Intravascular fluid compartment

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. 

Diuretics, typically spironolactone, form the main therapy, combined with restricted salt intake. Sodium restriction is usually unnecessary where fluid retention is mild, and if marked limitation (less than 40 mmol per day intake) is imposed, may lead to impaired nutrition and is poorly accepted. Diuretic treatment often requires reinforcement with loop diuretics. Treatment can be maintained if urinary sodium excretion is at least 30 mmol per day. Removal of ascites through diuresis requires fluid transfer through the intravascular fluid compartment. If diuresis is too intense the intravascular fluid volume is reduced and hypotension causes hepatorenal failure to follow. The aim should be, through monitoring weight loss, to restrict fluid removal to 0.5 kg per day. In this way the risks of hyponatraemia, renal and hepatic impairment should be reduced. 

Egress from Intravascular Fluid Compartment into Interstitial... 

The extracellular fluid (ECF) is the fluid outside the cell and is rich in sodium, chloride, and bicarbonate. O The ECF is approximately one-third of TBW (14 L in a 70-kg man or 12 Lin a 70-kg woman) and is subdivided into two compartments the interstitial fluid and the intravascular fluid. The interstitial fluid (also known as lymphatic fluid) represents the fluid occupying the spaces between cells, and is about 25% of TBW (10.5 L in a 70-kg man or 8.8 L in a 70-kg woman). The intravascular fluid (also known as plasma) represents the fluid within the blood vessels and is about 8% of TBW (3.4 L in a 70-kg man or 2.8 L in a 70-kg woman). The ECF is approximately one-third of TBW or 14 L in a 70-kg male. Because the exact percentages are cumbersome to recall, many clinicians accept that the ECF represents roughly 20% of body weight (regardless of gender) with 15% in the interstitial space and 5% in the intravascular space.6 Note that serum electrolytes are routinely measured from the ECF. 

Plasma volume expanders. If blood is lost during trauma, the loss of volume is more immediately threatening than the loss of red blood cells. Replacement with salt solutions does not work well because small solutes get rapidly filtrated into the interstitial fluid compartment. Only macromolecules are retained in the intravascular space and can prevent filtration of the diluted plasma due to their osmotic activity. Commonly used plasma expanders are metabolically inert polysaccharides such as dextran and hydroxyethyl-starch. 

Figure 46-1 Volume and distribution of total body water. Note that the intracellular and ECF compartments (ICF and ECF, respectively) are separated by cellular plasma membranes, and within the ECF, interstitial and intravascular fluids are separated by the capillary endothelium.The volumes indicated represent water and not total volume.

Body water and the electrolytes it contains are in a state of constant flux. Wc drink, we eat. we pass urine and wc sweat during all this it is important that wc maintain a steady state. A motor car s petrol tank might hold about 42 litres, similar to the total body water content of the average 70 kg male. If 2 litres were lost quickly from the tank it would hardly regi.steron the fuel indicator. However, if we were to lose the same volume from our intravascular comparimeni w e would be in serious trouble. We are vulnerable to changes in our fluid compartments, and a number of important homeostatic mechanisms... 

Plasma protein concentrations are dynamic parameters that depend on the biosynthesis, distribution between intravascular and extravascular fluid compartments, and elimination (degradation, catabolism, and loss) of the proteins. Table 8.3 lists some of the common causes for changes of plasma protein and albumin concentrations. A rise in plasma albumin is often due to dehydration, and this can be confirmed by associated increases of plasma globulins, blood hemoglobin, and hematocrit (packed cell volume). 

Proteins help to maintain the body s fluid and electrolyte balance. This means that proteins ensure that the proper types and amounts of fluid and minerals are present in each of the body s three fluid compartments. These fluid compartments are intracellular (contained within cells), extracellular (existing outside the cell), and intravascular (in the blood). Without this balance, the body cannot function properly. 

The body fluids do not constitute a homogeneous solution of electrolytes. Body fluids are classically categorized as intracellular and extracellular. The extracellular compartment is further divided into an intra- and extravascular compartment. The intravascular fluid constitutes the blood plasma, but even blood plasma does not form a homogeneous compartment. The composition of plasma varies with anatomical location and physiological conditions. The electrolyte compositions of plasma obtained from venous and arterial blood differ, and there are diurnal variations in the electrolyte concentrations of the plasma. 

One of the two main fluid compartments into which the body can be theoretically divided (the other being the intracellular fluid). Extracellular fluid can be subdivided into the intravascular fluid (i.e. plasma) and interstitial fluid (i.e. the fluid between the tissue cells). Plasma is separated from interstitial fluid by the capillary wall which acts as a semipermeable membrane, allowing the passage of water and small molecules, but not the larger molecules such as proteins. 

In arterioles, the hydrostatic pressure is about 37 mm Hg, with an interstitial (tissue) pressure of 1 mm Hg opposing it. The osmotic pressure (oncotic pressure) exerted by the plasma proteins is approximately 25 mm Hg. Thus, a net outward force of about 11 mm Hg drives fluid out into the interstitial spaces. In venules, the hydrostatic pressure is about 17 mm Hg, with the oncotic and interstitial pressures as described above thus, a net force of about 9 mm Hg attracts water back into the circulation. The above pressures are often referred to as the Starling forces. If the concentration of plasma proteins is markedly diminished (eg, due to severe protein malnutrition), fluid is not attracted back into the intravascular compartment and accumulates in the extravascular tissue spaces, a condition known as edema. Edema has many causes protein deficiency is one of them. 

Atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP) are members of a family of so-called natriuretic peptides, synthesized predominantly in the cardiac atrium, ventricle, and vascular endothelial cells, respectively (G13, Y2). ANP is a 28-amino-acid polypeptide hormone released into the circulation in response to atrial stretch (L3). ANP acts (Fig. 8) on the kidney to increase sodium excretion and glomerular filtration rate (GFR), to antagonize renal vasoconstriction, and to inhibit renin secretion (Ml). In the cardiovascular system, ANP antagonizes vasoconstriction and shifts fluid from the intravascular to the interstitial compartment (G14). In the adrenal cortex, ANP is a powerful inhibitor of aldosterone synthesis (E6, N3). At the hypothalamic level, ANP inhibits vasopressin secretion (S3). It has been shown that some of the effects of ANP are mediated via a newly discovered hormone, called adreno-medullin, controlling fluid and electrolyte homeostasis (S8). The diuretic and blood pressure-lowering effect of ANP may be partially due to adrenomedullin (V5). 

Albumin 5% and 25% concentrations are available. It takes approximately three to four times as much lactated Ringer s or normal saline solution to yield the same volume expansion as 5% albumin solution. However, albumin is much more costly than crystalloid solutions. The 5% albumin solution is relatively iso-oncotic, whereas 25% albumin is hyperoncotic and tends to pull fluid into the compartment containing the albumin molecules. In general, 5% albumin is used for hypovolemic states. The 25% solution should not be used for acute circulatory insufficiency unless diluted with other fluids or unless it is being used in patients with excess total body water but intravascular depletion, as a means of pulling fluid into the intravascular space. 

Osmotic effects are very important from a physiological standpoint. This is because biological membranes including the membrane of red blood cells behave like semipermeable membranes. Consequently, when red blood cells are immersed in a hypertonic solution (e.g., D5 A NS or D5NS), they shrink as water leaves the blood cells in an attempt to dilute and establish a concentration equilibrium across the blood cell membrane. Thus, when hypertonic solutions are administered into the blood stream, the fluid moves from interstitial and cellular space into the intravascular space. Conversely, when cells are placed in hypotonic environment (e.g., V2 NS), they swell because of the entry of fluid from the intravascular compartment, and may eventually undergo lysis. 

Excessively vigorous diuresis may lead to intravascular dehydration before removal of edema fluid from the rest of the extracellular compartment. This is especially dangerous if the patient has significant liver or kidney... 

FIGURE 3.4 Measured plasma concentrations of insulin in compartment 1 (intravascular space) after intravenous injection of a 25-mU/kg dose, and computer-derived estimates of insulin concentration in presumed splanchnic (compartment 2) and somatic (compartment 3) components of interstitial fluid space. The bar graph indicates the glucose infusion rate needed to maintain blood glucose concentrations at the basal level. (Reproduced with permission from ShenA in RS, Kramer KJ, Tobin JD, Insel PA, Liljenquist JE, Berman M, Andres R. J Clin Invest 1974 53 1481-92.)... 

High output left ventricular failure has been described after hysteroscopic lysis of adhesions using dextran as a distension medium. Prolonged surgical dissection of the uterine wall (the precise duration of the operation was not stated in the report) and the large volume of dextran and fluid (2 liters of 5% dextrose and an additional 800 ml of dextran) probably caused the dextran to enter into the systemic circulation, inducing a significant shift of fluid into the intravascular compartment (2). 

Pulmonary edema and coagulopathy following intrauterine instillation of 700 ml of 32% dextran 70 has been reported (3). The volume exceeded that recommended by the manufacturer (500 ml), and the installation time (2 hours) was in excess of that recommended (45 minutes). The authors pointed out that hyperosmolarity of the agent is such that if it enters the intravascular compartment, volume overload can result, since 100 ml of intravascular dextran 70 will osmotically expand the intravascular volume by 860 ml, by drawing interstitial fluid into the central compartment. This can further aggravate the risks of pulmonary edema and dilutional coagulopathy. 

Changes in posture cause fluid shifts within 10 minutes and thus alter the concentration of cells and large molecules, including albumin and total calcium (as part of it is protein-bound) in the vascular compartment. Standing decreases intravascular water and increases the total calcium concentration by 0.2 to 0.8 mg/dL (0,05 to 0.20 mmol/L), whereas... 

Based on the administration of 1 L of each solution which may not be an appropriate amount for clinical use) numbers are approximations arrows indicate direction of fluid shift and plus signs Indicate fluid retention In that compartment. After distribution, 60% of albumin (and associated fluid) Is In Interstitial compartment and 40% Is In Intravascular compartment. 

By causing redistribution (i.e., pulling fluid) from the intracellular space, hypertonic solutions cause rapid expansion of the intravascular compartment, which is essential for vital organ perfusion. In head-injured patients, this redistribution should decrease intracranial pressure because the vessels of the brain are more impermeable to sodium ions than vessels in other areas of the body. Additionally, hypertonic saline solutions have beneficial immunomodulating actions when compared with more isotonic solutions in experiments with animals. ... 

Isotonic crystalloids, such as 0.9% sodium chloride (normal saline) or lactated Ringer s solution, are used commonly for fluid resuscitation. A patient in septic shock typically requires up to 10 L of crystalloid solution during the first 24-hour period. These solutions distribute into the extracellular compartment. Approximately 25% of the infused volume of crystalloid remains in the intravascular space, whereas the balance distributes to extravascular spaces. Although this could impair diffusion of oxygen to tissues, clinical impact is unproven. 

Maternal water retention. Total body water increases by about 5 litres, mostly in the extracellular fluid. The volume of the intravascular compartment increases by more than a litre. 

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