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4c Blood Vessels, Lymphatic Structures, and Nerve Supply

4c Blood Vessels, Lymphatic Structures, and Nerve Supply

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785



Chapter Twenty-Six  Digestive System





with those of the stomach. In the adult human, the esophagus is about

25 centimeters (about 10 inches) long, and most of its length is within

the thorax, directly anterior to the vertebral bodies. Only the last

1.5 centimeters of the esophagus are located in the abdomen. The

empty esophagus is flattened; the lumen only opening slightly with

the passage of a food bolus.



26.5b  Histology

The esophageal mucosa is different from that in the abdominal GI

tract organs in that it is composed of thick, nonkeratinized stratified squamous epithelium (figure 26.10). This epithelium is better

suited to withstand the abrasions of the bolus as it moves through

the esophagus (see section 4.1e). Because the esophagus does not

absorb any nutrients, this thicker, protective epithelium supports

its function.

The submucosa is thick and composed of abundant elastic

fibers that permit distension during swallowing. It houses numerous mucous glands that provide a thick, lubricating mucus for the

epithelium.

The muscularis is composed of an inner circular layer and an

outer longitudinal layer. The muscularis of the esophagus is unique



in that it contains both skeletal and smooth muscle. The two layers of

muscle in the superior one-third of the muscularis layer are skeletal,

rather than smooth, to ensure that the swallowed material moves

rapidly out of the pharynx and into the esophagus before the next

respiratory cycle begins. (Remember that smooth muscle contracts

more slowly than does skeletal muscle; see section 10.9b.) Striated

and smooth muscle fibers intermingle in the middle one-third of the

esophagus, and only smooth muscle is found within the wall of the

inferior one-third. This transition marks the beginning of a continuous smooth muscle muscularis that extends throughout the stomach

and the small and large intestines to the anus. The outermost layer

of the esophagus is an adventitia, which adheres it to the posterior

body wall.

The superior esophageal sphincter (or pharyngoesophageal

sphincter) is a contracted ring of circular skeletal muscle at the superior end of the esophagus. It is the area where the esophagus and the

pharynx meet. This sphincter is closed during inhalation of air so it

enters the larynx and trachea instead of the esophagus.

The inferior esophageal sphincter (gastroesophageal, or c­ ardiac

sphincter) is a contracted ring of circular smooth muscle at the inferior end of the esophagus. This sphincter isn’t strong enough alone



Mucosa

Oropharynx

Laryngopharynx



Superior esophageal

sphincter



Muscularis

mucosae

Submucosa



Muscularis

Esophagus

Esophageal

hiatus



Diaphragm

Stomach



Inferior esophageal

sphincter



Adventitia

LM 11x

(b) Esophagus, transverse section



(a) Pharynx and esophagus



Figure 26.10

Histology of the Esophagus. (a) The esophagus extends inferiorly

from the pharynx and conducts swallowed materials to the stomach.

(b) A photomicrograph of a transverse section through the esophagus

identifies the tunics in its wall. (c) A photomicrograph shows the

esophageal mucosa.



Stratified squamous

epithelium



Mucosa

Lamina propria



(b) © Alfred Pasieka/Getty Images; (c) © McGraw-Hill Education/Al Telser,

photographer



Muscularis mucosae



LM 65x

(c) Esophageal mucosa



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Chapter Twenty-Six  Digestive System



Clinical View 26.1

Reflux Esophagitis and

Gastroesophageal Reflux Disease

Sometimes acidic chyme refluxes into the esophagus, causing the

burning pain and irritation of reflux esophagitis. Because the pain is

felt posterior to the sternum and can be so intense that it is mistaken

for a heart attack, this condition is commonly known as heartburn.



Unlike the stomach epithelium, the esophageal epithelium

is poorly protected against acidic contents and easily becomes

inflamed and irritated. Reflux esophagitis is seen most frequently

in overweight individuals, smokers, those who have eaten a very

large meal (especially just before bedtime), and people with hiatal

hernias (hī-ā′tăl her′nē-ă; rupture), in which a portion of the stomach

protrudes through the diaphragm into the thoracic cavity. Eating

spicy foods, or ingesting too much caffeine, may exacerbate symptoms. Preventive treatment includes lifestyle changes such as losing

weight, quitting smoking, limiting meal size, and not lying down until

2 hours after eating. Sleeping with the head of the bed elevated 4

to 6 inches, so that the body lies at an angle rather than flat, also

helps alleviate symptoms.



Barrett

esophagus



(a) Endoscopic view of a normal

esophagus.





Chronic reflux esophagitis can lead to gastroesophageal

reflux disease (GERD). Frequent gastric reflux erodes the esophageal tissue in this condition, so over time scar tissue builds up in the

esophagus, leading to narrowing of the esophageal lumen. In more

advanced cases, the esophageal epithelium may change from stratified squamous to a simple columnar epithelium, a condition known

as Barrett esophagus. Unfortunately, this metaplasia (the change

from one type of cell to another) increases the risk of cancerous

growths.



GERD may be treated with a series of medications.

Proton-pump inhibitors (e.g., omeprazole [Prilosec], esomeprazole

[Nexium]) limit acid secretion in the stomach by acting on the proton (hydrogen ion [H+]) pumps that help produce acid. Long term

use of proton pump inibitor’s have been linked with side effects

such as: osteoporotic fractures, decrease in cardiovascular health,

potential increase in bacterial infections of the digestive tract, and

most recently, dementia. Histamine (H2) blockers (e.g., famotidine

[Pepcid], nizatidine [Axid], ranitidine [Zantac]) also help limit acid

secretion in the stomach. Antacids (e.g., Tums, Rolaids) also help

neutralize stomach acid.



Columnar

secretory

epithelium



(b) An endoscopic view of the

esophagus shows the signs

of Barrett esophagus.



LM 400x



(a) © Gastrolab/Science Source; (b) Photo courtesy Interactive Atlas of Gastrointestinal



Microscopic appearance of Barrett esophagus.



Endoscopy by Edgar Jaramillo, available free of charge at www.gastrosource.com



© Dr. Edward Lee, Howard University



to prevent materials from refluxing back into the esophagus; instead,

the muscles of the diaphragm at the esophageal opening contract to

help prevent materials from regurgitating from the stomach into the

esophagus.





W H AT D I D YO U LE A R N ?

8





How do the esophageal tunics differ from the “default” tunic

pattern?



26.6  The Swallowing Process

✓✓Learning Objective



14. List and explain the three phases of swallowing.

Swallowing, also called deglutition (dē ′glu -̄ tish′ŭn; degluto  = to

swallow), is the process of moving ingested materials from the oral

cavity to the stomach. There are three phases of swallowing: the



voluntary phase, the pharyngeal phase, and the esophageal phase

(figure 26.11).

The voluntary phase occurs after ingestion. Food and saliva

mix in the oral cavity. Chewing forms a bolus that is mixed and manipulated by the tongue and then pushed superiorly against the hard

palate. Transverse palatine folds in the hard palate help direct the

bolus posteriorly toward the oropharynx.

The arrival of the bolus at the entryway to the oropharynx initiates the pharyngeal phase, when tactile sensory receptors trigger

the swallowing reflex, which is controlled by the swallowing center

in the medulla oblongata (see section 15.5c). The bolus passes quickly

and involuntarily through the pharynx to the esophagus. During this

phase, (1) the soft palate and uvula elevate to block the passageway

between the nasopharynx and oropharynx; (2) the bolus enters the

oropharynx; and (3) the larynx and laryngeal opening elevate toward

the epiglottis, ultimately covering and sealing the glottis to prevent

swallowed materials from entering the trachea (see section 25.3a).

Pharyngeal constrictors are skeletal muscle groups that contract

involuntarily and sequentially to ensure that the swallowed bolus



787



Chapter Twenty-Six  Digestive System







Hard palate



Bolus



Soft palate and uvula elevate

and close off nasopharynx



Soft palate and uvula return

to pre-swallowing position



Direction

of bolus

movement

Uvula

Oropharynx

Tongue

Epiglottis



Bolus



Epiglottis

closes over

laryngeal

opening



Larynx

Esophagus

Trachea



1 Voluntary phase: Bolus of food is pushed

by tongue against hard palate and then

moves toward oropharynx.



2 Pharyngeal phase (involuntary): As



bolus moves into oropharynx, the soft

palate and uvula close off nasopharynx,

and the larynx elevates so the epiglottis

closes over laryngeal opening.



Esophagus

Bolus



3 Esophageal phase (involuntary):



Peristaltic contractions of the esophageal

muscle push bolus toward stomach; soft

palate, uvula, and larynx return to their

pre-swallowing positions.



Figure 26.11

Phases of Swallowing. Swallowing, or deglutition, occurs as a result of coordinated muscular activities that force the bolus (1) into the pharynx from the oral

cavity, (2) through the pharynx, and (3) into the esophagus on the way to the stomach.



moves quickly (1 second elapses in this phase) through the pharynx

and into the esophagus.

The esophageal phase is also involuntary. It is the time (about

5 to 8 seconds) during which the bolus passes through the e­ sophagus

and into the stomach. This phase begins when the superior esophageal

sphincter relaxes to allow ingested materials into the esophagus. The

presence of the bolus within the lumen of the esophagus stimulates

peristaltic waves of muscular contraction that assist in propelling the

bolus toward the stomach. The inferior sphincter contracts after the bolus

passes into the stomach to prevent reflux of materials and fluids back

into the esophagus.





W H AT D I D YO U LE A R N ?

9





Briefly describe the three phases of swallowing.



26.7  Stomach



✓✓Learning Objectives

15. Describe the gross anatomy of the stomach.

16. Explain the histology of the stomach wall.

17. Compare and contrast the secretions of the stomach.

The stomach (stŭm′ ăk; stomachus = belly) is a muscular, J-shaped

sac that occupies the left upper quadrant of the abdomen, immediately

inferior to the diaphragm (figure 26.12). It continues the mechanical

and chemical digestion of the bolus. After the bolus has been completely processed in the stomach, the product is called chyme (kīm;

chymos = juice). Chyme has the consistency of a pastelike soup. The

stomach facilitates (1) mechanical digestion by contractions of its

thick muscularis layer that churns and mixes the bolus with gastric

secretions, and (2) chemical digestion through its gastric secretions

of acid and enzymes.



26.7a  Gross Anatomy

The stomach is composed of four regions:





The cardia (kar′dē-ă; heart) is the small, narrow, superior

entryway into the stomach lumen from the esophagus. The

internal opening where the cardia meets the esophagus is

called the cardiac orifice.

■The fundus (fŭn′dŭs; bottom) is the dome-shaped region lateral

and superior to the esophageal connection with the stomach. Its

superior surface contacts the diaphragm.

■The body (corpus) is the largest region of the stomach; it is

inferior to the cardiac orifice and the fundus.

■The pylorus (pı -̄ lōr′ŭs; gatekeeper) is a narrow, medially

directed, funnel-shaped pouch that forms the terminal region

of the stomach. The pylorus is divided into two parts: the

pyloric antrum (the more expanded region near the body)

and the pyloric canal (the more narrow region that attaches

to the duodenum). Its opening into the duodenum of the

small intestine is called the pyloric orifice. Surrounding

this pyloric orifice is a thick ring of circular smooth muscle

called the pyloric sphincter. The pyloric sphincter regulates

the entrance of chyme into the small intestine: It closes upon

sympathetic innervation and opens upon parasympathetic

innervation.





The inferior convex border of the stomach is the greater

curvature, whereas the superior concave border is termed the

lesser ­curvature. The greater omentum attaches to the greater curvature edge of the stomach, and the lesser omentum extends between

the lesser curvature and the liver.

Internally, the stomach lining is composed of numerous gastric

folds, or rugae (rū′jē; ruja = wrinkle). They are seen only when the

stomach is empty, allowing the stomach to expand greatly when it

fills and then return to its normal J-shape when it empties.



788



Chapter Twenty-Six  Digestive System



Simple

Esophageal- Stratified

stomach

squamous columnar

border

epthelium epithelium



Gastric

pits



Fundus



Esophagus



Cardia

Longitudinal

layer



Duodenum



Pyloric

orifice



Pyloric

sphincter



Lesser

curvature



Circular

layer



Muscularis



Oblique

layer

Body

Pyloric

canal



Greater curvature

(b) Esophageal-stomach border



Pylorus

Pyloric

antrum



LM 50x



Gastric folds



(a) Stomach regions, anterior view



Functions of the Stomach:

1. Digests mechanically

2. Digests chemically

Proteins

Lipids (limited)

3. Releases intrinsic factor

4. Releases gastrin



Figure 26.12

Gross Anatomy of the Stomach. The stomach is a muscular sac where mechanical and chemical digestion of the bolus occurs. (a) The major regions of the

stomach are the cardia, the fundus, the body, and the pylorus. Three layers of smooth muscle make up the muscularis tunic. (b) A photomicrograph of the abrupt

transition from stratified squamous epithelium in the esophagus to simple columnar in the stomach.

(b) © Victor P. Eroschenko



26.7b  Histology



26.7c  Gastric Secretions



The stomach is lined by a simple columnar epithelium, although little

absorption occurs in the stomach. This epithelium does not contain

goblet cells; instead, surface mucous cells (described later in this

section) secrete mucin onto the epithelial lining. Another feature that

distinguishes the stomach mucosa is that the lining is indented by

numerous depressions called gastric pits. The pits are continuous

with branched tubular glands, called gastric glands, which extend

through the length of the mucosa to its base (figure 26.13). The mucosa of the stomach is only 1.5 millimeters at its thickest point (about

the thickness of a nickel). The muscularis mucosae lies between the

base of the gastric glands and the submucosa, and it helps expel gastric gland secretory products when it contracts.

The muscularis of the stomach is composed of three smooth

muscle layers instead of two (the default pattern): an inner oblique layer,

a middle circular layer, and an outer longitudinal layer. The oblique

layer is best-developed at the cardia and the body of the stomach. The

presence of a third layer of smooth muscle assists the continued churning and blending of the bolus. The outermost tunic of the stomach is a

serosa, because the stomach is intraperitoneal.



Gastric juices are produced by cells in the gastric glands, and their

secretions are released into gastric pits, which funnel them to the

lumen of the stomach. Five types of secretory cells form the ­gastric

epithelium (figure 26.13b, c).

Surface mucous cells line the stomach lumen and extend into

the gastric pits. They continuously secrete mucin onto the gastric

luminal surface to prevent ulceration of the lining upon exposure to

the high acidity of the gastric fluid and protect the epithelium from

gastric enzymes.

Mucous neck cells are located immediately deep to the base

of the gastric pit and are interspersed among the parietal cells. These

cells produce an acidic mucin that differs structurally and functionally from the mucin secreted by the surface mucous cells. The acidic

mucin helps maintain the acidic conditions resulting from the secretion of hydrochloric acid by parietal cells.

Parietal cells (also called oxyntic cells) are located primarily in

the proximal and middle parts of the gastric gland. Their distinctive

features are small intracellular channels called canaliculi, which are

lined by microvilli. Hydrochloric acid secreted across the parietal

cells’ vast internalized surface helps denature proteins to facilitate

chemical digestion. Parietal cells also produce intrinsic factor, a

molecule that binds vitamin B12 in the stomach lumen and assists in

B12 absorption in the ileum of the small intestine.

Chief cells (also called zymogenic cells or peptic cells) are

housed primarily in the distal part of the gastric gland. These cells



W H AT D O YO U TH I N K ?

3





The stomach secretes gastric juices, which are highly acidic

and can break down and chemically digest food. What

prevents the gastric juices from eating away at the stomach

itself?



Chapter Twenty-Six  Digestive System







Gastric pit



Stomach lumen



Simple columnar epithelium



Blood vessel

Lymph vessel



Lamina propria



Mucosa



Muscularis mucosae

Submucosa



Artery

Vein

Oblique layer



Muscularis



Submucosal

nerve plexus



Circular layer



Myenteric

nerve plexus



Longitudinal layer

Serosa

(a) Stomach wall, sectional view

Stomach lumen



Gastric pit



Surface mucous cell

(secretes alkaline fluid

containing mucin)



Simple

columnar

epithelium



Mucous neck cell

(secretes acidic fluid

containing mucin)



Gastric pit



Parietal cell (secretes

intrinsic factor and

hydrochloric acid)

Gastric glands

Chief cell (secretes

pepsinogen and

gastric lipase)



Gastric gland



Enteroendocrine cell

(secretes gastrin into

blood)



LM 60x

(b) Stomach mucosa



(c) Gastric pit and gland



Figure 26.13

Histology of the Stomach Wall. (a) The stomach lumen contains invaginations within the mucosa called gastric pits that lead into gastric glands. (b) A

photomicrograph shows gastric glands and the cells lining the gastric pit. (c) A diagrammatic section of a gastric pit and gland shows their structure and the

distribution of different types of secretory cells.

(b) © McGraw-Hill Education/Al Telser, photographer



789



790



Chapter Twenty-Six  Digestive System



Clinical View 26.2

Peptic Ulcers

Normally a balance exists in the stomach between the acidic

gastric juices and the protective, regenerative nature of the

mucosa lining. When this balance is thrown off, the stage is set

for the development of a peptic ulcer­— a chronic, solitary erosion

of a portion of the lining of either the stomach or the duodenum.

Annually, over 4 million people in the United States are diagnosed

with an ulcer.



Gastric ulcers are peptic ulcers that occur in the stomach,

whereas duodenal ulcers are peptic ulcers in the superior part of

the duodenum, the first segment of the small intestine. Duodenal

ulcers are common because the first part receives the acidic

chyme from the stomach but has yet to receive the alkaline bile

and pancreatic juice that may neutralize chyme’s acidic content.



Symptoms of an ulcer include a gnawing, burning pain in

the epigastric region, which may be worse after eating; nausea;

vomiting; and extreme belching. Bleeding may also occur, and

the partially digested blood results in dark, tarlike stools. If left



untreated, an ulcer may erode the entire organ wall and cause

perforation, which is a medical emergency.



Irritation of the gastric mucosa (gastritis) has been linked to

many cases of peptic ulcer. Nonsteroidal anti-inflammatory drugs

(NSAIDs), such as ibuprofen and aspirin, are a common cause of

gastritis, and these drugs also impair healing of the gastric lining.

However, the major player in peptic ulcer formation is a bacterium

called Helicobacter pylori, which is present in over 70% of gastric

ulcer cases and well over 90% of duodenal ulcer cases. H. pylori

resides in the stomach and produces enzymes that weaken the

protective effects of the gastic mucus. As leukocytes enter the

stomach to destroy the bacteria, they also destroy the mucous

neck cells. This further irritates the stomach lining and creates an

ideal environment for continued H. pylori colonization.



Categories of medications that help include an antibiotic

taken for 2 weeks to eradicate H. pylori, and treatments that are

similar for gastric reflux, including antacids, proton-pump inhibitors,

a histamine (H2) blocker, and antacids.



Stomach



Duodenal ulcer



Gastric

ulcers

Mucosa

Submucosa

Muscularis

Serosa



Duodenum

(a) Common locations of gastric and duodenal ulcers



(b) Perforated gastric ulcer

(b) © Javier Domingo/Phototake



791



Chapter Twenty-Six  Digestive System





synthesize and secrete enzymes, primarily inactive pepsinogen, into

the lumen of the stomach. The acid content of the stomach then

converts inactive pepsinogen into the active enzyme pepsin, which

chemically digests denatured proteins in the stomach into smaller

fragments.

Enteroendocrine (en′ter-ō-en′dō-krin; enteron = gut, intestine) cells are endocrine cells widely distributed in the gastric

glands of the stomach. These cells secrete gastrin, a hormone that

enters the blood and stimulates the secretory activities of the chief

and parietal cells and the contractile activity of gastric muscle.

Enteroendocrine cells also produce other hormones, such as somatostatin, that modulate the function of nearby enteroendocrine and

exocrine cells.

There are numerous gastric pits in the mucosa of the stomach,

but the types of secretory cells differ in the various regions of the

stomach. The glands of the fundus and body contain all five of the

cell types discussed; however, the glands of the cardia and pylorus

contain mostly mucous neck cells. Parietal cells are abundant in

the glands of the fundus and body, but they are virtually absent in

pyloric glands. Thus, the glands of the fundus and body produce

highly acidic secretions, whereas those in the cardia and pylorus

produce mainly protective, alkaline mucin. The alkaline mucus

can help protect the adjacent regions of the digestive tract (esophagus and duodenum) from damage by stomach acid and digestive

enzymes.

Saliva, mucin, and gastric secretions contribute substantially

to the volume of ingested material. In fact, if a person eats 1 liter

of food, the amount of chyme that enters the small intestine is 3 to

4 liters, due to the volume of the secretions.





W H AT D I D YO U LE A R N ?

10



11





What are the four regions of the stomach, and where is each

located?

What are the five types of secretory cells in the stomach, and

what does each secrete?



26.8  Small Intestine

✓✓Learning Objectives



18. Describe the gross anatomy of the small intestine.

19. Compare and contrast the three regions of the small intestine.

20. Explain the microscopic structure of the small intestine.

The small intestine finishes the chemical digestion process and is responsible for absorbing up to 90% of the nutrients and water. Ingested

nutrients spend at least 12 hours in the small intestine as chemical

digestion and absorption are completed.



26.8a  Gross Anatomy and Regions

The small intestine, also called the small bowel, is a coiled, thinwalled tube about 6 meters (20 feet) in length in the unembalmed

cadaver. (It is much shorter in a living individual due to smooth

muscle tone.) It extends from the pylorus of the stomach to the

cecum of the large intestine, occupying a significant portion of

the abdominal cavity. The small intestine receives its blood supply primarily from branches of the superior mesenteric artery,

and it is innervated by the superior mesenteric plexus (see sections 23.3f and 18.5a, respectively). The small intestine consists

of three specific segments: the duodenum, the jejunum, and the

ileum (figure 26.14).



Accessory

organs



Liver

Gallbladder

Pancreas



Duodenojejunal

flexure

Duodenum



Large intestine

Ileocecal valve



Jejunum



Small

intestine



Ileum



Cecum



Figure 26.14

Gross Anatomy of the Small Intestine. The three regions of the small

intestine—duodenum, jejunum, and ileum—are continuous and “framed”

within the abdominal cavity by the large intestine.



The duodenum (dū′ō-dē′nŭm, dū- od′ĕ-nŭm; breadth of

twelve fingers) forms the initial or first segment of the small intestine. It is about 25 centimeters (10 inches) long and originates

at the pyloric sphincter. The duodenum is arched into a C-shape

around the head of the pancreas and becomes continuous with

the jejunum at the duodenojejunal flexure (flek′sher; fleksura =

bend). Most of the duodenum is retroperitoneal, although the very

initial portion is intraperitoneal and connects to the liver by the

lesser omentum. Within the wall of the duodenum is the major

duodenal papilla, through which bile (secretions released from

the liver and gallbladder) and pancreatic juice enter the duodenum (see figure 26.21a). A minor duodenal papilla also receives

an additional small amount of pancreatic juice via an accessory

pancreatic duct.

The jejunum (jĕ-jū′nŭm; jejunus = empty) is the middle region

of the small intestine. Extending about 2.5 meters (7.5 feet), it makes

up about two-fifths of the small intestine’s total length. The jejunum

is the primary region within the small intestine for chemical digestion

and nutrient absorption. It is intraperitoneal and suspended within the

abdomen by the mesentery proper.

The ileum (il′ē-ŭm; eiles = twisted) is the final or last region

of the small intestine. At about 3.6 meters (10.8 feet) in length, the

ileum forms about three-fifths of the small intestine. Its distal end

terminates at the ileocecal (il′ē-ō-sē′kăl) valve, a sphincter that controls the entry of materials into the large intestine (see figure 26.16).

The ileum is intraperitoneal and suspended in the abdomen by the

mesentery proper.

Internally, the mucosal and submucosal tunics of the small

intestine are thrown into circular folds (also called plicae [plī′kē;

fold] circulares) (figure 26.15a, b). Circular folds, which can be seen

with the naked eye, help increase the surface area through which

nutrients can be absorbed. In addition, the circular folds act like

“speed bumps” to slow down the movement of chyme and ensure that

it remains within the small intestine for maximal nutrient absorption.

Circular folds are more numerous in the duodenum and jejunum, and

least numerous in the ileum.

WHAT DO YOU THINK?

4





Why are the circular folds much more numerous in the

duodenum than in the ileum? How does the abundance of

circular folds relate to the main functions of the duodenum

and ileum?



792



Chapter Twenty-Six  Digestive System

Microvilli



Small

intestine



Simple columnar

epithelial cell with

microvilli (absorbs

nutrients)



Circular folds

Epithelium

Goblet cells



Mucosa

Submucosa

Muscularis

Inner circular layer

Outer longitudinal layer

Mucosa



Serosa

(a) Small intestine tunics



Goblet cell

(produces mucin)



Simple

columnar

epithelial cells

with microvilli



Intestinal gland



Lamina propria

Capillary

network



Circular fold



Unicellular gland

cell (synthesizes

enteropeptidase)



Lymphatic

nodule

Lacteal



Enteroendocrine

cell (secretes

hormones)



Muscularis mucosae

Lymph vessel

Venule

Arteriole



Submucosa



(c) Intestinal villus



Intestinal villi

Mucosa

Submucosa

Inner circular layer

Outer longitudinal layer

Serosa

(b) Section of small intestine

Simple columnar cell



Intestinal lumen



Muscularis



Functions of the Small Intestine:

1 . Completes chemical

digestion

2. Absorbs 90% of nutrients

and water



Microvilli



Villi



Simple columnar

epithelium

Intestinal

lumen



Lamina propria

TEM 18,000x



Goblet cells



(e) Microvilli



Figure 26.15

LM 70x

(d) Intestinal villi



Histology of the Small Intestine. The surface area of the small intestine wall is

vastly increased by specific structural modifications. (a) The wall is composed of four

concentric tunics. (b) Circular folds formed from the mucosa and submucosa are lined

by a dense covering of fingerlike projections called intestinal villi, which are formed

from mucosa only. (c) The structure of a single villus. (d) A photomicrograph shows

the internal structure of villi projecting into the intestinal lumen. (e) The plasma

membrane along the apical surface of the epithelium contains microvilli.

(d) © McGraw-Hill Education/Al Telser, photographer; (e) © Dr. Lee Peachey



Chapter Twenty-Six  Digestive System







26.8b  Histology

When circular folds are viewed at the microscopic level, smaller,

fingerlike projections of mucosa only, called villi, can be seen along

their surface. These villi further increase the surface area for absorption and secretion. Increasing the absorptive surface area even further

are microvilli (mı̄-krō-vil′ı̄; mikros = small) along the free surface of

the simple columnar cells (figure 26.15e and section 4.1e). Individual

microvilli are not clearly visible in light micrographs of the small

intestine; instead, they collectively appear as a brush border that

resembles a brightly staining surface on the apical end of the simple

columnar cells (figure 26.15d). Each villus contains an arteriole and

a venule, with a rich capillary network between them. The capillaries

absorb most nutrients. In the center of the villus is a lacteal, which is

responsible for absorbing lipids and lipid-soluble vitamins, which are

too large to be absorbed by the capillaries (see section 24.2a).

Between some of the intestinal villi are invaginations of mucosa called intestinal glands (also known as intestinal crypts or

crypts of Lieberkuhn) (figure 26.15c). These glands extend to the base

of the mucosa and slightly resemble the gastric glands of the stomach.

Lining them are simple columnar epithelial cells (with goblet cells),

unicellular glands, and enteroendocrine cells. The enteroendocrine

cells release hormones such as secretin, cholecystokinin, and

gastric inhibitory peptide. Some of these hormones temporarily

slow down digestive activities as material from the stomach begins

to enter the small intestine, thereby prolonging the time for stomach

emptying into the small intestine. The goblet cells produce mucin to

lubricate and protect the intestinal lining as materials being digested

pass through.

Distinctive histologic features characterize the three small

intestine regions. The proximal duodenum contains submucosal

glands (or Brunner glands), which produce a viscous, alkaline mucus

that protects the duodenum from the acidic chyme. Circular folds are

best-developed in the jejunum and nearly absent in the ileum. Additionally, villi are larger and more numerous in the j­ ejunum. The number of goblet cells in the mucosa increases from the duodenum to the

ileum, due to the increased need for lubrication as digested materials

are absorbed and undigested materials are left behind. Peyer patches

are abundant primarily in the ileum (see sections 24.4a and 26.4c).

Table 26.4 summarizes all of the small intestine structures that

aid in chemical digestion and absorption.





W H AT D I D YO U LE A R N ?

12





Compare and contrast the gross anatomic and histologic

characteristics that distinguish the duodenum, jejunum,

and ileum.



793



26.9  Large Intestine

✓✓Learning Objectives

21.

22.

23.

24.



Describe the gross anatomy of the large intestine.

Compare and contrast the large intestine regions.

Explain the microscopic structure of the large intestine.

Trace the movement of material through the large intestine.



The large intestine, also called the large bowel, forms a three-sided

perimeter in the abdominal cavity around the centrally located small

intestine (figure 26.16). From its origin at the ileocecal junction to its

termination at the anus, the large intestine has an approximate length

of 1.5 meters (5 feet) and a diameter of 6.5 centimeters (2.5 inches). It

is called the “large” intestine because its diameter is greater than that

of the small intestine. Recall from section 26.8 that the small intestine

absorbs much, but not all, of the digested material. On average, about

1 liter of remaining material passes from the small intestine to the

large intestine daily.

The large intestine absorbs most of the water and ions from

the remaining digested material. In so doing, the watery material

that first enters the large intestine soon solidifies and becomes feces.

The large intestine stores the feces until the body is ready to defecate

(expel the feces). The large intestine also absorbs a very small percentage of nutrients still remaining in the digested material.



26.9a  Gross Anatomy and Regions

The initial or first region of the large intestine is a blind sac called

the cecum (sē′kŭm; caecus = blind), which is located in the right

lower abdominal quadrant. This pouch extends inferiorly from the

ileocecal valve. Projecting inferiorly from the posteromedial region of the cecum is the vermiform (ver′mi-fōrm; vermis = worm)

appendix (ă-pen′diks; appendage), a thin, hollow, fingerlike sac

lined by lymphocyte-filled lymphatic nodules (see section 24.4a).

Both the cecum and the vermiform appendix are intraperitoneal

organs.

At the level of the ileocecal valve, the colon begins and forms

an inverted U-shaped arch. The colon is partitioned into four segments: the ascending colon, transverse colon, descending colon, and

sigmoid colon.

The ascending colon originates at the ileocecal valve and

extends superiorly from the superior edge of the cecum along the

right lateral border of the abdominal cavity. The ascending colon

is retroperitoneal, because its posterior wall directly adheres to the

posterior abdominal wall, and only its anterior surface is covered



Table 26.4



Small Intestine Structures Involved in Chemical Digestion and Absorption



Structure



Anatomy



Function(s)



Circular folds



Circular folds of mucosa and submucosa



Slow down the passage of materials undergoing digestion; increase

surface area for both chemical digestion and absorption



Villi



Fingerlike projections of mucosa



Increase surface area for both absorption and chemical digestion



Microvilli



Folded, fingerlike projections of plasma membrane on apical surface

of columnar epithelial cells



Increase surface area for both absorption and chemical digestion



Intestinal glands



Invaginations into mucosa between villi



Increase surface area for both absorption and chemical digestion;

enteroendocrine cells lining intestinal glands secrete digestive

hormones



Submucosal glands



Coiled tubular glands within submucosa with ducts opening into

intestinal lumen



Secrete alkaline mucin to protect and lubricate lining of small

intestine



794



Chapter Twenty-Six  Digestive System



Transverse colon



Figure 26.16

Gross Anatomy of the Large Intestine. (a) Anterior view of the large intestine, which

forms the distal end of the GI tract. (b) Details of the anal canal.



Ascending

colon



Descending

colon



Cecum

Vermiform

appendix



Rectum



Sigmoid

colon



Anal canal



Left colic flexure



Functions of the Large Intestine:

1. Absorbs most of the

water and ions from

remaining material

2. Solidifies and stores feces



Transverse mesocolon

Right colic

flexure

Omental appendices

Haustrum



Tenia coli



Superior

mesenteric

artery



Descending abdominal aorta

Inferior mesenteric artery



Rectum



Ileocecal

valve



Rectal valve



Cecum

Levator ani

muscle



Ileum



Anal canal



Vermiform appendix



Veins



Rectum



Sigmoid

mesocolon



Internal

anal sphincter

Anus



Anal canal

Anal columns

(a) Large intestine, anterior view



with peritoneum. As it approaches the inferior surface of the liver,

the ascending colon makes a 90-degree turn toward the left side of

the abdominal cavity. This bend in the colon is called the right colic

(kol′ik) flexure, or the hepatic flexure (figure 26.16a).

The transverse colon originates at the right colic flexure

and curves slightly anteriorly as it projects horizontally across

the anterior region of the abdominal cavity. A type of mesentery

called the transverse mesocolon connects the transverse portion

of the large intestine to the posterior abdominal wall. Hence, the

transverse colon is intraperitoneal. As the transverse colon approaches the spleen in the left upper quadrant of the abdomen, it

makes a 90-degree turn inferiorly. The resulting bend in the colon

is called the left colic flexure, or the splenic (splen′ik) flexure

(figure 26.16a).

The descending colon is retroperitoneal and found along the

left side of the abdominal cavity. It originates at the left colic flexure and descends vertically until it terminates at the sigmoid colon.



External

anal sphincter



Anal sinuses



(b) Anal canal



The sigmoid (sig′moyd; resembling letter S) colon originates at the

sigmoid flexure, where the descending colon curves and turns inferomedially into the pelvic cavity. The sigmoid colon is intraperitoneal

and has a mesentery called the sigmoid mesocolon. The sigmoid

colon terminates at the rectum.

The rectum (rek′tŭm; rectus = straight) is a retroperitoneal

structure that connects to the sigmoid colon. The rectum is a muscular tube that readily expands to store accumulated fecal material

prior to defecation. Three thick, transverse folds of the rectum,

called rectal valves, ensure that fecal material is retained during

the passing of gas (figure 26.16b). The rectum then terminates at

the anal canal.

The terminal few centimeters of the large intestine are

called the anal (ā′năl) canal (figure 26.16b). The anal canal passes

through an opening in the levator ani muscles of the pelvic floor

and terminates at the anus. The internal lining of the anal canal

contains relatively thin longitudinal ridges, called anal columns,



Chapter Twenty-Six  Digestive System







795



Clinical View 26.4



Clinical View 26.3

Appendicitis



Diverticulosis and Diverticulitis



Inflammation of the appendix is called appendicitis (ă-pen-disī′tis). Most cases of appendicitis occur because fecal matter

obstructs the appendix. As the tissue in its wall becomes

inflamed, the appendix swells, the blood supply is compromised, and bacteria may proliferate in the wall. Untreated,

the appendix may burst and release its contents into the

peritoneum, causing a massive and potentially deadly infection called peritonitis.



During the early stages of appendicitis, the smooth

muscle wall (innervated by the ANS) contracts and goes into

spasms. This pain thus is referred to the T10 dermatome around

the umbilicus (see section 19.1b and figures 19.3 and 19.4).



As the inflammation worsens and the parietal peritoneum becomes inflamed as well, the pain becomes sharp

and localized to the right lower quadrant of the abdomen.

Individuals with appendicitis typically experience nausea or

vomiting, abdominal tenderness in the inferior right quadrant,

a low fever, and an elevated leukocyte count. Acute appendicitis may be treated first with antibiotics. If the appendicitis fails

to resolve, the inflamed appendix may be surgically removed

in a procedure called an appendectomy.



Diverticulosis (dī′ver-tik′yū-lō′sis) is the presence of diverticula

(small “bulges”) in the intestinal lining. These are formed typically when the colon tightens and narrows in response to low

amounts of fiber or bulk in the colon. Diverticulitis is inflammation of diverticula. Diverticulitis may be life-threatening if

the diverticula rupture and the intestinal contents leak into

the abdominal cavity.

Diverticula



(a)



(b)



(a) An external view of the sigmoid colon showing diverticula. (b) An

endoscopic view of diverticula.

(b) Photo courtesy Interactive Atlas of Gastrointestinal Endoscopy by Edgar

Jaramillo, available free of charge at www.gastrosource.com



Inflamed appendix



Learning Strategy

Many of the segments of the colon are named for the direction in which

materials travel. So material ascends (travels superiorly) in the ascending

colon, material travels across in the transverse colon, and material descends

(travels inferiorly) in the descending colon.

Mesentery for appendix



internal and external anal sphincters, which close off the opening to the anal canal and relax (open) during defecation. The

internal anal sphincter is involuntary, whereas the external anal

sphincter is voluntary (see section 11.5).

Inflamed appendix

© UIG/Phototake



between which are small depressions termed anal sinuses. As

fecal material passes through the anal canal during defecation,

pressure exerted on the anal sinuses causes their cells to release

excess mucin. As a result, this extra mucus lubricates the anal

canal during defecation. At the base of the anal canal are the



26.9b  Histology

The mucosa of the large intestine is lined with simple columnar epithelium and goblet cells (figure 26.17). Unlike the small intestine, the

large intestine mucosa lacks intestinal villi; h­ owever, it contains numerous intestinal glands that extend to the muscularis mucosae. The

glands’ goblet cells secrete mucin to lubricate the undigested material

as it passes through, and the simple columnar epithelial cells continue

to absorb nutrients that were not absorbed during passage through

the small intestine. Many lymphatic nodules (see section 24.4a) and

lymphatic cells occupy the lamina propria of the large intestine.



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