Compartments, body fluid

Estimate the volumes of various body fluid compartments. 

The transcellular fluid includes the viscous components of the peritoneum, pleural space, and pericardium, as well as the cerebrospinal fluid, joint space fluid, and the gastrointestinal (GI) digestive juices. Although the transcellular fluid normally accounts for about 1% of TBW, this amount can increase significantly during various illnesses favoring fluid collection in one of these spaces (e.g., pleural effusions or ascites in the peritoneum). The accumulation of fluid in the transcellular space is often referred to as third spacing. To review the calculations of the body fluid compartments in a representative patient, see Patient Encounter 1. 

The body s normal daily sodium requirement is 1.0 to 1.5 mEq/kg (80 to 130 mEq, which is 80 to 130 mmol) to maintain a normal serum sodium concentration of 136 to 145 mEq/L (136 to 145 mmol/L).15 Sodium is the predominant cation of the ECF and largely determines ECF volume. Sodium is also the primary factor in establishing the osmotic pressure relationship between the ICF and ECF. All body fluids are in osmotic equilibrium and changes in serum sodium concentration are associated with shifts of water into and out of body fluid compartments. When sodium is added to the intravascular fluid compartment, fluid is pulled intravascularly from the interstitial fluid and the ICF until osmotic balance is restored. As such, a patient s measured sodium level should not be viewed as an index of sodium need because this parameter reflects the balance between total body sodium content and TBW. Disturbances in the sodium level most often represent disturbances of TBW. Sodium imbalances cannot be properly assessed without first assessing the body fluid status. 

Because disturbances in fluid balance are routinely encountered in clinical medicine, it is essential to have a thorough understanding of body fluid compartments and the therapeutic use of fluids. Similarly, disturbances in serum sodium, potassium, calcium, phosphorus, and magnesium are ubiquitous and must be mastered by all clinicians. Dysregulation of fluid and/or electrolyte status has serious implications regarding the concepts of drug absorption, volumes of distribution, and toxicity. Similarly, many medications can disrupt fluid and/or electrolyte balance as an unintended consequence. 

Volumes of the intracellular and extracellular body fluid compartments are kept constant by the osmotic pressure, which is created by the concentration of dissolved ions (electrolytes) in each compartment. The normal osmotic concentration is in the range of 280-310 mOsm/L. 

Relative concentrations of the weak acid acetaminophen (Pkg 9.5) and a weak base, diazepam (pKa 3.3), in some body fluid compartments. [ ], concentration U, un-ionized drug I, ionized drug. 

Flucytosine is currently available in North America only in an oral formulation. The dosage is 100-150 mg/kg/d in patients with normal renal function. It is well absorbed (> 90%), with serum concentrations peaking 1-2 hours after an oral dose. It is poorly protein-bound and penetrates well into all body fluid compartments, including the cerebrospinal fluid. It is eliminated by glomerular filtration with a half-life of 3-4 hours and is removed by hemodialysis. Levels rise rapidly with renal impairment and can lead to toxicity. Toxicity is more likely to occur in AIDS patients and those with renal insufficiency. Peak serum concentrations should be measured periodically in patients with renal insufficiency and maintained between 50 and 100 mcg/mL. 

Maintenance of water homeostasis is paramount to life for all organisms. In mammals, the maintenance of osmotic pressure and water distribution in the various body fluid compartments is primarily a function of the four major electrolytes, Na", K , Cl", and HCOi". In addition to water homeostasis, these electrolytes play an important role in the maintenance of pH, proper heart and muscle function, oxidation-reduction reactions, and as cofactors for enzymes. Indeed, there are almost no metabolic processes that are not dependent on or affected by electrolytes. Abnormal concentrations of electrolytes may be either the cause or the consequence of a variety of disorders. Thus determination of electrolytes is one of the most important functions of the clinical laboratory. Interpretation of abnormal osmolality and acid-base values requires specific knowledge of the electrolytes. Because of their physiological and clinical interrelationship, this chapter discusses determination. of electrolytes, osmolality, acid-base status, and blood oxygenation. 

TABLE 46-2 Electrolyte and Water Composition of Body Fluid Compartments "... 

Fig, 2 Water tank model of body fluid compartments. 

The water tank model illustrates the relative volumes of each of these compartments and can be used to help visualise some of the clinical disorders of lluid and electrolyte balance. It is important to realize that the assessment of the volume of body fluid compartments is not the undertaking of the biochemistry laboratory. The patient s state of hydration, i.e. the volume of the body fluid compartments, is assessed on clinical grounds. The term dehydration simply means that fluid loss has occurred from body compartments. Over-hydration occurs when lluid accumulates in body compartments. Figure 3... 

Fig. 4 Osmolality changes and water movement in body fluid compartments. The osmolality in different body compartments mu.st be equal. This is achieved by the movement of water across semipermeable membranes in response to concentration changes.

What will have happened to his body fluid compartments ... 

Assessment of the volumes of body fluid compartments is not carried out in the clinical biochemistry laboratory. This must be done clinically by history taking and examination. 

Arginine vasopressin (AVP) regulates renal water loss and thus causes changes in the osmolality of body fluid compartments. 

Fig. 1 The three types of fluid usually used In fluid therapy are shown here with the different contributions they make to the body fluid compartments.

Body Fluid Compartments Regulation of Water and Electrolyte Balance Movement of Water and Electrolytes Factors Regulating Movement Imbalances of Water and Electrolytes Water Depletion Water Excess Sodium Depletion Sodium Excess Potassium Depletion Potassium Excess Chloride... 

BODY FLUID COMPARTMENTS. Fluids—water, electrolytes, and other dissolved substances—are contained in two major compartments within the body. In order to gain this concept of body compartments, all the cells of the body must be thought of as a whole. Then, all fluid outside of the cells is termed extracellular fluid, while all fluid within the cells is termed intracellular fluid. Fluids in each compartment differ in composition. 

Dmgs distribute unevenly between red cells, white cells, plasma protein and plasma water. Once in the blood, a dmg diffuses throughout the various body fluid compartments at a rate and to an extent which depends on its physicochemical characteristics. The relative volumes of the fluid compartments into which the dmg can diffuse are shown in Figure 8.53. Total body water comprises 40% intracellular and 20% extracellular fluid (IGF and... 

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