Pathways of Energy Production According to Workload
The method of energy production (ATP resynthesis) is dictated by the intensity and nature of the performed work. The main energy substrates are glucose, fatty acids, and glycerol.
- Light work: The aerobic pathway dominates here. Oxygen is delivered to tissues in sufficient volume. The anaerobic phase occurs exclusively at the start of work due to a latent period: blood flow does not have time to instantly adapt to new demands.
- Heavy and static work: Oxygen delivery lags significantly behind the rapidly growing demands of muscle tissue. Under these conditions, anaerobic glycolysis comes to the forefront.
Oxygen Deficit and Oxygen Debt
At the very beginning of any physical activity, the circulatory system is just starting to ramp up. An oxygen deficit occurs, and the body must meet its demands through oxygen-free processes.
After work ceases, the person continues to breathe heavily, forming an oxygen debt. This is the excess oxygen consumption at rest that is used to resynthesize ATP molecules and process accumulated lactate (lactic acid). An important feature: if the work was prolonged and extremely intensive, the final magnitude of the oxygen debt may turn out to be greater than the deficit accumulated at the beginning.
Adaptation of Circulation, Respiration, and Blood Composition
Global adjustments in hemodynamics occur to optimize oxygen delivery. During light dynamic exercise, heart rate, blood pressure, cardiac output, and pulmonary ventilation increase strictly in proportion to the severity of the work. However, during heavy work, pulmonary ventilation increases at a faster rate due to the accumulation of lactate, which shifts blood pH to the acidic side and stimulates the respiratory center.
Blood composition also changes to improve gas transport:
- Erythrocytes are released into the vascular bed from depots.
- Leukocyte count increases (myogenic leukocytosis develops).
- Circulating plasma volume decreases as the liquid portion of the blood is filtered into surrounding tissues.
- The ability of hemoglobin to release oxygen to cells increases.
Neurohumoral Regulation
Energy supply is impossible without the participation of the central nervous system and two major endocrine axes:
- Sympathoadrenal system: The adrenal medulla secretes catecholamines into the blood (predominantly epinephrine, to a lesser extent norepinephrine). Epinephrine stimulates heart and vascular function, triggers cAMP production, and mobilizes fats and glycogen from tissue stores.
- Pituitary-adrenal mechanisms: The adenohypophysis increases the synthesis of adrenocorticotropic hormone (ACTH). This, in turn, causes the adrenal cortex to release corticosteroids. These hormones increase the performance capacity of muscle fibers and accelerate glycogenolysis in both muscles and the liver.
Energy Expenditure in Different Types of Labor
The level of energy expenditure serves as the basis for classifying occupational activity:
- Heavy physical labor: Requires 4500 to 6500 kcal/day.
- Mechanized labor: Costs the body 3000–4000 kcal/day.
Other forms of activity also exist (work in semi-automated and automated production, conveyor work, remote control, intellectual and operator work). In such forms, energy expenditures are lower and depend on the level of stress and static load.