Answer
People who engage in sustained programs of endurance exercise, such as long-distance runners or endurance cyclists, often experience adaptations in their cardiovascular system that can lead to a combination of unusually high stroke volume and a resting heart rate that is unusually low. These adaptations are a result of the body's response to the demands of aerobic endurance activities. Here's why this occurs:
**Increased Stroke Volume:**
Endurance exercise training leads to several cardiovascular adaptations that can increase stroke volume:
1. **Increased Cardiac Muscle Mass:** Endurance training can lead to an increase in the size and strength of the left ventricle, the heart chamber responsible for pumping oxygenated blood to the body. This increased muscle mass allows the left ventricle to hold more blood, resulting in a larger stroke volume.
2. **Enhanced Ventricular Filling:** Endurance training improves the ability of the heart to fill with blood during diastole (relaxation phase). This enhanced ventricular filling is due to greater venous return, increased blood volume, and improved ventricular compliance. A larger preload, as a result of increased ventricular filling, results in a larger stroke volume according to the Frank-Starling law of the heart.
3. **Improved Cardiac Contractility:** Endurance training can enhance the contractile strength of the heart. This allows the heart to pump more blood with each contraction, further contributing to an increased stroke volume.
**Low Resting Heart Rate:**
A resting heart rate that is unusually low is a common adaptation among endurance athletes and is known as "athlete's bradycardia." Several factors contribute to this phenomenon:
1. **Enhanced Parasympathetic Tone:** Endurance training increases the activity of the parasympathetic nervous system (rest-and-digest response). This leads to increased release of acetylcholine, which slows down the heart rate by reducing the rate of depolarization of the sinoatrial (SA) node—the heart's natural pacemaker.
2. **Enlarged Vagal Nerve:** Long-term endurance training can result in physical changes to the vagus nerve, which controls heart rate through the parasympathetic nervous system. This can lead to greater vagal influence on the heart, contributing to a lower resting heart rate.
3. **Increased Stroke Volume:** The increased stroke volume achieved through endurance training allows the heart to pump the same amount of blood at a slower rate, resulting in a lower heart rate at rest.
In summary, sustained programs of endurance exercise lead to adaptations in the cardiovascular system that result in an unusually high stroke volume and a resting heart rate that is unusually low. These adaptations include increased cardiac muscle mass, improved ventricular filling, enhanced cardiac contractility, heightened parasympathetic tone, and an increased vagal influence on the heart. These adaptations collectively contribute to improved cardiovascular efficiency and the ability to sustain aerobic activities for extended periods.
Work Step by Step
People who engage in sustained programs of endurance exercise, such as long-distance runners or endurance cyclists, often experience adaptations in their cardiovascular system that can lead to a combination of unusually high stroke volume and a resting heart rate that is unusually low. These adaptations are a result of the body's response to the demands of aerobic endurance activities. Here's why this occurs:
**Increased Stroke Volume:**
Endurance exercise training leads to several cardiovascular adaptations that can increase stroke volume:
1. **Increased Cardiac Muscle Mass:** Endurance training can lead to an increase in the size and strength of the left ventricle, the heart chamber responsible for pumping oxygenated blood to the body. This increased muscle mass allows the left ventricle to hold more blood, resulting in a larger stroke volume.
2. **Enhanced Ventricular Filling:** Endurance training improves the ability of the heart to fill with blood during diastole (relaxation phase). This enhanced ventricular filling is due to greater venous return, increased blood volume, and improved ventricular compliance. A larger preload, as a result of increased ventricular filling, results in a larger stroke volume according to the Frank-Starling law of the heart.
3. **Improved Cardiac Contractility:** Endurance training can enhance the contractile strength of the heart. This allows the heart to pump more blood with each contraction, further contributing to an increased stroke volume.
**Low Resting Heart Rate:**
A resting heart rate that is unusually low is a common adaptation among endurance athletes and is known as "athlete's bradycardia." Several factors contribute to this phenomenon:
1. **Enhanced Parasympathetic Tone:** Endurance training increases the activity of the parasympathetic nervous system (rest-and-digest response). This leads to increased release of acetylcholine, which slows down the heart rate by reducing the rate of depolarization of the sinoatrial (SA) node—the heart's natural pacemaker.
2. **Enlarged Vagal Nerve:** Long-term endurance training can result in physical changes to the vagus nerve, which controls heart rate through the parasympathetic nervous system. This can lead to greater vagal influence on the heart, contributing to a lower resting heart rate.
3. **Increased Stroke Volume:** The increased stroke volume achieved through endurance training allows the heart to pump the same amount of blood at a slower rate, resulting in a lower heart rate at rest.
In summary, sustained programs of endurance exercise lead to adaptations in the cardiovascular system that result in an unusually high stroke volume and a resting heart rate that is unusually low. These adaptations include increased cardiac muscle mass, improved ventricular filling, enhanced cardiac contractility, heightened parasympathetic tone, and an increased vagal influence on the heart. These adaptations collectively contribute to improved cardiovascular efficiency and the ability to sustain aerobic activities for extended periods.
