Open Type of Circulatory Systems
Several non-chordates have thus an open type of circulation - a system in which blood pumped by the heart empties through an artery into an open sinus filled with blood which lies between the ectoderm and endoderm. The blood filled space is considered to as haemocoel, particularly in arthropods. The fluid contained within the haemocoel, referred to as haemolymph or blood, is not circulated by capillaries but bathes the tissue directly. Figures show the organisation of the main vessels from two groups of non- chordates having open circulation, a Cray fish and a bivalve mollusc.
Figure: Invertebrate circulations (A) simplified diagram of the circulation in the crayfish, (B) Simplified diagram of the circulation in bivalve mollusc.
In several animals the haemocoel is large and constitutes 20 - 40% of body volume. In contrast, chordates along with a closed circulation contain a blood volume of 5 - 10% of the total body volume. Open circulatory systems have low pressures; seldom exceeding 5 - 10 mm Hg. Animals that have an open circulation generally have only limited capability to alter the velocity and distribution of blood flow. As a result, in bivalve molluscs and other animals that have an open circulation and use blood for gas transport, change in oxygen uptake is slow and rate of oxygen transfer is low per unit weight. In other words, their metabolism is extremely low, Inseots have, however, avoided this problem by evolving a tracheal system where gas transport to tissues occurs direct through air filled tubes or tracheae that bypass the blood. Accordingly, although insects have an open circulation, they have greater metabolic rate.