It is said that the record holder of hiccups in Guinness is Charles Osborne, an American. He hiccupped for 68 years from 1922 to 1990. I don't know how he lived that day.
How does burping happen?
To understand this problem, we must first recognize the diaphragm. Between our chest and abdomen, the diaphragm is a thick muscle like a lid and a piston, separating the chest from the abdomen. The diaphragm is the main respiratory muscle. When the diaphragm contracts, the top of the diaphragm drops, the upper and lower diameter of the thorax increases, so that the volume of the lung expands, the air pressure in the lung drops, and the external gas enters the lung through the respiratory tract to complete the inhalation; when the diaphragm contracts, the opposite happens.
When burping, the diaphragm contracts, and the air is inhaled into the lung quickly while the epiglottis closes the airway
Like other organs of the body, the diaphragm has nerve distribution and blood supply. Phrenic nerve is composed of afferent and efferent nerve fibers. When the cause of hiccups stimulates the afferent brain, the brain will send out instructions, causing the diaphragm to have paroxysmal and spasmodic contraction. When the air is rapidly absorbed into the lung, the epiglottis will be closed, the glottis will suddenly narrow, and the sound of "hiccups" will be produced when the air flow is through. It has been proved that stimulation of the afferent fibers of vagus or other brain nerves can cause transient excitation of phrenic nerve.
Therefore, burping is a normal physiological phenomenon. Burping may be caused when eating or drinking too fast, suddenly eating stimulating food, inhaling cold air, laughing, and changing posture make the pressure on intercostal muscles (also respiratory muscles, located between each two ribs) suddenly change.
Why do people burp?
We only understand the principle of burping, but we haven't figured out the reason of burping.
In recent years, a reasonable conjecture has been made that French scientists have found that burping is probably a legacy of amphibian ancestors. They found that human hiccups are similar to some amphibians that still have gills. When pumping water into gills, these animals also need to squeeze their mouths and close their glottis to prevent water from entering their lungs.
A breath research team in Canada also believes that just as breathing, coughing, walking and other behaviors all have neural circuits in operation, we may also have a neural circuit to make hiccups, which is probably left over from the past evolution stage. For example, when a tadpole is half developed, it has lungs that breathe air and gills that draw oxygen from the water at the same time. At this stage, the pressure pump activity at the mouth of the tadpole is almost the same as burping.
In addition, ultrasonic examination found that two month old fetus who had not yet breath movement also belched in the womb. Some theorists believe that the contraction of fetal respiratory muscles is to prepare for breathing after birth, while others believe that it is to prevent amniotic fluid inhalation. However, the opposition said that if the liquid is to be prevented from entering the lungs, the fetus should cough more.
According to evolutionary theory, the researchers believe that the reason why hiccups are preserved should have a role: one role may be to guide the sucking of infants to ensure that milk does not run into the lungs. Another possibility is to control the exhaust so that the gas can be discharged from the over full stomach.
But these are all conjectures, and no experiment can prove them clearly. But we can also speculate that people who are prone to hiccups are not 'underdeveloped' -- just like people with four wisdom teeth, they retain more ancestral traits.