Circulation

Circulation is the movement of substances within a space or system to distribute energy, matter, or information. The term is used in several disciplines, including biology, meteorology, economics, and information science, to describe how something is circulated through components connected by flow processes.

In animals, circulation typically refers to the circulatory system, where the heart pumps blood through arteries, capillaries, and veins. Blood travels in two main circuits: the pulmonary circuit, exchanging gases in the lungs, and the systemic circuit, delivering oxygen and nutrients to tissues. Lymphatic circulation carries interstitial fluid and immune cells, helping maintain fluid balance and defense.

In plants, circulation involves transport via xylem and phloem. The xylem moves water and minerals from roots to shoots, driven by transpiration and cohesion-tension. The phloem distributes sugars produced by photosynthesis through source-to-sink movements, often explained by the pressure-flow hypothesis.

Circulation in the atmosphere and oceans describes the large-scale movement of air and seawater driven by temperature, salinity, and Earth's rotation. Atmospheric circulation includes Hadley cells, mid-latitude westerlies, and jet streams, influencing climate and weather. Ocean circulation redistributes heat and nutrients through currents such as gyres and the thermohaline circulation.

In economics, money circulation refers to the movement of currency through an economy. The velocity of circulation measures how quickly money changes hands, influencing economic activity and inflation. Central banks influence circulation via policy, and the distinction between currency in circulation and bank deposits is common in monetary accounting.

In information science and media, circulation describes how information, publications, or content spreads among users and platforms, encompassing distribution, access, and readership metrics.