BL Session 9 - Digestive System and Liver

Question 1

Outline the features of the hepatic portal system.

Answer 1

• The portal circulatory systems differ from the typical circulatory route in that blood passes through two sets of smaller vessels before returning to the heart.
• Blood from the first set of capillaries collects in portal vessels (sometimes called portal veins) which then begin to branch again to supply a capillary network to a second location before entering a series of veins which will lead to the heart.

Question 2

What travels to the portal vein?

Answer 2

• Water
• Water soluble vitamins
• Electrolytes
• Carbohydrates (monosaccharides)
• Proteins – amino acids, dipeptides and tripeptides
• Intestinal hormones, particularly pancreatic hormones
• Toxins, including ammonia

Question 3

In terms of the Krebs cycle, why is the liver important?

Answer 3

• The liver is the only organ in the body in which the complete urea cycle (Krebs cycle) is expressed.
• It converts ammonia to urea which can then be excreted in the urine.

Question 4

What doesn’t travel through the portal vein?

Answer 4

• Lipids

I. Dietary lipid is mostly triglycerides, cholesterol and phospholipids

II. Digested by lipases, this process needs bile acids to form micelles

III. Lipids are taken into the cells and processed into chylomicrons

IV.Chylomicrons are taken up by lymphatics, called lacteals. Lacteals contain chyle

• Fat soluble vitamins (Vitamin ADEK)

Question 5

Outline the functions of the liver.

Answer 5

• Storage: Iron; Vitamin, B12, D, Vitamin K and Glycogen
• Anabolism/production

I. Albumin

II. Glycogen

III. Numerous coagulation factors

• Catabolism/breakdown/ toxin degradation

I. Drugs and poisons (cytochrome P450)

II. Hormones - Insulin, Glucagon, Oestrogen and Progesterone

III. Haemoglobin

IV. Can take over removal of aged red cells after splenectomy

• Filtering function – macrophages are critical for the execution of this function

Question 6

Outline the endocrine and exocrine functions of the liver.

Answer 6

• Exocrine function – bile is an exocrine secretion of the liver via the bile duct
• Endocrine function - it produces angiotensinogen, thrombopoetin, insulin like growth factor 1 (IGF – 1)

Question 7

Outline the structure of hepatocytes

Answer 7

• Constitute 80% of liver cell population
• Can regenerate remarkably!
• Compared to other cells contain:

I. Numerous mitochondria

II. A lot of peroxisomes (organelles full of oxidising agents)

III. Numerous free ribosomes

IV. A lot of rER

V. A lot of sER

VI. Numerous Golgi complexes

VII. Glycogen deposits

Question 8

Describe the structure and function of Kupffer cells.

Answer 8

• Kupffer cells are monocyte derived specialist macrophages that form part of the lining of the sinusoids.
• They patrol tiny vessels in the liver called sinusoids, recycling old red blood cells and ingesting pathogens.
• The endothelium of these vessels is perforated with large holes, allowing the Kupffer cells to migrate into liver tissue at sites of inflammation and damage.

Question 9

Describe the structure of Stellate (Ito) cells.

Answer 9

Stellate (Ito) cells are full of cytoplasmic vacuoles containing Vitamin A.

Question 10

What happens to stellate cells in the case of liver cirrhosis.

Answer 10

• In liver cirrhosis, hepatic stellate cells lose their vitamin A storage capability and differentiate into myofibroblasts.
• That synthesize and deposit collagen within the perisinusoidal space, resulting in liver fibrosis.
• This collagen surrounds the central vein, constricting it and leading to portal hypertension

Question 11

What is a sinusoid?

Answer 11

• A sinusoid is an irregular tubular space for the passage of blood, taking the place of capillaries and venules in the liver, spleen and bone marrow.
• Its endothelial cells gas large gaps (perforated)

Question 12

Describe the features of a liver acinus.

Answer 12

Question 13

Describe the features of a liver lobule.

Answer 13

Question 14

Describe the location of lymphatics in the liver.

Answer 14

Lymphatics arise from the periportal space of Mall and drain to the liver hilum and then onto the hepatic duct.

Question 15

Identify the components of the alimentary canal.

Answer 15

Question 16

Identify and describe the 4 layers of the gut wall.

Answer 16

• Mucosa (innermost)

I. Muscularis mucosae

II. Lamina propria

III. Epithelium

• Submucosa - a layer of connective tissue bearing glands, arteries, veins and nerves
• Two external muscle layers (muscularis externae) - circular and longitudinal muscle
• Serosa - a serous membrane

Question 17

Identify the functions of the gastrointestinal tract.

Answer 17

• To provide a port of entry for food into the body
• To mechanically disrupt the food
• To temporarily store the food
• To chemically digest the food
• To kill pathogens in the food
• To move the food along the tract
• To absorb nutrients from the resultant solution
• To eliminate residual waste material

Question 18

What is digestion?

Answer 18

Digestion: is the conversion of what we eat by physical and chemical disruption into a solution that is relatively sterile, neutral in pH and isotonic; from which we can absorb our nutrients (a few sugars, a few fatty acids, approx. 20 amino acids, some minerals and vitamins).

Question 19

What is the role of saliva?

Answer 19

• Starts digestion (amylase and lipase)
• Bacteriostatic (contains Immunoglobin A antibody (IgA))
• High calcium (protects teeth)
• Alkaline
• Assist swallowing
• Protects the mouth
• Maintains moisture

Question 20

Discuss physical and chemical disruption in the mouth and stomach.

Answer 20

- Begins in the mouth:

I. Physical - by action of teeth, tongue, and muscles of mastication

II. Chemical - by action of salivary amylase and lipase

- Continues in the stomach:

I. Physical - by churning (3 muscle layers and mucosal rugae)

II. Chemical - by acid (HCl) and enzymes (e.g. pepsin)

Question 21

Outline the role of the oesophagus.

Answer 21

• Mouth forms a bolus which enters the oesophagus
• Upper end oesophagus – voluntary control (some striated skeletal muscle)
• Lower end oesophagus – involuntary control (solely smooth muscle)
• Rapid peristaltic transport, which works even when upside down, transports bolus to stomach (8-9 seconds)

Question 22

Outline the oesophageal mucosa.

Answer 22

• Epithelium – stratified squamous non-keratinized (withstands abrasion).
• Lamina propria – loose connective tissue bearing blood & lymph vessels, some smooth muscle cells & many cells of immune system.
• Muscularis mucosae – thin layer of smooth muscle cells

Question 23

Outline the structure of the other layers of the oesophageal wall.

Answer 23

• Submucosa – subtending layer of connective tissue containing mucus-secreting glands.
• Muscularis externa – smooth muscle layers (inner – circular; outer – longitudinal) which move food by peristalsis.
• Adventitia – thin outermost layer of connective tissue (no peritoneal enfolding of this portion of GI tract).

Question 24

What is the role of the stomach?

Answer 24

• Acts as a necessary food store (we can eat faster than digest).
• Wall relaxes so pressure doesn’t rise (called receptive relaxation).
• Contracts rhythmically to mix and disrupt
• Secretes acid and proteolytic enzymes to break down tissues and disinfect.
• Protects its epithelium by secreting mucus.
• Produces hypertonic chyme by combined action of acid, enzymes and agitation.
• Delivers incompletely digested chyme slowly, and in a controlled way, to the duodenum.

Question 25

What are the 3 layers of smooth muscle in the stomach (part of the muscularis externa)

Answer 25

- Oblique - Circular - Longitudinal

Question 26

What are rugae?

Answer 26

Folds of gastric mucosa forming longitudinal ridges in the empty stomach

Question 27

Identify and describe the components of the gastric gland.

Answer 27

- The **gastric pit** is lined by mucus secreting cells, very much like goblet cells. - The **isthmus** is the region in which stem cells divide to populate the gland by upward or downward migration - **Parietal cells** secrete H⁺ ions into the lumen and HCO₃^- ions into nearby capillaries, which move it to surface mucous cells - **Chief cells** secret pepsinogens which are converted into pepsin which partly hydrolyse proteins - **Enteroendrocrine** **cells** include G cells which secrete gastrin. The wider gastric mucosa responds to this hormone by secreting acid.

Question 28

Outline the role of surface mucous cells in the gastric pit.

Answer 28

- Surface mucous cells are abundant in the **gastric pits.** - Their secreted mucus is **resistant to pepsin (enzymatic) degradation.** - The mucus is released in response to **distension, stomach contents**, and **acid secretion** from gastric glands. - **Alcohol or aspirin** can damage the mucous cells but they are quickly replaced by mitosis in deeper cells in the neck of the gastric pit - The secreted mucus contains **HCO₃^- ions** which neutralise the effect of H⁺ ions and thereby protect the stomach lining.

Question 29

Describe the structure of the duodenum

Answer 29

- Proximal portion of small intestine - Curves around head of pancreas - Walls contain **Brunner’s glands** which secrete bicarbonate–rich mucus, to neutralise acidic chyme.

Question 30

Outline the dilution and neutralisation of chyme.

Answer 30

- **Water** drawn in from ECF to render hypertonic chyme, isotonic - Liver releases **bile** (generally via the gall bladder). bile contains: water, alkali, bile salts (to emulsify fat) - Pancreas & liver secrete **alkali** to neutralise acidic chyme (precisely controlled) - Pancreas, liver and intestine secrete specific enzymes which act, with bile, to complete digestion of chyme (enzymes come to lie in ‘unstirred layer’)

Question 31

Outline the completion of digestion.

Answer 31

Enzymes from the pancreas and intestine... - Cleave peptides to amino acids - Cleave polysaccharides to monosaccharides - Break down and re-form lipids - Break down nucleic acids

Question 32

Discuss absorption.

Answer 32

- **Active process** - requires a lot of **energy** - **Slow process** - Requires a **large surface area** I. Gut is folded/ villi/ microvilli II. Adequate contact time (control of gut transit) - Good blood supply/drainage (latter via hepatic portal vein; all nutrients travel via the liver)

Question 33

Outline the components of the small intestine and explain what they do.

Answer 33

- Duodenum/Jejunum/Ileum – Most active absorption proximally - **Duodenum** absorbs iron - **Jejunum** absorbs most of sugars, amino acids and fatty acids - **Ileum** absorbs Vitamin B12, bile acids and remaining nutrients by the terminal Ileum

Question 34

What are plicae circulares?

Answer 34

- Circular folds of mucosa and submucosa project into the gut lumen - Seen in the jejunum

Question 35

What does the large intestine consist of?

Answer 35

- Caecum - Ascending colon - Transverse colon - Descending colon - Sigmoid colon - Right hepatic flexure - Left splenic flexure

Question 36

Outline the role of the large intestine.

Answer 36

- Continues water recovery over a 20 hour transit. - By end of large intestine contents semi-solid I. Contents await expulsion in colon (not rectum) II. At certain times rapidly propelled into rectum

Question 37

Outline the role of the bacteria of the large intestine.

Answer 37

- Colon contains most of the GI tract’s bacteria - 99% of these anaerobic and most lost in faeces (1011/g) - Involved in: I. Synthesis of vitamins K, B12, thiamine and riboflavin II. Breakdown of bile acids III. Conversion of bilirubin to non-pigmented metabolites - (all readily absorbed)

Question 38

What are the roles of the gut?

Answer 38

- Secretion - Movement (variable transport) - Absorption of both food and what the gut has added to it in order to digest it

Question 39

Outline the daily balance sheet of the gut.

Answer 39

- Ingest – 1kg - Mouth – 1.5L of saliva - Stomach – 2.5L of gastric secretions - Small intestine – 9L of water/alkali - Total = 14L (excluding fluid we drink)

Question 40

Outline the problems associated with fluid balance.

Answer 40

- It is a delicate balance (why diarrhoea & constipation so common) - Gut is dealing with large quantities of fluid. Decreased absorption, or increased secretion can result in: I. Life threatening dehydration (depletion of body fluids, not just what we recently drank/ingested) II. Life threatening electrolyte imbalance

Question 41

Outline the neural (somatic and autonomic) control of the gut.

Answer 41

- **Somatic** (innervating striated muscle) I. Ingestion (mouth and first ⅓ of oesophagus) II. Excretion (last sphincter of anus) - **Autonomic** nervous system controls the rest I. Post ganglionic neurones form plexuses A. One between muscle layers of gut wall B. One between submucosa and muscularis externa II. The ‘gut nervous system’ (the ‘gut brain’) III. Range of neurotransmitters

Question 42

Outline the paracrine control of the gut.

Answer 42

- Substances act locally - Histamine (controls production of acid in stomach) - Vasoactive substances (affect blood flow in gut)

Question 43

Outline the endocrine control of the gut.

Answer 43

Range of hormones control: - Secretion of stomach acid - Alkali secretion from liver and pancreas - Enzyme secretion

Question 44

Outline the role of gastrin.

Answer 44

- Released by: I. G cells of pyloric antrum of stomach II. Pancreas III. Duodenum - Promotes production of HCl (gastric acid) by the parietal cells of the stomach.

Question 45

Outline the role of cholecystokinin

Answer 45

- Synthesised and secreted by enteroendocrine cells of the duodenum - Promotes release of digestive enzymes from the pancreas - Promotes release of bile from gall bladder (stimulates it to contract) - Is a hunger suppressant

Question 46

Outline the role of secretin.

Answer 46

- Promotes HCO₃^- (bicarbonate) secretion from duct cells of pancreas - Promotes bile production by the liver - Inhibits secretion of acid by parietal cells of stomach

Question 47

What is the role of the epiglottis?

Answer 47

- The epiglottis is a flap of elastic cartilage covered with a mucous membrane. - It projects behind the tongue and closes during swallowing which prevents aspiration. - It forces the swallowed liquids or food to go down the oesophagus.

Question 48

Which poses more of a challenge to the swallowing mechanism - fluids or solids?

Answer 48

- Fluids because it's harder to form a bolus, hence you need more muscle and co-ordination and contraction. - Dysphasia - difficulty in swallowing (Alzheimer's and stroke)