David McKay, age 13, of Dotham, Ala., for his question:
WHAT MAKES THE HEART BEAT?
Circulating blood carries nourishment and oxygen to all parts of the body and wastes are then carried back. Handling the vital job of keeping the blood moving is the heart, a very remarkable organ that is made of a special kind of muscle.
The heart beats by contracting and relaxing. This regular action is handled automatically. You don't even have to think about the heat's action.
The automatic nervous system controls the heart's beat. Nervous impulses originate rhythmically in the sioatrial or S A node. Here is where the heartbeat starts with a contraction and where the pace is set. The S A node is called the heart's pacemaker.
The impulse from this first node then spreads through the atria and reaches a second node, the atrioventricular or A V node. A special part of this is called the atrioventricular bundle or the bundle of His. It conducts the beating impulse from the atria and the A V node to the rant of the heart. This system makes the heart contract as a single unit.
The heart is actually two pumps working together. Each side handles a different job. The right side takes blood from the body and pumps it to the lungs, where it picks up oxygen. The left side then collects the blood from the lungs and pumps it out to the body.
Blood entering the right side has carbon dioxide, a waste product that is collected by the blood from throughout the body. In the lungs the carbon dioxide is removed.
An adult's heart is about five inches long, 3 1/2 inches wide and about 2 1/2 inches thick. A man's heart weighs about 11 ounces while a woman's weighs about nine ounces.
The heart 1s located in a slanting position near the front and middle part of the chest. It is wider at the top than the bottom.
Veins bring blood to the heart while arteries carry it out to the body
A baby's heart weighs only about two thirds of an ounce at birth and by two years, its weight is only up to about an ounce and a half. It weighs about 3 1/2 ounces when the child is 9 and about seven ounces by the time the 16th birthday rolls around.
An adult's heart beats about 70 times each minute while an infant's heat rate is up to about 120 times per minute. A 7 year old child's heartbeat is about 90 times each minute.
There are two types of blood pressure: systolic pressure and diastolic pressure. The systolic pressure is the blood pressure when the heart is contracted. The diastolic pressure is the pressure when the heart relaxes between beats.
The most common disease of the heart and blood vessels is called atherosclerosis, and it happens during adult life. The wails of the arteries become thick. The condition also is called hardening of the arteries.