Exercise Physiology: Body Function Changes Under Stress
Exercise Physiology: Changes in Body Functions
Exercise can be categorized as light, heavy, or severe work. Light work requires up to 4 times the normal oxygen consumption, heavy work between 4-8 times, and severe work an oxygen consumption higher than 8 times the normal uptake (normal uptake = 3.5 mL/(kg·min)).
Energy Production During Exercise
As work increases, more ATP is required for muscle contraction. This can be produced via oxidative metabolism (Krebs cycle and ETC) or via anaerobic metabolism, a less efficient method where glucose is turned into lactic acid. The proportion of energy from aerobic vs. anaerobic metabolism varies based on the exercise type. Fast, explosive actions require anaerobic respiration, while slower, more sustainable exercise uses more aerobic respiration.
Carbohydrates provide most of the energy for muscular exercise, with a smaller contribution from fats. Proteins are generally not used in healthy individuals. Oxygen consumption does not rise immediately as work begins; rather, it increases gradually over several minutes until it matches the muscles’ needs. This creates an oxygen debt, which is repaid at the end of exercise, resulting in a slightly elevated respiratory rate for a prolonged period.
Cardiovascular Adaptations to Exercise
When exercise occurs, the cardiac system must adapt. At rest, cardiac output is 5 L/min, of which only up to 20% goes to the skeletal muscles. In heavy exercise, cardiac output can rise by as much as 80%, with a much greater amount now going to the muscles and less to other organs. The amount of blood flow to the brain remains relatively steady. As exercise begins, vagal activity declines, and sympathetic activity increases, increasing heart rate and mobilizing blood from large veins. Increased sympathetic activity leads to an increased inotropic effect, leading to increased stroke volume, which has a positive feedback effect as a result of Starling’s law.
The increased sympathetic activity of exercise also results in vasoconstriction in most vascular beds, with increased caliber. Meanwhile, the arterioles of the muscle tissue vasodilate, allowing for more blood to reach the muscles.
Fluid and Electrolyte Balance During Exercise
Prolonged exercise leads to water and electrolyte loss through sweat secretion to promote heat loss. The rate of this depends on the work rate, environmental temperature, and humidity. During exercise, gastric secretion is decreased to preserve fluids, but as the overall amount of water in the body decreases, it leads to increasing glucose concentrations, which decreases glucose efficiency. To counteract this, fluids with sodium and carbohydrates (electrolyte solutions) promote retention of fluids, increasing plasma volume and improving rehydration.
Hypobaric Pressure (Low Atmospheric Pressure)
Hypobaric pressure occurs when the atmospheric pressure is less than that at sea level. The highest living populations live at 17,500 ft above sea level, with approximately half the pressure of sea level. It is not possible to live at higher levels due to the risk of chronic hypoxia, as there is less oxygen per breath. At such high pressures, there are also changes to temperature control, heart rate, blood pressure, and many other aspects of the body.
Hyperbaric Conditions (High Atmospheric Pressure)
Hyperbaric conditions, in contrast, occur when the atmospheric pressure is higher than sea level (e.g., diving). For prolonged periods, hyperbaric conditions can result in too much oxygen, resulting in bronchopulmonary dysplasia (damage to lungs) and general cellular damage as a result of oxygen’s toxic nature. Similarly, increased pressure means each breath has more nitrogen; if this level gets too high, we can suffer from nitrogen narcosis, which exerts an effect similar to that of alcohol on the neurons of the CNS. Finally, we can see air embolisms in the blood with changes to pressure (the bends).
Effects of Weightlessness
Weightlessness can lead to muscle atrophy, slowing of the cardiovascular system, decreased RBC production, balance disorders, eyesight disorders, and a weakened immune system. Additionally, fluid redistribution can occur, along with loss of body mass and excess.