Explain the mechanism of blood circulation in cockroach.

Mechanism of blood circulation: 

1. Blood (haemolymph) circulates between sinuses and heart due to contraction and relaxation of heart and alary muscles.

2. The heart contracts (systole) and relaxes (diastole) alternatively. After diastole, there is a third phase in the heart cycle known as diastasis. During diastasis, heart remains in expanded state. 

3. During diastole, heart expands and the alary muscles contract, making the dorsal diaphragm flat. As a result, blood passes from perivisceral sinus to pericardial sinus through fenestrae and finally to the heart through ostia. 

4. During systole, contraction starts at the posterior end and the wave of contraction passes anteriorly. Due to this, blood is pushed towards the dorsal aorta. 

5. Ostia remain closed with the help of valves, during systole. As a result, blood flushes into head region from where it goes to perivisceral and perineural sinuses. 

6. During systole, alary muscles are relaxed and due to this, the dorsal diaphragm becomes convex. 

7. The volume of pericardial sinus is now reduced. This makes the blood to move from pericardial sinus to perivisceral sinus through fenestrae.