Intracellular fluid composition

Each cell is surrounded by a plasma membrane that separates the cytoplasmic contents of the cell, or the intracellular fluid, from the fluid outside the cell, the extracellular fluid. An important homeostatic function of this plasma membrane is to serve as a permeability barrier that insulates or protects the cytoplasm from immediate changes in the surrounding environment. Furthermore, it allows the cell to maintain a cytoplasmic composition very different from that of the extracellular fluid the functions of neurons and muscle cells depend on this difference. The plasma membrane also contains many enzymes and other components such as antigens and receptors that allow cells to interact with other cells, neurotransmitters, blood-borne substances such as hormones, and various other chemical substances, such as drugs. 

Most hydrophilic, or water-soluble, substances are repelled by this hydrophobic interior and cannot simply diffuse through the membrane. Instead, these substances must cross the membrane using specialized transport mechanisms. Examples of lipid-insoluble substances that require such mechanisms include nutrient molecules, such as glucose and amino acids, and all species of ions (Na+, Ca++, H+, Cl, and HC03). Therefore, the plasma membrane plays a very important role in determining the composition of the intracellular fluid by selectively permitting substances to move in and out of the cell. 

With active transport, energy is expended to move a substance against its concentration gradient from an area of low concentration to an area of high concentration. This process is used to accumulate a substance on one side of the plasma membrane or the other. The most common example of active transport is the sodium-potassium pump that involves the activity of Na+-K+ ATPase, an intrinsic membrane protein. For each ATP molecule hydrolyzed by Na+-K+ ATPase, this pump moves three Na+ ions out of the cell and two K+ ions into it. As will be discussed further in the next chapter, the activity of this pump contributes to the difference in composition of the extracellular and intracellular fluids necessary for nerve and muscle cells to function. 

All eukaryote cells are faced with differences in intracellular solute composition when compared with the external environment. Many eukaryotes live in seawater, and have cells which are either bathed in seawater directly, or have an extracellular body fluid which is broadly similar to seawater [3]. Osmoregulation and body fluid composition in animals has been extensively reviewed (e.g. [3,15-21]), and reveals that many marine invertebrates have body fluids that are iso-osmotic with seawater, but may regulate some electrolytes (e.g. SO2-) at lower levels than seawater. Most vertebrates have a body fluid osmotic pressure (about 320mOsmkg 1), which is about one-third of that in seawater (lOOOmOsmkg ), and also regulate some electrolytes in body fluids at... 

Body fluids are partitioned between the intracellular fluids (ICF), which constitute two-thirds of total body water, and extracellular fluids (ECF), which constitute one-third of total body water. The ECF consists of plasma and interstitial fluid plus lymph. The ionic composition differs substantially between ECF and ICF (Table 21.1). Sodium is the primary cation in ECF, whereas potassium is the principal intracellular cation. 

In this chapter, we will discuss a relatively small number of basic principles of water-solute interactions that provide a conceptual framework for understanding central aspects of macromolecular and micromolecular evolution. We will discover vital links between these two evolutionary processes through addressing the question of why the intracellular fluids have evolved to contain the types and amounts of solutes they do. The unifying view that can be developed through study of these basic principles will enable us to understand the unity that underlies the apparent diversity found in cellular fluids, in which total solute concentration varies by well over an order of magnitude, and solute composition likewise exhibits enormous variation among species. As emphasized... 

Figure 6.1. Compositions and concentrations of intracellular and extracellular fluids of selected marine animals having widely different total osmolarities. M = intracellular fluids of muscle tissue PI = plasma or hemolymph TMAO = trimethylamine-A-oxide Bet = glycine betaine FAA = free amino acids. (Data compiled from various sources for a comprehensive list of osmolyte compositions and concentrations in diverse species, see Kirschner, 1991 Somero and Yancey, 1997 Yancey et al., 1982.)...

Intracellular fluids (also called the cytosol) are quite different compositionally from plasma and interstitial fluids (Table 4, Figures 4 and 5). The internal pH of many cells is maintained near 6.9-7.0. via various membrane transport mechanisms such as Na" /H and CP/HCO exchangers, and various phosphate and protein buffers. In contrast to the plasma, the intracellular fluids have substantially lower concentrations of sodium, calcium, chloride, and bicarbonate and higher to substantially higher concentrations of potassium, magnesium,... 

Figure 9 Plots showing the calculated mineral saturation indices as a function of pH for hydroxylapatite, quartz, and various asbestos-forming minerals in electrolyte solutions approximating the electrol)he compositions of lung fluids (approximated by interstitial fluids, upper plot) and intracellular fluids (lower plot). Electrolyte concentrations used as input were taken from Table 4. The CO2 partial pressure was fixed at the value for venous plasma for each speciation at a different pH. Organic species such as amino acids and other organic acids were not included in the calculations, but likely would have the effect of decreasing the calculated saturation indices somewhat due to their...

The kidneys regulate and maintain the constant optimal chemical composition of the blood and the interstitial and intracellular fluids throughout the body. The mechanisms of differential reabsorption and secretion, located in the tubule of a nephron, are the effectors of regulation. The mechanisms operate under a complex system of control in which both extrarenal and intrarenal humoral factors participate. 

Physiological fluids within the human body can be divided into human intracellular fluid (hICF, with a volume of 271 for a 70 kg person) and human extracellular fluid (hECF, 131). Extracellular fluid can further be subdivided into two sub-compartments, that is human interstitial fluid (hISF, 9.51) and the liquid component of blood (plasma, 3.51 for a 70 kg person) (Tas, 2014). While the extracellular fluid represents the fluid outside cells, and intracellular fluid the fluid within cells, the interstitial fluid is the tissue fluid found between cells. There is a striking difference between the compositions of extracellular and intracellular fluids as presented in Table 7.7. Since the composition of blood plasma is close to that of hECF (Krebs, 1950), synthetic SBFs developed by various research groups frequently attempt to emulate these compositions (Table 7.8). 

Table 7.7 Composition of human intracellular fluid (hICF) and extracellular fluid (hECF) compartments in millimolar (Tas, 2014).

Life evolved in seawater. Therefore a consideration of the relative compositions of seawater and extracellular and intracellular fluids is relevant to an analysis of metal ion utilization. Seawater contains a high concentration of sodium ions, and, in lesser amounts, potassium, calcium, and magnesium ions. These cations are also found in living cells in varying amounts. It is necessary, however, for the cell to maintain pumps to keep the individual concentration of these ions within it to appropriate limits. For example, sodium ions are found in high concentrations in seawater and in extracellular fluids, but potassium ions are concentrated within living cells. Sodium ions must be pumped out of the cells, and systems are available to do this. Pumps are also available to control the intracellular concentrations of other cations. The transition metal ions, such as zinc, are also found in seawater, but in much, much lower concentrations, and they are, as described above, equally rare within cells. 

One of the main functions of the plasma membranes of living cells is to control the transport processes into and out of the cells of many substances and thus to regulate the composition of the intracellular fluid. The fluid usually contains solutes at concentrations which are quite different from their corresponding values in the bathing medium. This is achieved by the ability of the membrane to discriminate among various solutes so that some are allowed through, others are kept inside or outside the cell, and still others are carried actively. In addition, important processes such as oxidative metabolism, protein synthesis and several other synthetic processes are intimately connected with and dependent on membrane processes. In fact, continued existence of the cell is critically dependent on its having a functional plasma membrane. 

The parameters of interest in body composition analysis (bioelectric impedance analysis, BIA) are (a) TBW, (b) extracellular/intracellular fluid balance, (c) muscle mass, and (d) fat mass. Application areas are as diversified as sports, medicine, nutrition, and fluid balance in renal dialysis and transplantations. 

Compare the chemical compositions of plasma, interstitial fluid, and intracellular fluid. (Section 15.1)... 

Which two of the following have nearly the same chemical composition plasma, intracellular fluid, interstitial fluid ... 

Interest in the use of ion-selective electrodes in the biomedical field is a natural consequence of the electrolyte composition of bulk body and cell fluids (Table 2.1), a proportion of which is in the ionised form. In extracellular fluids, sodium is the principal cation with chloride as the major anion. In intracellular fluids, potassium is the major cation and phosphate the principal anion — except in erythrocytes where chloride predominates. Of special interest is ionised calcium, because of its importance in various physiological and biochemical processes such as bone formation, nerve conduction, cerebral function, cardiac conduction and contraction, membrane phenomena, muscle contraction and relaxation, blood coagulation, and enzyme activation [2—4]. 

The electrolyte concentrations of intra- and extracellular fluids differ in many ways. Among the most significant are the low sodium and chloride concentration and the high potassium concentration in intracellular fluid. These differences in electrolyte composition are not explained, but they do not result from the permeability of the cell membrane to sodium because radioactive sodium is rapidly distributed between extra- and intracellular fluid. Yet after radioactive sodium is injected, the sodium concentrations of the respective fluid systems are not altered. 

About two-thirds of the weight of the human body is water. Except for adipose tissue, all tissues contain a high proportion of water. The water is distributed into distinct compartments , the main subdivision being into water inside cells (intracellular water) and water outside cells (extracellular water). There is approximately twice as much intracellular water as extracellular water. The solutes in these two compartments are quite different and it is convenient to refer to two distinct fluids differing in composition, the intracellular fluid and the extracellular fluid. 

So far in this book, the emphasis has been on the chemistry of acid-base balance. We now proceed to consider the relationship of the concentrations of the chemicals important in the context of acid-base balance to the composition of plasma. In the wider context of the physiology of body fluids such as extracellular and intracellular fluids, there are two constraints which are of importance. The first constraint is physico-chemical and applies to any aqueous fluid. It is called the principle of electroneutrality , which states that, in an aqueous solution, the number of positive charges equals the number of negative charges. For reference, this important principle is included in Table 5.3. 

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. 

ECF, respectively). The ECF comprises interstitial fluid (15%) and plasma (5%). The total blood volume (TBV) is 8% of body water, the 3% of the body water associated with red cells being counted as part of the intracellular fluid volume. The localisation of a drug into blood cells is often studied since it provides a measure of how the compound will distribute into other tissues. Apart from water, the overall body composition is made up (approximately ) as follows water G0%, protein 18%, fat 15% and minerals 7%. 

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