Hem 01

FLUID COMPARTMENTS 1. 2. 3.

Body fluids and their volumes. How to measure them. Comparison of their compositions.

The body as an open system where the body exchanges material and energy with its surroundings e.g. H2O, Na, K, food … etc… .

Body fluids Water is the universal biological solvent. It was moved by osmosis according to osmotic gradients.

Body water content (total body Infants have low bodywater) fat, low bone mass, and is 73% or more total body water. Healthy males are about 60% total body water. Healthy females are around 50% total body water. @ Higher body fat. @ Smaller amount of skeletal muscle. Total body water content declines throughout life. In old age, only about 45% of body weight is water.

Water balance (water steady state) i.e. water gain (amount ingested = water loss (amount eliminated A- The average daily intake of water is about 2500 ml in the form of: 1- Ingested foods and drink (2200 ml/day). 2- Metabolic water (300 ml/day). However, intake of water is highly variable among different people and even within the same person on different days, depending on climate, habits and level of physical activity.

B) Also, the average output of water is about 2500 ml to achieve balance through: 1- Insensible water loss: a- Evaporation from the respiratory tract (400 ml/day). b- Diffusion through the skin (400 ml/day). 2- Water loss in sweat (about 100 ml/day). 3- Water loss in feces (about 100 ml/day). 4- Water loss by the kidneys (about 1500 ml/day).

Notes The insensible water loss through the skin occurs independently of sweating and is present even in people who are born without sweat glands. The average water loss by diffusion through the skin is minimized by the cholesterol-filled cornified layer of the skin, which provides a barrier against excessive loss by diffusion. When the cornified layer becomes denuded, as occurs with extensive burns, the rate of evaporation can increase as much as 10-fold reaching to 3 -5 L/day. In very hot weather or during heavy exercise, water loss in sweat occasionally increases to 1 to 2 L/hour. This would rapidly deplete the body fluids if intake were not also increased by activating the thirst mechanism.). The water loss in feces can increase to several liters a day in people with severe diarrhea. Urine volume can be as low as 0.5 L/day in a dehydrated person or as high as 20 L/day in a person who has been drinking tremendous amounts of water.

Total body water The percentage of total body water is ~ 60% of the body weight which is equal to ~ 45 liters for a 70 Kg man.

It is differentiated into: 1) Intracellular fluid (ICF) (IC Compartment): It is the liquid within the cell membrane i.e. that are found inside the 75 trillion cells. It represents 2/3 of total body water (~ 40% body weight) (~ 30L). It is responsible for metabolic reactions. In Protozoa, only intracellular fluid is present. 2) Extracellular fluid (ECF) (EC compartment): It fills the spaces between most cells of the body. It represents 1/3 of total body water (~ 20% body weight) (~ 15 liters). It is responsible for the nourishment of cells and also it serves in elimination of waste products.

Total body water (continue) ECF is differentiated into: a) Interstitial fluid: Water is about 15% body weight = to about 11 liters. b) The blood plasma (intravascular fluid): Water is about 4% body weight = ~ 3 liters. The pores of the capillary membranes are highly permeable to almost all solutes in the interstitial fluid except the proteins. So, plasma and interstitial fluids have about the same composition except for proteins, which have a higher concentration in the plasma. c) Transcellular fluids: Water is about 1% BW = ~ 1-2 liter. e.g. cerebrospinal fluid (CSF), eye humors (aqueous & vitreous), synovial fluid, gastrointestinal secretions (digestive juices), mucus, peritoneal fluid, pericardial fluid , tears, …. etc.

Constituents of extracellular and intracellular fluids The principle extracellular cation is Na+. The principle extracellular anions are chloride and bicarbonate. The principle intracellular cation is K+. The principle intracellular anions are phosphates [both inorganic (HPO42-, H2PO4-) and organic (ATP, etc.) and proteins. So, plasma and interstitial fluids have about the same composition except for proteins, which have a higher concentration in the plasma. Notes Solutes are broadly classified into: 1- Electrolytes – examples include inorganic salts, all acids and bases and some proteins. 2- Nonelectrolytes – examples include glucose, lipids, creatinine and urea, ….. etc. Electrolytes have greater osmotic power than nonelectrolytes.

Electrolyte concentration Expressed in milliequivalents per liter (mEq/L), a measure of the number of electrical charges in one liter of solution. mEq/L = (concentration of ion in [mg/L] / the atomic weight of ion) × number of electrical charges on one ion. For single charged ions, 1 mEq = 1 mOsm. For bivalent ions, 1 mEq = 1/2 mOsm.

Measurement of body fluid compartments For single charged ions, 1 mEq = 1 mOsm. For bivalent ions, 1 mEq = 1/2 mOsm.

Major cations and anions of the intracellular and extracellular fluids

Nonelectrolytes of the plasma

* Inject the suitable indicator with a known volume (V1) and concentration (C1)in the vein.

Wall of blood capillaries

* After a period, collect a sample of bloodand calculate the concentration of the indictor in the plasma (C2). * Apply the equation C1V1 = C2V2 to determine the volume of the specific fluid volume. # Determination of plasma volume: Can not penetrate the wall of blood vessels e.g. radioactive albumin 125Ialbumin) or Evans blue dye.

# Determination of extracellular fluid volume: Can penetrate the wall of blood vessels and can not penetrate the cell membrane e.g. radiated Na, Cl, thiosulphate …etc.

# Determination of total body water: Can penetrate the wall of blood vessels and cell membrane e.g. deuterium (heavy water) (2H2O), radioactive water (tritium, 3H2O) and antipyrine.

Plasma membrane

# Intracellular fluid volume (ICFV) Intracellular volume = total body water - extracellular volume # 5- Interstitial fluid volume (ISFV): The interstitial fluid volume is determined as the difference between ECF volume and plasma volume. To measure total blood volume, radioactive chromium (51Cr) is incubated with red blood cells then injected into animal. Or by the following equation: Total blood volume = plasma volume/ 1- hematocrit value

Markers (indicators) for measurement must share following qualities: 1- They disperse evenly in the compartment. 2- They remain in the compartment being measured. 3- Dispersion takes place only in the compartment. 4- They are not metabolized or excreted. 5- They are not toxic. 6- They are easily measurable (chemically, photoelectrically or by other means) 7- They do not alter water distribution.

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