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Ch. 1 Body Water
Transcript of Ch. 1 Body Water
This chapter examines the body fluids, the water distribution along different compartments; the ionic and osmotic composition of the intra- and extracellular compartments is discussed, and finally, the evaluation of water movements between the extracellular compartments is considered.
Body water content and water distribution
The distribution of water among the body fluids is shown. Approximately, the total body water represents 60% of the body weight of a person. An individual weighting 70 Kg will have about 42 liters of water. This water distributes 40% in the intracellular compartment and 20% in the extracellular compartment. The 60, 40, 20 rule is useful to know that 60% of the body weight is water, 40% is in the intracellular fluid, and 20% is in the extracellular fluid. The extracellular compartment contents the interstitial and the plasma fluids. These two compartments are separated by the capillary wall. The extracellular and intracellular compartments are separated by the cellular membrane that is normally water permeable.
Calculus of the body water volumes in the body compartments
The evaluation of the water content in each compartment of the body fluids is based in the dilution methods. This method consists in using a substance that is distributed exclusively in a particular compartment.
For example, tritiaded water serves to evaluate the total body water because tritiaded water distributes in all compartments of the body. In order to proceed with the calculus of the volume of water, the dilution method formula should be applied, is to say, the total amount of the substance added should be divided by the new concentration reached. The extracellular water is evaluated using 23Na or using inulin. The water content in the plasma is evaluated using 131I-albumin or using the dye Evans blue. The intracellular compartment is estimated by subtracting the extracellular volume to the total body water, and the interstitial water is estimated by subtracting the plasma water volume to the extracellular water.
Comparing the body compartments
The osmolarity values of the solutions in the body fluids are similar having approximately 300 milliosmols per liter of water. The Starling law that includes the hydrostatic pressure and the oncotic pressure could be applied only in the extracellular compartments, it is to say between plasma and intertitium, and in the particular case of the figure no net pressure for fluid movements is established. Ions are distributed asymmetrically among the extracellular and the intracellular compartments. Sodium ions are most concentrated in the extracellular compartments and potassium ions are most concentrated in the intracellular fluid. The cell membrane is permeable to water but relatively impermeable to solutes. On the other hand, the intercellular spaces in the capillary wall allow the movement of practically all solutes except large proteins and obviously the blood cells.
Osmolality of the plasma fluid
As stated previously, the osmolarity is similar in all body compartments having values near 300 miliosmols per kg of water. However, the ionic composition is different between the extracellular and the intracellular compartments. Ions are distributed asymmetrically. Whereas sodium is most concentrated in the extracellular fluid, potassium is most concentrated in the intracellular fluid. The principal anion in the extracellular fluid is chloride and the principal anion in the intracellular fluid is phosphate. The plasma osmolarity can be estimated approximately by multiply the plasma sodium concentration by 2. However, a more precise estimation of the plasma concentration can be obtained if glucose concentration and the blood urea nitrogen are known, by using the formula shown below.
Movements of fluids in the ECF
“Starling forces and equation”.
The fluid movement between extracellular compartments, it is to say between the plasmatic and the interstitial compartments are determined by the Starling equation. The hydrostatic pressure inside the capillary should produce the fluid movement toward the interstitial fluid. The inverse fluid movements will occur by the action of the hydrostatic pressure in the interstitial fluid. The oncotic pressure inside the capillary will induce the fluid movement inside the capillary and the contrary fluid movement will be produced by the oncotic pressure inside the interstitial fluid. In the Figure A we can see that the hydrostatic and the oncotic pressures in the interstitial fluid are equals. In this particular case we can evaluate the fluid movements analyzing only the pressures in the capillary. Let consider the left side of the figure A as the beginning and the right of the figure A as the end of the capillary. At the beginning of the capillary the pressure will be 7 mm of mercury favoring the fluid movement to the interstitium. If the oncotic pressure is similar at the final of the capillary, then the total calculated pressure of -13 mm of mercury will favors the fluid movement to the capillary. In the Figure B and based in the Starling equation, contrary the first example the calculated pressure, at the beginning of the capillary, favors the fluid movement outside the capillary and at the final no net movement of fluids is observed. The calculus should include the hydraulic conductance Kf such as the Starling equation states.