Think of the last time you went swimming. It is summertime at the lake; the little fishes are nibbling your toes and your brother is leisurely floating next to you. The cool, fresh water, having descended from the untouched purity of the frozen Canadian mountain peaks, is your oasis from the summer heat. You are playfully splashing in the shallows, and maybe your brother challenges you to a game every kid knows, and every fretful parent fears: the who-can-hold-their-breath-the-longest.
Ready? You both count down together; three, two, one, you take a deeeeep breath, and under you go. You decide to look around the new underwater world you have found yourself in, but all you see is the murky brown of silt that was kicked up from all your playful splashing. Seconds tick by, and your heart rate starts to decrease. Your cheeks start to puff out. Strategically, you let out some excess air, which lets you sink back a little lower and releases some of the pain you were experiencing in your chest. It feels like you have been down in the milky watery depths for an eternity. Your world starts to spin, your chest feels like it is going to explode, and finally – you know; you cannot wait another second. So you burst forth like a breaching whale, breaking the calm surface and sucking in the world around you.
And there is your brother, wading in the water next to you and grinning smugly. He says “Ha! Made you hold your breath,” and swims lithely away, while you are still too breathless to throw back the retort on the tip of your tongue; “You didn’t do it ‘cause you were scared to lose!”
This is an experience I am sure most are familiar with. The majority of mammals have a physiological diving response, which is triggered by contact with cold water on the face, as well as holding your breath (4)(9). The average untrained human can hold their breath from 44 to 71 seconds when in water from 10°C to 21°C (1). The technical term for this bit of child-like daredevilishness is static apnea, where static describes the fact that you are not moving, and apnea describes a suspension of external breathing (2). Your muscles have stopped the rise and fall of your rib cage and diaphragm, and therefore stopped the travel of air in and out of your lung cavity. However internal breathing, which occurs when red blood cells exchange carbon dioxide for oxygen at lungs, and then oxygen for carbon dioxide at the tissues, continues even when external breathing has stopped. This is possible because your blood continues to circulate, and because in a single breath, from inhale to exhale, your body only ever takes up about 5% of the 20% oxygen that is present in air. Hence why CPR is used when a patient is incapable of breathing on their own – the air delivered by a first responder’s lungs will still have more then enough oxygen needed to support a patient.
Getting back to your little game, shortly after submersion you have restricted your oxygen supply and carbon dioxide starts to build up in your tissues (3). Because of the rise of carbon dioxide gas in your blood, the partial pressure caused by the gas is making your blood pressure rise (4)(9)(10). At this point baroreceptors in your arterial wall will sense the increase in blood pressure, and chemoreceptors will detect the elevated carbon dioxide levels and depleted oxygen levels (4)(9). These receptors send a neurological emergency signal to your brain – it is as if the receptors are witnesses to the fire, and your brain is the fire alarm that they have just pulled. In response your brain signals the adrenal medulla, located on top of your kidneys, to release its mind-numbingly loud alarm bell in to your bloodstream – the hormone norepinephrine (5)(6).
After having taken the plunge and spending roughly 35 seconds in the milky depths, the first physiological response to static apnea is becoming apparent; your heart rate is decreasing (1)(4)(9)(10). Your heart rate can decrease from 10 to 25 percent in this short period (4). Since your blood is moving slower, the rate of exchange of oxygen at the tissues has slowed also.
This is where those hormonal signals that your brain triggered come in to play. Hormones act system-wide; they are released in to your blood and directly absorbed into your tissues. Your body’s response to this particular hormone is, firstly, vasoconstriction – literally the contraction of the muscles around your blood vessels, which restricts blood flow to nonessential areas and shunts blood to your vital organs (5)(6)(9)(10). The vessels become narrower and even close off in less important areas such as the skin – which is why a blue or purple colored face is noticeable when someone is choking, or laughing too hard (hence the expression “we laughed until we were blue in the face”) (1). The constriction of the blood vessels, combined with the slowing of the heart rate, allows for the blood that was shunted to your valuable inner organs to stay there longer, and therefore provide the essentials with oxygen for a longer period of time (10).
Secondly, at the cellular level, norepinephrine is acting on your mitochondria – the “organ” in your cells that makes energy (8). The mitochondria are being told to switch from the highly efficient-oxygen dependent process called oxidative phosphorylation, to the only option now available – glycolysis (3). If the two were vacations, oxidative phosphorylation would be like a three-month long biking tour of the French countryside; slow but culturally fulfilling, and glycolysis would be that ridiculously expensive weekend in Las Vegas that you remember only in bits and pieces. Glycolysis is energetically expensive for your body and therefore it can only be maintained for a very short time before your cells run out of juice. This is occurring in the unimportant cells – in the tissues where your body has already restricted blood flow (9)(10). Your more important inner organs, such as your brain and your heart are still functioning, for the most part, on their usual fuels (10).
Your cheeks have started to puff out – this is due to a build up of carbon dioxide in your lungs and airways (9). Your blood buffer system, which can temporarily stall carbon dioxide build up, has become saturated with carbon dioxide and carbonic acid (11). The carbonic acid comes from the buffering reaction of carbon dioxide and water at your tissues (12). As Le Chatelier’s equilibrium principles state, the high concentration of carbon dioxide at your tissues has shifted the equilibrium from the carbon dioxide and water, to form carbonic acid (12)(13). This is highly fortunate for you, as it allows your blood to act as a “carbon sink”. The carbonic acid then travels in the blood to the lungs, where the equilibrium shifts back, releasing carbon dioxide gas in to the lung cavity, as well as water (12)(13).
You let out a small amount of air, releasing some of the pressure that you could feel in your chest and enabling you to remain submerged for longer. As mentioned before, this pressure is the build up of carbon dioxide in your lung cavity (9). The high pressure building in your lungs is making it more and more kinetically unfavorable for carbon dioxide to leave your blood stream (13). It may be helpful to think of it in terms of Le Chatelier’s principles again – where the carbon dioxide in your lungs is like a 99B-line bus in the morning. There is simply no room for another person, or another carbon dioxide molecule, to fit inside. Your sad little carbon dioxide is left in the blood stream, unable to move outwards or onwards. By letting out a small volume, you are creating more space in your lungs for carbon dioxide to move out of your blood stream and fill the void, thus prolonging your underwater adventure.
However, this tactic can only work for a short time, and you start to feel that familiar painful tension building in you chest. Your vision starts to (figuratively) swim and you feel dizziness and nausea creeping up on you. These are all typical responses to oxygen depletion in the brain (14). You fight it – just a few more seconds, hold on, just a little longer, you have to show that cocky, good for nothing… until finally your animal instincts of self preservation kick in and your deprived limbs shoot to the surface. You are breathing in deeply and rapidly. Sucking in every inch of the glorious super-aqueous world. This period of elevated breath rate is due to a phenomenon called oxygen debt (3). Your body requires time to sense the oxygen now entering your lungs, and to deliver it to the tissues (3). The hormones in your blood are being absorbed by the tissues, their effect waning. Your blood vessels start to open up and your heavy breathing begins to slow, as you are returning the neglected cells to oxidative phosphorylation. After some time your blood starts to flow normally, your tissues are well serviced with oxygen, and all the excess carbonic acid in your blood has been released as carbon dioxide and water. Only then does your breath eventually calm.
Your pride has been wounded today, but take consolation in the fact that your body is at the peak of millions of years of evolution. As you slink off in to the water like the primordial sea creature you came from, revel in the awesomeness that is you and your display of physiological supremacy over other, lesser beings.
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