Your ability to sustain long periods of endurance exercise without exhaustion is based on maximum oxygen uptake, or VO2 max. This refers to your body's ability to consume oxygen while you perform high-intensity exercise. VO2 max varies by individual, ranging from elite triathletes to cardiac patients who are recovering from heart surgery. Different factors, including environmental and chemicals in your body, influence your VO2 max level.
Oxygen Delivery and Lactate
Without enough oxygen delivered to your body, your ability to sustain high-intensity, endurance exercise decreases because lactic acid, or lactate, begins to build up in your muscles. A high accumulation of lactic acid causes fatigue and muscle cramps to set in, decreasing your performance and VO2 max. As your body adapts to the exercise, your cardiovascular and pulmonary systems become more efficient in consuming more oxygen and removing carbon dioxide. Also, your cells' mitochondria -- the organelles that provide energy -- increase the rate of work, which enhances fat metabolism and decreases lactate buildup, according to "Medicine and Science in Sports and Exercise."
Cardiac output, which is one of the biggest factors that influences VO2 max, is the volume of blood that your heart pumps out in one minute. As you become more conditioned, the space in both ventricles of the heart increases and the walls of the chambers thicken. The elasticity of the heart also increases, which allows the heart to pump more blood out with greater force.
Pulmonary diffusion is the exchange of gases between your lungs and blood. Although it isn't a limiting factor in the average person, pulmonary diffusion is a major limiting factor that affects VO2 max for elite endurance athletes. This is because they have a very high cardiac output, which shortens the time period for blood to pick up oxygen in the lungs. This causes a low oxygen volume in the blood.
Sweating is your body's way of cooling down, however, you're also losing a large volume of water. Without rehydration, your muscles and central nervous system overheat, causing your heart rate to increase while pumping out the same volume of blood. This leads to fatigue, which decreases performance and oxygen intake. Take regular sips of water to avoid dehydration as you train.
Environmental factors, such as temperature, can play a major role in enhancing your VO2 max. According to a study published in 2010 in the "Journal of Applied Physiology," cyclists whose bodies have adapted to a heated environment had a 5 percent increase in VO2 max when they were tested in a cool environment. When they were tested in a hot environment, they had an 8 percent increase in VO2 max. The cycling group that wasn't exposed to a heated environment had no change in VO2 max.