Circulation Type

Most mollusks have an open circulatory system, which means that their blood is not restricted to blood vessels and flow around the body. They have a three chambered heart, two atria and one ventricle. Oxygen is collected in the gills or modified lungs and then is transfered the heart. The blood enters from the atrium and oxygenated blood mixes with the deoxygenated blood in the ventricles. The oxygenated blood is pumped by the heart to the cavity. Their blood filled space and main body cavity is known as the hemocoel. This is where most of the internal organs are contained in and where blood and coelomic fluid flow. The oxygenated blood bathes the the tissues and oxygen and nutrients exchange occurs. The deoxygenated blood is then taken to the gills where it is reoxygenated then taken to the heart again. Clams and mussels have this kind of circulatory system because they do not move a lot or at great speeds so their demand on oxygen is low.

Those that have a closed circulatory system are cephalopods, the blood remains within the blood vessels, and also include a capillary bed. It has a systemic heart which pumps the blood throughout the body and two brachial hearts. The brachial hearts are located near the gills where gas exchange occurs and the oxygenated blood enters the heart. The ventricles then pump the blood out of the heart into smaller vessels and eventually to the tissue capillaries. Cephalopods are fast moving and always active so a closed system allows for a greater demand of oxygen to keep the cells working.

Unlike humans, instead of being made up of hemoglobin like human, mollusks' blood is made of hemocyanins. They are respiratory proteins that is made up of copper atoms that helps bond to the oxygen molecule. When it is oxygenated, the blood is colourless and when it is deoxygenated it is blue.They are not bound to blood cells but are suspended directly in the blood.