The mouth

The mouth, or oral cavity, has boundaries of muscle and bone and is lined by mucous membrane. The lips protect its anterior opening, the cheeks form the lateral walls, the hard palate forms its anterior roof, and the soft palate forms its posterior roof. The uvula is a finger-like muscular projection hanging down from the midline of the soft palate. It helps prevent the entry of food and fluids into the nasal cavities.

The oropharynx

The oropharynx is the muscular canal forming the passage between the oral cavity and the major parts of the alimentary canal. Lined by mucous membrane, it is a tube about 13 cm long and is continuous with the nasopharynx above and the oesophagus below.

The oesophagus

The oesophagus is a muscular tube about 20–25 cm long, extending from the pharynx above to the stomach below. It lies behind the larynx and trachea, in the midline through the neck and thorax, and passes through the diaphragm to join the stomach. The oesophagus has an outer layer of fibrous tissue, a layer of involuntary muscle, a layer of connective tissue, and a lining of mucous membrane. The function of the oesophagus is to carry food to the stomach by means of peristaltic action.

Peristalsis is a wave-like progression of alternate contraction and relaxation of the muscle fibres of the oesophagus or intestines, by which contents are propelled along the alimentary canal. At rest, the opening between the oesophagus and pharynx (oesophageal sphincter) is closed. During swallowing the muscles contract and cause the sphincter to open, thereby allowing the bolus of food to pass down into the oesophagus. A wave of contraction in the circular muscle layer then propels the bolus down to the stomach. The bottom end of the oesophagus acts as a functional sphincter, which is normally in a state of tonic contraction. As the peristaltic wave approaches the sphincter, the muscle relaxes and allows food to enter the stomach. The sphincter then closes again and prevents regurgitation of gastric contents back into the oesophagus.

The stomach

The stomach is a hollow muscular organ that lies primarily in the upper left quadrant of the abdomen, beneath the diaphragm (Figure 33.3). It is commonly described as being ‘J shaped’, but the size and shape of the stomach varies according to its contents. The stomach is divided into four areas: the fundus (the upper portion); the cardia, where the oesophagus joins the stomach; the body, or main part of the stomach; and the pylorus (the narrowed lower portion).

Figure 33.3 The stomach

The ‘curvatures’ of the stomach are the lesser curvature, which is the medial border; and the greater curvature, which is the lateral border.

The opening of the oesophagus into the stomach is called the cardiac orifice, and is surrounded by a functional sphincter called the cardiac sphincter. The pyloric orifice is the opening between the stomach and the small intestine and is surrounded by the pyloric sphincter, which consists of a thickened layer of circular muscle and is normally partly open. Peristaltic waves in the stomach push some of the gastric contents through the orifice and into the duodenum. The orifice then closes.

The stomach wall consists of four layers:

The small intestine

The small intestine is a coiled muscular tube about 5–6 metres in length, extending from the pyloric end of the stomach to the large intestine. The small intestine is divided into several anatomically recognisable areas: the duodenum — the proximal section, which is the widest section and is about 20 cm long and curved; the jejunum, which is the middle section and is about 2.5 metres long; and the ileum, which is the distal section and is about 4 metres in length.

Covering the mucosa are very fine projections called villi (Figure 33.4). Intestinal glands in the mucous membrane secrete an intestinal juice containing enzymes to complete the digestion of food. Solitary and aggregated patches of lymphatic tissue (Peyer’s patches) are also found in the mucosa of the intestinal wall, especially in the lower ileum. An opening in the duodenum (ampulla of Vater) allows for the entry of the common bile duct, carrying bile from the liver, and the pancreatic duct, carrying pancreatic juice. At the junction of the ileum and the caecum of the large intestine is the ileo-caecal valve, which prevents a backward flow of contents from the large to the small intestine.

Figure 33.4 A villus

The large intestine

The large intestine is a muscular tube about 1.5 m in length and 6 cm in diameter, and extends from the end of the ileum to the anus (Figure 33.5). Lying in the abdominal and pelvic cavities, the large intestine may be divided into regions that are distinguished by their anatomical structure and position:

Figure 33.5 The large intestine

The wall of the large intestine has the same basic structure as that of the small intestine (the serosa, muscular layer, submucosa and mucosa).

The salivary glands

There are three pairs of salivary glands that secrete saliva into the mouth. The anatomical position of each pair is described above. Saliva is a watery fluid containing ions, mucin and the digestive enzyme salivary amylase. Salivation is largely initiated by sensory stimulation, including the presence of food in the mouth, and by taste and smell. Salivation may also be induced by the presence of irritating substances in the stomach or small intestine. Saliva has the following functions:

The pancreas

The pancreas is a soft gland, lying across the abdominal cavity behind the stomach. It is divided into a head, which fits into the curve of the duodenum; a central portion, or body; and a tail, which extends out to the spleen. The pancreatic duct runs centrally through the length of the pancreas, while smaller ducts carry the pancreatic juice secreted by the pancreas into the central duct. The pancreatic duct joins the common bile duct from the liver, to enter the duodenum (Figure 33.6).

Figure 33.6 The pancreas and neighbouring structures

The bulk of the tissue in the pancreas is composed of exocrine cells, which produce pancreatic juice. The pancreas secretes about 1200 mL of pancreatic juice daily. Pancreatic juice is a watery alkaline fluid rich in digestive enzymes. The enzymes and their actions are summarised in Table 33.1. The overall function of pancreatic juice is the digestion of nutrients. Scattered among the exocrine tissue are groups of hormone-secreting cells, the islets of Langerhans. The function of the islets of Langerhans is described in Chapter 41 on endocrine health, as these cells belong to the endocrine system.

TABLE 33.1 SUMMARY OF CHEMICAL DIGESTION

The liver

The liver is an organ situated in the upper part of the abdominal cavity, immediately beneath the diaphragm. The greater part of the liver lies in the right upper abdomen but the organ extends across to the left upper abdomen (Figure 33.7). The liver is divided into two parts, a large right lobe and a much smaller left lobe. Like the alimentary canal, the liver is almost entirely covered by a layer of peritoneum. Beneath this is a fibrous capsule, which is continuous with areolar connective tissue situated within the liver. The areolar tissue forms a tree-like structure, which carries branches of the hepatic artery, hepatic portal vein, bile ducts and lymphatic vessels. These vessels enter and leave the liver through the porta hepatis, a short transverse fissure on the inferior surface of the liver.

Figure 33.7 The liver

The hepatic artery carries oxygenated blood to the liver. The portal vein carries deoxygenated blood, rich in nutrients from the small intestine, to the liver. Three hepatic veins carry deoxygenated blood from the liver to the inferior vena cava. The right and left hepatic ducts carry bile, secreted by the liver, to the common hepatic duct. The latter combines with the cystic duct from the gall bladder to form the common bile duct, which drains into the duodenum. The biliary tract, which transports bile from the liver to the duodenum, consists of the left and right hepatic ducts, the common hepatic duct, the cystic duct, the gall bladder and the common bile duct (Figure 33.8).

Figure 33.8 The biliary tract

The gall bladder

The gall bladder is a small muscular sac that lies on the inferior surface beneath the right lobe of the liver.

Functions of the liver and gall bladder include:

Digestion in the mouth

In the mouth food is broken down physically by the process of chewing (mastication), and mixed with saliva to bring about the formation of a moist ball, or bolus. Chewing softens the food so that it passes more easily through the alimentary canal. The presence of food in the mouth, together with its taste and smell, stimulates the secretion of saliva, gastric and pancreatic juices and bile (by means of parasympathetic pathways). The enzyme salivary beta-amylase in saliva begins to digest starches.

After the food has been formed into a bolus it is passed through the pharynx and down the oesophagus into the stomach by the act of swallowing. Swallowing is a complex reflex regulated by a ‘swallowing centre’ in the medulla oblongata of the brain. Swallowing is initiated when the tongue muscles push the bolus upwards and backwards into the oropharynx. The soft palate is elevated and comes into contact with the posterior wall of the pharynx, thereby closing off the nasopharynx. The larynx is pulled upwards and forwards, and the bolus pushes the epiglottis back over the glottis to prevent food from entering the respiratory tract. The oesophageal sphincter opens and the bolus enters the oesophagus.

Peristaltic waves carry the bolus through the oesophagus, and the cardiac sphincter relaxes to allow food and fluids to enter the stomach. The cardiac sphincter contracts to close the cardiac orifice at the end of each wave of contraction of the oesophagus, then relaxes to open the orifice when the next wave of contraction begins.

Digestion in the stomach

The stomach stores the food and later releases it at a rate that is optimal for digestion. Food is mixed with gastric juice, thereby changing its consistency so that it will be more easily transported along the alimentary canal. The food is exposed to enzymes (pepsins) which begin the digestion of proteins, and the gastric juice converts ferric iron (Fe³⁺) to ferrous iron (Fe²⁺). When the stomach muscles are stretched by swallowed food, peristaltic contractions are stimulated, which results in a churning movement. When the food is mixed with gastric juice it develops a pasty consistency and becomes known as chyme. The rate of emptying of the stomach depends on the:

The average time for the stomach to empty after a meal is 4–6 hours. When the food has been well mixed in the stomach, peristalsis begins in the lower half of the stomach and forces the chyme through the pyloric sphincter. Because this sphincter is only partially opened, only small amounts of chyme enter it at one time. When the duodenum is filled with chyme a nervous reflex (the enterogastric reflex) occurs, which inhibits the vagus nerves from stimulating the stomach muscles, and slows the emptying of the stomach. This mechanism ensures that food does not enter the small intestine too rapidly, to enable its digestion.

Digestion in the small intestine

After it leaves the stomach, chyme is mixed with intestinal secretions as well as with bile and pancreatic juice. Digestion is completed and the products are absorbed through the villi of the intestinal wall. The wall of the small intestine is capable of several different types of movement:

Figure 33.9 Movements of the small intestineA: Peristalsis: food moves along the digestive tract, as neighbouring segments of the intestine contract and relax in turn. B: Segmentation: single segments contract and relax alternately. Because there are inactive segments between the active ones, the food mixes but does not move along the tract