Extracellular fluid

About 60% of the adult human body consists of fluid mainly water. But this fluid is divided into two categories
1. Intracellular fluid
2. Extracellular fluid

The fluid which is present inside the cell is known as intracellular fluid and this fluid is about two-thirds of the fluid. The fluid which is present outside the cell is known as extracellular fluid and it is one-third of the fluid.

The extracellular fluid is in constant motion and is transported in the circulating blood from where it mixes with the tissue fluids by the process of diffusion.


This fluid contains ions and nutrients which are responsible for the cells to maintain a healthy life.
Claude Bernard a French physiologist gives a name milieu interior to the extracellular fluid which means an internal environment of the body.
Cells need oxygen, amino acids, fatty acids, and many ions for normal functioning and they get all these substances from the extracellular fluid.

Difference between extracellular and intracellular fluid

In extracellular fluid mainly sodium, chloride, and bicarbonates are present and also contain oxygen, nutrients, and carbon dioxide while the intracellular fluid instead of sodium and chloride contains magnesium, potassium, and phosphate ions.


The transport of this fluid occurs in two stages throughout the body
1.movement of blood through the body
2. movement of fluid between capillaries and tissues through a membrane.
As the blood moves through the vessels and when it reaches the capillaries than the exchange of extracellular fluid between the plasma portion of the blood and interstitial fluid occurs.


The main element of the extracellular fluid is the interstitial fluid that surrounds the cells in the body. The other major element of the ECF is the intravascular fluid of the circulatory system called bloodstream plasma.  These constituents tend to be called fluid compartments. The transcellular fluid includes the aqueous humor in the attention, the synovial fluid in the joints.

The volume of extracellular fluid in an adult male of 70 kg, is 20% of body weight – about fourteen liters.

The interstitial fluid and the plasma constitute about 97% of the ECF, and a little percentage of the is lymph. Interstitial fluid is fluid that surrounds cells, providing them with nutrients and eliminating their waste products. Eleven liters of the ECF is interstitial fluid and the rest of the three liters is plasma. Plasma and interstitial fluid are very similar because drinking water, ions, and small solutes are constantly exchanged between them across the walls of capillaries, through skin pores and capillary clefts.

This solution makes up about 26% of the water in our body. The structure of interstitial fluid is dependent upon the exchanges between your cells in the biological tissue and the bloodstream.

The plasma that filters into the fluid will not contain red bloodstream cells or platelets because they are too large to feed but can contain some white blood cells to help the disease fighting capability.

After the extracellular fluid collects into small vessels it is known as to be lymph, and the vessels that make it back again to the bloodstream are called the lymphatic vessels. The lymphatic system profits protein and surplus interstitial fluid to the blood flow.

The ionic composition of the interstitial fluid and blood vessels plasma vary because of the Gibbs-Donnan effect. This causes hook difference in the focus of cations and anions between the two liquid compartments.

The function of extracellular fluid

This fluid provides a medium for the exchange of substances between the extracellular fluid and the cell. Substances that are present in the extracellular fluid includes gases, nutrients, and electrolytes, etc. The extracellular fluid also contains substances that are secreted from the cells in soluble form.


Homeostasis is the process that stabilizes the internal environment. Complex homeostatic mechanisms occur in the body to keep the composition of ECF stable. There is also a control system present at the cellular due to which the cell also keeps the composition of their internal environment stable.

Interaction between blood plasma, interstitial fluid, and lymph

The interstitial fluid and lymph interact with each other at the capillary level. As we know that capillaries are permeable so water can move freely in and out. At the arteriolar end of a capillary, the blood pressure is much greater than the hydrostatic pressure of the tissues due to which water from the capillary goes into the interstitial fluid.

There are pores in the capillary walls through which the water molecules come out. These pores are large enough so smaller molecule also moves freely through these pores.

The movement of water molecules out of the capillary to the interstitial fluid increases the concentration of substances that cannot pass the capillary level as the blood moves to the regular end of a capillary. Due to this process and pressure, the water moves back into the capillaries.  While the crystalloids substances that are present in the capillaries and interstitial fluids are equilibrated.

The capillary fluid is constantly renewed by blood due to which it dominates the equilibrium concentration achieved in the capillary bed. This equilibrium ensures that the watery environment of the body is stable and close to their ideal environment.

A small part of the capillary solution leak out of the capillaries that are not drawn back by colloid osmotic forces into the capillary. The amounts that leak out ranges from 2 to 4 liters per day for the body as a whole. The leak solution is collected by the lymphatic system. This solution is then drained into the subclavian vein where it mixes with the venous blood.  The lymph flows through lymph node where waste substances are removed from the lymph and many types of cells are added to the lymph such as white blood cells.


Extracellular fluid or ECCF is a fluid that is present outside the cell in living organisms. The main component of the extracellular fluid is the interstitial fluid that surrounds the cells. The extracellular fluid is the internal environment of multicellular organisms. Plasma and the interstitial fluid are the two substances that make at least 97% of the extracellular fluid.