Digestion lecture


Although lectures in BIOL454 on systems (such as circulation) are so advanced that digestion does not fit into that course, Dr. Bode's research on liver overlaps more with digestion than any of his topics.

At Mizzou, they did a lab on smooth muscle motility using rabbit gut.
The coordinator sacrificed a rabbit and brought pieces of gut with threads attached
Students calibrated the transducer with a weight (as we did)
A chamber, an air tube, and a transducer are used for the gut preparation
The demonstration involved manipulations like atropine to block motility
Contractions are monitored on the computer

Parasympathetic nervous system would promote digestion, sympathetic would put digestion and motility on hold. They act through the enteric nervous system, sometimes called the third part of the autonomic nervous system. Here, transmitters in addition to acetylcholine and norepinephrine come into play.

Input - output

One emphasis will be on how the human digestive system invests many juices (hydrolases = enzymes which catalyse hydrolysis)
Some glands have ducts and these are called exocrine glands.
This is in contrast with endocrine glands (ductless, for hormones, which are also involved in digestion)

Fig. 21.5

800 g food IN per day
1200 ml water
/
/ 7000 ml GLANDS
/
50 g solid OUT
100 g water

Overall anatomy

Fig. 21.2ab

(from mouth to stomach)

Mouth - teeth, lubrication, salivary amylase to disaccharide maltose - starch tastes sweet (only starch in mouth)
-ase enzymes
Pharynx swallowing
Esophagus - bolus, peristalsis
Cardiac oriface

Stomach

Stomach - gastric mucosa - mucus protect

Fig. 21.25

from parietal cell: HCl kill bacteria
stop amylase
From chief cell: pepsinogen ---(HCl, pepsin))--> pepsin (proteolytic)
optimum pH for pepsin is 2
(Inactive forms called zymogens)
Heartburn, antacids, ulcer (although it is now known that a specific bacterium, Helicobacter pylori, is associated with ulcer)
very little absorption in stomach - exceptions: aspirin, alcohol

Fig. 21.2ab again

Pyloric sphincter regulates emptying of acidic gastric juice to duodenum.
In duodenum, bile from liver and bicarbonate and enzymes from pancreas add to enzymes from small intestine

Intestine:

Fig. 21-02 e g

enzymes - lactase, maltase, sucrase, others
mitosis - since cells digest themselves
absorption - food and water

Villi (big) increase surface area. Note mitoses in crypts.

Here is a micrograph from our histology course dramatizing the tremendous increase in apical surface area of intestinal cells caused by the microvillar brush border.

The microvilli in the intestines have a special name, the brush border.
Protease (enteropeptidase turns trypsinogen into trypsin which, in turn, makes chymotrypsin and carboxypeptidase (and others)

Fig. 21.17

Fig. 21.29


How proteins are broken down and absorbed is complex.
Trypsin and chymotrypsin are endopeptidases.
Carboxypeptidase, exopeptidase, cuts carboxy terminal.
Aminopeptidase (shown on brush border) cuts amino terminal.
Di- and tri-peptidases are intracellular.
Intestinal cells digest themselves, and their enzymes go into intestinal lumen.

Fig. 21.15

how glucose gets absorbed
same story as for kidney, and, in fact, notice "lumen of kidney tubule or small intestine"
apical, cotransport with sodium
basolateral Na+-K+ATPase plus glucose facilitated transport

Pancreas

(no good text figure for this)

Pancreas is responsible for dumping in many of the enzymes
Unit of this exocrine gland is the acinus
Zymogen refers to precursor of enzyme.

"pro..." as in "procarboxypeptidase and "...ogen" as in "chymotrypsinogen" --a peptide fragment is cut off from a larger precursor protein to make active enzyme; there are many examples like this in biology, for instance prohormones cleaved to make active peptide hormones.
Pancreas puts out bicarbonate (alkaline) to neutralize stomach acid.
Optimum pH for for trypsin is 8.

Pancreas and common bile duct (from liver and gall bladder) dumping into duodenum.
When I took organic chemistry lab (1966-7) we used gall stones for a cholesterol extraction.
Note: Islets of Langerhans (endocrine tissue) in pancreas where alpha cells make glucagon and beta cells make insulin.

Liver

Very few enzymes.
Emulsify fats.
Iron recycling.
Eliminate some wastes to feces.
Detoxify.

Detoxification

Fig. 21.30

Hepatic portal system

Portal blood veins (circulatory system "wired" in series is unusual, another famous example being the hypothalamus of the brain which feeds to the pituitary gland and kidney cortex to medulla). Via hepatic portal vein pick-up from small intestine is first delivered to liver cels. There, "microsomal fraction" (how biochemists view the smooth endoplasmic reticulum) has enzymes to detoxify. Enzymes like those that detoxify drugs like barbiturates are increased on exposure to toxins (inducible). Alcohol -(alcohol dehydrogenase (ADH) )-> aldehyde - (aldehyde dehydrogenase)-> acetic acid. With AcetylCoA, acetic acid can add to fatty acid chains 2 carbons at a time. There is a fatty metamorphosis of the liver from one binge. Continued heavy drinking leads to scarring and cirrhosis.

Erythrocyte iron recycling, bile pigment (bilirubin) ->urobilinogen turns feces dark.
Also colors urine.
Hepatitis (disorder which spills bile into blood) - turns skin yellow (jaundice) (feces are not as dark, urine is darker)

Fat digestion

Fig. 21-19


Liver contributes to fat digestion via bile salts, salts of cholesterol, that emulsify fats
Triglycerides are broken to monoglycerides and free fatty acids.
Despite this breakdown, fats are reassembled, put in droplets with proteins and carried in lymph duct called lacteal.

Hormones

(table 21-2)

Local hormones control digestion - Many found later in other places
from stomach:
food stimulates gastrin which, in turn, stimulates gastric juice until there is a low (acidic) pH
from duodenum:
Cholecystokinin (CCK) - liver and pancreas
Secretin for bicarbonate release
Enterogastrones to slow gastric emptying

Quiz questions 2006:

1. What is the first, and only, enzyme to act on food in the mouth?

amylase

2. What structure is different in rats that prevents them from vomiting food or poison back up?

cardiac oriface

3. What ionic compound do parietal cells secrete?

HCl

4. Which organs contain the 3 portal systems we learned about this semester?

hypothalamus->anterior pituitary, intestine->liver, kidney cortex->kidney medulla

5. What does bicarbonate do to the pH of the stomach output to the intestine?

neutralize by raising pH

6. What combines with acetic acid derived from ethanol to build up fatty acid chains 2 carbons at a time (causing a fatty metamorphosis of the liver)?

coA

7. Hepatitis is a disease that spills what into the blood?

bile

8. A carboxypeptidase cuts which terminal of a protein?

carboxy, duh

9. A bolus of food moves down the esophagus via what movement?

peristalsis

10. Why must cells be produced by mitoses very readily in the intestines?

they digest themselves

Final questions 2006:

29. Give an exception to the general truth that functional proteins cannot be absorbed in the digestive system.

mother's antibodies in breast feeding (also prions)

30. Most absorption occurs in the intestine. Name a drug whose absorption begins in the stomach.

aspirin, alcohol

31. What is another word for inactive enzyme or precursor of an enzyme?

zymogen

32. In addition to acidity, what pathology causes ulcers in the stomach?

bacteria (Helicobacter pylori)

33. What does bile do to fats?

emulsify

34. Why do you have a lot of folds and "crypts" in the intestines?

to increase surface area

35. Amino acids are cotransported from the lumen along with what ion?

Na+

36. Where do the pancreas and common bile duct "dump" their contents?

duodenum

37. Why is it useful to have a portal vessel from the intestine to the liver?

to detoxify

38. What makes feces dark?

urobilogen

39. After passing the basolateral cell surface in the company of proteins, fats are picked up via which system?

lymphatic




This page was last updated 4/2/08

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