The fish circulation

As seen on the previous page, the major difference between the mammalian and fish circulatory system. The fish heart needs to generate the driving pressure for both the gills (lungs in mammals) and the body since they are connected in series, as seen in the figure to the left. 

In mammals both sides of the heart pump the same volume per time unit, but the pressure generation is very different in the right and left side. The right ventricle only generates a fraction of the pressure compared to the left ventricle (which is the pressure you measure during a physical exam. This pressure should normally be around 120/80 mmHg. To optimize the gas exchange in the lungs, the diffusion distance needs to me minimized (see formula for diffusion in the respiration module). A small diffusion distance means thin membranes, and this means that the blood pressure in the lung circulation should be low in order to avoid damage. In fish, the heart pumps blood first to the gills where the gas exchange takes places, and then the blood continues to the rest of the body. This is a fine balance, since the fish's gills (like the mammal's lungs) have to be thin walled to facilitate gas exchange ,and thus cannot tolerate high blood pressure. At the same time, the blood pressure will drop when the blood cells squeeze through the lamellae (see the respiration module), and the blood pressure that remains after the blood had passed through the gills has to be high enough to drive the blood around the body.