remains confined within cells.
• Substances used for dilution distribute beyond the intracellular compartment. Therefore, intracellular fluid volume is calculated indirectly by subtracting extracellular fluid volume from total body water.

Measurement of TBW

• Total body water is measured using the indicator dilution principle with substances that distribute throughout all body fluids.
• Deuterium oxide is commonly used because it equilibrates uniformly and provides reliable results.
• Tritium oxide and aminopyrine are alternative indicators for total body water estimation.
• Accurate measurement requires complete mixing and stable distribution of the indicator.

Ionic Composition Of Body Fluids

Basic Features

• The distribution of electrolytes differs between intracellular and extracellular compartments.
• Each compartment maintains electrical neutrality, where total cations equal total anions.
• Sodium, calcium, chloride, and bicarbonate are predominantly extracellular ions.
• Potassium, magnesium, phosphates, and proteins are mainly intracellular components.
• Most body potassium exists in an exchangeable form, contributing to osmotic activity.
• Only about sixty-five to seventy percent of sodium is exchangeable and osmotically active.
• Most calcium in bone and significant magnesium in bone and cells are nonexchangeable.

Table 8.4: Concentration (mmol/L of H₂O) of major ions in ECF (plasma) and ICF

Units for Measuring Solutes

• Solute concentration in body fluids is expressed using moles, representing number of molecules per unit volume.
• Equivalents indicate electrically active ions based on charge.
• Osmoles reflect total particle concentration contributing to osmotic pressure.

Moles

• The mole is the standard International System unit used to express the amount of a substance.
• One mole represents the molecular weight of a substance in grams.
• Each mole contains approximately 6 × 10²³ particles, including atoms, molecules, or ions. For example, one mole of potassium chloride equals the sum of atomic masses, about 74.5 grams.
• A millimole is one-thousandth of a mole, and 1 millimole of potassium chloride equals 74.5 milligrams. Concentration expressed in grams per liter does not indicate the actual number of molecules present.
• When chemical structure is known, concentration is expressed in moles per liter, which reflects particle number.
• A solution containing one mole of solute in one liter is termed a one molar solution.
• One molar solutions of different substances contain equal numbers of particles per liter. This uniformity allows accurate comparison of solute concentration across different chemical substances.

Equivalents

• Equivalents express the concentration of substances that exist as charged particles in body fluids.
• One equivalent equals one mole of an ion divided by its valency. For potassium chloride, one mole yields one equivalent each of potassium and chloride ions.
• Equivalent weight depends on atomic mass and ionic charge. For example, potassium has one equivalent equal to 39 grams, whereas calcium has one equivalent equal to 20 grams.
• A milliequivalent is one-thousandth of an equivalent and is commonly used clinically.
• Normality represents the number of gram equivalents present in one liter of solution.

Osmolarity and Osmolality

• Osmolarity refers to the number of osmoles of solute per liter of solution.
• Osmolality represents osmoles of solute per kilogram of solvent.
• These measures reflect the concentration of osmotically active particles in body fluids. Most solutes in the body are dissolved in water, which has a density close to one. Therefore, osmolarity and osmolality values are often similar in physiological conditions.
• Units commonly used are osmoles per liter and osmoles per kilogram.
• Milliosmoles are used for clinical measurements due to low concentrations.

pH of Body Fluids

• pH expresses hydrogen ion concentration as the negative logarithm of hydrogen ion activity.

• $\mathrm{pH} = -\log_{10}[H^+]$

• Lower pH indicates higher hydrogen ion concentration, while higher pH indicates lower concentration.
• The pK of a buffer equals the pH at which acid is half dissociated and half undissociated.
• Buffer systems are most effective when pH is close to their pK value.
• Normal blood pH ranges from 7.35 to 7.45 under physiological conditions.
• Values below this range indicate acidosis, whereas higher values indicate alkalosis.
• Even small deviations in pH can affect enzyme activity and cellular functions.
• Buffer systems, lungs, and kidneys maintain acid–base balance.

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