Why gas exchange in human is called negative pressure breathing?

Gas exchange in humans is called negative pressure breathing because it involves creating a pressure difference between the lungs and the atmosphere. When the diaphragm and intercostal muscles contract, the thoracic cavity expands, reducing the pressure inside the lungs relative to atmospheric pressure. This negative pressure allows air to flow into the lungs, facilitating the intake of oxygen and the expulsion of carbon dioxide during exhalation.