A single bout of aerobic exercise places a significant metabolic demand on the body, especially on the cardiovascular, respiratory, and muscular systems. With repeated exposure to this acute exercise stress, along with chronic training, the body develops functional and structural (morphological) adaptations across many systems. Understanding the acute effects of aerobic exercise gives you a foundation for recognizing these chronic adaptations.
During aerobic exercise, the cardiovascular system’s main job is to deliver oxygen and nutrients to working muscles and remove metabolic waste. The sections below describe key cardiovascular mechanisms during acute exercise.
Cardiac output () is the total amount of blood the heart pumps each minute. It depends on:
At the start of exercise, cardiac output rises quickly, then more gradually approaches a plateau. At maximal exercise, cardiac output can increase from a resting value of about L/min to as high as L/min in trained individuals.
Two primary physiological mechanisms regulate stroke volume:
During aerobic exercise, venous return increases (due to venoconstriction and the muscle pump). This raises end-diastolic volume, which increases the force of contraction and the amount of blood ejected (the Frank-Starling mechanism). Stroke volume increases with exercise intensity but typically plateaus at about 40-50% of maximal oxygen uptake.
As exercise begins, heart rate increases due to activation of the sympathetic nervous system. The rate of increase is proportional to exercise intensity.
Oxygen uptake () is the amount of oxygen consumed by tissues. It depends on:
Maximal oxygen uptake (max mL/kg/min) is the highest amount of oxygen an individual can utilize during exercise and is a key indicator of cardiorespiratory fitness.
The a- difference is the difference in oxygen content between arterial blood and venous blood. It reflects how much oxygen working muscles remove from the blood and is an important determinant of .
Another way to describe oxygen uptake is the metabolic equivalent (MET), which represents the energy cost of physical activity. One MET is defined as resting metabolic rate, or about mL per kg body weight per minute. Exercise intensity is often expressed in METs, with higher MET values indicating greater oxygen consumption.
During aerobic exercise, systolic blood pressure increases substantially because cardiac output rises. Diastolic blood pressure usually stays about the same or may decrease slightly due to vasodilation.
Mean arterial pressure (MAP) is the average blood pressure across the cardiac cycle:
During aerobic exercise, blood flow is redistributed to match the needs of active tissues:
Up to 90% of cardiac output can be directed to working muscles.
Aerobic exercise increases the demands on the respiratory system. To meet these demands, both tidal volume (air per breath) and breathing frequency increase.
Gas exchange is driven by partial pressure differences between alveolar air, arterial blood, and tissues. Oxygen diffuses from the blood into working muscles, while carbon dioxide diffuses from the muscles into alveolar air to be expired.
Oxygen is transported in the blood either dissolved in plasma or bound to hemoglobin. Because oxygen is not very soluble in plasma, only about 3% is carried this way. Most oxygen is carried by hemoglobin, which can bind about 1.34 mL of oxygen per gram of hemoglobin.
Carbon dioxide is transported in a similar overall pattern: a small amount is dissolved in plasma, while most is carried as bicarbonate ions (HCO₃⁻) or bound to hemoglobin. Formation of bicarbonate is important for maintaining blood pH during exercise.
Lactate is another metabolic by-product that can accumulate during high-intensity exercise. When lactate production exceeds lactate removal, blood lactate concentration rises, marking the onset of blood lactate accumulation (OBLA).
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