Digestive system (7)

Question 1

How is homeostasis involved in the digestive system?

Answer 1

> Digestive system is vital for homeostasis because it helps to ensure that cells are provided with nutritional requirements needed to maintain cellular functions
Balanced diet includes all of these: carbohydrates, lipids and proteins
DS reduces complexity of food molecules so we can absorb them into the bloodstream
Then transported to their site of action and used by cells to produce energy- this energy used for growth, repair, etc.

Question 2

What is the role of the gastrointestinal tract?

Answer 2

> Digests and absorbs food

> Nutrients become more available in each step

Question 3

What is the alimentary canal?

Answer 3

> Mouth
> Pharynx
> Oesophagus 
> Stomach 
> Small intestine 
> Large intestine

Question 4

What are the accessory digestive organs?

Answer 4

> Teeth
> Tongue 
> Gallbladder
> Salivary glands
> Liver
> Pancreas

Question 5

What are the 6 essential activities in the digestive process?

Answer 5

1) Ingestion (taking food into digestive tract)
2) Propulsion (swallowing and peristalsis- waves of contraction and relaxation of muscles in the organ walls)
3) Mechanical digestion (chewing)
4) Chemical digestion (catabolic breakdown of food e.g. by enzymes)
5) Absorption (movement of nutrients from GI tract to blood or lymph)
6) Defaecation (elimination of indigestible solid waste)

Question 6

What is the peritoneum?

Answer 6

> The serous membrane of the abdominal cavity (largest in the body)
Has 2 parts which are in close contact- visceral and parietal

Question 7

What is the visceral peritoneum?

Answer 7

> Covers the external surface of most digestive organs

Question 8

What is the parietal peritoneum?

Answer 8

Lines the abdominal wall

Question 9

What is the peritoneal cavity?

Answer 9

> Allows the digestive organs to glide across each other and along the body wall
Lubricates digestive organs

Question 10

What is the oral/buccal cavity?

Answer 10

> The mouth
Bounded by lips, cheeks, palate and tongue
The oral orifice is its anterior opening
Continuous with the oropharynx posteriorly

Question 11

How is the oral/buccal cavity made to withstand abrasions?

Answer 11

> Lined with thick stratified squamous epithelium which can withstand considerable friction
Gums, hard palate and dorsum of the tongue are slightly keratinised for extra protection against abrasion when eating

Question 12

What is the tongue?

Answer 12

> Occupies the floor of the mouth

> Fills the oral cavity when mouth is closed

Question 13

What are the functions of the tongue?

Answer 13

1) Gripping and repositioning food during chewing
2) Mixing food with saliva and forming the bolus (bolus is food mixed with saliva)
3) Begins process of deglutition (swallowing)
4) Initiation of speech

Question 14

What are the salivary glands?

Answer 14

> Extrinsic glands are parotid, submandibular and sublingual
Intrinsic glands are scattered throughout the oral mucosa (the moist membrane covering in the mouth)
They produce and secrete saliva

Question 15

What are the functions of saliva?

Answer 15

1) Cleanses the mouth
2) Moistens and dissolves food chemicals so they can be tasted
3) Aids in bolus formation
4) Contains amylase (an enzyme) which breaks down starch

Question 16

What are the steps of the digestive process?

Answer 16

1) Food is ingested
2) Mechanical digestion begins
3) Propulsion initiated by swallowing
4) Salivary amylase begins chemical breakdown of starch
5) Pharynx and oesophagus allow food to pass from mouth to stomach

Question 17

What occurs in the stomach?

Answer 17

Chemical breakdown of proteins begins and food is converted to chyme (an acidic fluid)

Question 18

What are the 4 parts of the stomach?

Answer 18

1) Cardiac region- Surrounds the cardiac sphincter
2) Fundus- dome shaped region beneath the diaphragm
3) Body- Midportion of the stomach
4) Pyloric region- Made up of antrum and canal which terminated at the pylorus (pylorus is continuous with duodenum through the pyloric sphincter).

Question 19

What is the stomach lining?

Answer 19

> Stomach is exposed to harshest conditions in the digestive tract so to keep it from digesting itself, it has a mucosal barrier with…
Thick coat of bicarbonate-rich mucus on the stomach wall
Epithelial cells that are joined by tight junctions
Gastric glands which have cells that are impermeable to HCL
Damaged epithelial cells replaced quickly

Question 20

How does digestion occur in the stomach?

Answer 20

> Holds ingested food
Degrades the food physically and chemically
Delivers chyme to the small intestine
Enzymatically digests protein with pepsin
Secretes intrinsic factor required for absorption of vitamin B12

Question 21

How does digestion occur in the small intestine?

Answer 21

> Has 3 parts- duodenum (25cm long), jejunum (2m long) and ilium (3m long)
Chemical digestion of food is completed here and virtuall all nutrient absorption absorption takes place here.
Chyme which passes from stomach to small intestine is released into duodenum where carbohydrates and proteins are partially digested and no fat digestion has taken place yet.
Chyme is hypertonic and has a low pH so mixing is required for proper digestion (required substances are supplied by the liver)

Question 22

What is the role of the villi in the small intestine?

Answer 22

> Intestinal wall contains series of folds (plicae circulares) which are lined with fingerlike projections called villi
Their covering consists of columnar epithelium
They increase the surface area for absorption
Each villus has a network of capillaries which carry absorbed nutrients to the hepatic portal circulation to be delivered to the liver

Question 23

What are intestinal hormones?

Answer 23

> Dueodenal endocrine cells produce hormones to coordinate the secretory activities of the stomach, duodenum, pancreas and liver

Question 24

What is gastrin?

Answer 24

> Intestinal hormone
Secreted by cells of duodenal walls in response to large amounts of incompletely digested proteins
Promotes gastric motility (process where food passes through GI tract)
Stimulates production of acids and enzymes

Question 25

What is secretin?

Answer 25

> Intestinal hormone
Released when pH in the duodenum falls when the acidic chyme arrives from the stomach
Main role is to increase secretion of bile and buffers by the liver and pancreas (to neutralise pH)

Question 26

What is cholecystokinin (CCK)?

Answer 26

> Intestinal hormone
Secreted when chyme arrives in duodenum
In pancreas, increases production of large numbers of digestive enzymes
In gallbladder, causes ejection of bile into duodenum

Question 27

What is gastric inhibatory peptide (GIP)?

Answer 27

> Released when fats and carbohydrates (especially glucose) enter the small intestine
Inhibits gastric activity and causes release of insulin from Islets of Langerhans

Question 28

What is the large intestine?

Answer 28

> Begins at end of ilium and finishes at anus

> 1.5m long

Question 29

What are the parts of the large intestine?

Answer 29

• Need diagram of large intestine
  1) Caecum- Expanded pouch where compaction begins
  2) Ileocaecal valve guards entrance between ilium and caecum
  3) Vermiform appendix- About 8-9cm long, is a fine tube which attaches to the caecum
  4) Colon- 4 parts
  1) Ascending colon- Begins at illeocaecal valve and ascends right side of the abdomen

Question 30

What are the parts of the large intestine?

Answer 30

*Need diagram of large intestine
1) Caecum- Expanded pouch where compaction begins
2) Ileocaecal valve guards entrance between ilium and caecum
3) Vermiform appendix- About 8-9cm long, is a fine tube which attaches to the caecum
4) Colon- 4 parts
1) Ascending colon- Begins at illeocaecal valve and ascends right side of the
abdomen to the liver where it becomes the transverse colon
2) Transverse colon- Extends across abdominal cavity towards the left side, below
the stomach. At spleen it becomes the descending colon
3) Descending colon- Travels down left side of abdomen until it becomes the
sigmoid colon which empties into the rectum
5) Rectum- End of digestive tract. Expandable organ for temporary storage of faeces
6) Two sphincters- Internal anal sphincter has smooth muscle cells not under voluntary control and the external anal sphincter which has skeletal muscle fibres and is under voluntary control

Question 31

What are the haustra of the colon?

Answer 31

The small pouches which give the colon its segmented appearance

Question 32

What are the functions of the large intestine?

Answer 32

> Main function is propulsion of faecal material towards anus
Digest enteric bacteria
Reclaims vitamins, water and electrolytes
Colon is not essential for life

Question 33

What is defaecation?

Answer 33

> Rectum is usually empty until a powerful peristaltic contraction forces faeces out of the sigmoid colon
The distention of the rectal walls triggers the defacation reflex which involves 2 positive feedback mechanisms

Question 34

What are the 2 positive feedback mechanisms involved in defacation?

Answer 34

1) Stretch receptors in the rectum stimulate series of peristaltic contractions in the sigmoid colon and the rectum which moves faeces towards the anus and increases distension of the anus
2) The stretch receptors also stimulate parasympathetic neurones in the sacral spinal cord and these neurones stimulate increased peristalsis in the descending colon and the sigmoid colon which pushes more faeces towards the rectum, further increasing distension.

Question 35

How does defaecation require conscious effort?

Answer 35

> Passage of faeces through anus requires relaxation of interal anal sphincter but as it relaxes, the external sphincter automatically closes.
Passage of faeces requires conscious effort- if command doesnt come then peristalsis stops until more rectal expansion triggers the defecation reflex again
Opening external sphincter also requires conscious effort such as tensing abdominal muscles or exhaling whilst closing the glottis- this raises intra-abdominal pressure which helps to force the faeces out of the rectum
This increase in pressure also forces extra blood into the network of veins in the rectal wall, causing them to stretch
Repeated stretching of these veins (straining at stool) can cause veins to be permanently distended leading to haemorrhoids.