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The digestive system is responsible for breaking down food we eat into smaller components so that nutrients can be easily absorbed by the body and the waste discarded.
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There are two types of digestion. Mechanical digestion is the physical breakdown of large pieces of food into smaller pieces through, chewing (mastication). While chemical digestion uses enzymes to break down this food mass further into small molecules which the body can separate and use.
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Saliva in our mouths plays a key role in initial digestion by moistening the food to help with the mechanical chewing and swallowing process. Saliva also contains an enzyme which starts the chemical digestion of starchy foods.
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Our salivary glands produce around 1.5 litres of saliva each day!
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Bolus is the name of the small round slurry mass produced for swallowing as a result of chewing and starch digestion.
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The pharynx, at the back of the throat, has a flap of tissue called the epiglottis that closes during swallowing to prevent food going down the trachea (windpipe).
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Once swallowed, bolus (food) travels down through the esophagus to the stomach, taking about 7 seconds to get there.
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Muscles in the esophagus tighten and relax to create a wave-like process called peristalsis which pushes food down the small tube, which is why your food never falls back out if you happen to be eating and swallowing upside down!
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Enzymes called proteases break down proteins within the stomach and small intestine. While in saliva, amylases break down carbohydrate and lipases break down fats.
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The adult stomach has a very small volume when empty but can expand to hold up to 1.5 litres of food when full.
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The inner wall of the stomach secretes hydrochloric acid to help kill bacteria and, along with proteases enzymes, aids in the digestion of food. To protect itself from the corrosive acid, the stomach lining must create a thick coating of mucus.
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Stomach rumblings (borborygmi) are caused by wave-like muscular contractions (peristalsis) at the walls of the stomach and small intestine. These are normal digestion movements, however the process is louder and more noticeable when the stomach is empty as the sound is not muffled.
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Some animals such as cows, giraffes and deer have stomachs with multiple compartments (not multiple stomachs as is commonly believed). While others like seahorses, lungfishes and platypuses have no stomachs at all.
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The small intestine is composed of a duodenum, jejunum, and ileum.
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Most of the digestion and absorption of food nutrients actually takes place in the small intestine. The stomach passes on a thick liquid called chyme and enzymes continue to break this down in the small intestine which absorbs the nutrients into the bloodstream.
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The pancreas secretes enzymes for use by the small intestine.
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On average, the human adult male's small intestine is 6.9 m (22 ft 6 in) long, and the female's 7.1 m (23 ft 4 in).
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The large intestine includes the cecum, appendix, colon, and rectum. It is the final part of the digestive system. It absorbs water from the remaining indigestible food matter, and passes any un-needed waste from the body.
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The large intestine is approximately 1.5 m (4.9 ft) long.
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The liver produces bile for the digestive system and processes the nutrients.
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The gall bladder stores the bile used to break down dietary fat.
