The digestive tract is a series of hollow organs – the mouth, oesophagus, stomach, small and large intestines, and anus – connected to form a long tube. The tract has muscular walls that rhythmically propel food along the tube (see Peristalsis), breaking it down and mixing it with digestive juices. Muscular activity is controlled by a network of nerves that covers the tract. Several muscular valves control the passage of food and prevent it from moving backwards.
The tongue, teeth, and saliva work together to start digestion and aid swallowing. Teeth chop and grind food, increasing the surface area over which digestive enzymes in saliva can act. Saliva also softens food so that the tongue can mould it into a bolus, or ball, for swallowing.
Swallowing begins when you push a bolus (ball of food) towards the oesophagus with your tongue. This action triggers two involuntary events: the soft palate, the back of the roof of the mouth, closes off the nasal cavity, and the epiglottis, a flap of cartilage, tilts downwards to seal the trachea (windpipe).
In their normal positions, the soft palate and the epiglottis allow air to pass from the nasal cavity into the trachea.
While swallowing, the epiglottis tilts to seal the trachea, the soft palate lifts to close off the nasal cavity, and the bolus enters the oesophagus.
Oesophagus and stomach
The throat leads into the oesophagus, a muscular tube that propels food to the stomach. In the stomach, solid food spends up to 5 hours being churned to a pulp and mixed with gastric juice to form chyme before being squirted into the small intestine. Liquids pass from the mouth to the intestine in a matter of minutes.
Small and large intestines
At about 6.5 m (22 ft) in length, the intestines form the longest part of the digestive tract. In the small intestine, which consists of the duodenum, jejunum, and ileum, food mixes with digestive juices, and nutrients and water are absorbed into the blood. In the large intestine, which is divided into the caecum, colon, and rectum, faeces are formed before passing out of the anus.
Food is propelled along the digestive tract by a sequence of muscular contractions called peristalsis. The muscular wall behind a piece of food squeezes to push it forwards into the next part of the tract, where the muscle is relaxed. Other types of muscular action churn food in the stomach and form faeces in the colon.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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