Final answer:
Tendon sheaths are elongated bursae that contain lubricating fluid to reduce friction where tendons operate near joints.
Step-by-step explanation:
The elongated bursae that wrap around tendons where there may be excessive friction are known as tendon sheaths. These structures are critical in reducing friction as they contain a lubricating fluid that allows for smooth movement of tendons across joints. Tendon sheaths are essentially extended bursae that create a frictionless environment for tendons, particularly in areas susceptible to a lot of movement. They serve the same purpose as bursae, which are fluid-filled sacs found in places where tissues like skin, muscle, or tendon glide over bone.
Bursae serve to prevent friction and are classified based on their locations, such as subcutaneous bursae located between the skin and underlying bones, submuscular bursae found between muscles, or bone and subtendinous bursae placed between bone and tendon. An example of a subtendinous bursa is the subacromial bursa that separates the tendon of shoulder muscle from the acromion of the scapula.
When these bursae become inflamed, such as from repetitive motions or pressure, this condition is referred to as bursitis. It can lead to pain, swelling, and joint stiffness. Common sites for bursitis include the shoulder, hip, knee, and elbow. Two well-known types of bursitis are subacromial bursitis at the shoulder and trochanteric bursitis at the hip.