The Digestive System
|The digestive system [Figure 9 – 1] is based around the alimentary canal,
a long muscular tube which extends through the body between the mouth and the
anus. Glands lying outside the alimentary canal produce enzymes which are
used in the digestive process.
These glands produce many litres of secretions in a day, but most of this volume is re-absorbed.
Figure 9 – 1 the digestive system
|Apart from the acts of eating and defaecation, the digestive process is not under the control of the conscious will; the movement of foods through the digestive system is effected by smooth muscle.
The digestive system is divided into four main areas following the passage of food: the mouth and oesophagus, the stomach, the small intestine, and the large intestine.
With the exception of the mouth and pharynx, the alimentary canal [Figure 9 – 2]
is formed from four layers of tissue:
Adventitia. (outer covering) This consists of loose fibrous tissue in the thoracic region, and peritoneum in the abdominal regions. (peritoneum is described separately).
Muscle layer. In most areas, this is smooth muscle, arranged in two layers enclosing blood vessels and nerve fibres.
Submucous layer. This is composed of loose connective tissue, together with blood vessels and nerve fibres.
Mucous membrane. (inner layer) This layer varies in nature along the alimentary canal. In areas where food has already been softened, the membrane is composed of columnar epithelium with glands which secrete digestive juices.
Although it mainly follows this same basic structure, the exact formation of the alimentary canal varies along its length.
Figure 9 – 2 the alimentary canal
The basic formation of the alimentary canal
[Figure 9 – 3] forms a lining for the abdominal cavity. It consists of
two layers of serous membrane:
Parietal layer. This lines the abdominal wall.
Visceral layer. This covers the abdominal
The potential space between the two layers is occupied by serous fluid to allow ease of movement.
Figure 9 – 3 the peritoneum
A sagittal section through the female abdomen, indicating the abdominal organs and the peritoneum
Potential spaces have been much exaggerated for clarity
peritoneum forms a closed sac, the abdominal organs being invaginated from
‘outside’. Some organs such as the stomach, spleen, liver, and intestines, are completely covered by double folds of peritoneum;.
other organs such as the pancreas and kidneys are only partially covered.
A fold of peritoneum
An omentum is a loose fold of peritoneum. The lesser omentum extends from the liver to
is the orifice by which food enters the alimentary canal.The mouth is bounded by
muscles and bones, and is lined with mucous membrane. It contains the tongue
and the teeth, which assist in the initial processing of food.
The digestive process begins as soon as food is placed in the mouth, although no nutrients are
is a muscular structure attached to the floor of the mouth.The tongue is involved in
the actions of chewing and swallowing, and holds the majority of nerve endings associated with the sense of taste.
serve the functions of cutting, tearing, and grinding food during the eating
process.A tooth [Figure 9 – 4] is formed mainly from an ivory-like substance named dentine,
in the centre of which is the pulp cavity.
The pulp cavity contains nerves and blood vessels. Enamel, which is extremely hard, protects
The teeth are set into the frontal opposing margins of the maxilla and mandible, and are arranged in a symmetrical layout.
[Figure 9 – 5]
There are 32 teeth in a full set for an adult:
Incisors. (8) These are the frontal cutting teeth.
Canines. (4) These slightly pointed teeth are used for tearing food.
Premolars (8) and molars. (12) These irregular shaped teeth are used for grinding food during chewing.
9 – 4 a tooth
A section through a tooth showing the structure
9 – 5 the teeth
The arrangement of the teeth
Chewing and Swallowing
of chewing breaks up the food, and mixes it with saliva. The tongue then pushes small pieces of food (boluses) into the pharynx for swallowing.
The start of a swallowing action is controlled by the conscious will, but the continuation of the
During the swallowing action, the soft palate prevents the food entering the nasal pharynx, and the epiglottis moves to prevent the entry of the food into the trachea.
presence (or anticipation) of food stimulates a reflex action secretion of saliva from three pairs of glands: the parotid in the cheeks, the submandibular below the angle of the jaw, and the sublingual beneath the tongue.
Saliva is a mixture of secretions, containing mucin to ease the act of swallowing, ptyalin which breaks down starch into maltose (a sugar), amylase – a digestive
is a muscular tube linking the lower part of the laryngopharynx to the
stomach, via an opening in the diaphragm.
The formation of the oesophagus
Food is propelled down the oesophagus by peristalsis, waves of muscular contraction, controlled by the autonomic nervous system.
Sphincter muscles at the top and bottom of the oesophagus prevent unwanted flows: the cricopharyngeal sphincter protects against air passing into the oesophagus during
[Figure 9 – 6] is a hollow organ, with muscular walls, situated in the epigastric, umbilical, and left hypochondriac regions of the abdominal cavity.
Figure 9 – 6 the stomach
The main areas of the stomach
|The composition of the stomach follows the general structure of the alimentary canal, but with some modifications:
The folds of the peritoneum enclosing the stomach continue to form the greater omentum.The muscle layer is formed from three complementary layers of muscle fibre which allow for a churning movement of stomach contents, as well as peristalsis.
The mucous membrane contains numerous gastric glands which secrete gastric juice.
Very little actual digestion takes place in the stomach; it acts more as a reservoir for
Within the stomach, food and gastric juice mix to form chyme. This then passes from the stomach
juice is secreted by the mucous lining of the stomach in response to the release of the hormone
gastrin, secretion of which is stimulated by the presence of food in the stomach.
Gastric juice contains around 0.5% Hydrochloric acid, and is thus strongly acidic. The enzymes
Gastric juice serves the purposes of initiating the breakdown of proteins, and of destroying bacteria in food.
The small intestine
[Figure 9 – 7] is a thin tube, approximately 5m in length, irregularly coiled in the abdomen.
Figure 9 – 7 the small intestine
The layout of the small intestine
The length and complexity of the small intestine have been much reduced for
|The small intestine is described in three parts – each continuous with the next:
Duodenum. This is the first 250mm, connecting to the stomach.
Jejunum. This is the central part, and is approximately 2m long.
Ileum. This is the final, largest part, and is about 3m long. It connects into the large intestine.
The walls of the small intestine follow the general structure of the alimentary canal, but with some
A mesentery attaches the majority of the small intestine to the posterior abdominal wall.
The mucous membrane of the small intestine contains small projections, approximately 0.5mm – 1mm
The villi serve the double purpose of increasing the absorbent surface area of the intestines,
The main digestive processes take place within the small intestine, aided by intestinal juice,
Additional substances from external glands assist the digestive processes:
Bile. This is secreted by the liver. It emulsifies fats.
Pancreatic juice. This is secreted by the pancreas. It assists in the breakdown of fats, carbohydrates, and proteins.
is a pale grey coloured gland, situated in the epigastric and left hypochondriac regions of the abdominal cavity. It secretes pancreatic juice into the duodenum.
Secretion is controlled partly by reflex nervous action, but more by a stimulating agent, secretin, which is released into the circulation whenever acidic stomach contents enter the duodenum.
Pancreatic juice is alkaline, to the extent that it neutralises the acidity from the stomach. It also contains three enzymes:
Trypsin. This assists in the digestion of proteins into amino acids.
Amylase. This breaks down carbohydrates into sugars.
Lipase. This breaks down fats into fatty acids and other simpler compounds.
The pancreas has a second function, as an endocrine gland; this is considered as part of the endocrine system.
[ Figure 9 – 8] is the largest gland in the body, occupying the majority of the right hypochondriac region of the abdominal cavity, and spreading across the epigastric region into the left hypochondriac region.
It is composed of many small lobules, each receiving blood from both the systemic and portal circulations. The liver is thus an initial recipient of substances absorbed from food.
The liver is covered by a thin capsule, and partially by a layer of peritoneum, which acts as a main support.
The liver is formed in four lobes:
The large right lobe and smaller left lobe form the majority, with the caudate and quadrate lobes as small areas on the posterior surface of the right lobe.
Blood vessels, lymph vessels, and nerve fibres, together with the hepatic ducts, join the liver at the portal fissure (or porta hepatis) on the posterior surface.
Figure 9 – 8 the liver
A frontal view of the liver
|The liver has a variety of functions:
Chemical breakdown. Amino acids, old tissue cells, alcohol, and hormones are broken down, mainly for later excretion.
Storage. Excess fats, glucose (as glycogen), vitamins, iron compounds, and copper compounds are stored, for release when intake does not meet the requirements of the body.
Synthesis. Vitamin A is synthesised from carotene. Some amino acids, plasma protein, and blood clotting factors are synthesised from amino acids available in the bloodstream.
Conversion. Glucose is converted intoglycogen in the presence of insulin. Glycogen is conversely converted to glucose by glucagon.
Stored fats are converted into glucose.
Heat production. The liver is the main heat producing organ in the body because of its considerable chemical activity.
Secretion. The liver secretes bile, which is used in the digestive process.
Bile is a slightly alkaline, greenish, viscous fluid. Bile contains complex salts, the pigment bilirubin as a by-product of old cell breakdown, as well as water, mucus, and cholesterol.
Bile Ducts and Gall Bladder
|The liver produces bile continuously. As bile is only required during digestion, that produced at other times is stored in the gall bladder.
Bile from the liver passes via the hepatic ducts which link with the cystic duct from the gall bladder to form the common bile duct.
When bile is not required for digestive purposes, it flows through the cystic duct into the gall bladder.
The gall bladder [Figure 9 – 9] is a hollow organ, formed from the same four layers of tissue as the
Its muscular layer allows it to contract and empty its contents into the duodenum, via the cystic duct and common bile duct.
Figure 9 – 9 the gall bladder
The arrangement of the bile ducts and gall bladder
|The common bile duct links with the pancreatic duct just outside the duodenum,
the connection to which is controlled by the sphincter of Oddi.
|The large intestine forms the final sections of the alimentary canal. It is much larger in section than the small intestine.
[Figure 9 – 10]
The large intestine is formed from the same four layers of tissue as the main alimentary canal. The muscle layer is formed in bands, to give the large intestine a slightly bulging appearance.
Figure 9 – 10 the large intestine
The layout of the large intestine
|The large intestine is described in seven parts, each continuous with the next:
Caecum. This is the first part, connecting to the end of the ileum at the ileocaecal valve.
The vermiform appendix is small closed tube leading from the inferior closed end of the caecum.
Ascending colon. This leads from the caecum, vertically up the right side of the abdomen to the right colic flexure or hepatic flexure, situated under the right lobe of the liver.
Transverse colon. This starts at the hepatic flexure, and leads across the abdominal cavity, in front of the stomach, to the left colic flexure or splenic flexure, anterior to the spleen.
Descending colon. This starts at the splenic flexure, and leads down the left side of the abdominal cavity.
Sigmoid colon. This is the continuation of the descending colon into the pelvic area.
Rectum. This is a slightly dilated section of the large intestine, and forms the end of the digestive areas.
Anal canal. This is the final part of the alimentary canal, leading to outside the body at the anus.
The main functions of the large intestine are those of absorbing water, minerals, and vitamins, and temporarily storing waste matter prior to excretion.
The chyme which passes into the caecum is liquid, although some water has been absorbed in the small intestine. The majority of water will have been absorbed by the time this chyme reaches the descending colon.
The absorption of water (particularly the reabsorption of most digestive enzymes) is an important factor in preventing the body becoming dehydrated. The residue remaining after final absorption has occurred is excreted as faeces.
The movement of matter along the large intestine takes place at infrequent intervals (not regular peristalsis). This is known as mass movement, and is often caused by ingesting food.
|When mass movement forces faeces into the rectum, the desire to defaecate is triggered.
The opening of the anus is controlled by two sphincter muscles.
The internal anal sphincter is composed of smooth muscle and is under autonomic control. The external anal sphincter is composed of striated muscle and is under conscious control.
When a decision is made to defaecate, the external anal sphincter is relaxed. Abdominal pressure is raised by a lowering of the diaphragm, a contraction of abdominal muscles, and by trying to force air out of the lungs against a closed epiglottis.
This forces faeces from the rectum out of the anus; a continuation of pressure then forces expulsion of faeces from the descending and sigmoid colons.
|The digestive process is a series of complex biochemical reactions which differ slightly between carbohydrates, proteins, and fats.|
Table 9 – 1 digestion of carbohydrates
The main biochemical activities involved in the digestion of carbohydrates
enzymes and activity
|Mouth||Salivary amylase begins the conversion of carbohydrates to sugars|
– as Hydrochloric acid in gastric juice stops the action of salivary amylase
|Amylase from pancreatic juice converts all starches to sugarsSucrase, maltase and lactase from the intestinal glands convert all sugars to glucose|
|Absorption||Glucose is absorbed into capillaries in the villi, and is carried to the liver via
the portal circulation
|Utilisation||Glucose provides energy for all cells when broken down in the presence of OxygenExcess glucose is stored, partly as glycogen in the liver and muscles, and partly as fat in adipose
Table 9 – 2 digestion of proteins
The main biochemical activities involved in the digestion of proteins
enzymes and activity
|Stomach||Hydrochloric acid in gastric juice converts pepsinogen to pepsin, which then converts proteins to less complex polypeptides|
|Enteropeptidase from intestinal juice converts inactive enzymes in pancreatic juice to those which then convert polypeptides to di- and tripeptidesPeptidase from intestinal glands breaks peptides into amino acids|
|Absorption||Amino acids are absorbed into capillaries in the villi, and are carried to the
liver via the portal circulation
|Utilisation||Amino acids are widely used throughout the body for cell repair and replacement, the production of secretions, and in producing substances such as blood clotting agentsExcess amino acids are either broken down in the liver and excreted as urea, used to provide heat and energy, or are stored as fat in adipose tissue|
Table 9 – 3 digestion of fats
The main biochemical activities involved in the digestion of fats
enzymes and activity
|Bile emulsifies fatsLipase from pancreatic juice converts fats to fatty acids and glycerol|
|Absorption||Fatty acids and glycerol are absorbed via the villi into lymph vessels, from where they pass into the thoracic duct, left subclavian artery, and into the
circulating blood. From this they reach the liver for reorganisation and
|Utilisation||Fats are used to provide heat and energy in the presence of OxygenExcess fats are stored in adipose tissue|
Disease and Injury
|Appendicitis is an inflammation of the vermiform appendix.
The initial cause may not be identifiable – bacterial infection, together with a blockage of the
Initial inflammation causes swelling and ulceration of the inner mucous membranes. This leads to pain – first in the centre of the abdomen, and then in the area around the appendix.
If the inflammation does not subside (with or without treatment), it will soon block the small lumen of the appendix and lead to an abscess. As this continues, the blood vessels associated with the appendix become occluded, the appendix begins to atrophy, and may well rupture – with serious consequences.
|Cholera is a totally debilitating infection by the vibrio cholerae bacterium.
It spreads through faeces into water and food.
The disease has a sudden onset and causes serious diarrhoea, stomach and leg cramps, violent vomiting, and exhaustion. Hypovolaemic shock can result if lost fluids are not
Cholera can be fatal, especially if not treated.
|Cirrhosis is a condition which affects the liver. Cirrhosis occurs when liver cells have become damaged by some other condition – chronic alcohol abuse, hepatitis, or other liver disease – and fibrous tissue begins to grow in a mesh between areas of regenerating liver cells.
The condition leads to a failure of normal liver function, which in turn causes a general degeneration in health, weight loss, vomiting, jaundice, a build-up of fluid in the abdomen, and oedema of the legs.
Cirrhosis is incurable and may eventually prove fatal.
|Coeliac disease is an inherited condition in which the small intestine develops an abnormal
sensitivity to gluten and other proteins found in grain foodstuffs.
The problem may develop at any time of life.
The villi in the small intestine become damaged and are then unable to absorb nutrients properly. As a result, there is a risk of malnutrition.
Indications of coeliac disease vary widely from person to person, but will probably include
|Crohn’s disease most commonly affects the ileum, although it may affect any part of the alimentary canal.
It is a chronic condition, causing thickening and inflammation to the walls of the digestive
The condition may be complicated by intestinal obstruction, and the creation of holes – fistulae
The cause is unknown, although infection may contribute, as may genetic factors.
An apical abscess occurs following decay of a tooth, with that decay extending through the pulp
An apical abscess is usually extremely painful, but if a chronic abscess forms, this may be
Dental caries is the decay and degeneration of teeth. Plaque – a mixture of food debris, bacteria, and saliva – forms a sticky coating over teeth. The bacteria then act on sugars in food and form
Initially, this decay is painless, but if it reaches the nerve in a tooth, toothache will be
Decayed tooth structure is not regenerated, so must be removed and replaced with artificial filling material if eventual complete destruction of a decayed tooth is to be prevented.
Gingivitis is an inflammation of gum tissue, causing swollen gums which are prone to bleeding.
The cause of gingivitis is almost always a build-up of plaque, particularly in the areas of the teeth
Knocked out tooth
A tooth which has been knocked out of its socket may be replaced, and may well re-connect to the gum tissue as long as certain conditions are met:
Replacement is within 24 hours, and preferably within one hour.
The tooth has not been contaminated, or more important, cleaned and disinfected.
The severed root structures have not dried out or been further damaged, and have not been
|Diverticula are small protrusions from the colon, sometimes caused by the pressure of chronic constipation resulting from a low fibre diet.
In the majority of cases, diverticula do not cause
Diverticulosis or uncomplicated diverticular disease is the term applied to relatively mild and intermittent symptoms of the presence of diverticula.
Diverticulitis is a potentially serious condition which results from faeces being trapped in a diverticulum. Bacteria multiply around the blockage causing inflammation and infection.
Symptoms of severe abdominal pain, fever, diarrhoea (or occasionally constipation), nausea and
There is a risk that diverticula may perforate or rupture and lead on to further infection and
|Gallstones are hard lumps of material which build up in the gall bladder or bile duct.
Most gallstones are around 6mm in diameter, although the size may vary from small sand-like
Gallstones tend to occur if the composition of the bile is abnormal, if the flow of bile is blocked,
A susceptibility to developing gallstones may be inherited.
Gallstones form when substances – often involving calcium compounds – precipitate out of the bile.
In the majority of cases, gallstones are asymptomatic. However, their existence may cause cholecystitis – inflammation of the gall bladder, with pain in the right hypochondriac
If gallstones block the bile duct, obstructive jaundice may result, with yellowing of the skin
|Gastritis is an inflammation of the mucous membrane in the stomach. This results in a sensation of indigestion and nausea.
Bleeding into the stomach may also cause vomiting of blood or blood
Gastritis has a variety of causes, including repeated use of medications such as Ibuprofen, swallowing of corrosive poisons, over-use of alcohol, infections, and extreme
|Gastroenteritis is an inflammation of the stomach and the intestines.
A common cause is infection by either the rotavirus or the Norwalk virus. Alternative causes are infection by bacteria or other organisms.
Gastroenteritis causes abdominal pain, fever, diarrhoea and vomiting. The condition usually passes quickly, but a prolonged or severe attack may lead to dehydration and hypovolaemic shock.
Food poisoning is a specific cause of gastroenteritis, associated with the ingestion of contaminated or inherently poisonous food or water.
Examples of causative agents include:
|Heartburn is a symptom of oesophageal
reflux – a gentle regurgitation of stomach contents into the oesophagus.
This occurs because of relaxation of the cardiac sphincter. There are many reasons for this,
The acid in the regurgitated material irritates the inner lining of the oesophagus, producing
Severe heartburn may mimic the pain of cardiac problems.
|Hepatitis is an inflammation of the liver, through bacteria, toxic substances, or viruses.
Overdoes of some medications, such as Paracetamol, may also cause hepatitis-like conditions.The viral types exist in several forms: A, B, C, D, E, F, and G have been identified to date.
The most common forms are
hepatitis A – spread from faeces into food and water,
hepatitis B and
Exact signs and symptoms vary with the type, but hepatitis usually causes abdominal pain, with nausea, diarrhoea and vomiting. The skin will probably be jaundiced.
If not treated, there is a risk that hepatitis will become chronic and lead to cirrhosis, eventually
occurs when part of an organ – usually abdominal – protrudes through a tear or hole in the muscular tissue which normally holds it in place.
Hernias often occur spontaneously, and may be present at birth, but may occur through muscular strain associated with heavy lifting, a chronic cough, or persistent straining against chronic constipation.
Hiatus hernia. This is a breakthrough of part of the stomach into the thoracic cavity – through an enlargement of the opening in the diaphragm through which the oesophagus passes.
The hernia itself rarely leads to symptoms, but it will tend to lead to oesophageal reflux and
Inguinal hernia. This is more common in males. It appears as a swelling in the groin area or scrotum, and occurs when a portion of the peritoneum or the large intestine breaks through the lower wall of the abdomen.
Femoral hernia. This is similar to an inguinal hernia, but is more common in females. It occurs when part of the intestine breaks through the abdominal wall via the opening taken by the femoral artery, appearing as a swelling in the upper thigh.
Umbilical hernia. This is caused by a weakness in the muscles around the navel, allowing protrusion of the intestine.
Incisional hernia. This occurs when a wound or operation incision has not healed fully and a weakness in the containing
Hernias are generally relatively minor problems, leading often to no more than a slight
Irritable Bowel Syndrome (IBS)
|Irritable bowel syndrome is a term used for a series of symptoms which occur in the digestive system, but without identifiable cause.
Symptoms are mainly caused by abnormal muscular contractions in the intestinal walls, sometimes brought on by external factors.
Symptoms include alternating diarrhoea and constipation, vomiting, abdominal pain and
|Mumps (epidemic parotitis) is a droplet spread viral infection which affects the parotid gland.Mumps may affect people of all ages, although children are most vulnerable.
Mumps brings fever, swelling of the parotid glands, a dry mouth with a ‘furred tongue’, and swollen lymph nodes in the neck. There may also be difficulty in swallowing and earache.
The condition may lead to encephalitis and orchitis (swollen testicles) – more commonly in
|Pancreatitis is a potentially fatal inflammation of the pancreas.
Acute pancreatitis is usually caused by gall bladder problems, such as gallstones blocking the pancreatic duct, or by chronic alcohol abuse. The exact mechanism of pancreatitis is not known, but it is believed that trapped digestive enzymes become active inside the pancreas and
Haemorrhage, oedema, and tissue damage result. The enzyme damage may spread to surrounding tissues, and may be distributed via the circulation to cause more widespread damage.
The condition is marked by severe pain in the upper abdomen, often occurring within twelve hours of a heavy meal with alcohol. This pain may extend to all abdominal areas, the back, and the shoulders, with nausea and vomiting – maybe leading to hypovolaemic shock.
Complications may include bowel obstruction, bleeding into the bowel, and jaundice resulting from bile duct obstruction.
Chronic pancreatitis is often associated with alcohol abuse or chronic pancreatic duct obstruction, although a cause may not be identifiable.
The condition may follow attacks of acute pancreatitis and usually begins with severe high abdominal pain – extending through to the back.
Attacks often last for 24 hours or more and tend to recur. Inflammation and fibrosis slowly destroy the glandular tissue with knock-on effects on the effectiveness of the overall
|A peptic ulcer is a breakdown in the mucous membrane lining the alimentary canal.
The basic cause is that this mucous membrane is damaged by the action of gastric acid or pepsin, either because these are present in abnormally high concentration, or because the normal protective mechanisms have broken down. Common ulcer sites are the stomach and duodenum.
Gastric ulcer. This occurs in the stomach. The main causes are the over-use of medications such as Aspirin or Ibuprofen, and infection by helicobacter pylori bacteria.
The ulcer may well produce no symptoms, but abdominal pain – increased after eating – and
Duodenal ulcer. This occurs in the duodenum. It is usually caused by over-production of gastric acid, and infection by helicobacter pylori bacteria is very common.
Abdominal pain – increased when the stomach is empty – and vomiting are the most likely
All gastric ulcers are at risk of haemorrhage, which will lead to vomiting of blood (haematemesis),
|Peritonitis is an inflammation of the peritoneum. This generally occurs as a result of some other condition, although a blood borne bacterial infection may be the cause.
Liver or kidney failure may cause peritonitis through a build-up of fluid in the abdominal cavity.
Bacteria may infect the peritoneum from the digestive system via a perforated ulcer, a ruptured appendix, from an inflamed pancreas, or because of a penetrating abdominal wound.
Peritonitis is indicated by abdominal pain (often sever and acute) and distension, a fever, and
|Typhoid is a bacterial infection of the intestines by the salmonella typhi bacillus. It is spread in excrement, by insects, and via food and water.
The bacteria enter the lymphoid tissue of the intestines. They then spread to the liver, spleen, and
Initially, typhoid manifests itself with a high fever and headache.
As the condition worsens, it brings nosebleeds, spots on the thorax and abdomen, a distended abdomen, and constipation which then possibly changes to diarrhoea, and finally
Intestinal haemorrhage and peritonitis may lead on to intestinal perforation which may be fatal.
|Vomiting is the body’s way of removing a possibly dangerous substance from the stomach, or simply removing stomach contents in reaction to some other problem.
There are many potential causes of vomiting, including: viral infection, reactions to medications,
The action is controlled from the vomiting reflex centre in the medulla oblongata. The mechanism involves a sharp downward contraction of the diaphragm, a pulling in of the
Foodstuffs and Vitamins
|In order to survive, the human body must receive a balanced diet of foods. This
balanced diet must include appropriate proportions of: carbohydrates,
proteins, fats, mineral salts, fibre, and water, together with vitamins.Failure to receive any of
the constituents of a balanced diet can result in malnutrition.
|Carbohydrates are formed from the elements Carbon, Hydrogen, and Oxygen. They vary in chemical complexity from simple sugars to complex cellulose:
Monosaccharides. These are simple sugars, which are readily absorbed by the body. eg. glucose, fructose.
Disaccharides. These, in molecular terms, are combinations of two monosaccharides. eg. sucrose, lactose.
Polysaccharides. These are complex molecular combinations of many monosaccharides. eg. starches, cellulose.
Not all polysaccharides can be digested by the human body; cellulose and other similar vegetable
Carbohydrates provide energy for heat and movement. Excess carbohydrate is converted to fat and stored.
|Proteins are combinations of amino acids, which are formed from the elements Carbon, Hydrogen, Oxygen, Nitrogen, Sulphur, and Phosphorus.
Proteins are split into their constituent amino acids during the digestive process, and absorbed
Amino acids are classed as either ‘essential’ or ‘non-essential’.
Essential amino acids cannot be synthesised in the body, and must be available from the diet.
Non-essential amino acids can be synthesised in the body from other amino acids, in a process called transamination.
Depending on their amino acid content, proteins are graded as either ‘first class’ – those
First class proteins are derived from animal products and soya beans. Second class proteins are mainly of vegetable origin.
Amino acids from proteins are used for:
Growth and repair of body cell tissues,
Production of hormones, enzymes, and antibodies,
Production of energy (only when insufficient is available from carbohydrates).
Amino acids provide the only usable source of Nitrogen to the body.
Excess protein is broken down; the nitrogenous part is excreted via the kidneys as urea, and the
|Fats are complex compounds formed from the elements Carbon, Hydrogen, and Oxygen. (although these are the same elements which form carbohydrates, the chemical composition is different).Fats are considered in two groups according to their source:
Animal fats. These are derived from meat, oily fish, and dairy products. All animal protein sources also contain some fat. Animal fats are mainly ‘saturated’. ie. their molecular
Vegetable fats. These are found in vegetable oils. Vegetable fats are mainly ‘polyunsaturated’.
ie. their molecular bonding structure is not fully utilised.
Vegetable fats are essential in the diet, and assist in the formation of prostaglandins and
Fats, in general, provide energy to the body. They also serve the purposes of:
Transporting the fat soluble vitamins,
Contributing to the secretions from sebaceous glands,
Assisting in the formation of cholesterol, and steroid hormones.
Excess fats are stored in the body’s fat depots – under the skin, and in the mesentery.
|Mineral salts are inorganic compounds which are essential for all body processes. Many mineral salts are required, although usually only in small quantities. [Table 9 – 4]|
Table 9 – 4 essential minerals
The more common
mineral elements, together with their sources and uses
products, green vegetables, some fish
of teeth and bones, mechanism of muscle contractions, mechanism of blood
some vegetables, a common artificial additive to table salt
of thyroid hormones
kidney, beef, eggs, grain, green vegetables
vegetables, milk, meat, nuts
of bone structure, regulation of muscular and nervous activity
of teeth and bones, regulation of body fluids
cellular activities, mechanism of muscle contractions, transmission of nerve
impulses, maintenance of body electrolyte levels
foods, table salt
of muscle contractions, transmission of nerve impulses, maintenance of body
|Fibre, or roughage, consists of indigestible vegetable substances, in the form of complex polysaccharides and cellulose.It is found in ‘whole meal’ grain products, bran, and many vegetables.Some fibre is partially digested by microbes in the large intestine to form gaseous products, or flatus.Fibre is necessary in the diet as it stimulates peristalsis and bowel movement. It may also contribute in the prevention of some gastro-intestinal disorders.|
|Water is essential to continuing life.
Approximately 60% of the total body weight is water. Unlike other foods, considerable quantities of water are lost through evaporation from the skin and the lungs.
Water is taken into the body either as a liquid, or as a constituent of other foods. A small amount is produced within the body as the result of Hydrogen from foods reacting with Oxygen.
Water provides a moist environment for living cells. It participates in, and facilitates, all
Water is a major constituent of blood, and acts as a transport medium for both benevolent and
Some water is excreted with faeces, but the majority of excess water is excreted as urine.
|A vitamin is an organic substance, found in minute quantities in specific foods, and which is essential for healthy survival, but which provides no actual energy or physical contribution to the body.
Vitamin deprivation causes specific disease depending on the vitamin involved.
Vitamins A, D, E, and K are fat soluble and excess amounts are stored in adipose tissue, mainly the liver. [Table 9 – 6]
The remaining vitamins are water soluble and are not stored; the effects of a reduced intake are
[Table 9 – 5]
Table 9 – 5 water soluble vitamins
The main water soluble vitamins
Figures given as daily need are approximate upper limits
roughage, yeast, milk, meat, yeast
oxidation of carbohydrates in cells
fatigue, loss of muscle tone, nerve changes, beriberi
eggs, beef, cereals, nuts, yeast
problems, skin disorders, digestive problems
(niacin, nicotinic acid)
grain, liver, sea food, beans
carbohydrate metabolism, assists production of sex hormones, reduces
assists utilisation of other vitamins and energy
sleep disorders, cardiovascular and neurone problems, eczema
liver, milk, green vegetables
in metabolism of amino acids
nervous disorders, learning disabilities, anaemia
dairy products, synthesised in the body
red blood cell production
nervous disorders, general weakness
eggs, synthesised in the body
cell growth, and synthesis of non-essential amino acids
muscle pains, weakness, insomnia
(folic acid, folacin)
eggs, milk, green vegetables, synthesised in the body
red blood cell production and DNA formation
vegetables (destroyed by cooking)
oxidation reactions, assists formation of collagen, teeth, and bones, assists
low infection resistance, anaemia, poor connective tissue formation
Table 9 – 6 fat soluble vitamins
The main fat soluble vitamins
Figures given as daily need are approximate upper limits
produce, liver, fish liver oils
epithelial tissues, corneas of eyes, assists formation of rhodopsin
low disease resistance, poor healing, night blindness
produce, eggs, liver, formed in skin by sunlight
Calcium and Phosphorus absorption from food, and bone and tooth formation
dental caries, osteomalacia, diarrhoea, insomnia
dairy produce, green vegetables, nuts
red blood cells, assists muscle and nerve maintenance
in the body, fruit, vegetables
clotting function of blood
Disease and Injury
|Malnutrition is the result of a failure to maintain a balance between the intake of foods required for health and foods actually taken.
The condition is usually associated with a much reduced and poor diet as a result of poverty or deprivation. However, it may also result from a straightforward imbalance in the intake of basic foods – perhaps through personal desire – or from a metabolic disorder which prevents the proper uptake of nutritional
The signs and symptoms vary widely, although weakness, a susceptibility to other diseases, and eventual death are common factors.
Anorexia nervosa is a psychological condition, which causes either a direct refusal to take food, or the use of laxatives or emetics to prevent the uptake of food.
The causes of the condition are complex, with factors such as emotional rejection being
|Deficiencies of individual vitamins in the diet will lead to various different general problems, as indicated in Table 9 – 5 andTable 9 – 6.Some vitamin deficiencies lead to specific conditions.
Beriberi is the result of a deficiency of vitamin B1. It is most common amongst those whose principal diet is based on rice.
Wet beriberi leads to excessive tissue fluid.
Dry beriberi lead to extreme wasting.
Degeneration of nervous tissue is a feature of both forms of the condition, with a common outcome of
Pellagra is a result of a deficiency of vitamin B3. It is most common amongst those whose principal diet is maize.
Pellagra leads to skin disease, diarrhoea, and depression.
Rickets is a result of a deficiency of vitamin D. It is primarily a childhood condition. The lack of vitamin D causes there to be insufficient Calcium salts in the bones, which are then
Osteomalacia is a softening of the bones occurring in adulthood through a lack of vitamin D.
Bone pain is the main symptom.
Scurvy is a result of a deficiency of vitamin C. It may occur in alcoholics, or in those who eat very few fresh vegetables or fruit.
Initial indications are swollen and bleeding gums, followed by a rash of fine bleeding spots.
Anatomy & Physiology for First Aiders
Preface | Introduction | The Body Covering | The Skeletal System | The Muscular System | The Circulatory System | The Respiratory System | The Nervous System | The Senses | The Digestive System | The Urinary System | The Endocrine System | The Reproductive System |Resource list |Copyright |Infection Control | Training Materials