Lecture 41 - Digestive System: Roles of the Small and Large Intestine and Coordination of Digestion

Question 1

Describe the path of chyme. Also, what are the 4 key points to remember about absorption and the intestines

Answer 1

As chyme moves through the small intestine to the large intestine, absorption occurs, and chyme is converted to feces (dehydration and compaction of chyme)

1. Not all ingested material can be digested or absorbed
2. A healthy intestine reabsorbs all the volume associated with digestive secretions
3. Old intestinal epithelial cells are continually shed into the lumen
4. Gut bacteria make up a significant proportion of fecal matter

Question 2

What are the 3 principal organs and 3 sphincters that make up the lower digestive tract?

Answer 2

Principal organs:
1. Small intestine - 2-2.5 cm in diameter and ~6 cm in length
2. Large intestine (colon) - 7.5 cm in diameter and ~1.5 cm in length
3. Rectum - variable diameter and 15 cm in length

Sphincters:
1. Pyloric
2. Ileocecal
3. Anal

Question 3

T/F? Chyme passes through accessory organs of the lower digestive system

Answer 3

False - they do not pass through the accessory organs, but their secretions/contents contribute to the functions of the tract

Ex. Liver, gallbladder, pancreas, appendix

Question 4

Describe the organization of the small and large intestine

Answer 4

The small intestine has a complete longitudinal muscle layer. Both peristalsis (propulsion) and segmentation (mixing/physical digestion) occur

The large intestine only contains bands of longitudinal muscle (taeniae coli). This arrangement allows for peristalsis and some compaction

Question 5

Categorize whether each of the 5 functions of the digestive system (propulsion, digestion, absorption, coordination, protection) occur in the small, large intestine, rectum, or all 3

Answer 5

All 3 organs - propulsion (peristalsis), protection (keeping pathogens out of the rest of the body), and coordination (feedback and feedforward loops)

Small and large intestine - absorption (nutrients, vitamins, water)

Only small intestine - digestion (physical (segmentation) and chemical (enzymes))

Question 6

What is the function of neural and hormonal activity in the small intestine?

Answer 6

They coordinate the activity of principal and accessory organs - the small intestine, esp. the duodenum, is a critical node in the coordination of activity throughout the digestive system (and beyond)

ex. The presence of chyme within the duodenum triggers changes in gastric activity

Question 7

What is the duodenum?

Answer 7

It is a secondary endocrine organ, secreting several different hormones. Each region of the small intestine secretes at least 1-2 hormones, but the duodenal enteroendocrine cells secrete at least 5. These hormones are secreted in response to different stimuli and produce distinct effects

Question 8

What is cholecystokinin (CCK)? What is its sensor and main effectors?

Answer 8

It is a peptide hormone released by the duodenum that controls the release of bile and pancreatic juice.

Sensor - released in response to chyme in the duodenum

Main effectors - gallbladder and duodenal papilla (hepatopancreatic sphincter)

Question 9

Describe the pathway for cholecystokinin (CCK) secretion starting from the liver

Answer 9

1. The liver secretes bile continuously
2. CCK triggers contraction of the gallbladder, emptying it of stored bile
3. CCK triggers relaxation of the heptopancreatic sphincter, allowing bile and pancreatic secretions to reach the digestive tract
4. Bile salts break apart lipid droplets by emulsification

This loop brings digestive enzymes and bile salts into contact with nutrients that need digestion

Question 10

How does CCK affect the pancreas?

Answer 10

It influences endocrine functions of the pancreas in a feedforward reflex. CCK in the bloodstream can act on the endocrine cells within the pancreas to enhance insulin secretion. This pathway allows insulin levels to rise in preparation for the glucose that is soon to be absorbed by the intestine

Question 11

Describe the difference in absorption between the small and large intestine

Answer 11

The small intestine absorbs a wider range of substances than the large intestine. Most nutrient absorption occurs in the small intestine, especially in the jejunum. Vitamin and water absorption also occurs

The large intestine absorbs very few nutrients, but does absorb some vitamins (especially in proximal sections) and water (especially in distal regions)

Question 12

Where is mucosa-associated lymphoid tissue (MALT) present?

Answer 12

The large intestine (and ileum) because of the increased numbers of gut bacteria in these regions. In general, MALT is more common wherever large numbers of bacteria are present in the lumen

Question 13

Which intestine (and its associated accessory organs) produces more secretions than the other?

Answer 13

The small intestine - the epithelium of the small intestine adds its own secretions (2L per day) to chyme arriving from the stomach:

• digestive enzymes
• water
• antibacterial substances and antibodies

In addition, 2L of fluid per day enters the duodenum from accessory organs

Question 14

T/F? The large intestine only secretes mucus

Answer 14

True

Question 15

Describe the general structural features of the small intestine. Describe the duodenum, jejunum, and ileum

Answer 15

It has circular folds covered in villi and its function is to increase the absorptive surface area

Duodenum - few folds, many mucous-secreting glands
Jejunum - many folds, many villi
Ileum - few folds, high MALT

Question 16

How is the mucosa of the small intestine organized? Within this organization, describe the function of intestinal crypts, goblet (mucosa) cells, and villi/microvilli

Answer 16

It’s organized into villi and crypts and covered by simple columnar epithelium with microvilli

Intestinal crypts - source of new epithelial cells and enteroendocrine cells. These crypts also contain cells that can secrete antibacterial substances

Goblet (mucous) cells - found in the epithelial layer through the villi

Villi and microvilli - structural specializations that increase surface area. Each villus contains capillaries and lymphatic vessels

Question 17

Describe the mucosa of the large intestine and rectum

Answer 17

They lack villi, but contains many mucous secreting glands and lymphoid tissue. The colonic mucosa is mostly flat, with many mucous-secreting glands. They function to reduce resistance (friction) and protects against harmful bacteria

It contains large amounts of MALT (lymphoid tissue), which helps curate and manage gut flora

Question 18

Describe the epithelium of the rectum, anal canal, and anus

Answer 18

Rectum - simple columnar epithelium

Anal canal - stratified squamous epithelium. The anal canal is subjected to high physical pressures (abrasion, dehydration)

Anus - keratinized stratified squamous epithelium. This portion can be exposed to air, so it needs to resist evaporation

Question 19

What are mass movements? What are they triggered by?

Answer 19

They are powerful coordinated peristaltic contractions within the colon that compact chyme into feces and push this mass into the sigmoid colon and rectum. These movements are often triggered or enhanced by stretch receptor activation in the stomach and duodenum (ie. gastrocolic reflex)

Question 20

What is the function of the rectum and its sphincters?

Answer 20

The rectum can temporarily store feces. It has stretch receptors and motor neurons in its ENS plexus. The opening at the anal canal is controlled by a dual sphincter: an internal smooth muscle component and an external ring of skeletal muscle

Both sphincters are contracted except during defecation

Question 21

What is the defecation reflex?

Answer 21

It triggers relaxation of the internal anal sphincter - the external anal sphincter can be voluntarily controlled

Question 22

Describe the innervation of the external anal sphincter

Answer 22

The external anal sphincter is innervated by somatic motor neurons, whose activity can be controlled by descending axons that come from primary motor cortex

Question 23

What are the 2 reflex arcs involved in the relaxation of the internal anal sphincter?

Answer 23

1. Short reflex - ENS only
2. Long reflex - CNS is involved via spinal cord ANS neurons