The cardiovascular system provides the link between pulmonary ventilation and oxygen usage at the cellular level. During exercise, efficient delivery of oxygen to working skeletal and cardiac muscles is vital for maintenance of ATP production by aerobic mechanisms. The equine cardiovascular response to increased demand for oxygen delivery during exercise contributes largely to the over 35-fold increases in oxygen uptake that occur during submaximal exercise. Cardiac output during exercise increases greatly owing to the relatively high heart rates that are achieved during exercise. Heart rate increases proportionately with workload until heart rates close to maximal are attained. It is remarkable that exercise heart rates six to seven times resting values are not associated with a fall in stroke volume, which is maintained by splenic contraction, increased venous return, and increased myocardial contractibility. Despite the great changes in cardiac output, increases in blood pressure during exercise are maintained within relatively smaller limits, as both pulmonary and systemic vascular resistance to blood flow is reduced. Redistribution of blood flow to the working muscles during exercise also contributes greatly to the efficient delivery of oxygen to sites of greatest need. Higher work rates and oxygen uptake at submaximal heart rates after training imply an adaptation due to training that enables more efficient oxygen delivery to working muscle. Such an adaptation could be in either blood flow or arteriovenous oxygen content difference. Cardiac output during submaximal exercise does not increase after training, but studies using high-speed treadmills and measurement of cardiac output at maximal heart rates may reveal improvements in maximal oxygen uptake due to increased stroke volumes, as occurs in humans. Improvements in hemoglobin concentrations in blood during exercise after training are recognized, but at maximal exercise, hypoxemia may reduce arterial oxygen content. More effective redistribution of cardiac output to muscles by increased capillarization and more efficient oxygen diffusion to cells may also be an important means of increasing oxygen uptake after training.
ADAPTATIONS IN THE CARDIOVASCULAR SYSTEM
Following training the cardiovascular system and its components go through various adaptations. Here are the most important:
The hearts mass and volume increase and cardiac muscle undergoes hypertrophy.
It is the left ventricle that adapts to the greatest extent. As well as the chamber size increasing as a result of endurance training, more recent studies show that the myocardial wall thickness also increases.
Resting heart rate can decrease significantly following training in a previously sedentary individual. During a 10-week exercise program, an individual with an initial resting heart rate of 80beats/min can reasonably expect to see a reduction of about 10beats/min in their resting heart rate.As mentioned earlier, highly conditioned athletes such as Lance Armstrong can have resting heart rates in the low 30s.
During submaximal exercise, heart rate is lower at any given intensity compared to pre-training. This difference is more marked at higher relative exercise intensities. For example, at low work rates there may only be a marginal difference in heart rate pre and post training. As intensity reaches maximal levels, the difference can be as much as 30beats/min following training.
Maximum heart rate tends to remain unchanged by training and seems to be genetically limited. However, there are some reports that maximum heart rate is reduced in elite athletes compared to untrained individuals of the same age.
Following an exercise bout, heart rate remains elevated before slowly recovering to a resting level. After a period of training, the time it takes for heart rate to recover to its resting value is shortened. This can be a useful tool for tracking the effects of a training program. However, it is not so useful to compare to other people as various individual factors other than cardiorespiratory fitness play a role in how quickly heart rate returns to a resting level.
Stroke volume increases at rest, during submaximal exercise and maximal exercise following training. Stroke volume at rest averages 50-70 ml/beat in untrained individuals, 70-90ml/beat in trained individuals and 90-110ml/beat in world-class endurance athletes.
This all-round increase in stroke volume in attributable to greater end-diastolic filling. This greater filling of the left ventricle is due to a) an increase in blood plasma and so blood volume (see below) and b) reduced heart rate which increases the diastolic filling time.
According to the Frank-Starling mechanism, this increased filling on the left ventricle increases its elastic recoil thus producing a more forceful contraction. So not only is the heart filled with more blood to eject, it expels a greater percentage of the end-diastolic volume (referred to as the ejection fraction) compared to before training.
If heart rate decreases at rest and during submaximal exercise and stroke volume increases, what is the net effect on cardiac output?
In actual fact, cardiac output remains relatively unchanged or decreases only slightly following endurance training. During maximal exercise on the other hand, cardiac output increases significantly. This is a result of an increase in maximal stoke volume as maximal heart rate remains unchanged with training. In untrained individuals, maximal cardiac output may be 14-20L/min compared to 25-35L/min in trained subjects. In large, elite athletes, maximal cardiac output can be as high as 40L.min.
Skeletal muscle receives a greater blood supply following training. This is due to:
- Increased number of capillaries
- Greater opening of existing capillaries
- More effective blood redistribution
- Increased blood volume
- Blood Pressure
Blood pressure can decrease (both systolic and diastolic pressure) at rest and during submaximal exercise by as much as 10mmHg in people with hypertension. However, at a maximal exercise intensity systolic blood pressure is decreased compared to pre-training.
It is interesting to note that although resistance exercises can raise systolic and diastolic blood pressure significantly during the activity, it too can lead to a long-term reduction in blood pressure.
Endurance training increase blood volume. While plasma volume accounts for the majority of the increase, a greater production of red blood cells can also a contributory factor. Recall that hematocrit is the concentration of hemoglobin per unit of blood. An increase in red blood cells should increase hematocrit but this is not the case. Because blood plasma increases to a greater extent than red blood cells, hematocrit actually reduces following training.