Haemocoel: The body cavity contains blood, so it is called haemocoel. The haemocoel is divided into two cavities by a transverse pericardial membrane. The dorsal cavity is called pericardial cavity (sinus) and the ventral cavity is called perivisceral (Meaning: Surrounding the viscera) cavity. The pericardial membrane is perforated.
Open Circulatory System: Cockroach has an open circulatory system. The blood is not always enclosed in a tube. Much of the time the blood flows in large spaces among the tissues in the body. There are no capillaries.
Haemolymph: The blood of haemolymph is colorless fluid. It contains white blood cells (which act as phagocytes).
Function: Function of blood is chiefly to transport the absorbed nutrients. It has no respiratory figment, so it has nothing to do with respiration. Reparation is performed by tracheal system.
The Blood Vessel: The blood vessel consists of a single, long tubular mid-dorsal blood vessel below the targa. Each segment of thorax and abdomen consists of a dorsal plate the tergum (plural terga) a ventral plate the sternum joined at the sides by soft cuticle the pleuron. The dorsal vessel is differentiated as (i) heart (ii) aorta.
(i) Heart: The part of the dorsal vessel in the abdomen is called heart. It is divided into eight chambers. Each chamber has a pair of lateral openings called ostia. Each ostium has a valve, to prevent backflow of the blood. On the sides of the heart there are alary muscles which help in the flow of blood.
(ii) Aorta: The part of the dorsal vessel that extends in thoracic and head regions is called aorta. It is without chambers. It opens in anterior by a funnel shaped opening into the haemocoel.
Circulation: When the alary muscles contract the heart chambers relax. Blood from the perivisceral cavity passes into the pericardial cavity. It enters the heart chambers through the ostia. When the heart contracts the ostium is closed by valve. The blood is forced along the dorsal aorta. The wave of contraction starts at the posterior end and work their way towards the anterior end. From the heart the blood flows into the aorta. From here the blood reaches the perivisceral cavity in the head region. Then moves backward where it bathes all the organs. It also circulates in the appendages and wings. The blood enters the pericardial cavity through the perforation in the pericardial membrane and again enters the heart through the ostia.