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Difference Between Alveoli and Alveolar Sac

Alveoli vs Alveolar Sac

The bodies of living organisms are made up of complex systems. Plants have a root-and-shoot system while animals have the digestive, cardiovascular, lymphatic, musculoskeletal, nervous, reproductive, bone, integumentary, and respiratory systems.

The respiratory system in animals helps them breathe. Amphibians, such as frogs, breathe through their skin, and fish have gills. Mammals, such as man, breathe through their respiratory system which is composed of the mouth, nose, pharynx, trachea, and lungs.

Humans take in air through their mouths and noses. It passes through the pharynx and the trachea before it reaches the lungs. Man only needs oxygen, and because the air that he takes in is composed of other gases such as carbon dioxide, it needs to be processed so that the body can take in only oxygen and dispose of the carbon dioxide.
This is done in the lungs particularly in the air sacs called alveolar sacs. Alveolar sacs form the end of small alveolar ducts that are the breathing airways filling the lungs. These airways or tubes divide and form two or three alveolar sacs at the end.

It is in the alveolar sacs where the exchange of oxygen and carbon dioxide takes place. It is thin-walled and is rich in capillaries. It is formed by a group or cluster of alveoli, and it is where the alveoli communicate.

There are approximately 700 million aveoli in the human lungs. They are composed of a layer of membranous tissue or epithelial layer and extracellular matrix which is enclosed in capillaries. Alveoli are made up of collagen and elastic fibers which allow them to be stretched when air is taken in and shrink back as carbon dioxide is expelled.

They are exceptionally used in mammalian lungs; other vertebrates use a totally different network. The alveoli release the carbon dioxide that the blood carries to them while the blood takes the oxygen from the alveoli for transportation to the cells of the body.

The word “alveoli” is the plural form of “alveolus,” a word which was derived from Latin which means “little cavity” and the Proto-Indo-European word “aulo” which means “hole” or “cavity” and from the Greek word “aulos” which means “pipe” or “tube.”


1. Alveoli are composed of epithelial layers and extracellular matrix enclosed in capillaries while alveolar sacs are the distal ends of alveolar ducts.
2. The alveoli sacs are formed by a group or cluster of alveoli, and it is there where they communicate while the alveoli are made up of collagen and elastic fibers.
3. The alveoli process the gases that the blood caries to the lungs expelling the carbon dioxide and letting the blood transport the oxygen to the cells in the body while it is in the alveolar sacs where this process takes place.
4. The word “alveolus” comes from the Latin word for “little cavity” while the term “alveolar sacs” is derived from the same word and is used to refer to the pouch where the alveoli are located.

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