Everyone knows that familiar wobbly feeling when you've bounced out of bed too fast. But with a slight thumping of your heart and a gentle squeezing of the veins in your legs, you've effortlessly got your blood back to where it is needed, and you're on your way. For a long time researchers assumed that thisbaroreceptor reflex', which helps our bodies maintain a stable blood pressure, evolved so that terrestrial creatures like us could deal with gravity constantly dragging our blood into our boots. But now it looks like fish are in on the trick too. Erik Sandblom and Michael Axelsson of Göteborg University in Sweden have shown conclusively that rainbow trout also have a baroreceptor reflex'(p. 821).

Sandblom has always loved fish and is intrigued by the mechanisms that allow these creatures to keep their blood pressure stable. Fish circulatory systems differ from mammalian systems: their heart only has two chambers instead of four. Fish oxygenate their blood by directing it through their delicate gills before pumping it around the body. Trout don't usually have to cope with blood pooling in their tails – their blood is the same density as the water they swim in – but they still need to regulate blood flow to their organs.

To find out how fish regulate their blood pressure, Sandblom and Axelsson needed to find a way to control blood flow in free-swimming fish. Studying a non-anaesthetised fish's intricate system of blood vessels is notoriously difficult, but Sandblom and Axelsson weren't put off. Sandblom popped down to the local fish farm, loaded up a shoal of rainbow trout and headed back to the lab. After months of intensive work, the team finally developed a method that enabled them to increase or decrease gill blood pressure. They were ready to test the trout's baroreceptor reflex'.

Placing a few trout in the spotlight, Sandblom put them through their paces. He first temporarily closed the ventral aorta that sends blood to the gills, and noticed that, within seconds, a reflex had kicked in and the trout's heart was thumping faster and its veins were constricting to compensate for the drop in blood pressure. The opposite happened when Sandblom blocked the dorsal aorta that takes blood away from the gills. The forced pressure increase in the gills caused the trout's heartbeat to gently decrease. Sandblom and Axelsson are convinced that this is truly a reflex response and that they have hit on what might be driving the trout'sbaroreceptor reflex'. Using the drugs prazosin and atropine, they went a step further and showed that the distinct blood vessel and heart rate responses are controlled by different nervous control systems.

Sandblom is pretty excited about his technique. It's a neat tool that can be used for many different purposes' he enthuses. He now wants to see what happens to the veins when fish are a little low on oxygen. Sandblom suspects that since fish live in a gravity-free environment the baroreceptor reflex'simply evolved to keep arterial blood pressure at a certain level, and not just to help us legged creatures defy gravity and saunter around on land.

Sandblom, E. and Axelsson, M. (
2005
). Baroreflex mediated control of heart rate and vascular capacitance in trout.
J. Exp. Biol.
208
,
821
-829.