WonderQuest:  On the web since 1997...      

Poison ivy itches only people, Octopuses need three hearts

Why does poison ivy itch? —Debbie and Steve, Pennsylvania, USA

Of what value is the blistering agent in poison ivy to the plant's survival? In the wild, deer and other herbivores don't get itchy blisters from touching poison ivy, do they? Even if they did, no animal would be smart enough to connect the itching with a certain plant. SO how and why did this plant evolve the itchy defense mechanism? —Glenn, Somewhere, World

Poison ivy itches because your confused immune system thinks your body is under attack. Consequently, killer T-cells in your blood stream release enzymes and toxins that lay waste to the surrounds. Fluid oozes from blood and lymphatic vessels, flows over skin, and kills cells—both good guys and those chemically bound to the sap’s active ingredient, urushiol. Your deadly T-cell liquid damages nerve cells and that makes you itch.

It’s a case of false alarm. Poison-ivy sap and its urushiol are basically harmless to everything but us. Deer, goats, horses, cattle, and many birds eat the foliage and fruits of the poison ivy plant. Flea beetles and armyworms chew their leaves, unaffected, says John Meyers of North Carolina State University. These parts are loaded with sap. Humans, though, are a different story. Eighty to 90 % of adults will get an allergic rash. All it takes is 50 micrograms (less than a grain of salt) of urushiol and at least a two-time exposure.

The human-blistering agent (urushiol) in the sap probably has no value to the plant’s survival: an accidental byproduct. Poison-ivy sap evolved as a gooey aid for injuries and a weapon against disease. The resinous sap helps heal plant wounds and may slow growth of infection-causing fungi and bacterial spores.

What to look out for to avoid the plant
Poison ivy: beware of three leaves, photo courtesy of Conservation Commission of Missouri	Poison ivy blossoms, photo courtesy of John Knouse	Poison ivy vine, photo courtesy of Melissa MB Wilkins

Further Reading:

Wayne’s Word: article on poison oak and ivy by W.P. Armstrong and W.L. Epstein

Poison ivy, Great Plains Nature Center

Poison Ivy, Sumac, and Oak Information Center

I know that the octopus has an extremely high blood pressure, but how high is high? What is a typical octopus systolic/diastolic reading? —Arthur, Los Altos Hills, California, USA

[NOAA] They make eye contact with you, reach toward you, respond to you. Octopuses mesmerize people.

People rarely measure an octopus’ blood pressure. I quizzed many experts to find this: The systolic and diastolic pressures of octopuses at rest are 27/15 millimeters of Mercury, says Martin Wells, a reader at the zoology department of Cambridge University and author of Civilization and the Limpet.

It is high for many marine animals but not for mammals. Humans’ systolic pressure (measured when the heart contracts to squeeze the blood out) is about 120 millimeters of mercury. The lowest (diastolic) pressure (measured when the heart relaxes) is about 80 millimeters. The blood pressure of an octopus is about a fifth that of humans. However, the octopus systolic pressure is twice that of a lobster. Mammals developed more efficient circulatory systems than non-mammals and have correspondingly higher blood pressures.

Circulation systems pump stuff through the body via blood. Blood vessels branch repeatedly and get tiny where exchanges take place, for example in the gills or lungs where the blood picks up oxygen and dumps carbon dioxide. The blood pressure drops as the blood spreads out into a jillion small streams (the capillaries). Animals face a problem: If the pump delivers the blood to the gills or lungs with a heady pressure, little force remains to distribute the blood to the rest of the body.

Most fishes never solved the problem and that’s why their blood pressure is low. The octopus managed a fairly good solution by evolving three hearts. It’s got two hearts to force blood through the two gills and then a main heart to force the blood everywhere else. They have another problem, though, that they share with lobsters and insects (but not fishes). The red blood cells are not equipped with hemoglobin (like ours and fish are) but rather with a poor oxygen carrier, called hemocyanin.

Consequently, oxygen deprived, octopuses drift through life along a lazy path. Even sex is not "an energetic procedure in octopuses," says Wells.

Their group (the cephalopods, which also include squids, cuttlefish, and the chambered nautilus) evolved when most life dwelled in the ocean—465 million years ago— before fishes swam the seas, before land plants developed spores, before vertebrates came ashore. Maybe the octopuses’ circulatory system is inefficient, their blood blue, and their blood pressure high. But they manage to kill sharks.

By the way, an octopus can squeeze through an opening no bigger than its eyeball.

Further Surfing:

Kimball’s Biology Pages: Animal circulatory systems

New York University: the molecular evolution of anthropoid and molluscan hemocyanin

(Answered Feb. 14, 2003)

Comment