Question

What lubricates the freely movable joint at the shoulder?

Answer 1

The joints can be of many types. Synovial joints allow movement and there exists a synovial cavity filled with fluid. The fluid allows free movement and provides lubrication.

Complete answer:
To avoid friction between them during movement, all articulations between bones that assist in a certain form of movement are positioned at a distance from each other. This gap is filled with synovial fluid between the two bones, which also helps minimise friction. The glenohumeral joint (a ball socket joint) in the shoulder is an example.
A viscous fluid found in the cavities of synovial joints is synovial fluid. The key function of synovial fluid with its egg white-like consistency is to decrease friction during motion between the articular cartilages of synovial joints. In the synovial membrane, which develops synovial fluid, the shoulder joints are encapsulated.
Joints are classified into fibrous, cartilaginous and synovial.
Fibrous joints do not allow any movement.This type of joint is present in skull bones which fuse end to end with the help of connective tissue. In cartilaginous joints bones are joined with the help to cartilage.It permits limited movement.

Note:
The places where two or more bones meet are called joints. There are many types of joints including fixed and mobile joints viz. ball-socket joint, hinge joint, pivot joint etc. Synovial fluid is also called non-Newtonian fluid or ‘synovia’.