Download Digestive tract

運動營養學專題研究
Sport Nutrition
Chapter 5
Gastric emptying, digestion, and
absorption
張振崗 教授
Jeukendrup A, Gleeson M. Sport nutrition: an introduction to energy
production and performance, 2nd Ed. Human Kinetics, 2010
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Chapter
16
The Digestive
System
PowerPoint® Lecture Slides
prepared by Jason LaPres
Lone Star College - North Harris
Copyright © 2010 Pearson Education, Inc.
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Digestive Tract

Digestive tract also called gastrointestinal
(GI) tract or alimentary canal


Is a muscular tube
Extends from oral cavity to anus:

Passes through pharynx, esophagus, stomach, and small
and large intestines
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Accessory Organs
Figure 16-1
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Digestive Tract
Figure 16-1
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Functions of the Digestive System -1
Ingestion


Occurs when materials enter digestive tract via
the mouth
Mechanical processing



Crushing and shearing
Makes materials easier to propel along digestive
tract
Digestion


The chemical breakdown of food into small
organic fragments for absorption by digestive
epithelium
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Functions of the Digestive System -2
Secretion


Is the release of water, acids, enzymes, buffers, and salts

By epithelium of digestive tract, and glandular organs
Absorption


Movement of organic substrates, electrolytes, vitamins,
and water

Across digestive epithelium

Into interstitial fluid of digestive tract
Excretion 排遺


Removal of waste products from body fluids
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Digestive Tract

Lining of the digestive tract protects surrounding
tissues against




Corrosive effects of digestive acids and enzymes
Mechanical stresses, such as abrasion
Bacteria either ingested with food or that reside in digestive
tract
Major layers of the digestive tract




Mucosa
Submucosa
Muscularis externa
Serosa
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The Structure of the Digestive Tract
Figure 16-2
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Segments of the Intestine
Figure 16-10
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The Intestinal Wall

Histology of the Small Intestine

Plicae circulares 環行皺襞 :


Transverse folds in intestinal lining
Are permanent features:


Intestinal villi:

A series of fingerlike projections:


in mucosa of small intestine
Covered by simple columnar epithelium:


do not disappear when small intestine fills
covered with microvilli
~3300 cm2 absorptive area
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The Intestinal Wall
Figure 16-11a
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The Intestinal Wall
Figure 16-11b
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The Intestinal Wall
Figure 16-11c
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Daily secretion of intestinal juices


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



Saliva: 1000 ml
Gastric secretions: 1500 ml
Pancreatic secretion: 1000 ml
Bile: 1000 ml
Small intestine: 2000 ml
Large intestine: 200 ml
Total: 6700 ml
Mostly reabsorbed
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Absorption of water

99% water absorption in small intestine, by
simple diffusion


Water flow toward compartment with lower
water concentration (higher solute concentration)
Osmole: number of solute particles




mOsm/kg (osmolality),
mOsm/L (osmolarity 滲透壓)
~290 mOsm/L for most body fluids
If ingested high osmolarity, water move into gut
lumen
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Osmosis 滲透
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Absorption of vitamins



Mostly in small intestine via diffusion
Digestion free vitamins from coenzymes
Fat-soluble vitamins absorbed along with FA



Water-soluble vitamins


Incorporated into chylomicrons
Transported through lymphatic system
Not retained to any great extent in body
Additional vit K, B12, thiamine, riboflavin
can be formed by bacteria in colon
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Absorption of minerals


Minerals not very well absorbed in human
intestine
Absorption rate, retention rate depend on
intake, storage, and requirement
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Regulation of gastric emptying


Gastric motility and secretion mostly
automatic
Stomach signals: usually positive feedback



Nervous signals from stretching and extension of
stomach wall, relax pyloric sphincter
Release of gastrin
Intestinal signals: usually negative feedback

Duodenum contains receptors for pH, distension
of duodenum, osmolarity
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Factors for gastric emptying


Fluid volume in stomach, exponential
Not affected by exercise up to 80% VO2max
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High osmolarity reduce gastric emptying

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
Short- or long-term high intensity exercise
reduce gastric emptying
Not important in 200-400 mOsm/L, typical for
most sport drinks
High energy density reduce gastric emptying
Temperature of food usually no effect
Psychological stress reduce gastric emptying
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Regulation of gastric emptying
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Rate of gastric emptying