Chapter 12: The digestive system Flashcards
1
Q
The digestive system
A
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.
2
Q
The process of the digestive system
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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.
3
Q
What happens after absorption?
A
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.
4
Q
Ingestion
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This is the taking of food into the alimentary tract, i.e., eating and drinking.
5
Q
Propulsion
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These mix and moves the contents along the alimentary tract.
6
Q
Digestion
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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.
7
Q
Absorption
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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.
8
Q
Elimination
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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
9
Q
Organs of the digestive system
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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
10
Q
Accessory organs
A
•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.
11
Q
The basic structure of the alimentary canal
A
•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.
12
Q
Adventitia or serosa
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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.
13
Q
Peritoneum
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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.
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Q
Muscle layer
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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.
15
Q
Submucosa
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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.
16
Q
Mucosa
A
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
17
Q
Mucous membrane
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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.
18
Q
Nerve supply
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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.
19
Q
The parasympathetic
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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
20
Q
The sympathetic supply
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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.
21
Q
Mouth
A
- 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.
22
Q
Tongue
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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
23
Q
The blood supply for the tongue
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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.
24
Q
Nerve supply for the tongue
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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
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Functions of the tongue
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•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.
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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.
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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.
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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.
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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.
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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)
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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Non-specific defense
Lysozyme and immunoglobulins present in saliva combat invading microbes.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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1 oral stage
With the mouth closed, the voluntary muscles of the tongue and cheeks push the bolus backward into the pharynx.
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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).
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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
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Functions of gastric juice
Water further liquefies the food swallowed.
• Hydrochloric acid:
– acidifies the food and stops the action of salivary amylase
– kills ingested microbes
– provides the acid environment needed for the action of pepsins.
•Pepsinogens are activated to pepsins by hydrochloric acid and by pepsins already present in the stomach. These enzymes begin the digestion of proteins, breaking them into smaller molecules. Pepsins have evolved to act most effectively at a very low ph.
•Intrinsic factor (a protein) is necessary for the absorption of vitamin B12 from the ileum. (Deficiency leads to pernicious anemia)
•Mucus prevents mechanical injury to the stomach wall by lubricating the contents. It also prevents chemical injury by acting as a barrier between the stomach wall and the corrosive gastric juice – hydrochloric acid is present in potentially damaging concentrations and pepsins would digest the gastric tissues.
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Secretion of gastric juice
There is always a small quantity of gastric juice present in the stomach, even when it contains no food. This is known as fasting juice. Secretion reaches its maximum level about 1 hour after a meal then declines to the fasting level after about 4 hours.
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1 cephalic phase
This flow of juice occurs before the food reaches the stomach and is due to reflex stimulation of the vagus (parasympathetic) nerves initiated by the sight, smell, or taste of food. When the vagus nerves have been cut (vagotomy), this phase of gastric secretion stops. Sympathetic stimulation, e.g., during emotional states, also inhibits gastric activity.
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2 gastric phases
When stimulated by the presence of food the enteroendocrine cells in the pylorus and duodenum secrete the hormone gastrin, which passes directly into the circulating blood. Gastrin, circulating in the blood which supplies the stomach, stimulates the gastric glands to produce more gastric juice. In this way secretion of digestive juice is continued after completion of a meal and the end of the cephalic phase. Gastrin secretion is suppressed when the pH in the pylorus falls to about 1.5.
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3 intestinal phases
* When the partially digested contents of the stomach reach the small intestine, two hormones, secretin, and cholecystokinin are produced by endocrine cells in the intestinal mucosa. They slow down the secretion of gastric juice and reduce gastric motility. By slowing the emptying rate of the stomach, the chyme in the duodenum becomes more thoroughly mixed with bile and pancreatic juice. This phase of gastric secretion is most marked following a meal with high-fat content.
* The rate at which the stomach empties depends largely on the type of food eaten. A carbohydrate meal leaves the stomach in 2–3 hours, a protein meal remains longer, and a fatty meal remains in the stomach longest.
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Functions of the stomach
These include:
•temporary storage allowing time for the digestive enzymes, pepsins, to act
•chemical digestion – pepsins break proteins into polypeptides
•mechanical breakdown – the three smooth muscle layers enable the stomach to act as a churn, gastric juice is added, and the contents are liquefied to chyme. Gastric motility and secretion are increased by parasympathetic nerve stimulation
•limited absorption – water, alcohol and some lipid-soluble drugs
•non-specific defense against microbes – provided by hydrochloric acid in gastric juice. Vomiting may occur in response to ingestion of gastric irritants, e.g., microbes or chemicals
•preparation of iron for absorption – the acid environment of the stomach solubilizes iron salts, essential for iron absorption in the small intestine
•production and secretion of intrinsic factor needed for absorption of vitamin B12 in the terminal ileum
•regulation of the passage of gastric contents into the duodenum. When the chyme is sufficiently acidified and liquefied, the pylorus forces small jets of gastric contents through the pyloric sphincter into the duodenum. The sphincter is normally closed, preventing backflow of chyme into the stomach
•secretion of the hormone gastrin
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Small intestine
The small intestine is continuous with the stomach at the pyloric sphincter. The small intestine is about 2.5 cm in diameter, a little over 5 meters long and leads into the large intestine at the ileocecal valve. It lies in the abdominal cavity surrounded by the large intestine. In the small intestine the chemical digestion of food is completed, and absorption of most nutrients takes place. The small intestine comprises three continuous parts.
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Duodenum
This is about 25 cm long and curves around the head of the pancreas. Secretions from the gall bladder and pancreas merge in a common structure – the hepatopancreatic ampulla – and enter the duodenum at the duodenal papilla. The duodenal papilla is guarded by a ring of smooth muscle, the hepatopancreatic sphincter (of Oddi).
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Jejunum
This is the middle section of the small intestine and is about 2 meters long.
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Ileum
This terminal section is about 3 meters long and ends at the ileocecal valve, which controls the flow of material from the ileum to the caecum, the first part of the large intestine, and prevents backflow.
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Structure of the small intestine
The walls of the small intestine are composed of four layers of tissue. Some modifications of the peritoneum and mucosa (mucous membrane lining).
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Peritoneum
The mesentery, a double layer of the peritoneum, attaches the jejunum and ileum to the posterior abdominal wall. The attachment is quite short in comparison with the length of the small intestine; therefore, it is fan-shaped. The large blood vessels and nerves lie on the posterior abdominal wall and the branches to the small intestine pass between the two layers of the mesentery.
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What is the mucosa consist of?
The mucosa is moist tissue that lines certain parts of the inside of your body. It is in your:
-Nose
-Mouth
-Lungs
-Urinary and digestive tracts
•Glands in this tissue release a thick fluid called mucus.
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Blood supply of the small intestine
The superior mesenteric artery supplies the whole of the small intestine. Venous drainage is by the superior mesenteric vein that joins other veins to form the portal vein. The portal vein contains a high concentration of absorbed nutrients and this blood passes through the liver before entering the hepatic veins and, ultimately, into the inferior vena cava.
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Intestinal juice
About 1500 mL of intestinal juice is secreted daily by the glands of the small intestine. It is slightly basic (alkaline) and consists of water, mucus, and mineral salts.
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Functions of the small intestine
The functions are:
- onward movement of its contents by peristalsis, which is increased by parasympathetic stimulation
- secretion of intestinal juice, also increased by parasympathetic stimulation
- completion of chemical digestion of carbohydrates, protein, and fats in the enterocytes of the villi
- protection against infection by microbes that have survived the antimicrobial action of the hydrochloric acid in the stomach, by both solitary and aggregated lymph follicles
- secretion of the hormones cholecystokinin (CCK) and secretin
- absorption of nutrients.
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Chemical digestion in the small intestine
When acid chyme passes into the small intestine it is mixed with pancreatic juice, bile, and intestinal juice, and is in contact with the enterocytes of the villi. The digestion of all nutrients is completed:
- carbohydrates are broken down into monosaccharides
- proteins are broken down into amino acids
- fats are broken down into fatty acids and glycerol.
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Pancreatic juice
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Pancreatic juice is secreted by the exocrine pancreas and enters the duodenum at the duodenal papilla. It consists of:
-water
-mineral salts
-enzymes:
-amylase
-lipase
-nucleases that digest DNA and RNA
-inactive enzyme precursors including:
-trypsinogen
-chymotrypsin
•Pancreatic juice is basic (alkaline, pH 8) because it contains significant quantities of bicarbonate ions, which are basic (alkaline) in solution. When acid stomach contents enter the duodenum, they are mixed with pancreatic juice and bile, and the pH is raised to between 6 and 8. This is the pH at which the pancreatic enzymes, amylase, and lipase, act most effectively.
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Digestion of proteins
Trypsinogen and chymotrypsin are inactive enzyme precursors activated by enterokinase, an enzyme in the microvilli, which converts them into the active proteolytic enzyme trypsin and chymotrypsin. These enzymes convert polypeptides to tripeptides, dipeptides, and amino acids. It is important that they are produced as inactive precursors and are activated only upon their arrival in the duodenum, otherwise, they would digest the pancreas.
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Digestion of carbohydrates
Pancreatic amylase converts all digestible polysaccharides (starches) not acted upon by salivary amylase to disaccharides.
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Digestion of fats
Lipase converts fats to fatty acids and glycerol. To aid the action of lipase, bile salts emulsify fats, i.e., reduce the size of the globules, increasing their surface area.
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Control of secretion
The secretion of pancreatic juice is stimulated by secretin and CCK, produced by endocrine cells in the walls of the duodenum. The presence in the duodenum of acid chyme from the stomach stimulates the production of these hormones
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Bile
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•Bile, secreted by the liver, is unable to enter the duodenum when the hepatopancreatic sphincter is closed; therefore, it passes from the hepatic duct along the cystic duct to the gall bladder where it is stored.
•Bile has a pH of around 8 and between 500 and 1000 mL is secreted daily. It consists of:
-water
-mineral salts
-mucus
-bile salts
-bile pigments, main bilirubin
-cholesterol
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Functions of the bile
The functions of bile; in summary, these are:
- emulsification of fats in the small intestine – bile salts
- making cholesterol and fatty acids soluble, enabling their absorption along with the fat-soluble vitamins – bile salts
- excretion of bilirubin (a waste product from the breakdown of red blood cells), most of which is in the form of stercobilin.
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Release from the gall bladder
After a meal, the duodenum secretes the hormones secretin and CCK during the intestinal phase of gastric secretion. They stimulate contraction of the gall bladder and relaxation of the hepatopancreatic sphincter, expelling both bile and pancreatic juice through the duodenal papilla into the duodenum. Secretion is markedly increased when chyme entering the duodenum contains a high proportion of fat.
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Intestinal secretions
* The principal constituents of intestinal secretions are water, mucus, and mineral salts.
* Most of the digestive enzymes in the small intestine are contained in the enterocytes of the epithelium that covers the villi. Digestion of carbohydrates, protein, and fat is completed by direct contact between these nutrients and the microvilli and within the enterocytes.
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Chemical digestion associated with enterocytes
•Alkaline intestinal juice (pH 7.8–8.0) assists in raising the pH of the intestinal contents to between 6.5 and 7.5. The enzymes that complete chemical digestion of food at the surface of the enterocytes are:
-peptidases
-lipase
-sucrase, maltase and lactase.
•Peptidases such as trypsin break down polypeptides into smaller peptides and amino acids. Peptidases are secreted in an inactive form from the pancreas (to prevent them from digesting it) and must be activated by enterokinase in the duodenum.
•The final stage of the breakdown of all peptides to amino acids takes place at the surface of the enterocytes.
•Lipase completes the digestion of emulsified fats to fatty acids and glycerol in the intestine.
•Sucrase, maltase, and lactase complete the digestion of carbohydrates by converting disaccharides such as sucrose, maltose, and lactose to monosaccharides at the surface of the enterocytes.
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Control of secretion
Mechanical stimulation of the intestinal glands by chyme is believed to be the main stimulus for the secretion of intestinal juice, although the hormone secretin may also be involved.
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Absorption of nutrients
* Absorption of nutrients from the small intestine through the enterocytes occurs by several processes, including diffusion, osmosis, facilitated diffusion and active transport. Water moves by osmosis; small fat-soluble substances, e.g., fatty acids and glycerol, can diffuse through cell membranes, while others are generally transported inside the villi by other mechanisms.
* Monosaccharides and amino acids pass into the blood capillaries in the villi. Fatty acids and glycerol enter the lacteals and are transported along lymphatic vessels to the thoracic duct where they enter the circulation.
* A small number of proteins are absorbed unchanged, e.g., antibodies present in breast milk and oral vaccines, such as poliomyelitis vaccine.
* Other nutrients such as vitamins, mineral salts and water are also absorbed from the small intestine into the blood capillaries. Fat-soluble vitamins are absorbed into the lacteals along with fatty acids and glycerol. Vitamin B12 combines with intrinsic factor in the stomach and is actively absorbed in the terminal ileum.
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The large intestine, rectum, and anal canal
The large intestine is about 1.5 meters long, beginning at the caecum in the right iliac fossa and terminating at the rectum and anal canal deep within the pelvis. Its lumen is about 6.5 cm in diameter, larger than that of the small intestine. It forms an arch around the coiled-up small intestine. For descriptive purposes, the large intestine is divided into the caecum, colon, sigmoid colon, rectum, and anal canal.
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The caecum
This is the first part of the large intestine. It is a dilated region which has a blind end inferiorly and is continuous with the ascending colon superiorly. Just below the junction of the two the ileocecal valve opens from the ileum. The vermiform appendix is a fine tube, closed at one end, which leads from the caecum. It is about 8–9 cm long and has the same structure as the walls of the large intestine but contains more lymphoid tissue. The appendix has no digestive function but can cause significant problems when it becomes inflamed (appendicitis).
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The colon
The colon has four parts that have the same structure and functions.
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The ascending colon
This passes upwards from the caecum to the level of the liver where it curves acutely to the left at the hepatic flexure to become the transverse colon.
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The transverse colon
This part extends across the abdominal cavity in front of the duodenum and the stomach to the area of the spleen where it forms the splenic flexure and curves acutely downwards to become the descending colon.
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The descending colon
This passes down the left side of the abdominal cavity then curves towards the midline. At the level of the iliac crest, it is known as the sigmoid colon.
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The sigmoid colon
This part describes an S-shaped curve in the pelvic cavity that continues downwards to become the rectum.
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The rectum
This is a slightly dilated section of the large intestine about 13 cm long. It leads from the sigmoid colon and terminates in the anal canal.
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The anal canal
This is a short passage about 3.8 cm long in the adult and leads from the rectum to the exterior. Two sphincter muscles control the anus; the internal sphincter, consisting of smooth muscle, is under the control of the autonomic nervous system and the external sphincter, formed by skeletal muscle, is under voluntary control.
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Structure
The four layers of tissue described in the basic structure of the gastrointestinal tract (Fig. 12.2) are present in the caecum, colon, rectum, and anal canal. The arrangement of the longitudinal muscle fibers is modified in the caecum and colon. They do not form a continuous layer of tissue but are instead collected into three bands, called taeniae coli, which run lengthways along the caecum and colon. They stop at the junction of the sigmoid colon and the rectum. As these bands of muscle tissue are slightly shorter than the total length of the caecum and colon, they give it a sacculated or puckered appearance
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Functions of the large intestine, rectum, and anal canal (absorption)
The contents of the ileum which pass through the ileocecal valve into the caecum are fluid, even though a large amount of water has been absorbed in the small intestine. In the large intestine absorption of water, by osmosis, continues until the familiar semisolid consistency of feces is achieved. Mineral salts, vitamins, and some drugs are also absorbed into blood capillaries from the large intestine.
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Microbial activity
* The large intestine is heavily colonized by certain types of bacteria, which synthesize vitamin K and folic acid. They include Escherichia coli, Enterobacter aerogenes, Streptococcus faecalis, and Clostridium perfringens. These microbes are commensals, i.e., normally harmless, in humans. However, they may become pathogenic if transferred to another part of the body, e.g., E. coli may cause cystitis if it gains access to the urinary bladder.
* Gases in the bowel consist of some of the constituents of air, mainly nitrogen, swallowed with food and drink. Hydrogen, carbon dioxide, and methane are produced by bacterial fermentation of unabsorbed nutrients, especially carbohydrates. Gases pass out of the bowel as flatus (wind).
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Mass movement
•The large intestine does not exhibit peristaltic movement as in other parts of the digestive tract. Only at long intervals (about twice an hour) does a wave of strong peristalsis sweep along the transverse colon forcing its contents into the descending and sigmoid colons. This is known as a mass movement, and it is often precipitated by the entry of food into the stomach. This combination of stimulus and response is called the gastrocolic reflex
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Defecation
* Usually the rectum is empty, but when a mass movement forces the contents of the sigmoid colon into the rectum the nerve endings in its walls are stimulated by a stretch. In infants, defecation occurs by reflex (involuntary) action. However, during the second or third year of life children develop voluntary control of bowel function. In practical terms, this acquired voluntary control means that the brain can inhibit the reflex until it is convenient to defaecate. The external anal sphincter is under conscious control through the pudendal nerve.
* Thus, defecation involves involuntary contraction of the muscle of the rectum and relaxation of the internal anal sphincter. Contraction of the abdominal muscles and lowering of the diaphragm increases the intra-abdominal pressure (Valsalva's maneuver) and so assists defecation. When the need to pass feces is voluntarily postponed, it tends to fade until the next mass movement occurs and the reflex is initiated again. Repeated suppression of the reflex may lead to constipation (hard feces) as more water is absorbed.
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Constituents of feces
•The feces consist of a semisolid brown mass. The brown color is due to the presence of stercobilin.
•Even though the absorption of water takes place in the small and large intestines, water still makes up about 60–70% of the weight of the feces. The remainder consists of:
-fiber (indigestible cellular plant and animal material)
-dead and live microbes
-epithelial cells shed from the walls of the tract
-fatty acids
-mucus secreted by the epithelial lining of the large intestine.
•Mucus helps to lubricate the feces and an adequate amount of dietary non-starch polysaccharide (NSP, fiber and previously known as roughage) ensures that the contents of the large intestine are sufficiently bulky to stimulate defecation.
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Pancreas
* The pancreas is a pale grey gland weighing about 60 grams. It is about 12–15 cm long and is situated in the epigastric and left hypochondriac regions of the abdominal cavity. It consists of a broad head, a body, and a narrow tail. The head lies in the curve of the duodenum, the body behind the stomach, and the tail lies in front of the left kidney and just reaches the spleen. The abdominal aorta and the inferior vena cava lie behind the gland.
* The pancreas is both an exocrine and endocrine gland.
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The exocrine pancreas
* This consists of many lobules made up of small acini, the walls of which consist of secretory cells. Each lobule is drained by a tiny duct, and these unite eventually to form the pancreatic duct, which extends along the whole length of the gland and opens into the duodenum. Just before entering the duodenum, the pancreatic duct joins the common bile duct to form the hepatopancreatic ampulla. The duodenal opening of the ampulla is controlled by the hepatopancreatic sphincter (of Oddi) at the duodenal papilla.
* The function of the exocrine pancreas is to produce pancreatic juice containing enzymes, some in the form of inactive precursors, that digest carbohydrates, proteins, and fats. As in the alimentary tract, parasympathetic stimulation increases the secretion of pancreatic juice and sympathetic stimulation depresses it.
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The endocrine pancreas
Distributed throughout the gland are groups of specialized cells called the pancreatic islets (of Langerhans). The islets have no ducts, so the hormones diffuse directly into the blood. The endocrine pancreas secretes the hormones insulin and glucagon, which are principally concerned with the control of blood glucose levels
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Blood supply of the pancreas
The splenic and mesenteric arteries supply the pancreas, and venous drainage is by veins of the same names that join other veins to form the portal vein.
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Liver
The liver is the largest gland in the body, weighing between 1 and 2.3 kg. It is situated in the upper part of the abdominal cavity occupying the greater part of the right hypochondriac region, part of the epigastric region, and extending into the left hypochondriac region. Its upper and anterior surfaces are smooth and curved to fit the undersurface of the diaphragm; its posterior surface is irregular in outline
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Organs associated with the liver
* Superiorly and anteriorly – diaphragm and anterior abdominal wall
* Inferiorly – stomach, bile ducts, duodenum, hepatic flexure of the colon, right kidney, and adrenal gland
* Posteriorly – esophagus, inferior vena cava, aorta, gall bladder, vertebral column, and diaphragm
* Laterally – lower ribs and diaphragm.
* The liver is enclosed in a thin inelastic capsule and incompletely covered by a layer of peritoneum. Folds of the peritoneum form supporting ligaments that attach the liver to the inferior surface of the diaphragm. It is held in position partly by these ligaments and partly by the pressure of the organs in the abdominal cavity.
* The liver has four lobes. The two most obvious are the large right lobe and the smaller, wedge-shaped, left lobe. The other two, the caudate and quadrate lobes, are areas on the posterior surface
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The portal fissure
* This is the name given to the region on the posterior surface of the liver where various structures enter and leave the gland.
* The portal vein enters, carrying blood from the stomach, spleen, pancreas, and the small and large intestines.
* The hepatic artery enters, carrying arterial blood. It is a branch from the coeliac artery, which branches from the abdominal aorta.
* Nerve fibers, sympathetic and parasympathetic, enter here.
* The right and left hepatic ducts to leave, carrying bile from the liver to the gall bladder.
* Lymph vessels leave the liver, draining lymph to abdominal and thoracic nodes.
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Structure of the liver
The lobes of the liver are made up of tiny functional units, called lobules, which are just visible to the naked eye. Liver lobules are hexagonal in outline and are formed by cuboidal cells, the hepatocytes, arranged in pairs of columns radiating from a central vein. Between two pairs of columns of cells are sinusoids (blood vessels with incomplete walls) containing a mixture of blood from the tiny branches of the portal vein and hepatic artery. This arrangement allows the arterial blood and portal venous blood (with a high concentration of nutrients) to mix and come into close contact with the liver cells. Amongst the cells lining the sinusoids are hepatic macrophages (Kupffer cells) whose function is to ingest and destroy worn out blood cells and any foreign particles present in the blood flowing through the liver.
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Carbohydrate metabolism
The liver has an important role in maintaining plasma glucose levels. After a meal when levels rise, glucose is converted to glycogen for storage under the influence of the hormone insulin. Later, when glucose levels fall, the hormone glucagon stimulates conversion of glycogen into glucose again, keeping levels within the normal range.
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Fat metabolism
Stored fat can be converted to a form in which it can be used by the tissues to provide energy.
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Deamination of amino acids
This process:
- removes the nitrogenous portion from amino acids that are not required for the formation of new protein; urea is formed from this nitrogenous portion and is excreted in urine
- breaks down nucleic acids (genetic material, e.g., DNA) to form uric acid, which is excreted in the urine.
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Transamination
Removes the nitrogenous portion of amino acids and attaches it to other carbohydrate molecules forming new non-essential amino acids.
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Synthesis of plasma proteins
These are formed from amino acids and include albumins, globulins, and blood clotting factors.
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Breakdown of erythrocytes and defense against microbes
This is carried out by phagocytic hepatic macrophages (Kupffer cells) in the sinusoids although the breakdown of red blood cells also takes place in the spleen and bone marrow.
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Inactivation of hormones
These include insulin, glucagon, cortisol, aldosterone, thyroid, and sex hormones.
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Production of heat
The liver uses a considerable amount of energy, has a high metabolic rate, and consequently produces a great deal of heat. It is the main heat-producing organ of the body.
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detoxification of drugs, toxins, and substances
These include ethanol (alcohol), waste products, and microbial toxins. Some drugs are extensively inactivated by the liver and are not very effective when given by mouth (orally), e.g., glyceryl trinitrate. This is because, after absorption from the alimentary tract, they travel in the blood to the liver where they are largely metabolized so that levels in the blood leaving the liver, and which enters the systemic circulation are inadequate to achieve therapeutic effects. This is known as ‘first-pass metabolism.
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Secretion of bile
The hepatocytes synthesize the constituents of bile from the mixed arterial and venous blood in the sinusoids. These include bile salts, bile pigments, and cholesterol.
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Storage
Stored substances include:
- glycogen
- fat-soluble vitamins: A, D, E, K
- iron, copper
- some water-soluble vitamins, e.g., vitamin B12.
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composition of the bile
Between 500 and 1000 mL of bile is secreted by the liver daily. Bile consists of:
- water
- mineral salts
- mucus
- bile pigments, main bilirubin
- bile salts
- cholesterol.
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Fat digestion
* The bile acids, cholic and chenodeoxycholic acid, are synthesized by hepatocytes from cholesterol, then secreted into bile as sodium or potassium salts. In the small intestine, they emulsify fats, aiding their digestion. Fatty acids are insoluble in water, which makes them very difficult to absorb through the intestinal wall. Bile salts make cholesterol and fatty acids more water-soluble, enabling both these and the fat-soluble vitamins (vitamins A, D, E, and K) to be readily absorbed.
* In the terminal ileum, most of the bile salts are reabsorbed and return to the liver in the portal vein. This enterohepatic circulation, or recycling of bile salts, ensures that large amounts of bile salts enter the small intestine daily from a relatively small bile acid pool
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Excretion of bilirubin
Bilirubin is one of the products of hemolysis of erythrocytes by hepatic macrophages (Kupffer cells) in the liver and by other macrophages in the spleen and bone marrow. Bilirubin is insoluble in water and is carried in the blood bound to the plasma protein albumin. In hepatocytes, it is conjugated (combined) with glucuronic acid and becomes water-soluble enough to be excreted in bile.
Microbes in the large
intestine converts bilirubin into stercobilin, which is excreted in the feces. Stercobilin colors and deodorizes the feces. A small amount is reabsorbed and excreted in urine as urobilinogen. Jaundice is yellow pigmentation of the tissues, seen in the skin and conjunctiva, caused by excess blood bilirubin.
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Bile ducts
The right and left hepatic ducts join to form the common hepatic duct just outside the portal fissure. The hepatic duct passes downwards for about 3 cm where it is joined by the cystic duct from the gall bladder. The cystic and hepatic ducts merge forming the common bile duct, which passes downwards behind the head of the pancreas. This is joined by the main pancreatic duct at the hepatopancreatic ampulla. It opens into the duodenum, at the duodenal papilla, which is controlled by the hepatopancreatic sphincter (of Oddi). The common bile duct is about 7.5 cm long and has a diameter of about 6 mm.
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gall bladder
The gall bladder is a pear-shaped sac attached to the posterior surface of the liver by connective tissue. It has a fundus or expanded end, a body or main part and a neck, which is continuous with the cystic duct.
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Structure of the gall bladder
The wall of the gall bladder has the same layers of tissue as those of the basic structure of the alimentary canal, with some modifications.
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Peritoneum
This covers only the inferior surface because the upper surface of the gall bladder is in direct contact with the liver and held in place by the visceral peritoneum that covers the liver.
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Muscle layer
There is an additional layer of oblique muscle fibers.
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Mucous membrane
This displays small rugae when the gall bladder is empty that disappears when it is distended with bile.
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Blood supply of the gall bladder
The cystic artery, a branch of the hepatic artery, supplies the gall bladder. Blood is drained away by the cystic vein that joins the portal vein.
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Functions of the gall bladder
•These include:
-reservoir for bile
-concentration of the bile by up to 10- or 15-fold, by absorption of water through the walls of the gall bladder
-release of stored bile.
•When the muscle wall of the gall bladder contracts, bile passes through the bile ducts to the duodenum. Contraction is stimulated by the hormone cholecystokinin (CCK), secreted by the duodenum, and the presence of fat and acid chyme in the duodenum.
•Relaxation of the hepatopancreatic sphincter (of Oddi) is caused by CCK and is a reflex response to contraction of the gall bladder.
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Metabolism
Metabolism constitutes all the chemical reactions that occur in the body, using nutrients to:
- provide energy by chemical oxidation of nutrients
- make new or replacement body substances.
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Catabolism
Catabolic processes break down large molecules into smaller ones releasing chemical energy, which is stored as adenosine triphosphate (ATP), and heat. Heat generated maintains core body temperature at the optimum level for chemical activity (36.8°C). Excess heat is lost, mainly through the skin.
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Anabolism
This is the building up, or synthesis, of large molecules from smaller ones and requires a source of energy, usually ATP.
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Metabolic pathways
* Anabolism and catabolism usually involve a series of chemical reactions, known as metabolic pathways. These consist of ‘small steps’ that permit controlled, efficient, and gradual transfer of energy from ATP rather than large intracellular ‘explosions. Metabolic pathways are switched on and off by hormones, providing control of metabolism and meeting individual requirements.
* Both catabolic and anabolic processes occur continually in all cells. Very active tissues, such as muscle or liver, need a large energy supply to support their requirements.
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Energy
* The energy produced in the body may be measured and expressed in units of work (joules) or units of heat (kilocalories).
* A kilocalorie (kcal) is the amount of heat required to raise the temperature of 1 liter of water by 1 degree Celsius (1°C). Daily, the body's collective metabolic processes generate a total of about 3 million kilocalories.
* 1 kcal = 4184 joules (J) = 4.184 kilojoules (kJ)
* The nutritional value of carbohydrates, protein, and fats are eaten in the diet may be expressed in either kilojoule per gram or kcal per gram.
* 1 gram of carbohydrate provides 17 kilojoules (4 kcal)
* 1 gram of protein provides 17 kilojoules (4 kcal)
* 1 gram of fat provides 38 kilojoules (9 kcal)
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Energy balance
Energy balance is important as it determines changes in body weight. Bodyweight remains constant when energy intake is equal to energy use. When intake exceeds requirement, bodyweight increases, which may lead to obesity. Conversely, body weight decreases when nutrient intake does not meet energy requirements.
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Metabolic rate
* The metabolic rate is the rate at which energy is released from the fuel molecules inside cells. As most of the processes involved require oxygen and produce carbon dioxide as waste, the metabolic rate can be estimated by measuring oxygen uptake or carbon dioxide excretion.
* The basal metabolic rate (BMR) is the rate of metabolism when the individual is at rest in a warm environment and is in the postabsorptive state, i.e., has not had a meal for at least 12 hours. In this state, the release of energy is sufficient to meet only the essential needs of vital organs, such as the heart, lungs, nervous system, and kidneys. Some of the many factors that affect metabolic rate. The postabsorptive state is important because the intake of food, especially protein, increases metabolic rate.
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Central metabolic pathways
Much of the metabolic effort of cells is concerned with energy production to fuel cellular activities. Certain common pathways are central to this function. Fuel molecules enter these central energy-producing pathways and in a series of steps, during which a series of intermediate molecules are formed, and energy is released, these fuel molecules are chemically broken down. The end results of these processes are production of energy and carbon dioxide and water (called metabolic water). Much of the energy is stored as ATP, although some is lost as heat. The carbon dioxide is excreted through the lungs and excess water excreted as urine.
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Carbohydrate metabolism
•Erythrocytes and neurons can use only glucose for fuel and therefore maintenance of blood glucose levels is needed to provide a constant energy source to these cells. Most other cells can also use other sources of fuel.
•Digested carbohydrate, mainly glucose, is absorbed into the blood capillaries of the villi of the small intestine. It is transported by the portal circulation to the liver, where it is dealt with in several ways:
-glucose may be oxidized to provide the chemical energy, in the form of ATP, necessary for the considerable metabolic activity which takes place in the liver itself
-some glucose may remain in the circulating blood to maintain the normal blood glucose of about 3.5–8 millimoles per liter (mmol/L) (63–144 mg/100 mL).
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Carbohydrate and energy release
Glucose is broken down in the body releasing energy, carbon dioxide, and metabolic water. Catabolism of glucose occurs in a series of steps with a little energy being released at each stage. The total number of ATP molecules which may be generated from the complete breakdown of one molecule of glucose is 38, but for this to be achieved the process must occur in the presence of oxygen (aerobically). In the absence of oxygen (anaerobically) this number is greatly reduced; the process is therefore much less efficient.
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Aerobic respiration
Aerobic catabolism of glucose can occur only when the oxygen supply is adequate and is the process by which energy is released during prolonged, manageable exercise. When exercise levels become very intense, the energy requirements of muscles outstrip the oxygen supply, and anaerobic breakdown then occurs. Such high levels of activity can be sustained for only short periods because there is an accumulation of wastes (mainly lactic acid) and reduced efficiency of the energy production process.
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Anaerobic catabolism
When oxygen levels in the cell are low, the molecule of the glucose still undergoes glycolysis and is split into two molecules of pyruvic acid, because glycolysis is an anaerobic process. However, the pyruvic acid does not enter the citric acid cycle or progress to oxidative phosphorylation; instead, it is converted anaerobically to lactic acid. The build-up of lactic acid causes the pain and cramps of overexercised muscles. When oxygen levels are restored, lactic acid is reconverted to pyruvic acid, which may then enter the citric acid cycle.
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Lactic acid
Some of the lactic acid produced by anaerobic catabolism of glucose may be oxidized in the cells to carbon dioxide and water but first, it must be changed back to pyruvic acid. If complete oxidation does not take place, lactic acid passes to the liver in the circulating blood where it is converted to glucose and may then take any of the pathways open to glucose.
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metabolic water
This is added to the considerable amount of water already present in the body; excess is excreted as urine by the kidneys.
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Protein metabolism
* Dietary protein consists of several amino acids. About 20 amino acids have been named and nine of these are described as essential because they cannot be synthesized in the body. The others are non-essential amino acids because they can be synthesized by many tissues. The enzymes involved in this process are called transaminases. Digestion breaks down dietary protein into its constituent amino acids in preparation for absorption into the blood capillaries of the villi in the small intestine. Amino acids are transported in the portal circulation to the liver and then into the general circulation, thus making them available to all body cells and tissues. Different cells choose from those available the amino acids required for building or repairing their specific type of tissue and for synthesizing their secretions, e.g., antibodies, enzymes, or hormones.
* Amino acids not required for building and repairing body tissues cannot be stored and are broken down in the liver.
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Amino acid pool
A small pool of amino acids is maintained within the body. This is the source from which the body cells draw the amino acids they need to synthesize their own materials, e.g., new cells or cell components, secretions such as enzymes, hormones, and plasma proteins.
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Endogenous
These are obtained from the breakdown of existing body proteins. In adults, about 80–100 g of protein are broken down and replaced each day. The entire intestinal mucosa is replaced about every 5 days.
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Deamination
Amino acids not needed by the body are broken down, or deaminated, mainly in the liver. The nitrogenous part, the amino group (NH2), is converted to ammonia (NH3) and then combined with carbon dioxide forming urea, which is excreted in the urine. The remaining part is used to provide energy, as glucose by gluconeogenesis, or stored as fat, if in excess of immediate requirements.
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Excretion
* The feces contain a considerable amount of protein within cells shed from the lining of the alimentary tract.
* Endogenous and exogenous amino acids are mixed in the ‘pool’ and the body is said to be in nitrogen balance when the rate of removal from the pool is equal to the additions to it. Unlike carbohydrates, the body has no capacity for the storage of amino acids except for this relatively small pool. Depicts what happens to amino acids in the body.
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Amino acids and energy release
Proteins, in the form of amino acids, are potential fuel molecules that are used by the body only when other energy sources are low, e.g., in starvation. To supply the amino acids for use as fuel, in extreme situations, the body breaks down muscle, its main protein source. Some amino acids can be converted directly to glucose, which enters glycolysis. Other amino acids are changed to intermediate compounds of the central metabolic pathways, e.g., acetyl coenzyme A or oxaloacetic acid, and therefore enter the system at a later stage.
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Fat metabolism
* Fat is synthesized from excess dietary carbohydrates and proteins, and stored in the fat depots, i.e., under the skin, in the momentum, or around the kidneys.
* Fats that have been digested and absorbed as fatty acids and glycerol into the lacteals are transported via the cisterna chyli and the thoracic duct of the lymphatic system to the bloodstream and so, by a circuitous route, to the liver. Fatty acids and glycerol circulating in the blood are used by the cells to provide energy and to synthesize some of their secretions. In the liver some fatty acids and glycerol are used to provide energy and heat, and some are recombined forming triglycerides, the form in which fat is stored. A triglyceride consists of three fatty acids chemically combined with a glycerol molecule. When required, triglycerides are converted back to fatty acids and glycerol and used to provide energy. The end products of fat metabolism are chemical energy, heat, carbon dioxide and water.
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Fatty acids and energy release
•When body tissues are deprived of glucose, as occurs in prolonged fasting, starvation, energy-restricted diets, or during strenuous exercise, the body uses alternative energy sources, mainly fat stores. Fatty acids may be converted to acetyl coenzyme A and enter the energy production pathway in that form. One consequence of this is an accumulation of ketone bodies, which are produced in the liver from acetyl coenzyme A when levels are too high for processing through the citric acid cycle. Ketone bodies then enter the blood and can be used by other body tissues, including the brain (which is usually glucose-dependent) as a source of fuel. However, at high concentrations, ketone bodies are toxic, particularly to the brain. Ketone bodies include acetone and some weak organic acids. Normally levels are low because they are used as soon as they are produced. When production exceeds use, in the situations mentioned above, levels rise causing ketosis. Ketosis is associated with acidosis, which can lead to coma or even death if severe. Excretion of excess ketone bodies is via:
-the urine (ketonuria)
-the lungs, giving the breath a characteristic sweet smell of acetone or ‘pear drops.
•In ketosis, compensation is required to maintain acid-base balance. This is achieved by buffer systems that excrete excess acid (hydrogen ions) by the lungs, through hyperventilation, or kidneys. In health, ketosis is self-limiting and ketone body production stops when fasting or exercise ceases. Ketoacidosis is associated with uncontrolled type 1 diabetes mellitus.
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Glycerol and energy release
The body converts glycerol from the degradation of fats into one of the intermediary compounds produced during glycolysis, and in this form, it enters the central metabolic pathways.
