Physiological changes during exercise

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Introduction0:00–0:46

During physical exercise, our organs and tissues are working hard to keep us moving; or, technically speaking, for our musculoskeletal system to do its job.
Now it’s fairly obvious that during exercise, skeletal muscles work, or contract, harder and faster than when we’re at rest, so they use a lot of energy in a short time, so they need a lot more blood and oxygen to keep going.
So organ systems like the cardiovascular and respiratory system have to make some quick physiological adjustments, to meet the skeletal muscles demand.
Moreover, the endocrine system also kicks things into high gear, by secreting hormones like cortisol and adrenaline, that speed up intracellular processes to keep us going.But before we delve into the specifics of that, let’s remember how muscle contraction works on a microscopic level.

Muscle Contraction0:46–1:38

So, skeletal muscles are made up of muscle fibers which are actually the skeletal muscle cells. We just call them “fibers” because they are long, multinucleated cells, meaning they have more than one nucleus.
Their structure also differs from other cells because their cytoplasm, sometimes also called sarcoplasm, is filled with stacks of long filaments called myofibrils, which are made up of contractile units called sarcomeres.
And finally, sarcomeres are made up of the thick myosin filaments, and thin actin filaments, which can slide over one another, shortening the sarcomeres.
So when all the sarcomeres in a muscle fiber do that in sync, that results in shortening of the muscle as a whole, or muscle contraction.
And this process is powered by energy in the shape of ATP molecules, where adenosine-triphosphate. The three phosphates in the molecule are linked in a chain, and between two adjacent phosphate molecules, there are high-energy phosphate bonds.

ATP1:38–3:35

ATP molecules attach to a part of the myosin filament called the myosin head. The myosin head is actually an ATPase, or an enzyme that can cleave an ATP molecule into ADP and phosphate ion, releasing the energy stored in the bonds.
After the energy is released, ADP detaches from the myosin head, so myosin can bind to actin filaments, forming cross-bridges that result in shortening of the muscle fiber.
Now, ATP molecules come from two sources: first, there’s a small stash spread out between myofibrils, which is just about enough to sustain muscle contraction for a single bout of exercise - like hitting a tennis ball with a racket.
But these ATP stores get depleted quickly, so if you want to keep playing tennis after the serve, muscle cells need to generate some more ATP.
ATP can be generated through a number of biochemical pathways. First one is substrate phosphorylation - which means a phosphate is added to a molecule, in this case, the leftover ADP.
In muscles, the phosphate comes from creatine phosphate, which splits into phosphate and creatine under the action of an enzyme called creatine phosphokinase.
Unfortunately, creatine phosphate also runs out rather quickly. So 10 to 30 seconds after the onset of exercise, ATP needs to be generated through anaerobic glycolysis, or the breakdown of glucose into pyruvate and lactic acid.
This process happens in the cytoplasm, and it doesn’t require oxygen but it only yields about 2 ATP molecules per molecule of glucose.
What’s more, in the absence of oxygen, pyruvate is converted to lactic acid in the cytoplasm, so this causes a buildup of lactic acid, which causes muscle fatigue after about 1 minute of intense exercise.
Lactic acid can also spill into the bloodstream, making blood PH take a dip. This is detected by peripheral chemoreceptors, which are specialized neurons located in the walls of the carotid arteries and the aortic arch.

Respiratory System3:35–4:53

When they register that blood PH dropped, these neurons fire more impulses, notifying the respiratory centers in the brainstem that they have to increase the respiratory rate and depth of breathing, all together called hyperventilation.
So more air, and, in turn, more oxygen reaches the alveoli, which are the tiny air sacs where gas exchange occurs. More oxygen in the alveoli leads to pulmonary vasodilation, meaning these tiny vessels of the pulmonary capillary bed start to widen, reducing the pulmonary vascular resistance, so more blood flows through.
A decrease in pulmonary vascular resistance and an increase in pulmonary blood flow in all three zones the lungs; the upper, the middle and the lower ones, allow blood to reach all of these zones almost equally.
As a result, we get a more even distribution of pulmonary perfusion, and the physiological dead space, or the number of alveoli that were not actively used for gas exchange, also decreases.
This increases efficiency in gas exchange between the alveoli and the pulmonary capillaries, so more oxygen gets in the blood, and more carbon dioxide leaves the blood.

Central Nervous System4:53–5:32

At the same time, when chemoreceptor firing rate increases, it also notifies the cardiac centers in the nucleus tractus solitarius located in the medulla oblongata, which signal the brain to turning down the parasympathetic stimulation to the heart, while increasing sympathetic stimulation - aka the fight or flight response.
Part of the fight or flight response is that brain signals the adrenal glands above the kidneys to release epinephrine, and when epinephrine gets to the heart, it binds to the adrenergic receptors of the heart muscle, making heart rate and contractility increase.
This means heart muscle fibers contract faster and stronger and the amount of blood the heart pumps out in a minute, increases as well.
Finally, epinephrine also causes systemic vasoconstriction, which means visceral blood vessels contract, so there’s reduced blood flow to the kidneys, liver and the gastrointestinal system.

Functional Hyperemia5:32–6:50

At the same time, it causes skeletal muscle blood vessels to dilate, in order to allow more blood to reach the working muscles.
The reason skeletal muscle blood vessels dilate is that with increased muscle activity, initially there’s an increased production of hydrogen ions from lactic acid, and mean arterial oxygen pressure, or PaO2, decreases, as muscles start using up oxygen, while mean arterial carbon dioxide pressure, or PaCO2, decreases, as muscle produce more carbon dyoxide.
Altogether, this causes the smooth muscles of the local blood vessels to relax, counteracting the effect of sympathetic vasoconstriction happening all around the body.
So when you put systemic vasoconstriction and muscular vasodilation together, there’s more blood for the muscles, and less for the other organs.
This mechanism is referred to as active or functional hyperemia, meaning there’s increased blood flow as a result of increased activity in a specific organ - in our case, the muscles.
This makes sense, since during exercise, digesting that burger can take a backseat while the muscles are doing their thing.At the end of the day, the abundance of oxygen-rich blood allows ATP to be generated aerobically.

Oxidative Phosphorylation6:50–7:47

So instead of being converted to lactic acid, pyruvate molecules enter the mitochondria and they’re converted to acetyl CoA, which can enter the Krebs cycle.
During the Krebs cycle, a lot of molecules called coenzymes receive hydrogen ions - so for example, the NAD+ coenzyme gets a hydrogen, making NADH, and FAD+ coenzyme becomes FADH2 the same way.
NADH and FADH2 can donate those hydrogen ions to the electron transport chain, which is made up of a number of complexes that pass the hydrogens from one to another to generate energy.
And this process is called oxidative phosphorylation, and it’s the final step in cellular respiration. And when all the biochemistry is said and done, in the presence of oxygen, ATP yield reaches 32 molecules per molecule of glucose.
This is good news - because as aerobic respiration keeps going on, it allows us to continue exercising for longer than just about one minute at a time.

Mean Arterial Partial Pressures7:47–8:34

That’s because the metabolism, or all the biochemical pathways, speed up, to the point where the level that the oxygen that’s consumed, and the carbon dioxide that’s produced as a result of cellular metabolism get in almost perfect balance with the oxygen we’re breathing in and the carbon dioxide we’re breathing out.
So the mean arterial oxygen, and mean carbon dioxide partial pressures, or PaO2 and PaCO2 for short, stay constant. In venous blood, however, oxygen partial pressure is low, but carbon dioxide partial pressure is high during exercise compared to rest.

Glycogenolysis and Lipolysis8:34–9:30

But eventually, like after you’ve been playing tennis for a while, blood glucose levels start to fall as it’s being used up by working muscles cells.
In response to low blood sugar, the pancreas releases glucagon. Glucagon makes the liver breakdown its stored glycogen into glucose, through a process called glycogenolysis.
And the epinephrine made by the adrenals a while back tells muscle cells to break down their glycogen stores into glucose through glycogenolysis as well, so that muscle cells always have enough glucose to breakdown for energy.
Glucagon and epinephrine also cause lipolysis in adipose tissue, meaning they promote the breakdown of lipids and fats into free fatty acids.
Free fatty acids get released into blood circulation, and then transported to the working muscle cells, where they undergo beta-oxidation, which converts free fatty acids to acetyl CoA in the mitochondria.
This keeps the Krebs’ cycle and oxidative phosphorylation for ATP regeneration.Finally, with regular physical exercises, the body systems adapt, which is to say they undergo long term specific changes to help increase the efficiency and capacity during exercises - so for example, the heart muscle hypertrophies, or enlarges, while the heart rate decreases.

Body Adaptation9:30–9:59

This allows for the same amount of blood per minute to be pumped even at rest, but the heart muscle uses less energy. That’s why everyone says exercise is good for your cardiovascular system!##SummaryAlright, as a quick recap.
During exercise, muscle cells use up more ATP for muscular contraction. ATP can be generated by breaking down glucose through anaerobic glycolysis, which generates enough energy for about one minute of exercise.

Review9:59–11:01

With continued exercise, the sympathetic nervous system gets activated, and epinephrine causes an increase in heart rate and heart contractility, to send more blood to the muscles.
At the same time, breathing rate and depth increase, to load that blood with oxygen. In the presence of oxygen, the energy required for contraction is generated through the Krebs’ cycle and oxidative phosphorylation.
Finally, after exercising for a long time, blood glucose levels start to fall, so glucagon is secreted by the pancreas. Glucagon, along with epinephrine, break down liver and muscle glycogen stores, respectively, increasing available glucose.
These two hormones also promote lipolysis, during which free fatty acids are released from adipose tissue and beta-oxidized in muscle cells, to help replenish more ATP for muscle contraction.