Any physical activity or movement of some sort, “activates” most of the physiological systems in the body, such as the respiratory, cardiovascular, and musculoskeletal systems, as well as hormonal and immune responses. Regular exercise results in the inherent chemistry of these systems responding, and adapting, to the increased activity, which benefits their efficiency and capacity.

What exactly happens during exercise in these various systems, and how does it promote overall health and wellness?

Effects of exercise on the respiratory (pulmonary) system:

The respiratory system provides oxygen to tissues in the body and removes carbon dioxide from tissues, through diffusion of oxygen and carbon dioxide between the blood and the alveoli in the lungs.

Any physical activity increases the body’s need for oxygen, that odourless and colourless gas in the air, that sustains life on Planet Earth. When exercising, pulmonary ventilation increases immediately. At lower levels of exercise, the increase in ventilation results from increases in tidal volume (taking deeper breaths). At higher intensities of exercise, the respiratory rate (taking faster breaths) increases in a linear fashion according to the demand for more oxygen.

For an unfit adult the pulmonary ventilation rates can increase from about 10 liters per minute at rest, to more than 100 liters per minute at maximal rates of exercise, while for a fit adult male the pulmonary ventilation rates can reach more than 200 liters per minute at maximal rates of exercise.

The oxygen consumption at maximal rates of exercise still leaves a safety margin below maximum breathing capacity. This means the respiratory system is not a limiting factor in the delivery of oxygen to the muscles during strenuous exercise. Long term regular exercise results in a stronger and more efficient respiratory system, with an improved ability to draw in deeper and longer breaths.

Effects of exercise on the cardiovascular system:

The cardiovascular system, consisting of the heart, blood vessels and blood, delivers oxygen and nutrients to all cells in the body. Exercising muscles need increased oxygen extraction from arterial blood as it passes through the muscle. The demand to get oxygenated blood quicker to the muscles leads to an increased heart rate – with faster contractions and a resulting increase in blood circulation. At the same time, more forceful heart contractions with each heartbeat, contributes to a greater amount of blood being pumped through the body and an increase in blood pressure, but only up to the point where it reaches maximum cardiac output.

Cardiac output refers to the total volume of blood that is pumped by the left ventricle of the heart, and can be calculated by the heart rate (number of beats per minute) and the stroke volume (volume of blood pumped per beat). Blood pressure refers to the pressure that is exerted against the arterial walls of the vascular system. Increases in blood pressure during exercise increases the blood flow through the body and the workload of the heart.

Regular exercise results in the cardiovascular system becoming more efficient at pumping blood and the delivery of oxygen, as is seen, for example, with a decreased heart rate of a fit person at rest, indicating that the stronger heart needs less heartbeats to circulate blood through the body.

Exercising not only results in increased blood flow, but also changes the pattern of blood flow dramatically. More blood is sent to the active skeletal muscles and to the skin as the body’s temperature increases. At rest the muscles and skin receive about 20% of cardiac output, which increases to about 80% during maximal rate of exercise. Increased metabolism and muscle contraction during exercise generates heat that needs to be dissipated via the skin. The increased blood supply to the skin acts as a heat exchanger between the warm blood and cooler outside air – this is vital for body heat loss during exercise. In addition, this is aided by the evaporation of sweat during strenuous exercise.

Effects of exercise on the musculoskeletal system:

The musculoskeletal system defines the body and provides movement when the muscles contract in response to stimuli from neurotransmitters, as well as electrical and hormonal stimuli. Muscular activities are fueled by the chemical energy provided by adenosine triphosphate (ATP), which is generated by metabolic processes and stored in all the cells of the body. Three energy systems are involved in generating ATP in muscles, namely the ATP-phosphocreatine (ATP-PCr) system, the glycolytic system, and the oxidative system (also known as the aerobic system), all of them providing energy during physical activities.

For the first few seconds of muscle contraction, energy is released from the ATP present in the cells and converted into a energy providing chemical called adenosine diphosphate (ADP). This breakdown of ATP is replenished when PCr is broken down to release both energy and a phosphate, that allows recycling of ADP back to ATP. This is the primary and high rate energy system for short duration high intensity exercises, such as sprinting, as ATP and PCr stores in the cells are rapidly depleted within 10-20 seconds. Beyond these first few seconds, new amounts of ATP must be formed.

During high intensity exercise of a longer duration, from 30 seconds to 2 minutes, such as a 800 meter race, the body relies on processes that break down glucose to provide energy. The glycolytic energy system then takes over as it uses glucose, stored in the blood, muscle and liver as glycogen, to produce ATP. This system however, produces lactate (lactic acid), which lowers the pH of blood and muscle and cause the production of ATP to stop. Lactate that accumulates in muscle results in the person experiencing fatigue.

For longer periods of exercise at a lower intensity, such as long-distance running, the oxidative energy system comes into play. This system uses oxygen to produce ATP in the mitochondria of the muscle cells, and can also produce ATP from fat and carbohydrate (and to a small extent protein) metabolism. This system supports muscle usage for as long as the nutrients in the body last and slows down the accumulation of lactic acid.

With all three energy systems at play, why do you get so damn tired when exercising?

 • The answer lies in the metabolic rate (the rate at which the body uses energy), in which the body’s ability for oxygen uptake and the forming of lactate in the blood play a role in experiencing fatigue during exercise.

• Oxygen uptake increases in direct proportion to the rate of work of the muscles, up to the point where the individual’s maximum oxygen uptake (VO2) max) potential is reached, and oxygen cannot be supplied to muscle fibers fast enough to produce all the ATP that is required for further muscle contraction. After exercise has stopped, hard breathing continues as extra oxygen is required to restore metabolic processes.

• When intensity of exercise increases, the rate in which lactate enters the blood from muscle will start to exceed the rate at which it is removed from the blood, called the lactate threshold. Lactic acid that accumulates in the muscle tissue makes muscle contraction painful and results in the onset of fatigue being experienced.

• The maximum oxygen uptake and lactate threshold are much higher for trained people than for unfit people. Fit individuals have a well-developed cardiovascular system that is well perfused with capillaries, resulting in good blood supply to muscles.

• In unfit people the rate at which ATP can be produced in the mitochondria is compromised.

The hormonal system’s response to exercise:

The endocrine system consists of glands that release hormones into the bloodstream, which control and integrate physiological functions in the body, ranging from metabolism and fluid balance to cardiovascular and pulmonary function. The endocrine functions adapt to repeated muscular exercise, and the effects of exercise persist after exercise has ended, continuing during the recovery period.

In response to exercise, many hormones are secreted at an increased rate to ensure the production and mobilization of energy stores, to meet the enhanced metabolic demands. At the same time, insulin (a hormone that regulates glucose uptake by muscles and tissues that use glucose for energy) is secreted at a suppressed rate, while reproductive and growth functions are also inhibited during vigorous exercise, to preserve energy for sustaining the fundamental process of movement and exercise that ensure the body’s survival. Regular daily exercise increases muscle insulin sensitivity and helps to lower blood glucose and insulin levels.

The hormone epinephrine is released during exercise, which increases the amount of blood that the heart pumps and widens blood vessels to enhance the flow of oxygen-rich blood. These effects are supported by the circulating thyroid hormone, T3, which in addition, increase the circulating blood volume, by direct stimulation of erythropoietin synthesis, leading to increased red cell mass. T3 also helps to maintain functional integrity of the vascular endothelium by inducing nitric oxide synthesis.

The increased release of certain hormones during exercise can positively affect your mental state, for example increased testosterone levels can increase confidence and libido. Increased levels of endorphins can reduce sensitivity to pain, as well as reducing tension and anxiety, by inducing a sense of euphoria.

The immune system’s response to exercise:

Like so many other systems in the body, the immune system is very responsive to exercise, depending on the intensity and duration of the physical activity.

Exercise of moderate to vigorous intensity plays an important role in strengthening the body’s immune response, which enhances the ability to fight harmful bacteria, viruses, infections and diseases. Studies have shown that regular exercise has an overall anti-inflammatory and antioxidant effect, reducing the risk of infections.

In contrast, very high intensity exercise such as competition events and other high endurance events, place high amounts of physiological and metabolic stress on the body, and can result in the immune system being less effective in fighting pathogens and diseases, leaving the body at an increased risk for inflammation and other health conditions, even during the recovery period.

The immune system becomes less effective with aging and the accompanying immune dysregulation due to aging is known as immunosenescence. Research has indicated that regular exercise by older people can improve the functioning of the immune system and delay the onset of immunosenescence.

Exercise also affects the gut microbiome, which forms part of the immune surveillance system. The gastrointestinal tract is colonized by trillions of micro-organisms and the composition and diversity of these gut bacteria are influenced by a variety of factors, such as diet, exercise, genetics, age, gender, antibiotics, health, and disease. Recent studies have indicated that exercise and physical fitness contribute to a diversified gut microbiome and enhances the number of microbial communities.

When people’s metabolism or immune systems do not respond to the benefits of exercise as expected, especially those with diabetes or prediabetes, an imbalance in the micro-organisms in the gut (called dysbiosis) may play a role. In a study by the University of Hong Kong amongst a group of men who were prediabetic as well as obese or overweight, those with a healthier gut microbiome responded more positively to the benefits of exercise in terms of increasing insulin sensitivity and lowering blood glucose levels.

Conclusion:

The chemistry of exercise has a significant impact on the whole body. Long-term changes occur in the body’s metabolism, while the improved functioning of physiological processes in the body leads to improved health and fitness, while lowering the risk of chronic diseases.

Sources:

Physical activity and health: A Report of the Surgeon General. Published 17 November 1999. Chapter 3 of the Report: Physiologic responses and long-term adaptations to exercise. Centers for Disease Control and Prevention. USA. (www.cdc.gov)

Physiological effects of exercise. Published 1 December 2004. BJA Education. Oxford Academic. (www.academic.oup.com)

Exercise and metabolism: what role does the gut microbiome play? Published 29 April 2020. Gut Microbiota for Health. (www.gutmicrobiotaforhealth.com)

Exercise physiology. Published online and updated 20 October 2015. Medscape. (www.emedicine.medscape.com)

The compelling link between physical activity and the body’s defense system. Published May 2019 edition of Journal of Sport and Health Science. ScienceDirect. (www.sciencedirect.com)

Regular exercise benefits immunity – even in isolation. Published 31 March 2020. Science News, ScienceDaily. (www.sciencedaily.com)

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