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Flashcards in Chaper 21 Deck (246):
1

Opening

Fistula

2

The digestive system is also referred to as?

The gastrointestinal system

3

Long tube passing through the body. The tube has muscular walls lined with epithelium and is closed off by a skeletal muscle sphincter at each end.

The gastrointestinal tract

4

The primary function of the GI tract

To move nutrients, water, electrolytes from the external environment into the body's internal environment

5

Autodigestion

When enzymes digest the cells of the GI tract itself

6

What must our digestive system do?

Proteins and complex carbohydrates, so our digestive systems must secrete powerful enzymes to digest food into molecules that are small enough to be absorbed into the body. At the same time these enzymes must not digest the cells of the GI tract itself

7

What happens if protective mechanisms against auto digestion fail?

We may develop raw patches known as peptic ulcers on the walls of the GI tract

8

Another challenge the digestive system faces daily is mass balance which is?

Matching fluid input with output

9

To maintain homeostasis, the volume of fluid entering the GI tract by intake or secretion must?

Equal the volume leaving the lumen

10

People ingest about 2 liters of fluid a day. In addition, the exocrine glands and cells secrete?

7 liters or so of enzymes, mucus, electrolytes, and water into the lumen of the GI tract. That volume of secreted fluid is the equivalent of one-sixth of the body's total body water (42 liters), or more than twice the plasma volume of 3 liters, and it must be reabsorbed or the body would rapidly dehydrate

11

Normally reabsorption is very efficient, and only about _____ mL of fluid is lost in the feces

100 mL

12

Vomiting and diarrhea can become an emergency when?

GI secretions that would normally be reabsorbed are lost to the environment

13

A final challenge the digestive system faces is repelling foreign invaders. It is counterintuitive, but the largest area of contact between the internal environment and the outside world is?

In the lumen of the digestive system. And as a result, the GI tract with a total surface area about the size of a tennis court, faces daily conflict between the need to absorb water and nutrients, and the need to keep bacteria, viruses, and other pathogens from entering the body.

14

Because of the large surface area of the GI tract and the need to keep foreign invaders out, the epithelium of the GI tract is assisted by?

An array of physiological defense mechanisms, including mucus, digestive enzymes, acid, and the largest collection of lymphoid tissue in the body, the gut-associated lymphoid tissue (GALT)

15

By one estimate ____% of all lymphocytes in the body are found in the small intestine

80%

16

How does the body meet the sometimes conflicting physiological challenges it faces?

By coordinating the four basic processes of the digestive system:
1) Digestion
2) Absorption
3) Motility
4) Secretion

17

Functions of the Digestive System

1) Motility
2) Secretion
3) Digestion
4) Absorption
5) Storage & elimination

18

What is digestion?

Chemical and mechanical breakdown of food into absorbable units

19

What is absorption?

Movement of material from GI lumen to ECF

20

What is motility?

Movement of material through the GI tract as a result of muscle contraction

21

What is secretion?

Movement of material from cells into lumen or ECF

22

Chemical and mechanical breakdown of foods into smaller units that can be taken across the intestinal epithelium into the body

Digestion

23

Active process or passive transfer of substances from the lumen of the GI tract to the extracellular fluid

Absorption

24

Refers to the transepithelial transfer of water & ions from the ECF to the digestive tract lumen as well as to the release of substances synthesized by GI epithelial cells

Secretion

25

Motility and secretion are continuously regulated to maximized?

The availability of absorbable material

26

Motility is regulated because?

If food moves through the system too rapidly, there is not enough time for everything in the lumen to be digested and absorbed

27

Secretion is regulated because?

If digestive enzymes are not secreted in adequate amounts, food in the GI tract cannot be broken down into an absorbable form

28

When digested nutrients have been absorbed and have reached the body's cells what happens?

Cellular metabolism directs their use or storage

29

Some of the same chemical signal molecules that alter digestive motility and secretion also participate in?

The control of metabolism, providing an integrating link between the two steps

30

The digestive system begins with the?

Oral cavity (mouth and pharynx), which serves as a receptacle for food

31

In the oral cavity the first stages of digestion begin with?

Chewing & the secretion of saliva by three pairs of salivary glands: sublingual glands under the tongue, submandibular glands under the mandible (jawbone), and parotid glands lying near the hinge of the jaw

32

Once swallowed, food moves into the GI tract. At intervals along the tract there are?

Rings of muscle function as sphincters to separate the tube into segments with distinct functions

33

How does food move through the tract?

Propelled by waves of muscle contraction

34

Along the GI tract secretions are added to food by?

Secretory epithelium, the liver, and the pancreas, creating a soupy mixture known as chyme

35

Where does digestion take place?

Primarily in the lumen of the tube

36

The products of digestion are absorbed across the?

Epithelium and pass into the extracellular compartment. From there, they move into the blood or lymph for distribution throughout the body. Any waste remaining in the lumen at the end of the GI tract leaves the body through the opening known as the anus

37

What is the difference between digestion and metabolism?

Digestion takes place in the GI tract lumen, which is external to the body; metabolism takes place int he body's internal environment

38

What is the difference between absorption and secretion?

Absorption moves material from the GI lumen into the ECF; secretion moves substances from the cells or the ECF into the lumen

39

Narrow tube that travels through the thorax to the abdomen

Esophagus

40

When you swallow food it passes into the?

Esophagus

41

The esophageal walls are?

Skeletal muscle initially but transition to smooth muscle about two-thirds of the way down the length

42

Just below the diaphragm, the esophagus ends at the?

Stomach, a bag-like organ that can hold as much as 2 liters of food and fluid when fully (if uncomfortably) expanded

43

The stomach is divided into 3 sections

1) The upper fundus
2) The central body
3) The lower antrum

44

The stomach continues digestion that began in the mouth by?

Mixing food with acid and enzymes to create chyme

45

The pylorus (gatekeeper) or opening between the stomach and the small intestine is guarded by the?

Pyloric valve. This thickened band of smooth muscle relaxes to allow only small amounts of chyme into the small intestine at any one time

46

The stomach acts as an intermediary between the?

Behavioral act of eating and the physiological events of digestion and absorption int he intestine

47

Integrated signals and feedback loops between the intestine and stomach regulate?

The rate at which chyme enters the duodenum. This ensures that the intestine is not overwhelmed with more than it can digest and absorb

48

Most digestion takes place in the?

Small intestine

49

The small intestine is divided into 3 sections

1) The duodenum (the first 25 cm)
2) The jejunum
3) The ileum (the latter two together are about 260 cm long)

50

Digestion is carried out by?

Intestinal enzymes, aided by exocrine secretions from two accessory glandular organs: the pancreas and the liver

51

Secretions from the pancreas and liver enters the?

Initial section of the duodenum through ducts. A tonically contracted sphincter (the sphincter of Oddi) keeps pancreatic fluid and bile from entering the small intestine except during a meal

52

Digestion is essential completed in the?

Small intestine, and nearly all digested nutrients and secreted fluids are absorbed there, leaving about 1.5 liters of chyme per day to pass into the large intestine

53

The proximal section of the large intestine

The colon

54

What happens in the colon?

Watery chyme is converted into semisolid feces as water and electrolytes are absorbed out of the chyme and into the ECF

55

The terminal section of the large intestine

The rectum

56

What happens when feces are propelled into the rectum?

Distension of the rectal wall triggers a defecation reflex. Feces leave the GI tract through the anus, with its external sphincter of skeletal muscle, which is under voluntary control

57

The portion of the GI tract running from the stomach to the anus is collectively called the?

Gut

58

In a living person, the digestive system from mouth to anus is about?

450 cm (nearly 15 feet) long. Of this length, 395 cm (about 13 feet) consists of the large and small intestines

59

The GI tract wall consists of 4 layers

1) An inner mucosa facing the lumen
2) Submucosa
3) Layers of smooth muscle known collectively as muscular is externa
4) A covering of connective tissue called serosa

60

The mucosa, the inner lining of the GI tract is created from?

1) A single layer of epithelial cells
2) The lamina propria, subepithelial connective tissue that holds the epithelium in place
3) The muscarlaris mucosae, a thin layer of smooth muscle

61

The entire wall is crumpled into folds called ______ in the stomach, and ______ in the small intestine

Rugae

62

The intestinal mucosal also projects into the lumen in small fingerlike extensions known as?

Villi

63

Some of the deepest invaginations form?

Gastric glands

64

The most variable feature of the GI tract that changes from section to section

Epithelial cells

65

The epithelial cells of the GI tract include?

1) Transporting epithelial cells (called enterocytes in the small intestine)
2) Endocrine and exocrine secretory cells
3) Stem cells

66

Transporting epithelial cells secrete?

Ions & water into the lumen, and absorb ions, water, and nutrients into the ECF

67

At the mucosal (apical) surface, secretory cells release?

Enzymes, mucus, and paracrine molecules into the lumen

68

At the serosal (basolateral) surface, secretory cells secrete?

Hormones into the blood or paracrine messengers into the interstitial fluid, where they act on neighboring cells

69

The cell-cell junctions that tie epithelial cells together?

Vary

70

In the stomach and colon, the cell-cell junctions form?

A tight barrier so that little can pass between the cells.

71

In the small intestine the junctions are?

Not as tight as they are in the stomach and colon.

72

The intestinal epithelium is considered leaky because?

Some water & solutes can be absorbed between the cells (paracellular pathway) instead of through them. These junctions have plasticity and that their "tightness" and selectivity can be regulated to some extent

73

The GI stem cells are?

Rapidly dividing, undifferentiated cells that continuously produce new epithelium in the crypts and gastric glands

74

As stem cells of the GI tract divide what happens?

The newly formed cells are pushed toward the luminal surface of the epithelium.

75

The average life span of the GI epithelial cells is?

Only a few days, a good indicator of the rough life such cells lead

76

As with other types of epithelium, the rapid turnover & cell division rate in the GI tract make these organs susceptible to?

Developing cancer

77

The lamina proporia is?

Subepithelial connective tissue that contains nerve fibers and small blood and lymph vessels. Also contains wandering immune cells, such as macrophages and lymphocytes, patrolling for invaders that enter through breaks in the epithelium

78

Absorbed nutrients pass into the blood and lymph here

Lamina proporia

79

In the intestine, collections of lymphoid tissue adjoining the epithelium form?

Small nodules and larger Peyer's patches that create visible bumps in the mucosa. These lymphoid aggregations are a major part of the gut-assoiciated lymphoid tissue (GALT)

80

The third region of the mucosa, the muscularis mucosae, separates the?

Mucosa from the submucosa

81

The muscularis mucosa is a?

Thin layer of smooth muscle, and contraction of this layer alters the effective surface area for absorption by moving the villi back and forth

82

The submucosa is

The layer of the gut wall adjacent to the mucosa. Composed of connective tissue with larger blood and lymph vessels

83

The submucosa also contains the submucosal plexus which is?

On of the two major nerve networks of the enteric nervous system. Innervates cells in the epithelial layer as well as smooth muscle of the muscularis mucosae

84

The enteric nervous system helps coordinate?

Digestive function

85

The outer layer of the GI tract, the muscularis externa, consists of?

Primarily two layers of smooth muscle: an inner circular layer and an outer longitudinal layer

86

Contraction of the muscularis externa inner circular layer does what?

Decreases the diameter of the lumen

87

Contraction of the muscularis externa outer longitudianl layer does what?

Shortens the tube

88

The second nerve network of the enteric nervous system, the myenteric plexus lies?

Between the longitudinal and circular layers. Controls & coordinates the motor activity of the muscularis externa

89

The outer covering of the entire digestive tract is?

The serosa, a connective tissue membrane that is continuation of the peritoneal membrane (peritoneum) lining the abdominal cavity

90

The peritoneum also forms sheets of?

Mesentery that hold the intestines in place so that they do not become tangled as they move

91

Is the lumen of the digestive tract on the apical or basolateral side of the intestinal epithelium? On the serosal or mucosal side?

The lumen of the digestive tract is on the apical or mucosal side of the intestinal epithelium

92

Name the 4 layers of the GI tract wall, starting at the lumen and moving out.

1) Mucosa
2) Submucosa
3) Muscularis externa
4) Serosa

93

Name the structures a piece of food passes through as it travels from mouth to anus

Mouth > Pharynx > Esophagus > Stomach (fundus, body, antrum) > Small intestine (duodenum, jejunum, ileum) > Large intestine (colon, rectum) > Anus

94

Why is the digestive system associated with the largest collection of lymphoid tissue in the body?

Because the GI tract has a large, vulnerable surface area facing the external environment, it needs the immune cells of lymphoid tissue to combat potential invaders

95

Motility in the gastrointestinal tract serves two purposes

Moving food from the mouth to the anus and mechanically mixing food to break it into uniformly small particles. This mixing maximizes exposure of the particles to digestive enzymes by increasing particle surface area

96

Gastrointestinal motility is determined by?

The properties of the tract's smooth muscle and modified by chemical input from nerves, hormones, & paracrine signals

97

A series of contractions that begin in the empty stomach and end in the large intestine

The migrating motor complex

98

Responsible for forward movement

Peristaltic contractions

99

Responsible for mixing

Segmental contractions

100

Spontaneous depolarizations in GI smooth muscle

Slow waves

101

Muscle contraction in the gastrointestinal tract occur in three general patterns

1) Peristaltic contractions
2) Segmental contractions
3) The migrating motor complex

102

The migrating motor complex is what type of function?

A housekeeping function that sweeps food remnants and bacteria out of the upper GI tract and into the large intestine

103

Peristalsis is?

Progressive waves of contraction that move from one section of the GI tract to the next

104

In peristalsis, circular muscles contract just behind a mass, or bolus, of food. This contraction does what?

Pushes the bolus forward into a receiving segment, where the circular muscles are relaxed. The receiving segment then contracts, continuing the forward movement

105

Peristalsis in the esophagus does what?

Propels material from pharynx to the stomach

106

Peristalsis in the stomach does what?

Contributes to food mixing in the stomach, but in normal digestion intestinal peristaltic waves are limited to short distances.

107

What influences peristalsis in all regions of the GI tract?

Hormones, paracrine signals, and the autonomic nervous system

108

What happens in segmental contractions

Short (1-5 cm) segments of intestine alternately contract and relax. In the contracting segments, circular muscles contract while longitudinal muscles relax. These contractions may occur randomly along the intestine or at regular intervals

109

What is the purpose of segmental contractions alternating contractions?

It churns the intestinal contents, mixing them and keeping them in contact sequentially, in an oral-to aboral direction, digested material is propelled short distances

110

Motility disorders are among the?

More common gastrointestinal problems. They range from esophageal spasms and delayed gastric (stomach) emptying to constipation and diarrhea

111

A chronic functional disorder characterized by altered bowel habits and abdominal pain

Irritable bowel syndrome

112

Most of the gastrointestinal tract is composed of?

Single-unit smooth muscle, with groups of cells electrically connected by gap-junctions to create contracting segments

113

Difference between tonic contractions and phasic contractions

Tonic contractions that are sustained for minutes or hours occur in some smooth muscle sphincters and int he anterior portion of the stomach.
Phasic contractions with contraction-relaxation cycles lasting only a few seconds, occur in the posterior region of the stomach and in the small intestine

114

What is slow wave potentials?

Cycles of smooth muscle contraction and relaxation are associated with spontaneous cycles of depolarization and depolarization

115

How do slow wave potentials differ from myocardial pacemaker potentials?

The GI waves have a much slower rate and do not reach threshold with each cycle

116

The likelihood of a slow wave firing an action potential depends on what?

Input from the enteric nervous system.

117

When a slow wave does reach threshold what happens?

Voltage-gated calcium channels in the muscle fiber open, calcium enters, and the cell fires one or more action potentials

118

Contraction of smooth muscle, like that of cardiac muscle, is graded according to what?

The amount of calcium that enters the fiber

119

The longer the duration of the slow wave, the longer the?

1) The more action potentials fire
2) The greater the contraction force in the muscle
3) The longer the duration of contraction

120

Both amplitude and duration of a slow wave can be modified how?

By neurotransmitters, hormones, or paracrine molecules

121

Slow wave frequency varies by region of the digestive tract, ranging from ___waves/min in the stomach to ___waves/min in the duodenum

3
12

122

Slow waves originate in a network of cells called the?

Interstitial cells of Cajal

123

These modified smooth muscle cells lie between smooth muscle layers and the intrinsic nerve plexuses, and they may act as an intermediary between the neurons and smooth muscle

Interstitial cells of Cajal

124

These cells function as the pacemakers for slow wave activity in different regions of the GI tract

Interstitial cells of Cajal

125

In a typical day, ___ liters of fluid pass through the lumen of an adult's GI tract-equal to the contents of three 3-liter soft drink bottles! Only about ___ liters of that volume enter the GI system through the mouth. The remaining ___ liters of fluid come from body water secreted along the enzymes and mucus. About half of the secreted fluid comes from?

9 liters
2 liters
7 liters
Accessory organs and glands such as the salivary glands, pancreas, and liver. The remaining 3.5 liters are secreted by epithelial cells of the digestive tract itself

126

A large portion of the 7 liters of fluid secreted by the digestive system each day is composed of?

Water and ions, particularly sodium, potassium, chloride, bicarbonate, and hydrogen

127

The ions of the digestive system are first secreted into what?
What about water?

Lumen of the tract, then reabsorbed. Water follows osmotic gradients created by the transfer of solutes from one side of the epithelium to the other.

128

How does water move in the digestive tract?

Through the epithelial cells via membrane channels or between cells (the paracellular pathway)

129

Gastrointestinal epithelial cells have distinct apical and basolateral membranes. Each cell surface contains?

Proteins for active transport, facilitated diffusion, and ion movement through channels. The arrangement of channels and transporters on the apical and basolateral membranes determines the direction of movement of solutes and water across the epithelium

130

Many of the membrane transporters of the GI tract are similar to those of the?

Renal tubule

131

The basolateral membrane of the GI tract contains?

The ubiquitous sodium-potassium pump

132

What does motility include?

1) Ingestion
- putting food into the mouth
2) Mastication
- Chewing, mixing with saliva
3) Deglutition
-Swallowing
4) Peristalsis & Segmentation
- Squeezing by muscular contraction along and through the GI tract

133

Cotransporters in the GI tract include?

1) Sodium-potassium-2 chloride symporter (NKCC)
2) Chloride-bicarbonate exchangers (Cl- - HCO3)
3) Sodium-hydrogen exchanger (NHE)
4) Hydrogen-potassium pump

134

Ion channels of the GI tract include?

1) The apical sodium channel (ENaC) epithelial sodium channel
2) Potassium channels
3) Chloride channels, such as the gated chloride channel known as the cystic fibrosis transmembrane conductance regulator, or CFTR chloride channel

135

Defects in this cause the disease cystic fibrosis

Cystic fibrosis transmembrane conductance regulator (gated chloride channel)

136

1) Acid secretion by?
2) Bicarbonate secretion by?
3) Isotonic sodium chloride secretion by?

1) Stomach
2) Pancreas and duodenum
3) Intestines and salivary glands

137

What secretes hydrochloric acid into the lumen of the stomach?

Parietal cells deep in the gastric glands

138

Acid secretion in the stomach averages ____ liters per day and can create a luminal pH as low as __. The cytoplasmic pH of the parietal cells is about ____, which means the cells are pumping ____ against a gradient that is 2.5 million times more concentrated in the lumen

1-3 liters per day
low as 1
7.2

139

Describe the parietal cell pathway for acid secretion

(FIGURE 21.5A)
The process begins when hydrogen from water inside the partial cell is pumped into the stomach lumen by an hydrogen potassium pump in exchange for potassium entering the cell. Chloride then follows hydrogen through open chloride channels, resulting in net secretion of hydrochloric acid by the cell. While acid is being secreted into the lumen, bicarbonate made from carbon dioxide and the hydroxide (OH-) from water is absorbed into the blood. The buffering action of bicarbonate makes blood leaving the stomach less acidic, creating an alkaline tide

140

Bicarbonate secretion into the duodenum does what?

Neutralizes acid entering from the stomach

141

A small amount of bicarbonate is secreted by?
But most comes from the?

Small amount- duodenal cells
Most- pancreas, which secretes a watery solution of sodium bicarbonate (NaHCO3). They also secrete hormones from islet cells tucked among the acinar cells

142

The exocrine portion of the pancreas consists of?

Lobules called acini that open into ducts whose lumens are part of the body's external environment

143

The acinar cells secrete?

Digestive enzymes, and the duct cells secrete the sodium bicarbonate (NaHCO3) solution

144

Bicarbonate production requires?

High levels of the enzyme carbonic anhydrase

145

In the pancreas and/or duodenum, bicarbonate produced from carbon dioxide and water is secreted by?

An apical chloride bicarbonate exchanger. Chloride enters the cell on a basolateral NKCC cotransporter and leaves via an apical CFTR (Cystic fibrosis transmembrane conductance regulator) channel. Luminal chloride then re-enters the cell in exchange for bicarbonate entering the lumen. Hydrogen ions produced along with bicarbonate leave the cell on basolateral sodium hydrogen exchangers. The hydrogen thus reabsorbed into the intestinal circulation helps balance bicarbonate put into the blood when parietal cells secrete hydrogen into the stomach

146

In the pancreatic duct cell and duodenal cell sodium and water movement in these tissues is?

A passive process, driven by electrochemical and osmotic gradients

147

For intestinal and colonic crypt cells and salivary gland acini, and pancreatic duct cell or duodenal cell, the net movement of negative ions from the ECF to the lumen attracts what?

Sodium which moves down its electrochemical gradient through leaky junctions between the cells. The secretion of sodium and bicarbonate into the lumen creates an osmotic gradient, & water follows by osmosis. The net result is secretion of a watery sodium bicarbonate solution

148

Is the movement of hydrogen into the stomach active or passive?

Active

149

Crypt cells in the small intestine and colon secrete?

An isotonic sodium chloride solution that mixes with mucus secreted by goblet cells to help lubricate the contents of the gut

150

In the crypt cells in the small intestine and colon, the active chloride secretion is similar to?

The pancreatic cells

151

In the crypt cells in the small intestine and colon, chloride from the ECF enters cells via?

NKCC transporters, then exits into the lumen via apical CFTR channels. Sodium & water follow along the paracellular pathway, with the end result being secretion of isotonic saline solution.

152

Digestive enzymes are secreted by?

Either by exocrine glands (salivary glands and the pancreas) or by epithelial cells in the mucosa of the stomach and small intestine

153

Enzymes are?

Proteins which means that they are synthesized on the rough ER, packaged by the golgi complex into secretory vesicles, and then stored in the cell until needed. On demand they are released by exocytosis

154

Many intestinal enzymes are?

Not released free into the lumen but remain bound to the apical membranes of intestinal cells, anchored by transmembrane protein stalks or lipid anchors

155

Some digestive enzymes are secreted in an inactive proenzyme form known collectively as?

Zymogens

156

Zymogens must be activated in the?

GI lumen before they can carry out digestion. This late activation allows enzymes to be stockpiled in the cells that make them without damaging those cells.

157

Zymogen names often have?

The suffix-ogen added to the enzyme name such as pepsinogen

158

The control pathways for enzyme release vary but include.

A variety of neural, hormonal, and paracrine signals. Usually stimulation of parasympathetic neurons in the vagus nerve enhances enzyme secretion

159

Mucus is?

A viscous secretion composed primarily of glycoproteins called mucins

160

The primary functions of mucus are?

To form a protective coating over the GI mucosa and to lubricate the contents of the gut.

161

Where is mucus made?

In specialized exocrine cell called mucous cells in the stomach, serous cells in salivary glands, and goblet cells in the intestine

162

These cells make up between 10-24 percent of the intestinal cell population

Goblet cells

163

The signals for mucus release include?

Parasympathetic innervation, a variety of neuropeptides found in the enteric nervous system, & cytokines from immunocytes

164

Parasitic infections and inflammatory processes in the gut also cause substantial increase in?

Mucus secretion as the body attempts to fortify its protective barrier

165

Saliva is a complex hyposmotic fluid secreted by the?

Salivary glands of the oral cavity.

166

The salivary glands, like the exocrine pancreas, are organized into?

AAcini and ducts

167

The components of saliva include?

Water, ions, mucus, and proteins such as enzymes and innumoglobulins

168

The salivary glands are?

Exocrine glands, with a secretory epithelium that opens to the outside environment through a duct

169

Saliva is controlled by the?

Autonomic nervous system

170

Primary stimulus for secretion of saliva

Parasympathetic innervation, but there is also some sympathetic innervation to the glands.

171

Bile is a?

Nonenzymatic solution secreted from hepatocytes or liver cells

172

The key components of bile are?

1) Bile salts, which facilitate enzymatic fat digestion
2) Bile pigments, such a bilirubin, which are the waste products of hemoglobin degradation
3) Cholesterol, which is excreted in the feces

173

Bile salts, which act as detergent to solubilize fats during digestion, are made from?

Steroide Bile acids combined with amino acids

174

Bile is secreted into?

Hepatic ducts that lead to the gallbladder which stores and concentrates the bile solution

175

During a meal, contraction of the gallbladder sends bile into the?

Duodenum through the common bile duct, along with a watery solution of bicarbonate and digestive enzymes from the pancreas

176

The gallbladder is an organ that is not essential for normal digestion, and if the duct becomes blocked by gallstones, the gallbladder can be?

Removed without creating long term problems

177

Chewing and churning create what?

Smaller pieces of food with more surface area exposed to digestive enzymes

178

The pH at which different digestive enzymes function best reflects?

The location where they are most active

179

Enzymes that act in the stomach work well at?

Acidic pH

180

Those enzymes secreted in the small intestine work best in?

Alkaline pH

181

Most absorption takes place in the?

Small intestine, with additional absorption of water & ions in the large intestine

182

The surface area for absorption is greatly increased by?

The presence of fingerlike villi and by the brush border on the luminal surface of enterocytes, created from numerous microvilli on each cell

183

The crypts also add to surface area, but the crypt cells are specialized for?

Fluid and hormone secretion

184

Once absorption of nutrients & ions across the GI epithelium, most of the nutrients enters?

The capillaries within the villi. The exception is fats, which mostly enter lymph vessels called lacteals

185

Digestion and absorption are not directly regulated except in a few instances. Instead they are influenced primarily by?

Motility and secretion in the digestive tract, the two processes that in turn are regulated by hormones, the nervous system, and local control mechanisms

186

About half the calories the average American ingests are in the form of?

Carbohydrates, mainly starch & sucrose (table sugar)

187

Intestinal carbohydrate transport is redistricted to?

Monosaccharides, which means that all complex carbohydrates & disaccharides must be digested if they are absorbed

188

We are unable to digest cellulose because?

We lack the necessary enzymes.

189

The complex carbohydrates we can digest are? How?

Starch & glycogen. The enzyme amylase breaks long glucose polymers into smaller glucose chains & into disaccharide maltose. Maltose & other disaccharides are broken down by intestinal brush border enzymes known as disaccharidases (maltase, sucrase, lactase)

190

The end products of carbohydrate digestion are?

Glucose, galactose, and fructose

191

Intestinal glucose and galactose absorption uses

Transporters identical to those found in the renal proximal tubule

192

What are the transporters for glucose and galactose?

The apical sodium glucose symporter (SGLT) & the basolateral GLUT-2 transporter. These transporters move galactose as well as glucose

193

Fructose absorption is not?

Sodium dependent. Fructose moves across the apical membrane by facilitated diffusion on the GLUT-5 transporter & across the basolateral membrane on GLUT-2

194

Unlike carbohydrates, which are ingested in forms ranging from simple to complex, most ingested proteins are?

Polypepties or larger. Not all proteins are equally digested by humans

195

The enzymes for protein digestion are classified into two broad groups

Endopeptidases and exopeptidases

196

What are endopeptidases, more commonly called proteases

They attack peptide bonds in the interior of the amino acid chain and break a long peptide chain into smaller fragments

197

Proteases (endopeptidases) are secreted as?

Inactive proenzymes from epithelial cells in the stomach, intestine, and pancreas and are activated in the GI tract lumen

198

Examples of proteases include

Pepsin secreted in the stomach, & trypsin & chymotrypsin secreted in the pancreas

199

Exopeptidases release?

Single amino acid from peptides by chopping them off the ends, one at a time

200

The most important digestive exopeptidases are two isozymes of carboxypeptidase secreted by the?

Pancreas

201

The primary products of protein digestion are?

Free amino acids, dipeptides, all of which can be absorbed

202

Most free amino acids are carried by?

Sodium dependant cotransport proteins similar to those in the proximal tubule of the kidney

203

Dipeptides and tripeptides are carried into the mucosal cell on the oligopeptide transporter PepT1 that uses hydrogen dependent cotransport. Once inside the cell the oligopeptides have two possible fates

Most are digested by cytoplasmic peptidases into amino acids, which are then transported across the basolateral membrane & into the circulation.
Those that are not digested are transported intact across the basolateral membrane on a hydrogen dependent exchanger

204

The transport system that moves oligopeptides also is responsible for?

Intestinal uptake of certain drugs, including beta-lactam antibiotics, angiotensin-converting enzyme inhibitors, and thrombin inhibitors

205

Some peptides larger than three amino acids are absorbed by?

Transcytosis after binding to membrane receptors on the luminal surface of the intestine

206

In newborns, peptide absorption takes place primarily in?

Intestinal crypt cells

207

One of the most common antigens responsible for food allergies is?

Gluten, a component of wheat

208

Fats and related molecules in the Western diet include.

Triglycerides, cholesterol, phospholipids, long-chain fatty acids, and the fat-soluble vitamins

209

Nearly ___% of our fat calories come from triglycerides because they are the primary form of lipid in both plants and animals

90%

210

Fat digestion is complicated by the fact that most lipids are not particularly water soluble. As a result, the aqueous chyme leaving the stomach contains a coarse emulsion of large fat droplets, which have less surface area than smaller peptides. To increase the surface area available for enzymatic fat digestion, the liver secretes?

Bile salts into the small intestine. Bile salts help break down the coarse emulsion into smaller, more stable particles

211

Bile salts, like phospholipids of cell membranes, are amphipathic meaning that they have both a hydrophobic region and a hydrophilic region. The hydrophobic region of bile salts associate with the?
The polar side?

Surface of lipid droplets, while the polar side chains interact with water, creating a stable emulsion of small, water-soluble fat droplets

212

Enzymatic fat digestion is carried out by?

Lipases, enzymes that remove two fatty acids from act triglyceride molecule. Result is one monoglyceride & two fatty acids

213

The bile salt coating of the intestinal emulsion complicates digestion because?

Lipase is unable to penetrate the bile salts. For this reason fat digestion also requires colipase, a protein cofactor secreted by the pancreas

214

Fat digestion requires a protein cofactor secreted by the pancreas called co-lipase, what is the function?

Co-lipase displaces some bile salts, allow lipase access to fats inside the bile salt coating

215

Phospholipids are digested by?

Pancreatic phopholipase

216

As enzymatic and mechanical digestion proceed, fatty acids, bile salts, monoglycerides, phospholipids, & cholesterol from?

Small disk-shaped micelles

217

Because fats are lipophilic many are absorbed by?

Simple diffusion

218

Fatty acids and monoglycerides move out of their micelles and diffuse where?

Across apical membrane into the epithelial cells

219

What happens once monoglycerides and fatty acids are inside the enterocytes?

They move to the smooth endoplasmic reticulum where they recombine into triglycerides

220

Macromolecules broken down: Complex form of protein?
Building blocks of protein?

Protein/polypeptide
Amino acids

221

Macromolecules broken down: Complex form of
carbohydrates?
Building blocks?

Polysaccharides: Glycogen, starch
Building blocks: Monosaccharides: glucose, fructose, galactose

222

Macromolecules broken down: Complex form of fat?
Building blocks?

Complex form of fat: Triglyceride
Building blocks: 3 fatty acids + glycerol

223

When food comes into the mouth, the feedforward cephalic vagal reflex begins?

Secretion in the stomach

224

In the stomach parietal cells deep in the gastric glands secrete?

Gastric acid (HCl)

225

What is the stomachs gastric acid (HCl)? (4 things)

It has multiple functions:
1) Activates pepsin, the enzyme that digests proteins
2) It denatures the proteins by breaking disulfide & hydrogen bonds that hold the protein in its tertiary structure. This is important because unfolding the protein chains makes the peptide bonds between amino acids accessible to pepsin
3) Helps kill bacteria & other ingested microorganisms
4) Inactivates salivary amylase, which stops carbohydrate digestion that began in the mouth

226

What are the enzymes secreted by the stomach?

1) Chief cells in the gastric gland secrete the inactive enzyme pepsinogen, which is cleaved to activate pepsin in the lumen of the stomach by the action of hydrogen
2) Gastric lipase is co-secreted with pepsin

227

What is pepsin?

An endopeptidase that carries out the initial digestion of proteins. Particularly effective on collagen and therefore plays an important role in digesting meat (protein)

228

Less than ___% of fat digestion takes place in the stomach

10%

229

What are the paracrines secreted by the stomach?

1) Enterochromaffin-like (ECL) cells secrete histamine, a paracrine signal that promotes acid secretion by parietal cells.
2) D-cells secrete somatostatin (SS), known as growth hormone-inhibiting hormone. This is the primary negative feedback signal for gastric phase secretion. It shuts down acid secretion directly & indirectly & also inhibits pepsinogen secretion

230

Hormone secretion in the stomach

G-cells found deep in the gastric glands, secrete the hormone gastrin into the blood. Gastrin release is stimulated by the presence of amino acids and peptides in the stomach, by dissension of the stomach, and by neural reflexes mediated by gastrin-releasing peptide

231

Acid in the stomach lumen stimulates?
Acid also triggers?

1) Pepsinogen release from chief cells through a short reflex. In the lumen acid converts pepsinogen into pepsin, & protein digestion begins
2) Triggers somatostatin release from D cells which acts via negative feedback to inhibit secretion of gastric acid, gastrin, histamine, & pepsinogen

232

The stomach has 3 general functions

1) Storage. The stomach stores food and regulates its passage into the small intestine, where most digestion & absorption takes place
2) Digestion- The stomach chemically and mechanically digests food into the soupy mixture of uniformly small particles called chyme
3) Protection- Protects the body by destroying many of the bacteria & other pathogens that are swallowed with food or trapped in airway mucus. At the same time the stomach must protect itself from being damaged by its own secretions

233

Before food even arrives, digestive activity in the stomach begins with the?

Long vagal reflex of the cephalic phase. Then once food enters the stomach, stimuli in the gastric lumen initiate a series of short reflexes that constitute the gastric phase of digestion.
In gastric phase reflexes, distension of the stomach and the presence of peptides air amino acids in the lumen activate endocrine cells & enteric neurons. Hormones, neurocrine secretions, & paracrine molecules then influence motility and secretion

234

What happens when food arrives from the esophagus to the stomach?

The stomach relaxes & expands to hold the increased volume. This neurally mediated reflex is called receptive relaxation

235

What happens if we ingest more than we need form a nutritional standpoint? Why is this important?

The stomach must regulate the rate at which food enters the small intestine.
Without this regulation the small intestine would not be able to digest & absorb the load presented to it, & significant amounts of unabsorbed chyme would pass into the large intestine. The epithelium of the large intestine is not designed for large-scale nutrient absorption, so most of the chyme would pass out in the feces, resulting in diarrhea.

236

While the upper stomach is quietly holding food, the lower stomach is busy with digestion. In the distal half of the stomach, a series of?

Peristaltic waves pushes the food down towards the pylorus, mixing food with acid and digestive enzymes. As large food particles are digested to the more uniform texture of chyme, each contractile wave squirts a small amount of chyme through the pylorus into the duodenum. Enhanced gastric motility during a meal is primarily under neural control & is stimulated by dissension of the stomach

237

The secretory cell of the gastric mucosa, mucous neck cells, secrete what?

1) Mucus-physical barrier between lumen & epithelium
2) Bicarbonate- Buffers gastric acid to prevent damage to epithelium

238

The secretory cell of the gastric mucosa, Parietal cells, secrete what?

1) Gastric acid (HCl)- Activates pepsin; kills bacteria
2) Intrinsic factor- Complexes with vitamin B12 to permit absorption

239

The secretory cell of the gastric mucosa, enterochromaffin-like cell (paracrine), secretes what?

Histamine- Stimulates gastric acid secretion

240

The secretory cell of the gastric mucosa, chief cells, secretes what?

1) Pepsin(ogen) (enzyme)- Digests proteins
2) Gastric lipase (enzyme)- Digests fat

241

The secretory cell of the gastric mucosa, D-cells, secretes what?

Somatostatin- Inhibits gastric acid secretion

242

The secretory cell of the gastric mucosa, G-cells, secretes what?

Gastrin- Stimulates gastric acid secretion

243

Under normal conditions, the stomachs gastric mucosa protects itself from acid & enzymes by a?

Mucus-bicarbonate barrier. Mucus cells in the neck of gastric glands secrete both substances

244

Describe how the stomachs gastric mucosa layer protects itself?

The mucus forms a physical barrier, & the bicarbonate creates a chemical buffer barrier underlying the mucus

245

What is the pH of bicarbonate in the stomach?

Close to 7, even when the pH in the lumen is 2.

246

When does mucus secretion increase?

When the stomach is irritated, such as by the ingestion of aspirin or alcohol