Body Fluid Analysis Notes

Body Fluids Introduction

• Body is formed by solids and fluids. The fluid part is more than 2/3 of the whole body. Water forms most of the fluid part of the body.
• In human beings, the total body water varies from 45-75% of body weight. In a normal young adult male, the body contains 60-65% of water and 35-40% of solids.
• In a normal young adult female, the water is 50-55% and solids are 45-50%. In females, water is less because of more amount of subcutaneous adipose tissue. In thin persons, water content is more than in obese persons.
• In old age, water content is decreased due to an increase in adipose tissue. The total quantity of body water in an average human being weighing about 70 kg is about 40 liters.

Read And Learn More: Medical Physiology Notes

Body Fluids Significance

1. In Homeostasis:

• Body cells survive in the fluid, medium called the internal environment or milieu interieur.
• Growth and functions of cells depend upon the availability of certain materials like glucose, amino acids, lipids, vitamins, ions, oxygen, etc. in proper quantities in the internal environment.
• Water forms not only the major constituent of the internal environment but also plays an important role in homeostasis.

2. In Transport Mechanism

• Body water forms the transport medium by which nutrients and other essential substances enter the cells and unwanted substances come out of the cells.
• 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 the characteristic form and texture of various tissues.

5. In Temperature Regulation: Water plays a vital role in the maintenance of normal body temperature.

Body Fluids Compartments

Distribution Of Body Fluids: Total water in the body is about 40 liters. It is distributed into two major compartments:

1. Intracellular fluid (ICF): Its volume is 22 liters and it forms 55% of the total body water
2. Extracellular fluid (ECF): Its volume is 18 liters and it forms 45% of the total body water.

ECF is divided into 5 subunits:

1. Interstitial fluid and lymph (20%)
2. Plasma (7.5%)
3. Fluid in bones (7.5%)
4. Fluid in dense connective tissues like cartilage proteins, fatty acids and other lipids, hormones, and (7.5%)
5. Transcellular fluid (2.5%) that includes:
   • Cerebrospinal fluid
   • Intraocular fluid
   • Digestive juices
   • Serous fluid-intrapleural fluid, pericardial fluid, and peritoneal fluid
   • Synovial fluid in joints
   • Fluid in the urinary tract.

The volume of interstitial fluid is about 12 liters. The volume of plasma is about 2.75 liters. The total volume of other subunits of ECF is about 3.25 liters. Water moves between different compartments.

Body Fluids Composition

• 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:
  • The inorganic substances present in body fluids are sodium, potassium, calcium, magnesium, chloride, bicarbonate, phosphate, and sulfate.
  • ECF contains large quantity of sodium, chloride, bicarbonate, glucose, fatty acids, and oxygen.
  • ICF contains large quantities of potassium, magnesium, phosphates, sulfates, and proteins. The pH of ECF is 7.4. The pH of ICF is 7.0. The differences between ECF and ICF are given in the Table.

Body Fluids Measurement

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 Dilution Method

• Principle
  • A known quantity of a substance such as a dye is administered into a specific body fluid compartment.
  • These substances are called marker substances or indicators. After administration into the fluid, the substance is allowed to mix thoroughly with the fluid compartment.
  • Then, a sample of fluid is drawn and the concentration of the marker substance is determined.
  • Radioactive substances or other substances whose concentration can be determined by using a colorimeter are generally used as marker substances.
• Formula to Measure the Volume of Fluid by Indicator Dilution Method
• • The quantity of fluid in the compartment is measured by using the formula V = M/C
  • V = the volume of fluid in the compartment
  • M = mass or total quantity of marker substance injected
  • C = concentration of the marker substance in the sample fluid
  • Correction factor: Some amount of marker substance is lost through urine during distribution. So, the formula is corrected as follows:

 

• 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: The 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
     carbon dioxide
  3. Should not be excreted rapidly
  4. Should be excreted from the body completely within a reasonable time
  5. Should not change the color of the body fluid
  6. Should not alter the volume of body fluid.
• Marker Substances Used to Measure Fluid Compartments: The marker substance used to measure different fluid compartments are listed in Table.

Measurement Of Total Body Water

• The volume of total body water (fluid) is measured by using a marker substance, which is distributed, through all the compartments of body fluid. Such substances are listed in Table.
• Deuterium oxide and tritium oxide mix with fluids of all the compartments within few hours after injection.
• Since plasma is part of total body fluid, the concentration of marker substances can be obtained from a sample of plasma. And, the formula for indicator dilution method is applied to calculate total body water.
• 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

• The substances, which pass through the capillary membrane but do not enter the cells, are used to measure ECF volume. Such marker substances are listed in Table.
• These substances remain only in the ECF and do not enter the cell (ICF). 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.
• The indicator dilution method is applied to calculate ECF volume. 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 subcompartments of ECF. 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

Sucrose space = Mass Amount lost in urine/Concentration of sucrose in plasma

= 150-10 mg/ 0.01 mg/mL = 14,000 mL

Therefore, the ECF volume 14 liters.

Measurement Of Plasma Volume: The substance, which binds with plasma proteins strongly and diffuses into the interstitium only in small quantities or does not diffuse at all is used to measure plasma volume. Such substances are listed in Table.

Measurement Of Interstitial Fluid Volume: The 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: The 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: The concentration of body fluids is expressed in three ways:

1. Osmolality
2. Osmolarity
3. Tonicity.

1. Osmolality:

• The 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 H₂O).

2. Osmolarity

• Osmolarity is another term to express osmotic concentration. It is the number of particles (osmoles) per liter of solution (osmoles/L).
• The osmotic pressure in solutions depends upon osmolality. However, in practice, the osmolarity and not osmolality is considered to determine the osmotic pressure because of the following reasons:

1. The measurement of the weight (kilogram) of water in solution is a difficult process
2. The difference between osmolality and osmolarity is very much negligible and it is less than 1%.

• • Often these two terms are used interchangeably. Change in the osmolality of ECF affects the volume of both ECF and ICF.
  • When the osmolality of ECF increases, water moves from ICF to ECF. When the osmolality decreases in ECF, water moves from ECF to ICF.
  • Water movement continues until the osmolality of these two fluid compartments becomes equal.
• Mole and Osmole
  • 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.
  • It is the molecular weight of a substance in grams divided by number of freely moving particies liberated in solution of each molecule. One milliosmole (mOsm) is 1/1000 of an osmole.

3. Tonicity

• Usually, the movement of water between the fiuid 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.
• On the contrary, 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.
• The osmolality that causes the movement of water from one compartment to another is called effective osmolality and the effective osmoles are responsible for this.

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

1. Isotonic fluid
2. Hypertonic fluid
3. Hypotonic fluid.

1. Isotonic Fluid:

• The fluid which has the same effective osmolality (tonicity) as body fluids is called isotonic fluid. The examples are 0.9% sodium chloride solution (normal saline) and 5% glucose solution.
• The 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 the inside and outside the cell across the cell membrane.

2. Hypertonic Fluid

• The fluid which has greater effective osmolality than the body fluids is called hypertonic fluid. An example is 2% sodium chloride solution.
• When the red blood cells or other cells are placed in hypotonic fluid, water moves into the cells (endos- miosis)resulting in shrinkage of the cells (crenation).

3. Hypotonic Fluid

• The fluid, which has less effective osmolality than the body fluids is called hypotonic fluid. An example is 0.3% sodium chloride solution.
• When the red blood cells or other cells are placed in a hypertonic fluid, water moves out of the cells (endosmosis) and causes swelling of the cells. Now the red blood cells become globular (spherocytes) 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.

Applied Physiology

Dehydration Definition:

• Dehydration is defined as excessive loss of water from the body. Body requires certain amount of fluid intake water intake is about 1 L. This varies with the age and daily for normal functions.
• Minimum daily requirement of activity of the individual. The most active individuals need 2-3 L of water intake daily. Dehydration occurs when fluid loss is more than what is consumed.

Dehydration Classification: Basically, dehydration is of three types:

1. Mild dehydration: It occurs when fluid loss is about 5% of total body fluids. Dehydration is not very serious and can be treated easily by rehydration
2. Moderate dehydration: It occurs when fluid loss is about 10%. Dehydration becomes little serious and immediate treatment should be given by rehydration
3. Severe dehydration: It occurs when fluid loss is about 15%. Dehydration becomes severe and requires hospitalization and emergency treatment. When fluid loss is more than 15%, dehydration becomes very severe and life-threatening.

On the basis of ratio between water loss and sodium loss, dehydration is classified into three types:

1. Isotonic dehydration: Balanced loss of water and sodium as in the case of diarrhea or vomiting
2. Hypertonic dehydration: Loss of more water than sodium as in the case of fever
3. Hypotonic dehydration: Loss of more sodium than water as in the case of excess use of diuretics.

Dehydration Causes:

1. Severe diarrhea and vomiting due to gastrointestinal drinking water alone is ineffective because it cannot disorders
2. Excess urinary output due to renal disorders
3. Excess loss of water through urine due to endocrine disorders such as diabetes mellitus, diabetes insipidus, and adrenal insufficiency
4. Insufficient intake of water
5. Prolonged physical activity without consuming an adequate amount of water in hot environment
6. Excess sweating leading to heat frustration (extreme loss of water, heat, and energy). Severe sweating and dehydration occur while spending longer periods on regular basis in the saunas
7. Use of laxatives or diuretics in order to loose weight quickly. This is common in athletes.

Dehydration Signs And Symptoms:

• Mild and moderate dehydration
  1. Dryness of the mouth
  2. Excess thirst
  3. Decrease in sweating
  4. Decrease in urine formation
  5. Headache
  6. Dizziness
  7. Weakness
  8. Cramps in legs and arms.
• Severe dehydration
  1. Decrease in blood volume
  2. Decrease in cardiac output
  3. Low blood pressure
  4. Hypovolemic cardiac shock
  5. Fainting
• Very severe dehydration
  1. Damage of organs like the brain, liver, and kidneys
  2. Mental depression and confusion
  3. Renal failure
  4. Convulsions
  5. Coma.

Aging Effects On Dehydration; Elders are at higher risk for dehydration even if they are healthy. It is because of increased fluid loss and decreased fluid intake. In some cases severe dehydration in old age may be fatal.

Dehydration Treatment; Treatment depends upon the severity of dehydration. In mild dehydration, the best treatment is drinking of water and stopping fluid loss. However, in severe dehydration compensate the salt loss. So the effective treatment for severe dehydration is oral rehydration therapy.

• Oral rehydration therapy
  • Oral rehydration therapy (ORT) is the treatment for dehydration in which an oral rehydration solution (ORS) is administered orally. ORS was formulated by world health organization (WHO).
  • The solution contains anhydrous glucose, sodium chloride, potassium chloride and trisodium citrate.
  • In the case of very severe dehydration, proper treatment is the intravenous administration of necessary water and electrolytes.

Water Intoxication Or Overhydration

Water Intoxication Or Overhydration Definition: Water intoxication is a condition characterized by great increase in the water content of the body. It is also called overhydration, hyperhydration, water excess, or water poisoning.

Water Intoxication Or Overhydration Causes: Water intoxication occurs when more fluid is taken than can be excreted. Water intoxication due to drinking excess water is rare when the body’s systems are functioning normally. But there are some conditions that can produce water intoxication.

1. Heart failure in when heart cannot pump blood properly
2. Renal disorders in which the kidney fails to excrete enough water in urine
3. Hypersecretion of antidiuretic hormone as in the case of syndrome of inappropriate hypersecretion of antidiuretic hormone (SIADH)
4. Intravenous administration of unduly large amounts of medications and fluids that the person’s body can excrete
5. Infants have greater risk of developing water intoxi- cation in the first month of life when the filtration mechanism of the kidney is underdeveloped and cannot excrete the fluid rapidly
6. Water intoxication is also common in infants having swimming practice since they are more prone to drink too much of water while swimming
7. An adult (whose heart and kidneys are functioning normally) can develop water intoxication if the person consumes about 8 liters of water every day regularly.

Water Intoxication Or Overhydration Signs And Symptoms

1. Since the brain is more vulnerable to the effects of water intoxication, behavioral changes appear first
2. The person becomes drowsy and inattentive
3. Nausea and vomiting occur
4. There is sudden loss of weight followed by weakness and blurred vision
5. Anemia, acidosis, cyanosis, hemorrhage, and shock are also common
6. Muscular symptoms such as weakness, cramps, twitching, poor coordination, and paralysis develop
7. Severe conditions of water intoxication result in:
   • Delirium (an extreme mental condition characterized by a confused state and illusion)
   • Seizures (sudden uncontrolled involuntary muscular contractions)
   • Coma (profound state of unconsciousness in which the person fails to respond to external stimuli and cannot perform voluntary actions).

Water Intoxication Or Overhydration Treatment; Mild water intoxication requires only fluid restriction. In very severe cases the treatment includes

1. Diuretics to increase water loss through urine
2. ADH receptor antagonists to prevent ADH-induced reabsorption of water from renal tubules
3. Intravenous administration of saline to restore sodium.