Cockroaches do not have a four-chambered organ like the human heart, but they possess a functional circulatory pump called the dorsal vessel. This muscular structure is the central component of their open circulatory system, moving fluid throughout their body. Understanding this organ and the system it supports is key to grasping the unique physiology of these common insects.
The Dorsal Vessel: The Cockroach “Heart”
The cockroach’s primary circulatory organ, the dorsal vessel, is a long, muscular tube running along the insect’s back, beneath the body wall. It is divided into two regions: the heart, the posterior section, and the aorta, the anterior portion extending toward the head. The heart is a segmented tube, often composed of 13 funnel-shaped chambers, though this number can vary.
The heart functions as the main pumping mechanism, driving the circulatory fluid forward toward the head. Along the heart are small, valve-like openings in each segment called ostia. These ostia allow fluid to enter the heart from the surrounding body cavity when the heart is relaxed.
When the heart muscle contracts, the ostia close, and the fluid is pushed forward through the chambers and into the aorta. The aorta is a simple tube that opens into the head region. This muscular contraction is assisted by fan-like alary muscles that help anchor the heart and drive circulation.
Hemolymph: The Circulatory Fluid and Open System
The circulatory fluid in a cockroach is called hemolymph, which differs significantly from mammalian blood. Hemolymph is not confined to arteries and veins but flows freely within the insect’s body cavity, known as the hemocoel. This arrangement defines the cockroach’s open circulatory system, contrasting with the closed systems of vertebrates.
Hemolymph includes a clear plasma and specialized cells called hemocytes. Hemocytes play a role in immune defense, encapsulating foreign invaders and coagulating at injury sites. The plasma carries nutrients, hormones, and metabolic waste products like uric acid, distributing them to the tissues and organs.
Hemolymph lacks the high concentration of oxygen-carrying pigments, such as hemoglobin, meaning the fluid is usually colorless or pale. Its role focuses on transport functions other than respiration. The dorsal vessel moves this fluid around the hemocoel, bathing the internal organs directly.
Oxygen Delivery: A Separate System
The cockroach’s circulatory system is functional because oxygen transport is managed by an entirely separate, dedicated system. This is the tracheal system, a sophisticated network of air-filled tubes permeating the insect’s body. This difference in oxygen delivery is why their “heart” and fluid do not require the same pumping power or respiratory pigments as a human’s.
Air enters the cockroach’s body through small, paired external openings along the sides of the thorax and abdomen called spiracles. There are typically ten pairs of spiracles. Each spiracle acts as a tiny, valved doorway, often controlled by muscles to regulate air intake and minimize water loss.
Once inside, the air travels through increasingly fine tubes, starting with the tracheae. These tubes branch extensively, narrowing into microscopic tracheoles that reach deep into the tissues and individual cells. This direct delivery mechanism allows oxygen to diffuse straight from the tracheoles into the surrounding cells.
Because oxygen is supplied directly to the tissues, the circulatory system is freed from the burden of gas transport. The cockroach’s heart and hemolymph are specialized for nutrient, hormone, and waste movement. This efficient, direct-to-cell oxygen delivery system supports the cockroach’s metabolism without relying on blood-based respiration.