MOUTH. SALIVARY GLANDS.
SMALL AND LARGE INTESTINE
Our exploration of the digestive tract will follow the path of food from the mouth to the anus. The mouth opens into the oral cavity. We can summarize the functions of the oral cavity as follows: (1) analysis of material before swallowing; (2) mechanical processing through the actions of the teeth, tongue, and palatal surfaces; (3) lubrication by mixing with mucus and salivary secretions; and (4) limited digestion of carbohydrates and lipids.
The oral cavity, or buccal cavity (Figure 24-6), is lined by the oral mucosa, which has a stratified squamous epithelium. Only the regions exposed to severe abrasion, such as the superior surface of the tongue and the opposing surfaces of the hard palate, are covered by a layer of keratinized cells. The epithelial lining of the cheeks, lips, and undersurface of the tongue is relatively thin, nonkeratinized, and delicate. Although nutrient absorption does not occur in the oral cavity, the mucosa inferior to the tongue is thin enough and vascular enough to permit the rapid absorption of lipid-soluble drugs. Nitroglycerin is sometimes administered via this route to treat acute angina attacks.
The mucosae of the cheeks, or lateral walls of the oral cavity, are supported by pads of fat and the buccinator muscles. Anteriorly, the mucosa of each cheek is continuous with that of the lips, or labia (singular, labium). The vestibule is the space between the cheeks (or lips) and the teeth. Ridges of oral mucosa, the gums, or gingivae, surround the base of each tooth on the alveolar processes of the maxillary bones and mandible. The gingivae in most regions are firmly bound to the periostea of the underlying bones.
The roof of the oral cavity is formed by the hard and soft palates; the tongue dominates its floor (Figure 24-6). The floor of the mouth inferior to the tongue receives extra support from the mylohyoid muscle. The hard palate is formed by the palatine processes of the maxillary bones and the horizontal plates of the palatine bones. A prominent central ridge, or raphe, extends along the midline of the hard palate. The mucosa lateral and anterior to the raphe is thick, with complex ridges. When your tongue compresses food against your hard palate, these ridges provide traction. The soft palate lies posterior to the hard palate. A thinner and more-delicate mucosa covers the posterior margin of the hard palate and extends onto the soft palate.
The posterior margin of the soft palate supports the dangling uvula and two pairs of muscular pharyngeal arches (Figure 24-6). On either side, a palatine tonsil lies between an anterior palatoglossal arch and a posterior palatopharyngeal arch. A curving line that connects the palatoglossal arches and uvula forms the boundaries of the fauces, the passageway between the oral cavity and the oropharynx.
The tongue (Figure 24-6) manipulates materials inside the mouth and is occasionally used to bring foods (such as ice cream) into the oral cavity. The primary functions of the tongue are (1) mechanical processing by compression, abrasion, and distortion; (2) manipulation to assist in chewing and to prepare the material for swallowing; (3) sensory analysis by touch, temperature, and taste receptors, and (4) secretion of mucus and an enzyme, lingual lipase.
We can divide the tongue into an anterior body, or oral portion, and a posterior root, or pharyngeal portion. The superior surface, or dorsum, of the body contains a forest of fine lingual papillae. We described the structure of the lingual papillae in Chapter 17, in our discussion of taste buds and taste sensations. The thickened epithelium covering each papilla assists in the movement of materials by the tongue. A V-shaped line of circumvallate papillae roughly indicates the boundary between the body and the root of the tongue, which is situated in the pharynx (Figure 24-6b).
The epithelium covering the inferior surface of the tongue is thinner and more delicate than that of the dorsum. Along the inferior midline is the lingual frenulum (frenulum, a small bridle), a thin fold of mucous membrane that connects the body of the tongue to the mucosa covering the floor of the oral cavity. Ducts from two pairs of salivary glands open on either side of the lingual frenulum. The lingual frenulum prevents extreme movements of the tongue. However, if your lingual frenulum is too restrictive, you cannot eat or speak normally. When properly diagnosed, this condition, called ankyloglossia, can be corrected surgically.
The tongue's epithelium is flushed by the secretions of small glands that extend into the lamina propria of the tongue. These secretions contain water, mucus, and the enzyme lingual lipase. Lingual lipase begins the enzymatic breakdown of lipids, specifically triglycerides, before you have swallowed the food.
Your tongue contains two groups of skeletal muscles: (1) intrinsic tongue muscles and (2) extrinsic tongue muscles. All gross movements of the tongue are performed by the relatively large extrinsic muscles, which we detailed in Chapter 11. The smaller intrinsic muscles alter the shape of your tongue and assist the extrinsic muscles during precise movements, as in speech. Both intrinsic and extrinsic tongue muscles are under the control of the hypoglossal nerve (N XII).
Three pairs of salivary glands (Figure 24-7a) secrete into the oral cavity. Each pair of salivary glands has a distinctive cellular organization and produces saliva with slightly different properties:
1. The large parotid salivary glands lie inferior to the zygomatic arch beneath the skin that covers the lateral and posterior surface of the mandible. Each gland has an irregular shape, extending from the mastoid process of the temporal bone across the outer surface of the masseter muscle. The parotid salivary glands produce a thick, serous secretion containing large amounts of salivary amylase, an enzyme that breaks down starches (complex carbohydrates). The secretions of each parotid gland are drained by a parotid duct (Stensen's duct), which empties into the vestibule at the level of the second upper molar.
2. The sublingual salivary glands are covered by the mucous membrane of the floor of the mouth. These glands produce a watery, mucous secretion that acts as a buffer and lubricant. Numerous sublingual ducts (Rivinus' ducts) open along either side of the lingual frenulum.
3. The submandibular salivary glands are situated in the floor of the mouth along the inner surfaces of the mandible within a depression () called the mandibular groove. The submandibular glands (Figure 24-7b) secrete a mixture of buffers, glycoproteins called mucins, and salivary amylase. The submandibular ducts (Wharton's ducts) open into the mouth on either side of the lingual frenulum immediately posterior to the teeth (Figure 24-6a).
Movements of the tongue are important in passing food across the opposing surfaces of the teeth. The occlusal , or opposing, surfaces of your teeth perform chewing, or mastication, of food. Mastication breaks down tough connective tissues in meat and the plant fibers in vegetable matter, and it helps saturate the materials with salivary secretions and enzymes.
Figure 24-8a is a sectional view through an adult tooth. The bulk of each tooth consists of a mineralized matrix similar to that of bone. This material, called dentin, differs from bone in that it does not contain living cells. Instead, cytoplasmic processes extend into the dentin from cells in the central pulp cavity. The pulp cavity receives blood vessels and nerves from the root canal, a narrow tunnel located at the base, or root, of the tooth. Blood vessels and nerves enter the root canal through the apical foramen to supply the pulp cavity.
The root of each tooth sits within a bony socket called an alveolus. Collagen fibers of the periodontal ligament extend from the dentin of the root to the alveolar bone, creating a strong articulation known as a gomphosis. A layer of cementum covers the dentin of the root, providing protection and firmly anchoring the periodontal ligament. Cementum is very similar in histological structure to bone, and it is less resistant to erosion than is dentin.
The neck of the tooth marks the boundary between the root and the crown. The crown is the exposed portion of the tooth. A shallow gingival sulcus surrounds the neck of each tooth. The mucosa of the gingival sulcus is very thin, and it is not tightly bound to the periosteum. The epithelium is bound to the tooth over an extensive area. This epithelial attachment prevents bacterial access to the lamina propria of the gingiva and the relatively soft cementum of the root. When you brush and massage your gums, you stimulate the epithelial cells and strengthen the attachment. If the epithelial attachment breaks down, bacterial infection of the gingivae can occur, a condition called gingivitis.
The dentin of the crown is covered by a layer of enamel. Enamel contains calcium phosphate in a crystalline form; it is the hardest biologically manufactured substance. Adequate amounts of calcium, phosphates, and vitamin D during childhood are essential if the enamel coating is to be complete and resistant to decay.
Tooth decay generally results from the action of bacteria that inhabit your mouth. Bacteria adhering to the surfaces of the teeth produce a sticky matrix that traps food particles and creates deposits of plaque. Over time, this organic material can become calcified, forming a hard layer of tartar, or dental calculus, which can be difficult to remove. Tartar deposits most commonly develop at or near the gingival sulcus, where brushing cannot remove the relatively soft plaque deposits.
Types of Teeth
The alveolar processes of the maxillary bones and the mandible form the upper and lower dental arches, respectively. There are four types of teeth within these arches, each with specific functions (Figure 24-8b):
1. Incisors are blade-shaped teeth located at the front of the mouth. Incisors are useful for clipping or cutting, as when you nip off the tip of a carrot stick. These teeth have a single root.
2. The cuspids, or canines, are conical, with a sharp ridgeline and a pointed tip. They are used for tearing or slashing. You might weaken a tough piece of celery by the clipping action of the incisors but then take advantage of the shearing action provided by the cuspids. Cuspids have a single root.
3. Bicuspids, or premolars, have flattened crowns with prominent ridges. They are used for crushing, mashing, and grinding. Bicuspids have one or two roots.
4. Molars have very large, flattened crowns with prominent ridges. They excel at crushing and grinding. You can usually shift a tough nut or sparerib to your bicuspids and molars for successful crunching. Molars typically have three or more roots.
During development, two sets of teeth begin to form. The first to appear are the primary dentition: deciduous teeth (deciduus, falling off), also known as primary teeth, milk teeth, or baby teeth. Most children have 20 deciduous teeth—five on each side of the upper and lower jaws (Figure 24-9a). These teeth will later be replaced by the adult secondary dentition, or permanent dentition (Figure 24-9b). Adult jaws are larger and can accommodate more than 20 permanent teeth. Three additional molars appear on each side of the upper and lower jaws as the individual ages. These teeth extend the length of the tooth rows posteriorly and bring the permanent tooth count to 32.
On each side of the upper or lower jaw, the primary dentition consists of two incisors, one cuspid, and a pair of deciduous molars. These deciduous teeth are gradually replaced by the permanent dentition.
As replacement proceeds, the periodontal ligaments and roots of the primary teeth are eroded until eventually the deciduous teeth either fall out or are pushed aside by the eruption, or emergence, of the secondary teeth. The adult premolars take the place of the deciduous molars, and the definitive adult molars extend the tooth row as the jaw enlarges. The third molars, or wisdom teeth, may not erupt before age 21. When wisdom teeth fail to erupt, it is because they either develop in inappropriate positions or because there is inadequate space on the dental arch. Any teeth that develop in locations that do not permit their eruption are called impacted teeth. Impacted wisdom teeth can be surgically removed to prevent formation of abscesses in later life.
The muscles of mastication close your jaws and slide or rock your lower jaw from side to side. Chewing is not a simple process, as it can involve any combination of mandibular elevation, depression, protraction, retraction, and medial-lateral movement. (Try classifying the movements involved the next time you eat.)
During mastication, you force food from the oral cavity to the vestibule and back, crossing and recrossing the occlusal surfaces. This movement results in part from the action of the masticatory muscles, but control would be impossible without the aid of the buccal, labial, and lingual muscles. Once you have shredded or torn the material to a satisfactory consistency and moistened it with salivary secretions, your tongue begins compacting the debris into a bolus, or small oval mass. You can swallow a compact, moist, cohesive bolus relatively easily.
CONCEPT CHECK QUESTIONS
1. Which type of epithelium lines the oral cavity?
2. The digestion of which nutrient would be affected by damage to the parotid glands?
3. Which type of tooth is most useful for chopping off bits of relatively rigid foods?
The pharynx serves as a common passageway for solid food, liquids, and air. We described the epithelial lining and divisions of the pharynx—the nasopharynx, the oropharynx, and the laryngopharynx—in Chapter 23. Food normally passes through the oropharynx and laryngopharynx on its way to the esophagus. Both of these divisions have a stratified squamous epithelium similar to that of the oral cavity, and the lamina propria contains scattered mucous glands and the lymphoid tissue of the pharyngeal, palatal, and lingual tonsils. Beneath the lamina propria lies a dense layer of elastic fibers, bound to the underlying skeletal muscles.
· The pharyngeal constrictors push the bolus toward the esophagus.
· The palatopharyngeus and stylopharyngeus elevate the larynx.
· The palatal muscles elevate the soft palate and adjacent portions of the pharyngeal wall.
These muscles cooperate with muscles of the oral cavity and esophagus to initiate the process of swallowing, which pushes the bolus along the esophagus and into the stomach.
The pharynx is a chamber shared by the digestive and respiratory systems. It extends between the internal nares and the entrances to the larynx and esophagus. The curving superior and posterior walls of the pharynx are closely bound to the axial skeleton, but the lateral walls are flexible and muscular.
1. The nasopharynx is the superior portion of the pharynx. It is connected to the posterior portion of the nasal cavity through the internal nares and is separated from the oral cavity by the soft palate (Figure 23-3c). The nasopharynx is lined by the same pseudostratified ciliated columnar epithelium as that in the nasal cavity. The pharyngeal tonsil is located on the posterior wall of the nasopharynx; on each side, one of the auditory tubes opens into the nasopharynx.
2. The oropharynx (oris, mouth) extends between the soft palate and the base of the tongue at the level of the hyoid bone. The posterior portion of the oral cavity communicates directly with the oropharynx, as does the posterior inferior portion of the nasopharynx. At the boundary between the nasopharynx and the oropharynx, the epithelium changes from a pseudostratified columnar epithelium to a stratified squamous epithelium.
3. The narrow laryngopharynx, the inferior portion of the pharynx, includes that portion of the pharynx that lies between the hyoid bone and the entrance to the larynx and esophagus (Figure 23-3c). Like the oropharynx, it is lined by a stratified squamous epithelium that can resist mechanical abrasion, chemical attack, and pathogenic invasion.
The esophagus is a hollow muscular tube with a length of approximately 25 cm (1 ft) and a diameter of about 2 cm (0.75 in.) at its widest point. The primary function of the esophagus is to carry solid food and liquids to the stomach.
The esophagus begins posterior to the cricoid cartilage, at the level of vertebra C6. From this point, the narrowest portion of the esophagus, it descends toward the thoracic cavity posterior to the trachea, passes inferiorly along the dorsal wall of the mediastinum, and enters the abdominopelvic cavity through the esophageal hiatus, an opening in the diaphragm. The esophagus then empties into the stomach anterior to vertebra T7.
Within the neck, the esophagus receives blood from the external carotid and thyrocervical arteries. In the mediastinum, it is supplied by the esophageal arteries and branches of the bronchial arteries. As it passes through the esophageal hiatus, the esophagus receives blood from the inferior phrenic arteries, and the portion adjacent to the stomach is supplied by the left gastric artery. Blood from esophageal capillaries collects into the esophageal, inferior thyroid, azygos, and gastric veins.
The esophagus is innervated by parasympathetic and sympathetic fibers from the esophageal plexus. Resting muscle tone in the circular muscle layer in the upper 3 cm (1 in.) of the esophagus normally prevents air from entering your esophagus. A comparable zone at the inferior end of the esophagus normally remains in a state of active contraction. This condition prevents the backflow of materials from the stomach into the esophagus. Neither region contains a well-defined sphincter muscle comparable to those located elsewhere along the digestive tract. Nevertheless, the terms upper esophageal sphincter and lower esophageal sphincter (cardiac sphincter) are often used in recognition of the similarity in function.
Histology of the Esophagus
· The mucosa of the esophagus contains a nonkeratinized stratified squamous epithelium similar to that of the pharynx and oral cavity.
· The mucosa and submucosa are thrown into large folds that extend the length of the esophagus. These folds allow for expansion during the passage of a large bolus; except when you swallow, muscle tone in the walls keeps the lumen closed.
· The muscularis mucosae consists of an irregular layer of smooth muscle.
· The submucosa contains scattered esophageal glands that produce a mucous secretion that reduces friction between the bolus and the esophageal lining.
· The muscularis externa has inner circular and outer longitudinal layers. In the upper third of the esophagus, these layers contain skeletal muscle fibers; in the middle third, there is a mixture of skeletal and smooth muscle tissue; along the lower third, there is only smooth muscle.
· There is no serosa, but an adventitia of connective tissue outside the muscularis externa anchors the esophagus in position against the dorsal body wall. Over the 1-2 cm between the diaphragm and stomach, the esophagus is retroperitoneal, with peritoneum covering the anterior and left lateral surfaces.
Swallowing, or deglutition, is a complex process whose initiation can be voluntarily controlled, but it proceeds automatically once it begins. Although you are consciously aware of, and voluntarily control, swallowing when you eat or drink, swallowing can also occur unconsciously, as saliva collects at the back of the mouth. Each day you swallow approximately 2400 times. We can divide swallowing into buccal, pharyngeal, and esophageal phases:
1. The buccal phase begins with the compression of the bolus against the hard palate. Subsequent retraction of the tongue then forces the bolus into the pharynx and assists in the elevation of the soft palate, thereby isolating the nasopharynx (Figure 24-11a,b). The buccal phase is strictly voluntary. Once the bolus enters the oropharynx, reflex responses are initiated, and the bolus is moved toward the stomach.
2. The pharyngeal phase begins as the bolus comes into contact with the palatoglossal and palatopharyngeal arches and the posterior pharyngeal wall (Figure 24-11c,d). The swallowing reflex begins as tactile receptors on the palatal arches and uvula are stimulated by the passage of the bolus. The information is relayed to the swallowing center of the medulla oblongata over the trigeminal and glossopharyngeal nerves. Motor commands originating at this center then target the pharyngeal musculature, producing a coordinated and stereotyped pattern of muscle contraction. Elevation of the larynx and folding of the epiglottis direct the bolus past the closed glottis, while the uvula and soft palate block passage back to the nasopharynx. It takes less than a second for the pharyngeal muscles to propel the bolus into the esophagus. During this period the respiratory centers are inhibited, and breathing stops.
3. The esophageal phase of swallowing begins as the contraction of pharyngeal muscles forces the bolus through the entrance to the esophagus (Figure24-11e-h). Once within the esophagus, the bolus is pushed toward the stomach by a peristaltic wave. The approach of the bolus triggers the opening of the lower esophageal sphincter, and the bolus then continues into the stomach.
Primary peristaltic contractions are coordinated by afferent and efferent fibers within the glossopharyngeal and vagus nerves. For a typical bolus, the entire trip takes about 9 seconds to complete. Liquids may make the journey in a few seconds, arriving ahead of the peristaltic contractions with the assistance of gravity.
A relatively dry or poorly lubricated bolus travels much more slowly, and a series of secondary peristaltic waves may be required to push it all the way to the stomach. Secondary peristaltic waves are local reflexes triggered by the stimulation of sensory receptors in the esophageal walls. These receptors relay information by way of the submucosal and myenteric plexuses, producing peristaltic contractions in the absence of CNS instructions. You cannot swallow a completely dry bolus, because friction with the walls of the esophagus will make peristalsis ineffective. (For this reason, you cannot swallow an entire slice of processed white bread without taking a drink.
CONCEPT CHECK QUESTIONS
1. What is unusual about the muscularis externa of the esophagus?
2. Where in the human body would you find the fauces?
3. What is occurring when the soft palate and larynx elevate and the glottis closes?
Small and large intestine
Learning objectives. After studying this lesson, you should be able to describe the structure of the stomach, small and large intestine.
Terms to remember: Stomach, Chyme, Doudenum, Jejunum, Ileum, Peristalsis, Secum, Colon
The stomach performs four major functions: (1) the bulk storage of ingested food, (2) the mechanical breakdown of ingested food, (3) the disruption of chemical bonds in food material through the action of acids and enzymes, and (4) the production of intrinsic factor, a glycoprotein whose presence in the digestive tract is required for the absorption of vitamin B12. The mixing of ingested substances with the secretions of the glands of the stomach produces a viscous, highly acidic, soupy mixture of partially digested food. This material is called chyme.
Anatomy of the Stomach
The stomach has the shape of an expanded J. A short lesser curvature
forms the medial surface of the organ, and a long greater curvature forms the
lateral surface. The anterior and posterior surfaces are smoothly rounded. The
shape and size of the stomach are extremely variable from individual to
individual and even from one meal to the next. In an "average"
stomach, the lesser curvature has a length of approximately
We can divide the stomach into four regions
is the smallest part of the stomach. It consists of the superior, medial
portion of the stomach within
2. The fundus. The portion of the stomach superior to the junction between the stomach and esophagus is the fundus. The fundus contacts the inferior and posterior surface of the diaphragm.
3. The body. The area between the fundus and the curve of the J is the body of the stomach. The body is the largest region of the stomach, and it functions as a mixing tank for ingested food and secretions produced within the stomach. Gastric glands (gaster, stomach) in the fundus and body secrete most of the acids and enzymes involved in gastric digestion.
4. The pylorus. The pylorus is the curve of the J. The pylorus is divided into a pyloric antrum (antron, cavity), which is connected to the body, and a pyloric canal that empties into the duodenum, the proximal segment of the small intestine. As mixing movements occur during digestion, the pylorus frequently changes shape. A muscular pyloric sphincter regulates the release of chyme into the duodenum. Glands in the pylorus secrete mucus and important digestive hormones, including gastrin, a hormone that stimulates the activity of gastric glands.
The stomach's volume increases at mealtime, then decreases as chyme enters the small intestine. When your stomach is
relaxed (empty), the mucosa is thrown into prominent folds called rugae
(wrinkles). Rugae are temporary features that let the
gastric lumen expand. As your stomach fills, the rugae
flatten out. When your stomach is full, the rugae
almost disappear. When empty, your stomach resembles a muscular tube with a
narrow, constricted lumen. When full, it can expand to contain 1-
Musculature of the Stomach
The muscularis mucosae and muscularis externa of the stomach contain extra layers of smooth muscle cells in addition to the usual circular and longitudinal layers. The muscularis mucosae generally contains an outer, circular layer of muscle cells. The muscularis externa has an inner, oblique layer of smooth muscle. The extra layer of smooth muscle strengthens the stomach wall and assists in the mixing and churning activities essential to the formation of chyme.
A simple columnar epithelium lines all portions of the stomach. The epithelium is a secretory sheet that produces a carpet of mucus that covers the interior surfaces of the stomach. The alkaline mucous layer protects epithelial cells against the acids and enzymes in the gastric lumen.
The stomach receives blood from (1) the left gastric artery; (2) the splenic artery, which supplies the left gastroepiploic artery; and (3) the common hepatic artery, which supplies the right gastric, gastroduodenal, and right gastroepiploic arteries.
Shallow depressions called gastric pits open onto the gastric surface. The mucous cells at the base, or neck, of each gastric pit are actively dividing, replacing superficial cells that are shed into the chyme. The continuous replacement of epithelial cells provides an additional defense against the acidic gastric contents. A typical epithelial cell has a life span of 3-7 days, but exposure to strong alcohol or other chemicals can increase the rate of cell turnover.
In the fundus and body of the stomach, each gastric pit communicates with several gastric glands that extend deep into the underlying lamina propria. Gastric glands are dominated by two types of secretory cells: (1) parietal cells and (2) chief cells. Together they secrete about 1500 ml of gastric juice each day.
Parietal Cells. Parietal cells are especially common along the proximal portions of each gastric gland. These cells secrete intrinsic factor and hydrochloric acid (HCl). Intrinsic factor is a glycoprotein that facilitates the absorption of vitamin B12 across the intestinal lining.
The parietal cells do not produce HCl in the cytoplasm, because it is such a strong acid that it would erode a secretory vesicle and destroy the cell. Instead, H+ and Cl, the two ions that form HCl, are transported independently by different mechanisms.
Hydrogen ions are generated inside the cell as the enzyme carbonic anhydrase converts carbon dioxide and water to carbonic acid. The carbonic acid promptly dissociates into hydrogen ions and bicarbonate ions. The hydrogen ions are actively transported into the lumen of the gastric gland. The bicarbonate ions are ejected into the interstitial fluid by a countertransport mechanism that exchanges intracellular bicarbonate ions for extracellular chloride ions. The chloride ions then diffuse across the cell and through open chloride channels in the cell membrane to the lumen of the gastric gland.
The bicarbonate ions released by the parietal cell diffuse through the interstitial fluid into the bloodstream. When gastric glands are actively secreting, enough bicarbonate ions enter the circulation to increase the pH of the blood significantly. This sudden influx of bicarbonate ions has been called the alkaline tide.
THE SMALL INTESTINE AND ASSOCIATED GLANDULAR ORGANS
Your stomach is a holding tank where food is saturated with gastric juices and exposed to stomach acids and the digestive effects of pepsin. These are preliminary steps, for most of the important digestive and absorptive functions occur in your small intestine, where the products of digestion are absorbed. The mucosa of the small intestine produces only a few of the enzymes involved. The pancreas provides digestive enzymes as well as buffers that assist in the neutralization of acidic chyme. The liver and gallbladder provide bile, a solution that contains additional buffers and bile salts, compounds that facilitate the digestion and absorption of lipids.
The Small Intestine
The small intestine plays the primary role in the digestion and
absorption of nutrients. The small intestine averages
A rather abrupt bend marks the boundary between
the duodenum and the jejunum.
At this junction, the small intestine reenters the
peritoneal cavity, supported by a sheet of mesentery. The jejunum is about
is the third and last segment of the small intestine. It is also the longest,
The small intestine fills much of the peritoneal cavity, and its position is stabilized by a broad mesentery attached to the dorsal body wall.
Movement of the small intestine during digestion is restricted by the stomach, the large intestine, the abdominal wall, and the pelvic girdle. Blood vessels, lymphatics, and nerves reach these segments of the small intestine within the connective tissue of the mesentery. The primary blood vessels involved are branches of the superior mesenteric artery and the superior mesenteric vein.
The intestinal lining bears a series of transverse folds called plicae, or plicae circulares. Unlike the rugae in the stomach, each plica is a permanent feature that does not disappear when the small intestine fills with chyme. There are roughly 800 plicae along the length of the small intestine, and their presence greatly increases the surface area available for absorption.
The mucosa of the small intestine is thrown into a series of fingerlike projections, the intestinal villi. The intestinal villi are covered by a simple columnar epithelium that is carpeted with microvilli. Because the microvilli project from the epithelium like the bristles on a brush, these cells are said to have a brush border.
If the small intestine were a simple tube with smooth walls, it would
have a total absorptive area of roughly 3300 cm2 (
The lamina propria of each villus contains an extensive network of capillaries. These capillaries originate in a vascular network within the submucosa. They transport respiratory gases and carry absorbed nutrients to the hepatic portal circulation for delivery to the liver. The liver adjusts the nutrient concentrations of the blood before it reaches the general systemic circulation.
In addition to capillaries and nerve endings, each villus contains a lymphatic capillary called a lacteal (lacteus, milky). Lacteals transport materials that are unable to enter blood capillaries. For example, absorbed fatty acids are assembled into protein-lipid packages too large to diffuse into the bloodstream. These packets, called chylomicrons, reach the venous circulation by way of the thoracic duct, which delivers lymph into the left subclavian vein. The name lacteal refers to the pale, cloudy appearance of lymph that contains large quantities of lipids.
Contractions of the muscularis mucosae and smooth muscle cells within the villi move the villi back and forth, exposing the epithelial surfaces to the liquefied intestinal contents. This movement improves the efficiency of absorption, because local differences in the nutrient concentration of the chyme will be quickly eliminated. Movements of the villi also squeeze the lacteals, thus assisting in the movement of lymph out of the villi.
Between the columnar epithelial cells, goblet cells eject mucins onto the intestinal surfaces. At the bases of the villi are the entrances to the intestinal crypts (also known as intestinal glands or crypts of Lieberkuhn). These glandular pockets extend deep into the underlying lamina propria. Near the base of each intestinal gland, stem cell divisions produce new generations of epithelial cells. These new cells are continuously displaced toward the intestinal surface. Within a few days, they will have reached the tip of a villus, where they are shed into the intestinal lumen. This ongoing process renews the epithelial surface and the subsequent disintegration of the shed cells adds enzymes to the chyme.
Several important brush border enzymes enter the intestinal lumen in this way. Brush border enzymes are integral membrane proteins located on the surfaces of intestinal microvilli. The enzymes have important digestive functions: Materials in contact with the brush border are attacked by these enzymes, and the breakdown products are absorbed by the epithelial cells. Once the epithelial cells are shed, they disintegrate within the lumen, and the intracellular and brush border enzymes enter the chyme. There they continue to function until proteolytic enzymes break them apart. Enterokinase, also called enteropeptidase, is a brush border enzyme that enters the lumen in this way. Enterokinase does not directly participate in digestion, but it activates proenzymes secreted by the pancreas. (We shall consider the functions of enterokinase and other brush border enzymes in a later section.) Intestinal crypts also contain enteroendocrine cells responsible for theproduction of several intestinal hormones, including gastrin, cholecystokinin, and secretin.
The regions of the small intestine have histological specializations related to their primary functions. The duodenum contains few plicae; the villi are numerous but shorter and stumpier than those of the jejunum. There are numerous mucous glands in the duodenum, both within the epithelium and beneath it. In addition to the intestinal crypts, the submucosa contains submucosal glands, or Brunner's glands, which produce copious quantities of mucus when chyme arrives from the stomach. Mucus produced by these glands protects the epithelium from the acidic chyme. It also contains buffers that help elevate the pH of the chyme. Along the length of the duodenum, the pH of the chyme goes from 1-2 to 7-8. The submucosal glands also secrete the hormoneurogastrone, which inhibits gastric acid production. Urogastrone, or epidermal growth factor (EGF), stimulates the division of epithelial cells along the digestive tract as well as stem cell activity in other areas.
Jejunum. Plicae and villi are prominent over the proximal half of the jejunum. As materials approach the ileum, the plicae and villi become smaller and continue to diminish in size to the end of the ileum. This reduction parallels the reduction in absorptive activity; most nutrient absorption has occurred before ingested materials reach the ileum. One rather drastic surgical method of promoting weight loss is the removal of a significant portion of the jejunum. The reduction in absorptive area causes a marked weight loss, but the side effects can be very troublesome.
Ileum. The ileum adjacent to the large intestine lacks plicae altogether, and the scattered villi are stumpy and conical. The ileum also contains 20-30 masses of lymphoid tissue called aggregate lymphoid nodules, or Peyer's patches. The lymphocytes in these nodules protect the small intestine from bacteria that are normal inhabitants of the large intestine. Lymphoid nodules are most abundant in the terminal portion of the ileum, near the entrance to the large intestine.
After chyme has arrived in the duodenum, weak peristaltic contractions move it slowly toward the jejunum. These contractions are myenteric reflexes not under CNS control. Their effects are limited to within a few centimeters of the site of the original stimulus. These short reflexes are controlled by motor neurons in the submucosal and myenteric plexuses. In addition, some of the smooth muscle cells contract periodically, even without stimulation, establishing a basic contractile rhythm that then spreads from cell to cell.
The stimulation of the parasympathetic system in- creases the sensitivity of these myenteric reflexes and accelerates both local peristalsis and segmentation. More-elaborate reflexes coordinate activities along the entire length of the small intestine. Two examples are triggered by the stimulation of stretch receptors in the stomach as it fills. The gastroenteric reflex stimulates motility and secretion along the entire length of the small intestine; the gastroileal reflex triggers the relaxation of the ileocecal valve. The net result is that materials pass from the small intestine into the large intestine. Thus the gastroenteric and gastroileal reflexes accelerate movement along the small intestine—the opposite effect of the enterogastric reflex.
Hormones released by the digestive tract can enhance or suppress reflex responses. For example, the gastroileal reflex is triggered by stretch receptor stimulation, but the degree of ileocecal valve relaxation is enhanced by gastrin, which is secreted in large quantities when food enters the stomach.
MATURATION OF THE SMALL INTESTINE
The exact timing of the cellular morphogenesis of the gut is difficult to establish, especially as it undergoes a proximodistal gradient in maturation. Developmental differences between parts of the small intestine or colon have not yet been correlated with age. The endodermal cells of the small intestine proliferate and form a layer some three to four cells thick with mitotic figures throughout. From 7 weeks, blunt projections of the endoderm have begun to form in the duodenum and proximal jejunum; these are the developing villi which increase in length until in the duodenum the lumen becomes difficult to discern. The concept of occlusion of the lumen and recanalization which is described in many accounts of development does not match the cytodifferentiation which occurs in the gut epithelia. Thus it is no longer thought that there is secondary recanalization of the gut lumen. By 9 weeks the duodenum, jejunum and proximal ileum have villi and the remaining distal portion of ileum develops villi by 11 weeks. The villi are covered by a simple epithelium. Primitive crypts, epithelial downgrowths into the mesenchyme between the villi, appear between 10 and 12 weeks similarly along a craniocaudal progression. Brunner's glands are present in the duodenum from 15 weeks and the muscularis mucosa can be seen in the small intestine from 18 weeks.
Whereas mitotic figures are initially seen throughout the endodermal layer of the small intestine prior to villus formation, by 10–12 weeks they are limited to the intervillous regions and the developing crypts. It is believed that an ‘adult' turnover of cells may exist when rounded-up cells can be observed at the villus tips, in position for exfoliation. The absorptive enterocytes have microvilli at their apical borders before 9 weeks. An apical tubular system appears at this time composed of deep invaginations of the apical plasma membrane and membrane-bound vesicles and tubules; many lysosomal elements (meconium corpuscles) appear in the apical cytoplasm. These latter features are more developed in the ileum than jejunum, are most prominent at 16 weeks, and diminish by 21 weeks. There are abundant deposits of glycogen in the fetal epithelial cells, and it has been suggested that prior to the appearance of hepatic glycogen the intestinal epithelium serves as a major glycogen store. Goblet cells are present in small numbers by 8 weeks, Paneth's cells differentiate at the base of the crypts in weeks 11 and 12, and enteroendocrine cells appear between weeks 9 and 11. M cells (membrane or microfold cells) are present from 14 weeks.
Meconium can be detected in the lumen of the intestine by the 16th week. It is derived from swallowed amniotic fluid, which contains vernix and cellular debris, salivary, biliary, pancreatic and intestinal secretions, and sloughed enterocytes. As the mixture passes along the gut, water and solutes are removed and cellular debris and proteins concentrated. Meconium contains enzymes from the pancreas and proximal intestine in higher concentrations in preterm than full-term babies.
The muscularis layer is derived from the splanchnopleuric mesenchyme as it is in other parts of the gut. Longitudinal muscle can be seen from 12 weeks. At 26–30 weeks the gut shows contractions without regular periodicity; from 30–33 weeks repetitive groups of regular contractions have been seen in preterm neonates.
The small intestine possesses only a dorsal mesentery. The movement of the root of this dorsal mesentery, and the massive lengthening of its enteric border in order to match the longitudinal growth of the gut tube, reflect the spiralizing of the midgut loop in the umbilical coelom. The specific regions of adherence of the serosa and parietal peritoneum of the small intestine in the peritoneal cavity are given on page 1216.
In the neonate the small intestine forms an oval-shaped mass with its greater diameter transversely orientated in the abdomen, rather than vertically as in the adult. The mass of the small intestine inferior to the umbilicus is compressed by the urinary bladder, which is anterior at this point. The small intestine is 300–350 cm long at birth and its width when empty is 1–1.5 cm. The ratio between the length of the small and large intestine at birth is similar to the adult ratio. The mucosa and submucosa are fairly well developed and villi are present throughout the small intestine, however, some epithelial differentiation is incomplete. The muscularis is very thin, particularly the longitudinal layer, and there is little elastic tissue in the wall. There are few or no circular folds in the small intestine, and the jejunum and ileum have little fat in their mesentery.
The layers of tissue in the large intestinal wall (see Fig. 67.3; Fig. 67.49) resemble those in the small intestine (Ch. 66), except that villi and circular folds are absent and the glands (crypts) are longer.
Fig. 67.49 The microstructure of the colonic wall and its epithelial cells. Note the aggregations of lymphocytes (blue) and undifferentiated epithelial cells (white).
The mucosa is pale, smooth, and, in the colon, raised into numerous crescent-shaped folds between the sacculi. In the rectum it is thicker, darker, more vascular, and more loosely attached to the submucosa.
The luminal surface of all but the anorectal junction is lined by columnar cells, mucous (goblet) cells, and occasional microfold (M) cells that are restricted to the epithelium overlying lymphoid follicles. Columnar and mucous cells are also present in the intestinal glands (crypts) which additionally contain stem cells and neuroendocrine cells. In general, the glands lack Paneth cells, but some may be present in the caecum.
Columnar (absorptive) cells
Columnar (absorptive) cells are the most numerous of the epithelial cell types. They are responsible for ion exchange and other transepithelial transport functions including water resorption, particularly in the colon. Although there is some variation in their structure, they all bear apical microvilli, which are shorter and less regular than those on enterocytes in the small intestine. All cells have typical junctional complexes around their apices, and these limit extracellular diffusion from the lumen across the intestine wall.
Mucous (goblet) cells
Mucous cells have a similar structure to those of the small intestine, but are more numerous. They are outnumbered by absorptive cells for most of the length of the colon, but they are equally frequent towards the rectum, where their numbers increase further.
Microfold (M) cells
Microfold cells are similar to those of the small intestine: they are flattened or cuboidal cells with long, blunt microfolds rather than typical microvilli, and they are restricted to epithelium overlying lymphoid follicles.
Stem cells are the source of the other epithelial cell types in the large intestine. They are located at or near the bases of the intestinal glands, where they divide by mitosis. They provide cells that migrate towards the luminal surface of the intestine: their progeny differentiate, undergo apoptosis and are shed after approximately 5 days.
Neuroendocrine cells are situated mainly at the bases of the glands, and secrete basally into the lamina propria.
Intestinal glands (crypts)
The crypts are narrow perpendicular tubular glands which are longer, more numerous and closer together than those of the small intestine. Their openings give a cribriform appearance to the mucosa in surface view. The glands are lined by low columnar epithelial cells, mainly goblet cells, between which are columnar absorptive cells and neuroendocrine cells. Epithelial stem cells at their bases give rise to all three cell types.
The lamina propria is composed of connective tissue that supports the epithelium. It forms a specialized pericryptal myofibroblast sheath around each intestinal gland. Solitary lymphoid follicles within the lamina propria are most abundant in the caecum, appendix and rectum, but are also present scattered along the rest of the large intestine. They are similar to those of the small intestine; efferent lymphatic vessels originate within them. Lymphatic vessels are absent from the lamina propria core between crypts.
The muscularis mucosae of the large intestine is essentially similar to that of the small intestine: it has prominent longitudinal and circular layers.
The submucosa of the large intestine is similar to that of the small intestine.
The muscularis externa has outer longitudinal and inner circular layers of smooth muscle. The longitudinal fibres form a continuous layer but, with the exception of the uniform outer muscle layer of most of the appendix, macroscopically these are aggregated as longitudinal bands or taeniae coli (see Figs 67.3 and 67.12). Between the taeniae coli the longitudinal layer is much thinner, less than half the circular layer in thickness. The circular fibres form a thin layer over the caecum and colon, and are aggregated particularly in the intervals between the sacculi. In the rectum they form a thick layer and in the anal canal they form the internal anal sphincter. There is an interchange of fascicles between circular and longitudinal layers, especially near the taeniae coli. Deviation of longitudinal fibres from the taeniae to the circular layer may, in some instances, explain the haustration of the colon.
The serosa or visceral peritoneum is variable in extent. Along the colon the peritoneum forms small fat-filled appendices epiploicae which are most numerous on the sigmoid and transverse colon but generally absent from the rectum. Subserous loose connective tissue attaches the peritoneum to the muscularis externa.
THE LARGE INTESTINE
The horseshoe-shaped large intestine begins at the end of the ileum and ends at the anus. The large intestine lies inferior to the stomach and liver and almost completely frames the small intestine. The major functions of the large intestine include (1) the reabsorption of water and compaction of intestinal contents into feces, (2) the absorption of important vitamins liberated by bacterial action, and (3) the storing of fecal material prior to defecation.
The large intestine, or the large bowel , has an
average length of about
Material arriving from the ileum first enters an
expanded pouch called the cecum. The ileum attaches to the
medial surface of the cecum and opens into the cecum at the ileocecal
valve. The cecum collects and stores chyme and begins
the process of compaction. The slender, hollow vermiform appendix (vermis, worm), or simply appendix, is attached to the posteromedial surface of the cecum.
The appendix is generally about
The colon has a larger diameter and a thinner wall than the small intestine. Distinctive features of the colon include the following:
· The wall of the colon forms a series of pouches, or haustra (singular, haustrum). Cutting into the intestinal lumen reveals that the creases between the haustra affect the mucosal lining as well, producing a series of internal folds. Haustra permit the expansion and elongation of the colon rather like the bellows that allow an accordion to lengthen.
· Three separate longitudinal ribbons of smooth muscle—the taenia coli , are visible on the outer surfaces of the colon just beneath the serosa. These bands correspond to the outer layer of the muscularis externa in other portions of the digestive tract. Muscle tone within these bands creates the haustra.
· The serosa of the colon contains numerous teardrop-shaped sacs of fat called epiploic appendages.
Regions of the Colon
We can subdivide the colon into four regions: the ascending colon, the transverse colon, the descending colon, and the sigmoid colon:
5. The ascending colon begins at the superior border of the cecum and ascends along the right lateral and posterior wall of the peritoneal cavity to the inferior surface of the liver. At this point, the colon makes a sharp bend to the left at the right colic flexure, or hepatic flexure. This flexure marks the end of the ascending colon and the beginning of the transverse colon. The ascending colon is retroperitoneal, and only its lateral and anterior surfaces are covered by the peritoneum.
6. The transverse colon curves anteriorly from the right colic flexure and crosses the abdomen from right to left. It is supported by the transverse mesentery and is separated from the anterior abdominal wall by the layers of the greater omentum. As the transverse colon reaches the left side of the body, it passes inferior to the greater curvature of the stomach. Near the spleen, the colon makes a 90° turn at the left colic flexure, or splenic flexure, and becomes the descending colon.
7. The descending colon proceeds inferiorly along the left side until reaching the iliac fossa. The descending colon is retroperitoneal and firmly attached to the abdominal wall. At the iliac fossa, the descending colon curves at the sigmoid flexure and becomes the sigmoid colon.
The sigmoid flexure is the start of
the sigmoid colon (sigmeidos, the Greek letter S), an S-shaped segment that is only about
The large intestine receives blood from tributaries of the superior mesenteric and inferior mesenteric arteries, and venous blood is collected by the superior mesenteric and inferior mesenteric veins.
The rectum forms the
The last portion of the rectum, the anorectal canal, contains small longitudinal folds, the rectal columns. The distal margins of the rectal columns are joined by transverse folds that mark the boundary between the columnar epithelium of the proximal rectum and a stratified squamous epithelium like that in the oral cavity. Very close to the anus, or anal orifice (the exit of the anorectal canal), the epidermis becomes keratinized and identical to the surface of the skin.
There is a network of veins in the lamina propria and submucosa of the anorectal canal. If venous pressures there rise too high due to straining during defecation, the veins may become distended, producing hemorrhoids. The circular muscle layer of the muscularis externa in this region forms the internal anal sphincter. The smooth muscle cells of the internal anal sphincter are not under voluntary control. The external anal sphincter guards the anus. This sphincter, which consists of a ring of skeletal muscle fibers, is under voluntary control.
Although the diameter of the colon is roughly three times that of the small intestine, its wall is much thinner. The major characteristics of the colon are the lack of villi, the abundance of goblet cells, and the presence of distinctive intestinal glands. The glands in the large intestine are deeper than those of the small intestine, and they are dominated by goblet cells. The mucosa of the large intestine does not produce enzymes; any digestion that occurs results from enzymes introduced in the small intestine or from bacterial action. The mucus is important in providing lubrication as the fecal material becomes less moist and more compact. Mucous secretion occurs as local stimuli, such as friction or exposure to harsh chemicals, trigger short reflexes involving local nerve plexuses. Large lymphoid nodules are scattered throughout the lamina propria and submucosa.
The muscularis externa of the large intestine is unusual because the longitudinal layer has been reduced to the muscular bands of the taenia coli. However, the mixing and propulsive contractions of the colon resemble those of the small intestine.
Examples of the tests
Fifty years old patient with the symptoms of the obstructive jaundice was suspiced to the cancer of the Vater’s papilla. The Vater’s papilla is situated:
A: in the upper part of duodenum
B: in horizontal part of duodenum
C: in ascending part of duodenum
E: in the descending part of duodenum
D: in duodeno-jejunal flexure
Where will escaped gastric contents drain to if a gastric ulcer perforates (a) through the anterior stomach wall (b) through the posterior stomach wall?
(a) Into the greater peritoneal sac between the anterior abdominal wall and greater omentum. (b) Into the lesser sac.
Where the angular incisura of the stomach located ?
a. lesser curvature
b. greater curvature
c. cardiac ostium
d. pyloric ostium
What do touch the back stomach wall?
g. left liver lobe
h. right liver lobe
i. anterior abdominal wall
Describe relation of the stomach and peritoneum:
A. Extraperitoneal position
B. Mesoperitoneal position
C. Intraperitoneal position and presence of ligaments
D. Intraperitoneal position without any ligament
E. peritoneum does not cover the stomach
Describe relation of the sigmoid colon and peritoneum:
a. Extraperitoneal position
b. Mesoperitoneal position
c. Intraperitoneal position and presence of mesentery
d. Intraperitoneal position without mesentery
e. peritoneum does not cover it
Which folds does rectal ampulla have?
f. longitudinal folds
g. transverse folds
h. circular folds
i. semilunar folds
An obstruction of the cystic duct would result in:
Inability to digest protein
Increased sugar in the chyme
An inability to absorb water-soluble vitamins
The longer bile stays in the gallbladder, the
More dilute it becomes
Higher the concentration of bile salts becomes
Lower the concentration of bile pigments becomes
More water it contains
All of the above
A blockage of the opening in the duodenal papilla would
Interfere with neutralization of gastric chyme
Impair fat digestion
Decrease protein digestion
Decrease carbohydrate digestion
Any of the above
Decreased levels of bile salts in the bile would interfere with
Digestion of disaccharides
Digestion of complex carbohydrates
Digestion of vitamins
5. An intestinal hormone that stimulates the gallbladder to release bile is
6. The fusion of the hepatic duct and the cystic duct forms the
Hepatic portal vein
Common pancreatic duct
Common bile duct
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