Nutrition and Digestion Flashcards
What are the layers of the walls of the GI tract?
Mucosa - the lining of the GI tract, comprised of 3 layers, including a thin layer of muscle
Submucosa – connective tissue, where blood vessels and nerves lie
Muscularis – Layers of smooth muscle and enteric nervous system
Serosa – this is the visceral layer of the peritoneum
Some parts of the GI tract have modifications to these standard layers.
What comprises the upper GI tract?
Mouth (oral/buccal cavity), pharynx, peristalsis, oesophogus, stomach, gastric mucosa
Describe the function and properties of the oral cavity.
Key for mastication, speech, starting digestion, some absorption.
Comprised of:
Lips, cheeks, soft and hard palates;
Tongue (skeletal muscle) with taste buds on papillae;
Salvitory glands - submandibular, parotid and sublingual (secrete salivary amylase);
Teeth.
Describe the function and properties of the Pharynx.
Divided into 3 parts: nasopharynx, oropharynx, laryngopharynx.
Swallowing involves the moving of a food bolus fromthe buccal cavity to the oesophagus through the pharynx by peristalsis.
What is peristalsis?
Contraction of muscles behind a food bolus to push it down the oesophagus into the stomach.
Describe the function and properties of the oesophagus.
First segment of the true digestive tract; about 25cm long extending from pharynx to the stomach, transversing the diaphragm, posterior to the heart and trachea.
3 segments - cervical, thoracic, abdominal.
Lined with stratified squamous epithelium (thick and robust) until last 1cm which is columnar epithelium.
Muscles are voluntary (striated) in the upper third;
Involuntary (smooth) in the lower third;
Mixed in the middle third.
Upper oesophageal sphincter - muscular, primarily cricopharyngeus, stops air getting into lower gut.
Lower oesophageal sphincter - thickened muscular layer in lower oesophagus and cardia of stomach (intrinsic) as well as the diaphragm (extrinsic). Prevents acid/food reflux.
Describe the properties of the stomach.
Variable size, usually collapsed but can hold up to 1.5l.
Lies under the diaphragm in central/left midline; divided into cardia/fundus/body/atrium. Oesophagus enters in the cardia, exits to the small intestine (duodenum) at the pylorus (pyloric sphincter).
Stomach wall is 4 adapted layers (mucosa, submucosa, muscularis externa, serosa).
Muscles lie in oblique layers, very strong and effective.
The mucosa is folded into RUGAE (folds) and within these are gastric pits.
What are the different cells in the gastric mucosa?
Glands: secrete mucous which protects the mucosa from the acid environment ofthe stomach.
Chief cells: secrete enzymes of gastric juice (pepsin).
Parietal Cells: secrete hydrochloric acid and intrinsic factor (important for b12 absorption).
Endocrine cells: secrete grelin (hormone which promotes appetite) and gastrin (digestive hormone.
What is the function of the stomach?
Food reservoir: stores food until ready to be digested
Digestion: started by gastric acids and juices and physically broken down by churning
Secretes intrinsic factor: allows b12 absorption
Some absorption: water, alcohol, some drugs
Endocrine: ghrelin and gastrin secretion
What is the lower GI tract?
Generally, beyond the pylorus (lower stomach). Includes the small intestine and large intestine (colon) as well as pancreas, gallbladder, liver.
What is the small intestine?
Approx 2.5cm wide tube, 6-8m long.
Sits concertina’d and coiled in central abdomen.
Divided into 3 - duodenum, jejunum, ileum.
What is the duodenum?
Shortest of the 3 parts of the small intestine, 25 cm long. Split into 4 sections.
Biliary tract enters the GI tract here.
Transitions into the jejunum at the DJ flexure (where it abruptly turns).
What is the jejunum?
2.5m long middle part of the small intestine (after the DJ flexure).
Transitions into the ileum.
What is the ileum?
3.5m long part of the small intestine. After the jejunum.
Ileum ends at the ileo-caecal valve in the RIF.
What is the small intestine mucosa?
The mucosa of the SI is folded into villi to increase surface area for absorption; there are millions of villi in health, which gives a carpet like appearance.
Each vilius contains blood vessels and lymph vessel.
Surface cells (enterocytes) have microvilli - this is known as the ‘brush border’.
Digestive enzymes are found here.
Other cells:
Mucus secreting goblet cells,
Enteroendocrine cells,
Stem cells - found in deep crypts adjacent to villi
What is the anatomy of the large intestine (colon)?
Diameter is about 6cm, length 1.5m.
Divided into sections:
Caecum;
Colon (ascending, transverse, descending, sigmoid);
Rectum;
Anal Canal.
What is the rectum?
Last 15-20cm of large bowel, repository for stool, ends at the anal canal where there is a transition to squamous mucosa.
Anal sphincters – internal (smooth muscle) and external (striated muscle).
What is the function and properties of the wall of the colon?
Multiple mucus-secreting glands.
No villi but crypts.
Muscles are grouped into dense strips (taeniae coli) and rings. These ares horter than the bowel and mean pouches (haustra) are formed.
What is the appendix?
Connected to Caecum of large intestines (right side), variable position.
Vestigial but may have role in gut microflora.
What is the peritoneum?
What are the layers?
It’s a continuous membrane that covers most abdominal organs.
Layers:
Visceral - lines organs (is their serosa),
Parietal - lines walls of abdominal cavity.
What are features and the function of the gallbladder?
Lies below the liver, internally mucosa form rugae.
Stores bile, which is crucial for fat absorption.
Empties when triggered by gut hormone - CCK.
What is the pancreas?
15cm long, head lies within curve of duodeum, tail touches spleen.
Exocrine and endocrine function.
What is the exocrine part of the pancreas?
Majority of the tissue contributes to it.
Have an acinar arrangement like the liver.
Complex ductal collecting system that ends at the pancreatic duct which empties into the duodenum.
Secrete pancreatic juice i.e. Digestive enzymes and sodium bicarbonate.
What is the endocrine part of the pancreas?
Islands of endocrine cells ‘islet of langerhans’.
Several kinds of cells.
Secrete hormones systemically into capillaries. Most important is insulin (from beta cells) and glucagon (from alpha cells).
What are general anatomical features of the liver?
Large, lobulated exocrine and blood-processing gland, with vessels and ducts entering and leaving at the porta.
Enclosed by a thin Collagen Tissue capsule, mostly covered by mesothelium.
Collagen tissue of the branching vascular system provides gross support.
Parenchymal cells are supported by fine reticular fibres.
What are the vessels that enter and leave the liver and their function?
Portal vein: bringing food-rich blood from the gut.
Hepatic artery: bringing arterial blood.
Hepatic veins: taking away processed blood into the vena cava.
Lymphatics: taking away some lymph.
Hepatic ducts: removing bile to the gallbladder and gut.
Describe the blood supply of the liver.
25% from the hepatic artery (which comes off the aorta), supplies arterial blood.
75% from the portal vein, brings food-rich blood from the gut.
Hepatic veins take blood away to vena cava after processing.
Very little nerve supply at sinusoidal level but some sympathetic and parasympathetic of perivascular structures.
What is a liver lobule?
Microanatomy of liver shows large glandular cells throughout the liver substance.
The cells are arranged in perforated plates, one cell wide.
Between the plates are sinusoidal blood channels 9-l2 μm wide, lined by endothelial cells.
Scattered in the glandular mass are blood vessels, alone and accompanied by other vessels.
The distribution of these vessels defines the classic hepatic lobules.
What are the types of lobar vessels?
Central vein/terminal hepatic venule - very thin wall, lies in the centre of a lobule, with sinusoids converging towards and opening into it.
Sublobular/intercalated vein - thicker wall, lies alone at the periphery of the lobule.
Branch of portal vein - again at the periphery of the lobule, but accompanied by one or more small hepatic arteries/arterioles, one or more bile ducts/ductules lined by cuboidal epithelium, and lymphatics.
Portal vein, artery, and bile duct constitute a portal triad; the area in which they lie is a portal area.(The lymphatics are ignored for this naming).
What is the portal triad?
Portal vein, artery, and bile duct constitute a portal triad; the area in which they lie is a portal area.
What is hepatic lobular blood flow?
Comes from branches of the portal vein and hepatic artery; from the periphery towards the centre.
It is in the sinusoids, between the cell plates.
Blood collected in central veins goes to sublobular veins, then to collecting veins, and then hepatic veins leaving the liver.
Intralobular bile flow is from the lobule’s centre towards the peripheral bile ducts, and runs, within any one cell plate, between the liver cells in bile canaliculi.
What is a liver acinus?
A functional unit in the liver of three or so lobules. Explains differences in exposure to blood supply among various parts of lobules.
These differences are reflected in varied functional activities and degrees of susceptibility to toxic agents - metabolic zonation.
Acinus territory has one final branch of the portal vein as it’s axis and is subdivided into:
1. periportal (closest to portal triads),
2. intermediate,
3. perivenous (close to the central vein) zones, with the initial periportal zone being roughly spheroid, and isolated from periportal zones of adjacent acini.
