Flashcards in Digestive System Deck (72):
What is meant by digestion?
Chemical breakdown of ingested food into absorbable molecules.
What is meant by absorption?
Movement of nutrients, water and electrolytes through the epithelial lining of the gut into blood or lymph.
Describe the enamel of the tooth.
- crystalline rods or prisms of calcium
phosphate & carbonate
- no cells
- hardest tissue of the body
- no sensation
Describe the dentine of the tooth.
Similar to bone but cells (odontoblasts)
occur nearby in the pulp instead of scattered
through the dentine
Describe the pulp of the tooth.
- soft tissue
- blood vessels, nerves, lymphatics
What is the cementum of the tooth?
calcified connective tissue covering the root
Describe the periodontal ligament of the tooth?
- collagen fibres linking the bone of the
socket (alveolar bone) to the cementum
- rapid turnover
What happens in periodontal disease?
The periodontal ligament becomes loose when bacteria infect them
What are the two types of papilli? Which contains taste buds?
Fungiform (taste buds) and filiform
What are the four main tastes?
Sweet, sour, salty, bitter
WHat are the three types of muscles found in the tongue?
Transverse, vertical, longitudinal
What are the gustatory pores?
Pores between taste buds
Do taste buds have nerves?
What is meant by circumvallate?
Being any of the 12 big papillae with many taste buds found near the back of the tongue
What are the three pairs of salivary glands? What do they each secrete?
Parotid: serous fluid (watery)
Submandibular: (mixture of serous fluid and mucous)
Sublingual: mucous (mucous
What do the major salivary glands secrete in response to?
Parasympathetic stimulation induced by seeing, smelling,
tasting or thinking about food.
What is saliva made up of?
Water, mucous and enzymes
Enzymes include: amylase (breaks down starchy debris around teeth) and lysozyme (antibacterial)
What is the general arrangement of abdominal gastrointestinal organs?
- Parietal peritoneum
- Visceral peritoneum
- Peritoneal cavity
Intraperitoneal organs: within the peritoneal cavity
Retroperitoneal organs: behind the peritoneum
What is the greater omentum?
A large fold of the visceral peritoneum that hangs down from the stomach
What is the mesocolon?
A broad, mesofold of peritoneum, which connects the transverse colon to the posterior wall of the abdomen.
How does the peritoneum assist the digestive system?
It needs a good blood and lymph supply. Peritoneum supplies this. It also holds the organs to limit their movement while the person is moving.
Describe the steps of gaining surface area in the gut tube.
Simple tubular glands (Food goes into glands and pits)
Finger-like projections (villi)
Throughout the length of the gut tube we can recognise _____ tunics making up the wall.
What do the other regions of the tube apart from the small intestine differ principally in?
Specialised features of the mucous membrane. The outer three tunics are less variable.
What is the first layer of the gut wall? Describe what it consists of.
Mucosa (mucous membrane) consisting of:
• epithelium, (specialised for protection or absorption or secretion or combinations of all three).
• lamina propria, (a soft fibrous bed of loose connective tissue on which the epthelium rests; carries nerves and blood capillaries, populated with defensive cells).
• muscularis mucosae, (two thin layers of smooth muscle, inner circular and outer longitudinal). This provides the mucosa with some movement independent of the external
muscle coat (e.g. contractions squeeze secretions from glands or lymph along lacteals).
What is the second layer of the gut wall?
Submucosa, a thick bed of loose connective tissue carrying larger blood vessels, lymphatic vessels and nerves (submucosal plexus). It connects the mucosa to the external muscle coat, but allows some movement between the two. For removing products of digestion.
What is the third layer of the gut wall?
Muscularis externa (external smooth muscle), in two layers to produce peristalsis. The inner layer is circular, the outer is longitudinal. The myenteric nerve plexus occurs between the layers.
What is the lowest level of the gut wall?
Serosa, a slippery outer covering for the gut tube (except oesophagus). It is two layered, with outer mesothelium sitting on a bed of connective tissue. The serosa is also known as the visceral peritoneum. Where a structure is not in contact with the body cavity, the outermost connective tissue layer is referred to as the adventitia.
What is secreted in the mouth, small intestine and large intestine?
Mouth: water, saliva, enzymes
Small intestine: bile and enzymes
Large intestine: mucous
Describe the esophagus
A muscular tube about 25cm long, extending from pharynx to stomach. Travels posterior to trachea. The tube is normally empty with its lumen collapsed, and expands to accommodate food / water.
What are the functions of the esophagus?
- transport (travel time 5 seconds for food, 1 second for fluid, due to peristalsis)
- no absorption, little secretion, no digestion
How is the epithelium of the mucous membrane of the oesophagus tube different to the general gut wall?
- Thick, many-layered (stratified squamous) with sacrificial outer layers for protection against abrasive fragments of food
- Cells are replaced by division in basal layers, then slow migration outwards. Old cells are shed from the surface. Entire epithelium is renewed every 7 days.
How is the external muscle of the oesogphagus tube different from the general gut wall?
Contains, in addition to the usual smooth muscle, some skeletal muscle in the upper third of the esophagus to allow rapid contraction and voluntary control of swallowing
How is the serosa of the oesophagus tube different from the general gut wall?
Over most of its length the esophagus does not lie in a body cavity, so lacks a serosa. Instead it is covered with a fibrous adventitia which attaches it to neighbouring organs eg. trachea
What are the 4 main functions of the digestive system?
Secretion, digestion, absorption, transport
What are the functions of the circular muscle layer and longitudinal muscle layer of the oesophagus?
C: contraction above and relaxation below bolus
L: oesophagus shortens during peristalsis
Describe the stomach.
- A J-shaped bag on the left side, an enlargement of the gut tube. Capacity about 1.5 litres.
- Primary function is storage, since food can be eaten more quickly than it can be digested and absorbed.
- Four regions (cardia, fundus, body, pylorous)
- Well developed muscular sphincter at outlet (pyloric sphincter)
- When empty, lined with longitudinal folds (rugae)
How is the epithelium of the mucosa of the stomach different from that of the general gut wall?
Simple columnar - mucous secreting. Forms many pits lined with mucous-secreting cells, plus gastric glands which open into the pits
How does the external muscle of the stomach different from that of the general gut wall?
Three layers rather than two, (with addition of innermost oblique layer)
Inner oblique, middle circular, outer longitudinal
What are the functions of the stomach?
- Secretion of acid, enzymes, mucus (total volume 2 - 3 litres per day).
Food + gastric juice = chyme
- Digestion of proteins by pepsin
- Absorption of water, ions, some drugs (aspirin, alcohol)
- Transport (mixing waves due to extra layer of muscle, every 20 sec)
- Protection (against its own secretions and microbes)
List the cells in the mucosa of the stomach.
- Surface mucous cells: Secrete insoluble alkaline mucus
which protects the mucosa from acid and pepsin.
- Undifferentiated cells: Stem cells dividing to generate
- Parietal cells: Secrete HCl which kills microbes and living cells. Also secretes intrinsic factor.
- Mucous neck cells: Secrete soluble acid mucus at mealtimes.
- Chief cells: Secrete pepsinogen and gastric lipase. Pepsinogen is converted into the protein-splitting enzyme
pepsin by acid in the lumen of the gland.
- Gastrin cells (enteroendocrine): Gastrin enters blood stream as a hormone, stimulates secretion of acid and pepsinogen, increases muscular contractions of stomach, relaxes pyloric sphincter.
What is the liver?
The liver is an enormous gland, made of epithelial cells (hepatocytes) derived from embryonic endoderm.
What are the functions of the liver? What are these functions carried out by?
All liver functions are carried out by hepatocytes which are multi-talented cells, each performing more than 500 different metabolic functions, including: glycogen/glucose storage and release, recycling of red blood cells, bile synthesis and secretion, synthesis of plasma proteins and removal of toxins from blood.
What does every hepatocyte require?
- access to nutrient-laden blood drained from the intestinal wall
- access to oxygenated blood from the systemic circuit
- access to ducts which drain bile to the gall bladder
What is the function of the fenestrated endothelial cell of liver sinusoid?
These cells act as a filter and allow passage of the watery part of the blood (lymph) but exclude RBC's
List the functions of the liver
• Produce Bile.
– a greenish fluid that emulsifies fats
• Detoxify drugs, metabolites, and poisons
• Store excess nutrients and vitamins
• Synthesize blood plasma proteins -
albumins, globulins, and proteins required for
• Recycle components of aged erythrocytes
Describe the liver lobules
Plates of hepatocytes are stacked together forming liver lobules. Each lobule is hexagonal in cross-section, about 2mm long by 1mm wide.
Describe the dual function nature of the pancreas
- An exocrine gland manufacturing the precursors of digestive enzymes and secreting them as alkaline “pancreatic juice” via a duct system leading to the duodenum. Enzyme precursors are converted to their active form once they arrive in the duodenum. Most food substances are digested by pancreatic enzymes (proteins, carbohydrates, lipids and nucleic acids). Also secrete bicarbonate.
- An endocrine gland. Islets of Langerhans make up 1% of the pancreas and secrete hormones into the bloodstream. The hormones (insulin & glucagon) regulate blood glucose levels.
How can the pancreas be compared to a tree?
Each leaf is a secretory unit (acinus). The duct system forms twigs and branches, where intercalated ducts are the twigs and interlobar ducts are small branches. The main pancreatic duct is the trunk.
What is the size of the small intestine? What occurs in it? What does it receive at its upper end?
The small intestine is a tube about 3cm in diameter and a little over 3m long. Most digestion and absorption occurs here. At its upper end it receives exocrine secretions from the liver (i.e. bile, stored in the gallbladder) and pancreas (pancreatic juice, containing digestive enzyme precursors).
Describe the three regions of the small intestine
• duodenum (L duodeni = 12) refering to its length in finger-breadths, 25cm long. C-shaped tube, not suspended by mesentery, receives biliary and pancreatic ducts. Originates at the pyloric sphincter.
• jejunum (L jejunus = empty), 1m long. Primary region for chemical digestions and nutrient absorption.
• ileum (L ileum = twisted), 2m long, terminates at the ileocecal valve.
How are the four layers of the gut tube different in the small intestine?
• the mucosa is specialised to greatly increase the surface area available for secretion and absorption.
• the submucosa just downstream of the pyloric sphincter contains mucous glands (glands of Brunner).
List the orders of folding to increase surface area
• gross convolutions
• plicae are circular folds 1cm high. Each plica is covered with mucosa and has a core of submucosa.
• villi are fingers 1mm high. Covering is epithelium, core is lamina propria.
• microvilli are fingers 1 micrometre high. They form a “brush border” on the surface of individual absorptive cells. Each microvillus is covered with cell membrane and filled with cytoplasm.
Why is the pancreas a mixed gland?
Exhibits both endocrine and exocrine functions
What is the ileocecal valve?
A sphincter that controls the entry of materials into the large intestine. The ileocecal valve controls intermittent flow of chyme from ileum into cecum.
Describe the cells found in the mucosa of the small intestine
- Columnar absorptive cells, (enterocytes): Absorb the small molecules resulting from digestion
- Goblet cells: Secrete mucus
- Enteroendocrine cells: Secrete hormone secretin (and others) into capillaries of the lamina propria
- Undifferentiated cells: Stem cells dividing to generate new epithelium
- Paneth cells: Secrete bactericidal enzyme lysozyme and are phagocytic
Describe the transport that occurs to and from the small intestine in vessels.
- Transport of carbohydrates (absorbed as monosaccharides), proteins (absorbed as amino acids), water and electrolytes. Venules in the submucosa are tributaries of the hepatic portal vein.
- Transport of absorbed lipids. Lymphatic vessels eventually drain into the venous system.
Describe the replacement of epithelium in the small intestine.
The epithelium is short-lived. Cell division occurs deep in the glands, then the entire sheet (except Paneth cells) moves slowly up the gland walls and up the sides of the villi. Old cells are shed from the villous tip. The entire journey lasts 5 days.
What is the lymph vessel in each villus of the small intestine called? How is the lymph moved?
The lymph vessel within each villus = lacteal. Lymph is “milked” along the lacteal by contraction of smooth muscle fibres in the lamina propria which shorten the villus. These
fibres arise from the muscularis mucosae.
What is the release of secretin from the small intestine stimulated by? What does it stimulate? What is gastrin secreted by? What does it stimulate?
Release of secretin from enteroendocrine cells is stimulated by arrival of acid chyme. Secretin in bloodstream stimulates release of pancreatic juice. Cholecystokinin similar. Gastrin is secreted by enteroendocrine cells of small intestine and stomach; it stimulates gastric glands to secrete acid and enzymes.
Describe the large intestine and list its 7 parts.
A soft muscular tube about 1.5m long.
1) Vermiform appendix
3) Ascending colon
4) Transverse colon
5) Descending colon
6) Sigmoid colon
7) Rectum, Anal Canal, Anus
What are the functions of the large intestine?
- absorption of salts (electrolytes) and water, (1 litre/day compared with 8 litre/day absorbed in the small intestine)
- conversion of chyme into feces. Bacteria ferment remaining carbohydrates that are difficult to digest such as plant cell walls that contain cellulose and lignin. Free fatty acids (butyrate and propionate) are formed as a consequence of fermentation and they protect the enterocytes.
- bacteria produce some vitamins (B12 & K - important for blood clotting) which are absorbed
- secretion of mucus to lubricate feces
- stores fecal matter
What is the caecum? What happens in it?
The cecum is a dilated pouch, (much larger in other mammals, especially herbivores). Here bacteria (not enzymes) are responsible for digestion. Humans produce no enzymes capable of digesting the cellulose of plants.
What is the appendix?
Vestigial worm-like appendage of the caecum
What does faeces contain? What gives rise to its colour?
Contain bacteria (30%), undigested dietary fibre (30%), cells shed from the intestinal lining, and mucus. Their colour is from pigments derived from bacterial decomposition of bilirubin
How is the mucosa of the gut wall different in the large intestine?
- Has no villi but many intestinal glands that increase SA (also called “crypts of Lieberkuhn”).
- Surface epithelial cells are enterocytes (absorptive) similar to small intestine.
- Intestinal glands contain mostly goblet cells (mucus), no Paneth cells.
- Cell renewal as in the small intestine, entire epithelium replaced every 4-5 days.
- Many clusters of lymphocytes (lymphoid nodules) in the lamina propria, reflecting large bacterial content of colon. Protection from bacteria getting into the body via the epithelium.
How is the external muscle of the large intestine different to the general gut wall?
outer longitudinal muscle is thickened in three strips. The strips (teniae coli) contract to pull
the intestinal tube into sac-like pockets (haustra coli). Haustra change shape and position.
List and describe the different cells found in the mucosa of the colon
- Columnar absorptive cells, (enterocytes): Similar to those in the small intestine, absorbing mostly water
- Goblet cells: Secrete mucus to lubricate the passage of
- Undifferentiated cells: Stem cells dividing to generate new
- White blood cells: Mostly lymphocytes which provide defence against bacteria invading from the lumen of the colon
Describe the rectum and the anus
The rectum makes up the final 20cm of the gut tube; its
last 2cm are called the anal canal. The canal is closed by two sphincters. The inner one is an involuntary smooth-muscle sphincter; the outer one is under voluntary control, and is skeletal muscle.
When is the urge to defecate felt? What causes this?
The urge to defecate is felt when the rectum fills to about 25% of its capacity. Stretching of the rectal wall initiates a reflex contraction of teniae coli in the descending colon and rectum. Shortening of this part of the gut tube further
increases pressure in the rectum. The internal anal sphincter relaxes (opens) as part of the reflex.
When does the external sphincter of the anus relax? Describe this action in children.
- If the time is convenient the external sphincter is then relaxed voluntarily; if not it is maintained in the contracted state and the defecation reflex subsides.
- Children take some time to learn how to control
the external sphincter.