Cirkulation

Cirkulation refers to the movement of fluid or material through a system. In biology the term most often denotes the circulatory system, the transport network that distributes blood throughout an animal’s body and removes waste products. The vertebrate circulatory system consists of the heart, arteries, veins and capillaries. Blood is pumped by the heart in two circuits: the pulmonary circuit bringing deoxygenated blood to the lungs, and the systemic circuit delivering oxygenated blood to tissues. Exchange occurs across capillary walls, where oxygen and nutrients diffuse into tissues and carbon dioxide and wastes are collected for removal. Blood flow is regulated by neural and hormonal signals, local metabolic demands, and the mechanical properties of vessels. Cardiac output, the product of heart rate and stroke volume, roughly 4–6 liters per minute in a resting adult, increases with activity.

In plants, circulation refers to the vascular transport of water, minerals and sugars through xylem and phloem. In both kingdoms, circulation is essential for metabolism, temperature regulation and homeostasis.

Other uses include the circulation of information or goods in libraries, publishing and commerce, which measures how widely and quickly items are distributed or borrowed.

Historically, the concept of circulation was established through the study of the cardiovascular system, with William Harvey’s description of systemic circulation in 1628 and the later recognition of pulmonary circulation.