Digestion

The human digestive system is a long coiled tube extending from the mouth to the anus, with parts listed in the direction of food passage. The sequence is the oral or buccal cavity, the oesophagus, the stomach, the small intestine (duodenum, jejunum, and ileum), and the large intestine (ascending colon, transverse colon, descending colon, caecum, and rectum). The sigmoid colon is also part of the large intestine. Useful dimensions include the jejunum, the middle region of the small intestine, at about $2.5$ metres; the ileum, the last region, at about $3.6$ metres; and the large intestine, shorter at about $1.5$ metres but much wider, about $6.5$ cm across. Digestion occurs at three primary sites: the oral cavity, the stomach, and the small intestine. This number is important, as questions frequently test the initial site of digestive processes.

The mouth performs four functions before enzymatic action begins. It selects food using taste, smell, and sight, primarily through the tongue's taste buds. It grinds food through mastication with molar teeth, a mechanical process that aids passage down the oesophagus and increases surface area for enzymatic attack. It lubricates food with saliva. Finally, it initiates digestion, a function that often surprises students. The human jaw has 20 deciduous teeth in childhood and 32 permanent teeth in adulthood; questions may involve identifying teeth from diagrams. The appendix is a small projection from the caecum, the first part of the large intestine. The ileocolic sphincter, located at the end of the ileum, regulates the passage of undigested material into the large intestine.

Swallowing involves three examinable phases. The oral phase is voluntary. Once food reaches the back of the mouth, the pharyngeal phase, an involuntary action, prevents aspiration into the lungs: the tongue moves up and back against the roof of the mouth, the soft palate rises to close the nasal opening, and the epiglottis, a cartilage flap, swings horizontally over the glottis as the larynx rises. The oesophageal phase then involves involuntary peristalsis, moving the bolus down. The cardiac sphincter, a muscular ring, is located at the junction of the oesophagus and stomach, preventing backflow and opening in response to a peristaltic wave.