The digestive tract is coupled to the lungs at the esophagus where pressure is less than atmospheric. The rest of the digestive tract is mostly above atmospheric, food being propelled by peristalsis . Peristalsis is a wave of smooth muscle contractions where there is a contraction above and relaxation below a segment of the tract. Pressure in the stomach increases when stretched by eating. Trapped air in the stomach causes belching.
Gas is generated in the gut by bacterial action on fibre.
Fibre is important because it keeps things moving. He used a sharpened goose quill to puncture the artery and the goose's trachea for the flexible connection to the glass tube. This is the low pressure side. The difference in the heights of mercury, gives the pressure above atmospheric pressure. In 1849 Eugene Bourdon invented the Bourdon tube, which is an oval tube bent into a circular arc.