Download small intestine - Zanichelli online per la scuola

David Sadava, David M. Hillis,
H. Craig Heller, May R. Berenbaum
La nuova
biologia.blu
Anatomia e fisiologia dei viventi S
Nutrition, Digestion,
and Absorption
What Do Animals Require from Food?
Animals are heterotrophs: they derive their nutrition by
eating other organisms.
Heterotrophs depend on this synthesis and have evolved a
variety of adaptations to take advantage of it.
The Human Digestive System
How Is the Vertebrate Gastrointestinal Structure?
Tissues of the vertebrate gut are arranged in layers:
• Lumen—the gut cavity.
• Mucosa—layer of epithelial cells that secrete mucus,
digestive enzymes, or hormones; some absorb nutrients
through microvilli; in the stomach some secrete
hydrochloric acid.
• Submucosa—has blood and lymph vessels and nerves.
How Is the Vertebrate Gastrointestinal Structure?
• Two layers of smooth
muscle are outside the
submucosa:
• Circular muscle layer
• Longitudinal muscle layer
How Is the Vertebrate Gastrointestinal Structure?
Nerve nets in the submucosa and between the smooth
muscle layers are called the enteric nervous system.
The peritoneum is a membrane that surrounds the gut
and lines the wall of the cavity.
It includes connective and epithelial tissues that secrete
lubricating fluids so organs can easily slide against each
other in the body cavity.
How Does the Vertebrate Gastrointestinal System Function?
Mechanical activity in digestion:
• Food is chewed and mixed with saliva.
• The tongue pushes a bolus to the soft palate, initiating
swallowing—food passes into the esophagus.
• Food is kept out of the trachea by the closed larynx and the
epiglottis.
Chemical
digestion:
Salivary glands
secrete amylase that
breaks down
carbohydrates.
How Do Animals Ingest and Digest Food?
Digestion begins with the teeth.
Mammalian teeth:
• Enamel, composed of calcium phosphate covers the crown
• Dentine, (bony material) in the crown and root
• Pulp cavity, contains blood vessels, nerves, and dentine-producing cells
How Does the Vertebrate Gastrointestinal System Function?
Peristalsis: waves of muscle contractions that move food toward
the stomach.
The upper esophagus is skeletal muscle; the rest is smooth muscle.
As food reaches the smooth muscle,
the esophagus contracts and pushes
the food toward the stomach.
The lower esophageal sphincter
prevents food from moving backward
into the esophagus.
The pyloric sphincter controls the
passage of food into the intestine.
How Does the Vertebrate Gastrointestinal System Function?
Gastric pits in the
stomach have three
types of secretory
cells: chief cells,
parietal cells, and
epithelial cells that
secrete mucus.
Mucus protects the
tissues from the acids
and enzymes.
How Does the Vertebrate Gastrointestinal System Function?
Chief cells secrete pepsinogen, the inactive form or
zymogen of the protease pepsin.
The low pH of the stomach
converts it to the active form.
Pepsin activates other
pepsinogen molecules—a
process called autocatalysis.
Parietal cells secrete HCl and
keep the stomach pH below 1.
How Does the Vertebrate Gastrointestinal System Function?
Segmentation movements: movements of the stomach
and small intestine are not as coordinated.
Segments of the gut periodically contract but do not
generate a peristaltic wave of contraction that moves the
food in one direction.
Food moves back and forth and mixes with digestive
juices.
How Does the Vertebrate Gastrointestinal System Function?
Chyme is a mixture of gastric juices and partly digested
food.
The stomach walls contract and move chyme to the
bottom of the stomach.
The pyloric sphincter allows small amounts to enter the
small intestine.
Most chemical digestion occurs in the small intestine.
The small intestine has three sections:
• Duodenum—the initial section and site of most digestion
• Jejunum and ileum—carry out most absorption.
How Does the Vertebrate Gastrointestinal System Function?
In humans, the gut wall has folds with finger-like projections
called villi.
Surface cells of villi have smaller projections called
microvilli.
The microvilli give the intestine an enormous surface area for
absorbing nutrients.
Ducts of the Gallbladder and Pancreas
The liver synthesizes bile salts from cholesterol
and secretes them as bile.
Bile flows through the
hepatic duct to the
duodenum and through
the cystic duct to the
gallbladder where bile is
stored.
Bile salts emulsify fats in
the chyme.
Lipophilic ends of molecules merge with fat droplets and
keep them from sticking together, forming micelles.
How Does the Liver Work?
Absorbed nutrients go to the liver.
Blood leaving the digestive tract goes to the liver via the
hepatic portal vein.
The liver controls fat metabolism by lipoprotein production.
Lipoproteins transport fats in the blood.
• High-density lipoproteins (HDLs) remove cholesterol
from tissue and carry it to liver
• Low-density lipoproteins (LDLs) transport cholesterol in
body
• Very-low-density lipoproteins (VLDLs) transport
triglycerides to fat cells.
How Does the Pancreas Work?
The pancreas is both an endocrine and exocrine gland.
Exocrine: it secretes digestive juices to the duodenum via
the pancreatic duct.
In the duodenum, the zymogen trypsinogen is activated by
enterokinase to produce the active protease trypsin.
Trypsin can activate other zymogens.
The pancreas also secretes HCO3– to neutralize chyme in
the intestine.
Major Digestive Enzymes of Humans
In the small intestine, epithelial cells secrete various
enzymes to cleave peptides, disaccharides, and lipids.
Many humans stop
producing lactase after
childhood, and cannot
digest lactose.
The lactose is
metabolized by bacteria
in the large intestine,
causing gas, diarrhea,
and cramps.
How Does the Vertebrate Gastrointestinal System Function?
Contents of the small intestine pass into the large intestine,
or colon.
The colon absorbs water and ions and produces feces.
Too much water absorption in the colon leads to
constipation; too little leads to diarrhea.
How Is the Flow of Nutrients Controlled and Regulated?
Digestion is governed by neuronal and hormonal controls.
Many autonomic reflexes coordinate activity in different
regions of the digestive tract.
The enteric
nervous system
coordinates
movement of
food; it also
exchanges
information with
the CNS.
How Is the Flow of Nutrients Controlled and Regulated?
Digestive hormones:
• Secretin from the duodenum causes the pancreas to
secrete bicarbonate ions.
• Cholecystokinin causes the gallbladder to release bile,
stimulates the pancreas, and slows stomach action.
• Gastrin is released by the stomach, stimulating stomach
movements and secretion of digestive juices.
How Is the Flow of Nutrients Controlled and Regulated?
Insulin is released by the
pancreas during the absorptive
period, when blood glucose
rises.
Insulin promotes uptake and
utilization of glucose for
metabolic activities, or synthesis
of glycogen and fat.
If blood glucose level is very
low, glucagon is released and
causes the liver to break down
glycogen and begin
gluconeogenesis.
What Are the Major Diseases?
Nutrient deficiency leads to malnutrition; chronic
malnutrition leads to deficiency diseases:
• Scurvy—lack of vitamin C
• Beriberi—lack of thiamin (vitamin B1); this disease led to
the discovery of vitamins
• In pernicious anemia, vitamin B12 (cobalamin) is not
absorbed in the stomach.
Mineral deficiencies can also lead to disease:
• Iodine deficiency leads to hypothyroidism and goiter
• Iron deficiency leads to anemia.
How Do Animals Ingest and Digest Food?
Heterotrophs acquire nutrition in different ways:
• Saprobes absorb nutrients from dead organic matter (e.g.,
protists and fungi).
• Detritivores or decomposers actively feed on dead organic
matter.
• Predators feed on living organisms.
Digestion usually begins in a body
cavity.
Gastrovascular cavities connect to the
outside through a single opening—
jellyfish and other cnidarians.
Tubular guts have an opening at each
end. A mouth takes in food, and wastes
are eliminated through the anus.
How Do Animals Ingest and Digest Food?
Food is broken up in the mouth cavity
by teeth, radula (snails), or
mandibles (arthropods).
Most birds have gizzards with small
stones for grinding food.
Food particles move from the
stomach into the intestines.
Most digestion occurs in the
intestine; nutrients, water, and ions
are absorbed across its walls.
The last segment recovers ions and
water and stores undigested waste
as feces.
A muscular rectum expels feces.
Stomachs and crops are storage
chambers that allow for gradual
digestion.
How Do Animals Ingest and Digest Food?
Surface area is increased in the parts of the gut that
absorb nutrients.
The earthworm gut has an infolding of the gut wall, or
typhlosole.
Sharks have a spiral valve—walls of the spiral have a
large surface area.
Adapted from
Life: The Science of Biology, Tenth Edition, Sinauer Associates, Sunderland, MA, 2014
Inc. All rights reserved