Poison ivy itches only people, Octopuses need three hearts
Why does poison ivy itch?
óDebbie 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?
óGlenn, 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 cellsóboth good guys and those
chemically bound to the sapís active ingredient, urushiol. Your deadly T-cell
liquid damages nerve cells and that makes you itch.
Itís 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
plantís 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
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
Wayneís Word: article on
poison oak and ivy by W.P. Armstrong and W.L. Epstein
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?
Altos Hills, California, USA
[NOAA] They make eye contact with you, reach toward you,
respond to you. Octopuses mesmerize people.
People rarely measure an octopusí 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. Humansí 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 thatís why their blood pressure is
low. The octopus managed a fairly good solution by evolving three hearts. Itís
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 oceanó465 million
years agoó before fishes swam the seas, before land plants developed spores,
before vertebrates came ashore. Maybe the octopusesí circulatory system is
inefficient, their blood blue, and their blood pressure high. But they manage to
By the way, an octopus can squeeze through an opening no bigger than its
Kimballís Biology Pages: Animal circulatory systems
New York University: the molecular evolution of anthropoid and molluscan
(Answered Feb. 14, 2003)