The digestive system consists of a 6 m long convoluted alimentary tube which is supported by several accessory organs of digestion, the liver, gallbladder and pancreas. It functions to extract nutrients from food in a number of processes including mastication (chewing), swallowing, mechanical and chemical break down of food, absorption and defecation (elimination of waste).
The mouth is where digestion begins and is formed by the hard and soft palates above, by the closed lips infront and by the buccinators (muscles of the cheeks) and mucosa of the cheeks to the sides. The tongue and teeth are contained within the mouth; the tongue forms its floor,and the teeth form two archs along the front and sides of the mouth deep to the lips and cheeks.
The hard palate is formed by the palatine and maxillae bones. The maxilla houses the upper teeth, whilst the lower teeth are held in the mandible. The soft palate is muscular and hangs from the back of the hard palate, separating the mouth and pharynx. From either side of the soft palate the palatoglossal and palatopharyngeal folds run downwards, the palatine tonsils lie in between.
Within the digestive system the mouth functions to take in the food (ingest), taste and sense the texture and temperature of food, hold and manipulate the food under the teeth, facilitate chewing (mechanical digestion), the start of chemical digestion and swallowing.
The tongue is composed of intrinsic and extrinsic muscles. The intrinsic muscles can be found just under the mucosa of the tongue, and the extrinsic muscles form the bulk of the tongue and attach to the hyoid bone and the mandible (lower jaw). Its surface is covered in taste buds and projections called papillae, which increase its surface area. For more information on taste buds see the Special Senses chapter.
Within the digestive system the tongue functions to taste and sense the texture and temperature of food, manipulate the food under the teeth for chewing, compresses the food into a small round bolbus for swallowing and aids in the action of swallowing.
The deciduous (primary) teeth erupt during a child's first or second year. They are replaced by permanent teeth between the ages of 6 and 20. There are four different types of teeth;
Each tooth is made up of three parts;
In the centre of each tooth is a pulp cavity filled with pulp (nerves and vessels), surrounding which is the dentin. The dentin of the crown is covered with enamel, whilst that of the root is covered with cement. Each root is embedded in the alveolus of the maxilla or mandible and secured in place by a periodontal ligament. They contain a root canal for the transmission of nerves and vessels and at their apex (inferior ends) a foramen for the exit and entry of those nerves and vessels.
Saliva is a substance consisting mainly of water, with small amounts of digestive enzymes, mucus and salts. It is produced by minor and major salivary glands. The minor glands include the labial, buccal, palatoglossal, palatal and lingual glands. The three major glands are the parotid, submandibular and sublingual glands;
The salivary glands produce saliva continuously in small amounts to keep our mouth lubricated. A reflex stimulates the salivary glands to produce larger amounts of saliva, when food is smelt, seen, tasted or even thought about. Saliva functions to soften and lubricate the food so that it can be swallowed, and contains a number of enzymes that start to chemically break down the food such as salivary amylase and lingual lipase. Amylase breaks down carbohydrates (into maltose and dextrin) and lipase breaks down fats. Saliva also cleanses the teeth and mouth and keeps the bacteria population under control.
The pharynx is a muscular tube consisting of a number of constrictor muscles lined with a mucous membrane that joins the nasal and buccal cavities with the oesophagus and larynx. It consists of three parts, the nasopharynx, oropharynx and laryngopharynx. The nasopharynx is situated behind the nasal cavities, the oropharynx behind the buccal cavity and the laryngopharynx behind the larynx.
When food has been chewed, it is carried to the back of the mouth, formed into a small round bolbus and swallowed. During swallowing the soft palate moves backwards to block the nasal cavity and the epiglottis moves downwards to block off the entrance to the larynx. Food can then be pushed safely by the constrictor muscles along the oropharynx and laryngopharynx into the oesophagus.
The oesophagus is a 25 cm long muscular tube which joins the inferior end of the pharynx (laryngopharynx) to the stomach. It originates in the neck at the level of C6, the common carotid arteries and back of the thyroid gland lying by its sides. It descends through the thorax behind the trachea and in front of the vertebral column. At the level of T10 it passes into the abdomen by passing through the diaphragm at the oesophageal aperture where it soon enters the cardiac region of the stomach.
The oesophagus can be named in three sections according to its location, the cervical, thoracic and abdominal parts. The walls of the oesophagus are made up of four layers;
At the inferior end of the oesophagus is a constriction known as the cardiac sphincter formed by muscles fibers in the wall of the oesophagus as well as external muscule fibers from the diaphragm. Within the wall of the inferior end of the oesophagus is a ring of smooth muscle known as the lower oesophageal sphincter. It remains contracted except when swallowing, and is thought to help prevent the reflux of food from the stomach back into the oesophagus. An external sphincter formed by the encircling fibers of the diaphragm as the oesophagus passes through the oesophageal aperture contracts during inspiration and when their is an increase in intra-abdominal pressure and also prevents the gastro-oesophgeal reflux.
The oesophagus is innervated by the vagus nerve (cranial nerve), splanchnic nerves and sympathetic trunks.
The oesophagus forces food into the stomach by powerful waves of contractions along its muscular walls called peristalsis. Food may also be moved back up the oesophagus during vomiting.
The stomach is a large distendible muscular sac, which varies in size and shape from individual to individual. It lies to the left of the upper abdomen immediately below the diaphragm. Its upper end (cardiac region) is continuous with the lower end of the oesophagus and its lower end (pyloric canal) is continuous with the first part of the small intestine (duodenum).
It has greater and lesser curvatures and is divided into the fundus, body, pyloric antrum and pyloric canal. Both openings in and out of the stomach are surrounded by a sphincter. The cardiac sphincter lies at the junction between the stomach and oesophagus and is discribed with the oesophagus above. The pyloric sphincter is the strongest sphincter and lies at the opening between the stomach and the duodenum.
The stomach wall consists of 4 layers;
The stomach functions to store and mechanically churn food to break it down and mix it with the digestive enzymes. The pyloric antrum is the most active area of the stomach in this respect, it contracts and relaxes to break up the food and send it in waves to the small intestine via the pyloric canal and sphincter.
The stomach can contain 1500 ml of liquified food which will remain in the stomach for 1-3 hours, depending on the nature of the food and the muscularity of the stomach. The stomach secretes gastric juice which contains water, mucus, hydrochloric acid and the enzyme pepsinogen.
Hydrochloric acid has no direct digestive role, but will lower the pH of the stomach, kill micro-organisms and convert pepsinogen to pepsin, which digests proteins. Mucous lines the stomach to protect itself from these acidic digestive juices.
A peptic ulcer is an area of the stomach or duodenal lining which becomes eroded by stomach acid. They result when the mechanisms used to protect the lining of the stomach or duodenum fail. Ulcers may become perforated, forming a hole in the stomach wall, causing a possibly fatal situation.
At least 90% of all peptic ulcers are caused by Helicobacter pylori, but other factors, including stress, alcohol and aspirin, can also cause them.
The small intestine is the first part of the gut; it extends for 6 m from the pylorus of the stomach to the caecum. It is the longest and most convoluted part of the alimentary canal and lies within the boundaries of the large intestine, within the centre of the abdominal cavity. Its walls are composed of the same 4 layers as the stomach.
The small intestine is split into three parts; the duodenum, jejunum and ileum;
The first, shortest and widest part of the small intestine, it leaves the pylorus of the stomach and forms a C-shaped curve around the head of the pancreas.
When it receives food, it secretes hormones which trigger the release of bile from the gallbladder and liver, and pancreatic enzymes from the pancreas. These chemicals enter the duodenum on its medial side via the hepato-pancreatic ampulla and take part in the further chemical digestion of the food.
The jejunum and ileum are covered with peritoneum, a thin serous coating which continues as a mesentery to anchor the intestines to the back of the abdominal wall. The duodenum however, lies behind the peritoneum, which covers the abdominal wall.
The small intestine is the main digestion and absorption site for proteins, carbohydrates and fats using intestinal and pancreatic juices. The muscular wall uses peristalsis to knead the food, bringing it in contact with the intestinal wall. Its internal surface is permanently folded, and covered in villi, projections which increase the surface area of the small intestine to that of a tube 500 m long.
The large intestine is 1.5 m long. It receives the ileum of the small intestine and traverses the abdomen to terminate in the anus. It is composed of the same layers as the small intestine and is split into seven parts;
A narrow, blind-ended, worm-like tube that opens into the caecum below the ileocaecal junction. It is only fixed at one end and therefore may lie in many different positions;
The transverse colon is the horizontal continuation of the ascending colon. When it reaches the inferior surface of the spleen it forms the left colic flexure by turning downwards, to continue as the descending colon.
The transverse colon in our model is a U-shaped colon however, it is normal for its course to vary from individual to individual;
The large intestine (apart from the rectum) is also anchored to the posterior abdominal wall by peritoneum.
The large intestines function to absorb water and salts, and excrete waste. The waste is very fluid when it enters the large intestine but once the water has been absorbed, only cellulose and bacteria remain. Peristalsis only occurs 3-4 times a day to move waste into the sigmoid colon. Defecation occurs when waste passes from the sigmoid colon into the rectum, this causes a reflex contraction of the rectal muscles and the waste is expelled. This movement can be inhibited voluntarily by the external anal sphincter.
The peritoneum is a continuous serous membrane that lines the abdominal walls (parietal peritoneum) and contents (visceral peritoneum). In the male it forms a closed sac, but in the female the uterine tubes form an opening.
The peritoneum covering the stomach extends from the greater curvature as a flap that hangs over the front of the intestines. It then doubles back on itself to cover the transverse colon, forming the greater omentum. The lesser omentum extends between the liver and the lesser curvature of the stomach. The large and small intestines are anchored to the back of the abdominal wall via similar peritoneal mesenteries.
The peritoneum functions to prevent friction as well as hold the abdominal contents in place, carry vessels and prevent the spread of infection.
The digestive system is assisted by and closely related to the liver, gallbladder and pancreas. These organs also have other functions that will be discussed in more detail in the relevant chapters.
The liver is the largest gland in the body and lies inferior to the right hand side of the diaphragm, but does not usually extend below the costal margin. It has four lobes, the right lobe is the largest and lies over the right colic flexure of the large intestine, and the left lobe is smaller and lies over the stomach. The small caudate and quadrate lobes lie vertically in between the right and left lobes. The liver is suspended from the diaphragm by several ligaments.
The liver is very important as it has many essential functions. The liver acts as a storage unit for fats, vitamin A, D and B12, iron and glucose (in the form of glycogen). It breaks down fats, amino acids, sugars, toxins (including alcohol and drugs) and old cells. It produces heat, vitamin A, plasma proteins (including prothrombin and fibrinogen essential inclotting of blood), antibodies and heparin.
Two of the substances which the liver produces that are involved in digestion are urea and bile.
The gallbladder is a 7-10 cm long brown-green sac found under the inferior surface of the right lobe of the liver. Behind the liver, the gallbladder constricts to become the cystic duct, which runs for 3-4 cm before joining the common hepatic duct to form the bile duct.
If the bile secreted by the liver is not needed immediately, it travels down the cystic duct and is stored in the gallbladder. The gallbladder can hold around 40 ml of bile and will continually absorb water to further concentrate it. When food enters the duodenum, the gallbladder will contract to force bile through the hepato-duodenal ampulla via the bile duct.
The pancreas is a 12-15 cm long tadpole-shaped organ that lies horizontally deep to the stomach. The duodenum encircles the head, the tail contacts the spleen and the body lies in between. It is covered in front by parietal peritoneum. Each lobule of the pancreas is drained into the centrally running main pancreatic duct. This joins with the bile duct to enter the duodenum at the hepatico-pancreatic ampulla.
The pancreas functions to produce an enzyme-rich pancreatic juice which helps break down proteins to amino acids, starch to maltose and fats into fatty acids and glycerol. It also has an endocrine role as specialised cells produce the hormones glucagon and insulin. These hormones control blood sugar levels by initiating the conversion of glycogen to glucose and vice versa. This function is discussed in more detail in the Endocrine chapter.