Final answer:
The insert inside the facepiece of an SCBA that covers the mouth and nose is called a respiratory mouthpiece, responsible for delivering clean air in hazardous environments.
Step-by-step explanation:
An insert inside the facepiece of an SCBA (Self-Contained Breathing Apparatus) that fits over the user's mouth and nose is known as a respiratory mouthpiece. This component allows the wearer to breathe clean air in environments that are smoke-filled, toxic, or otherwise hazardous. The SCBA is a critical piece of equipment for firefighters and others working in dangerous conditions, using technology developed partly by NASA to ensure lightweight, durable, and effective protection. Pulmonary ventilation occurs during inhalation when the thoracic cavity expands, creating negative pressure in the lungs, allowing air to enter. The respiratory system works efficiently, with air being warmed, humidified, and filtered through the nasal cavity before reaching the lungs.