The cells of the human body live in a carefully regulated fluid environment. The fluid inside the cells,
the intracellular fluid (ICF), occupies what is called the intracellular compartment, and the fluid
outside the cells, the extracellular fluid (ECF), occupies the extracellular compartment. The barriers
that separate these two compartments are the cell membranes.
For life to be sustained, the body must rigorously maintain the volume and composition of the
intracellular and extracellular compartments. To a large extent, such regulation is the result of
transport across the cell membrane. It will be discussed how cell membranes regulate the
distribution of ions and water in the intracellular and extracellular compartments.
The intracellular and extracellular fluids
Total-body water is the sum of the ICF and ECF volumes
Table 5.1. Approximate water distribution in adult humans. All of the above
values are approximate and for illustration only. The volumes are rounded to the
nearest liter, assuming a BW of 70 kg for both sexes, an Hct for men of 45%, and
an Hct for women of 40%. BW: body weight; Hct: hematocrit.
1
,The prototypical 70 kg male has ~42 L of TBW (60% of 70 kg). Of these 42 L, ~60% (25 L) is
intracellular and ~40% (17 L) is extracellular.
Extracellular fluid is composed of:
Blood plasma
Interstitial fluid
Transcellular fluid
Figure 5.1. Fluid compartments of a prototypical adult human male weighing 70 kg. TBW is divided into four major
compartments: ICF (green), interstitial fluid (blue), blood plasma (red), and transcellular water such as synovial fluid (tan).
Color codes for each of these compartments are maintained throughout this summary.
Plasma volume
2
, Of the ~17 L of ECF, only ~20% (~3 L) is contained within the cardiac chambers and blood vessels,
that is, within the intravascular compartment. The total volume of this intravascular compartment is
the blood volume, ~6 L. The extracellular 3 L of the blood volume is the plasma volume. The balance,
~3 L, consists of the cellular elements of blood: erythrocytes, leukocytes, and platelets. The fraction
of blood volume that is occupied by these cells is called the hematocrit. The hematocrit is
determined by centrifuging blood that is treated with an anticoagulant and measuring the fraction of
the total volume that is occupied by the packed cells.
Interstitial fluid
About 75% (~13 L) of the ECF is outside the intravascular compartment, where it bathes the
nonblood cells of the body. Within this interstitial fluid are two smaller compartments that
communicate only slowly with the bulk of the interstitial fluid:
1. Dense connective tissue (e.g., cartilage and tendons)
2. Bone matrix
The barriers that separate the intravascular and interstitial compartments are the walls of capillaries.
Water and solutes can move between the interstitium and blood plasma by crossing capillary walls
and between the interstitium and cytoplasm by crossing cell membranes.
Transcellular fluid
Finally, ~5% (~1 L) of ECF is trapped within spaces that are completely surrounded by epithelial cells.
This transcellular fluid includes the synovial fluid within joints and the cerebrospinal fluid surrounding
the brain and spinal cord. Transcellular fluid does not include fluids that are, strictly speaking, outside
the body, such as the contents of the gastrointestinal tract or urinary bladder.
ICF is rich in K+, whereas ECF is rich in Na+ and Cl−
3
the intracellular fluid (ICF), occupies what is called the intracellular compartment, and the fluid
outside the cells, the extracellular fluid (ECF), occupies the extracellular compartment. The barriers
that separate these two compartments are the cell membranes.
For life to be sustained, the body must rigorously maintain the volume and composition of the
intracellular and extracellular compartments. To a large extent, such regulation is the result of
transport across the cell membrane. It will be discussed how cell membranes regulate the
distribution of ions and water in the intracellular and extracellular compartments.
The intracellular and extracellular fluids
Total-body water is the sum of the ICF and ECF volumes
Table 5.1. Approximate water distribution in adult humans. All of the above
values are approximate and for illustration only. The volumes are rounded to the
nearest liter, assuming a BW of 70 kg for both sexes, an Hct for men of 45%, and
an Hct for women of 40%. BW: body weight; Hct: hematocrit.
1
,The prototypical 70 kg male has ~42 L of TBW (60% of 70 kg). Of these 42 L, ~60% (25 L) is
intracellular and ~40% (17 L) is extracellular.
Extracellular fluid is composed of:
Blood plasma
Interstitial fluid
Transcellular fluid
Figure 5.1. Fluid compartments of a prototypical adult human male weighing 70 kg. TBW is divided into four major
compartments: ICF (green), interstitial fluid (blue), blood plasma (red), and transcellular water such as synovial fluid (tan).
Color codes for each of these compartments are maintained throughout this summary.
Plasma volume
2
, Of the ~17 L of ECF, only ~20% (~3 L) is contained within the cardiac chambers and blood vessels,
that is, within the intravascular compartment. The total volume of this intravascular compartment is
the blood volume, ~6 L. The extracellular 3 L of the blood volume is the plasma volume. The balance,
~3 L, consists of the cellular elements of blood: erythrocytes, leukocytes, and platelets. The fraction
of blood volume that is occupied by these cells is called the hematocrit. The hematocrit is
determined by centrifuging blood that is treated with an anticoagulant and measuring the fraction of
the total volume that is occupied by the packed cells.
Interstitial fluid
About 75% (~13 L) of the ECF is outside the intravascular compartment, where it bathes the
nonblood cells of the body. Within this interstitial fluid are two smaller compartments that
communicate only slowly with the bulk of the interstitial fluid:
1. Dense connective tissue (e.g., cartilage and tendons)
2. Bone matrix
The barriers that separate the intravascular and interstitial compartments are the walls of capillaries.
Water and solutes can move between the interstitium and blood plasma by crossing capillary walls
and between the interstitium and cytoplasm by crossing cell membranes.
Transcellular fluid
Finally, ~5% (~1 L) of ECF is trapped within spaces that are completely surrounded by epithelial cells.
This transcellular fluid includes the synovial fluid within joints and the cerebrospinal fluid surrounding
the brain and spinal cord. Transcellular fluid does not include fluids that are, strictly speaking, outside
the body, such as the contents of the gastrointestinal tract or urinary bladder.
ICF is rich in K+, whereas ECF is rich in Na+ and Cl−
3
