The Gastrointestinal System

Question 1

What is the function of the digestive system?

Answer 1

• breaking down ingested food and fluids into molecules small enough to be absorbed into the bloodstream
• removing waste products from the body

Question 2

Describe the structure and function of the teeth

Answer 2

• the 20 decidua (milk) teeth erupt into the oral cavity from around the 6th month of life
• from the age of 6 these are gradually pushed out and by the age of 24 they will have been replaced by 32 permanent teeth
• two types of teeth -> incisors and canines for cutting and biting
  - > premolars and molars for grinding and chewing
• made up of crown, root, neck and covered with enamel
• inside the tooth is the pulp cavity containing blood, lymph vessels and nerve cells, surrounding this is dentine

Question 3

Describe the structure and function of the palate

Answer 3

• the hard palate
  - -> situated at the front of the roof of the mouth
  - -> formed by the maxilla and palatine bones
• the soft palate
  - -> the back of the roof of the mouth
  - -> composed of muscle
  - -> continuous with the pharynx
  - -> at the rear is the uvula and either side of which are fold of mucous membrane called pillars of faucets
  - -> between these the palatine tonsils can be seen which are collections of lymphoid tissue

Question 4

Describe the structure and function of the tongue

Answer 4

• large muscular structure occupying the floor of the mouth
• attached posteriorly to the hyoid bone and inferiorly by the frenulum to the floor of the mouth
• surface is covered with stratified epithelium containing containing projections (papillae) of which some contain taste buds
  - -> these nerve endings extend into the soft palate, pharynx and epiglottis
  - -> surface also contains glands that secrete digestive enzymes
• the muscular action of the tongue enables food to be chewed, shaped and swallowed
• opening into the mouth are ducts from the salivary glands

Question 5

What are the salivary glands?

Answer 5

• three pairs of exocrine glands in the mouth
• parotid glands -> situated beneath and slightly to the front of the ears, they terminate close to the second upper molar teeth
• submandibular glands -> situated beneath the angle of the jaw, the ducts open into the mouth beneath the tongue to either side of the frenulum
• sublingual glands -> lie above the submandibular glands, a number of ducts leave the gland and open into the floor of the mouth

Question 6

Describe the composition of saliva and its functions

Answer 6

• saliva is made up of the combined secretions of the six salivary glands and small mucus-secreting glands found scattered around the mouth
  - -> it contains 99.5% water and 0.5% chemical solutes (sodium and postassium ions) also present are organic substances (mucus, immunoglobulins, lysozyme and salivary amylase)
  - > the functions of salvia are:
  - to dissolve foods so that their taste can be determined
  - to keep the mouth moist
  - to lubricate food for easier swallowing
  - to begin the chemical breakdown of the food
• the presence of lysozyme and immunoglobulins helps to prevent tooth decay and infection in the mucous membranes of the mouth
• the mucus aids lubrication of the food so that it can be more easily chewed and swallowed
• saliva secretion is under control of the autonomic nervous system
• a small amount is secreted constants to keep the mucous membranes of the mouth, pharynx and oesophagus lubricated, however production increases with the presence and smell of food

Question 7

Describe the structure and function of the pharynx

Answer 7

• the oropharynx and laryngopharynx are associated with the digestive tract
• food passes from the mouth to the pharynx and then to the oesophagus as a continuous process
• food taken into the mouth is manipulated into position by the cheeks and tongue so that it can be chewed and mixed with saliva to form a soft mass
• saliva contains salivary amylase which begins to break down starch Ito disaccharides and polysaccharides
• this mass or bolus of food can then be swallowed

Question 8

Describe the process of swallowing food

Answer 8

• voluntary movement of the bolus of food into the rear of the mouth by the action of the tongue against the palate
• involuntary passage through the pharynx
  - -> the soft palate rises to occlude the nasopharynx, the tongue and the pharyngeal folds close the way back into the mouth and the larynx is lifted upwards and forward so that the epiglottis covers the opening to the trachea
  - -> the bolus is then moved into the oesophagus
  - -> during this process coordinated by the deglutition centre, breathing is interrupted for a few seconds
• peristaltic action moves the bolus of food down the oesophagus into the stomach
• salivary amylase is unable to complete the breakdown of disaccharides and polysaccharides in the mouth but it’s action continues until it is neutralised by gastric acids in the stomach

Question 9

Describe the microstructure of the digestive tract

Answer 9

• the mucosa
  - -> innermost layer of the digestive tract
  - -> consists of a lining of epithelium supported by connective tissue
  - -> the epithelial cells contain mucus-secreting cells
  - -> in areas where there is a risk of damage (oesophagus, anal canal) the epithelium is stratified
  - -> in selected areas of the digestive tract the epithelium contains glands which secrete the hormones that control digestion
  - -> the layer of connective tissue contains blood vessels (transport absorbed substances round the body) nerve cells (innervate the tract) and lymph vessels (protect the body from organisms that may have entered with the food)
• the submocosa
  - -> consists of connective tissue (contains blood vessels, nerves and lymphoid tissue)
  - -> Meissner’s plexus is located in this layer, it has an important role to play in controlling the release of gastric secretions
• the muscle layer
  - -> composed of 2 layers of smooth muscle (inner layer of circular fibres, outer layer of longitudinal fibres)
  - -> involuntary muscle action breaks up the food, mixes it with digestive secretions and propels it along the tract by peristaltic action
  - -> the plexus of Auerbach is contained in this layer, it contains autonomic nerve fires, it is responsible for the degree of contraction or relaxation of the muscles of the digestive tract
• the serosa
  - -> the outermost layer of the digestive tract
  - -> consists of connective tissue and epithelium, it is a continuation of the peritoneum
  - -> peritoneum consists of two layers
  - parietal layer -> lines abdominal cavity
  - viscera layer -> covers the organs and supports their position
  - -> the cavity between the parietal and visceral peritoneum is a potential space containing serous fluid which makes it possible for the two layers to slide over one another without friction

Question 10

Describe the structure and function of the oesophagus

Answer 10

• it is a thin-walled muscular tube which runs from the pharynx to the superior position of the stomach
• it passes through the diaphragm at the oesophageal hiatus and bends sharply upwards before entering the stomach
  - -> this is thought to prevent regurgitation of stomach contents into the oesophagus
• two sphincters are present at either end of the oesophagus
  - -> superiorly the cricophayngeal sphincter - prevents air entering the oesophagus during inspiration
  - -> inferiorly the cardiac sphincter - prevents the regurgitation of gastric contents
• blouses of food are transported through the oesophagus by peristalsis

Question 11

Describe the structure and function of the stomach

Answer 11

• the stomach is an enlarged J-shaped section of the digestive tract, continuous with the oesophagus above and the duodenum below, commonly described in 3 parts
  - -> the fundus - above the level of the cardiac sphincter
  - -> the body - make up most of the stomach
  - -> pylorus - lower end which leads to the duodenum
• the fundus and body walls contain little muscles and are used as a reservoir for food
• the walls of the pylorus contain more muscle and in this region strong muscular contractions churn up the food and mix it with gastric juices
• food is then removed from the stomach by peristalsis through the pyloric sphincter into the duodenum
• the size of the stomach varies according to the amount of food present - the walls of the fundus and body are arranged in folds (rugae) and are capable of great distension
• gastric secretions (gastric juices) are produced in the stomach and continue the process of digestion

Question 12

Describe the composition and function of the gastric juice

Answer 12

• approx 2-3 L produced daily
• composed of water - liquify the food
• mucus - further lubricate it
• hydrochloride acid - makes the stomach contents highly acidic (pH 1.5 -3.5) and is a major defence against ingested harmful micro-organisms
  - -> it also enables enzymes to begin the process of protein digestion by denaturing proteins and activating the enzymes
• pepsinogen - precursor of pepsin, responsible for the breakdown of proteins
• intrinsic factor - absorption of vitamin B12
• secreted mucus for lubrication of food

Question 13

Describe what happens to food once it has entered the stomach (digestive function)

Answer 13

• once food enters the stomach gentle mixing waves (peristalsis) break it up, mix it with gastric secretions and reduce it to a semi-fluid mass known as chyme
• peristalsis is created by the alternate contraction and relaxation of portions of the stomach wall
• as digestion proceeds the waves become stronger and they move the contents down towards the pylorus
• the pyloric sphincter remains almost closed so only small fluid portions leave the stomach at one time
• the majority of the chyme reaching the sphincter is rolled back into the body of the stomach where chemical and mechanical breakdown continue
• it takes 2-6 hours after eating a meal for the stomach to be emptied of food
• proteins stay in the stomach the longest, while carbohydrates are over on mor quickly into the small intestine
• absorption of nutrients does not usually begin until chyme enters the small intestine, however, water, alcohol, some electrolytes and certain drugs such as aspirin are absorbed from the stomach

Question 14

Describe how gastric and muscular secretions are controlled

Answer 14

• they are controlled by both hormonal and nervous stimulation
• before the ingestion of food during the cephalic stage, the sight and smell of food stimulate the autonomic nervous system to begin peristalsis and the production of gastric secretions
• once food enters the stomach distension and a rise in pH stimulate the production of the hormone gastrin which further stimulates gastric secretions
  - -> this phase is known as the gastric phase and continues until the stomach is emptied and the pH falls to 1.5
• finally during the intestinal phase, the presence of food in the duodenum stimulates the production of three hormones
  - -> gastric inhibitory peptide - inhibits both gastric secretions and gastric motility
  - -> secretin - inhibits gastric secretions
  - -> cholecystokinin (CCK) - inhibits stomach emptying, preventing over distension of the duodenum and allowing more time for gastric digestion, the enterogastric reflex (neural reflex) also effects this process

Question 15

Describe the structure and function of the small intestine

Answer 15

• situated centrally in the abdominal cavity and opens into the large intestine
• most digestion and absorption takes place here
• divided into 3 parts
  - -> the duodenum - shortest section of the small intestine, at its midpoint two openings enter, one from the bile duct and the other from the pancreas, the pylorus empties into the duodenum which I turn empties into the jejunum
  - -> the jejunum - middle section, empties into the ileum
  - -> the ileum - final section, joins the large intestine at the ileocaecal sphincter

Question 16

Describe the microstructure of the small intestine

Answer 16

• follows general 4 layer pattern of the digestive tract, however both the mucosal and submucosal layers are modified to facilitate digestion and absorption
• mucosal layer
  - -> folded to increase the surface area available for absorption
  - -> the surface layer of epithelial cells are further folded to produce villi surmounted with microvilli (known as the brush border)
  - -> a network of blood capillaries and lymph vessels are present in each villus to transport absorbed substances around the body
  - -> interspersed throughout the epithelial layer are goblet cells secreting mucus
• submucosal layer
  - -> contains intestinal glands and provides cells to renew the epithelium
  - -> contains digestive enzymes to complete the digestion of carbohydrates, fats and proteins
• small intestine continuously secretes intestinal secretions at a pH of approx 7.5 which contains water, mucus and the digestive enzyme enterokinase

Question 17

Describe the digestive functions of the small intestine

Answer 17

• chyme enters the small intestine from the stomach and is mixed with intestinal secretions, bile and pancreatic juice
• as this mixture passes over the brush border of the intestinal lining, further enzymes are secreted from the epithelial layer
• all these contribute to the breakdown of proteins, carbohydrates and fats into their smallest components (amino acids, monosaccharides, fatty acids and glycerol)
• this is completed at the chyme moves through the length of the small intestine
• approx 90% of these substances are absorbed into the bloodstream, along with electrolytes, vitamins and most of the water ingested or released as part of the digestive process
• the rate at which chyme is emptied from the stomach governs its passage along the small intestine, this is partly due to peristalsis by predominately segmentation (both processes controlled by autonomic nervous system)
• segmentation is a localised contraction of the intestinal wall which results in the chyme being separated into small parcels
  - -> subsequent relaxation of the wall at this point, at the same time as the contraction of the wall around the centre of the parcels mixes the food thoroughly allowing digestive enzymes to work efficiently
• it will take 3-5 hrs for chyme to pass through the small intestine

Question 18

Describe the structure and function of the large intestine

Answer 18

• extends from the ileum to the rectum
• it has a larger lumen than the small intestine and is divided into 2 sections
  - -> caecum - the ileocaecal sphincter separates the ileum from the caecum
  - it is here that the appendix is attached, it is a small tube with a blind end that leads from the caecum, it is a dilated section of the large intestine that is continuous the ascending colon
  - -> colon - the ascending colon is situated of the right side of the abdomen and ascends to a point beneath the liver where it bends towards the left, forming the transverse colon
  - this section of colon passes around the base of the spleen and in front of the duodenum to the left side of the abdomen where it bends downwards to form the descending colon
  - the descending colon passes down the left side of the abdomen until it enters the pelvis where it becomes known as the sigmoid colon
  - the sigmoid colon is S-shaped and moves towards the midline to join the rectum

Question 19

Describe the microstructure of the large intestine

Answer 19

• similar to the digestive tract but differs in the structure of the muscle layer
• portions of the longitudinal layer are thickened to form three bands
  - -> the taeniae coli - located at regular intervals around the length of the large intestine
  - -> these bands of muscle are slightly shorter than the overall length of the colon and give the colon a puckered appearance
• the walls of the large intestine therefore form pouches known as haustra

Question 20

Describe the digestive functions of the large intestine

Answer 20

• by the time the chyme reaches the large intestine the majority of nutrients have been absorbed, however more water is absorbed here as are some further electrolytes and vitamins
• mucus s secreted by the large intestine but no enzymes are produced
• bacteria are found in the large intestine which play a role in the production of certain B vitamins and vitamin K
  - -> they also prepare the chyme for its elimination in the form of faeces
• any remaining carbohydrates are fermented by the Bactria present and produce hydrogen, carbon dioxide and methane
  - -> these gases are the constituents of flatus present in the colon
• further bacteria break down bilirubin to urobilinogen which gives faeces their colour
• the colon empties these faeces into the rectum for storage until elimination
• movement of chyme through the large intensive is governed by the rate of gastric emptying like the small intestine
  - -> as food enters the large intestine it is moved up the ascending colon by the contraction of the muscle layer in each individual haustrum as it is filled
• peristalsis only occurs at a very slow rate except after the ingestion of food which initiates a strong wave from the middle of the transverse colon to the rectum, this results in mass movement of chyme whenever food is eaten

Question 21

Describe the structure and function of the rectum and anus

Answer 21

• the final portion of the digestive tract consists of a dilated section called the rectum
• and a short canal leading to the exterior of the body, the anus
• the anus contains two sphincters, internal sphincter (involuntary control) and the external sphincter (voluntary control) both are responsible for the elimination of faeces

Question 22

Describe the process of defecation

Answer 22

• the rectum is normally empty but mass movement of chyme in the colon moves faeces into it
• distension of the rectum stimulates stretch receptors in the rectal walls initiating the defecation reflex
• the internal sphincter relaxes but as the external sphincter is under voluntary control the defecation reflex can be inhibited in which case the faeces are fed back into the colon until the next episode of mass movement
• when a voluntary decision is made to allow defecation the external sphincter relaxes and with increased intra-abdominal pressure from the abdominal muscles and the lowering of the diaphragm, defecation occurs

Question 23

Describe the blood supply and lymphatic drainage to the digestive system

Answer 23

• the blood supply to the abdominal digestive organs comes from the coeliac artery (stomach, duodenum, liver, gall bladder, pancreas) and superior (small intestine and large intestine to the transverse colon) and inferior mesenteric arteries (descending colon, rectum)
• rectal arteries supply the remaining rectum and anus
• venous return is via the hepatic-portal system
  - -> veins from the structures of the digestive organs join together to form the portal vein which then passes through the liver before returning the deoxygenated blood to the heart

Question 24

Describe the changes that take place during pregnancy

Answer 24

• increased appetite and thirst in early pregnancy due to hCG affecting the hypothalamus and decreasing the osmotic threshold for thirst
• in advanced pregnancy both appetite and capacity for food intake decline owing to the upward gastric displacement and pressure from the gravid uterus, reduces capacity for large meals
• changes in taste buds, experience food cravings and aversions
  - -> pica - extreme craving usually of non-nutritious substances (rocks, soap coal etc)
• nausea and vomiting, usually in the first trimester
• gums often become hyperaemic, oedematous and spongy due to the effects of oestrogen on blood flow and connective tissue constituency
  - -> gums therefore bleed more easily and are vulnerable to abrasive food and vigorous tooth brushing
  - -> can develop pregnancy gingivitis
• saliva also becomes more acidic in pregnancy which can lead to tooth erosion
• excessive production of saliva can occur (ptyalism)
• heartburn is a common condition experienced in pregnancy
  - -> the effects of progesterone on the tone of the cardiac sphincter means its competence is impaired and regurgitation of gastric acid is more likely
• acid secretion in the stomach tends to decrease, secretion of peptin also decreases
• gastric tone and motility decrease
• in advanced pregnancy the stomach drapes loosely over the fundus which tends to delay gastric emptying, the delay of chyme released from the stomach may increase the likelihood of heartburn and nausea
• progesterone induced relaxation of the smooth muscle decreases gut tone and motility thus transit time in the gut increases which may enhance absorption but leads to bloating and abdominal distension
• the relaxation of the smooth muscle in the colon leads to increased water absorption and increases the incidence of constipation
• raised levels of angiotensin and aldosterone also increase sodium and water absorption from the colon