Open Circulatory System: Structure, Function & Examples

In an open circulatory system, blood (hemolymph) is pumped by a heart into body cavities (hemocoel/sinuses) where it directly bathes tissues and organs—not confined to vessels. It returns to the heart through openings called ostia.

Key Features

• Low pressure, slow flow
• No capillaries; exchange occurs directly across tissue surfaces
• Hemolymph carries nutrients, hormones, immune cells—but typically no respiratory pigment (oxygen diffuses via tracheae in insects)

Main Processes

1. Gas exchange: O₂/CO₂ diffuse directly between hemolymph and tissues (or tracheal system).
2. Nutrient/waste transport: Hemolymph distributes absorbed nutrients; collects metabolic wastes for excretory organs (Malpighian tubules, green glands).

Where It's Found

Most arthropods (insects, crustaceans, arachnids) and many mollusks (clams, snails). Contrasts with closed systems (vertebrates, annelids, cephalopods) where blood stays in vessels.