Cardiovascular responses to acute exercise

What is exercise? How can we define it scientifically? Simply put, exercise is the physical exertion of the body that results in expenditure of energy, and that, in the long term, enhances or maintains fitness and health. In this video, we will explore the immediate physiological responses of the body to different types and intensities of exercise. A single bout of exercise can mean a leisurely hike on Saturday morning, or running a record breaking marathon. Most of us will never even come close to achieving the feats of physical performance displayed by elite athletes, participating in Olympic games or world championships. But, from a physiological perspective, the fundamental responses of the body to exercise are similar in everyone.

Our bodies are made up of 11 organ systems that function in a coordinated, integrated way that allows us to maintain optimal health. A single bout of exercise profoundly changes the function of most of these systems. You don’t have to be a physiologist to recognise that the responses of the musculoskeletal, cardiovascular, and respiratory systems, which will be discussed here, are particularly evident during exercise. But why is that? Our body’s cells, whether muscle cells, brain cells, or liver cells, require oxygen and nutrients in order to carry out their functions. ATP, adenosine triphosphate, is a molecule that acts as the energy currency of the cell.

Using ATP allows us to drive the chemical reactions in the cell that are fundamental to life, including muscle contraction. Cells produce the ATP that drives these reactions by using oxygen, absorbed in the lungs from inhaled air, and fuels, in the form of nutrients absorbed from digested food or released from energy stores, such as adipose tissue. In the process, carbon dioxide is produced, as is heat. This is aerobic cellular metabolism. We can also produce ATP using metabolic pathways that do not require oxygen. This is anaerobic cellular metabolism and causes the production of substances, including lactic acid.

The job of the cardiovascular system is to deliver blood to all tissues of the body, in order that they receive oxygen and nutrients in proportion to their needs. The blood is also the means by which we remove metabolic wastes from our tissues and dissipate heat produced by these thermogenic chemical reactions. When we exercise, the metabolic needs of working muscles change. Their demand for oxygen and nutrients increases, along with the need to remove metabolic wastes. Essentially, they need more blood. So how do we deliver more blood to the tissues? The average human being has five litres of blood.

The pumping activity of the heart means that these five litres are constantly being circulated from the heart to the lungs and back, so that the blood can pick up oxygen and deliver carbon dioxide, and from the heart to all the body tissues and back, so that the tissues can use oxygen and nutrients for metabolism. When you’re lying down or sitting quietly at rest, the heart is pumping five litres of blood, the total blood volume, from each side of the heart, every minute. This is called the cardiac output.

As soon as you increase physical activity, a cardiac output of 5 litres per minute is no longer sufficient and you will not be able to exercise for very long without giving up through sheer fatigue. This is the reason why the heart rate needs to increase with exercise. We need to increase the cardiac output in order to match the metabolic needs of the tissues. Cardiac output is driven by heart rate, the number of times the heart beats per minute, and stroke volume, the volume of blood ejected from the heart every time the heart beats. During exercise, both of these variables increase. Hand in hand with this increased cardiac output, we also see an increase in systolic blood pressure.

Depending on whether you’re exercising at low, moderate, or maximal intensity, the cardiac output can change quite modestly, perhaps by less than two-fold when walking, right up to an incredible seven-fold or more in elite athletes who are exercising maximally. But this is only part of the story. Many changes take place within the working muscles themselves during exercise. Our blood vessels are very dynamic. They literally change their size in response to tissue activity. When you’re sitting at rest, many of the blood vessels in your muscles are narrowed or even closed. This limits the blood flow through the muscles when activity is low.

However, as soon as muscle activity increases, the blood vessels sense metabolic changes in the tissues, causing them to widen, and capillary beds to open, thus allowing a hugely increased blood flow. Again, the purpose of this is to serve the metabolic needs of the tissues. We see major increases in blood flow to the working muscles, the heart, and the skin during exercise, increasing with increasing exercise intensity. Flow to the muscles and heart drives their increased activity, while increased flow to the skin, in combination with sweating, helps to dissipate the increased heat produced during exercise. That is, it helps you to thermoregulate.

Conversely, we see a modest decrease in blood flow to organs such as the kidneys and the digestive tract during exercise, which again helps to facilitate increased blood flow to regions where it is needed when we exercise.