Circulatory System

The haemocoel — an open circulatory system

Unlike vertebrates, the cockroach (like all insects) has an open type of circulatory system: the blood (haemolymph) is not confined within a closed network of vessels but bathes the internal organs directly in a large body cavity called the haemocoel. The haemocoel is not the true embryonic coelom; it is formed by the breakdown of embryonic cavity walls and the merging of the blastocoel with remnants of the coelom, so that organs lie bathed directly in circulating haemolymph rather than suspended in a coelomic fluid separated from blood.

Two thin, horizontal, perforated sheets of connective tissue and muscle, the dorsal diaphragm and the ventral diaphragm, stretch across the haemocoel and divide it into three communicating chambers or blood sinuses:

• Pericardial sinus (dorsal-most) — houses the tubular heart.
• Perivisceral sinus (the largest, middle chamber) — bathes the gut, fat body and reproductive organs.
• Perineural sinus (ventral-most) — houses the ventral nerve cord.

Haemolymph percolates from one sinus to the next through the perforations in the diaphragms, driven onward by the diaphragms' own rhythmic undulations as well as by the heartbeat, so that the whole body cavity forms a single continuous circulatory space.

The heart

The heart is a single, slender, muscular tube lying mid-dorsally in the pericardial sinus, directly beneath the terga, running the length of the thorax and abdomen. It is divided along its length into 13 chambers, each chamber narrowing behind into the next through a valve. Each chamber bears a pair of lateral openings called ostia, guarded by valves that permit haemolymph to flow only inward — from the pericardial sinus into the heart — and prevent backflow. At its front end the heart continues into a simple tube, the aorta, which runs forward through the thorax and opens freely into the head sinus, releasing haemolymph around the brain and sense organs.

Alary muscles and the heartbeat

Each heart chamber is anchored to the body wall and to the dorsal diaphragm by a pair of fan-shaped alary muscles. Rhythmic contraction of the alary muscles pulls the dorsal diaphragm and heart wall outward, enlarging (dilating) each chamber — this is diastole, during which haemolymph is drawn in from the pericardial sinus through the open ostia. This is immediately followed by contraction of the heart's own circular muscle — systole — which closes the ostial valves and drives the haemolymph forward, chamber by chamber, toward the aorta and out into the head. This alternating diastole–systole wave, repeated chamber after chamber, produces a slow peristaltic pulse that keeps haemolymph flowing forward through the heart and out into the open sinuses, from where it seeps back posteriorly through the body to be drawn in again at the ostia — a continuous, one-way, low-pressure circuit.

Haemolymph and the fat body …