Body Fluids PDF

Dr. Zakaria Rajeh Total Body Water 1. Intracellular fluid (ICF): approximately 2/3 of total body water 2. Extracellular fluid (ECF): approximately 1/3 of total body water a. Interstitial fluid (ISF): approximately 3/4 of the extracellular fluid b. Vascular compartment: contains the bloo...

Dr. Zakaria Rajeh Total Body Water 1. Intracellular fluid (ICF): approximately 2/3 of total body water 2. Extracellular fluid (ECF): approximately 1/3 of total body water a. Interstitial fluid (ISF): approximately 3/4 of the extracellular fluid b. Vascular compartment: contains the blood volume which is plasma and the cellular elements of blood, primarily red blood cells Plasma volume (PV): approximately 1/4 of the extracellular fluid  It is important to remember that membranes can serve as barriers.  The 2 important membranes are shown below.  The cell membrane is a relative barrier for Na+, while the capillary membrane is a barrier for plasma proteins. FUNCTIONS OF THE BODY WATER: (1) It is a medium for the chemical and enzymatic reactions. (2) It is a medium for the physical processes e.g. diffusion and filtration. (3) It is an ionizing medium (regulating pH and body fluids osmolality). (4) It regulates the body temperature through heat absorption and distribution and evaporation. (5) It is a lubricant in the joints and the potential spaces (e.g. the pleura). (6) It is a refractive medium in the eye. (7) The cerebrospinal fluid is a mechanical buffer that protects the brain. (8) It is the medium for exchange of O2 and CO2 in the lungs and tissues. COMPOSITION OF THE ECF The composition of the ECF is almost the same elsewhere except for the protein concentration which is much higher in the plasma than in the interstitial fluid (about 17 mEq/liter & 5 mEq /liter respectively).  The main cation is Na/ (about 142 mEq/liter) while the main anion is Cl (about 106 mEq/liter).  Other cations include K (about 4.2 mEq/liter) and small amount of Ca and Mg while other anions include HCO3- (about 24 mEq/liter) ,proteins and small amounts of HPO4 and SO4.  The ECF also contains non-electrolytes such as :  glucose, cholesterol, urea, uric acid, creatinine, bile pigments and phospholipids.  Its pH is about 7.4 and its osmolality is about 300 mOsm /liter. COMPOSITION OF THE ICF  The main cations include K and Mg (about 140 mEq/liter and 20 mEq/liter respectively) together with a small amount of Na and very little Ca.  The main anions include HPO4- and protein together with small amounts of Cl, HCO3 and SO4.  Its pH is less than that of the ECF (about 7) due to its low HCO3 content, while its osmolality is equal to that of the EC. FLUID EXCHANGE BETWEEN ECF and ICF:  Normally, the cell membranes are highly permeable to water, so fluid exchanges continuously across them resulting in an equal osmolality in both the ECF and ICF. This condition is maintained as follows :  If a hypertonic NaCl solution is i.v. injected. the ECF osmolality increases. This causes water transfer from the ICF to the ECF till an osmotic equilibrium is established and the osmolality of both fluids become equal again. As a result, the ICF volume decreases, so the cells are dehydrated and shrink.  If a hypotonic NaCl solution (or water) is i.v. injected, the ECF osmolality decreases.This causes water transfer from the ECF to the ICF till an osmotic equilibrium is established and the osmolality of both fluids become equal again.  As a result, the ICF volume increases, so the cells are swollen and may rupture.  Injection of isotonic solutions mainly increase the ECF volume (resulting in edema), but they do not affect the osmolality of the body fluids. OSMOSIS ACROSS SELECTIVELY PERMEABLE MEMBRANES—“NET DIFFUSION” OF WATER:  The most abundant substance that diffuses through the cell membrane is water.  The amount that normally diffuses in the two directions is balanced so zero net movement of water occurs.  Therefore, the volume of the cell remains constant.  However, under certain conditions, a concentration difference for water can develop across a membrane.  When this concentration difference for water develops, net movement of water does occur across the cell membrane, causing the cell to swell or shrink, depending on the direction of the water movement.  Osmosis is the diffusion of water across a semipermeable or selectively permeable membrane. Water diffuses from a region of higher water concentration to a region of lower water concentration.  The concentration of water in a solution is determined by the concentration of solute; the greater the solute concentration, the lower the water concentration.  It is the number of particles that is crucial.  There are 2 compartments separated by a membrane that is permeable to water but not to solute.  Side B has the greater concentration of solute (circles) and thus a lower water concentration than side A. As a result, water diffuses from A to B, and the height of column B rises, and that of A falls.  If a solute does not easily cross a membrane, then it is an “effective” osmole for that compartment, i.e., it creates an osmotic force for water.  For example, plasma proteins do not easily cross the capillary membrane, so they serve as effective osmoles for the vascular compartment.  Sodium does not easily penetrate the cell membrane, but it does cross the capillary membrane, thus it is an effective osmole for the extracellular compartment. 2 Requirements for osmosis:  Must be difference in[solute] on the 2 sides of the membrane.  Membrane must be impermeable to the solute. Osmotic pressure  Osmotic pressure: The amount of pressure required to stop osmosis of the sodium chloride solution.  Osmosis of water from chamber B into chamber A causes the levels of the fluid columns to become farther and farther apart, until eventually a pressure difference develops between the two sides of the membrane that is great enough to oppose the osmotic effect. WATER BALANCE  This is the balance between the daily amounts of water gain and water loss. Under normal conditions, both are equal (2300 ml/day each) so that the body is in a normal water balance, and the TBW is kept constant. WATER GAIN  This is normally about 2300 ml/day and is derived from 2 sources :  Exogenous water (by the oral route) : This is the main source of water gain. It averages 2000 ml/day, and it includes the volumes of :  Water and other fluids that are drunk (about 1400 ml/day).  Water present in the eaten food e.g. water in meat, vegetables and bread (about 600 ml/day).  Endogenous water (which is formed inside the body as a result of oxidation of H2 in the foodstuffs). It is normally about 300 ml/day. WATER LOSS In the comfortable zone of atmospheric temperature (about 20 °C), young adult individuals normally lose about 2300 ml of water/day as follows : (a) 1400 ml in the urine. (b) 100 ml in the feces. (c) 350 ml by evaporation from the respiratory tract. (d) 450 ml from the skin (mostly as insensible perspiration i.e. by nonsensed diffusion and evaporation through the skin). Water loss through the skin & respiratory tract is thus commonly called the insensible water loss. THANK YOU…