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

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)

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)