Fluid and Electrolyte Balance
A Percentage of body weight made up by water (Table 14-1):
TABLE 14-1
| Body Water | Adult Male (% body weight) | Adult Female (% body weight) | Infant (% body weight) |
| Total body water | 60 ± 15 | 50 ± 15 | 80 |
| Intracellular | 45 | 40 | 50 |
| Extracellular | 15-20 | 15-20 | 30 |
| Interstitial | 11-15 | 11-15 | 24 |
| Intravascular | 4.5 | 4.5 | 5.0 |
From Wilkins RL, et al: Egan’s Fundamentals of Respiratory Care, ed 8. St. Louis, Mosby, 2003.
TABLE 14-2
| Regulation | Average Daily Volume (mL) | Maximum Daily Volume |
| Water Losses | ||
| Insensible | ||
| Skin | 700 | 1500 ml |
| Lung | 200 | |
| Sensible | ||
| Urine | 1000-1200 | 2000+ ml/hr |
| Intestinal | 200 | 8000 ml |
| Sweat | 0 | 2000+ ml/hr |
| Water Gain | ||
| Ingestion | ||
| Fluids | 1500-2000 | 2000+ ml/hr |
| Solids | 500-600 | 1500 ml/hr |
| Body metabolism | 250 | 1000 ml |
From Wilkins RL, et al: Egan’s Fundamentals of Respiratory Care, ed 8. St. Louis, Mosby, 2003.
4. Obese persons: ≤45%. Fat is much less vascular than muscle, thus the greater the fat content, the lower the water content.
B In the average adult, total body fluid volume is approximately 40 L.
II Normal Intake and Output of Fluids
III Composition of the Intravascular Space (Figure 14-1)
A Normally all fluid compartments in the body are in electrostatic balance as described by the law of electroneutrality (i.e., cation and anion concentrations are equal).
1. Cation mean values (mEq/L):
| Na+ | 142 |
| K+ | 5 |
| Ca+2 | 5 |
| Mg+ 2 | 3 |
| Total | 155 |
| HCO3− | 27 |
| Cl− | 103 |
| Protein | 16 |
| Organic acids | 6 |
| PO4−3 | 2 |
| SO4−2 | 1 |
| Total | 155 |
B Plasma protein concentrations can be subdivided into:
| Mean values | |
| Albumin | 4.8 g/100 ml |
| Globulin | 2.5 g/100 ml |
| Fibrinogen | 300 mg/100 ml |
C The following neutral substances also are present in vascular fluid:
| Mean values | |
| Glucose | 90 mg/100 ml |
| Lipids | 600 mg/100 ml |
D Anion gap: The difference between the commonly measured anions and cations, reflecting the quantity of unmeasured anions:
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1. Normally the anion gap ranges from approximately 15 to 20 mEq/L.
2. An increased anion gap is caused by a metabolic acidosis in which fixed acids accumulate in the body.
3. An increased anion gap indicates an increase in unmeasured anions.
4. However, a metabolic acidosis caused by HCO3− loss does not cause an increased anion gap because a HCO3− loss is associated with a Cl− gain, maintaining electrical neutrality. This type of acidosis is referred to as a hyperchloremic acidosis.
5. Causes of anion gap and nonanion gap metabolic acidosis are listed in Box 14-1.
6. A decrease in the anion gap is caused by a metabolic alkalosis in which increased levels of base accumulate in the body (Box 14-2).
1. Another approach to defining electrolyte imbalances and, as a result, metabolic acid-base disturbances is by assessing the strong ion difference (SID).
2. The SID represents the amount of anions necessary to balance the number of cations (primarily Na+) to maintain electrical neutrality.
3. SID is regulated by the amount of renal reabsorption of Na+: the greater the Na+ reabsorption, the greater the SID, and the lesser the Na+ reabsorption, the lesser the SID.
4. Protein is the other major component of the SID; however, unlike HCO3− it is not regulated with respect to maintenance of ion or acid-base balance.
5. Thus the SID is the amount of HCO3− and protein, the two major body buffers, needed to balance the cations.
6. As noted in Figure 14-1, the SID varies based on fluid compartment evaluated.
7. Because protein is not readily diffusible across the cell membrane, the intracellular SID is greater than the extracellular SID.
8. An increase in the SID is observed during metabolic alkalosis, and a decrease is seen during metabolic acidosis.
IV Composition of Extravascular (Interstitial) Fluid (see Figure 14-1)
A The extravascular concentrations of most substances are about the same as their intravascular concentrations.
B The major exception is protein. Because protein is not freely diffusible across the capillary membrane, extravascular protein concentrations are less than one third intravascular concentrations.
C As with all other compartments, electrostatic balance is maintained.
V Composition of the Intracellular Compartment (see Figure 14-1)
A Electrostatic balance is maintained within the intracellular space.
B However, the composition of anions and cations differs considerably from intravascular levels.
1. Cations, mean values (mEq/L):
| Na+ | 10 |
| K+ | 140 |
| Ca+2 | 0 |
| Mg+2 | 30 |
| Total | 180 |
2. Anions, mean values (mEq/L):
| HCO3− | 10 |
| Cl− | 4 |
| Protein | 61 |
| PO4−3 | 11 |
| SO4−2 | 2 |
| Other anions | 92 |
| Total | 180 |
A The following mechanisms are responsible for the movement of fluid and dissolved substances across membranes:
B Facilitated diffusion (Figure 14-2) occurs from an area of high concentration of the diffusing substance to an area of low concentration. However, a carrier substance is necessary for movement to occur across the membrane.
1. No energy is expended compared with active transport.
2. Glucose moves across cell membranes by facilitated diffusion. Insulin allows a rapid attachment of glucose to the intramembrane carrier substance.
C Active transport is the movement of a substance from an area of low concentration to an area of high concentration (Figure 14-3).
