Question

The demands of exercise necessitate numerous physiological changes in animals, especially in circulation and muscle metabolism. Consider prolonged exercise activity (e.g. long-distance migration or a marathon run in human exercise).

1. Describe the changing demands in muscle metabolism as exercise progresses (from seconds to minutes to hours). How will the fuel used by locomotory muscles change?
2. Describe the associated changes in respiration and circulation in order to meet the increased demand of skeletal muscle. What specific changes in cardiac output and capillaries at the muscle bed take place, and how are these regulated?

Answer 1

Prolonged exercise activity, like marathon running, require a sustained and regular supply of energy to the skeletal muscles. Since, the two organ systems lnvolved inlong distance running are the cardiovascular and the locomotor systems, the main physiological changes take place in these two. To meet the muscles' energy requirements, the circulatory and respiratory systems have to work together.

In the beginning of any race, the first 30 to 60 seconds, the muscles rely on the phosphogenic pathway of energy production. The breakdown of glucose from the blood or the stored glycogen in the muscles, occurs by glycolysis without the use of oxygen. But as the demand for energy increases, the aerobic pathway becomes the chief energy source and fats dominate as the fuel.

Humans metabolize food to produce energy in the form of ATP in the cells. For exercise, human body places a high demand on ATP to fulfil its energy requirements and to support all the corresponding changes in the body. In the beginning, it is the phosphogenic pathway and then the anaerobic glycolytic pathway takes over. The anaerobic pathway provides energy for about 30 seconds to 3 minutes into the race. Therafter, the aerobic pathway takes over as the main source of energy. Aerobic pathway is the slowest of the three pathways and is fully dependent on oxygen. Low intensity exercise like the steady pace of a marathon runner, gets most of the energy from the aerobic pathway which can use fats, carbohydrates as well as proteins as fuel.

Marathon runners obtain above average aerobic capacities, almost 50% more than that of normally active individuals. The physiological mechanisms that aerobic capacity or VO2max consist of are blood circulation and the use of oxygen within the muscle cells. The transportation of large amounts of blood to and from the lungs to reach all the body tissues depends on a high cardiac output and sufficient levels of hemoglobin in the blood. For effective supply of oxygen to the cells, the blood supply must be efficient. The muscle blood supply shouls be efficient too, that is, the capillary bed in the muscles should br dense and rich with blood.In fact, a 20 fold increase in the local blood flow in the skeletal muscles is necessary for endurance athletes to meet the their muscles' oxygen demand.The myoglobins present in the skeletal muscles extract oxygen from the blood and help to store them and use them. The supply of oxygen rich blood to the muscles and the muscles' capacity to use O2 and organic fuels in the body to produce ATP define the aerobic capacity of the marathon runners.

Marathon runners often have enlarged dimensions of heart and decreased resting heart rates that help them to achieve greater aerobic capacities. Due to enlarged dimensions of the heart, there is a higher stroke volume. Initially, when the heart rate increases, there is a decrease in the stroke volume. The maximum heart rate decreases with age and hence the aerobic capacity cannot continuously increase.

The initial increase in blood volume during marathon running can later lead to decreased blood volume as a result of increase in the core body temperature, pH changes in the skeletal muscles, and the increased dehydration associated with cooling during running.Oxygen affinity of the blood depends on the blood plasma volume and an overall decrease in blood volume. Dehydration, temperature and pH differences between the lungs and the muscle capillaries can limit the ability of the runner to use their aerobic capacity.

Other physiological changes affecting marathon runner's VO2max are pulmonary diffusion of O2, mitochondria enzyme activity and capillary density. Another important physiological feature is the fuel store in the body. In the beginning, glucose is the main fuel but later on fats stored in the body become the main fuel for the skeletal muscles in marathon runners.