1.8 Digestive system

Question 1

What is digestion?

Answer 1

The step where nutrients are broken down into their monomers

Question 2

What is absorption?

Answer 2

Absorption is the step where these monomers from digestion pass through the intestinal cells to reach the bloodstream.

Question 3

What are the main organs of the gastrointestinal tract?

Answer 3

Mouth, Esophagus, Stomach, Small intestine (duodenum (stomach exit), jejunum, ileum), Large intestine (cecum, ascending colon, transverse colon, descending colon, rectum and anus)

Question 4

What are the supporting organs of the GI tract?

Answer 4

• Salivary glands
• Liver
• Gallbladder
• Pancreas

Question 5

What are the layers of the GI tract?

Answer 5

Mucosa, submucosa, muscularis propria, serosa

Question 6

What is the mucosa?

Answer 6

Where absorptive function occurs. It faces the lumen of the GI tract where there are nutrients mixed with GI secretions.

The mucosa is constituted of one single layer of cells, the enterocytes that form an epithelium.

The mucosa is attached to the lamina propria which is a connective tissue, and to a layer of smooth muscle called the muscularis mucosae.

Question 7

What is the submucosa?

Answer 7

Contains the submucosal plexus. Contains inflammatory cells, autonomous nerve fibres and ganglion cells.

Question 8

What is the muscularis propriar?

Answer 8

Smooth muscle cells organised in circular (inner) and longitudinal (outer) layer. Propels food through the gut by contractile peristaltic mechanisms controlled by the myenteric plexus.

Question 9

What is the serosa?

Answer 9

Consists of an outer squamous epithelium (mesothelium) and a loose connective tissue.

Protective layer against external aggressions.

Question 10

What is the oesophageal anatomy?

Answer 10

Connects mouth with stomach

Changes shape and can adapt its diameter to the size of the food ingested. When there is no food, esophagus is a collapsed tube.

External layer of GI tract is called adventia

Muscular propria in the esophagus contains both smooth and strated muscles. Smooth muscle= involuntary movements and striated muscle= voluntary movements

The bolus is food that has been
masticated and is going from the mouth, down the esophagus and down to the stomach.

The chyme is the bolus that is processed in the stomach , mixed with various juices as we will see and then exits the
stomach to go to the small intestine.

Question 11

What is the main role of the oesophagus?

Answer 11

move bolus down to the stomach

Question 12

Why is there a uni-directional function in the oesophagus?

Answer 12

• 2 esophageal sphincters (1st avoids food going down trachea and lower sphincter avoids stomach reflux)
• peristaltic movements, which are reflex contractions of the circular and longitudinal muscles.

Question 13

What do the gastric glands in the mucosa consist of?

Answer 13

• Parietal cells: produces hydrochloric acid (HCl). The pH in the stomach
  varies from 4-5 at rest to 1.5-3.5 after a meal (high-protein meal requires a lower pH). Also secrete the intrinsic factor (Vit-B12 absorption)
• Neck cell: secrete bicarbonate to buffer the pH, mucus and water
• Pit cells : secrete gastric mucus
• Chief (Zymogenic) cells: secrete digestive enzymes (pepsinogen; gastric lipase and amylase)
• Endocrine cells: secrete hormones (e.g.; ghrelin, somatostatin)
• HCl secretion- necessary to break down secondary and tertiary proteins, to kill bacteria /pathogens and to activate digestive enzymes (e.g.; pepsinogen into pepsin)

Question 14

How is chyme released to the duodenum?

Answer 14

Via the rhythmic opening of the pyloric sphincter

Question 15

What is the anatomy of the small intestine?

Answer 15

Duodenum: short (28 cm long),
encircles the pancreas and receives the pancreatic juice and the bile from the gallbladder to (nearly) complete the digestion. All components are in their simplest form (amino acids, monosaccharides and emulsified fatty acids)
- Jejunum (2/5 of the small
intestine) and Ileum : main sites of absorption.
- Mesentery : connecting membrane between the intestine and the abdominal walls. Provides insulation (fat) and supplies blood to the intestine

Question 16

What is the vascularisation of the small intestine?

Answer 16

• Blood IN: abdominal aorta (mesenteric artery) + hepatic artery (pancreatiduodenal artery). Vessels go through the mesentery that gives branches entering the intestine wall.
• Blood OUT: superior mesenteric vein.
  Connects with the portal vein and drains into the liver

Question 17

Contraction and motility of the small intestine

Answer 17

• Peristaltic contractions
• Segmenting contractions: localised circumferential contractions to mix and churn the intestinal chyme.
• Decreasing gradient of contractions from duodenum to ileum.

Question 18

What do the salivary glands do?

Answer 18

First step of the digestive process

Contains antimicrobial enzymes (lysozyme; IgA) and ezymes such as amylase (starch),
lipase (fats, only activated in stomach)

Question 19

What does the pharynx do?

Answer 19

- Swallowing (voluntary &
involuntary action)
- Swallowing initiates
peristalsis in the esophagus
- Ends at the epiglottis that closes to avoid choking (closes pathway to lungs)

Question 20

What does the small intestine specialise in?

Answer 20

Absorption of nutrients

Small intestine is enormous, with on average of 4500 square meters of organ surface that can absorb nutrients.

Question 21

What are the parts of the small intestine resposible for absorption?

Answer 21

Absorptive surface is the mucosa which is arranged in plicae circulares. These folds are made off special extensions of the mucosa that are called villi.

Intestinal villi- Projection of the mucosa into the lumen, in contact with the chyme (and the intestinal microbiota). Covered by an epithelium consisting of enterocytes connected by tight junctions (defense against pathogens)

The basolateral part contains the basolateral membrane facing the blood vessels (hepatic vein) and the lymphatic system.

Other components of the villus (goblet cells, stem cells and endocrine cells)

Question 22

What is the function of the brush border?

Answer 22

Digestion: secretion of disaccharidase and peptidase

Absorption: receptors and transport proteins for the passage of sodium ions, D-glucose and amino acids

Question 23

What are the parts of the large intestine(colon) ?

Answer 23

Ascending, transverse, descending, rectum and anus

Question 24

What are the 3 main functions of the large intestine?

Answer 24

reabsorb water

maintain osmolarity (secretion/absorption of electrolytes (Na+, Cl-)

store fecal material (indigestible matter, mucus, dead bacteria)

Question 25

How is motility stimulated in the large intestine?

Answer 25

by mastication and hormones gastrin and cholescystokinin

Question 26

What is the vascularisation of the large intestine?

Answer 26

Blood in via mesenteric arteries (+internal ileac artery)

blood out via the mesenteric veins joining the portal vein

Question 27

What is the intestinal water content?

Answer 27

1.5 to 2 litres of chyme (food + gastric secretion + pancreatic secretions + bile)
Important to reabsorb water (chyme is reduced to 150ml in the colon)

Question 28

What is the main function of the liver?

Answer 28

Metabolism of proteins, carbohydrates and fats
Synthesise cholesterol and bile acids
Transport and storage of carbohydrates (energy balance)
Transport of bilirubin (from breakdown of erythrocytes)

Question 29

What does the liver consist of?

Answer 29

Organised in lobules (clusters of cells) consisting of:
Hepatocytes; bile ductules (collecting bile acids); sinusoids (expansion of the portal vein) allowing direct accessibility of the hepatocytes to the bloodstream; Kupffer cells (phagocyte debris

Question 30

What is the vascularisation of the liver?

Answer 30

Blood in: fully oxygenated (from heart) via the hepatic artery +
partially oxygenated (venous blood) via the portal vein (from pancreas, spleen, stomach, small and large intestines). Portal vein divides into 2 parts (porta hepatis) going to each lobe

Blood out: hepatic vein merges with the vena cava (to the heart)

Question 31

What is the structure of hepatocytes?

Answer 31

Mitochondria (energy needed for synthesis and metabolism)

Endoplasmic reticulum highly developed (needed for enzymes synthesis)

Lysozyme (in cytoplasm): aids iron storage and digestion of foreign particles

Synthesise cholesterol and primary bile acids which derive from cholesterol

Bile acids: chenodeoxycholic acid and cholic acid (dehydroxylated into deoxycholic acid by intestinal bacteria). Synthesis stimulated by CCK and secretin

Question 32

What is the function of the gallbladder?

Answer 32

Liver produces about 1 L of bile daily
Gallbladder volume: 150ml
Hepatic bile is concentrated in the gallbladder where water, chloride and bicarbonate are reabsorbed.

Question 33

What is the main function of bile?

Answer 33

Main function is to emulsify lipids: bile acids form micelles around lipids (will aid lipase action in the duodenum).
Absorption of Vit A, D, E and K.
Excretion of bilirubin (Hb product from erythrocytes breakdown)

Question 34

What is bile composed of?

Answer 34

Water (98%), bile acids (salts),
Bilirubin (yellow colour)
Cholesterol
Lecithin (amphipathic mix of glycerophospholipids (e.g. phosphatidylcholine)

Question 35

What are the types of bile acids?

Answer 35

Steroids acids, derived from cholesterol metabolism
Primary acids: liver (chenodeoxycholic acid and cholic acid
Secondary acids: intestines (taurocholic acid & glycocholic acid/ taurochenodeoxycholic acid & glycochenodeoxycholic acid/ lithocholic acid)

Question 36

What is the anatomy of the gallbladder?

Answer 36

Right and left hepatic ducts (primary bile acids) merge to form the common bile duct.

Common bile duct goes through the head of the pancreas where it joins the pancreatic duct (80%) in the hepatopancreatic ampulla.

The hepatopancreatic ampulla ends in the lumen of the duodenum (duodenal papilla).

Question 37

What cells does the pancreas consist of?

Answer 37

Acinar cells and islet cells

Question 38

What do acinar cells do?

Answer 38

Exocrine function, produce digestive enzyme precursors

Synthetises pancreatic enzymes in membrane-wrapped zymogen granules (stored in cytoplasm).

Question 39

What do islet cells do?

Answer 39

Endocrine function, produce hormones that regulate circulating (blood) glucose levels
Alpha cells: secrete glucagon (when glucose is low- for gluconeogenesis and glycogen breakdown, increases glucose levels)
Beta cells: secrete insulin (when glucose is high- for glucose storage& utilization- decreases glucose levels)
Delta cells: secrete somatostatin (inhibits both glucagon and insulin release) (& VIP (vasoactive intestinal polypeptide))

Question 40

What enzymes do acinar cells secrete?

Answer 40

Trypsin (and trypsin inhibitor), chymotrypsin, carboxypeptidase (proteins); amylase (carbs); pancreatic lipase, phospholipase A2, carboxylester hydrolase (lipids).
Secretion is induced by gastrin (stomach), CCK, secretin, Ach and VIP (duodenum, in response to swallowing)

Question 41

What is the peristaltic reflex?

Answer 41

Reflex that propagates an unidirectional wave of muscular contraction and relaxation down the gut wall.

Question 42

What does the peristaltic reflex depend on?

Answer 42

Enteric Nervous System (ENS) – Myenteric Plexus

Question 43

How is the peristaltic reflex regulated?

Answer 43

Regulated by the Autonomous Nervous System:
Parasympathetic system tends to increase peristalsis
Sympathetic system tends to inhibit peristalsis

Question 44

How does the peristaltic reflex work?

Answer 44

Combination of propulsive segments and receiving segments

Question 45

What are the steps of the peristaltic reflex?

Answer 45

1. Mechanical stretch of the gut wall induces serotonin secretion (5-HT) from the villi epithelial cells.
2. Serotonin activates the sensory afferent neurons of the Myenteric Plexus + activation of an inhibiting loop in the submucosa plexus (stop serotonin secretion)`
   Sensory neurons make synapses on interneurons in the myenteric plexus.
   3.Two pathways with opposite actions depending on muscle chemoreceptors, neural networks and segmentation (i.e.; propulsive or receiving).
   Excitatory motor neurons: release Acetylcholine (Ach) & Substance P (P)
   Inhibitory motor neurons: release Nitric Oxide (NO) & Vasoactive Intestinal peptide (VIP).
   In the propulsive segment: interneurons activate the excitatory neurons for the circular muscles (contraction) but inhibit the excitatory neurons for the longitudinal muscles. Interneurons also activate the inhibitory neurons for the longitudinal muscles (relaxation)
   In the receiving segment: interneurons inhibits the excitatory neurons for the circular muscle and activate the inhibitory neurons (relaxation). Interneurons activate the excitatory neurons for the longitudinal muscles and inhibits the inhibitory neurons (contraction).