Emergency! Heart-pounding action.

9:00 P.M. Blood-curdling sirens rip through the crisp evening air. Seconds later, an ambulance pulls up. The patient, a stabbing victim, needs immediate medical treatment. Paramedics whisk him inside.

These people aren't actors on the hit TV series ER. They're the players in a real emergency room at Elmhurst Hospital in New York City, where the drama - and trauma - can be heart-stopping, literally.

Here, 24 hours a day, doctors and nurses treat patients with stabbings, heart attacks, and other conditions that can traumatize the body, especially its circulatory system - the heart and its network of blood vessels (see diagrams, p. 18). This system carries oxygen and nutrients, vital for life, to all parts of the body, including the brain. Deprived of oxygen, the brain can suffer irreparable damage in four to six minutes. So when something disrupts the body's circulation, E.R. doctors and nurses work fast.

9:03 P.M. Just as my nightbeat reporting begins, the E.R. team attends to the newly arrived case, Mark Franklin,* a 15-year-old with a stab wound to his abdomen.

Mark doubles over in pain; blood oozes down his side. "This guy tried to rob me," he gasps. "He had a knife up his sleeve ... and when I fought back he stabbed me." As Mark tells his story, a doctor cuts off his clothes and checks for other wounds.

The real danger: shock, a potentially life-threatening drop in the amount of blood or oxygen reaching vital organs.

9:04 P. M. Dr. Larry Stiefel, the E.R. doctor in charge, calls out orders in a language I don't quite understand. "Set up a normal saline IV, and let it flow wide open!" Translation: Get lots of fluid flowing through Mark's veins - fast!

To get fluid in, Stiefel wraps a tourniquet around Mark's arm and prepares to stick a needle into one of his bulging veins. "This will hurt for just a second," Stiefel says as the needle pierces Mark's skin. The needle and an attached tube hook up to a one-liter bag of intravenous (IV) fluid, a salt-water solution that flows into Mark's veins (that's why it's called intra-venous). This solution mimics the blood's concentrations of sodium and chloride (two substances that help keep cells functioning properly), and replaces the lost volume of blood.

9:06 P.M. Stiefel checks for signs of shock:

* A drop in blood pressure, the pressure of blood pushing against the walls of blood vessels, would mean the volume of blood is too low.

* An increased heart rate would signal that the heart is beating faster and faster to pump the low volume of blood through the system.

Both Mark's blood pressure and heart rate are steady, Stiefel says, after taking readings with a stethoscope and blood-pressure cuff, and hooking up the teen to a cardiac (heart) monitor.

9:17 P.M. Now at he knows Mark is in no immediate danger, Stiefel turns to examine the wound. He measures its depth by carefully probing the wound with a sterile swab. Mark grimaces; I feel faint. "Sorry, almost done," Stiefel says. "We just need to make sure the knife didn't puncture anything important." Massive bleeding on the inside (internal bleeding) is usually more serious than a surface wound because the bleeding can go undetected. Lucky for Mark, the wound is minor - only 0.5 cm deep, not enough to puncture his intestines, liver, or spleen.

9:50 P.M. With IV tubes poking through his skin, heart monitor beeping steadily, and his wound "dressed" with sterile gauze, Mark finally calms down. "We'll keep him here overnight just for observation," Stiefel says.

Meanwhile, 10 more patients have checked into the E.R. Like a quarterback ready to make a play, Stiefel claps his hands. "OK," he says. "Next." He leaves to tend to his next case. I head to the vending machines for a juice break.

10:25 P.M. Bam! The E.R.'s automatic doors swing open. Gene Sloan, a 75-year-old man with a history of heart trouble, is rushed in on a stretcher. Sloan looks worried. He says he's fainted three times in the past two days.

Dr. Neal Oster, another E.R. physician, hooks Sloan up to an electrocardiogram (EKG), a machine that gives a readout of the heart's electrical activity, in addition to the rhythm of its beats. Sloan's heart rate is 20 beats per minute. It should be 80, Oster says. The problem, says the doc, is with Sloan's natural pacemaker - a bundle of cells in the heart that sends out electrical signals to trigger cardiac muscle contractions ("beats"). Sloan's pacemaker fails to send a regular impulse. So even though his blood vessels contain plenty of fluid, that fluid and its vital contents aren't getting pumped around.

"If he hadn't come in he might have gone into cardiac arrest [a ceasing of heart function]," says Oster. Those fainting spells were warnings that his heart "wasn't pumping enough blood to his head," he explains. Oster's recommendation: Sloan needs an artificial (battery-operated) pacemaker.

11:00 P.M. Doctors quickly scrub and give Sloan an anesthetic so he doesn't feel pain. They make a tiny puncture in a Sloan's neck. Through the vein, they carefully insert a thin wire and thread it to the edge of Sloan's heart. With the device in place firing electrical signals, Sloan's heart contracts normally again - pumping blood through his body every three-quarters of a second.

12:00 A.M. In the recovery room, Sloan looks weary, but his spirits are up. He tells his sister to go home and get some rest. I could use some shut-eye too.

12:05 A.M. As I leave the E.R., completely drained, I realize I've only been there three hours. Tonight's E.R. staff has at least nine hours to go. An ambulance heads up the street, sirens blaring. Whoopwhirrrr. The emergency room lights shine bright as day.

THE HEART OF THE MATTER

At the heart of saving lives is making sure a patient's circulatory system is working right. In a healthy person

* The heart, the central pump, squeezes oxygen-rich blood (red) through arteries to tiny capillaries in all body tissues.

* Cells in these tissues take in the oxygen and nutrients, and dump wasters like carbon dioxide.

* The capillaries connect to form veins, which carry this oxygen-depleted, waste-laden blood (blue) back to the right side of the heart.

* The heart pumps this blood to the lungs to dump the carbon dioxide and pick up more oxygen.

* The blood then returns to the left side of the heart to be pumped around the system again.