What is a Bursa? Anatomy, Locations, and Function

Brittany W., Nadine James
  • Author
    Brittany W.

    Brittany has a bachelor's degree in biological sciences and biochemistry.

  • Instructor
    Nadine James

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

What is a bursa? Learn the bursa definition and bursa function. This includes bursa anatomy, bursa locations in the body, bursae types, and bursitis definition. Updated: 09/21/2021

What is a Bursa?

A bursa is defined as a fluid-filled sac located within the joints of the body. The plural word for bursa is bursae. A bursa is designed to provide a cushion, reduce friction, and dissipate force in relatively tight spaces with multiple moving parts. Every component of a joint must coordinate movement without causing damage to other internal structures.

What Is Bursa?

Bursae are fluid-filled sacs located around a joint that cushion bones when muscles, tendons, or skin rub against the bone. They are very important in movement of the bones because they help reduce friction. Without them, movement would be difficult and even painful. Now let's try and relate this to something you may know about.

Let's say your friend told you about an incident that happened last week in the factory where he works. One of the machines made a very loud grinding sound and then suddenly stopped working. On inspection, your friend tells you that the washers that lie between the metal parts of one of the machines broke. The grinding was the sound made when metal rubbed on metal. In this example, the washers were crucial to having the machine work. Likewise, the bursae are crucial for having the joint work properly.

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  • 0:54 Anatomy
  • 1:55 Types and Function
  • 2:43 Pathology
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Bursa Function

Generally speaking, bursae function to reduce friction and dissipate force within joints. They work as a cushion to absorb shock and facilitate musculoskeletal movement. There are different types of bursae with specific functions that will be discussed in more detail throughout this lesson.

Bursa Anatomy

Concerning bursa anatomy, the thin outer lining of a bursa is called a synovial membrane. It is a layer of connective tissue that lines bursae, joint cavities, and tendon sheaths. This membrane is made up of cells that produce synovial fluid, which is a slippery, viscous, and lubricating.

Imagine an empty plastic bag with a zip closure. The two sides of the plastic bag may produce friction if rubbed together, which could easily cause damage if the action persisted over a long period of time. However, the two sides would easily glide back and forth if a small amount of mayonnaise or maple syrup were added inside the bag. The addition of a thick and slippery substance facilitates movement of the bag and anything that might surround it. For the purposes of this example, synovial fluid is to a synovial membrane as maple syrup is to a plastic bag.

Bursae are relatively small structures, making them a perfect fit for the confined space of a joint. On average, bursae maintain a diameter of 4 cm and a thickness of 2 mm in an adult human. Bursae are often surrounded by layers of tendons, ligaments, muscles, and skin.

Bursa Locations in the Body

Bursae have many locations in the body. Most of them are found within large joints, including:

  • Shoulders
  • Elbows
  • Hips
  • Knees

An adult human has approximately 140 to 160 bursae. The iliopsoas bursa is the largest in the body, and it is located at the joint where the pelvis meets the femur. It gets its name from the iliopsoas muscle, which inserts at the femur when the iliacus and psoas major muscles combine.

Types of Bursae

As mentioned, there are different types of bursae with specific functions in the body. Bursae are typically named based on their location.

Adventitious Bursae

Adventitious bursae, also called accidental bursae, form due to repeated irritation. They exist to relieve repeated friction from sustained pressure and stress. This is most easily exemplified when a person wears shoes that are slightly too small for their feet. The repeated irritation caused by friction between the shoe, skin, and underlying bone would cause an adventitious bursa. Examples include:

  • Bunions
  • Bony prominences that stick out and cause irritation when rubbed

Subcutaneous Bursae

Subcutaneous bursae, also known as superficial bursae, are located just below the skin to facilitate frictionless motion of the skin when above bone. Examples include:

  • The patella bursa of the knee
  • The olecranon bursa of the elbow

Synovial Bursae

Synovial bursae, the most common type of bursa, are located within or among the synovial lining of joint cavities. They exist in the spaces between bone and tendons or bone and ligaments to facilitate frictionless joint movement. Examples include:

  • The subcoracoid bursa of the upper back and shoulder
  • The subacromion bursa of the upper back and shoulder

Anatomy

Wherever there are tendons moving across a bony surface, there is a bursa. There are approximately 150 bursa found in the body. Most are present at birth, but a bursa may form in an area where there is friction. Bursae are filled with a thin layer of thick fluid, like the consistency of a raw egg white, and a lining of synovial fluid, which also works as a shock absorber for the joint and supplies molecules to the cartilage. This is known as dissipating force. Normally, the bursae are thin and provide a very slippery surface so there will be very little friction when there is movement of the joint.

Think of a slide on a playground. If a child has a piece of wax paper and sits on it to slide, they go down very quickly, don't they? This would represent the function of a bursa. But if the child has on shorts and tries to slide down, the skin can cause more friction, and the child can hardly move down the slide. If no bursae are present at the joint, bending your elbow would be like trying to go down a slide with shorts on.

Types and Function

There are four different types of bursa in the body. Three are present at birth. One, known as the adventitious type, develops in any area where there is constant use and friction. An example of the adventitious type you may have seen is on the feet of older women. It is called a bunion. Bunions form usually on the side of the great toe and are caused from wearing high heel shoes. The shoe rubs on the foot and creates the adventitious bursa, sometimes called the accidental bursa.

The other three are subcutaneous, synovial, and sub-muscular. They are named for their location in the body. Therefore, the subcutaneous bursae lie just under the skin, while synovial reside in the joint, and sub-muscular are of course under the muscle.

Pathology

Bursitis happens when the bursae are irritated and become inflamed. They lose their functional ability and become a problem instead of a help to the joint. The bursae become larger, and the fluid is thicker. This causes increased friction, basically the opposite of the bursae's primary function. Any movement of the inflamed bursa will cause pain.

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Video Transcript

What Is Bursa?

Bursae are fluid-filled sacs located around a joint that cushion bones when muscles, tendons, or skin rub against the bone. They are very important in movement of the bones because they help reduce friction. Without them, movement would be difficult and even painful. Now let's try and relate this to something you may know about.

Let's say your friend told you about an incident that happened last week in the factory where he works. One of the machines made a very loud grinding sound and then suddenly stopped working. On inspection, your friend tells you that the washers that lie between the metal parts of one of the machines broke. The grinding was the sound made when metal rubbed on metal. In this example, the washers were crucial to having the machine work. Likewise, the bursae are crucial for having the joint work properly.

Anatomy

Wherever there are tendons moving across a bony surface, there is a bursa. There are approximately 150 bursa found in the body. Most are present at birth, but a bursa may form in an area where there is friction. Bursae are filled with a thin layer of thick fluid, like the consistency of a raw egg white, and a lining of synovial fluid, which also works as a shock absorber for the joint and supplies molecules to the cartilage. This is known as dissipating force. Normally, the bursae are thin and provide a very slippery surface so there will be very little friction when there is movement of the joint.

Think of a slide on a playground. If a child has a piece of wax paper and sits on it to slide, they go down very quickly, don't they? This would represent the function of a bursa. But if the child has on shorts and tries to slide down, the skin can cause more friction, and the child can hardly move down the slide. If no bursae are present at the joint, bending your elbow would be like trying to go down a slide with shorts on.

Types and Function

There are four different types of bursa in the body. Three are present at birth. One, known as the adventitious type, develops in any area where there is constant use and friction. An example of the adventitious type you may have seen is on the feet of older women. It is called a bunion. Bunions form usually on the side of the great toe and are caused from wearing high heel shoes. The shoe rubs on the foot and creates the adventitious bursa, sometimes called the accidental bursa.

The other three are subcutaneous, synovial, and sub-muscular. They are named for their location in the body. Therefore, the subcutaneous bursae lie just under the skin, while synovial reside in the joint, and sub-muscular are of course under the muscle.

Pathology

Bursitis happens when the bursae are irritated and become inflamed. They lose their functional ability and become a problem instead of a help to the joint. The bursae become larger, and the fluid is thicker. This causes increased friction, basically the opposite of the bursae's primary function. Any movement of the inflamed bursa will cause pain.

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Frequently Asked Questions

What is the function of a bursa?

Bursae function to facilitate musculoskeletal movement. They do this by providing a cushion, reducing friction, and dissipating force to and within the body.

Does bursitis go away?

Depending on the cause, bursitis can be acute (lasting for a short period of time) or chronic (requiring prolonged care). Treatment options include rest, medication, physical therapy, and surgical drainage.

Where is the bursa located?

Bursae are located all over the body. Most commonly, they can be found in the joints of the shoulders, elbows, hips, and knees.

What is the bursa in the human body?

A bursa in the human body is a fluid-filled sac. It is usually found within the major joints of the body.

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