Spiders, like many other living organisms, possess a heart. While their circulatory system differs significantly from that of mammals, this organ plays a central role in their survival. Understanding their unique circulatory system provides insight into diverse animal adaptations.
The Spider’s Heart and Its System
Spiders have a heart located in their abdomen, along the dorsal midline above the intestine. This tubular heart is a simple, muscular tube, unlike the multi-chambered organ found in vertebrates. It rhythmically contracts and relaxes to propel fluid throughout the spider’s body.
Small openings, called ostia, are along the sides of this tubular heart. These ostia function as one-way valves, allowing hemolymph, the spider’s equivalent of blood, to enter the heart from the surrounding body cavity. When the heart contracts, these ostia close, forcing the hemolymph forward and backward through arteries.
The heart operates within a pericardial sinus, a muscular cavity that surrounds it. Elastic muscles around this cavity contract, enlarging the tubular heart and drawing hemolymph in. The heart then pumps this fluid through branching arteries that extend into various parts of the spider’s body, including the legs and the central nervous system.
Hemolymph: Spider “Blood”
Spiders do not have true blood like humans; instead, their bodies are filled with a fluid called hemolymph. This hemolymph serves multiple functions, including the transport of oxygen, nutrients, hormones, and waste products throughout the body. It also plays a role in immune defense, containing cells called hemocytes that help protect against pathogens and aid in wound healing.
A key component of spider hemolymph is hemocyanin, a respiratory protein responsible for oxygen transport. Unlike the iron-based hemoglobin that gives human blood its red color, hemocyanin contains copper atoms. When oxygenated, hemocyanin gives spider hemolymph a faint blue or sometimes greenish tint.
Beyond transport, hemolymph is crucial for spider movement. Spiders use hydrostatic pressure, generated by their hemolymph, to extend their legs. While they have muscles to flex their legs inward, they lack extensor muscles; the increase in hemolymph pressure within their cephalothorax forces their legs outward, enabling movement.
Open vs. Closed Circulatory Systems
Spiders possess an open circulatory system, where hemolymph is not entirely confined within blood vessels. Instead, the heart pumps hemolymph into open spaces or sinuses that directly bathe the internal organs and tissues. This allows for a direct exchange of gases, nutrients, and waste products between the hemolymph and the cells.
In contrast, humans and other vertebrates have a closed circulatory system. In this system, blood remains enclosed within a continuous network of arteries, veins, and capillaries, and is pumped by a more complex, multi-chambered heart. The exchange of substances occurs across the thin walls of capillaries, rather than by direct bathing of tissues.
The open circulatory system is efficient for spiders given their size and metabolic demands. It requires less metabolic energy to operate compared to a closed system because it involves fewer vessels and simpler structures. This simplicity means less energy is expended on pumping and circulating the fluid, allowing spiders to allocate energy to other vital processes.