Cardiovascular Response to Exercise: What are the physiological changes that occur?

The Cardiovascular System

The components of the cardiovascular system are the heart, a pumping muscle, and an interconnected network of blood vessels, which are divided into three types: arteries, veins, and capillaries. The heart pumps blood around a closed circuit of vessels as it flows through the "circulations" of the body almost continuously.¹ 

The cardiovascular system's primary functions include supplying oxygen and nutrients to the body, removing carbon dioxide (CO2) and metabolic waste products, maintaining body temperature and acid-base balance, and transporting hormones from the endocrine glands to their respective target organs.² The cardiovascular system also needs to be able to adapt to changes in skeletal muscle activity as rapidly as possible in order to be functional and efficient.

Exercise and the cardiovascular system

Whenever the human body is subjected to any form of physical activity, many interconnected changes occur, especially in the cardiovascular system. When the body engages in regular exercise training, certain adaptations take place in our cardiovascular systems, resulting in an increase in the efficiency and capabilities of the body in general. 

The extent of these alterations is determined by the intensity and duration of exercise, the force or load applied during exercise, and the initial level of fitness of the individual. These adaptations to exercise are also reversible through a process called detraining.³ Therefore, exercise can be used to determine the adequacy of cardiac function, especially for the diagnosis and prognosis for assessing patients with suspected or known ischaemic heart disease. We call it an exercise tolerance test or an exercise stress test.

The entire amount of blood pumped out by the left ventricle of the heart in one minute is referred to as a person's cardiac output. It is considered to be a product of heart rate and stroke volume (the amount of blood ejected by the heart each beat). Exercise causes an increase in oxygen consumption, so the heart has to pump more blood to fulfill this demand, but only to the extent of its maximum capability. Increases in heart rate and stroke volume are responsible for the increase in cardiac output, observed at intensities up to 50-60 percent of a person's maximum heart rate.

Changes in heart rate during exercise

The heart rate increases linearly in response to the increasing intensity of exercise, up until the point where the oxygen supply meets the demand of the muscles. After reaching this point, your heart rate will plateau for the balance of your workout. 

Your maximum heart rate is roughly 220 beats per minute (bpm) minus your age. So, a 50-year-old man's maximum heart rate would be 170 beats per minute (220 – 50 = 170bpm). 

The amount of effort necessary to reach maximum heart rate varies from person to person. An unfit individual, for example, may hit their maximum heart rate when jogging at 8km/h, but a healthy person may reach their heart rate maximum while running at 20km/h. Therefore, if you are a beginner, it is important to know what intensity is easy for you to reach a healthy and safe maximum heart rate. 

When participating in low-moderate level aerobic fitness exercise, heart rates return to normal within 10-20 minutes after the exercise. Similarly, if the intensity of the exercise fluctuates, then heart rates will also fluctuate. In anaerobic training, work periods are typically short (5 - 30 seconds) while intensity is very high. Given the brief duration of each work period and the use of energy obtained through the utilization of anaerobic pathways, heart rates do not increase much and therefore only exhibit mild increases during each work period.⁴

Changes in stroke volume during exercise

Stroke volume refers to the amount of blood that is ejected by the heart with each beat. When lying down, and to a lesser extent when sitting, the values are higher than when standing. This is because while a person is lying or sitting, it is much simpler for blood to return to and fill the heart since the influence of gravity on blood flow is not as strong. The level also increases as soon as a person begins to exercise, and continues to increase with increasing intensity of the activity. 

In most cases, the increased volume of blood returning to the heart is the primary cause of the increase in stroke volume. The total stroke volume grows only up to around 40-60 percent of a person's maximal oxygen consumption (VO2 max), after which it reaches a plateau. This is in contrast to heart rate. However, recent research has revealed that stroke volume in highly trained individuals can continue to expand even when working at near-peak rates.⁵ As a result of low to moderate intensity aerobic fitness exercise, stroke volume returns to resting levels in a manner consistent with heart rate recovery. While in an anaerobic exercise, stroke volume was also altered, but aerobic training always has a greater impact on increasing the level of stroke volume than anaerobic training.⁶

Changes in blood pressure during exercise

Systolic pressure (the force of your heart on your arteries per beat) is the predominant contributor to an increase in mean arterial blood pressure during dynamic exercise, whereas diastolic pressure (the force of your heart on your arteries between beats) remains similar to resting levels during this period. In healthy individuals, systolic blood pressure increases linearly with increasing intensity of exercise, reaching a maximum between 200 and 240 millimeters. In this case, an increase in cardiac output is responsible for the observed increase in mean arterial pressure, which outweighs a concurrent decrease in total peripheral resistance. This is because cardiac output multiplied by total peripheral resistance equals cardiac output plus mean arterial pressure. 

When working at the same pace as nonhypertensive adults, hypertensive (in other words, high blood pressure) patients frequently experience a greater increase in both systolic and diastolic blood pressures. Due to a lesser reduction in total peripheral resistance, the mean arterial pressure in these patients is higher than in the general population. 

Post-exercise hypotension is defined as a drop in blood pressure below pre-exercise resting levels that occurs within the first two to three hours after exercise.⁷ Because of this, physical activity may be critical in lowering blood pressure in hypertension patients, as seen by alteration in blood pressure after an exercise session.

Intensity of Exercise

The NHS recommends getting at least 150 minutes per week of moderate-intensity activity or 75 minutes per week of vigorous aerobic activity. All adults should aim to do strengthening activities that work all the major muscle groups (legs, hips, back, abdomen, chest, shoulders, and arms) at least 2 days a week. It is also better to spread these exercises evenly over 4 to 5 days a week or every day. 

Moderate activity will raise your heart rate and make you breathe faster and feel warmer. These activities include brisk walking, water aerobics, riding a bike, dancing, etc. Most moderate activities can become vigorous if you increase your effort. Examples include running, swimming, walking up the stairs, sports, etc.⁸ The whole point is to spend less time sitting and more time being active. Kids should have at least 60 minutes of moderate to vigorous exercise per day.

Exercise and cardiovascular health in the long run

According to the British Heart Foundation, cardiovascular disease (CVD) is the leading cause of death globally, accounting for a quarter of all deaths in the UK. This totals to more than 160,000 deaths each year - an average of 460 deaths per day, or 1 every 3 minutes in the UK.⁹ It has been shown in multiple epidemiological studies that lower levels of physical activity are associated with an increased risk of most CVD risk factors, such as high blood pressure, excess body fat, dyslipidemia and metabolic syndrome, depression, and type 2 diabetes.¹⁰ High levels of physical activity have also been shown to reduce the risk of CVD mortality in high-risk populations, including those with type 2 diabetes and the elderly.¹¹ 

A sedentary lifestyle, as the researchers point out, is a major contributor to poor cardiovascular health, whereas exercise has numerous health benefits. Long-term cardiovascular benefits of exercise include decreased blood pressure and a lower risk of heart disease. Cardiovascular health and longer life can be improved by regular moderate-intensity exercise performed several times a week.

Summary

The best lifestyle choice to reduce your risk of CVD and boost your overall wellbeing is to follow the NHS recommendation and aim for at least 150 minutes of moderate-intensity activity per week for optimal cardiovascular health. Your cardiovascular system should adjust in as little as 2 weeks if you are an experienced athlete, or up to 4 weeks if you are just getting started with exercising.¹² Not only does regular exercise reduce your risk of CVD, it also helps to improve your sleep, sexual health, and mental health. It is never too late to start. You can start slowly and find ways to incorporate more physical activity into your life.