A circulatory system transports essential substances like nutrients and waste products within an animal’s body. There are two primary types: open and closed. In a closed system, the fluid (blood) remains entirely within vessels like arteries, veins, and capillaries. An open circulatory system (OCS) is different; its transport fluid, hemolymph, is pumped from vessels directly into a large body cavity called a hemocoel. The hemolymph freely bathes the organs before returning to the heart through specialized openings, providing specific advantages for certain animal groups.
The Economics of Structural Simplicity
The most apparent advantage of an open circulatory system is the significant reduction in material and energy required to build and maintain it. Unlike a closed system requiring an intricate, high-pressure network of fine capillaries, the OCS needs only a few vessels to direct fluid flow. This simpler vascular infrastructure translates directly into lower biological building costs during growth and development.
This simpler plumbing operates under very low pressure compared to the high-pressure demands of a closed system. The heart, often a simple muscular tube, requires substantially less energy to pump the hemolymph just into the hemocoel rather than forcing it through a long, high-resistance capillary bed. Low pressure also means that the vessels themselves do not need the thick, reinforced walls required in a pressurized system, further minimizing resource expenditure.
The dispersed nature of the hemolymph within the hemocoel also simplifies the process of repairing damage. A puncture in the hemocoel is far less catastrophic than a rupture in a major artery or vein of a high-pressure system. The fluid pressure is so low that internal bleeding is less severe, and the body does not need to allocate extensive resources to rapidly seal a high-pressure leak in an intricate vessel network.
Functional Sufficiency for Low Metabolic Demand
The OCS is perfectly sufficient for animals with low overall metabolic rates or small size. In these organisms, the distance the hemolymph needs to travel is short enough that slower circulation is still adequate for nutrient and waste exchange via diffusion. This sufficiency becomes a distinct advantage compared to a high-speed closed system.
For many arthropods, particularly insects, the open circulatory system is freed from the most demanding transport task: oxygen delivery. Insects utilize a separate, highly efficient respiratory system called the tracheal system, which delivers oxygen directly to the tissues through a network of tubes. By offloading this high-demand function to the tracheal system, the OCS only needs to manage the slower transport of nutrients, hormones, and waste products.
This division of labor provides significant energy savings by eliminating the need for a high-pressure, oxygen-transporting system. The OCS is well-suited for animals with intermittent activity patterns, such as clams or insects. These organisms can sustain short bursts of activity while saving considerable energy during long periods of rest.
Utilizing Hemolymph for Hydrostatic Action
A unique advantage of the open circulatory system is that hemolymph can be used for mechanical, non-transport functions, acting as a hydraulic fluid. The hemocoel allows the animal to transiently increase fluid pressure in specific areas using internal muscle contraction. This action creates a hydrostatic skeleton that assists in movement and structural change.
In many spiders, for instance, leg extension is achieved not primarily by muscle action, but by a rapid increase in hemolymph pressure within the limb. This hydraulic extension is a fast and energy-efficient way to move, allowing for rapid actions like jumping or running. The pressure generated by muscle contraction against the contained fluid volume provides the force needed for rapid locomotion.
The mechanical use of hemolymph is also indispensable during molting, or shedding the exoskeleton. To escape the old, rigid cuticle and expand the new, soft one before it hardens, the animal must quickly inflate its body. This inflation is accomplished by rapidly increasing the internal volume and pressure of the hemolymph. This fluid pressure acts to split the old exoskeleton and stretch the new one to its full size.