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MEDSCI142 > Digestive System Lecture 33 > Flashcards

Digestive System Lecture 33 Flashcards

1
Q

Which of the four tunics is the most variable?

A

Mucosa

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2
Q

Which tunic is responsible for peristalsis?

A

Muscularis Externa

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3
Q

What co-ordinates peristalsis?

A

Myenteric plexus, which is heavily influenced by the autonomic nervous system

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4
Q

What is the main function of the submucosa?

A

Support

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5
Q

What cells are in the serosa?

A

Mesothelial cells, which produces serous fluid to reduce abrasion between neighboring organs.

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6
Q

Stem cells in the epithelium of mucous membrane

A

Structure:

Thick and many-layered (stratified squamous epithelium).
The outer layers are sacrificial, providing protection against abrasive fragments of food.

Cell Replacement:
Cells are continuously replaced through division in the basal layers (stem cells in the basal layer).
The new cells migrate outward slowly, replacing older cells.
Old cells are shed from the surface.
The entire epithelium is renewed every 7 days.

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7
Q

What type of muscle is found in the external muscle layer of the upper third of the esophagus?

A

In addition to smooth muscle, the upper third of the esophagus contains skeletal muscle, allowing for rapid contraction and voluntary control of swallowing.

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8
Q

What replaces the serosa in the esophagus?

A

The esophagus is covered by a fibrous adventitia, as it does not lie in a body cavity and therefore lacks a serosa.

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9
Q

What are the layers of the gut?

A

Mucosa (epithelium, lamina propria, muscularis mucosae), submucosa, muscularis externa, serosa/adventitia

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10
Q

What does the visceral peritoneum cover?

A

It covers the abdominal organs.

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11
Q

Where is the transverse colon located?

A

Near the stomach and small intestines.

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11
Q

What suspends the small intestines in the abdominal cavity?

A

The parietal peritoneum.

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12
Q

What is the omentum?

A

A sheet of fatty tissue structure that overlays all gut organs, suspended by the transverse colon, containing lymphatic tissue.

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13
Q

Is the omentum visible in rats?

A

No, the omentum is not visible in rats but present in humans.

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14
Q

What must be moved during dissections or surgeries to view underlying gut organs?

A

The omentum.

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15
Q

What type of tissue does the omentum contain?

A

Lymphatic tissue.

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16
Q

What is the role of the omentum in infections?

A

It migrates chemotactically towards the infection site to isolate and contain it.

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17
Q

What is the texture of the omentum similar to?

A

Fat seen in sheep heart dissections.

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18
Q

Where does food travel after being swallowed?

A

Food travels dorsally (posterior) through the pharynx into the oesophagus.

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19
Q

What prevents food from entering the trachea during swallowing?

A

The epiglottis closes like a trap door, sealing off the glottis.

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20
Q

How is food transported through the oesophagus?

A

Food is transported rapidly through peristalsis.

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21
Q

What kind of epithelium lines the oesophagus to handle friction and abrasion?

A

Stratified squamous epithelium (multi-layered, flattened cells). Stratified squamous epithelium has a sacrificial outer layer for protection, which is sloughed off with each swallow.

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22
Q

What happens to the outermost cells of the oesophageal epithelium during swallowing?

A

The outermost cells are sloughed off and replaced by basal cells.

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23
Q

What are the three components of the oesophageal mucosa?

A

Stratified squamous epithelium
Lamina propria
Muscularis mucosae (smooth muscle).

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23
Q

What is found in the submucosa of the oesophagus?

A

Larger blood vessels for structural support.

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24
Q

What are the two layers of muscle in the oesophageal muscularis externa?

A

Inner circular muscle layer
Outer longitudinal muscle layer.

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25
Q

What type of muscle is found in the upper oesophagus and why?

A

Skeletal muscle for voluntary swallowing.

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26
Q

What is adventitia and where is it found?

A

Adventitia is a connective tissue layer found where the oesophagus is not in contact with serous fluid.

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27
Q

When does the oesophagus transition to having a serosa layer?

A

When the oesophagus enters the abdominal cavity and is surrounded by the peritoneum.

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28
Q

What type of muscle is found in the lower oesophagus and why?

A

Contains smooth muscle for involuntary movement of food through peristalsis.

29
Q

Functions of the eosophagus

A
  • transport (travel time 5 seconds for food, 1 second for fluid, due to peristalsis)
  • protection
  • no absorption, little secretion, no digestion
30
Q

Structure of the stomach

A
  • A J-shaped bag on the left side, an enlargement of the gut tube.
  • Capacity about 1.5 litres.
31
Q

What is the primary function of the stomach?

A

Storage, as food can be eaten more quickly than it can be digested and absorbed.
Storage
* Secretion of acid, enzymes, mucus (total volume 2 - 3 litres per day).
Food + gastric juice = chyme
* Digestion of proteins by pepsin
* Absorption of water, ions, some drugs (aspirin, alcohol)
* Protection (against its own secretions and microbes)
* Transport (mixing waves, every 20 sec)

32
Q

Rugae

A

Longitudinal folds that increase the surface area of the stomach.
Transient structures (short period of time): Present when the stomach is empty, flatten out when the stomach is full.

33
Q

What are the 4 regions of the stomach?

A

Cardia: At the entrance of the stomach.
Fundus: The roof of the stomach.
Body: Main section of the stomach.
Pylorus: The end section, connects to the small intestine.

34
Q

What is pyloric sphincter?

A

A thickening of the inner circular layer of the muscularis externa.
Controls the rate and timing of chyme release into the duodenum (C-shaped, first part of the small intestine).
Opens and closes to regulate chyme movement from the stomach to the duodenum.

35
Q

Lower oesophageal sphincter

A

Located at the entrance to the stomach, not as well-developed as the pyloric sphincter.
Failure of this sphincter results in gastric reflux, where acidic contents from the stomach move back into the oesophagus, causing heartburn.

36
Q

Oesophageal epithelium

A

Protects against friction/abrasion, but not against acid and enzymes from the stomach.
This is why heartburn is felt when acidic stomach contents reflux into the oesophagus, as the epithelium is not equipped to handle acid.

37
Q

Referred sensation of heartburn

A

The oesophagus is located behind the heart, leading to the sensation of “heartburn” even though the heart isn’t affected.

38
Q

Chyme

A

The semi-digested food mixture in the stomach.
Acidic in nature.

39
Q

Epithelium of the Mucosa

A

In the stomach, the mucosa forms gastric pits, which are deep invaginations or folds.
These pits are lined with mucus-secreting cells, which produce mucus to protect the stomach lining from the highly acidic gastric juices.
At the bottom of the gastric pits are gastric glands that secrete digestive enzymes (like pepsin) and hydrochloric acid (HCl). These glands open into the gastric pits, allowing the secretions to mix with the stomach contents.

40
Q

External Muscle (Muscularis Externa)

A

The muscularis externa in the stomach has three layers of muscle, instead of the usual two found in other parts of the gut.
Innermost oblique layer: This is unique to the stomach and helps churn and break down food mechanically by moving it in multiple directions.
Middle circular layer: Like the rest of the gut tube, this layer helps with peristalsis.
Outer longitudinal layer: Works in conjunction with the circular layer for coordinated contractions.
The presence of the additional oblique layer enhances the stomach’s ability to mix and churn food, turning it into chyme, which is then gradually released into the small intestine for further digestion.

41
Q

Parietal Cells

A
  • Large cells that produce hydrochloric acid (HCl) and intrinsic factor.
  • HCl is secreted as separate hydrogen (H⁺) and chloride (Cl⁻) ions to prevent autodigestion (self-destruction).
  • HCl serves to sterilize food and create an acidic environment that aids in digestion by activating enzymes.
  • Intrinsic factor is critical for vitamin B12 absorption, which is necessary for red blood cell production (haematopoiesis). A deficiency in intrinsic factor can lead to a specific type of anaemia.
42
Q

Chief Cells

A

Secrete pepsinogen, an inactive enzyme precursor that is activated by the acidic environment into pepsin, which cleaves proteins.
This prevents autodigestion within the chief cells.
Chief cells also produce gastric lipase, though this is less significant in comparison to pepsinogen.

43
Q

Surface Mucus Cells

A

Produce an insoluble, alkaline mucus that protects the stomach lining from acid and pepsin.
These cells are continuously replaced to maintain protection.

44
Q

Enteroendocrine Cells

A

Produce hormones such as gastrin, which stimulates acid and pepsin secretion, increases stomach muscle contraction, and relaxes the pyloric sphincter.
Gastrin acts as an activator, initiating various digestive processes in the stomach when triggered by food intake, such as stomach stretch or pH changes.

45
Q
A
46
Q

Gastric epithelium

A

Layer of cells that lines the inner surface of the stomach

47
Q

Location of undifferntiated Stem cells

A

In the middle of the gastric gland. These are multipotent cells that have the ability to differentiate into various cell types.

48
Q

Location of Surface mucous cells

A

Located at the top, and they produce mucus to protect the lining of the stomach from acid. Stem cells can differentiate into these cells, and these cells migrate upwards towards the gastric surface.

49
Q

Location of mucous neck cells

A

Located lower down in the gland and produce mucus but in a different environment. Stem cells can also differentiate into these cells, which move downwards.

50
Q

Location of parietal cells

A

Responsible for secreting hydrochloric acid (HCl) and intrinsic factor, and they also migrate downwards from the stem cell zone.

51
Q

Location of Chief cells

A

At the base of the gland and are responsible for producing pepsinogen (the inactive form of the enzyme pepsin). These cells also migrate downwards.

52
Q

Location of Enteroendocrine Cells

A

Produce hormones such as gastrin, which stimulates acid and pepsin secretion, increases stomach muscle contraction, and relaxes the pyloric sphincter.
Gastrin acts as an activator, initiating various digestive processes in the stomach when triggered by food intake, such as stomach stretch or pH changes.

53
Q

What are hepatocytes?

A

Multi-talented cells which carry out all liver functions, each performing more than 500 different metabolic functions

54
Q

What are the metabolic functions hepatocytes perform?

A

Glycogen/glucose storage and release, recycling of red blood cells, bile synthesis and secretion, synthesis of plasma proteins and removal of toxins from blood.

55
Q

What does each hepatocyte require?

A
  • Access to nutrient-laden blood drained from the intestinal wall
  • Access to oxygenated blood from the systemic circuit
  • Access to ducts which drain bile to the gall bladder
56
Q

Bile Synthesis and Function

A

Bile is synthesized in the liver by hepatocytes and plays an essential role in digestion.
It helps to emulsify fats, breaking down large fat globules into smaller ones, increasing the surface area for pancreatic lipases to digest fats.
Without bile, ingested fats would not be properly absorbed and would pass through in the stool, leading to steatorrhea (fatty poop).
Bile produced in the liver is stored and concentrated in the gallbladder but synthesized in the liver.

57
Q

Blood Supply to Hepatocytes

A

Hepatocytes receive two types of blood supplies:
Nutrient-rich blood from the intestinal wall, delivered via the hepatic portal vein.
Oxygenated blood from the systemic circuit, essential for the survival of hepatocytes.

58
Q

Liver Lobules

A

The liver is composed of hexagonal units called lobules, which are the subunits of the liver.
Hepatocytes are organized in radiating columns from a central vein within the lobule, demonstrating a highly organized structure.
Surrounding each lobule is connective tissue septa with portal areas at the edges containing the portal triad.

59
Q

Bile Canaliculi

A

Bile canaliculi are small channels between hepatocytes where bile flows away from the cells towards the bile ductules.
These canaliculi merge into larger bile ducts, eventually draining bile out of the liver and into the bile duct for storage or release.

59
Q

Portal Triad

A

The portal triad consists of:
A bile duct (carries bile away from the liver).
A branch of the hepatic artery (supplies oxygenated blood to hepatocytes).
A branch of the portal vein (delivers nutrient-rich blood from the intestines).

60
Q

Blood Supply to the Liver

A

The liver receives two types of blood:
Nutrient-rich, deoxygenated blood from the gut via the hepatic portal vein, which flows toward the center of the lobule.
Oxygenated blood from the hepatic artery, which supplies oxygen to the hepatocytes to maintain their function.
Both types of blood mix and flow through specialized vessels called sinusoids towards the central vein at the center of the lobule.

61
Q

Sinusoids

A

Sinusoids are wide, leaky blood vessels lined with fenestrated endothelium, allowing the exchange of substances between the blood and the hepatocytes.
Sinusoids are wider than regular capillaries, capable of holding multiple red blood cells at a time, which slows down blood flow, giving more time for nutrients and oxygen to diffuse to the hepatocytes.
Sinusoids allow nutrient-rich and oxygenated blood to flow past the hepatocytes, facilitating efficient exchange of materials.

62
Q

Bile Flow

A

Bile is secreted by the hepatocytes into small channels called bile canaliculi that run between adjacent hepatocytes.
Bile flows in the opposite direction of blood, moving away from the center of the lobule and into the bile ducts for storage in the gallbladder or release into the intestine.

63
Q

Central Vein

A

Blood that has passed through the sinusoids drains into the central vein, a large vessel at the center of each lobule.
This mixed blood (oxygenated and deoxygenated) is then returned to the heart via the inferior vena cava.

64
Q

Lymph Space of Disse

A

Between the endothelial cells lining the sinusoids and the hepatocytes is the lymph space of Disse, which allows additional exchange of substances. This space plays an important role in the lymphatic drainage of the liver.

65
Q

Microvilli on Hepatocytes

A

The surfaces of hepatocytes facing the sinusoids are lined with microvilli, which increase the surface area for the exchange of materials, enhancing absorption and secretion processes.

66
Q

Sinusoid

A

Specialized type of capillary found in the liver, playing a crucial role in filtering blood and exchanging materials with hepatocytes, the liver’s main functional cells. Sinusoids are lined with fenestrated endothelium, meaning the endothelial cells have pores, making them leaky. This allows nutrients, waste, and other molecules to pass between the blood and hepatocytes while keeping red blood cells within the sinusoid.
Unlike regular capillaries, which are typically only wide enough for one red blood cell to pass through at a time, sinusoids are much wider, accommodating several red blood cells at once. This increased width slows down the blood flow, allowing more time for interaction between the blood and the hepatocytes. The slower movement is essential for efficient filtration and metabolic exchange.

Between the endothelial lining of the sinusoids and the hepatocytes lies the space of Disse, which is a small space where lymphatic fluid accumulates. This space will become more relevant in advanced study, as it plays a role in material exchange between the blood and the liver cells.

Additionally, hepatocytes have microvilli on their surfaces, increasing their surface area, which enhances the absorption and secretion of substances. These microvilli are critical for processes such as bile production and the uptake of nutrients from the blood.

67
Q

Is blood going towards or away from the centre of the lobule?

A

Towards, bile opposite.

67
Q
A
68
Q
A
69
Q
A

MEDSCI142 (33 decks)

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