Digestion – Gastrointestinal System
38. General principles of gastrointestinal function. Motility. Enteric nervous system.
Autonomic and hormonal control of the gastrointestinal tract.
39. Mastication and swallowing. Motor function of the stomach. Regulation.
40. Movements of the small intestine. Movements of the colon. Defecation.
41. Secretion of the saliva. Composition and function of the saliva. Regulation of salivary secretion.
42. Gastric secretion. Regulation of gastric secretion.
43. Pancreatic secretion. Regulation.
44. Secretion of the bile by the liver. Secretion of the small intestine. Regulation.
45. Basic principles of gastrointestinal absorption. Absorption of water, ions and vitamins.
46. Digestion and absorption of proteins in the gastrointestinal tract.
47. Digestion and absorption of fats in the gastrointestinal tract.
48. Digestion and absorption of carbohydrates in the gastrointestinal tract.
49. Liver. Metabolic functions of the liver. Excretion of bilirubin in the bile.
50. Dietary balances. Energy available in foods. Daily requirements for proteins, fats and
carbohydrates. Energetics and metabolic rate.
51. Vitamins and minerals. Functions and daily requirements. Trace elements.
52. Carbohydrate metabolism. General role of glucose in carbohydrate metabolism. Storage of glycogen
in liver and muscle (glycogenesis). Glycogenolysis. Gluconeogenesis. Hormonal regulation.
53. Protein metabolism. Hormonal regulation of protein metabolism.
54. Lipid metabolism. Regulation of fat utilization.
55. Body temperature. Heat production and heat loss.
Regulation of body temperature -role of the hypothalamus.
38. General principles of gastrointestinal function. Motility. Enteric nervous system.
Automatic and hormonal control of the gastrointestinal tract.
The alimentary tract provides the body with a continual supply of
water, electrolytes, vitamins, and nutrients. To achieve this
requires (1) movement of food through the alimentary tract; (2)
secretion of digestive juices and digestion of the food; (3)
absorption of water, various electrolytes, vitamins, and digestive
products; (4) circulation of blood through the gastrointestinal
organs to carry away the absorbed substances; and (5) control of
all these functions by local, nervous, and hormonal systems.
Each part of the alimentary tract is adapted to its specific
functions: some to simple passage of food, such as the
oesophagus; others to temporary storage of food, such as the
stomach; and others to digestion and absorption, such as the small
intestine. In this chapter, we discuss the basic principles of
function in the entire alimentary tract; in the following chapters,
we discuss the specific functions of different segments of the tract.
General Principles of Gastrointestinal Motility
Motilin is secreted by the stomach and upper duodenum during
fasting, and the only known function of this hormone is to increase
gastrointestinal motility. Motilin is released cyclically and
stimulates waves of gastrointestinal motility called interdigestive myoelectric complexes that move through the
stomach and small intestine every 90 minutes in a fasted person. Motilin secretion is inhibited after ingestion by
mechanisms that are not fully understood.
Two types of movements occur in the gastrointestinal tract: (1) propulsive movements, which cause food to move
forward along the tract at an appropriate rate to accommodate digestion and absorption, and (2) mixing movements,
which keep the intestinal contents thoroughly mixed at all times. The basic propulsive movement of the
gastrointestinal tract is peristalsis, a contractile ring appears around the gut and then moves forward; this is analogous
to putting one's fingers around a thin distended tube, then constricting the fingers and sliding them forward along the
tube. Any material in front of the contractile ring is moved forward. due to syncytial smooth muscle tubes; stimulation
1
,at any point in the gut can cause a contractile ring to appear in the circular muscle, and this ring then spreads along the
gut tube. Mixing movements differ in different parts of the alimentary tract. In some areas, the peristaltic contractions
themselves cause most of the mixing.
Neural Control of Gastrointestinal Function-Enteric Nervous System
The gastrointestinal tract has a nervous system
all its own called the enteric nervous system. It
lies entirely in the wall of the gut, beginning in
the oesophagus and extending all the way to
the anus. The number of neurons in this enteric
system is about 100 million, almost exactly
equal to the number in the entire spinal cord.
This highly developed enteric nervous system
is especially important in controlling
gastrointestinal movements and secretion. The
enteric nervous system is composed mainly of
two plexuses, an outer plexus lying between
the longitudinal and circular muscle layers,
called the myenteric plexus or Auerbach's
plexus, and (2) an inner plexus, called the
submucosal plexus or Meissner's plexus, that
lies in the submucosa. The nervous
connections within and between these two
plexuses are also shown in Figure 62-4.
The myenteric plexus controls mainly the gastrointestinal movements, and the submucosal plexus controls mainly
gastrointestinal secretion and local blood flow. The extrinsic sympathetic and parasympathetic fibres that connect to
both the myenteric and submucosal plexuses. Although the enteric nervous system can function independently of these
extrinsic nerves, stimulation by the parasympathetic and sympathetic systems can greatly enhance or inhibit
gastrointestinal functions, as we discuss later. The submucosal plexus, in contrast to the myenteric plexus, is mainly
concerned with controlling function within the inner wall of each minute segment of the intestine. For instance, many
sensory signals originate from the gastrointestinal epithelium and are then integrated in the submucosal plexus to help
control local intestinal secretion, local absorption, and local contraction of the submucosal muscle that causes various
degrees of infolding of the gastrointestinal mucosa.
Types of Neurotransmitters Secreted by Enteric Neurons
In an attempt to understand better the multiple functions of the gastrointestinal enteric nervous system. Two of them
with which we are already familiar are (1) acetylcholine and (2) norepinephrine. Others are (3) adenosine
triphosphate, (4) serotonin, (5) dopamine, (6) cholecystokinin, (7) substance P, (8) vasoactive
intestinal polypeptide, (9) somatostatin, (10) leu-enkephalin, (11) met-enkephalin, and (12) bombesin. The specific
functions of many of these are not known well enough to justify discussion here, other than to point out the following.
Autonomic Control of the Gastrointestinal Tract
Parasympathetic Stimulation Increases Activity of the Enteric Nervous System
Except for a few parasympathetic fibres to the mouth and pharyngeal regions of the alimentary tract, the cranial
parasympathetic nerve fibres are almost entirely in the vagus nerves. These fibres provide extensive innervation to the
oesophagus, stomach, and pancreas and somewhat less to the intestines down through the first half of the large
intestine. The sacral parasympathetics originate in the second, third, and fourth sacral segments of the spinal cord and
pass through the pelvic nerves to the distal half of the large intestine and all the way to the anus. The sigmoidal, rectal,
and anal regions are considerably better supplied with parasympathetic fibres than are the other intestinal areas. These
fibres function specially to execute the defecation reflexes. The postganglionic neurons of the gastrointestinal
parasympathetic system are located mainly in the myenteric and submucosal plexuses. Stimulation of these
parasympathetic nerves causes general
increase in activity of the entire enteric nervous system. This in turn enhances activity of most gastrointestinal
functions.
Hormonal Control of Gastrointestinal Motility
The gastrointestinal hormones are released into the portal circulation and exert physiological actions on target cells
with specific receptors for the hormone. The effects of the hormones persist even after all nervous connections
between the site of release and the site of action have been severed. Table 62-1 outlines the actions of each
gastrointestinal hormone, as well as the stimuli for secretion and sites at which secretion takes place.
2
, 39. Mastication and swallowing. Motor function of the stomach. Regulation.
Mastication (Chewing)
Most of the muscles of chewing are innervated by the motor branch of the fifth cranial nerve, and the chewing
process is controlled by nuclei in the brain stem. Stimulation of specific reticular areas in the brain stem taste
centres will cause rhythmical chewing movements. Also, stimulation of areas in the hypothalamus, amygdala,
and even the cerebral cortex near the sensory areas for taste and smell can often cause chewing. Much of the
chewing process is caused by a chewing reflex. The presence of a bolus of food in the mouth at first initiates
reflex inhibition of the muscles of mastication, which allows the lower jaw to drop. The drop-in turn initiates a
stretch reflex of the jaw muscles that leads to rebound contraction. This automatically raises the jaw to cause
closure of the teeth, but it also compresses the bolus again against the linings of the mouth, which inhibits the jaw
muscles once again, allowing the jaw to drop and rebound another time; this is repeated again and again.
Chewing is important for digestion of all foods, but especially important for most fruits and raw vegetables
because these have indigestible cellulose membranes around their nutrient portions that must be broken before
the food can be digested. Also, chewing aids the digestion of food for still another simple reason: Digestive
enzymes act only on the surfaces of food particles; therefore, the rate of digestion is absolutely dependent on the
total surface area exposed to the digestive secretions. In addition, grinding the food to a very fine particulate
consistency prevents excoriation of the gastrointestinal tract and increases the ease with which food is emptied
from the stomach into the small intestine, then into all succeeding segments of the gut.
Swallowing (Deglutition)
Swallowing is a complicated mechanism, principally because
the pharynx sub serves respiration and swallowing. The
pharynx is converted for only a few seconds at a time into a
tract for propulsion of food. It is especially important that
respiration not be compromised because of swallowing. In
general, swallowing can be divided into (1) a voluntary stage,
which initiates the swallowing process; (2) a pharyngeal stage,
which is involuntary and constitutes passage of food through
the pharynx into the oesophagus; and (3) an oesophageal stage,
another involuntary phase that transports food from the pharynx
to the stomach.
Voluntary Stage of Swallowing
When the food is ready for swallowing, it is "voluntarily"
squeezed or rolled posteriorly into the pharynx by pressure of
the tongue upward and backward against the palate, as shown
in Figure 63-1. From here on, swallowing becomes entirely-or
almost entirely-automatic and ordinarily cannot be stopped.
Pharyngeal Stage of Swallowing
As the bolus of food enters the posterior mouth and pharynx, it stimulates epithelial swallowing receptor areas all
around the opening of the pharynx, especially on the tonsillar pillars, and impulses from these pass to the brain stem to
initiate a series of automatic pharyngeal muscle contractions. The trachea is closed, the oesophagus is opened,
and a fast peristaltic wave initiated by the nervous system of the pharynx forces the bolus of food into the
upper oesophagus, the entire process occurring in less than 2 seconds.
Nervous Initiation of the Pharyngeal Stage of Swallowing
The most sensitive tactile areas of the posterior mouth and pharynx for initiating the pharyngeal stage of
swallowing lie in a ring around the pharyngeal opening, with greatest sensitivity on the tonsillar pillars.
Impulses are transmitted from these areas through the sensory portions of the trigeminal and
glossopharyngeal nerves into the medulla oblongata, either into or closely associated with the tractus
solitarius, which receives essentially all sensory impulses from the mouth. In summary, the pharyngeal
stage of swallowing is principally a reflex act. It is almost always initiated by voluntary movement of food
into the back of the mouth, which in turn excites involuntary pharyngeal sensory receptors to elicit the
swallowing reflex.
Oesophageal Stage of Swallowing
3
, The oesophagus functions primarily to conduct food
rapidly from the pharynx to the stomach, and its
movements are organized specifically for this function. The
oesophagus normally exhibits two types of peristaltic
movements: primary peristalsis and secondary
peristalsis. Primary peristalsis is simply continuation of
the peristaltic wave that begins in the pharynx and spreads
into the oesophagus during the pharyngeal stage of
swallowing. This wave passes all the way from the pharynx
to the stomach in about 8 to 10 seconds. Food swallowed
by a person who is in the upright position is usually
transmitted to the lower end of the oesophagus even more
rapidly than the peristaltic wave itself, in about 5 to 8
seconds, because of the additional effect of gravity pulling
the food downward.
Motor Functions of the Stomach
The motor functions of the stomach are threefold: (1) storage of large quantities of food until the food can be
processed in the stomach, duodenum, and lower intestinal tract; (2) mixing of this food with gastric
secretions until it forms a semifluid mixture called chyme; and (3) slow emptying of the chyme from the
stomach into the small intestine at a rate suitable for proper digestion and absorption by the small intestine.
Anatomically, the stomach is usually divided into two major parts: (1) the body and (2) the antrum.
Physiologically, it is more appropriately divided into (1) the "orad" portion, comprising about the first two
thirds of the body, and (2) the "caudad" portion, comprising the remainder of the body plus the antrum.
Regulation???
40. Movement of the small intestine. Movement of the colon. Defecation.
Movements of the Small Intestine
The movements of the small intestine, like those elsewhere in
the gastrointestinal tract, can be divided into mixing
contractions and propulsive contractions. To a great extent,
this separation is artificial because essentially all movements
of the small intestine cause at least some degree of both
mixing and propulsion. The usual classification of these
processes is the following.
Mixing Contractions (Segmentation Contractions)
When a portion of the small intestine becomes distended with
chyme, stretching of the intestinal wall elicits localized
concentric contractions spaced at intervals along the intestine
and lasting a fraction of a minute.
The contractions cause "segmentation" of the small intestine, as shown in Figure 63-3. That is, they divide the
intestine into spaced segments that have the appearance of a chain of sausages. As one set of segmentation
contractions relaxes, a new set often begins, but the contractions this time occur mainly at new points between the
previous contractions. Therefore, the segmentation contractions "chop" the chyme two to three times per minute, in
this way promoting progressive mixing of the food with secretions of the small intestine.
The maximum frequency of the segmentation contractions in the small intestine is determined by the frequency of
electrical slow waves in the intestinal wall, which is the basic electrical rhythm described in Chapter 62. Because this
frequency normally is not over 12 per minute in the duodenum and proximal jejunum, the maximum frequency of the
segmentation contractions in these areas is also about 12 per minute, but this occurs only under extreme conditions of
stimulation.
4
38. General principles of gastrointestinal function. Motility. Enteric nervous system.
Autonomic and hormonal control of the gastrointestinal tract.
39. Mastication and swallowing. Motor function of the stomach. Regulation.
40. Movements of the small intestine. Movements of the colon. Defecation.
41. Secretion of the saliva. Composition and function of the saliva. Regulation of salivary secretion.
42. Gastric secretion. Regulation of gastric secretion.
43. Pancreatic secretion. Regulation.
44. Secretion of the bile by the liver. Secretion of the small intestine. Regulation.
45. Basic principles of gastrointestinal absorption. Absorption of water, ions and vitamins.
46. Digestion and absorption of proteins in the gastrointestinal tract.
47. Digestion and absorption of fats in the gastrointestinal tract.
48. Digestion and absorption of carbohydrates in the gastrointestinal tract.
49. Liver. Metabolic functions of the liver. Excretion of bilirubin in the bile.
50. Dietary balances. Energy available in foods. Daily requirements for proteins, fats and
carbohydrates. Energetics and metabolic rate.
51. Vitamins and minerals. Functions and daily requirements. Trace elements.
52. Carbohydrate metabolism. General role of glucose in carbohydrate metabolism. Storage of glycogen
in liver and muscle (glycogenesis). Glycogenolysis. Gluconeogenesis. Hormonal regulation.
53. Protein metabolism. Hormonal regulation of protein metabolism.
54. Lipid metabolism. Regulation of fat utilization.
55. Body temperature. Heat production and heat loss.
Regulation of body temperature -role of the hypothalamus.
38. General principles of gastrointestinal function. Motility. Enteric nervous system.
Automatic and hormonal control of the gastrointestinal tract.
The alimentary tract provides the body with a continual supply of
water, electrolytes, vitamins, and nutrients. To achieve this
requires (1) movement of food through the alimentary tract; (2)
secretion of digestive juices and digestion of the food; (3)
absorption of water, various electrolytes, vitamins, and digestive
products; (4) circulation of blood through the gastrointestinal
organs to carry away the absorbed substances; and (5) control of
all these functions by local, nervous, and hormonal systems.
Each part of the alimentary tract is adapted to its specific
functions: some to simple passage of food, such as the
oesophagus; others to temporary storage of food, such as the
stomach; and others to digestion and absorption, such as the small
intestine. In this chapter, we discuss the basic principles of
function in the entire alimentary tract; in the following chapters,
we discuss the specific functions of different segments of the tract.
General Principles of Gastrointestinal Motility
Motilin is secreted by the stomach and upper duodenum during
fasting, and the only known function of this hormone is to increase
gastrointestinal motility. Motilin is released cyclically and
stimulates waves of gastrointestinal motility called interdigestive myoelectric complexes that move through the
stomach and small intestine every 90 minutes in a fasted person. Motilin secretion is inhibited after ingestion by
mechanisms that are not fully understood.
Two types of movements occur in the gastrointestinal tract: (1) propulsive movements, which cause food to move
forward along the tract at an appropriate rate to accommodate digestion and absorption, and (2) mixing movements,
which keep the intestinal contents thoroughly mixed at all times. The basic propulsive movement of the
gastrointestinal tract is peristalsis, a contractile ring appears around the gut and then moves forward; this is analogous
to putting one's fingers around a thin distended tube, then constricting the fingers and sliding them forward along the
tube. Any material in front of the contractile ring is moved forward. due to syncytial smooth muscle tubes; stimulation
1
,at any point in the gut can cause a contractile ring to appear in the circular muscle, and this ring then spreads along the
gut tube. Mixing movements differ in different parts of the alimentary tract. In some areas, the peristaltic contractions
themselves cause most of the mixing.
Neural Control of Gastrointestinal Function-Enteric Nervous System
The gastrointestinal tract has a nervous system
all its own called the enteric nervous system. It
lies entirely in the wall of the gut, beginning in
the oesophagus and extending all the way to
the anus. The number of neurons in this enteric
system is about 100 million, almost exactly
equal to the number in the entire spinal cord.
This highly developed enteric nervous system
is especially important in controlling
gastrointestinal movements and secretion. The
enteric nervous system is composed mainly of
two plexuses, an outer plexus lying between
the longitudinal and circular muscle layers,
called the myenteric plexus or Auerbach's
plexus, and (2) an inner plexus, called the
submucosal plexus or Meissner's plexus, that
lies in the submucosa. The nervous
connections within and between these two
plexuses are also shown in Figure 62-4.
The myenteric plexus controls mainly the gastrointestinal movements, and the submucosal plexus controls mainly
gastrointestinal secretion and local blood flow. The extrinsic sympathetic and parasympathetic fibres that connect to
both the myenteric and submucosal plexuses. Although the enteric nervous system can function independently of these
extrinsic nerves, stimulation by the parasympathetic and sympathetic systems can greatly enhance or inhibit
gastrointestinal functions, as we discuss later. The submucosal plexus, in contrast to the myenteric plexus, is mainly
concerned with controlling function within the inner wall of each minute segment of the intestine. For instance, many
sensory signals originate from the gastrointestinal epithelium and are then integrated in the submucosal plexus to help
control local intestinal secretion, local absorption, and local contraction of the submucosal muscle that causes various
degrees of infolding of the gastrointestinal mucosa.
Types of Neurotransmitters Secreted by Enteric Neurons
In an attempt to understand better the multiple functions of the gastrointestinal enteric nervous system. Two of them
with which we are already familiar are (1) acetylcholine and (2) norepinephrine. Others are (3) adenosine
triphosphate, (4) serotonin, (5) dopamine, (6) cholecystokinin, (7) substance P, (8) vasoactive
intestinal polypeptide, (9) somatostatin, (10) leu-enkephalin, (11) met-enkephalin, and (12) bombesin. The specific
functions of many of these are not known well enough to justify discussion here, other than to point out the following.
Autonomic Control of the Gastrointestinal Tract
Parasympathetic Stimulation Increases Activity of the Enteric Nervous System
Except for a few parasympathetic fibres to the mouth and pharyngeal regions of the alimentary tract, the cranial
parasympathetic nerve fibres are almost entirely in the vagus nerves. These fibres provide extensive innervation to the
oesophagus, stomach, and pancreas and somewhat less to the intestines down through the first half of the large
intestine. The sacral parasympathetics originate in the second, third, and fourth sacral segments of the spinal cord and
pass through the pelvic nerves to the distal half of the large intestine and all the way to the anus. The sigmoidal, rectal,
and anal regions are considerably better supplied with parasympathetic fibres than are the other intestinal areas. These
fibres function specially to execute the defecation reflexes. The postganglionic neurons of the gastrointestinal
parasympathetic system are located mainly in the myenteric and submucosal plexuses. Stimulation of these
parasympathetic nerves causes general
increase in activity of the entire enteric nervous system. This in turn enhances activity of most gastrointestinal
functions.
Hormonal Control of Gastrointestinal Motility
The gastrointestinal hormones are released into the portal circulation and exert physiological actions on target cells
with specific receptors for the hormone. The effects of the hormones persist even after all nervous connections
between the site of release and the site of action have been severed. Table 62-1 outlines the actions of each
gastrointestinal hormone, as well as the stimuli for secretion and sites at which secretion takes place.
2
, 39. Mastication and swallowing. Motor function of the stomach. Regulation.
Mastication (Chewing)
Most of the muscles of chewing are innervated by the motor branch of the fifth cranial nerve, and the chewing
process is controlled by nuclei in the brain stem. Stimulation of specific reticular areas in the brain stem taste
centres will cause rhythmical chewing movements. Also, stimulation of areas in the hypothalamus, amygdala,
and even the cerebral cortex near the sensory areas for taste and smell can often cause chewing. Much of the
chewing process is caused by a chewing reflex. The presence of a bolus of food in the mouth at first initiates
reflex inhibition of the muscles of mastication, which allows the lower jaw to drop. The drop-in turn initiates a
stretch reflex of the jaw muscles that leads to rebound contraction. This automatically raises the jaw to cause
closure of the teeth, but it also compresses the bolus again against the linings of the mouth, which inhibits the jaw
muscles once again, allowing the jaw to drop and rebound another time; this is repeated again and again.
Chewing is important for digestion of all foods, but especially important for most fruits and raw vegetables
because these have indigestible cellulose membranes around their nutrient portions that must be broken before
the food can be digested. Also, chewing aids the digestion of food for still another simple reason: Digestive
enzymes act only on the surfaces of food particles; therefore, the rate of digestion is absolutely dependent on the
total surface area exposed to the digestive secretions. In addition, grinding the food to a very fine particulate
consistency prevents excoriation of the gastrointestinal tract and increases the ease with which food is emptied
from the stomach into the small intestine, then into all succeeding segments of the gut.
Swallowing (Deglutition)
Swallowing is a complicated mechanism, principally because
the pharynx sub serves respiration and swallowing. The
pharynx is converted for only a few seconds at a time into a
tract for propulsion of food. It is especially important that
respiration not be compromised because of swallowing. In
general, swallowing can be divided into (1) a voluntary stage,
which initiates the swallowing process; (2) a pharyngeal stage,
which is involuntary and constitutes passage of food through
the pharynx into the oesophagus; and (3) an oesophageal stage,
another involuntary phase that transports food from the pharynx
to the stomach.
Voluntary Stage of Swallowing
When the food is ready for swallowing, it is "voluntarily"
squeezed or rolled posteriorly into the pharynx by pressure of
the tongue upward and backward against the palate, as shown
in Figure 63-1. From here on, swallowing becomes entirely-or
almost entirely-automatic and ordinarily cannot be stopped.
Pharyngeal Stage of Swallowing
As the bolus of food enters the posterior mouth and pharynx, it stimulates epithelial swallowing receptor areas all
around the opening of the pharynx, especially on the tonsillar pillars, and impulses from these pass to the brain stem to
initiate a series of automatic pharyngeal muscle contractions. The trachea is closed, the oesophagus is opened,
and a fast peristaltic wave initiated by the nervous system of the pharynx forces the bolus of food into the
upper oesophagus, the entire process occurring in less than 2 seconds.
Nervous Initiation of the Pharyngeal Stage of Swallowing
The most sensitive tactile areas of the posterior mouth and pharynx for initiating the pharyngeal stage of
swallowing lie in a ring around the pharyngeal opening, with greatest sensitivity on the tonsillar pillars.
Impulses are transmitted from these areas through the sensory portions of the trigeminal and
glossopharyngeal nerves into the medulla oblongata, either into or closely associated with the tractus
solitarius, which receives essentially all sensory impulses from the mouth. In summary, the pharyngeal
stage of swallowing is principally a reflex act. It is almost always initiated by voluntary movement of food
into the back of the mouth, which in turn excites involuntary pharyngeal sensory receptors to elicit the
swallowing reflex.
Oesophageal Stage of Swallowing
3
, The oesophagus functions primarily to conduct food
rapidly from the pharynx to the stomach, and its
movements are organized specifically for this function. The
oesophagus normally exhibits two types of peristaltic
movements: primary peristalsis and secondary
peristalsis. Primary peristalsis is simply continuation of
the peristaltic wave that begins in the pharynx and spreads
into the oesophagus during the pharyngeal stage of
swallowing. This wave passes all the way from the pharynx
to the stomach in about 8 to 10 seconds. Food swallowed
by a person who is in the upright position is usually
transmitted to the lower end of the oesophagus even more
rapidly than the peristaltic wave itself, in about 5 to 8
seconds, because of the additional effect of gravity pulling
the food downward.
Motor Functions of the Stomach
The motor functions of the stomach are threefold: (1) storage of large quantities of food until the food can be
processed in the stomach, duodenum, and lower intestinal tract; (2) mixing of this food with gastric
secretions until it forms a semifluid mixture called chyme; and (3) slow emptying of the chyme from the
stomach into the small intestine at a rate suitable for proper digestion and absorption by the small intestine.
Anatomically, the stomach is usually divided into two major parts: (1) the body and (2) the antrum.
Physiologically, it is more appropriately divided into (1) the "orad" portion, comprising about the first two
thirds of the body, and (2) the "caudad" portion, comprising the remainder of the body plus the antrum.
Regulation???
40. Movement of the small intestine. Movement of the colon. Defecation.
Movements of the Small Intestine
The movements of the small intestine, like those elsewhere in
the gastrointestinal tract, can be divided into mixing
contractions and propulsive contractions. To a great extent,
this separation is artificial because essentially all movements
of the small intestine cause at least some degree of both
mixing and propulsion. The usual classification of these
processes is the following.
Mixing Contractions (Segmentation Contractions)
When a portion of the small intestine becomes distended with
chyme, stretching of the intestinal wall elicits localized
concentric contractions spaced at intervals along the intestine
and lasting a fraction of a minute.
The contractions cause "segmentation" of the small intestine, as shown in Figure 63-3. That is, they divide the
intestine into spaced segments that have the appearance of a chain of sausages. As one set of segmentation
contractions relaxes, a new set often begins, but the contractions this time occur mainly at new points between the
previous contractions. Therefore, the segmentation contractions "chop" the chyme two to three times per minute, in
this way promoting progressive mixing of the food with secretions of the small intestine.
The maximum frequency of the segmentation contractions in the small intestine is determined by the frequency of
electrical slow waves in the intestinal wall, which is the basic electrical rhythm described in Chapter 62. Because this
frequency normally is not over 12 per minute in the duodenum and proximal jejunum, the maximum frequency of the
segmentation contractions in these areas is also about 12 per minute, but this occurs only under extreme conditions of
stimulation.
4
