Hemolymph

Hemolymph is the circulatory fluid of many invertebrates, particularly arthropods (such as insects, spiders, and crustaceans) and most mollusks. It serves as the transport medium for nutrients, waste products, hormones, and immune cells, and it often contributes to hydrostatic support and wound healing. In many organisms with an open circulatory system, hemolymph is not entirely confined to vessels but fills the body cavity (the hemocoel) and bathes internal tissues. Some species have more restricted or partly closed systems, but hemolymph typically remains in contact with organs rather than being fully enclosed in vessels.

Composition: Hemolymph consists of a plasma-like liquid and cellular components called hemocytes. Hemocytes participate in phagocytosis, encapsulation of parasites, clotting-like responses, and tissue repair. Hemolymph may contain respiratory pigments such as hemocyanin (a copper-containing protein that often gives blue color) or, in some mollusks, hemoglobin. In insects, the primary gas exchange occurs via the tracheal system, while hemolymph carries other materials.

Functions: It transports nutrients (amino acids, sugars, lipids), wastes, and signaling molecules; distributes immune factors and cells; participates in humoral and cellular immune responses (including melanization and antimicrobial peptides); and can contribute to osmoregulation and wound healing.

Circulation: The heart, with ostia, pumps hemolymph into the hemocoel; hemolymph returns to the heart through ostial openings. Movement and body movements aid flow in open systems.

Overview: Hemolymph is a defining feature of invertebrate physiology, enabling internal transport and defense in organisms without a closed vertebrate-type circulatory system.