Interstitial Fluid vs. Extracellular Fluid Illustration

Interstitial Fluid vs. Extracellular Fluid: Overview (2024)

by | Updated: Jan 4, 2024

Interstitial fluid and extracellular fluid are two essential components of the body’s fluid environment, playing key roles in maintaining overall physiological balance.

Extracellular fluid, as the name suggests, is located outside of cells, encompassing not just the interstitial fluid but also other components such as blood plasma. Interstitial fluid, on the other hand, is a specific subset of the extracellular fluid, situated in the ‘interstices’ or spaces between cells and tissues.

Despite their shared characteristics, these two types of fluids serve distinct roles within the complex systems of the human body.

This article aims to provide a comprehensive overview of both interstitial and extracellular fluid, delving into their definitions, composition, functions, and roles in health and disease.

What is the Difference Between Interstitial and Extracellular Fluid?

Extracellular fluid is the body fluid found outside cells, including components like blood plasma and interstitial fluid. Interstitial fluid, a subset of extracellular fluid, fills the spaces between cells and tissues. The key difference lies in their locations: extracellular fluid includes all fluid outside cells, while interstitial fluid is specifically located between cells.

Extracellular Fluid

Extracellular fluid, as its name implies, is the body fluid that exists outside of the cells. It comprises approximately one-third of the body’s total water content.

This fluid is mainly divided into two categories: the fluid that exists within the blood vessels, known as plasma, and the fluid outside the blood vessels, also called interstitial fluid.

Other components of the extracellular fluid include the cerebrospinal fluid, which bathes the brain and spinal cord, and the synovial fluid found in joints.

Role of Extracellular Fluid

The primary role of the extracellular fluid is to provide a consistent environment for the body’s cells. This is achieved through the transport of nutrients, oxygen, and hormones to cells and the removal of metabolic waste from cells.

It also assists in the distribution of heat throughout the body and plays a role in buffering pH levels, making it a crucial component in maintaining homeostasis.


The composition of extracellular fluid is carefully regulated. It is primarily made up of the following:

  • Water
  • Electrolytes (i.e., sodium, chloride, and bicarbonate
  • Nutrients
  • Signaling molecules
  • Waste products

Its electrolyte composition differs significantly from intracellular fluid, primarily because it contains high concentrations of sodium and chloride but low concentrations of potassium and magnesium.

Any disturbance in the volume or composition of the extracellular fluid can lead to clinical conditions.

For example, excessive loss of extracellular fluid, as in severe burns or prolonged vomiting and diarrhea, can lead to dehydration and electrolyte imbalances, affecting organ function.

Therefore, understanding the role and management of extracellular fluid is important in health and disease.

Interstitial Fluid

Interstitial fluid, also known as tissue fluid, is a subset of extracellular fluid that occupies the spaces between cells and tissues, accounting for approximately 16% of the body’s total weight.

It is an integral part of the body’s internal environment, serving as the link between blood and cells for the exchange of substances.

Interstitial fluid is derived from blood plasma. The hydrostatic and osmotic pressures in the capillaries cause the fluid to be filtered out of the blood vessels, delivering nutrients, oxygen, and hormones to the cells.

It simultaneously collects waste products from the cells for removal, which then reenter the bloodstream or the lymphatic system.

Fluid Compartment Plasma Interstitial Fluid Illustration


The composition of the interstitial fluid is similar to that of blood plasma but with a lower protein concentration. It contains:

  • Water
  • Electrolytes (i.e., sodium, chloride, bicarbonate)
  • Biomolecules
  • Amino acids
  • Sugars
  • Fatty acids
  • Coenzymes
  • Hormones
  • Neurotransmitters
  • Waste products

The body maintains the volume and composition of the interstitial fluid within precise limits. Any imbalances can lead to edema, where excess fluid accumulates in the interstitial spaces.

Monitoring and understanding the dynamics of interstitial fluid is important for maintaining health and diagnosing certain pathological conditions.

FAQs About Interstitial and Extracellular Fluid

Is Interstitial Fluid the Same as Extracellular Fluid?

No, interstitial fluid is not the same as extracellular fluid, though it is a subset of it. Extracellular fluid refers to all fluid found outside of cells, including both interstitial fluid and plasma, the liquid component of blood.

Where is Interstitial Fluid Found in the Body?

Interstitial fluid is found in the interstices, the microscopic spaces between the cells and tissues throughout the body.

It surrounds the body’s cells, providing a medium for the exchange of substances between cells and blood.

What is the Difference Between Intracellular and Interstitial Fluid?

Intracellular fluid is the fluid found inside the cells, making up about two-thirds of the body’s total water content. Interstitial fluid, on the other hand, is a component of the extracellular fluid residing in the spaces between cells.

Their main difference lies in their location and their specific compositions due to the different roles they play in the body’s physiological processes.

What is Interstitial Fluid Also Known As?

Interstitial fluid is also commonly referred to as tissue fluid, indicating its presence within and around the body’s tissue cells.

Where is Extracellular Fluid Found in the Body?

Extracellular fluid is found in all areas outside of the cells. It is primarily located within blood vessels as plasma and in the spaces between cells as interstitial fluid.

It can also be found in specific locations such as the cerebrospinal fluid in the brain and spinal cord, synovial fluid in the joints, aqueous humor in the eyes, and pericardial fluid around the heart, among others.

Final Thoughts

Both extracellular and interstitial fluid play vital roles in maintaining the body’s physiological balance. While they share common characteristics, the differentiation between them is defined by their location within the body and their specific functions.

Extracellular fluid encompasses all fluid outside of cells, including plasma and interstitial fluid, and is responsible for the delivery of nutrients and removal of waste.

Interstitial fluid, a component of the extracellular fluid, specifically exists in the spaces between cells, facilitating the exchange of substances between blood and cells.

Understanding their distinct roles and the mechanisms that regulate their composition and volume is crucial for health maintenance and disease diagnosis.

John Landry, BS, RRT

Written by:

John Landry, BS, RRT

John Landry is a registered respiratory therapist from Memphis, TN, and has a bachelor's degree in kinesiology. He enjoys using evidence-based research to help others breathe easier and live a healthier life.


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