Open Circulatory System: Structure, Function, and Examples

Definition

An open circulatory system lacks continuous vessels; hemolymph (blood + interstitial fluid) is pumped by a dorsal vessel (heart) into body cavities (hemocoel/sinuses), directly bathing tissues.

Key Features

• No capillaries: Exchange occurs across sinuses.
• Low pressure & flow velocity: Limits metabolic rate and body size.
• Heart: Simple muscular tube with ostia (valved openings) for hemolymph entry during diastole.
• Hemolymph: No RBCs; oxygen transported via diffusion/tracheae (insects) or hemocyanin (mollusks/crustaceans).

Examples

• Arthropods: Insects, spiders, crustaceans.
• Mollusks: Most (except cephalopods, which have closed systems).

Comparison to Closed Systems

Feature	Open	Closed
Vessels	Absent (sinuses only)	Continuous (arteries, capillaries, veins)
Pressure	Low	High
O₂ transport	Diffusion / tracheae / hemocyanin	Hemoglobin in RBCs
Metabolic capacity	Low	High
Complexity	Simple	Complex

Open systems suffice for small, low-metabolism invertebrates; closed systems enable larger size and higher activity.