Gastrointestinal Tract

Question 1

What is digestion? And what are the types of digestion?

Answer 1

It converts large (usually insoluble) molecules into smaller molecules that can be absorbed across gut epithelium into blood capillaries or lacteals. There is mechanical (food is physically broken up in mouth or by peristalsis) , chemical (enzymes) and microbial (enzymic but specifically by gut microbials) digestion.

Question 2

Wat is the general structure, components of the GI tract?

Answer 2

Mouth to anus; accessory organs being pancreas, liver and gall bladder; pancreas has an exocrine and endocrine function.

Question 3

What is the function of the teeth?

Answer 3

20 primary and 32 permanent. They cut, tear and crush the food. The K9s cut and tear the food. The teeth are involved in mechanical breakdown.

Question 4

What is the function of the tongue?

Answer 4

It manipulates and masticates food. Its involved in speech. They have lingual papillae which increase the SA of the tongue and therefore allow for more contact and friction with the food. They also have chemosensory receptors that allow for taste.

Question 5

What is the function of the salivary glands?

Answer 5

They aid mastication and swallowing by lubricating the food. There are three pairs and they have an exocrine function as substances are secreted into the mouth. Activity of the exocrine glands are activated by the parasympathetic nervous system which can be activated through smell and sight. It signals the brain that will release acetylcholine which stimulates the salivary glands to start secreting saliva and the stomach to start releasing gastric acid.
There are three types of salivary gland:
- parotoid - salivary amylase
- lingual - lingual lipase
- submandibular - lingual lipase and mucus.

Question 6

What is the function of the oesophagus?

Answer 6

Links mouth to stomach. Has peristaltic contractions that causes the food to move one way to the stomach. The food that enters the stomach is controlled by the epiglottis which is a flap covering the trachea and also the cardiac sphincter at the base of the oesophagus.

Question 7

What is the function of the stomach?

Answer 7

Mechanical digestion - peristalsis from the circular and longitudinal smooth muscle layers churn and break up food.
Chemical digestion - histamine is secreted in response to acetylcholine coming from the vagus nerve. the histamine then acts on the parietal cells to secrete gastric acid.

Question 8

What are specialised cells in the stomach?

Answer 8

Chief cells - secrete gastric lipase (HCl). They also secrete pepsinogen. The gastric bypass causes the stomachs pH to drop to 2, causing pepsinogen to denature and become pepsin, beginning digestion of polypeptides.
Parietal cells - secrete gastric acid also. Secrete the intrinsic factor which is a compound facilitating the absorption of vitamin B12.

Question 9

What is the gastric pit in the stomach lining?

Answer 9

Invaginations to the stomach lining, and at the base of these pits are the specialised cells. When neurotransmitters and hormones are released, they fuse into the blood stream, bind to receptors at the base of the basal lateral side of parietal cells. (BL side points AWAY from the gastric pit).

Question 10

What is the function of the liver?

Answer 10

Synthesises bile from cholesterol. It can produce 400-800ml of bile a day.

Question 11

What is the function of the gall bladder?

Answer 11

Storing and releasing the bile, it can only store 30-60ml at any time. Movement of lipids and AAs in chyme from the stomach to the duodenum causes the specialised cells (enterochromafin like cells) lining the duodenum to secrete two hormones into the blood stream and to the liver and gall bladder. They act on the site and stimulate the contraction of the gall bladder, pushing the bile into the bile duct and into the duodenum.

Question 12

What is bile?

Question 13

What is the function of the pancreas?

Answer 13

Exocrine cells secreting digestive enzymes.
Endocrine cells secreting hormones like insulin and glucagon. Beta cells secrete insulin and alpha cells secrete glucagon. They are not secreted into the pancreatic duct, but instead the basal lateral side, so the hormones can go around the rest of the body. This is important in the overall regulation of energy status after a meal.

Question 14

What are the villi on the small intestinal wall?

Answer 14

Finger like projections that increase the surface of the SI. They have a columnar epithelial outer membrane. Underneath is lacteal and blood capillaries. They are also called enterocytes. The side of the enterocyte facing the interior lumen is called the apical membrane, the side facing the capillary and lacteal is the basal lateral.
Villi also contain goblet cells that secrete mucus to act as a protective layer for the epithelium protecting it from heavy metals, pesticides or bacteria.
The blood capillaries absorb AAs and glucose. The lacteal duct will absorb products of lipid digestion (triglycerides, cholesterol and fat soluble vitamins).
The base of the villi have CBC stem cells that produce epithelial cells, some form enterocytes, some form enteroendocrine cells that secrete hormones to regulate the digestive tract.

Question 15

What is the function of the large intestine?

Answer 15

The start has the caecum and appendix. Caecum is a reservoir of chyme, the appendix is a tube structure connected to the caecum and is a reservoir for beneficial gut bacteria.
It also facilitates microbial digestion of nutrients that weren’t absorbed in the SI.

Question 16

What is the function of the rectum and anus?

Answer 16

Rectum is a 12cm chamber at the end of the descending colon and fills as the colon gets full. The entry of the stool into the rectum stimulates defecation. The anal sphincter regulates the stool leaving the body (egestion)

Question 17

How is glucose absorbed?

Answer 17

Absorbed by a sodium dependant glucose transporter (SGLUT1) on the apical membrane. A sodium membrane binds to the SGLUT1, and a glucose or galactose molecule will bind to the sodium. They are co-transported into the enterocyte. Absorption of too many sodium ions is toxic, so, a sodium potassium pump is located on the basal lateral side. As sodium ions move into the blood capillaries, potassium moves into the enterocyte.

Question 18

How are amino acids absorbed?

Answer 18

Absorbed by a sodium dependent amino acid transporter on the apical membrane. A sodium ion will bind to it, allowing an amino acid to bind to the sodium. They are co transported into the enterocyte and the AA is facilitated out. As sodium moves into the blood capillary, a potassium ion will move in via the sodium potassium pump.

Question 19

How are Di and Tripeptides absorbed?

Answer 19

Absorbed by a hydrogen dependent di- and tri- peptide transporter (PepT1) on the apical membrane. A hydrogen ion will bind to the PepT1 and a peptide can now bind also. They are co transported into the enterocyte and intracellular peptidases hydrolyse it into its individual AAs. A high hydrogen content is toxic, so, a hydrogen sodium pump on the apical membrane exchanges hydrogen into the lumen for a sodium into the enterocyte, then the sodium is facilitated out via the sodium potassium pump on the basal lateral membrane.

Question 20

How are lipids absorbed?

Answer 20

Lipids are digested by salts that break down the large fat globules to small ones called micelles. Co lipase binds to micelles and help anchor the lipase close to it. lipase breaks down poly-glycerides (di- + tri-) into fatty acids and monoglycerides. Phospholipase A2 hydrolyses phospholipids. Cholesterol esterase breaks cholesterol esters into free cholesterol.
The micelle will be close to the apical membrane, and absorbed into it via a scavenger receptor (cholesterol) and fatty acid binding proteins. Once in enterocyte, they reassemble, forming a chylomicron. It moves to the basal lateral membrane and is removed from the cell via exocytosis. The chylomicrons go into the lacteal duct.

Question 21

What is the function of amylase?

Answer 21

breaks down starch and glycogen to maltose, maltotriose and limited dextran’s.

Question 22

What is the function of lipase and co-lipase? And cholesterol esterase?

Answer 22

Breaks down triacylglycerides (TAG), phospholipids and cholesterol esters into diacylglycerides, monoacylglycerides, free fatty acids and free cholesterol.

Question 23

What is the function of trypsinogen, proelastase?

Answer 23

Breaks down proteins and polypeptides into oligopeptides.

Question 24

What is the function of nucleases?

Answer 24

Breaks down DNA and RNA into nucleotides

Question 25

What are the layers of the GI tract

Answer 25

mucosa, submucosa, muscularis externa, serosa

Question 26

What is the mucosa layer in the GI tract?

Answer 26

Inner lining. Mucous membrane lined with epithelial cells that stay moist from glandular secretions, mainly mucus secreted by goblet cells. It also contains lamina propria which is a thin layer of connective tissue.

Question 27

What is the submucosa layer in the GI tract?

Answer 27

Layer after muscosa. dense irregular connective tissue containing large blood and lymph vessels. Some regions have exocrine glands to secrete buffers and enzymes into the lumen of the digestive tract. The vessels facilitate absorption.

Question 28

What is the muscularis externa layer of the GI tract?

Answer 28

Located behind the submucosa. Layer of smooth muscle with inner circular layer and outer longitudinal layer. When contracted, they narrow the lumen. The longitudinal layer push food along the GI tract.

Question 29

What is the serosa layer of the GI tract?

Answer 29

Absent in oral cavity, pharynx, oesophagus and rectum. Dense network of collagen fibres that form a sheath called the adventitia which firmly connects the GI tract to the spinal cord.