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HUBS192 > Lecture 25- Secretion > Flashcards

Lecture 25- Secretion Flashcards

1
Q

What secretions were focused on in this lecture?

A
  • Salivary
  • Gastric
  • Pancreatic
  • Biliary (Liver, but also involves the gallbladder)
  • Intestinal
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2
Q

What are exocrine secretions produced by?

A
  • Epithelia that lines the GI tract

- All secretions into the lumen of the GI tract are exocrine as the lumen is continuous with the outside world

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3
Q

What are the components and functions of exocrine secretion in the GI system?

A
  • Mucus for protection and lubrication. Also aids in mechanical digestion
  • Electrolyte solution which serves to dilute food & provides optimal pH. This is essential for chemical digestion of food
  • Digestive enzymes which are essential for chemical digestion of food and also aid absorption
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4
Q

Describe the general change in pH that occurs in the GI tract?

A
  • In the stomach pH is low due to acid
  • As move further down into small intestine becomes closer to 7 and stabilizes throughout the rest of the system due to bicarbonate neutralizing the acid
  • Related to the optimum pH of the enzymes throughout
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5
Q

How much do we secret each day and how does this relate to the plasma?

A
  • The plasma is the source of all fluids we secrete, there is 3L of it
  • We secret 8L per day
  • Therefore, reabsorption is vital to ensure we do not lose fluid constantly
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6
Q

What are the three pairs of salivary glands? Which secretes the most?

A
  • Parotid (secrets 5%)
  • Sublingual (secrets the most, makes up 70% of salivary secretions)
  • Submandibular (secrets 25%)
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7
Q

How much saliva do we produce per day? What amount of this is stimulated as opposed to basal (at rest)?

A
  • Produce 1.5 L fluid per day
  • Basal secretion - 0.3 ml/min
  • Stimulated secretion - 1.5 ml/min
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8
Q

What is the composition of the salivary secretions like and the function of each component?

A

Mucus:
-Lubrication

Dilute solution of NaHCO3/NaCl (sodium bicarbonate):

  • Dilution of food
  • Optimal pH for digestive enzymes

Digestive Enzymes:

  • Lingual lipase (digest fats)
  • a-amylase (digest carbohydrates)
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9
Q

How much a-amylase is produced in the mouth, what happens to it?

A

Enough is produced to digest all the carbohydrates we ingest, however, this doesn’t occur as food is quickly moved to the stomach where amylase denatures due to low/ acidic pH

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10
Q

What are the functions of salivary secretions?

A

Not essential, but…aids:

  • Talking
  • Chewing and swallowing (this dissolves food and lubricates)

Important for hygiene as:

  • Irrigation flushes out any left over food material
  • Xerostomia (dry mouth from reduced or absent saliva)
  • Avoids dental problems

Also for digestion:

  • Dissolves food allowing tasting as particles are exposed to taste buds
  • Lingual lipase (Fats)
  • a amylase (Starch)
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11
Q

How is salivary secretions regulated?

A

Nervous via:

  • Thought, smell, sight of food
  • Presence of food in mouth

Autonomic Nervous system:

  • Parasympathetic stimulates secretion of copious quantities of fluid
  • Sympathetic results in small volumes of viscous (thick) fluid
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12
Q

How much does the stomach secret per day? (volume of gastric secretions)

A

2-3 L per day

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13
Q

How much and what does the stomach secret between meals?

A
  • Slow rate (15-30 ml/h) of secretion

- Surface cells secret mucous

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14
Q

How much and what does the stomach secret while eating?

A
  • Superimposed on basal rate
  • Mucous cells secret mucus
  • Parietal cells secret 150 mls per hour of HCl acid, pH = 1.0 and also Intrinsic factor
  • Chief cells secret pepsinogen
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15
Q

What is pepsinogen and what is important about the position of the chief cells to influence how it is activated?

A

Chief cells which produce pepsinogen sit at the bottom of the gastric gland. They secret pepsinogen which is a precursor to the enzyme for proteins: pepsin. The pepsinogen travels up towards the lumen on the GI tract through the region containing parietal cells secreting HCl. This HCl activates the pepsinogen turning it into pepsin before it reaches the lumen. It can now begin the digestion of proteins.

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16
Q

What does intrinsic factor produced by parietal cells in the stomach do?

A

Aids absorption of Vitamin B12 in small intestine

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17
Q

What does mucus in the stomach do?

A

Protect squishy/ soft epithelial cells from abrasion (lubricates) and acid

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18
Q

What are the functions of gastric acid?

A
  • Dilutes food
  • Denatures protein (loosening it up so that enzymes can act to digest)
  • Activates pepsinogen to pepsin
  • Creates optimum pH for pepsin action
  • Protection (protects us from harmful things that may be in our food)
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19
Q

How is the bicarbonate and hydrogen ions needed for the parietal cells to secrete acid (HCl) sourced in the stomach?

A
  • Carbon dioxide combines with water to give carbonic acid (H2CO3) via the enzyme carbonic anhydrase
  • Dissociation of carbonic acid then gives H+ and bicarbonate (HCO3-)
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20
Q

Where is the enzyme carbonic anhydrase found?

A

Floating in the cytoplasm of parietal cells

21
Q

What are the steps in the secretion of acid (HCl) by the parietal cells in the stomach?

A

(Note: before this already have formation of bicarbonate and Hydrogen atoms occurring- separate process but a vital first step)

  1. Secretion of acid (H+) via Hydrogen Potassium ATPase
    -Happens on the Apical (facing the lumen of GI tract) membrane of parietal cells
    -Pumps H+ ions into lumen in exchange of
    potassium ions (K+) (this is to keep a charge balance)
    -K+ return to lumen through membrane channel
  2. Source of chloride (Cl-) via anion counter transporter
    - Happens on the serosal membrane (facing blood) of parietal cells
    - Ejects HCO3- (bicarbonate) into interstitial fluid
    - Imports Cl- ions into cell (exchange is occurring)
  3. Secretion of chloride (Cl-)
    - Cl- diffuse across cell
    - Enters lumen via Cl- channel in apical membrane
  4. Hydrogen ions and chloride ions combine to form HCl
22
Q

How is gastric secretion regulated?

A
  • Coordinated with the eating and arrival of food

- Nervous system (divided by region which is controlling)

23
Q

What are the three phases invovled in regulating gastric secretion?

A
  • Cephalic phase: head controls secretion
  • Gastric phase: stomach controls secretion
  • Intestinal phase: intestine controls secretion
24
Q

What is the cephalic (head) stage in controlling gastric secretion?

A
  • Is invovled in the preparation for food arrival
  • Stimuli such as the thought, smell or sight of food and also chewing action + taste works to activate the higher centers (centeral nervous system)
  • Signal is then sent down through the vagus nerve (parasympathetic nervous system) to regulate the action of the enteric nervous system
  • The enteric nervous system stimulates parietal cells, chief cells and goblet cells to prep for making acid, proteins and mucus (the stuff that is needed to digest food)
  • The enteric nervous system also stimulates secretion of the hormone gastrin into the blood which amplifies the actions above by further stimulating the parietal and chief cells (feeding back)
25
Q

What percentage of secretion associated with a meal is to do with the cephalic stage?

A

20%

26
Q

What is the gastric (stomach) stage in controlling gastric secretion?

A

-Ensures sufficient secretion to handle
ingested food

  • Works as a response to stimuli in the stomach such as the stretch/distension stomach wall (detected by stretch receptors), products of digestion contained in the stomach lumen and elevated pH (detected by chemoreceptors)
  • Under both nervous and hormonal regulation
  • The Local nervous reflex is carried out by the enteric nervous system
  • The external nervous reflex is carried out by the parasympathetic nervous system
  • Both work to stimulate secretion from mucus + chief + parietal cells, motility (mixing patterns) and gastrin secretion (the things needed to digest food material)
  • Gastrin further stimulates secretion and motility
27
Q

What percentage of secretion associated with a meal is to do with the gastric stage?

A

70% (most)

28
Q

What is the intestinal stage in controlling gastric secretion?

A
  • Controls delivery to small intestine
  • Works in response to stimuli in the duodenum such as distention (picked up by stretch receptors) and the arrival of acid chyme + lipids and carbohydrates (picked up by chemoreceptors)
  • Under both nervous and hormonal regulation
  • Hormones that work are GIP, CCK and Secretin. GIP and CCK release into blood stream is stimulated by the presence of lipids and carbohydrates. Secretin on the other hand is released due to the decrease in pH
  • The nervous response is via the enterogastric reflex
  • Both nervous and hormones act to inhibit secretion (chief cells + parietal cells) and motility (peristalsis) in the stomach thus slowing the emptying in the small intestine.
29
Q

What percentage of secretion associated with a meal is to do with the intestinal stage?

A

10%

30
Q

What function of the pancreas are we particularly interested in?

A

The exocrine function not the endocrine (glucose regulation)

31
Q

What is the volume and composition of the secretion from the pancreas?

A
  • 1 – 1.5 L per day
  • Acinar cells produce enzymes for the chemical digestion of food

-Duct cells secret alkaline fluid (bicarbonate) which works to neutralize acid and maintain the optimum pH for enzymes

32
Q

What does the pancreas provide in terms of digestive enzymes (name them)?

A
  • Lipolytic: lipase, phospholipase (digest fats)
  • Amylytic: pancreatic amylase (digest carbohydrates)
  • Proteolytic: Trypsin, chymotrypsin, carboxypeptidase (digest proteins)
  • Nucleolytic: ribonuclease, deoxyribonuclease (digest nucleotides)
33
Q

Is is secretion stimulated in the pancreas?

A

-Secretion of digestive enzymes by acinar cells stimulated by hormone cholecystokinin
(CCK)
-Arrival of lipids and carbohydrates in duodenum stimulates CCK secretion

34
Q

What are proteolytic enzymes secreted as?

A

As inactive precursors which are then activated in the duodenum

35
Q

Trypsin precursor is…

A

Trypsinogen

36
Q

Chymotrypsin precursor is…

A

Chymotrypsinogen

37
Q

Carboxypeptidase precursor is…

A

Procarboxypeptidase

38
Q

In the small intestine how are inactive precursors to enzymes activated?

A

-Enterokinase (=enteropeptidase) is found bound to the duodenal membrane
-This Converts trypsinogen to trypsin
-Trypsin can then convert other enzymes to active
form

39
Q

Why do enzymes have to initially be in a precursor form until they are at the site they need to be used?

A

Can’t have active ones sitting and digesting the pancreas (most of our body cells are proteins)

40
Q

What is the secretion of alkaline (bicarbonate rich) fluid from the small intestine duct cells stimulated by?

A

Hormone secretin, secretin which is itself stimulated by

arrival of acid chyme in duodenum

41
Q

What is the function of the alkaline fluid secreted from the pancreas?

A
  • Neutralises acid chyme delivered from stomach

- Creates optimum pH (6.7-9.0)for pancreatic and intestinal digestive enzyme

42
Q

What is the volume and composition of biliary secretions?

A

-0.5 L per day

Consists of:
-Products associated with digestion such as Bile salts
and bicarbonate rich fluid via ducts
-Excretory products such as Bile pigments (waste products) and Cholesterol

43
Q

What are the functions of biliary secretions?

A
  • Bile salts invovled in Fat digestion
  • HCO3 (bicarbonate) rich fluid neutralizes acid
  • Bile pigments aid in excretion
44
Q

Where is bile created, stored and delivered to?

A
  • Bile secreted constantly by liver
  • Stored and concentrated in gallbladder (as remains pressure builds up)
  • Delivered to duodenum with arrival of food
45
Q

How is bile secretion controlled?

A

Initial delivery of bile is under hormonal control:

  • Release of the hormone CCK (cholecystokinin) triggers ejection of bile from the gall bladder (contraction)
  • This is coupled with relaxation of the hepatopancreatic ampulla
  • Bile passes into the duodenum where bile salts break apart lipid droplets by emulsification

-This primarily how it occurs but the hormone secretin also has a mild role in releasing bile from the liver

In addition bile can stimulate its own secretion via enterohepatic circulation

46
Q

What is CCK produced in response to?

A

Products of digestion in duodenum

47
Q

What is enterohepatic circualtion?

A
  • Bile is metabolically expensive to produce
  • Therefore, 95% of bile salts once released are reabsorbed in the ileum
  • They can then be transported back to liver in enterohepatic circulation (hepatic portal vein)
  • Bile salts are then reabsorbed by the liver and secreted again stimulating bile secretion
48
Q

What is secreted from the small intestine?

A

-1.5 L per day
-Mucus for lubrication
-Isosmotic fluid (Alkaline - mixture of NaCl and
NaHCO3) to helps neutralize acid and dilute food to aid digestion
-Digestive enzymes (cells are shed every seven days therefore the enzymes contained in the brush border are released and reabsorbed)

49
Q

What is secreted from the large intestine?

A

Mucus for Lubrication

HUBS192 (38 decks)

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