High blood pressure is a silent killer, with no known cause in 90% of sufferers! As humans are the only mammals known to naturally suffer this disease, it's proved difficult to track down the causes. But Ken Olson has found that despite the differences between man and trout, the fish could turn out to be our greatest ally against hypertension(p. 457). The fish's venous system has muscular tone; a major player in hypertension!

When our ancestors left the sea, they had to raise their blood pressure to keep delivering blood to organs against the effects of gravity. Increased blood pressure has led us to develop tightly sealed blood vessels, and a lymph system to recirculate blood products that have seeped out. Although fish with low blood pressure shouldn't need either of these adaptations, Olson wondered if fish blood vessels are impermeable to blood proteins, and whether their secondary circulatory system might double up as the fish equivalent of a lymph system.

Olson decided to answer both these questions by testing how the fish responded to changes in blood volume. He monitored the red blood cell levels(hematocrit) in spleenectomised fish. When their blood volume increases the spleenless fish can't raise their red blood cell levels to maintain their hematocrit levels, so their hematocrit falls. Measuring the fall in hematocrit gives a good measure of the change in the fish's blood volume.

First he tested whether the fish's secondary circulatory system functioned like a mammalian lymph system. Olson figured that if he increased the fluid levels in the fish's circulatory system the fish could reduce its blood volume to normal levels if the secondary system functioned as a lymph system by slowly leaking fluid across the capillaries. Olson boosted the fish's blood volume with saline solution or trout blood plasma, and watched how quickly the fish lost the excess volume. Fish injected with saline recovered faster than fish injected with plasma, and the plasma-injected fish lost the extra volume much faster than expected. The fish recovered too quickly for the secondary circulation to function as a lymph system, and their vessels were permeable to plasma proteins.

But what happens when the fish experience a sudden drop in blood volume?Olson monitored the fishes' hematocrit levels continuously after the fish had lost blood, and was amazed to see the fish recovered half the lost blood volume almost instantaneously! Their recovery was simply too fast for the fluid to have leaked in through the blood vessel walls.

This was when Olson realized that the fish have venous tone. He explains that the only way the fish could replace the lost fluid so quickly was if the tissue's microcirculatory system stored fluid and the vessels had muscle tone. As soon as the fish suffers a sudden drop in blood volume, the storage system's tiny vessels contract, rapidly delivering the fluid to make up the blood loss.

When Olson set out to lay to rest a few long-standing problems in the fish cardiovascular system he had no idea that he would stumble across the ideal comparative system for hypertensive research, but he hopes his discovery will help him to begin unraveling the complex hormonal networks that regulate human blood pressure.

Olson, K. R., Kinney, D. W., Dombkowski, R. A. and Duff, D. W. (
). Transvascular and intravascular fluid transport in the rainbow trout: revisiting Starling's forces, the secondary circulation and interstitial compliance.
J. Exp. Biol.