Chapter 12: The digestive system Flashcards
The digestive system
Describes the alimentary canal, its accessory organs and a variety of digestive processes that prepare food eaten in the diet for absorption. The alimentary canal begins at the mouth, passes through the thorax, abdomen and pelvis and ends at the anus. It has a basic structure which is modified at different levels to provide for the processes occurring at each level.
The process of the digestive system
The digestive processes gradually break down the foods eaten until they are in a form suitable for absorption. For example, meat, even when cooked, is chemically too complex to be absorbed from the alimentary canal. Digestion releases its constituents:
amino acids, mineral salts, fat, and vitamins. Digestive enzymes responsible for these changes are secreted into the canal by specialized glands, some of which are in the walls of the canal and some outside the canal, but with ducts leading into it.
What happens after absorption?
After absorption, nutrients provide the raw materials for the manufacture of new cells, hormones, and enzymes. The energy needed for these and other processes, and for the disposal of waste materials, is generated from the products of digestion.
Ingestion
This is the taking of food into the alimentary tract, i.e., eating and drinking.
Propulsion
These mix and moves the contents along the alimentary tract.
Digestion
This consists of:
- mechanical breakdown of food by, e.g., mastication (chewing)
- chemical digestion of food into small molecules by enzymes present in secretions produced by glands and accessory organs of the digestive system.
Absorption
This is the process by which digested food substances pass through the walls of some organs of the alimentary canal into the blood and lymph capillaries for circulation and use by body cells.
Elimination
Food substances that have been eaten but cannot be digested and absorbed are excreted from the alimentary canal as feces by the process of defecation
Organs of the digestive system
Also known as the gastrointestinal (GI) tract, this is essentially a long tube through which food passes. It commences at the mouth and terminates at the anus, and the various organs along its length have different functions, although structurally they are remarkably similar. The parts are:
- mouth
- pharynx
- esophagus
- stomach
- small intestine
- large intestine
- rectum and anal canal
Accessory organs
•Various secretions are poured into the alimentary tract, some by glands in the lining membrane of the organs, e.g., gastric juice secreted by glands in the lining of the stomach, and some by glands situated outside the tract. The latter are the accessory organs of digestion, and their secretions pass through ducts to enter the tract. They consist of:
-three pairs of salivary glands
-the pancreas
-the liver and biliary tract.
•The organs and glands are linked physiologically as well as anatomically in that digestion and absorption occur in stages, each stage being dependent upon the previous stage or stages.
The basic structure of the alimentary canal
•The layers of the walls of the alimentary canal follow a consistent pattern from the esophagus onwards. This basic structure does not apply so obviously to the mouth and the pharynx, which are considered later in the chapter.
•In the organs from the esophagus onwards, modifications of the structure are found which are associated with specific functions. The basic structure is described here and any modifications in structure and function are described in the appropriate section.
•The walls of the alimentary tract are formed by four layers of tissue:
-adventitia or serosa – outer covering
-muscle layer
-submucosa
-mucosa – lining.
Adventitia or serosa
This is the outermost layer. In the thorax, it consists of loose fibrous tissue and in the abdomen, the organs are covered by a serous membrane (serosa) called the peritoneum.
Peritoneum
Peritoneum, large membrane in the abdominal cavity that connects and supports internal organs. It is composed of many folds that pass between or around the various organs.
Muscle layer
With some exceptions, this consists of two layers of smooth (involuntary) muscle. The muscle fibers of the outer layer are arranged longitudinally, and those of the inner layer encircles the wall of the tube. Between these two muscle layers are blood vessels, lymph vessels, and a plexus (network) of sympathetic and parasympathetic nerves, called the myenteric plexus. These nerves supply the adjacent smooth muscle and blood vessels.
Submucosa
This layer consists of loose areolar connective tissue containing collagen and some elastic fibers, which bind the muscle layer to the mucosa. Within it are blood vessels and nerves, lymph vessels, and varying amounts of lymphoid tissue. The blood vessels are arterioles, venules, and capillaries. The nerve plexus is the submucosal plexus, which contains sympathetic and parasympathetic nerves that supply the mucosal lining.
Mucosa
This consists of three layers of tissue:
- mucous membrane formed by columnar epithelium is the innermost layer and has three main functions: protection, secretion, and absorption
- lamina propria consisting of loose connective tissue, which supports the blood vessels that nourish the inner epithelial layer, and varying amounts of lymphoid tissue that protects against microbial invaders
- muscularis mucosa, a thin outer layer of smooth muscle that provides involutions of the mucosal layer, e.g., gastric glands
Mucous membrane
A mucous membrane, also known as a mucosa (plural: mucosae), is a layer of cells that surrounds body organs and body orifices. It is made from ectodermal tissue.
Nerve supply
The alimentary canal and its related accessory organs are supplied by nerves from both divisions of the autonomic nervous system, i.e., both parasympathetic and sympathetic parts. Their actions are generally antagonistic to each other and at any time one has a greater influence than the other, according to body needs, at that time. When digestion is required, this is normally through increased activity of the parasympathetic nervous system.
The parasympathetic
One pair of cranial nerves, the vagus nerves, supplies most of the alimentary canal and the accessory organs. Sacral nerves supply the most distal part of the tract. The effects of parasympathetic stimulation on the digestive system are:
- increased muscular activity, especially peristalsis, through increased activity of the myenteric plexus
- increased glandular secretion, through increased activity of the submucosal plexus
The sympathetic supply
This is provided by numerous nerves that emerge from the spinal cord in the thoracic and lumbar regions. These form plexuses (ganglia) in the thorax, abdomen, and pelvis, from which nerves pass to the organs of the alimentary tract. The effects of sympathetic stimulation on the digestive system are to:
- decrease muscular activity, especially peristalsis, because there is reduced stimulation of the myenteric plexus
- decrease glandular secretion, as there is less stimulation of the submucosal plexus.
Mouth
- The oral cavity is lined throughout with mucous membrane, consisting of stratified squamous epithelium containing small mucus-secreting glands.
- The part of the mouth between the gums and the cheeks is the vestibule and the remainder of its interior is the oral cavity. The mucous membrane lining of the cheeks and the lips is reflected onto the gums or alveolar ridges and is continuous with the skin of the face.
- The palate forms the roof of the mouth and is divided into the anterior hard palate and the posterior soft palate. The hard palate is formed by the maxilla and the palatine bones. The soft palate, which is muscular, curves downwards from the posterior end of the hard palate and blends with the walls of the pharynx at the sides.
Tongue
The tongue is composed of voluntary muscle. It is attached by its base to the hyoid bone and by a fold of its mucous membrane covering, called the frenulum, to the floor of the mouth. The superior surface consists of stratified squamous epithelium, with numerous papillae (little projections). Many of these contain sensory receptors (specialized nerve endings) for the sense of taste in the taste buds
The blood supply for the tongue
The main arterial blood supply to the tongue is by the lingual branch of the external carotid artery. Venous drainage is by the lingual vein, which joins the internal jugular vein.
Nerve supply for the tongue
The nerves involved are:
- the hypoglossal nerves (12th cranial nerves), which supply the voluntary muscle
- the lingual branch of the mandibular nerves, which arise from the 5th cranial nerves, are the nerves of somatic (ordinary) sensation, i.e., pain, temperature and touch
- the facial and glossopharyngeal nerves (7th and 9th cranial nerves), the nerves of taste
Functions of the tongue
•The tongue plays an important part in: -chewing (mastication) -swallowing (deglutition) -speech -taste •Nerve endings of the sense of taste are present in the papillae and widely distributed in the epithelium of the tongue.
Teeth
- The teeth are embedded in the alveoli or sockets of the alveolar ridges of the mandible and the maxilla. Babies are born with two sets, or dentitions, the temporary or deciduous teeth and the permanent teeth. At birth, the teeth of both dentitions are present, in immature form, in the mandible and maxilla.
- There are 20 temporary teeth, 10 in each jaw. They begin to erupt at about 6 months of age and should all be present by 24 months.
- The permanent teeth begin to replace the deciduous teeth in the 6th year of age and this dentition, consisting of 32 teeth, is usually complete by the 21st year.
Functions
Teeth have different shapes depending on their functions. Incisors and canine teeth are the cutting teeth and are used for biting off pieces of food, whereas the premolar and molar teeth, with broad, flat surfaces, are used for grinding or chewing food.
Structure
•Although the shapes of the different teeth vary, the structure is the same and consists of:
-the crown – the part that protrudes from the gum
-the root – the part embedded in the bone
-the neck – the slightly narrowed region where the crown merges with the root.
•In the center of the tooth is the pulp cavity containing blood vessels, lymph vessels and nerves, and surrounding this is a hard ivory-like substance called dentine. The dentine of the crown is covered by a thin layer of very hard substance, enamel. The root of the tooth, on the other hand, is covered with a substance resembling bone, called cementum, which secures the tooth in its socket. Blood vessels and nerves pass to the tooth through a small foramen (hole) at the apex of each root.
Blood supply of the teeth
Most of the arterial blood supply to the teeth is by branches of the maxillary arteries. The venous drainage is by several veins which empty into the internal jugular veins.
Nerve supply of the teeth
The nerve supply to the upper teeth is by branches of the maxillary nerves and to the lower teeth by branches of the mandibular nerves. These are both branches of the trigeminal nerves (5th cranial nerves)
Salivary glands
Salivary glands release their secretions into ducts that lead to the mouth. There are three main pairs: the parotid glands, the submandibular glands and the sublingual glands. There are also numerous smaller salivary glands scattered around the mouth.
Parotid gland
These are situated one on each side of the face just below the external acoustic meatus. Each gland has a parotid duct opening into the mouth at the level of the second upper molar tooth.
Submandibular glands
These lie one on each side of the face under the angle of the jaw. The two submandibular ducts open on the floor of the mouth, one on each side of the frenulum of the tongue.
Sublingual glands
These glands lie under the mucous membrane of the floor of the mouth in front of the submandibular glands. They have numerous small ducts that open into the floor of the mouth.
Structure of the salivary glands
The glands are all surrounded by a fibrous capsule. They consist of several lobules made up of small acini lined with secretory cells. The secretions are poured into ductulus that join up to form larger ducts leading into the mouth.
The blood supply for salivary glands
Arterial supply is by various branches from the external carotid arteries and venous drainage is into the external jugular veins.
Composition of saliva
Saliva is the combined secretions from the salivary glands and the small mucus-secreting glands of the oral mucosa. About 1.5 liters of saliva is produced daily and it consists of:
- water
- mineral salts
- salivary amylase; a digestive enzyme
- mucus
- antimicrobial substances; immunoglobulins and the enzyme lysozyme.
Secretion of saliva
The secretion of saliva is controlled by the autonomic nervous system. Parasympathetic stimulation causes profuse secretion of watery saliva with a relatively low content of enzymes and other organic substances. Sympathetic stimulation results in the secretion of small amounts of saliva rich in organic material, especially from the submandibular glands. Reflex secretion occurs when there is food in the mouth and the reflex can easily become conditioned so that the sight, smell and even the thought of food stimulates the flow of saliva.
Chemical digestion of polysaccharides
Saliva contains the enzyme amylase that begins the breakdown of complex sugars, including starches, reducing them to the disaccharide maltose. The optimum pH for the action of salivary amylase is 6.8 (slightly acid). Salivary pH ranges from 5.8 to 7.4 depending on the rate of flow; the higher the flow rate, the higher is the ph. Enzyme action continues during swallowing until terminated by the strongly acidic gastric juices (pH 1.5–1.8), which degrades the amylase.
Lubrication of food
The high-water content means that dry food entering the mouth is moistened and lubricated by saliva before it can be made into a bolus ready for swallowing.
Cleaning and lubricating the mouth
An adequate flow of saliva is necessary to clean the mouth, and to keep it soft, moist, and pliable. This helps to prevent damage to the mucous membrane by rough or abrasive food.
Non-specific defense
Lysozyme and immunoglobulins present in saliva combat invading microbes.
Taste
The taste buds are stimulated only by chemical substances in solution and therefore dry foods only stimulate the sense of taste after thorough mixing with saliva. The senses of taste and smell are closely linked and involved in the enjoyment, or otherwise, of food.
Pharynx
In humans, it is a hollow structure (or muscular cavity) lined with a moist tissue. This is typical of all structures within our alimentary and digestive tracts.
Blood supply of the pharynx
The blood supply to the pharynx is by several branches of the facial arteries. Venous drainage is into the facial veins and the internal jugular veins.
Nerve supply of the pharynx
This is from the pharyngeal plexus and consists of parasympathetic and sympathetic nerves. Parasympathetic supply is mainly by the glossopharyngeal and vague nerves and sympathetic from the cervical ganglia.
Esophagus
Relatively straight muscular tube through which food passes from the pharynx to the stomach. The esophagus can contract or expand to allow for the passage of food. Anatomically, it lies behind the trachea and heart and in front of the spinal column.
Structure of the esophagus
There are four layers of tissue. As the esophagus is almost entirely in the thorax the outer covering, the adventitia consists of elastic fibrous tissue that attaches the esophagus to the surrounding structures. The proximal third is lined by stratified squamous epithelium and the distal third is by columnar epithelium. The middle third is lined by a mixture of the two.
Arterial
The thoracic region is supplied mainly by the paired esophageal arteries, branches from the thoracic aorta. The abdominal region is supplied by branches from the inferior phrenic arteries and the left gastric branch of the coeliac artery.
Venous drainage
From the thoracic region, venous drainage is into the azygos and hemiazygos veins. The abdominal part drains into the left gastric vein. There is a venous plexus at the distal end that links the upward and downward venous drainage, i.e., the general and portal circulations.
Formation of a bolus
When food is taken into the mouth it is chewed (masticated) by the teeth and moved around the mouth by the tongue and muscles of the cheek. It is mixed with saliva and formed into a soft mass or bolus ready for swallowing. The length of time that food remains in the mouth largely depends on the consistency of the food. Some foods need to be chewed longer than others before the individual feels that the bolus is ready for swallowing.
Swallowing
This occurs in three stages after chewing is complete and the bolus has been formed. It is initiated voluntarily but completed by a reflex (involuntary) action.
1 oral stage
With the mouth closed, the voluntary muscles of the tongue and cheeks push the bolus backward into the pharynx.
2 pharyngeal stages
The muscles of the pharynx are stimulated by a reflex action initiated in the walls of the oropharynx and coordinated by the swallowing center in the medulla. Involuntary contraction of these muscles propels the bolus down into the esophagus. All other routes that the bolus could take are closed. The soft palate rises and closes off the nasopharynx; the tongue and the pharyngeal folds block the way back into the mouth, and the larynx is lifted up and forward so that its opening is occluded by the overhanging epiglottis preventing entry into the airway (trachea).
3 esophageal stages
•Peristaltic waves pass along the esophagus only after swallowing begins. Otherwise, the walls are relaxed. Ahead of a peristaltic wave, the cardiac sphincter guarding the entrance to the stomach relaxes to allow the descending bolus to pass into the stomach. Usually, constriction of the cardiac sphincter prevents reflux of gastric acid into the esophagus. Other factors preventing gastric reflux include:
-the attachment of the stomach to the diaphragm by the peritoneum
-the acute angle formed by the position of the esophagus as it enters the fundus of the stomach, i.e., an acute cardio-esophageal angle
-increased tone of the cardiac sphincter when intra-abdominal pressure is increased and the pinching effect of diaphragm muscle fibers.
•The walls of the esophagus are lubricated by mucus which assists the passage of the bolus during the peristaltic contraction of the muscular wall.
Stomach
The stomach is a J-shaped dilated portion of the alimentary tract situated in the epigastric, umbilical, and left hypochondriac regions of the abdominal cavity.
Organs associated with the stomach
- Anteriorly – left lobe of the liver and anterior abdominal wall
- Posteriorly – abdominal aorta, pancreas, spleen, left kidney, and adrenal gland
- Superiorly – diaphragm, esophagus, and left lobe of the liver
- Inferiorly – transverse colon and small intestine
- To the left – diaphragm, and spleen
- To the right – liver, and duodenum.
Structure of the stomach
- The stomach is continuous with the esophagus at the cardiac sphincter and with the duodenum at the pyloric sphincter. It has two curvatures. The lesser curvature is short, lies on the posterior surface of the stomach and is the downward continuation of the posterior wall of the esophagus. Just before the pyloric sphincter it curves upwards to complete the J shape. Where the esophagus joins the stomach, the anterior region angles acutely upwards, curves downwards forming the greater curvature and then slightly upwards towards the pyloric sphincter.
- The stomach is divided into three regions: the fundus, the body and the pylorus. At the distal end of the pylorus is the pyloric sphincter, guarding the opening between the stomach and the duodenum. When the stomach is inactive the pyloric sphincter is relaxed and open, and when the stomach contains food, the sphincter is closed.
Walls of the stomach
• The four layers of tissue that comprise the basic structure of the alimentary canal are found in the stomach but with some modifications.
Muscle layer
•This consists of three layers of smooth muscle fibers:
-an outer layer of longitudinal fibers
-a middle layer of circular fibers
-an inner layer of oblique fibers.
•In this respect, the stomach is different from other regions of the alimentary tract as it has three layers of muscle instead of two. This arrangement allows for the churning motion characteristic of gastric activity, as well as peristaltic movement. Circular muscle is strongest between the pylorus and the pyloric sphincter.
Mucosa
When the stomach is empty the mucous membrane lining is thrown into longitudinal folds or rugae, and when full the rugae are ‘ironed out’ giving the surface a smooth, velvety appearance. Numerous gastric glands are situated below the surface of the mucous membrane and open onto it. They consist of specialized cells that secrete gastric juice into the stomach.
Blood supply of the stomach
Arterial supply to the stomach is by the left gastric artery, a branch of the coeliac artery, the right gastric artery, and the gastroepiploic arteries. Venous drainage is through veins of corresponding names into the portal vein.
Gastric juice and functions of the stomach
The gastric muscle generates a churning action that breaks down the bolus and mixes it with gastric juice, and peristaltic waves that propel the stomach contents towards the pylorus. When the stomach is active the pyloric sphincter closes. Strong peristaltic contraction of the pylorus forces chyme, gastric contents after they are sufficiently liquefied, through the pyloric sphincter into the duodenum in small spurts. Parasympathetic stimulation increases the motility of the stomach and secretion of gastric juice; sympathetic stimulation has the opposite effect.
Gastric juice
About 2 liters of gastric juice are secreted daily by specialized secretory glands in the mucosa. It consists of:
- water
- mineral salts
- mucus secreted by mucous neck cells in the glands and surface mucous cells on the stomach surface
- inactive enzyme precursors: pepsinogens secreted by chief cells in the glands