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I tested this out with my friends, and I find that after they hold their breath and can't hold it anymore, they exhale air, instead of inhaling air.
Interestingly, they all try to inhale in as much air as possible before starting to hold their breath. When I told them to exhale as much air as possible before starting to hold their breath, they inhaled air after they can't hold it anymore.
It's understandable that when one exhales then holds his breath, he needs air and thus he inhales afterwards. But when one inhales then holds his breath, shouldn't he inhale again after "using up all that air he inhaled before holding hi breath"?
This is more about basic physics than biology. When you hold your breath, you normally take in one last long breath and keep it in as long as possible, Your lungs are therefore already full of gas (remember that the oxygen used by our lungs is only ~22% of the total volume of air you inhale). Therefore, when you release that breath and want to take in a new one you need to first empty your lungs out in order to refill them.
In addition, you need to have your lungs empty in order to be able to expand them and draw in more air. There is no biological reason for any of this, it is the same principle as trying to use a turkey baster:
If you use it once and pull up some liquid into it, you will have to let air out before you can use it again. Your lungs work in the same way. Test this by exhaling all the air from your lungs and then holding your breath without inhaling. When you want to start breathing again, you'll be able to breathe directly in without needing to exhale first.
If you inhale on top of inhaled air this is more work. There is more dead air, air which is not as useful due to the lower concentration gradient. And we breathe more to exhale carbon dioxide than we require oxygen. Low oxygen levels only push us to breathe when oxygen levels are a good deal lower, however tiny changes in carbon dioxide causes changes in our ventilation rate almost immediately.
It is also more work as our muscles require more energy to inflate an already inflated chest. This is the case in COPD patients. Try this: take a deep breath in and now holding that air in, breathe normally on top of that air. Feel how difficult it is and how breathless you feel.
It's understandable that when one exhales then holds his breath, he needs air and thus he inhales afterwards. But when one inhales then holds his breath, shouldn't he inhale again after "using up all that air he inhaled before holding hi breath"?
Before we hold our breathe we first inhale some air inside.
So, after holding it for some times, carbondioxide (CO2) which can be toxic to our body will be released after completing metabolic processes inside our body which will be pushed to our thorax, ready to be expelled.
The partial pressure of this gas (pCO2), is the primary fast (minutes or hours) trigger for breath; where partial pressure of Oxygen (pO2) is a slow trigger (hours or days). This explains why patients with severe re-acutized cronic obstructive pulmonary disease (COPD) stop spontaneous breathing, even if their pO2 falls, because of augmented pCO2.
The pCO2 is sensed in the human body in the Central Nervous System (CNS), precisely in the ponto-bulbar centers of breathing. The pO2 is sensed at periferical strucrures as the glomi aorticus and carotideus.
What’s the secret to holding your breath?
H ow long can you hold your breath? I’m trying it right now. The first 30 seconds are easy. I’m ready to give up at 45 seconds but I push on through, and it seems to get easier for a while. But as the second hand ticks past a minute, I know I’m on borrowed time. My heart is pounding. I let out a tiny breath and this helps. Eventually I give in, expelling the spent air in my lungs and taking a huge gasp. (And continue to gasp for a few more breaths, prompting my husband to ask what on earth I’m doing). I manage one minute and 12 seconds. I’m quite impressed with myself.
Breath-holding ability becomes extremely important in some sports, particularly freediving. In 2006 I was filming a programme about the anatomy and physiology of the lungs for a BBC series called, slightly oddly, Don’t Die Young. I was lucky enough to meet Sam Kirby (now Sam Amps), who was captain of the UK freedive team. At a pool in Bristol she taught me some simple exercises to help me hold my breath for longer while swimming underwater. By the end of the session I hadn’t cracked freediving – I’d cracked one of Sam’s precious monofins on the bottom of the pool, and I think I’d managed a prodigious 90 seconds of breath-holding, enough to let me swim a width. Sam swam three widths with ease. She could hold her breath for five minutes, while swimming. Five!
I asked how she did it: very slow breathing for several minutes prior to each dive, then a big, deep breath before diving in. She also said training helped her resist the urge to breathe for far longer than most people.
Some have suggested that the ability to voluntarily hold your breath is evidence of a watery episode in human evolution. It’s even been said that humans have an ability to lower heart rate and metabolic rate in order to breath-hold for even longer. Other anatomical and physiological bits and bobs – our hairlessness, the distribution of our subcutaneous fat, and even our tendency to walk on two legs – have been linked to an aquatic phase of evolutionary development. Unfortunately, the cobbled-together “aquatic ape hypothesis” fails to hold water. It’s a romantic notion that may appeal to us, but with the cold light of day falling on the scientific evidence, it’s revealed to be nothing more than a fiction.
Looking at voluntary breath-holding, it turns out that we’re certainly not unique among non-aquatic mammals in being able to hold our breath. (Having said that, it’s a difficult thing to investigate in other mammals as, unlike humans, they tend not to comply when you ask them to breath-hold). And experimental evidence shows that heart rate doesn’t drop during breath-holding. At least, it doesn’t if you’re breath-holding on land. When you’re submerged in cold water it’s a different story: cooling the face does lead to a slower heart rate in most people. But, once again, this isn’t evidence of an aquatic ape ancestry, as it turns out to be a very general characteristic of air-breathing vertebrates. This reduction in heart rate is just one of the physiological responses that are sometimes described together as the “mammalian diving reflex”. But physiological responses that could be useful in diving are also – and perhaps even more importantly – useful for not drowning.
While our ability to breath-hold may not be all that special, when we compare ourselves with other animals, it’s now proving very useful in one particular area of medicine. Radiotherapy for breast cancer involves directing radiation, very precisely, at the tumour. This may require several minutes’ worth of radiation, and so it’s usually done in short bursts, between breaths. But if the patient can keep her chest perfectly still for several minutes, it means that the entire dose can be delivered, in the right place, in one go. The problem, of course, is that most people, just like me, struggle to hold their breath for much longer than a minute. But doctors at University Hospital Birmingham have recently performed careful experiments that show that, if patients are ventilated with oxygen-rich air before attempting a breath-hold, they can manage to hold their breath for an impressive five-and-half-minutes.
Surprisingly, the trick seems to lie, not in fooling the body’s usual sensors for low oxygen or high carbon dioxide levels in the blood, but in fooling the diaphragm. When you breathe in, you’re contracting the muscle of your diaphragm, pulling it flat so that the volume of your chest increases – and air is drawn into your lungs. When you hold your breath, you keep your diaphragm in this contracted state. Artificially raising oxygen levels and reducing carbon dioxide levels before a breath-hold, as in the Birmingham radiotherapy experiments, may work by delaying fatigue in the diaphragm. And – not so useful if you’re trying to keep your chest perfectly still – breathing out a little air lets the diaphragm relax a little, and helps you to prolong a breath-hold, exactly as I found when attempting my breath-hold. And so it’s your diaphragm, the main muscle of breathing, that is also in charge when it comes to reaching the breakpoint of your breath-hold. Eventually, even if you’ve fooled it for a while, the signals from the diaphragm are just too strong, and you have to give in – and take a breath.
How is carbon dioxide produced in the body?
We all know that carbon dioxide is actually a waste product of various metabolic and biochemical processes that occur inside our bodies, but how exactly is it produced in the first place? The answer to this question lies in&hellip
Talking about respiration, the first thing that comes to mind is the nose and lungs. If you&rsquore more of a nerd, however, you might have a diagram of the entire respiratory tract pop up in your head&hellip
The most commonly used diagram to illustrate the human respiratory system (Photo Credit: OpenStax College / Wikimedia Commons)
However, cellular respiration is not what you see in the image above. As its name implies, it is something that occurs on a cellular level inside our bodies. More specifically, it&rsquos a bunch of metabolic processes and reactions that go on inside the cells of organisms to convert biochemical energy derived from vital nutrients into a source of energy to fuel cellular activity.
Although many biochemical reactions occur within our bodies all the time, the one that occurs inside our cells and is responsible for giving us energy is probably the most important of all. The reactants involved in this reaction are mainly sugars, carbohydrates, fats and proteins, and since it occurs in the presence of oxygen, it&rsquos known as aerobic respiration.
This biochemical reaction occurs inside our bodies&rsquo cells and produces carbon dioxide gas as a byproduct. In answer to our earlier question, that&rsquos how carbon dioxide is produced inside the body. Since glucose, fats and proteins are all used as fuel sources for this reaction, the rate of carbon dioxide production is less than the rate of oxygen consumption. In simple words, we produce less carbon dioxide than the amount of oxygen we consume.
4 Answers 4
It is mainly due to air entrainment.
If you blow through a tight mouth, there is smaller volume of air but a higher velocity. This pulls in and mixes with a lot of ambient air (Venturi or Bernoulli effect) - in fact typically the air stream is only 40% body warmth and 60% ambient so it will be markedly colder. As an experiment you pucker and blow through a tube held to your mouth, this excludes the ambient air and you will get reduced airflow but at the higher temperature again.
With a wide mouth there is hardly any air entrainment. Its temperature will be almost same as its temperature in your lungs which is higher than the ambient temperature.
I don't think it's quite that physical. The pressures involved aren't that high
When you breathe slowly on your hand the air is war, moist and 37C so feels warm compared to the surroundings, if you blow through a small opening the flow of air increases the cooling and evaporation from your skin.
I think most reasons given above are false. It is our common experience that cold air comes when we exhale through a narrow opening of mouth. (Experience it yourself). It is due to adiabatic expansion of air. When a gas is allowed to expand suddenly, it does so by absorbing heat energy. When air is suddenly exhaled out into a larger volume through the narrow opening, air undergoes adiabatic expansion. When we place our hand near the out flowing air heat energy is being absorbed from our hand. Hence we feel cold. Opposite is in the case when a gas is compressed. Heat energy is liberated. (try this by tightly closing your mouth with your hand and exhaling air out into a small volume compressing it. You feel that the air is hot). So the exact reason is the adiabatic expansion or compression according to the size of the mouth.I wonder if the case is different in any other cold countries. In India it is as explained above.
The total sense of hot or cold is set by the integration of the response of all the sensors in the skin, +- evenly distributed, involved in the experiment.
The sense of hot is proportional to the area and the sense of cold is proportional to the perimeter*$delta r$ , for the same pressure (see the answer of user1355).
The outcome of the experiment can be described by the ratio hot/cold or _(area)/(perimeter*$delta r$)_ $=pi R^2/2 delta r pi R=R/2delta r$, i.e proportional to $R$ .
Let Experiment open mouth have radius $10 R$ and Experiment almost closed mouth have radius $R$.
The ratio of the outcomes of the two experiments (open/closed) is 10 .
If someone has doubts, and I do have, that the perimeter can contribute significantly to a colder sensation than the overall outcome becames more contrasted (ratio of areas, proportional to $R^2$) and the above example will give the value 100.
An array of thermometers can set the question, imo. One measure can be defined as the sum of all the temperatures.
The description in the question is correct, supported by experimentation (I did it ), and have a physical and rational interpretation.
Why do we exhale after we hold our breath? - Biology
Are You Holding Your Breath? Here's How (and Why) to Stop Pausing
Breathe in. Breathe out. The yogis say the in- and out-going breaths are the two guards of the City of Life. When the guards are well coordinated, the city&rsquos defenses are strong. When the guards are disorganized and disconnected, the city comes under attack.
If we study the breath we soon realize that our breathing habits not only reflect our state of being, but have a profound effect on it as well. The manner in which we breathe influences our entire being: our mental-emotional states, the nervous system, hormonal balance, muscular tension, and all the functions of body and mind. Bad habits cause strain and stress good habits keep us strong and healthy, and create a sense of well-being.
The yogis say the in- and out-going breaths are the two guards of the City of Life.
A smooth, even, quiet, diaphragmatic breath passing through the nose is the optimal breathing pattern from the standpoint of both good health and spiritual practice. It is the ideal breath in most situations, short of running a marathon or being chased by a tiger. Unfortunately, however, if you closely observe your breathing habits and those of others, you&rsquoll notice this ideal breath is rare. Most of us assume that our guards are doing their job, but we seldom check up on them. It doesn&rsquot occur to us that they might be showing up late for work or are poorly trained. But, in fact, inefficient guards are quite common. Four deviations from the ideal breathing pattern are particularly prevalent.
First, if the diaphragm is restricted the breath is shallow and confined to the chest. This overstimulates the sympathetic nervous system and results in the physiological symptoms of stress and feelings of anxiety. It also tends to keep us locked into our worries and anxious thoughts. Second, a noisy breath indicates that the nasal passages are obstructed. A third problem is breathing with jerks during inhalation and exhalation. Pausing at the end of inhalation or exhalation is the fourth and often the most thorny problem. Yogis have discovered that all of these patterns disrupt the rhythm of the lungs, disturb the nervous system, and have an adverse effect on the heart.
He Who Pauses Is Lost
The fourth problem&mdashpausing at the end of inhalation or exhalation&mdashis often the most difficult to correct. For one thing, pausing at the end of inhalation is a natural response to mental stress. If someone asks you a difficult question, you might naturally inhale, hold your breath, close your eyes, and think intently for a second or two before trying to answer. Pausing briefly any time your posture is slumped or after a deep sigh is natural, but according to the yogis, any habitual pause in the breathing pattern drains the life force.
How can even breathing be mastered? How can we train our guards and fortify the city?
There are practices in hatha yoga in which the breath is retained or held, but those techniques are effective and safe only if a healthy, even, breathing pattern has been mastered. That means perfecting the steady, unbroken flow of inhalation and exhalation. How can even breathing be mastered? How can we train our guards and fortify the city? Can we learn to breathe without even a hint of a pause at the end of inhalation and exhalation?
The secret lies in the abdomen. First, notice how you breathe with a relaxed abdomen. Sit reasonably straight in a chair. Don't slump, but don't pitch yourself forward, either. Make sure your lower abdomen is not restrained by tight clothing. Now breathe so that your lower abdomen moves out during each inhalation and relaxes during each exhalation. Make sure your chest does not move at all.
Notice that for even, relaxed breathing, the critical moments occur at the changing of the guard&mdashat the end of inhalation and at the end of exhalation. The in and out breaths must be smoothly merged. It may help to visualize a circular pattern.
Imagine that you are observing a car on a Ferris wheel. You will observe an ascent of the car, a leveling off of its upward motion, then a descent, followed by the leveling off of the downward motion before the next ascent.
If the upward motion represents inhalation and the downward motion exhalation, you have a wonderful model for how the breath can flow without a pause. The breath slows just as the vertical motion decelerates&mdashat the transitions after exhalation and inhalation. Imagine how the ascent of a car on a Ferris wheel (inhalation) slows at its peak and merges smoothly into the descent (exhalation).
Although the image of the Ferris wheel is still useful, the actual pattern of breathing is elliptical rather than circular. That means there is more time spent during the inhalation and exhalation phases than during the transitions between the two. In practice there may be pauses at both junctures. We'll look at them separately because they present different challenges of the breath.
Transition Between Inhalation and Exhalation
The end of inhalation is the least troublesome of the two because normal neurological impulses operate to smooth the transition between the end of inhalation and the beginning of exhalation. Some nerve impulses keep impinging on the muscle fibers of the diaphragm as exhalation begins, slightly inhibiting its relaxation, until the diaphragm finally releases, yielding completely to the motion of exhalation. If you make the transition from inhalation into exhalation in slow motion&mdashever so slowly initiating your exhalation&mdashyou will feel a slight hesitation as you start to exhale. This feeling reflects the continuing flow of nerve impulses into the diaphragm even as it is beginning to relax for exhalation.
With a little effort you can tune this mechanism to make a smooth transition from inhalation into exhalation.
With a little effort you can tune this mechanism to make a smooth transition from inhalation into exhalation. If you do not have healthy breathing habits, however, you might find yourself holding your breath at the end of inhalation. In that case it is better to concentrate on breathing evenly in the hatha yoga postures (asanas) until the postures themselves correct your bad breathing habits.
For example, it is easier to merge inhalation evenly with exhalation while you are repeating a posture such as the cobra (raising with the inhalation, lowering with the exhalation) than while you are simply sitting and watching the breath. If you are trying to &ldquofix&rdquo your breath while sitting, either your mind may get caught up in extraneous thoughts, or you may try so hard to breathe evenly that the concentration itself causes you to hold your breath&mdashjust as students sometimes hold their breath while concentrating on difficult problems.
Transition Between Exhalation and Inhalation
Making a smooth transition between the end of exhalation and the beginning of inhalation is more of a problem. As exhalation proceeds to completion, the diaphragm relaxes completely. This can be experienced when you sigh. Try it. Take in a fairly deep breath, and let it go completely. If you don't make any noise or restrain exhalation, the diaphragm relaxes completely.
The problem with a completely relaxed exhalation is that the beginning of inhalation creates a perceptible jerk as the neurons that innervate the diaphragm start firing nerve impulses. It is like starting a car on a cold morning&mdashit cranks in fits and coughs before it runs smoothly. You can try to begin inhaling as soon as exhalation ends, but most people will still have difficulty avoiding a jerk.
The remedy for this is to press in gently with the abdominal muscles as exhalation proceeds, maintain tension with these muscles throughout exhalation, and tail this tension into the beginning of inhalation. This technique is easier described than learned.
Begin by emphasizing the exhalation. Purse the lips so that only a small amount of air can escape and blow gently, as if you are blowing up a balloon. Notice that the abdominal muscles are responsible for the forced exhalation. Keep blowing as long as you can. Because you are emphasizing exhalation, the beginning of inhalation will be passive&mdashthe chest and the abdomen will spring open of their own accord. As you continue to inhale, retain a lingering tension in your abdominal muscles. Allowing the motion of exhalation (contraction of the abdomen) to overlap with the motion of inhalation (contraction of the diaphragm) prevents a gasp.
The Perfect Breath
Now we&rsquore ready to combine our techniques into the perfect breath. First, experiment with posture. Sit perfectly straight and breathe evenly. Try to breathe without jerkiness or pauses. Now slump forward, allowing your lower back to collapse. Notice three things: inhalation is more labored, exhalation starts with a gasp, and it is more difficult to use the abdominal muscles smoothly to aid exhalation. Breathing evenly is impossible. To breathe evenly, you have to sit up straight.
Now we&rsquore ready to combine our techniques into the perfect breath.
And so&mdashin a straight and upright posture&mdashbegin breathing through the nose in two-second exhalations and two-second inhalations. Do this without noise, pauses, or jerks. Remember how observing the flow of breath can be like tracing an ellipse. Smoothly decelerate your rate of inhalation and merge it into exhalation exactly as you would round off an ellipse at the top of a chalkboard. Then use your abdominal muscles to smoothly accelerate your exhalation as you imagine drawing the chalk down the ellipse. Smoothly decelerate your exhalation and merge it into the inhalation as you carry your mark around the bottom of the ellipse: allow some tension to linger in the abdomen as the exhalation merges into the inhalation.
Here&rsquos another technique: think of the abdomen moving down and in for exhalation, and up and out for inhalation&mdashdown, in, up, out, down, in, up, out. Another technique is to mentally repeat the words hum-m-m during exhalation and so-o-o during inhalation&mdashhum, so, hum, so, hum, so. Or imagine a scooping feeling from the lower abdomen, pushing in from the lowest portion of the abdomen, and then moving up to the middle and upper position of the abdominal wall toward the end of exhalation.
If you are still having trouble, work with the asanas, especially those that strengthen the diaphragm (like the crocodile) and abdominal muscles. When the body is strong, flexible, and relaxed, proper breathing will follow with minimal effort.
A smooth, even breath builds our vital energy. From the habit of such a breath arises a calm and balanced mind and a healthy body. When our twin guards are well trained and constantly at our service, the city grows stronger and stronger, and the vicissitudes of life are no longer able to shake its foundation. It&rsquos then that the body, breath, and mind become fit instruments for spiritual practice.
Expired Air Resuscitation
In movies, a person is often seen passed out from drowning or gasping for breath. They then have a savior enter the scene dramatically, and breathe into the victim&rsquos mouth, until the rising and falling of the victim&rsquos chest resumes its normal rhythm. Colloquially called the kiss of life, the technical term is expired air resuscitation (EAR).
Expired Air Resuscitation being performed on a person. (Photo Credit : BruceBlaus / Wikimedia Commons)
This is a form of artificial breathing or ventilation wherein the victim requires assistance in normal breathing. Air is forced down their respiratory tract until it reaches the lungs. This is done a number of times until that person can breathe on their own or until medical help arrives.
EAR is different than CPR, although the two are often used together for better results.
How to practice deep breathing
Deep breathing instead involves taking slower, longer breaths from your stomach to counter the short, rapid breaths that you default to when stressed or anxious.
Rhoads likes to teach deep breathing by first having someone activate their sympathetic nervous system. You can do this by sitting comfortably, closing your eyes and imagining an extremely stressful situation. Notice how your body responds: Your chest might tighten, your breathing might grow shallower and your heart might beat faster.
Next, turn your attention to your breath. Focus on breathing from your stomach, pushing your stomach out each time you inhale. Take longer breaths, counting to at least three for each inhalation and exhalation. Keep doing this even though it may feel uncomfortable at first. After a while, you will start to notice your body feeling more relaxed.
“Noticing the differences for yourself in how your body feels is more powerful than anyone describing it to you,” Rhoads says.
Deep breathing may be simple, but it isn’t necessarily easy. It can quiet your nervous system in a short amount of time, though it probably won’t provide instant relief from all anxiety. The more you practice, the better you’ll get at it and the more you’ll be able to use it in times of stress to help calm yourself down.
Question Why do I see my breath when it's cold outside?
Cold air causes the warm moisture in our breath to condense into tiny droplets of water that appear like a small, misty cloud.
Airman Keith Miller, 52nd Security Forces Squadron, catches his breath during Operation Saber Crown. Airman 1st Class Nathanael Callon, photographer. Spangdahlem Air Base Photos, U.S. Air Force.
Many people think seeing your breath has everything to do with temperature, but the spectacle has just as much to do with the amount of moisture in the atmosphere.
Because our bodies contain nearly 70% water, the air in our lungs is almost completely saturated with water vapor (water in gas form) and is the same temperature as our bodies (98.6oF). Cold air cannot hold as much moisture as warm air. So when one exhales a warm, saturated breath on a cold day the cold air rapidly lowers the temperature of our breath, whereupon the combination briefly reaches dew point. At dew point, air can no longer hold water vapor when air is cooled beyond dew point water vapor turns to liquid form, the physical process known as condensation. It is this liquid form of your breath – minuscule droplets of water – that creates the fleeting, misty cloud we see when breathing in cold weather.
Bison herd sunrise at -20 degrees F, Yellowstone National Park. Jacob W. Frank, photographer, 2017. National Park Service, NPS Flickr Photostream.
Seeing your breath requires just the right combination of temperature and humidity. Though it is pretty common to see your breath in cold weather (usually below 45oF), the next time you have fun making breath clouds, you’ll know it’s because of the exact science of atmospheric moisture and temperature.
Snowboarders’ breath on a cold day, Saint-Adolphe-d’Howard, Canada. External link Alain Wong, photographer, 2016. Wikimedia Commons.
Published: 11/19/2019. Author: Science Reference Section, Library of Congress
Why do we exhale nearly 75% of inhaled oxygen?
According to BBC, we inhale 21% of oxygen but we exhale approximately 16%. That means that we exhale nearly 75% of inhaled oxygen.
Why such a large percentage and doesn't that make breathing highly inefficient?
The simplest answer is that our lungs don't transfer 100% of the oxygen because we don't need that much the lungs have evolved to transfer exactly as much oxygen as needed.
A more complex answer is that the body must maintain a certain partial pressure of oxygen in the blood. A normal pO2 for arterial blood (aka freshly oxygenated blood) is about 80-100 mmHg, compared to deoxygenated venous blood of about 40 mmHg. So the lung has evolved to transfer as much oxygen as needed to reach that level of oxygen saturation. Typically, 95-100% of the hemoglobin in arterial blood is oxygen-bound when it leaves the lung, so the lung is actually really good at oxygenation.
As to efficiency, I don't know if this is the most efficient way. Maybe a biomedical engineer could design a more efficient lung. Evolution doesn't necessarily produce the best theoretical design, it just selects for improvements on what existed in previous species. So I guess blame amphibians for not having better primitive lungs.
Birds already have much more efficient lungs than mammal lungs, so. you don't even need an engineer to improve on the relatively simple mammalian design. Blame our synapsid ancestors for slacking while archosaurs were putting in the evolutionary work!
The lungs are not like a vacuum pump for oxygen. They just bring the air in contact with blood. Some oxygen goes from the air to the blood, some oxygen goes from the blood to the air. In an equilibrium both processes happen at the same rate, if one side has more oxygen then more oxygen will go from there to the other side. If you inhale gas with 0% oxygen then your blood has more oxygen - your lungs will lose oxygen that was already in your blood. For the same reason the lungs can't remove all the oxygen from air, you would reach equilibrium earlier. Under normal conditions the air we inhale has a lot of oxygen, of course, and the net flow is going into the blood.
We could use more of the oxygen we inhale, but breathing has another function: Get rid of CO2. Here the role is reversed: Our body produces it, we need a low concentration in the air to get rid of it. If you hold your breath too long then your lungs get closer to an equilibrium and cannot get rid of it any more. That's bad for the body, so it normally makes you breath even before you got all the oxygen that you could possibly get from the previous breath.
ELI5: Why do people exhale with an "Ahhh" after drinking something refreshing?
When you take a long, deep drink you are holding your breath. When you finish, exhaling ensures your airway is clear. The "ahh" sound is just a social behavior that is learned to express satisfaction (such as vs "eww").
The first 25 seconds of this dutch comedian illustrates it perfectly
People are social creatures. We communicate in verbal and nonverbal messages all the time.
Saying this is like telling the rest of the room that I am OK and that I feel joy or pleasure. Do people do this when they KNOW they are alone or out of earshot of others. I bet not.
We do the same thing with yawns. Ever give a loud yawn?
We do the same things when we get hurt. we shout out. There must be a pretty strong evolutionary reason for this since it could encourage a predator. Perhaps calling to allies?
We do the same thing during sex. Some people (especially some women) CAN'T be quiet during sex. It sends a message to the lover and everyone else in the village of what is going on.