The anatomy of the digestive system

Starting with the teeth, travelling through the gastrointestinal tract and ending with the rectum and anus, we've put together 14 images that take you on a tour through the digestive system.

Image research and captions by Benjamin Thompson, adapted from Wellcome Images" target="_blank">Wellcome Images.

Milk teeth

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Teeth are calcified structures found within the jaw, used to break down food into smaller particles. The white part of the tooth is the enamel, made of a very hard mineral known as hydroxyapatite. Underneath is a substance called dentine. Less brittle than enamel, it is used to add support to the tooth. The centre of the tooth comprises the pulp. This area is filled with soft tissue containing blood vessels and nerves. The cementum covers the root of the tooth. Humans have four types of teeth: incisors, which are found at the front of the mouth and are designed for cutting food; canines, which grip and tear food; and premolars and molars, which chew and grind food. The image above shows a child’s deciduous teeth, also known as baby or milk teeth. These fall out during childhood to be replaced by a single set of permanent teeth.

Credit: Wellcome Photo Library, Wellcome Images.

Anatomy of the tongue

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The tongue is the primary organ of taste, and its upper surface is covered in taste buds. The structure of a taste bud can be seen in the top right of the image. The tongue is also used to move food around the mouth during chewing. The tongue consists of eight muscles: four that act to change its shape and four that are used to change its position. The tongue has a rich network of nerves and blood vessels and is known as a ‘muscular hydrostat’ - made mainly of muscle, with no skeletal support. Its shape is maintained by the water within its muscles.

Credit: Medical Art Service, Munich/Wellcome Images.

Gustatory centre of the brain

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Humans are able to distinguish between five different tastes: sweet, sour, bitter, salty and umami. These are detected by the different taste buds present on the tongue. Contrary to popular belief, there are not specific areas of the tongue that detect different flavours. Some areas are more sensitive to particular tastes than others, however. The taste buds are structures known as papillae and contain taste cells. These cells detect the presence of flavour molecules, which induces a nerve signal that alerts the brain to the flavour’s presence. The nerve signals are transported to the brain via three cranial nerves.

Credit: Medical Art Service, Munich/Wellcome Images.

Fruit bat tongue

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Scanning electron micrograph of the tongue of a fruit bat. This image shows the papillae, small bumps that cover the surface of the tongue, which efficiently scoop up nectar and pollen.

Credit: Kevin MacKenzie, University of Aberdeen, Wellcome Images.

Illustration of swallowing

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The act of swallowing food is a complex process controlled by multiple nerves and muscles. It is divided into three phases: oral, pharyngeal and oesophageal. During the oral phase, food is moistened by saliva and broken down by the teeth. This ball of food, known as the ‘bolus’, is moved by the tongue to the back of the mouth and into the oesophagus (foodpipe). At this time, the nasopharynx and larynx are closed to prevent food from entering the trachea (windpipe). A wave of contractions in the smooth muscle behind the bolus moves the food down the oesophagus and prevents it from returning to the mouth. Food enters the stomach via the oesophageal sphincter.

Credit: Medical Art Service, Munich/Wellcome Images.

Stomach

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The stomach contains a variety of chemicals and molecules designed to break down food. The stomach consists of four separate layers of muscle and connective tissue. The uppermost layer, or ‘epithelium’, has multiple folds, as can be seen in the image above. Within these folds are numerous ducts that produce mucus. This mucus is essential to protect the stomach from its acidic contents. Food enters the stomach via the oesophageal sphincter and becomes mixed with both stomach acid and digestive enzymes. The digestive enzymes in the stomach work optimally at an acidic pH. The stomach acid provides this environment and also kills many microorganisms. Pepsin is the main digestive enzyme. It breaks down the proteins in food into smaller chains of amino acids, known as peptides. The partially digested food, known as chyme, leaves the stomach via the pyloric sphincter and enters the duodenum - the first section of the small intestine. Here, more enzyme-based degradation occurs.

Credit: Medical Art Service, Munich/Wellcome Images.

Liver, pancreas and gall bladder

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The image above shows a cross-section of some organs that are important in the digestion of food. The large organ at the top is the liver. This is the largest internal organ and has a multitude of functions, including the storage of glucose as glycogen, the breakdown of toxins and the production of bile, which is needed for the digestion of fats. The yellow organ in the centre of the image is the pancreas. This organ secretes a number of important hormones into the bloodstream, including insulin. The pancreas also releases digestive enzymes into the small intestine to aid in the breakdown of partially digested food. The small organ to the left of the pancreas is the gall bladder, which stores bile produced by the liver.

Credit: Medical Art Service, Munich/Wellcome Images.

Structure of the jejunal wall

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Once foods have left the first section of the small intestine they enter middle section, known as the jejunum. A diagram showing the cross-section of this part of the intestine can be seen above. Partially digested food is passed through the jejunum via a wave of muscular contractions, known as ‘peristalsis’. Muscles contracting behind the ball of food push it slowly though the intestines and prevent it from going backwards. The inside of the small intestines are covered in finger-like projections called villi, which are themselves covered in microvilli. These projections drastically increase the surface area within the small intestine and are needed to absorb nutrients released.

Credit: Medical Art Service, Munich/Wellcome Images.

Villi in the small intestine

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A scanning electron micrograph of villi within the small intestine. Food particles can be seen within the crevices between the villi. This picture is 450 micrometres (0.45 millimetres) wide.

Credit: Professor Alan Boyde, Wellcome Images.

Cross-section of villi in the small intestine

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This image shows a cross-section of two intestinal villi. The rich network of blood vessels within each can be seen. Molecules such as amino acids or sugars pass through the intestinal wall by diffusion. By having a large surface area, the intestine increases the likelihood of absorbing these molecules. The blood vessels transport these nutrients away.

Credit: Medical Art Service, Munich/Wellcome Images.

Goblet cell

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Goblet cells are found within many parts of the human body, including the respiratory tract, the eyes and the small intestine. The cells release mucous globules that provide lubrication and protection to the body’s surfaces. These globules can be seen in the image above, stained blue. They are condensed within the goblet cell, but once released they absorb water, expanding in around 20 milliseconds.

Credit: University of Edinburgh, Wellcome Images.

Bacteria in the intestines

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The surface of the lower human digestive tract is covered by an enormous number of microorganisms. These are known as the ‘gut flora’. There are hundreds of different species present in the gut, and it is believed that there are ten times as many bacterial cells present than eukaryotic cells that make up the human body. Although it is possible to live without gut flora, the microorganisms perform a variety of important functions, including carbohydrate digestion and preventing disease-causing bacteria from growing. The image above shows pathogenic E. coli bacteria binding to the intestinal lining. The red staining shows the nuclei of the intestinal cells (large) and bacteria (small).

Credit: S Schuller, Wellcome Images.

Colonic crypt

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Once food has been digested within the small intestine, the remaining indigestible matter passes into the large intestine. This section of the digestive tract comprises the cecum, colon and rectum. Water and salts are absorbed in the large intestine, in addition to vitamins that are produced by bacteria that live there. The image above shows a cross-section of a colonic crypt. Mucin cells (stained red) within the crypt produce mucous that passes into the interior of the colon through crypt lumen, the area in the centre surrounded by blue. This mucus helps to ease the passage of faeces through the colon.

Credit: Michela Schaeppi, Wellcome Images.

Rectum

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The final portion of the large intestine is known as the rectum, which leads to the anus. Faeces are stored temporarily in the rectum before defecation.

Credit: Medical Art Service, Munich/Wellcome Images.