Body Fluids- Compartments, Composition, Concentration .

Body Fluid Introduction .

[1]. Body is formed by solids and fluids. Fluid part is more than two third of the whole body. Water forms most of the fluid part of the body. In human beings, the total body water varies from 45% to 75% of body weight. 

[2]. In a normal young adult male, body contains 60% to 65% of water and 35% to 40% of solids. In a normal young adult female, the water is 50% to 55% and solids are 45% to 50%. 

[3]. In females, water is less because of more amount of subcutaneous adipose tissue. In thin persons, water content is more than that in obese persons.  

[4]. In old age, water content is decreased due to increase in adipose tissue. Total quantity of body water in an average human being weighing about 70 kg is about 40 L. 

 Significance of Body Fluids .

1. In Homeostasis .

[1]. Body cells survive in the fluid medium called internal environment or ‘milieu interieur’. Internal environment contains substances such as glucose, amino acids, lipids, vitamins, ions, oxygen, etc. which are essential for growth and functioning of the cell. 

[2]. Water not only forms the major constituent of internal environment but also plays an important role in homeostasis. 

2. In Transport Mechanism .

[1]. Body water forms the transport medium by which nutrients and other essential substances enter the cells; and unwanted substances come out of the cells. 

[2]. Water forms an important medium by which various enzymes, hormones, vitamins, electrolytes and other substances are carried from one part to another part of the body. 

 3. In Metabolic Reactions .

Water inside the cells forms the medium for various metabolic reactions, which are necessary for growth and functional activities of the cells. 

4. In Texture of Tissues .

Water inside the cells is necessary for characteristic form and texture of various tissues. 

5. In Temperature Regulation .

Water plays a vital role in the maintenance of normal body temperature. 

Compartments of  Body Fluids – Distribution of body Fluids . 

Body fluid compartments and movement of fluid between different compartments.

 

Total water in the body is about 40 L. It is distributed into two major compartments: 

1. Intracellular fluid (ICF): Its volume is 22 L and it forms 55% of the total body water 

2. Extracellular fluid (ECF): Its volume is 18 L and it forms 45% of the total body water. ECF is divided into 5 subunits: 

i. Interstitial fluid and lymph (20%) 

ii. Plasma (7.5%) 

iii. Fluid in bones (7.5%) 

iv. Fluid in dense connective tissues like cartilage (7.5%) 

v. Transcellular fluid (2.5%) that includes: 

a. Cerebrospinal fluid 

b. Intraocular fluid 

c. Digestive juices 

d. Serous fluid – intrapleural fluid, pericardial fluid and peritoneal fluid 

e. Synovial fluid in joints 

f. Fluid in urinary tract. 

Volume of interstitial fluid is about 12 L. Volume of plasma is about 2.75 L. Volume of other subunits of ECF is about 3.25 L. Water moves between different compartments . 

Composition of body Fluids . 

Body fluids contain water and solids. Solids are organic and inorganic substances. 

Organic substances .

Organic substances are glucose, amino acids and other proteins, fatty acids and other lipids, hormones and enzymes. 

Inorganic substances .

[1]. Inorganic substances present in body fluids are sodium, potassium, calcium, magnesium, chloride, bicarbonate, phosphate and sulfate.  

[2]. ECF contains large quantity of sodium, chloride, bicarbonate, glucose, fatty acids and oxygen. 

[3]. ICF contains large quantities of potassium, magnesium, phosphates, sulfates and proteins. 

[4]. The pH of ECF is 7.4. The pH of ICF is 7.0. 

Differences between extracellular fluid (ECF)
and intracellular fluid (ICF) .

Extracellular fluid (ECF) and intracellular fluid (ICF)

 

Measurement of Body Fluid Volume .

Total body water and the volume of different compartments of the body fluid are measured by indicator dilution method or dye dilution method. 

Indicator of  Dilution Method .

Principle .

[1]. A known quantity of a substance such as a dye is administered into a specific body fluid compartment. These substances are called the marker substances or indicators. 

[2]. After administration into the fluid compartment, the substance is allowed to mix thoroughly with the fluid. Then, a sample of fluid is drawn and the concentration of the marker substance is determined .

[3]. Radioactive substances or other substances whose concentration can be determined by using colorimeter are generally used as marker substances . 

Uses of Indicator Dilution Method .

Indicator dilution or dye dilution method is used to measure ECF volume, plasma volume and the volume of total body water. 

Characteristics of Marker Substances .

Dye or any substance used as a marker substance should have the following qualities: 

1. Must be nontoxic .

2. Must mix with the fluid compartment thoroughly within reasonable time .

3. Should not be excreted rapidly .

4. Should be excreted from the body completely within reasonable time .

5. Should not change the color of the body fluid .

6. Should not alter the volume of the body fluid. 

Measurement of Total  Body Water .

[1]. Volume of total body water (fluid) is measured by using a marker substance which is distributed through all the compartments of body fluid. Deuterium oxide and tritium oxide mix with fluids of all the compartments within few hours after injection. 

[2]. Since plasma is part of total body fluid, the concentration of marker substances can be obtained from sample of plasma. The formula for indicator dilution method is applied to calculate total body water. 

[3]. Antipyrine is also used to measure total body water. But as it takes longer time to penetrate various fluid compartments, the value obtained is slightly low. 

Measurement  of  Extracellular fluid Volume .

[1]. Substances which pass through the capillary membrane but do not enter the cells, are used to measure ECF volume. . These substances remain only in ECF and do not enter the cell (ICF). 

[2]. When any of these substances is injected into blood, it mixes with the fluid of all sub-compartments of ECF within 30 minutes to 1 hour. Indicator dilution method is applied to calculate ECF volume. 

[3]. Since ECF includes plasma, the concentration of marker substance can be obtained in the sample of plasma. Some of the marker substances like sodium, chloride, inulin and sucrose diffuse more evenly throughout all sub-compartments of ECF. 

[4]. So, the measured volume of ECF by using these substances is referred as sodium space, chloride space, inulin space and sucrose space. 

Example for Measurement of ECF Volume 

Quantity of sucrose injected (Mass) : 150 mg .

Urinary excretion of sucrose : 10 mg .

Concentration of sucrose in plasma : 0.01 mg/mL .

Measurement  of  Plasma Volume .

The substance which binds with plasma proteins strongly and diffuses into interstitium only in small quantities or does not diffuse is used to measure plasma volume. 

Measurement of interstitial Fluid Volume .

Volume of interstitial fluid cannot be measured directly. It is calculated from the values of ECF volume and plasma volume. 

Interstitial fluid volume = ECF volume – Plasma volume 

Measurement of Intracellular Fluid Volume .

Volume of ICF cannot be measured directly. It is calculated from the values of total body water and ECF. 

ICF volume = Total fluid volume – ECF volume. 

 Concentration of body fluids

Concentration of body fluids is expressed in three ways: 

1. Osmolality .

2. Osmolarity .

3. Tonicity.  

Osmolality .

Measure of a fluid’s capability to create osmotic pressure is called osmolality or osmotic (osmolar) concentration of a solution. In simple words, it is the concentration of osmotically active substance in the solution. Osmolality is expressed as the number of particles (osmoles) per kilogram of solution (osmoles/kg H2 O) . 

 Osmolarity 

[1]. Osmolarity is another term to express the osmotic concentration. It is the number of particles (osmoles) per liter of solution (osmoles/L). Osmotic pressure in solutions depends upon osmolality. 

[2]. However, in practice, the osmolarity and not osmolality is considered to determine the osmotic pressure because of the following reasons: 

i. Measurement of weight (kilogram) of water in solution is a difficult process 

ii. Difference between osmolality and osmolarity is very much negligible and it is less than 1%. 

[3]. Often, these two terms are used interchangeably. Change in osmolality of ECF affects the volume of both ECF and ICF.  

[4]. When osmolality of ECF increases, water moves from ICF to ECF. When the osmolality decreases in ECF, water moves from ECF to ICF. 

[5]. Water movement continues until the osmolality of these two fluid compartments becomes equal. 

Mole and Osmole .

[1]. A mole (mol) is the molecular weight of a substance in gram. Millimole (mMol) is 1/1000 of a mole. One osmole (Osm) is the expression of amount of osmotically active particles. 

[2]. It is the molecular weight of a substance in grams divided by number of freely moving particles liberated in solution of each molecule. One milliosmole (mOsm) is 1/1000 of an osmole. 

Tonicity .

isotonic, hypertonic and hypotonic solutions on red blood cells

 

[1]. Usually, movement of water between the fluid compartments is not influenced by small molecules like urea and alcohol, which cross the cell membrane very rapidly. These small molecules are called ineffective osmoles. 

[2]. On the contrary, the larger molecules like sodium and glucose, which cross the cell membrane slowly, can influence the movement of water. Therefore, such molecules are called effective osmoles

[3]. Osmolality that causes the movement of water from one compartment to another is called effective osmolality and the effective osmoles are responsible for this. 

[4]. Tonicity is the measure of effective osmolality. In terms of tonicity, the solutions are classified into three categories: 

i. Isotonic fluid .

ii. Hypertonic fluid .

iii. Hypotonic fluid. 

1. Isotonic Fluid .

[1]. Fluid which has the same effective osmolality (tonicity) as body fluids is called isotonic fluid. Examples are 0.9% sodium chloride solution (normal saline) and 5% glucose solution. 

[2]. Red blood cells or other cells placed in isotonic fluid (normal saline) neither gain nor lose water by osmosis . This is because of the osmotic equilibrium between inside and outside the cell across the cell membrane. 

2. Hypertonic Fluid .

[1]. Fluid which has greater effective osmolality than the body fluids is called hypertonic fluid. Example is 2% sodium chloride solution. 

[2]. When red blood cells or other cells are placed in hypertonic fluid, water moves out of the cells (exosmosis) resulting in shrinkage of the cells (crenation).  

3. Hypotonic Fluid .

[1]. Fluid which has less effective osmolality than the body fluids is called hypotonic fluid. Example is 0.3% sodium chloride solution. When red blood cells or other cells are placed in hypotonic fluid, water moves into the cells (endosmosis) and causes swelling of the cells .  

[2]. Now the red blood cells become globular (sphereocytic) and get ruptured (hemolysis). 

Maintenance of Water Balance .

Body has several mechanisms which work together to maintain the water balance. The important mechanisms involve hypothalamus and kidneys . 

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