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What keeps the heart beating; Why
aircraft fly off a carrier when coming into port
Q: What keeps the heart pumping all the time? A friend of mine says
sometimes he is afraid that his heart will stop beating suddenly! Does he
have some reason? Luis, Lisboa, Portugal

The body's #1 pacemaker (1 in the diagram) is a bundle of neurons in the
upper right part of the heart, called the sinoatrial (SA) node. This nerve
bundle controls the beating of the heart. Drawing courtesy of Rod Nave,
copyright, used with permission.
A: The heart's natural pacemaker keeps the heart pumping
— 100,000 beats a day and in an average 77-year
life expectancy, more than 2.8 billion times — without fail. It's a remarkable
'self-firing' bioelectrical-chemical device.
The pacemaker functions much like a repeating
circuit that produces a flashing light on an emergency flashlight, says
physicist
Rod Nave, professor at Georgia State University.
The pacemaker triggers heartbeats in a complicated process, simplified below to
describe the main features:
- Nerve cells in the pacemaker (#1 in the
diagram) charge like the positive plate of a capacitor connected to a
battery, as channels in the cell membrane:
- release fewer positive
potassium ions to the outside and
- allow more positive sodium and
calcium ions into the cells.
The positive charge gradually (over about a
half second) piles up until the voltage reaches nerve-cell firing threshold. The cells fire; current flows through the heart to trigger a
heartbeat. Click here to see an
animated figure
showing how the current flows. After the nerve cells fire, the cell's
electrical charge returns to nearly its rest-state charge, which is slightly negative.
- The electrical signal
flowing through
the heart triggers a release of calcium within each of the
heart's muscle cells. The released calcium "causes the
cell and the heart, as a whole, to contract," says physiologist
Sandor
Gvörke, professor at the Ohio
State University College of Medicine. The heart beats.
Almost immediately, gates close the calcium channels, and block release of
calcium until the next beat.
The muscles
relax.
- The process repeats for the next beat.
If for some reason, the main pacemaker (#1 in the diagram) doesn't function
or the signal gets blocked, then the secondary pacemaker (#2, the AV node) kicks
in. And if that fails, the tertiary pacemakers (#3, the Bundle of His and
the Purkinje fibers) will spontaneously fire to trigger the heart beat.
Finally, if all three pacemakers fail to deliver the trigger signal, the
individual muscle cells can contract.
"The individual muscle cells can contract rhythmically on their own, and
touching heart cells will
beat in unison,"
emails cardiovascular researcher
Gail Sullivan, assistant research professor of medicine at the University of
Virginia Health Science Center.
The pacemaker and its several backups keep the heart beating.
Further Reading:
The sinoatrial node: the body's natural pacemaker by Rod Nave,
HyperPhysics
Molecular defect that may cause heart failure by Sandor Gvörke,
Proceedings of the National Academy of Sciences.
Cardiac pacemaker, Wikipedia
Life expectancy, efmoody.com
Q: Why do Navy aircraft fly off the aircraft carrier before the ship comes
into port? (Steve, Rochester, New York)
 The
CVN-65 Enterprise in the Virginia Capes during carrier qualifications, Dec. 11,
1995. Photo courtesy of Photographer's Mate Airman Nicholas H. Griseto
A: "Aircraft generally fly off whenever the carrier returns to
homeport, and that could be after a short exercise of just a few weeks, when
they practice take-off and landings or in war games, or it could be from a
longer operational deployment, when they may fly combat or perform other
overseas missions,"
Lt. Trey Brown
says in a telephone conversation from the Navy Pentagon in Washington DC.
Aircraft fly off, because the aircraft home base is not at the carrier homeport.
"It's cheaper and simpler for the aircraft to fly off, and head home than to be
lifted off the carrier with cranes."
For example, on May 2, 2006, the nuclear aircraft carrier CVN-65
Enterprise left her home port at Norfolk, Virginia for a six months
deployment in the Middle East and the Western Pacific. During that time
its combat aircraft made 8,300 sorties of which more than 2000 were combat
missions supporting strikes in Iraq, Afghanistan, the Philippines and the Horn
of Africa. Every few weeks, she made port calls, but the planes did not
fly off. At the end of the six months on Nov. 18, 2006, she returned to
her home port. Shortly before coming into Norfolk, her F-14 Tomcats and
FA-18 Hornets flew off the carrier, and streaked across the skies for their home
base at the Naval Air Station Oceana, about 20 miles east.
"The air wing and the carrier are separate navy commands," Brown elaborates
in an email, "and each does most of its training separate from the other. The
aircraft will launch from their home airfield to practice dog fighting or
bombing runs or whatever, and the carrier will sometimes go to sea without its
air wing, so its sailors can practice driving the ship, etc, and so they can
work with pilots who are still training, but not necessarily pilots from their
air wing."
Further Reading:
Navy Office of
Information
Aircraft Carrier Locations, Go Navy
(Answered Jan. 15, 2007)
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