How Does the Heart Respond to Exercise?
We all do it, you go to the gym and you sweat, maybe leave a little red and blotchy, breathing heavily. Other than this face value assessment, do you think any further on what’s happening in your body during exercise? Here’s a little look inside…
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.
Why Do These Systems Change?
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. Think about it as the electricity that the light bulb needs in order to switch on.
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. Sound complicated? Think about it in simple terms that you already know: your body uses oxygen and food to fuel its movement.
In the process, carbon dioxide is produced, as is heat. If you want to get fancy, this is called aerobic cellular metabolism, but is another thing we know from school: oxygen in, CO2 out.
Fun fact! Your body 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. You know that bad boy, its what makes your muscles sore as your progress through an exercise or even the next day.
Cardiovascular – how does your heart cope with exercise?
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. Basically, it’s the dispatch operator, giving out the oxygen your bringing in by breathing to each area of the body in the right quantities. 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? (answer: it’s not DPD!)
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: 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.
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. Just take a second to think about that. SEVEN times the speed. So that’s five litres of blood every 9 seconds!
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, because it’s not needed.
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 the 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. So when you’re wiping that sweat off in spin class, think about why it’s there.
Your heart will also slightly decrease, the blood flow to organs such as the kidneys and the digestive tract during exercise, because it understands that these are not being used to the same capacity as other parts of the body. This helps facilitate increased blood flow to regions where it is needed when we exercise. Amazing right?
This blog was written by Anna Clayton: Anna works at Bury Physiotherapy Clinic as a Senior Musculoskeletal Physiotherapist and has recently completed a MSc in Advanced Physiotherapy. She teaches regular Pilates classes including a Pink Pilates; specifically for breast cancer patients. At the clinic she offers patients acupuncture alongside other treatment techniques to help people back to normal day to day activities, sports and hobbies - she is all for functional movement! Anna enjoys keeping fit and active with regular running (the odd half or full marathon), occasional cycling and was a rower and heptathlete in her youth.