Sacs

A sac is a general term for a closed pouch or cavity formed by membranes or tissues that stores, protects, or separates substances. The word originates from the Latin sacculus, meaning a small bag. Sacs are common throughout biology and medicine and can be fluid-filled, gas-filled, or contain solids.

In human anatomy, several sacs play important roles. The amniotic sac surrounds and protects a developing fetus, while the pericardial sac encloses the heart. The pleural sacs surround the lungs, creating a potential space for movement with minimal friction. Synovial sacs, or bursae, are fluid-filled cushions around joints, reducing friction during movement. Alveolar sacs in the lungs are clusters of air sacs that participate in gas exchange, and various hollow organs such as the gallbladder and urinary bladder function as fluid-containing sacs.

Pathology often involves sac-like structures. A cyst is typically a fluid-filled sac lined by epithelium; examples include ovarian cysts and renal cysts. An abscess is a pus-filled sac that forms during infection, while Baker’s cyst refers to a fluid-filled swelling behind the knee.

Other uses of the term appear in different fields. In mycology, the asci of sac fungi are sac-like cells that contain spores, illustrating how the concept of a sac spans multiple disciplines. Overall, sacs are versatile anatomical and conceptual features that vary in structure and function across organisms and contexts.