The danger of over-simplification threatens when complex physiological processes, which are incompletely studied and understood, have to be simplified to make them exciting for the viewer. Moreover each programme may repeat and so propagate the original simplification. One simplification can be the merging of breath-holds on land visually dull with those during immersion visually thrilling. Another is that humans decrease heart rate during breath-holding to reduce metabolic rate and hence prolong breath-hold duration.
Metabolic rate the rate of O2 consumption is normally measured at the mouth from at least one breath, using an oxygen-filled spirometer or a Douglas bag. Expired air measurements alone are adequate to measure metabolic rate at rest or during exercise, because neither the arterial partial pressure of oxygen PaO2 falls nor that of carbon dioxide PaCO2 rises.
Measuring metabolic rate during breath-holding presents more of a technical challenge. Firstly, because it is only averaged over the breath-hold and secondly, because blood gases do deteriorate during breath-holding. So the true metabolic rate can only be measured from the oxygen extracted from both expired air and blood Hong et al.
The key and classic papers are more than 40 years old and so far all have found that metabolic rate in resting humans does not decrease during breath-holding. In Stevens et al. They proposed this was because the oxygen taken up from the gas in the unventilated lungs was not replaced by carbon dioxide gas produced as metabolism continued. Instead, while gaseous oxygen is consumed, carbon dioxide remains dissolved in the blood and tissues because breath-holding abolishes the partial pressure gradients driving CO2 from alveolar blood to alveolar gas.
The rate of change of buoyancy, i. Stevens confirmed this in three subjects by showing the change in buoyancy was almost exactly equal to the rate of oxygen consumption measured by subjects breathing from an O2-filled spirometer before and after the breath-hold.
Stevens et al. Strictly speaking, they were also immersed, but the water temperature is unknown! The true rate of oxygen consumption must be higher than this, when also accounting for the additional fall in blood oxygen content during breath-holding.
Alternatively, if the lungs are over-filled with oxygen at the start of the breath-hold i. Stevens also used breath-holds with preoxygenation but without hyperventilation and found that the measured mean rate of oxygen consumption was higher ml O2,STPD min—1. This measure is nearer the true metabolic rate, but still fails to allow for any influences on oxygen carriage of changes in PaCO2.
They measured no change in the volume of gaseous CO2 in the lungs during breath-holding confirming that all metabolically produced CO2 remains dissolved. Finally, in , Hong et al. A definitive study of validated measurements of true metabolic rate before and during breath-holding using a larger number of normal subjects would be welcome.
Typically people get better and better in repeated breath holds, in part because their spleens are squeezing more oxygen-carrying red blood cells into circulation.
During the third and final breath hold, they took continuous measurements of parameters including heart rate, oxygen levels in the brain, and oxygen levels in the leg muscles. The diving response is enhanced if your face is submerged in water, but it happens even on dry land. Your heart rate drops, and the blood vessels leading to non-essential parts of the body like your leg muscles constrict in order to redirect crucial blood and oxygen to the brain.
The subjects held their third breath for an average of two minutes and 37 seconds, which strikes me as incredibly good for normal untrained people. The gray area indicates when they started and stopped the breath hold. On the far left, you can see the blue dots which represent the average value increasing as the subjects prepare for the breath hold.
The subjects were specifically forbidden from hyperventilating before the breath hold which blows off a bunch of carbon dioxide, allowing you to hold your breath for longer , but they were given a second warning and a second countdown, and told to take a deep but not maximal breath right before starting.
Within about ten seconds after starting the breath hold, heart rate is dropping. It ends up decreasing by 27 beats per minute, reaching its low point after 83 seconds on average. This is fairly similar to what you see in elite free divers, except they reach their minimum heart rate within 30 to 60 seconds. There are two individuals who quit early; one of them fainted, and the other got dizzy and was on the verge of fainting.
More on them below. The next parameter is tissue oxygenation in the leg muscles, as measured with near-infrared spectroscopy , which basically involves shining infrared light through the skin and measuring how much is absorbed by oxygen-rich hemoglobin.
Studies show that about 30 seconds of breath-holding can lead to a lowered heart rate and lower cardiac output. Once your body's heart rate goes down during breath-holding, it tries to compensate by raising your blood pressure to get blood pumped to the body. This happens as our blood vessels constrict. This blood pressure increase usually happens after three minutes of breath-holding, once the oxygen level in our blood starts dropping.
The dangerous risk of holding your breath underwater is the chance of passing out in the water. According to experts at Emory, when children hold their breath underwater, the pressure in their chests causes their blood vessels to cut off blood flow to the right side of the heart.
When this happens, the heart can't pump blood, which leads to the reflex to faint. Adults who hold their breath for extended periods underwater are also at risk of passing out.
Because of accidental drownings related to healthy children and adults holding their breath underwater, the U. Centers for Disease Control and Prevention recommend never practicing voluntary apnea. Be sure to teach your kids to stay safe when swimming and never play breath-holding games with friends.
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