Lesson

Information of the Heart

Information on the Heart

The existence of the heart was well known to the ancient Greeks, who gave it the name Kardia, still surviving in modern words such as cardiac and tachycardia. Aristotle believed that the heart was the seat of a person’s soul. The Romans modified Kardia to Cor, the latter word still surviving in cordial greetings. The old Teutonic word herton was also derived from Cor and gives us heart via the medieval heorte.

Heart Location

If you answered left chest, you’re wrong! The heart is situated almost dead centre in the middle of the chest. However, the apex or tip of the heart is shifted towards the left chest wall and hits against the ribs during contraction. Consequently, the rhythm is best detected on the left side, just below the pectoralis.

Size of the Heart

This depends. Are you a human? Well, then it is generally about the size of your fist. This is not really very big when you think about the job it does. In some animals, such as horses, the heart size to body size ratio is much greater. This helps explain why horses are such great endurance athletes! The heart is also bigger in champion endurance athletes, due to genetics and training.

In a sense, the heart is really two hearts, the left heart and the right. Both sides pump the same amount of blood, but to different locations at different pressures. The right side (right ventricle) pumps oxygen-depleted blood that has returned from the body to the lungs for reoxygenation. This is a short trip and requires little pressure development, so the right ventricle is rather thin walled like a fireplace bellows. The left side (left ventricle) is the real workhorse, pumping oxygenated blood that has returned from the lungs (the right and left side of the heart are thus connected) to the entire body. That means moving blood through an incredible maze of blood vessels, from the top of the head to the toes! Consequently it must develop more pressure each beat (about 120mmHg at rest). The left heart muscle is thicker as a result, just as your bicep would become thicker if you had to lift heavy weights with it all day.

Heart Rate Control

Unlike skeletal muscle, which is under voluntary control, the heart is an involuntary muscle. Most of us cannot just tell our heart to slow down or speed up (biofeedback training not withstanding). The beating frequency (heart rate) is controlled by the balance of stimulation coming from the sympathetic andparasympathetic branches of the autonomic nervous system. Both nervous inputs to the heart converge on a small area of tissue on the right atrium called the sinoatrial node.

Parasympathetic (rest and recover) stimulation tends to slow down the rate, while sympathetic (fight or flight) input increases the rate (and the force of contraction). Normally, there is a balance between the two inputs, leaning toward the parasympathetic side.

So the average untrained person has a resting heart rate of about 70 as a result of some constant parasympathetic stimulation. Training affects vagal stimulation first, it increases vagal stimuli at rest resulting in a slower resting heart rate. Also, less sympathetic stimulation takes place during periods of activity. Elite endurance athletes may have resting heart rates of 35 to 40. Values of 28 have been reported!

Heart rate is usually in direct ratio to body size. An elephant has a very low HR of 10-15 bpm at rest, whereas a budgerigar may have a resting HR as high as 1,000 bpm. Imagine the size of the stroke volumeof the elephant!

The initiation of activity results first in a withdrawal of the parasympathetic stimulation (up to a heart rate of about 100), followed by an increase in sympathetic stimulation with more intense activity up to the maximum heart rate.

Will training make maximal heart rate increase?

No, the maximum heart rate is not increased by training! (As we get older, our maximum heart rate decreases). The major difference in the endurance-trained heart is a bigger stroke volume. The trained heart gets bigger and pumps more blood each beat.

However, even without any nervous input, the heart will beat automatically, due to some unique features of its membrane physiology. This intrinsic rate is 110-120 bpm. A purely parasympathetic stimulation will result in a heart rate of about 30.