There are two effects.
The first is an increase in circulation results in an increase of metabolism. Especially the short term reactions. These include serotonin. An increased heart rate adds to this effect.
Because serotonin is such a powerful neurotransmitter, its levels take a while to return to normal, thus the "feeling good" afterwards since other metabolic processes are slowing down, yet you still have a high supply of serotonin.
Intense periodic exercise (even just 30 minutes a day), causes the brain to become "used" to serotonin, thus producing more serotonin receptors on the neurons to counter the "addiction". Thus, periodically raising your temporal serotonin levels through daily exercise, results an increase in "happiness" (although this evens out eventually), and a mild "depression" (caused by addiction to your own brain chemical) when you miss a few exercise sessions in a row. People who train often can attest to this.
serotonin and dopamine
Vitamin A
serotonin
yes
Breathing.
Exercise (very important!), get plenty of sleep, sunshine, and eat a balanced diet. Some fruits and veggies can help boost serotonin levels.
exercise will stimulate the muscles of the intestinal walls for increased peristalsis
To increase the oxygen intake to stimulate the heart and lungs. You can try exercises like pursed-lip breathing and deep belly breathing to open your airways and increase the amount of oxygen in your body.
Exercise and sunlight can help, but ultimately he'll need medical help if he has a serious problem.
Overabundance of serotonin can cause serotonin syndrome.
Hydroxyzine decreases serotonin levels as it a serotonin antagonist.
exercise, sleep early, good diet, avoid junk, take some anti depressant medecine, or supplements and after that talk with a doctor