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Interstitial Fluid Compartments & Pressure

Dhanalakshmi Harikrishnan, Nadine James
  • Author
    Dhanalakshmi Harikrishnan

    Obtained a Masters degree in Zoology from Madras University, Chennai. Possess three years of professional experience as Subject Matter Expert, creating learning contents like learning videos, text book solutions, assessments etc.

  • Instructor
    Nadine James

    Nadine has taught nursing for 12 years and has a PhD in Nursing research

Understand interstitial fluid and where interstitial fluid is found in the body. Learn about interstitial fluid movement due to hydrostatic and osmotic pressure. Updated: 03/07/2022

Body Fluid Compartments

Water accounts for around 60% of the total body weight in an adult human. The fluid is distributed throughout the body, in organs, organ systems, and tissues. Total body water is the amount of water in these tissues. The percentage of water in the body varies as the body grows. So, water is more abundant in infants and children than in adults. About 70% of the body mass in infants is water, 50%-60% in normal adults, and 45% in older adults.

Total body water gets distributed inside as well as outside of the cells into two body fluid compartments. The water content present on the inside of the cells is called the intracellular fluid (or ICF), and the water content present on the outside of the cells forms the extracellular fluid (or ECF). The cell membrane (semi-permeable membrane) separates the intracellular fluid from the extracellular fluid. Most of the fluid inside the cells is two-thirds of the total body water, so the remaining one-third would be the extracellular fluid. If total body water accounts for 60% of body weight, then 40% is the intracellular fluid and 20% is the extracellular fluid.


The two major compartments of body fluid are Intracellular & extracellular, which is divided into plasma & interstitial fluid

This image shows the cross-section of a blood vessel with different body fluid compartments.


The intracellular fluid (ICF) refers to all fluids present within the cells, including cytosol as well as fluid in the cell nucleus. The cytosol is the matrix that holds cellular organelles in place. So, the cytosol and organelles together form the cytoplasm. The extracellular fluid (ECF) refers to the fluids present outside the cells. The ECF is further divided into the fluid inside vessels and fluid outside vessels. Inside the vessels would be the plasma and outside is the interstitial fluid. The interstitial fluid is larger being around three-quarters of the total ECF, so the plasma is a quarter. The interstitial fluid and plasma are separated by a capillary membrane, which has pores that allow solutes to pass through, except for proteins as they are large molecules. So they stay in the plasma and the interstitial fluid has very little protein. The plasma has more cations like sodium and lesser anions like chloride. So, the predominant cation in the ECF is sodium and the predominant anions would be chloride and bicarbonate along with plasma proteins in the plasma.

The cell membrane is selectively permeable. It is freely permeable to water, but is impermeable to a lot of solutes. So the composition of the ICF and the ECF are different. In the ICF, the major cations are potassium and magnesium and the anions are organic phosphates like ATP, ADP, and proteins. Proteins are found inside cells and in plasma. However, there is very little protein in the interstitial fluid.


Body fluid compartments

Pictorial representation of body fluid compartments


What is Interstitial Fluid?

The human body is mostly made up of fluid. In order to talk about all this fluid, scientists have divided it up into categories. Fluid in our bodies pretty much have to be inside our cells or outside of them. So there are two main fluid compartments (areas) in the body, intracellular and extracellular. The intracellular (IC) compartment contains the fluid that bathes the inside of the cells of the body. The extracellular (EC) compartment is the fluid that lies outside of the cells. The extracellular compartment is further divided into two areas - intravascular (fluid inside the blood vessels) and interstitial (fluid outside the blood vessels). This last type of fluid is the focus of this lesson.

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What is Interstitial Fluid?

What is interstitial fluid? Interstitial space refers to the fluid-filled spaces between blood vessels, while the interstitial fluid definition is the fluid that fills interstitial space. So, interstitial fluid is a type of ECF as it is found outside the interstitial cells. It is also known as intercellular or tissue fluid. The interstitial fluid forms when plasma fluid is filtered through the capillary membrane. Hence, it is an ultrafiltrate of plasma and its composition is similar to plasma.

Sugars, fatty acids, amino acids, coenzymes, hormones, neurotransmitters, minerals such as sodium, chloride, calcium, magnesium, and potassium, and metabolic waste products are all found in interstitial fluid. Intercellular exchange and blood in the particular tissue determine the composition of interstitial fluid. This implies that the interstitial fluid content varies depending on the tissues and locations of the body.

Interstitial fluid is not static as the movement of fluid takes place across the capillary membrane, which separates the interstitial fluid and plasma. Fluid from the plasma gets filtered through the capillary membrane to form the interstitial fluid. Water, ions, and tiny solutes are constantly transferred between plasma and interstitial fluid through capillary walls, via pores, and capillary clefts, making them compositionally similar.

Where Is Interstitial Fluid Found?

Where is interstitial fluid found? "Inter" means between or among, so interstitial means between the parts of a tissue, but not within the blood vessel. Hence, interstitial fluid is a type of ECF that lies between cells of the tissue, but is not inside the blood vessels. It contains nutrients from capillaries and collects waste products released by the cellular metabolism. It is involved in the supply of oxygen and nutrients to cells as well as the removal of waste materials from the cells. The lymph vessels absorb the old interstitial fluid, which is replaced by the newly formed interstitial fluid.

Movement of Interstitial Fluid

The interstitial fluid flows from the capillary to the interstitial space. Interstitial fluid flow is the transport of fluid across the extracellular matrix of tissues, particularly between blood and lymphatic vessels. Interstitial fluid nourishes the matrix and cells in the interstitial space. A cell membrane or plasma membrane functions as a barrier and separates the intracellular compartment from the interstitial space. Water passes through this barrier readily, but small and large molecules do not pass through.

Fluids are constantly moving across the capillary membrane, in and out of the cell. The amount of fluid that moves across the capillary membrane is determined by hydrostatic pressure, osmotic pressure, and permeability of the membrane. The fluid shift is a process in which the body fluids move between the fluid compartments. This is caused by hydrostatic as well as the osmotic pressure gradients in the human body.

Interstitial Fluid Hydrostatic Pressure

The pressure that causes fluid to flow out of capillaries is known as hydrostatic pressure. It causes an outward pressure that will drive fluid out of the capillaries. There are two hydrostatic pressures involved in the movement of fluid from vessel to interstitial space: capillary hydrostatic pressure, and interstitial fluid hydrostatic pressure.

The pressure generated by blood on the capillary wall is known as capillary hydrostatic pressure (CHP). The interstitial fluid hydrostatic pressure (IFHP) is a type of interstitial pressure that increases as fluid leaves the capillary and enters tissues. CHP in the arteries is considerably more than IFHP because the lymphatic system continually absorbs extra fluid from the tissues to maintain normal fluid levels in the tissues. As a result, fluid exits the capillary and enters the interstitial fluid.

Interstitial Fluid Osmotic Pressure

Interstitial Fluid Osmotic pressure is the total pressure that causes the reabsorption of fluids or the movement of fluid from the interstitial fluid back into the capillaries. Osmotic concentration gradients are responsible for osmotic pressure. The osmotic pressure difference is the major factor for the inward flow of fluid (from the interstitial space to the capillaries). This difference is due to the fact that plasma has a higher protein concentration than interstitial fluid.

The difference in the concentration of solute and water in the blood, as well as interstitial fluid, is known as an osmotic concentration gradient. Due to the concentration gradient, water moves across a semipermeable membrane from the region of higher water concentration (and lower in solute concentration) to the region of lower water concentration (and higher solute concentration). Plasma proteins play a critical role in the osmotic concentration gradient. Solutes move across the capillary wall along with their concentration gradient (high concentration to low concentration). As a result of this movement, plasma and interstitial fluid concentrations should be comparable. Plasma proteins are referred to as colloids as they are large molecules and are distributed evenly.

Composition

The interstitial fluid bathes the outside of the blood vessels and makes up about 75% of the total amount of EC fluid (plasma is the other 25%). Age will decrease the amount of fluid the body holds in the two major compartments. As children age to puberty, the fluid levels go down. A young adult has about 15% of his or her total body fluid as interstitial fluid, but the percentages will continue to decline with age.

Interstitial fluid contains water and dissolved solutes and proteins. The solutes are sugar, salts, acids, hormones, neurotransmitters, wastes and electrolytes. An electrolyte is an element or compound that breaks up into ions when dissolved in fluid and are essential to maintaining healthy body functions. The most common electrolytes are sodium, chloride, calcium, and bicarbonate.

Movement of Fluids and Solutes

Do the body's fluids stay in one place in the body or do they move? It is very important that the fluids in the two compartments remain fairly constant. But they do move inside and outside cells. There is a cell membrane that decides what can go into and out of the cells. It is semipermeable, meaning some things can pass freely through the membrane while others cannot.

Think of a window screen; it lets in air and some dust but keeps out leaves and bugs. The screen represents a semipermeable membrane. The cell membrane allows some molecules and fluids in but keeps others out. So how do the two compartments maintain fluid levels? How do solutes come in or out of the cell?

Think of an amusement park. The park only allows so many people on a ride. There is a gate that keeps the right amount of people through to get on the ride. The rest of the people are outside the gate. When the ride is over, new people can come through the gate. When the cell needs a particular molecule or ion in the cell, it is permitted to enter the cell, just like opening the gate to the amusement ride. This cellular process is called facilitated diffusion (which is a type of transportation of molecules or ions in and out of a cell).

Facilitated Diffusion
facilitated diffusion

Facilitated diffusion (FD) does not require energy because the solutes move along the concentration gradient (the process of solutes moving from an area of higher to lower numbers of solutes). But FD does require a helper called a carrier. The carrier is usually a protein that has the permission to cross the semipermeable membrane of the cell. So in our amusement park example, the carrier would be the person who opens the gate to let you get on the ride.

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Video Transcript

What is Interstitial Fluid?

The human body is mostly made up of fluid. In order to talk about all this fluid, scientists have divided it up into categories. Fluid in our bodies pretty much have to be inside our cells or outside of them. So there are two main fluid compartments (areas) in the body, intracellular and extracellular. The intracellular (IC) compartment contains the fluid that bathes the inside of the cells of the body. The extracellular (EC) compartment is the fluid that lies outside of the cells. The extracellular compartment is further divided into two areas - intravascular (fluid inside the blood vessels) and interstitial (fluid outside the blood vessels). This last type of fluid is the focus of this lesson.

Composition

The interstitial fluid bathes the outside of the blood vessels and makes up about 75% of the total amount of EC fluid (plasma is the other 25%). Age will decrease the amount of fluid the body holds in the two major compartments. As children age to puberty, the fluid levels go down. A young adult has about 15% of his or her total body fluid as interstitial fluid, but the percentages will continue to decline with age.

Interstitial fluid contains water and dissolved solutes and proteins. The solutes are sugar, salts, acids, hormones, neurotransmitters, wastes and electrolytes. An electrolyte is an element or compound that breaks up into ions when dissolved in fluid and are essential to maintaining healthy body functions. The most common electrolytes are sodium, chloride, calcium, and bicarbonate.

Movement of Fluids and Solutes

Do the body's fluids stay in one place in the body or do they move? It is very important that the fluids in the two compartments remain fairly constant. But they do move inside and outside cells. There is a cell membrane that decides what can go into and out of the cells. It is semipermeable, meaning some things can pass freely through the membrane while others cannot.

Think of a window screen; it lets in air and some dust but keeps out leaves and bugs. The screen represents a semipermeable membrane. The cell membrane allows some molecules and fluids in but keeps others out. So how do the two compartments maintain fluid levels? How do solutes come in or out of the cell?

Think of an amusement park. The park only allows so many people on a ride. There is a gate that keeps the right amount of people through to get on the ride. The rest of the people are outside the gate. When the ride is over, new people can come through the gate. When the cell needs a particular molecule or ion in the cell, it is permitted to enter the cell, just like opening the gate to the amusement ride. This cellular process is called facilitated diffusion (which is a type of transportation of molecules or ions in and out of a cell).

Facilitated Diffusion
facilitated diffusion

Facilitated diffusion (FD) does not require energy because the solutes move along the concentration gradient (the process of solutes moving from an area of higher to lower numbers of solutes). But FD does require a helper called a carrier. The carrier is usually a protein that has the permission to cross the semipermeable membrane of the cell. So in our amusement park example, the carrier would be the person who opens the gate to let you get on the ride.

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Frequently Asked Questions

Where is interstitial fluid found in the body?

Body fluids are divided into intercellular fluid and extracellular fluid. Intercellular fluid is present inside the cells, whereas extracellular fluid is present outside the cells. Extracellular fluid is further divided into plasma and interstitial fluid. Plasma is found within the blood vessels, whereas interstitial fluid is present outside the blood vessels. Therefore, interstitial fluid is the body fluid present between blood vessels and cells.

What is the meaning of interstitial fluid?

Interstitial fluid is a type of extracellular fluid, found in the spaces around cells, but not in the capillaries. It is formed from the substances that are released from the blood capillaries.

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