Heart Basics


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What’s in a Name?

The existence of the heart was well known to the 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 the soul and the center of man. 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.

Where is it Located?

Dumb question right? Well if you answered left chest, you’re wrong! The heart is situated almost dead center in the middle of the chest nested between the two lungs. 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.

How Big is it?

It is generally about the size of your fist. This is not really very big when you think about the size of the body it serves. 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.  The average untrained heart can pump about 15 to 20 liters of blood per minute at max.  Large, elite athletes like champion rowers may have a maximal cardiac output of nearly 40 liters/min.  This is a huge flow moving through a pump the size of your fist!  To get some perspective on these output rates, go to your kitchen sink and turn on the water full blast.  Now find a milk jug or something that will give you a measure of volume.  I bet you find that your faucet does not flow as fast as the heart can pump.

In a sense, the heart is really two linked pumps, the left heart and the right. Both sides pump the same amount of blood, but to different locations at different pressures. The right side pump (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.

How Does it Pump Blood?

Classically, we have been taught that the heart squeezes blood through the aorta by decreasing the external circumference of the heart. This view is supported by the fact that during heart surgery (with the chest cracked open), the heart does pump in this manner. However, under normal conditions, the heart operates within the thoracic cavity in a closed, fluid-filled volume. There is now growing evidence to indicate that during exercise, the heart performs more like a piston or a vacuum pump, with little change in external circumference. As we learn more about the dynamics of heart function, it is evident that this model is critical to the efficiency of the heart as a pump. More recent models of heart performance indicate that the heart takes advantage of vacuum effects and fluid inertia as heart rate increases during exercise. One reason why artificial hearts have performed so poorly is that they have tried to use a design based on erroneous assumptions about how the human heart pumps. The classical view of heart pumping mechanics will die slowly, due to its pervasiveness. However, it seems reasonable to say that the heart performs more like a vacuum pump then like a hand squeezing the juice out of a lemon.

ls the Heart Rate?

Now this is a tough question to answer without using a little physiology lingo. 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 and parasympathetic branches of the autonomic nervous system. Both nervous imputs to the heart converge on a small area of tissue on the right atrium called the sino-atrial 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). At rest, there is normally a balance between the two inputs leaning toward the parasympathetic side. However, even without any nervous input, the heart will beat automatically due to some unique features of its membrane physiology. This intrinsic rate is quite slow however (about 20 bpm). A purely parasympathetic stimulation will result in a heart rate of about 30. So the average untrained person has a resting heart rate of ~70 bpm as a result of some constant sympathetic stimulation. With training, the balance between parasympathetic and sympathetic stimulation tends to shift in favour of the parasympathetic, resulting in a slower resting heart rate. Elite endurance athletes may have resting HRs of 35 to 40. Values of 28 bpm have been reported!

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.  A number of studies have demonstrated that maximal heart rate actually tends to DECREASE with high volumes of endurance training.  The average of a number of studies is about a 7 beat reduction in maximal heart rate after training compared to the untrained state.   Anecdotally, it also appears that even in athletes, periods of very high volume can transiently cause a further reduction in the maximal heart rate, or perhaps more correctly a reduction in the capacity of the sympathetic nervous system to maximally mobilize the heart rate.  We have tested junior XC skiers before and after a 10 day training camp filled with an abnormally (for them) high training volume.  On average, the team showed a slight reduction in VO2 max despite being very fit, and their maximal heart rate during a VO2 max test was perhaps 4 beats per minute lower.  The athletes were very fit, but could not fully mobilize; the lacked that last gear.  After a few days of relative rest, they were back to normal.

Will Training Make My Maximal Heart Rate Increase?

The answer to this question has just been answered.  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.  So, that small reduction in maximal heart rate is more than compensated.


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