Spiders possess a heart, but its structure and function are significantly different from the four-chambered muscular pump found in mammals. As arachnids, spiders have a circulatory system that is evolutionarily distinct from vertebrates, operating on different biological principles. This system is perfectly suited to the spider’s small, segmented body plan.
The Spider’s Heart: Structure and Location
The organ responsible for propelling fluid is not a compact, centralized structure, but rather a long, muscular tube known as the dorsal vessel. This vessel is situated longitudinally along the top, or dorsal, side of the spider’s abdomen, also called the opisthosoma. The heart is encased within a thin-walled sac called the pericardium.
The tubular heart is not a continuous, closed vessel, but contains small, valve-like openings along its sides called ostia. These ostia are crucial for circulation, allowing the fluid to flow inward from the surrounding body cavity when the heart is relaxed. When the heart muscle contracts, the ostia close to prevent backflow, ensuring the fluid is propelled forward through the body.
The contraction of the dorsal vessel pushes the circulatory fluid out into arteries, which branch off toward the front of the spider. An anterior aorta extends from the heart to supply the cephalothorax, or prosoma, with fluid. This simple, peristaltic pumping action is regulated by its own nerve center, allowing the heart to beat independently of the spider’s brain.
An Open Circulatory System
The spider’s heart and arteries form only one part of what is classified as an open circulatory system. In this type of system, the circulating fluid is not continuously contained within a network of vessels like the arteries, veins, and capillaries of humans. Instead, the fluid is pumped into open spaces that directly bathe the internal organs.
The dorsal vessel discharges the fluid into arteries, which quickly terminate, releasing the fluid into a large body cavity called the hemocoel. The hemocoel acts as the primary reservoir for the fluid, surrounding and directly supplying the tissues and organs with nutrients and oxygen. This direct contact between the fluid and the cells eliminates the need for an extensive network of microscopic capillaries.
After circulating around the organs, the fluid gradually collects in open sinuses. Coordinated movements of the spider’s body muscles help to push the fluid back toward the abdominal region. Once gathered around the heart, the fluid re-enters the dorsal vessel through the ostia, completing the circuit.
Hemolymph: More Than Just Blood
The fluid circulating through the spider’s open system is called hemolymph, which serves the combined functions of blood and interstitial fluid in vertebrates. Hemolymph is responsible for the transport of nutrients, hormones, and waste products throughout the arachnid’s body. For oxygen transport, spiders use a specialized protein known as hemocyanin.
Hemocyanin is a copper-based respiratory pigment, unlike the iron-based hemoglobin found in human blood. When hemocyanin binds to oxygen, the fluid takes on a faint blue tint. This protein is dissolved directly into the hemolymph rather than being stored within specialized blood cells.
Hemolymph also performs a mechanical function, acting as a hydraulic fluid to assist with movement. Spiders lack extensor muscles in some of their main leg joints, relying instead on changes in internal fluid pressure to extend their limbs. By contracting muscles in the prosoma, the spider increases the hemolymph pressure, forcing the fluid into the legs and causing them to straighten.