1. Human Physiology Chapter 15 The Mechanisms of Body Function
Vander’s
Human Physiology
The Mechanisms of Body Function
Tenth Edition by
Widmaier • Raff • Strang
© The McGraw-Hill Companies, Inc.
Figures and tables from the book, with additional comments by:
John J. Lepri, Ph. D.,
The University of North Carolina at Greensboro

2. Figure 15-1
Digestion of food and absorption of nutrients are accomplished in a long tube connected to the external world at both ends; secretion and motility of “the tube” are major themes in understanding the gut.

3. Figure 15-2
The four processes carried out by the GI tract: digestion, secretion,
absorption, and motility.

4. Figure 15-3
Many functions in the gut are found in specific locations along its length. Most of the absorption of nutrients occurs in the small intestine, so most of digestion is accomplished there or upstream.

6. Figure 15-4 Digestive secretions from the liver and the pancreas are
delivered into the duodenum of the small intestine through
the sphincter of Oddi.

7. Figure 15-5 Digestive secretions are mostly water, with the average
amounts indicated
here. Note that only
100 ml are excreted
in feces, so the
mechanisms for water
absorption are efficient
(recall the kidneys’
primary role in water and
osmotic homeostasis).

8. Figure 15-6
The gut wall has a layered organization, with the absorptive cells lining the lumen and neural and muscular components below. Blood and lymph vasculature is abundant to transport absorbed nutrients.

9. Figure 15-7 By projecting into the lumen, the villi increases
nutrients
By projecting
into the lumen,
the villi increases
the surface area
for absorption of
nutrients.
Microvilli [aka brush
border] fringe the
villi to further increase
surface area.

11. Figure 15-9 A molecular model of a bile salt, with the
cholesterol-derived
“core” in yellow.
A space-filling model
of a bile salt. The
non-polar surface
helps emulsify fats, and
the polar surface
promotes water solubility.

12. Figure 15-10
Bile salts and
phospholipids convert large fat globules into smaller pieces with polar surfaces that inhibit reaggregation.

13. Figure 15-11 Emulsified fat globules are small enough that lipase
enzymes gain access to
degrade triglycerides
to monoglycerides and
fatty acids, which enter
the absorptive cells by
simple diffusion or aggregate to form loosely held micelles, which readily break down.

14. Figure 15-12 Big Droplets of Fat Small Droplets of Fat Micelles
Fatty Acids and
Monoglycerides
Chylomicron Assembly
Distribution and Processing

15. Figure 15-13
The enteric nervous system coordinates digestion, secretion, and motility to optimize nutrient absorption. Its activity is modified by information from the CNS and from local chemical and mechanical sensors.

17. Figure 15-14
Difficult to see food bolus (raw liver?)– can a brighter color be used (blueberry yogurt)?
The swallowing reflex is coordinated by the medulla oblongata, which stimulates the appropriate sequence of contraction and relaxation in the participating skeletal muscle, sphincters, and smooth muscle groups.

18. Figure 15-15
The coordinated sequence of contraction and relaxation in the upper esophageal sphincter, the esophagus, and the lower esophageal sphincter is necessary to deliver swallowed food to the stomach.

19. Figure 15-16 Specialized cells in the stomach synthesize and
secrete mucous
fluid, enzyme precursors,
hydrochloric acid,
and hormones.
Note esophageal sphincter correction.
The abundant smooth muscle in the
stomach is responsible for gastric motility.

20. Figure 15-17
Chief cells synthesize and secrete the protease precursor known as pepsinogen.
Parietal cells synthesize and secrete the hydrochloric acid responsible for the acidic pH in the gastric lumen.

21. Figure 15-18 F O D
“Epithelial cell” label might confuse.
Acid production by the parietal cells in the stomach depends on the generation of carbonic acid; subsequent movement of hydrogen ions into the gastric lumen results from primary active transport.

22. Figure 15-19 One inhibitory and three stimulatory signals that alter
acid secretion by
parietal cells
in the stomach.

23. “HCL” typo on text proofs (“L” should be lowercase “l”)

24. Figure 15-21
The acidity in the gastric lumen converts the protease precursor pepsinogen to pepsin; subsequent conversions occur quickly as a result of pepsin’s protease activity.

25. Figure 15-22 Waves of smooth muscle contraction mix and propel the
ingested contents of the gastric lumen, but only a small amount of the material enters the small intestine (duodenum) as a result of each wave cycle.

26. Figure 15-23
Rhythmic waves of smooth muscle contraction in the gut are the result of waves of action potentials moving along via gap junctions.

27. Figure 15-24
Delivery of acid and nutrients into the small intestine initiates signaling that slows gastric motility and secretion which allows adequate time for digestion and absorption in the duodenum.

28. “HCL” typo on text proofs (“L” should be lowercase “l”)

29. Figure 15-25
The exocrine cells in the pancreas play a central role in the production of digestive enzymes; the endocrine functions of the pancreas will be discussed at length in Chapter 16.

30. Figure 15-26
Were digestive enzymes synthesized in their active form, they would digest the very cells that make them. Hence, inactive precursors (e.g., trypsinogen) become activated (trypsin).

31. Figure 15-27
Secretin’s receptors are found in the pancreas, which responds with additional bicarbonate delivery: gastric motility and secretion are inhibited.

32. Figure 15-28 Cholecystokinin’s receptors are located:
in the pancreas, which
responds with additional
enzyme delivery
in the gallbladder, which
contracts to deliver more
bile
in the sphincter
of Oddi, which relaxes to
facilitate delivery of the enzymes and bile salts

33. Figure 15-29
Bile formation by cells in the liver includes 6 components:
bile salts, lecithin, bicarbonate ions, cholesterol, bile pigments, and trace metals.
The bile is funneled into the gallbladder and then delivered
into the duodenum upon stimulation from CCK.

34. Figure 15-30
Up to 95% of the cholesterol-based bile salts are “recycled” by reabsorption along
the intestine.

35. Figure 15-31
Cholecystokinin (CCK) stimulates the gallbladder, which responds by contracting and
delivering more
bile to the duodenum
through the sphincter
of Oddi, which relaxes (opens) in response to CCK.

36. Figure 15-32 Most of the contractions of the small intestine are
of the mixing and churning
actions portrayed here as
segmentation contractions;
peristalsis and the
downstream movement
of materials is infrequent.
Figure legend in text has mispelled peristalsis.

37. Types of GI reflexes
Migrating myoelectric complex the cessation of segmentation is replaced by MMC beginning in the lower portion of stomach, repeated peristaltic waves travel about 2 feet along the small intestine and die out.
Gastroileal reflex Segmentation intensity in the ileum increases during the period of gastric emptying.
Intestino-intestinal reflex Large distension of he intestine or bacterial infections lead to a complete cessation of the intestinal movement.

38. Figure 15-33
In the large intestine, active transport of sodium, coupled with osmotic absorption of water, are the primary activities. Microbes here are active in the production of vitamin K.

39. Mass movement
3 to 4 times a day, generally following a meal, a wave of intensive contraction spreads rapidly over the transverse segment of the large intestine toward the rectum.
It usually coincides with the gastroileal reflex.

40. Figure 15-34 Video endoscopy has greatly enhanced our
understanding of normal processes in the gut,
and reveals complications resulting from disease.

41. The End.