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Today we're going to take a look at the processes that keep you in motion, and see how today's exercise generates changes in your body. As soon as you decided to go for a run, your adrenal glands began flooding your body with epinephrine – otherwise known as adrenaline. This hormone tells your heart to beat faster and harder, which increases blood flow to your muscles and lungs. It also makes it easier to breathe and absorb oxygen, as well as creating the "fight or flight" reaction, known as an adrenaline rush.
Take a moment to pay attention to your posture; you may think that your leg muscles are doing all the work, but it's the muscles in your backside that keep you upright. Humans are the only mammals in the world that run on two legs, an evolution that allowed us to carry things, such as spears, while we ran. Nowadays, we tend to carry phones instead!
At this comfortable pace, your body is performing aerobic exercise; your cells use oxygen to turn glucose into energy. Glucose is the body's preferred source of energy. Your body may tap into fat stores too, but you still need glucose to burn it. Your muscles keep a little bit of glucose stored so that it's readily available, but by now you've already used your initial stockpile. This means your body is diverting energy away from non-urgent places, such as your digestive system, and you're now getting glucose from your liver. If you put on a burst of speed, the higher intensity means your cells don't have enough oxygen to keep up the pace for long so they've compensated by transitioning to anaerobic exercise. This is a less efficient method of making fuel, and creates lactic acid, which is often a source of running-related pain and soreness. This isn't the only source of pain though; microscopic muscle damage also occurs. This will eventually lead to stronger, denser muscles.
As your cells turn glucose into energy, the chemical reaction raises your body temperature and causes you to sweat. When the water in sweat evaporates, it leaves behind a cooled surface. This is why running feels so much warmer in humid climates: your sweat doesn't evaporate as quickly when the air is full of moisture. Your blood vessels have also expanded to let more blood flow close to the surface of your skin, and carry heat away from your body's core.
Hormones are responsible for nearly all of your bodily functions. They come from specialised glands located throughout your body, as well as your pancreas. These make up the endocrine system. Along with epinephrine, which gets your heart and lungs ready for action, norepinephrine raises your blood pressure to bring resources to your muscles as quickly as possible. Insulin and glucagon come from the pancreas; glucagon tells your live to put glucose into the bloodstream, while insulin tells your cells to grab that glucose so it can be burned for fuel. Endorphins come from the pituitary gland in the brain and act as natural painkillers, causing that state known as the "runner's high".
This circulatory activity floods nutrients to your brain's tiniest blood vessels, making them stronger, and even creating new ones! This improved blood flow will affect your mental acuity now and in the future.
Small scale muscle damage in the form of micro-tears trigger the release of human-growth hormone, or HGH. This facilitates muscle recovery and strengthening, and allows cells to take all of the resources they need to repair themselves. It also produces new muscle tissue, so your muscles get denser, stronger, and more durable. Added muscle tissue will demand more energy, boosting your metabolism. Running also improves insulin sensitivity, meaning that your energy goes up, and blood sugar goes down. In a single run, your glucose uptake could increase by as much as 40%.
Now you've stopped running, your body will switch from performance to recovery mode. It will heal damaged tissue, replaced resources, and get rid of by-products such as lactic acid. After intense or extended exercise, your muscles tighten and contract as they heal. Stretching while they're still warm increases your range of motion and lessens your risk of injury. Your body need simple carbohydrates to replace the glucose you've burned, and lean protein to help you heal. Some Olympic athletes swear by chocolate milk!