Digestive System

Question 1

Phylogeny of the Digestive System

Answer 1

The intracellular digestion occurred in protozoa and sponges. The digestion was entirely intracellular with food particles being enclosed within a food vacuole by phagocytosis. Digestive enzymes would then be added to the vacuole (compartmentalisation to digest the food without digesting the cell itself). The limits of this is only small particles can be phagocytosed to be ingested, every cell must be capable of secreting the necessary enzymes and absorbing the products of digestion into the cytoplasm.

Question 2

Gastrovascular Cavity

Answer 2

The digestive compartment with a single opening (mouth). This functions in both digestion and the distribution of nutrients. These are involved in extracellular digestion of large food masses, specialisation of cell lining the lumen for forming digestive enzymes and absorbing the nutrients from food. This allows organisms to ingest food much larger than any of its cells could take in directly.

Question 3

Alimentary Canal

Answer 3

This evolution produces a complete mouth to anus tube. The tube between 2 openings (mouth and anus) and food moves in 1 direction. Valves and sphincters are used to control the flow of food along the digestive tract. There is an area for extracellular digestion of larger food particles and increased regional specialisation which allows for the sequencing of processes e.g. first extracellular digestion which then moves enzyme breakdown to absorption areas.

Question 4

Stages of Digestion

Answer 4

1. Ingestion - the selective intake of food.
2. Digestion - through both mechanical (occurs first) and chemical (occurs after mechanical) breakdown of food.
3. Absorption - the uptake of nutrient molecules into epithelial cells of the digestive tract and then to blood and lymph.
4. Compaction - the absorption of water and consolidation of the indigestible waste left over from the originally consumed food.
5. Defecation - the elimination of the waste (indigestible material).

Question 5

Major Regions of the Alimentary Canal

Answer 5

First there is a reception area for food including the mouth parts and salivary glands. A conduction tube to the storage and early digestion area is then found in the form of an esophagus. The storage and early digestive area is the stomach. There is typically a grinding area (gizzard in birds) to mechanically breakdown food more. An area of terminal digestion and absorption occurs after this usually as a small intestine. Finally an area for water absorption and the concentration of solids is found in the form of the large intestine.

Question 6

Human Digestive System

Answer 6

1. Reception - the mouth and pharynx.
2. Conduction - esophagus.
3. Storage and early digestion - stomach.
4. Terminal digestion and absorption - small intestine (SI).
5. Water absorption and solid concentration - large intestine (LI).
6. Elimination of wastes - rectum and the anus.

Question 7

Mechanical Digestion

Answer 7

This is a form of digestion which is characterised by the physical breakdown of food. It involves the cutting and grinding with teeth and the churning and mixing which occurs in the stomach and SI. This breakdown of food assists to increase the surface area to volume ratio which will assist in later processes of chemical digestion.

Question 8

Chemical Digestion

Answer 8

This is a form of digestion which is done via enzymes produced by glands associated with the digestive tract. It involves amylase (breaks down polysaccharides to monosaccharides), protease/pepsin/peptidases (breaks down polypeptides to amino acids) and lipase (breaks down fats to monoglycerides and fatty acids). Certain nutrients can be absorbed without digestion e.g. vitamins, free amino acids, minerals, cholesterol, water etc.

Question 9

Layers of the Digestive Tract Wall

Answer 9

Lumen - the central space which contains the food being digested.
Mucosa - the inner layer modified according to the digestive organ (epithelium, lamia propria (CT), muscularis mucosa (smooth muscle).
Submucosa - the layer of loose CT containing nerve, blood, mucus secreting glands and lymphatic vessels.
Muscularis Externa - the typically 2 layers of smooth muscle with an inner circular layer and and outer longitudinal layer.
Serosa or Adventitia - a thin layer of CT and mesothelium or a layer of fibrous CT.
Supply - there is also typically many blood and nerve vessels found around the walls of the gut.

Question 10

Motility

Answer 10

The alimentary tract has 2 opposing layer of smooth muscle with in inner circular layer and an outer longitudinal layer. The 2 mechanisms to move food along and within the gut is peristalsis and segmentation which is controlled by sphincters and valves. Peristalsis is waves of contraction of circular muscle behind to the food bolus and relaxing the muscle in front. This helps food move along the gut. Segmentation occurs when alternate rings of smooth muscle contract to divide and squeeze the contents back and forth. This is involved in mixing food within the gut and doesn’t move the contents along that much.

Question 11

Peritoneum

Answer 11

A 2-layered serous membrane which binds viscera to the body wall or suspends viscera from the body wall. This comes in 2 forms of the parietal and visceral. The parietal outer layer lines the abdominal cavity wall. The visceral inner layer turns inward from the body wall and wraps around the abdominal viscera forming the serosa. Between both the layers there is peritoneal cavity which is lubricated by peritoneal fluid.

Question 12

Peritoneum & Organs

Answer 12

Organs can either be retroperitoneal or intraperitoneal. Retroperitoneal organs are attached to the posterior body wall and have parietal peritoneum overlying them e.g. pancreas. Intraperitoneal organs aren’t sitting in the peritoneal cavity but are wrapped in visceral peritoneum.

Question 13

Mesenteries

Answer 13

These are large sheets of thin CT that loosely suspend the stomach and intestines from the abdominal wall. This allows for the movement of organs which provides a framework and passage for blood vessels, nerves and lymphatic vessels. The greater omentum hangs down from the stomach, the lesser omentum runs between the stomach and liver while the mesocolon runs between the curvature of the large intestine.

Question 14

Arterial Circulation

Answer 14

The celiac trunk is the first branch of the aorta which supplies the lower esophagus, stomach, duodenum, pancreas, liver and spleen. The superior mesenteric artery supplies most of the SI and the proximal half of the LI. The inferior mesenteric artery supplies the distal half of the large intestine as well as the rectum. All of these arteries are branches of the aorta.

Question 15

Hepatic Portal Circulation

Answer 15

A portal system is where blood flows from 1 capillary bed to another before returning to the heart. This particular portal vein delivers venous blood from organs of the gastrointestinal tract and spleen to the liver.

Question 16

Enteric Nervous System

Answer 16

This neural control system of the gut functions independently of the CNS. It is composed of 2 networks and regulates the digestive tract motility, secretion and blood flow through reflexes that don’t require the spinal cord. The 2 networks are the myenteric plexus and the submucosal plexus. The myenteric plexus is between the 2 layers of the muscularis externa and controls the frequency and strength of contraction of the muscularis externa. The submucosal plexus is found in the submucosa and controls glandular secretions and the movements of muscularis mucosa. There are also chemoreceptors and mechanoreceptors (detect stretch).

Question 17

Autonomic Nervous System

Answer 17

The parasympathetic nervous system is from the cranial nerves (facial (VII), glossopharyngeal (IX), Vagus (X)) and sacral nerves. This division of the ANS increases the secretions from glands and gut motility. The sympathetic nervous system is from the thoracic and upper lumbar spinal nerves. This division of the ANS decreases secretions from glands and gut motility.

Question 18

Herbivores vs Carnivores

Answer 18

A carnivores teeth when a straight line is drawn through the center of the open mouth the line should also intersect with the articulation point of the jaw and the skull. This means that the teeth of a carnivore will close from the back first and then the front after (scissors). The same line in a herbivores mouth will be below the articulation point of the jaw and skull. This means that a herbivores teeth will all close simultaneously. Humans have the same structure as a herbivore.

Question 19

Basic Gut Characteristics

Answer 19

Typically the epithelium is simple columnar epithelium (look out for changes to this). There are adaptations for increased surface area (villi). Typically the gut wall consists of 2 layers of smooth muscle (look out for changes in this). The outer layer can either be serosa or adventitia. Serosa is moist, wet and thin which allows for the movement of organs whereas adventitia is fibrous CT which attaches these organs to surrounding tissues.

Question 20

The Oral Cavity

Answer 20

Also known as the Buccal cavity. It consists of an upper and lower lip, tongue, salivary glands, the soft and hard palate and the uvula. This is the site at which ingestion occurs which mechanical digestion occurring through chewing and chemical digestion. It is also involved in speech and respiration for other organ systems. It is lined with stratified squamous epithelium with areas subject to food abrasion have keratinised (gums, hard palate) the other areas have non-keratinised (soft palate, insides of cheeks, lips).

Question 21

Teeth

Answer 21

There are 32 adult ones and 20 deciduous (baby) ones which are divided evenly between the maxilla and mandible. There are 8 incisors used for biting, cutting and stripping. There are 4 canines used for seizing, piercing and tearing. There are 8 premolars and 12 molars which are used for grinding and crushing. The hominoidea superfamily all have a 2123/2123 dental formula (the number of teeth in 1/4 of the jaw).

Question 22

Mastication

Answer 22

When food enters the buccal cavity it stimulates receptors that trigger an involuntary chewing reflex. This makes food easier to swallow, exposes more surface area for the action of digestive enzymes and speeds up chemical digestion. This is a form of mechanical digestion.

Question 23

Lips & Cheeks

Answer 23

These are lined with stratified squamous epithelium. These provide a seal in which food can be stored for mastication (suckling in babies) to occur.

Question 24

Tongue

Answer 24

These are an adaptation of vertebrates (not all vertebrates) which has intrinsic (within) and extrinsic (attached to) muscles (these assist in food manipulation and speech), lingual papillae (tastebuds used as chemosensors), neuroepithelial cells (attached to lingual papillae in order to comprehend taste in the brain), lingual glands (serous and mucous glands that secrete saliva) and the whole structure is covered in stratified squamous epithelium (partially keratinised).

Question 25

Salivary Glands

Answer 25

These are intrinsic and come in lingual (in the tongue), labial (inside the lips) and buccal (inside the cheek) form. These produce lingual lipase and lysozyme as hormones and secrete saliva at a fairly constant rate. These can be extrinsic and come in parotid, submandibular and sublingual forms which sit outside the oral cavity and secrete into it through ducts. These secrete 1-1.5L salvia/day. These both contain mucous (secrete mucous) and serous (contain secretory granules and secrete fluid containing enzymes and electrolytes) glands.

Question 26

Saliva

Answer 26

This moistens and lubricates food. It contains water and some other substances which are salivary amylase (enzyme that digests carbohydrates), lingual lipase (enzyme that digests fat only slightly), mucus (binds and lubricates food for swallowing), lysozyme (enzyme that kills bacteria), antibodies (inhibit bacterial growth) and electrolytes. The release of this substance is controlled by the brainstem.

Question 27

Salivation

Answer 27

The process of releasing saliva which is controlled by the brainstem (medulla oblongata and Pons). This process is stimulated by the present of food (odour, sight, thought) and is a response from the autonomic nervous system (ANS). The parasympathetic division controls the motor response via the facial (CN VII) and glossopharyngeal (CN IX) nerves which stimulates salivary glands to produce more thin, enzyme rich saliva. The sympathetic division stimulates the salivary glands to produce less saliva but the saliva produced will be thicker and mucus rich.

Question 28

Pharynx

Answer 28

This is made up of 3 regions the nasopharynx (posterior to nasal cavity), oropharynx (posterior to oral cavity), laryngopharynx (posterior to larynx). This region contains constrictor muscles which force food downward during swallowing.

Question 29

Swallowing

Answer 29

This is controlled by a center in the medulla oblongata and occurs in 2 phases. The buccal phase occurs under voluntary control whereas the pharyngoesophageal phase is involuntarily controlled.

Question 30

Buccal Phase of Swallowing

Answer 30

The first swallowing phase where mastication occurs simultaneously with breathing so epiglottis doesn’t block airway. After mastication is finished the tongue pushes the bolus of food to the back of the oropharynx, the soft palate rises to close off the nasopharynx, the tongue lifts up and the epiglottis closes to move food down the esophagus. This is voluntarily controlled.

Question 31

Pharyngoesophageal Phase of Swallowing

Answer 31

The second swallowing phase which is involuntarily controlled via peristalsis. During peristalsis there is a contraction and relaxation of the inner circular layer of muscle to push food down the esophagus. Contraction occurs behind the bolus of food and relaxation in front of it to move the food further down. This is controlled by the myenteric plexus a nerve plexus between the layers of muscularis externa.

Question 32

Esophagus

Answer 32

This extends from the pharynx to the stomach and passes through the diaphragm. This is involved in the conduction of food to the digestive region. This region is lined by stratified squamous epithelium, has a lamina propria and has muscularis mucosa surrounding the layers to complete the mucosa. The mucosa is folded into longitudinal ridges when empty. There is muscularis externa consisting of an inner circular layer and an outer longitudinal layer making up the submucosa (also includes glands). The outer layer is adventitia above the diaphragm and serosa below the diaphragm. It contains sphincters with an upper (formed by inferior pharyngeal muscles) and lower (also known as cardiac formed by thickening of circular muscles of muscularis externa) esophageal sphincters.

Question 33

Muscularis Externa of Esophagus

Answer 33

This is made up of both skeletal and smooth muscle. In the upper 1/3 of the esophagus this is purely skeletal muscle, the the middle 1/3 there is a combination of both skeletal and smooth muscle and in the lower 1/3 it consists only of smooth muscle. The skeletal muscle is faster than smooth and therefore can move the bolus of food quickly.

Question 34

Gastro-Esophageal Junction

Answer 34

There is a transition from the stratified squamous epithelium in the esophageal mucosa to simple columnar epithelium of the gastric mucosa.

Question 35

Stomach

Answer 35

This is a bag-like organ which is adapted for storage (stretch) and has 4 major regions which are the fundic region, cardiac region, body and pyloric region. Throughout the inside of this organ there are gastric rugae (folds). This mechanically breaks up food particles by muscular action (churning) and liquified food to form chyme. This is also the place where the chemical digestion of fat and protein really begins. This also contains 3 layers of muscularis externa consisting of an outer longitudinal layer, a middle circular layer and an inner oblique layer. There is a pyloric sphincter at the end which allows chyme to enter the duodenum created by a thickening of the muscularis externa.

Question 36

Gastric Mucosa

Answer 36

This consists of simple columnar epithelium folded into gastric pits that lead into glands which secrete gastric juice (water + HCl + pepsin). There are tight junctions between epithelial cells to prevent gastric juice from digesting the CT of the lamina propria. This also has mucous cells and mucous neck cells that secrete mucus and resists the action of acid and enzymes on the stomach tissue, parietal cells that secrete HCl, intrinsic factor and ghrelin and chief cells that secrete pepsinogen and gastric lipase.

Question 37

Pepsin

Answer 37

Chief cells secrete pepsinogen which is a zymogen (a digestive enzyme secreted as an inactive protein). The pepsinogen will then react with the HCl secreted by parietal cells to form the active enzyme pepsin which allows for the breakdown/digestion of proteins. This also has a autocatalytic effect and coverts more pepsinogen into pepsin.

Question 38

Gastric Motility

Answer 38

There are peristaltic contracts which begin the the fundus and progresses to the pyloric region. It churns food and mixes it with gastric juice which allows for both mechanical and chemical digestion to occur. Each wave of peristalsis squirts 3mL of chyme into the duodenum in order to digest nutrients in small amounts and neutralise the stomach acid. The parasympathetic division increases gastric secretion and motility when food is eaten while the sympathetic suppresses gastric secretion and motility when the stomach empties.

Question 39

Digestion in the Oral Cavity

Answer 39

Mechanical digestion of food through chewing. Salivary amylase begins the digestion of carbohydrates (starch) (polysaccharides -> oligosaccharides + maltose). Lingual lipase begins the digestion of fats.

Question 40

Digestion in the Stomach

Answer 40

Mechanical digestion of food through churning. Pepsin partially digests proteins into shorter peptide chains. Gastric lipase and lingual lipase begins fat digestion (activated by lower pH in stomach). A small amount of absorption occurs here however most of the absorption happens after chyme leaves this area.

Question 41

Small Intestine

Answer 41

This is made up of 3 parts the duodenum, jejunum and ileum. The duodenum receives food and chemicals (stomach contents, pancreatic juice and bile), stomach acid is neutralised, pepsin is inactivated by the increased pH, fats are emulsified by bile acids and chemical digestion occurs via pancreatic enzymes. The jejunum is where most digestion and nutrient absorption occurs. The ileum is a thinner, less muscular and vascular end of this organ. The ileum releases digested food into the cecum where there is the ileocecal valve which prevents the backflow of food and bacteria in the large intestine flowing into the ileum.

Question 42

Gastroduodenal Junction

Answer 42

This is the join between the duodenum with the stomach. There is a pyloric sphincter which controls the release of stomach contents into the duodenum made by a thickening of the circular muscularis externa. The epithelia of the stomach changes into large finger-like projections called villi and folded down into intestinal crypts in order to increase the surface area. In the submucosa underneath the epithelium there are duodenal glands which secrete bicarbonate-rich mucus in order to neutralise the stomach acid and protect the mucosa. The muscularis externa is relatively common for digestive organs with a thick inner circular layer and a thin outer longitudinal layer. The food moves down the organ through segmentation and peristalsis.

Question 43

Accessory Glands

Answer 43

The advantage of these organs is that they are kept outside of the walls of the small intestine so it can be much larger and produce more enzymes. The liver is one of these as it secretes bile through hepatic ducts which works in fat digestion and absorption by emulsifying fats. The gallbladder stores and concentrates bile. The pancreas secretes pancreatic amylase (breaks down carbohydrates) and pancreatic lipase (digests fat), it secretes zymogens which are inactive pepsins which are activated once secreted in. Bicarbonate is released into the small intestine to neutralise the stomach acid. Insulin and glucagon are also released to maintain blood sugar levels.

Question 44

Plicae Circulares

Answer 44

These are the circular folds of the mucosa and submucosa of the small intestine (SI). These form transverse to spiral rides which start at the duodenum and carry on to the middle of the ileum. They cause chyme to flow in a spiral pathway to increase the contact of chyme with mucosa and ensures the movement is slow. This promotes the thorough mixing of enzymes and chyme and nutrient absorption which is increased by a factor of 2 to 3 by these folds.

Question 45

Villi

Answer 45

These are fingerlike projections formed by folds in the epithelia. These structures increase the surface area by a factor of 10. There are 2 types of these cells in these structures with some involved in absorption (enterocytes) and other involved in secreting mucus (goblet cells). In the core of each of these projections there is an entering arteriole (providing nutrients and oxygenated blood), a capillary network (arteriole and venule connect and nutrients are absorbed), a venule (removes waste and deoxygenated blood) and a lymphatic vessel into which the broken down lipids are absorbed. The epithelium of these are simple columnar and the enterocytes all contain microvilli which are projections of the cell membrane that contain brush border enzymes in the plasma membrane and undergo contact digestion. These increase absorption by a factor of 20. The lamina propria also extends into the core of this structure.

Question 46

Carbohydrate Digestion & Absorption in the SI

Answer 46

Amylase secreted by the pancreas converts the starch (polysaccharides) into oligosaccharides and disaccharides. The brush border enzymes of microvilli will then convert the oligosaccharides and disaccharides into monosaccharides (mainly glucose). The monosaccharides are absorbed into the intestinal epithelia cells and moved into capillaries of villi via facilitated diffusion after which they move through the hepatic portal system to the liver.

Question 47

Protein Digestion & Absorption in the SI

Answer 47

Proteases secreted by the pancreas convert polypeptides into oligopeptides. The pancreatic proteases continue with the support of brush border enzymes of microvilli convert oligopeptides to peptides which are further broken down into amino acids. The amino acids are then absorbed through the intestinal epithelia and moved into the villi capillaries via facilitated diffusion after which they move through the hepatic portal system to the liver.

Question 48

Fat Digestion & Absorption in the SI

Answer 48

Bile and agitation are secreted by the liver which covert the fat globules into emulsification droplets (mechanical digestion). The lipase from the pancreas convert fats (triglycerides) into 2 free fatty acids (FFAs) and a monoglyceride. Micelles in bile transport lipids to the surface of the intestinal absorptive cells where they diffuse into the epithelia. Inside epithelial cells these are packaged into secretory vesicles and released into the core of the villus where they are taken up by lacteal (lymphatic capillary) into lymph and flow into larger and larger lymphatic vessels after which they reenter the bloodstream.

Question 49

Other Nutrients Digested & Absorbed in the SI

Answer 49

Nucleic acids are broken down by nucleases from the pancreas which convert DNA and RNA into nucleotides while brush border enzymes from microvilli covert nucleotides into phosphate ions, ribose or deoxyribose sugar and nitrogenous bases to be absorbed. Most vitamins are absorbed without change however vitamin B12 isn’t absorbed well unless bound to Intrinsic Factor from the stomach. Minerals are absorbed along the entire length of the SI. Most water is absorbed in the SI (8L) via osmosis with some remaining water also being absorbed in the large intestine (LI) (0.8L).

Question 50

Large Intestine (LI)

Answer 50

The small intestine enters this organ at the ileocecal valve. There is an ascending colon, transverse colon, descending colon, sigmoid colon and the rectum. The initial part of this organ is supplied by the superior mesenteric artery while the latter part is supplied by the inferior mesenteric artery. The muscularis externa contains 3 bands of longitudinal (transverse) muscle known as taeniae coli which when contracted results in the formation of pouches called haustrum (haustra) for segmentation (promotes absorption of water and electrolytes) to occur. The mucosa also folds to form intestinal crypts (glands) but no villi. There are absorptive cells (water), secreting cells (goblet cells for mucus), bacteria (digests cellulose and other carbohydrates and synthesise vitamins B and K). The submucosa is mostly lymphatic tissue for protection from bacteria.

Question 51

Colon

Answer 51

This consists of the rectum, anal canal and anus. The rectum is deposited into by the large intestine. The anal canal has an internal anal sphincter made up of the smooth muscle of muscularis externa. The anus an external anal sphincter made up of skeletal muscle of the pelvic diaphragm allowing for voluntary control of elimination.

Question 52

Anorectal Junction

Answer 52

This is the join between the rectum and anal cavity and anus. The epithelium transitions from simple columnar and intestinal glands in the rectum to non-keratinised stratified squamous epithelium in the anus.

Question 53

Neural Control of Defecation

Answer 53

Stretching of the walls of the organs will lead to peristalsis as an intrinsic defecation reflex within the myenteric plexus moving the fecal matter down the tube. There is a parasympathetic defecation reflex in the spinal cord which intensifies peristalsis and the relaxation of the internal anal sphincter which is stimulated by stretch of the colon. THere is voluntary control of the external anal sphincter which is the final process required for defecation or prevent defecation until it is safe/appropriate to do so.