is ahyphal tip that penetrates the host but not the cell
membrane.
Haversian system(osteon) A structural composition
of compact bone that contains a Haversian canal, a
longitudinally arranged vascular channel in the center
of the osteon containing blood vessels and nerves;
Haversian lamella, the circumferentially arranged lay-
ers of osteoid and osteocytes surrounding the canal;
and the canaliculi, fine channels through the bone that
provide a link between osteocytes.
heart A muscular organ composed of cardiac muscle
that pumps blood throughout the body.
heat Kinetic energy in the process of being trans-
ferred from one object to another due to a temperature
difference. It moves in one of three ways: radiation,
conduction, or convection.
heath hen Along with the passenger pigeon, the
heath hen (Tympanuchus cupido cupido), a relative of
the prairie chicken, is an example of extinction by
human disturbance. Last observed on March 11, 1932,
at James Green’s farm in West Tisbury, Martha’s Vine-
yard, Massachusetts, it became the victim of habitat
conversion to cropsand pastureland, as well as being
hunted as game and for sport. It was a ground-
dwelling/nesting meadows seedeater ranging from
Maine to Chesapeake Bay.
See alsoEXTINCT SPECIES.
heat-shock proteins (HSPs) A family of closely
related proteins, widely distributed in virtually all
organisms (plants, animals, microorganisms, and
humans). Even though they are found in widely differ-
ent sources, they show structural similarity. HSP
expression increases in response to physiological
stresses such as rise in temperature, pH changes, and
oxygen deprivation. Many of these stresses can disrupt
the three-dimensional structure, or folding, of a cell’s
proteins, and so HSPs bind to those damaged proteins,
helping them refold back into their proper shapes.
They also help newly synthesized polypeptides fold
and prevent premature interactions with other pro-
teins. HSPs, also called chaperones, aid in the trans-
port of proteins throughout the cell’s various
compartments and aid in the destruction of peptides
specific to tumors or pathogens.hectare A unit of land measurement. One hectare
(ha) is equal to 2.5 acres.hectare 157Illustration of a section through a whole human heart showing
the internal organization of chambers, valves, connecting arter-
ies, and veins. The heart can be regarded as being two pumps:
the right side (left on image) serving the lungs and the left side
pushing blood around the general circulation. Oxygenated blood
entering the heart from the lungs passes through the left atrium
into the left ventricle (bottom right) before being distributed
via the aorta (top, center). Deoxygenated blood enters the right
atrium through the venae cavae (top left) prior to being pumped
to the lungs from the right ventricle.(Courtesy © David Gifford/
Photo Researchers, Inc.)