Open or closed Double or single

Open circulation In an open circulation there is some pumping of body fluid which is partially contained in vessels Blood is pumped into the body space surrounding the tissues and then seeps back in to the heart as pressure rises in the body

Closed circulation The blood fluid is contained in vessels and nutrients and oxygen are exchanged with the tissues through the thin walls of blood capillaries. Blood circulates one way around the system driven by a pump

Single circulation Some animals like fish have a closed circulation where blood passes from the heart to the gas exchange surface then to the tissues and back to the heart. Blood passes through the heart once for every circuit of the body

Double circulation Blood passes from the heart to the gas exchange system (pulmonary circulation) and then back to the heart to be pumped around the rest of the body (systemic circulation). Blood passes twice through the heart for every once around the body.

Comparing double and single circulation

Questions What is the disadvantage of a single circulation? Why is a single circulation adequate for fish? What is the advantage of a double circulation?

Answers Blood pressure is reduced as blood passes through the narrow capillaries in the gills-this slows the rate of flow to the rest of the body meaning a slower delivery of oxygen and nutrients to the rest of the body. Fish do not maintain a warm body temperature and are not as active as mammals so the amount of oxygen and nutrients delivered is sufficient for their needs Blood pressure can be increased by the heart after oxygenation so blood flows to the tissues more quickly. Pressure can be higher in the systemic circulation than the pulmonary