Intracellular and Extracellular Fluids

The water in the body can be divided into two compartments: intracellular and extracellular fluid. About 55% of the water is inside cells, and the remainder is outside. The extracellular fluid, or ECF, can in turn be subdivided into plasma, lymphatic fluid, and interstitial fluid, but for now we can lump all the ECF together into one compartment. Similarly there are subcompartments within cells, but it will suffice for now to treat cells as uniform bags of fluid. The wall that separates the intracellular and extracellular fluid compartments is the outer cell membrane, also called the plasma membrane of the cell.

Both organic and inorganic substances are dissolved in the intracellular and extracellular water, but the compositions of the two fluid compartments differ. Table 2-1 shows simplified compositions of ECF and intracellular fluid (ICF) for a typical mammalian cell. The compositions shown in the table are simplified by including only those substances that are important in governing the basic osmotic and electrical properties of cells. Many other kinds of inorganic and organic solutes beyond those shown in the table are present in both the ECF and ICF, and many of them have important physiological roles in other contexts. For the present, however, they can be ignored.

The principal cation (positively charged ion) outside the cell is sodium, although there is also a small amount of potassium, which will be important to consider when we discuss the origin of the membrane potential of cells. Inside cells, the situation is reversed, with a small amount of sodium and potassium being the principal cation. Negatively charged chloride ions, which are present at a high concentration in ECF, are relatively scarce in ICF. The major anion (negatively charged ion) inside cells is actually a class of molecules that bear a net negative charge. These intracellular anions, which we will abbreviate A-, include protein molecules, acidic amino acids like aspartate and glutamate, and inorganic ions like sulfate and phosphate. For the purposes of this

Table 2-1 Simplified compositions of intracellular and extracellular fluids for a typical mammalian cell.

Internal

External

Can it

concentration

concentration

cross plasma

(mM)

(mM)

membrane?

K+

125

5

Y

Na+

12

120

N*

Cl-

5

125

Y

A-

108

0

N

H2O

55,000

55,000

Y

Membrane potential = -60 to -100 mV

*As we will see in Chapter 3, this "No" is not as simple as it first appears.

Membrane potential = -60 to -100 mV

*As we will see in Chapter 3, this "No" is not as simple as it first appears.

book, the anions of this class outside cells can be ignored, and we will simplify the situation by assuming that the sole extracellular anion is chloride.

It will also be important to consider the concentration of water on the two sides of the membrane, which is also shown in Table 2-1. It may seem odd to speak of the "concentration" of the solvent in ECF and ICF. However, as we shall see when we consider the maintenance ofcell volume, the concentration of water must be the same inside and outside the cell, or water will move across the membrane and cell volume will change.

Another important consideration will be whether a particular substance can cross the plasma membrane that is, whether the membrane is permeable to that substance. The plasma membrane is permeable to water, potassium, and chloride, but is effectively impermeable to sodium (however, we will reconsider the sodium permeability later). Of course, if the membrane is to do its job properly, it must keep the organic anions inside the cell; otherwise, all of a cell's essential biochemical machinery would simply diffuse away into the ECF. Thus, the membrane is impermeable to A-.

As described in Chapter 1, there is an electrical voltage across the plasma membrane, with the inside of the cell being more negative than the outside. The voltage difference is usually about 60-100 millivolts (mV), and is referred to as the membrane potential of the cell. By convention, the potential outside the cell is called zero; therefore, the typical value of the membrane potential (abbreviated Em) is -60 to -100 mV, as shown in Table 2-1. A major concern of the first section of this book will be the origin of this electrical membrane potential. In later sections, we will discuss how the membrane potential influences the movement of charged particles across the cell membrane and how the electrical energy stored in the membrane potential can be tapped to generate signals that can be passed from one cell to another in the nervous system.

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