The body, most specifically the cardiovascular and muscular systems, experience significant metabolic demands from just one single bout of aerobic exercise. There are immediate and acute responses by the body, as well as chronic adaptations, to the stresses of aerobic exercise.
The acute responses by the cardiovascular and respiratory system are experienced in the transition between rest and the start of aerobic exercise.
Immediately during the start of exercise, the cardiac output (Q) increases because of an increase in stroke volume (SV) and heart rate (HR). Cardiac output is the amount of blood pumped in liters per minute, and is the product of stroke volume and heart rate: [Q = SV x HR]. Obviously, the heart rate increases during exercise, but most people don't realize that it actually starts to pick up right before the start of exercise as a reflex. The sympathetic nervous system sends an anticipatory stimulation to get the heart ready for exercise. The stroke volume increases beginning at the onset of exercise, and similarly to heart rate, can increase with the anticipation of exercise. Stroke volume can increase up to 50% - 60% of the resting value.
Oxygen uptake (VO2), which is the amount of oxygen consumed by the body's tissues, also increases to accommodate the metabolic demands. At rest, VO2 is 1 met = 3.5 mL/kg/min. The maximum value for VO2 can be anywhere between 25 - 80 mL/kg/min, depending on age and conditioning level.
There is also an increase in blood pressure and blood flow. Systolic blood pressure is the pressure in the arteries during the heart's contraction, and should increase during exercise. Diastolic blood pressure is the pressure in the arteries during the rest between heart contractions, and can increase or decrease during exercise. However, diastolic blood pressure should never increase over 20 mmHg.
During exercise, there is vasodilation in the active muscles, meaning the blood vessels leading to and in the active muscles dilate to allow more blood to come through and more oxygen to be transported. At the same time, there is vasoconstriction in the other organ systems as a mechanism of prioritization.
Respiration also increases during aerobic exercise, obviously, to meet the new oxygen demands. Specifically, there are significant increases in the amount of oxygen delivered to the tissues, carbon dioxide returned to the lungs, and minute ventilation, which is the volume of air breathed per minute. Minute ventilation increases through an increase in breathing frequency and tidal volume, which is the amount of air inhaled and exhaled per breath. There is also an increase in the diffusion of oxygen from the capillaries into the tissues, as well as an increase in diffusion of carbon dioxide from the blood into the lungs.
After long term aerobic training, the body adapts to become more efficient at meeting the metabolic demands.
The changes to the cardiovascular system include increased maximal cardiac output (Qmax), increased stroke volume (SV), and reduced heart rate (HR) at rest and during sub maximal exercise. There is also and increase in the density of muscle fiber capillaries to support the delivery of oxygen and removal of carbon dioxide.
The maximum cardiac output increases as a result of stroke volume increasing a very significant amount. The increase in stroke volume is achieved from increases in the heart's contractility, elasticity, and chamber volume, as well as an increase in the thickness of the left ventricle, which is the space that holds blood before it's pumped out into the arteries to deliver oxygen and other nutrients. Therefore, the heart can literally fill up with more blood before each beat, at both rest and during exercise.
The increase in stroke volume allows resting heart rate to decrease. If the heart is pumping more blood per beat at rest, it doesn't have to pump as frequently to meet the same resting cardiac output demands. Highly conditioned athletes, for example, have resting heart rates ranging from 40-60 ppm, compared to the average person's resting heart rate of 60-100 bpm.
The more advanced capillary functions ultimately allow more efficiently delivered oxygen, nutrients, and hormones, as well as an increased means for the removal of heat and metabolic byproducts.
↑ Chamber volume and thickness of left ventricle
Can literally fill up with more blood
↓ HR at rest and during submax exercise
Doesn't need to pump as much if not at max because the SV is so much more efficient
↑ Capillarization/Number of capillaries
↑ Delivery of oxygen, nutrients, hormones
↑ Removal of heat and metabolic byproducts
Respiratory adaptations are specific to the exercise type and upper or lower extremity involvement. If the training focuses on the lower extremities, such as running, its unlikely that you will see any adaptations during upper extremity exercises.
If the athlete is training at a maximal level, then there will be an increase in breathing frequency and tidal volume, which is the volume of air displaced with each full respiratory cycle (inhale and exhale).
Bone and Connective Tissue