Chapter 24:  The Digestive System     PART 1

 

Substances are used as raw materials for synthesizing essential compounds (anabolism) or are decomposed to provide the energy that cells need to continue functioning (catabolism).

 

The catabolic reactions require two essential ingredients: (1) oxygen and (2) organic molecules, such as carbohydrates, fats, or proteins, that can be broken down by intracellular enzymes.

 

The digestive system provides both the fuel that keeps all the body's cells running and the building blocks needed for cell growth and repair.

The digestive system consists of a muscular tube, the digestive tract , also called the gastrointestinal (GI) tract or alimentary canal , and various accessory organs . The oral cavity (mouth), pharynx , esophagus , stomach , small intestine , and large intestine make up the digestive tract.

 

 Accessory digestive organs include the teeth, tongue, and various glandular organs , such as the salivary glands, liver, and pancreas, which secrete into ducts emptying into the digestive tract.

 

 

Functions of the Digestive System

 

Digestion refers to the chemical breakdown of food into small organic fragments suitable for absorption by the digestive epithelium.

 

Secretion is the release of water, acids, enzymes, buffers, and salts by the epithelium of the digestive tract and by glandular organs.

 

Absorption is the movement of organic substrates, electrolytes (inorganic ions), vitamins, and water across the digestive epithelium and into the interstitial fluid of the digestive tract.

 

Excretion is the removal of waste products from body fluids. The digestive tract and glandular organs discharge waste products in secretions that enter the lumen of the tract. Most of these waste products, after mixing with the indigestible residue of the digestive process, will leave the body. The ejection of materials from the digestive tract, a process called defecation, or egestion , eliminates materials as feces .

 

The lining of the digestive tract also plays a protective role by safeguarding surrounding tissues against (1) the corrosive effects of digestive acids and enzymes; (2) mechanical stresses, such as abrasion; and (3) bacteria that either are swallowed with food or reside in the digestive tract.

 

When bacteria reach the underlying layer of areolar tissue, the lamina propria , they are attacked by macrophages and other cells of the immune system.

 

The Digestive Organs and the Peritoneum
The abdominopelvic cavity contains the peritoneal cavity.

 

Because a thin layer of peritoneal fluid separates the parietal and visceral surfaces, relative movement can occur without friction and resulting irritation. About seven liters of fluid is secreted and reabsorbed each day, although the volume within the peritoneal cavity at any one time is very small. Liver disease, kidney disease, and heart failure can cause an increase in the rate at which fluids move into the peritoneal cavity. The accumulation of fluid creates a characteristic abdominal swelling called ascites. The distortion of internal organs by this fluid can result in symptoms such as heartburn, indigestion, and lower back pain.

Mesenteries
Portions of your digestive tract are suspended within the peritoneal cavity by sheets of serous membrane that connect the parietal peritoneum with the visceral peritoneum. These mesenteries are double sheets of peritoneal membrane.

 

Mesenteries also stabilize the positions of the attached organs and prevent your intestines from becoming entangled during digestive movements or sudden changes in body position.

 

 

 (b) A diagrammatic view of the organization of mesenteries in an adult. As the digestive tract enlarges, mesenteries associated with the proximal portion of the small intestine, the pancreas, and the ascending and descending portions of the colon fuse to the body wall. (c) An anterior view of the empty peritoneal cavity, showing attachment sites where fusion occurs. (d) A sagittal section of the mesenteries of an adult. Notice that the pancreas, duodenum, and rectum are retroperitoneal

 

 

 

 

 

The lesser omentum stabilizes the position of the stomach and provides an access route for blood vessels and other structures entering or leaving the liver. The falciform  ligament helps stabilize the position of the liver relative to the diaphragm and abdominal wall.

 

The greater omentum hangs like an apron from the lateral and inferior borders of the stomach.

 

All but the first 25 cm of the small intestine is suspended by the mesentery proper , a thick mesenterial sheet that provides stability, but permits some independent movement.

 

A mesocolon is a mesentery associated with a portion of the large intestine

 

The transverse mesocolon , which supports the transverse colon, and the sigmoid mesocolon , which supports the sigmoid colon, are all that remains of the original embryonic mesocolon.

 

An inflammation of the peritoneal membrane produces symptoms of peritonitis a painful condition that interferes with the normal functioning of the affected organs. Physical damage, chemical irritation, and bacterial invasion of the peritoneum can lead to severe and even fatal cases of peritonitis. In untreated appendicitis, peritonitis may be caused by the rupturing of the appendix and the subsequent release of bacteria into the peritoneal cavity. Peritonitis is a potential complication of any surgery in which the peritoneal cavity is opened or of any disease that perforates the walls of the stomach or intestines.

 

Histological Organization of the Digestive Tract
The major layers of the digestive tract include (1) the mucosa , (2) the submucosa , (3) the muscularis externa , and (4) the serosa .

 

 

The Mucosa
The inner lining, or mucosa , of the digestive tract is a mucous membrane consisting of an epithelium, moistened by glandular secretions, and a lamina propria of areolar tissue.
The Digestive Epithelium

 

The oral cavity, pharynx, and esophagus (where mechanical stresses are most severe) are lined by a stratified squamous epithelium, whereas the stomach, the small intestine, and almost the entire length of the large intestine (where absorption occurs) have a simple columnar epithelium that contains goblet cells. Scattered among the columnar cells are enteroendocrine cells , which secrete hormones that coordinate the activities of the digestive tract and the accessory glands. The lining of the digestive tract is often thrown into longitudinal folds.  The folding dramatically increases the surface area available for absorption.

 

The life span of a typical epithelial cell varies from two to three days in the esophagus to up to six days in the large intestine. The lining of the entire digestive tract is continuously renewed through the divisions of epithelial stem cells, keeping pace with the rate of cell destruction and loss at epithelial surfaces. This high rate of cell division explains why radiation and anticancer drugs that inhibit mitosis have drastic effects on the digestive tract. Lost epithelial cells are no longer replaced, and the cumulative damage to the epithelial lining quickly leads to problems in absorbing nutrients. In addition, the exposure of the lamina propria to digestive enzymes can cause internal bleeding and other serious problems

 

This submucosal plexus , or plexus of Meissner , contains sensory neurons, parasympathetic ganglionic neurons, and sympathetic postganglionic fibers that innervate the mucosa and submucosa

 

The Muscularis Externa is a region dominated by smooth muscle cells. Like the smooth muscle cells in the muscularis mucosae, those in the muscularis externa are arranged in an inner, circular layer and an outer, longitudinal layer.

 

The Movement of Digestive Materials
The muscular layers of the digestive tract consist of visceral smooth muscle tissue , a type of smooth muscle introduced in Chapter 10 . The smooth muscle along the digestive tract shows rhythmic cycles of activity due to the presence of pacesetter cells . These smooth muscle cells undergo spontaneous depolarization, and their contraction triggers a wave of contraction that spreads through the entire muscular sheet. Pacesetter cells are located in the muscularis mucosae and muscularis externa, the layers of which surround the lumen of the digestive tract. The coordinated contractions of the muscularis externa play a vital role in the movement of materials along the tract, through peristalsis , and in mechanical processing, through segmentation .

 

The muscularis externa propels materials from one portion of the digestive tract to another by contractions known as peristalsis. Peristalsis consists of waves of muscular contractions that move a bolus , or small oval mass of digestive contents, along the length of the digestive tract.

 

 

Peristalsis.
Peristalsis propels materials along the length of the digestive tract.

 

Segmentation
Most areas of the small intestine and some portions of the large intestine undergo cycles of contraction that produce segmentation . These movements churn and fragment the bolus, mixing the contents with intestinal secretions. Because they do not follow a set pattern, segmentation movements do not push materials along the tract in any one direction.

 

Control of the Digestive Function
The activities of the digestive system are regulated by neural, hormonal, and local mechanisms

 

 

 

The Regulation of Digestive Activities.
The major factors responsible for regulating digestive activities: neural mechanisms; hormonal mechanisms;  local mechanisms.

 

Neural Mechanisms
The movement of materials along your digestive tract, as well as many secretory functions, is controlled primarily by neural mechanisms.

 

These reflexes are also called myenteric reflexes , and the term enteric nervous system is often used to refer to the neural network that coordinates the myenteric reflexes along the digestive tract.

 

Short reflexes control activities in one region of the digestive tract. The control may involve coordinating local peristalsis and triggering the secretion of digestive glands.

 

Sensory information from receptors in the digestive tract is also distributed to the CNS, where it can trigger long reflexes , which involve interneurons and motor neurons in the CNS.

 

Long reflexes may involve parasympathetic motor fibers in the glossopharyngeal, vagus, or pelvic nerves that synapse in the myenteric plexus.

 

Hormonal Mechanisms
The sensitivity of the smooth muscle cells to neural commands can be enhanced or inhibited by digestive hormones. Your digestive tract produces at least 18 hormones that affect almost every aspect of digestive function, and some of them also affect the activities of other systems. The hormones ( gastrin, secretin , and others), which are peptides produced by enteroendocrine cells in the digestive tract, reach their target organs by distribution in the bloodstream

 

Local Mechanisms
Prostaglandins, histamine, and other chemicals released into interstitial fluid may affect adjacent cells within a small segment of the digestive tract.  For example, the release of histamine in the lamina propria of the stomach stimulates the secretion of acid by cells in the adjacent epithelium.

 

The Oral Cavity, or buccal cavity

 

 

The Oral Cavity.
(a) anterior view  (b) A sagittal section.

 

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 gland secretions; and (4) limited digestion of carbohydrates and lipids.

 

The posterior margin of the soft palate supports the uvula, a dangling process that helps prevent food from entering the pharynx prematurely.

 

The Tongue  functions of the tongue are (1) mechanical processing by compression, abrasion, and distortion; (2) manipulation to assist in chewing and to prepare material for swallowing; (3) sensory analysis by touch, temperature, and taste receptors, and (4) secretion of mucins and the enzyme lingual lipase .
We can divide the tongue into an anterior body , or oral portion , and a posterior root , or pharyngeal portion .

 

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. The smaller intrinsic muscles change the shape of the 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 (XII).

 

Salivary Glands
Three pairs of salivary glands secrete into the oral cavity

 

The Salivary Glands.
(a) A lateral view, showing the relative positions of the salivary glands and ducts on the left side of the head. For clarity, the left ramus and body of the mandible have been removed. (b) The submandibular gland secretes a mixture of mucins, produced by mucous cells, and enzymes, produced by serous cells.

 

The large parotid salivary glands lie inferior to the zygomatic arch deep to 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 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.

 

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 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

 

Saliva
Your salivary glands produce 1.0–1.5 liters of saliva each day. Saliva is 99.4 percent water, and the remaining 0.6 percent includes an assortment of electrolytes (principally and ), buffers, glycoproteins, antibodies, enzymes, and waste products. The glycoproteins, called mucins , are primarily responsible for the lubricating action of saliva. About 70 percent of saliva originates in the submandibular salivary glands, 25 percent in the parotids, and the remaining 5 percent in the sublingual salivary glands.

 

Buffers in the saliva keep the pH of your mouth near 7.0 and prevent the buildup of acids produced by bacterial action. In addition, saliva contains antibodies (IgA) and lysozymes that help control populations of oral bacteria.

 

The saliva produced when you eat has a variety of functions, including:

 

Lubricating the mouth.

 

Moistening and lubricating materials in the mouth.

 

Dissolving chemicals that can stimulate the taste buds and provide sensory information about the material.

 

Initiating the digestion of complex carbohydrates before the material is swallowed. The enzyme involved is salivary amylase , which is also known as ptyalin or alpha–amylase . Although the digestive process begins in the oral cavity, it is not completed there, and no absorption of nutrients occurs across the lining of the cavity. Saliva also contains a small amount of lingual lipase that is secreted by the glands of the tongue.

 

The mumps virus most often targets the salivary glands, especially the parotid salivary glands, although other organs can also become infected. Infection typically occurs at 5–9 years of age. The first exposure stimulates the production of antibodies and, in most cases, confers permanent immunity; active immunity can be conferred by immunization. In post– adolescent males, the mumps virus can also infect the testes and cause sterility. Infection of the pancreas by the mumps virus can produce temporary or permanent diabetes; other organ systems, including the central nervous system, are affected in severe cases.

 

Control of Salivary Secretions
Salivary secretions are normally controlled by the autonomic nervous system. Each salivary gland receives parasympathetic and sympathetic innervation. The parasympathetic outflow originates in the salivatory nuclei of the medulla oblongata and synapses in the submandibular and otic ganglia. Any object in your mouth can trigger a salivary reflex by stimulating receptors monitored by the trigeminal nerve (V) or by stimulating taste buds innervated by cranial nerves VII, IX, or X. Parasympathetic stimulation accelerates secretion by all the salivary glands, resulting in the production of large amounts of saliva.

 

For example, chewing with an empty mouth, the smell of food, or even thinking about food will initiate an increase in salivary secretion rates; that is why chewing gum is so effective at keeping your mouth moist. The presence of irritating stimuli in the esophagus, stomach, or intestines will also accelerate the production of saliva, as will the sensation of nausea

 



The Pharynx  serves as a common passageway for solid food, liquids, and air.

 

The Esophagus

 

 

The Esophagus.
(a) A transverse section through the esophagus. (b) The esophageal mucosa.  (c) A color–enhanced SEM of the transition between the esophageal and gastric mucosae at the lower esophageal sphincter.

 

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 From this point, where it is at its narrowest, the esophagus descends toward the thoracic cavity posterior to the trachea. It 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.  The esophagus is innervated by parasympathetic and sympathetic fibers from the esophageal plexus.

 

Resting muscle tone in the circular muscle layer in the superior 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.

 

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.

 

The muscularis mucosae consists of an irregular layer of smooth muscle.

The submucosa contains scattered esophageal glands , which produce a mucous secretion that reduces friction between the bolus and the esophageal lining.

 

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Swallowing , or deglutition , is a complex process whose initiation can be voluntarily controlled, but that 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

 

 

The Swallowing Process.
This sequence, based on a series of X rays, shows the stages of swallowing and the movement of materials from the mouth to the stomach