Fluid compartments
There are three fluid compartments in the human body. These are:
Intracellular fluid (ICF)
Intravascular fluid or plasma
The water is inside the blood vessels, such as the arteries, veins, and capillaries. This fluid compartment contains the least amount of fluid, only 8% of the total.
Intercellular or interstitial fluid (IF)
The water found in between the cells is called the intercellular fluid. It contains 25% of the total body water.
Extracellular fluid
In some medical literature, the combination of intravascular and intercellular fluids is known and referred to collectively as the extracellular fluid compartment.
Relationships between the three fluid compartments
Physical barriers separate the three fluid compartments. But then, their contents still shift from one place to another. It depends on the amount and concentration of sodium chloride (salt) in each of them.
Osmosis governs this shifting of fluid from one compartment to another. The law of osmosis states that fluid moves from a lower salt concentration to the region where the salt concentration is higher. This difference in salt concentrations produces the so-called osmotic gradient. Thus, if the salt concentration is higher inside the blood vessels, water shifts from inside and between the cells into the intravascular compartment, increasing blood pressure.
Since there are more solutes or salts in compartment B than A, fluid moves from A to B.
Infusion of pure water into the patient’s vascular system dilutes the salt concentration inside the blood vessels. Higher salt concentration inside the cells causes the water to shift from the blood vessels and between cells into the intracellular fluid, causing the cells to swell. Thus, some parts of the body, such as the lower legs, begin to swell. For this very reason, intravenous fluids use a saline solution containing salts as a vehicle and not pure water.
Roles of electrolytes in the fluid compartments
In the three fluid compartments, there are various kinds of electrolytes. The most common ones are sodium (Na+), chloride (Cl-), and potassium (K+). Typically, the concentrations of sodium and chloride inside the blood vessels and between cells are high. At the same time, their amounts in the intracellular compartments are low. The purpose of this is to ensure that water stays in the extracellular spaces and does not enter the cells.
If intravenous fluids use pure water, the concentration of sodium and chloride in the intravascular space will be diluted and lowered, forcing water to enter the cells and the areas in between them. Swelling of cells takes place, and the tissues will soon swell as well. Hence, saline is used in intravenous fluids and not pure water. It contains salts needed to hold water in the extracellular spaces.
Preventing the entry of too much water inside the cells is critical in the maintenance of human lives. Thus, there is another mechanism that lowers the concentration of sodium inside the cells. It is via the so-called sodium-potassium pump. This mechanism pumps out the extra sodium molecules that are inside the cells. They finally settle into the intercellular spaces. Thus, the concentrations of sodium inside the cells are maintained at low levels, preventing water from entering in large and uncontrollable amounts.
Use of intravenous fluids (IVF)
When there is a problem in one fluid compartment, using intravenous fluids corrects it. On most occasions, saline, also known as saline solution, is used. It is composed of sodium chloride and sterile water. The concentration of the sodium chloride, or saline solution, varies, and each formulation is used differently, depending on a medical condition. The most common one is the 0.9% sodium chloride solution.
Types of saline solution
0.9% Normal Saline (NS, 0.9NaCl, or NSS)
This type of saline solution is the most commonly used. It hydrates patients who experience bleeding, vomiting, diarrhea, metabolic acidosis, or shock. In medical situations like these, the intravascular fluid has collapsed. Thus, there is a need to give IVF with a high concentration of sodium chloride. It immediately restores the intravascular blood volume and blood pressure.
Lactated Ringers (LR, Ringers Lactate, or RL)
This type of saline solution is mainly similar to the blood plasma concentration. It helps burns and traumatized patients. It corrects acute blood loss, electrolyte imbalance, or metabolic acidosis. The fluid contains sodium chloride, potassium chloride, calcium chloride, and sodium lactate in sterile water. It helps maintain the intravascular blood volume that has not collapsed significantly.
0.45% Normal Saline (Half Normal Saline, 0.45NaCl, .45NS)
This saline solution treats too much elevation of salt in the blood or diabetic ketoacidosis. Using this IVF, the high level of sodium ions in the blood will be diluted and reduced. However, patients with burns, trauma, or liver disease do not need it. Be very careful in using this half-normal saline because it may cause fluid overload or pulmonary edema.
Differences between osmosis and the sodium-potassium pump
Osmosis and sodium-potassium pump (SPP) are the two mechanisms that control the movements of fluids and electrolytes in the three compartments. However, there are three main differences between the two: (1) osmosis depends on the concentration of sodium chloride while SPP does not, (2) osmosis does not use energy to do its function while the SPP uses energy, and (3) osmosis is involved in the movement of fluid while SPP is involved in the movement of electrolytes.
Summary of the three fluid compartments
| Points of Comparison | Intravascular Fluid or Plasma | Intercellular or Interstitial Fluid | Intracellular Fluid |
| Location of water | Inside the blood vessels, such as artery, veins, capillaries | Spaces in between the cells | Inside the cells |
| Percentage of total body water | 8% | 25% | 67% |
| Part of extracellular fluid | Yes | Yes | No |
| Concentration of sodium (Na+) and chloride (Cl–) | High | High | Low |
| Concentration of potassium (K+) | Low | Low | High |
