You have probably observed that after crying for a long time, it’s very difficult to take a deep breath without that tracheal contraction that makes you go huh-huh-huh. The following comments explain why this occurs.
Intense crying can cause less oxygen to enter the brain, therefore the contractions are like yawning, it’s supposed to allow more oxygen to enter.
Oxygen levels/anxiety has too many inconsistencies to be wholly correct. For staters, a baby is more likely to just go blue if he/she has widespread hypoxia. Cerebral hypoxia may lead to a reduction in level of consciousness. Anxiety also seems inconsistent in that those “hiccups” may persist despite the subject ceasing crying.
Other random observations that would support a diaphragmatic nature/spasms to the “hiccups”
Excessive crying is essentially prolonged periods of forced exhalations which naturally requires the diaphragm to contact for long periods of time – could explain spasms of a muscle after prolonged use. Methods to alleviate the “post crying hiccups” are oddly quite similar to alleviating “normal” hiccups: valsalva manoeuvre, etc…
When we’re stressed or upset, we produce stress hormones and our breathing becomes fast. We’re often told to “take deep breaths” to relax us. That’s why it’s a fundamental part of relaxation activities like yoga and meditation, our rate of breathing has direct effects on the chemicals we produce which determine how we feel.
When most of us are asked to take a deep breath, we inhale quickly, pushing our chests out and our shoulders up. But when we do that we only fill the top parts of our lungs. What we should be doing is taking in a long and deep breath, with the aim of filling our lungs with as much air as possible. The kind of breath you’d need if you were playing a wind instrument. Air right to the bottom of the lungs, shoulders stay where they are, tummy pushes out. When we do this, we’re stretching and controlling our diaphragm, a key muscle responsible for the rate at which we’re breathing. We’re making our body think everything’s fine, there’s no danger, and to be at ease. So our body responds accordingly, balancing out those chemicals and hormones.
The huh-huh-huh after intense crying is our body doing this for itself. It’s stretching the diaphragm (which has been working overtime while you’ve been crying) and forces deep breaths. This has effects on the production and levels of cortisol, adrenaline and other chemicals and hormones we produce when things are scary or sad.
It calms us down.
Pro tip: hiccups are a spasm of the diaphragm. To get rid of hiccups, breath deeply (the method above) and hold for as long as you possibly can, then exhale sharply. While you’re holding your breath, really push the air to the bottom of your lungs. If you can even feel a hiccup while you’re holding your breath, then you don’t have enough air in there. Once you eventually exhale, the hiccups will be gone. It’s the hiccup equivalent of pulling your foot back when your calf cramps, and is the basis for the “receiving a fright” hiccup cure.
Bonus tip: breathing in this way when you’re stressed will calm you down and make you feel normal again. But breathing is also linked with sleep hormones, so if you breathe in this way when you’re not stressed, and already feeling normal, you’ll soon find yourself yawning.