User:LittleHow/Sigh (respiration)

A sigh is a spontaneous large breath of greater volume than those preceding or following it. They are more common in newborns than adults. They can occur during sleep both in REM and non-REM stages. Those associated by others as a sign of sadness, and in many languages the phase "sigh of relief" is associated with relaxation or relief. However they they also have a respiratory physiological function in resetting the homeostasis of respiratory regulation.

Physiology
A sigh is made by coupling a normal breath with a gasp at the end of its inspiration. This “breath on top of a breath” gives it a biphasic shape. A similar type of gasp can be artificially induced by passively inflating the lung in a phenomenon called the “paradoxical reflex of Head”. A less common pattern of sigh also exists in which "the secondary inspiratory airflow clearly begins in expiration, such that a “sawtooth” flow pattern is generated." Sighs are followed by apnea and this reduced breathing provides a ‘‘compensatory’’ response to the increased ventilation provided by the sigh.

Sighs increase the activity of the intercostal muscles during inspiration, and the abdominal muscles during expiration. They can prevent airlessness (atelectasis) in hypoventilated parts of lungs.

Newborns
Newborn infants sigh roughly seven times every ten minutes. Such sighs were relatively larger in infants than those in adults and have a characteristic biphasic inspiratory flow in which there is an abrupt change in flow rate halfway through the in-breath. These sighs have been suggested to occur as an "inflation-augmenting" reflex that increases lung volume following brief "interruption of ventilation". This inflation acts to restore the functioning of alveoli that have collapsed or filled with fluid, and reopening small airways in the lung.

Emotional
Research finds that people sigh more when faced with insoluble puzzles after making futile solution attempts at their solution. Such sighs are I proposed to function as a means of creating a pause in which the futile activity, plan or desire can be discarded before turning to something new. Sighs occur in nonhuman mammals such as rats in which case they have been suggested to function as a social signal of safety. In humans sighs occur after periods of stress and during period of relief.

Hyperventilation
Breathing that is irregular and punctuated by deep sighs can lead to hyperventilation.