Digestive System
Chapter 24
Peritoneum
FIGURE 24.51.Recall that the abdominopelvic cavity is lined with a serous membrane called the peritoneum.
A.The visceral peritoneum surrounds organs.
B.The parietal peritoneum lines the wall of the abdominopelvic cavity and covers certain organs.
C.The peritoneal cavity, which is in-between the visceral and parietal peritoneum, is filled with peritoneal fluid. The peritoneal fluid functions to reduce friction between organs.
2.Some organs are covered by (located behind) the parietal peritoneum and are called retroperitoneal. Examples are the kidneys, adrenal glands, pancreas, ascending colon, descending colon, and the urinary bladder.
3.Mesenteries[meaning middle intestine] are a double layer of peritoneum that extends from the abdominopelvic wall to the organs within the lumen of the abdominopelvic cavity.
A.Functions of the mesenteries.
1)Mesenteries supports the abdominal organs.
2)Mesenteries are a bridgeway for blood vessels and nerves to reach the abdominopelvic organs.
3)Mesenteries are a site of fat storage.
B.Important mesenteries of the digestive tract. [Analogy: As water flows around a rock in a stream, the serous membrane flows around the organs]
1)The coronary ligament (all double layers of peritoneum not called mesenteries are called ligaments) attaches the liver to the diaphragm.
2)The falciform [L. sickle + form; falsi-fOrm]ligament attaches the liver to the anterior abdominal wall.
3)The lesser omentum[meaning membrane of the bowels] attaches the liver to the lesser curvature of the stomach.
4)The greater omentum attaches the greater curvature of the stomach to the transverse colon. The greater omentum folds over on itself to form a space called the omental bursa.
5)The transversemesocolon attaches the transverse colon to the posterior abdominal wall.
6)The mesentery proper attaches the small intestine to the posterior abdominal wall.
Oral Cavity
FIGURE 24.61.The oral cavity (mouth) is the part of the digestive tract bounded by the lips, fauces (opening into the pharynx), cheeks, and palate.
2.The vestibule is the part of the oral cavity between the lips or cheeks and the alveolar processes.
Lips and Cheeks
1.The lips are muscle (orbicularis oris) covered by keratinized stratified squamous epithelium. They are important in communication.
2.The cheeks are muscle (mostly the buccinator) covered internally by moist stratified squamous epithelium. They function in mastication (chewing) by holding food against the teeth, and they function in communication.
Palate
1.The hard palate, composed of the maxillary bones and the palatine bones, functions to separate the oral cavity from the nasal cavity.
2.The soft palate and uvula, composed of muscles, function to separate the oral cavity and the nasal cavity. They also close off the opening to the nasal cavity during swallowing.
Tongue
FIGURE 10.13 (p. 333)1.The tongue functions to move food about during mastication and swallowing and is also involved in communication. The tongue consists of intrinsic tongue muscles, which change the shape of the tongue. [Tongue rolling and folding is a dominant trait.] The extrinsic tongue muscles attach to the tongue and move it about as a unit.
2.The anterior two thirds of the surface of the tongue is covered with papillae that give it a rough texture. Some of the papillae contain taste buds. The posterior one third of the tongue does not have papillae and has only a few taste buds.
3.The frenulum is a mucous membrane that attaches the tongue to floor of the mouth. If it extends too far toward the tip of the tongue it can interfere with normal movement of the tongue and normal speech, resulting in a person who is "tongue tied".
4.There are many superficial blood vessels under the tongue, making this location ideal for administering certain medications, such as nitroglycerin, to increase blood flow to the heart.
Tonsils
FIGURE 22.5 (p. 789)1.Tonsils consists of lymphatic tissue and they are a part of the immune system, which helps to protect against microorganisms.
2.There are three sets of tonsils.
A.The lingual tonsils are embedded within the posterior part of the tongue.
B.The palatine tonsils are in the lateral wall of the fauces.
C.The pharyngeal tonsil is located in the nasopharynx (back of the throat superior to the uvula). The pharyngeal tonsil is often referred to as the adenoids.
Teeth
1.The teeth mechanically breakup food, and are involved in communication and defense.
2.Types of teeth.
FIGURE 24.7A.Deciduous teeth are the first set of teeth. Usually they are all present by 2 years of age.
B.Permanent teeth replace the deciduous teeth between 6 to 17 years of age. The last permanent teeth are molars called wisdom teeth, and they appear between 17 to 25 years of age. Wisdom teeth may have to be removed because of lack of room.
Type of Tooth / Deciduous / Permanent / FunctionIncisor / 8 / 8 / Cutting
Canines / 4 / 4 / Tearing
Premolar / 0 / 8 / Grinding
Molar / 8 / 12 / Grinding
TOTAL / 20 / 32
3.The teeth extend into the alveoli (sockets) of the mandible and the maxillary bones. The gums or gingiva (dense fibrous connective tissue) cover the alveoli and jaw bones.
4.General parts of a tooth.
FIGURE 24.8A.The crown is the part of the tooth above the gum line. The crown is the cutting or chewing surface of the tooth. [The clinical crown is the part of the crown exposed in the oral cavity. The anatomical crown is the enamel covered part of the tooth.]
B.The neck is the constricted part of the tooth at the gum line.
C.The root is the part of the tooth below the gum line that extends into the alveolus.
5.Detailed description of a tooth.
A.Teeth consist of a hard bony material called dentin.
1)Enamel covers the dentin of the [anatomical] crown. Caries or cavities are usually a breakdown of the enamel caused by acids produced by bacteria. Fluoride treatment increases the resistance of the enamel to acid.
2)Cementum covers the dentin of the root and neck. It anchors the tooth to the periodontal ligaments.
3)The periodontal ligaments extend from the tooth to the bone of the alveoli. They hold the tooth in place. Periodontal gum disease is destruction of the periodontal ligaments caused by bacterial activity. The result is the tooth falls out. This kind of tooth loss can be reduced by regular cleaning and flossing of the teeth.
B.The pulp cavity contains pulp (connective tissue), blood vessels, and nerves.
C.The root canal is a passageway for blood vessels and nerves into the pulp cavity. The opening into the root canal is the apical foramen. If an infection occurs inside the tooth, the top of the tooth is removed, pulp is removed from the root canal and replaced with a packing material (a procedure called a “root canal”), and an artificial crown is attached to the tooth.
Mastication
1.Mastication is the process of chewing food.
2.Mastication mixes food with saliva to form a bolus of food. Breaking the food down into smaller pieces increases the surface area of the food, which promotes digestion by various secretions.
3.The muscles of mastication move the mandible, providing the force necessary for the teeth to cut, tear, and grind food.
FIGURE 10.11 (p. 334)4.Mastication is controlled primarily by the mastication reflex, which alternately causes the muscles of mastication to contract and relax. Mastication can be voluntarily started and stopped.
Salivary Glands
FIGURE 24.91.The salivary glands produce saliva, which moistens food and begins the process of digesting carbohydrates.
2.There are three pairs of salivary glands.
A.The parotid salivary glands are located anterior to the ear. They are the largest salivary glands and can become swollen in mumps. [Duct opens by upper 2nd molar. A site of plaque buildup.]
B.The submandibular glands are located along the border of posterior half of each mandible. [Duct opens near midline under the tongue. Source of saliva that squirts out of the mouth.]
C.The sublingual glands are located in the floor of the mouth. [Each has 8-20 ducts that open into the floor of the mouth. Saliva from the submandibular and sublingual glands contribute to plaque buildup.]
4.The salivary glands produce approximately 1 - 1.5 L of saliva per day.
A.Saliva is mostly water. The water makes it possible to taste food and makes the food easier to swallow.
B.Salivary amylase starts starch digestion by splitting starch into maltose (a disaccharide) and isomaltose.
C.Lysozyme (an enzyme) and IgA (an antibody) kill some bacteria. The mouth still harbors many bacteria, and human bites can easily cause infections.
D.Mucus is a secretion of the mucous membranes, consisting of mucin, epithelial cells, white blood cells, and inorganic salts suspended in water. Mucin is a proteoglycan (polysaccharides attached to a protein core) that makes mucus very slippery. Mucus acts as a lubricant.
5.Regulation of saliva secretion.
A.Stimulation of saliva production.
1)Taste or smell stimuli that act through conditioned reflexes [Pavlov’s dogs].
2)Tactile stimuli in the mouth [Old cowboy movies - when lost in the desert the old timer tells the greenhorn to put a pebble in his mouth].
3)Thoughts (e.g., lemon) or nausea.
4)Irritating substances in the stomach and small intestine. Saliva may help to dilute the irritating substance [Common experience to feel increased saliva secretion just before you throw up].
B.Inhibition of saliva production.
1)Dehydration.
2)Fear, anxiety, or intense mental effort.
Pharynx
1.The pharynx (G. throat), or throat, is divided into three interconnected parts.
FIGURE 23.2 (p. 827)A.The nasopharynx is connected to the nasal cavity.
B.The oropharynx is connected to the oral cavity.
C.The laryngopharynx is connected to the esophagus.
2.Pharyngeal constrictors are muscles that move food through the pharynx to the esophagus.
FIGURE 10.14b (p. 338)Esophagus
FIGURE 24.11.The esophagus is a muscular tube that connects the laryngopharynx with the stomach. [Superior part is skeletal muscle, inferior part is smooth muscle.]
2.Sphincters.
A.The upper esophageal sphincter regulates movement into the esophagus.
B.The lower esophageal sphincter (cardiac sphincter) regulates movement into and out of the stomach.
3.The esophagus is lined with moist, stratified squamous epithelium that provides protection against abrasion.
4.Mucous glands produce mucus that lubricates the surface of the epithelium.
Liver
FIGURE 24.171.The liver is located in the right-upper and left-upper quadrants of the abdominopelvic cavity.
2.The liver is divided into four lobes: two major lobes (right and left) and two minor lobes (caudate and quadrate). [Gray, 38th Ed, p. 1796-1798]
A.The division of the liver into four external lobes is based on surface markings formed by the ligaments of the liver [the falciform ligament, fissures for the ligamentum teres and ligamentum venosum, and the porta (porta hepatis)].
B.The internal divisions of the liver are based on the branching pattern of blood vessels and ducts entering or exiting the liver (see below). Based on this pattern, the “internal” left lobe consists approximately of the “external” left lobe, caudate lobe, and quadrate lobe. The “internal” right lobe consists of the remainder of the liver.
C.The “internal” lobes are further subdivided into 7 to 9 segments, depending on the classification scheme used. There are no surface markings for these divisions, which must be determined using radiographic techniques.
D.Liver transplants typically involve transplanting the whole liver from a recently deceased person. Sometimes the liver is split into right and left “internal” lobes to provide two recipients. [Gray, 38th Ed, p. 1808]
E.There is a shortage of livers for transplantation into children. One procedure removes two segments [#2 and 3] from an adult “external” left lobe, achieving a better size match. In some cases, a living parent donates part of a liver.
3.The porta (gate), on the inferior surface of the liver, is where vessels, ducts, and nerves enter or exit the liver.
4.Blood enters the porta (gate) of the liver from two sources.
FIGURE 24.20A.The hepatic artery, which brings oxygenated blood from the aorta (a major blood vessel arising from the heart).
B.The hepatic portal vein drains the digestive tract and carries deoxygenated, nutrient rich blood to the liver.
5.The liver segments are subdivided into lobules.
FIGURE 24.19A.The tissue of the lobules consists of hepatic cords, which are columns of hepatocytes, the functional cells of the liver.
B.On the outside of the lobules are portal triads, which consist of the branches of the hepatic artery, hepatic portal vein, and hepatic duct (to be discussed below).
C.Branches of the hepatic artery and hepatic portal vein empty into thehepatic sinusoids, which are enlarged blood capillaries, located between the hepatic cords.
1)The blood from the hepatic arteries supplies the hepatocytes with oxygen.
2)The blood from the hepatic portal veins brings the hepatocytes nutrients that are processed.
D.The hepatic sinusoids connect to a central vein in the center of the lobule. The central veins from the lobules empty into the hepatic veins.
E.Blood leaves the liver through the hepatic veins, which donot pass through the porta. The hepatic veins join the inferior vena cava as it passes by the posterior side of the liver.
Given the following structures:
1.central vein
2.hepatic artery
3.hepatic portal vein
4.hepatic sinusoid
5.hepatic vein
Make a list of the structures in the order blood passes through them as the blood moves from the GI tract into and out of the liver (note that not all of the structures listed need to be used).
3, 4, 1, 56.Bile ducts.
FIGURE 24.18A.Bile, the secretion of the liver, is produced by the hepatocytes.
B.Bile from the hepatocytes enters bile canaliculi, which lie between the hepatic cords. The bile canaliculi join the small hepatic ducts of the portal triads.
C.The small hepatic ducts within the liver tissue join together to form the right and left hepatic ducts. The right hepatic duct drains the right half of the liver and the left hepatic duct drains the left half of the liver. [Note that the halves do not correspond to the externally marked left and right lobes. The left half of the liver is the left, caudate, and quadrate lobes]
D.The right and left hepatic ducts join to form the common hepatic duct.
E.The cystic duct from the gallbladder joins the common hepatic duct to form the common bile duct.
F.The common bile duct empties into the hepatopancreatic ampulla, an enlarged duct formed by the union of the common bile duct and the main pancreatic duct.
1)The hepatopancreatic ampulla empties through the major duodenal papilla into the duodenum.
2)The opening of the hepatopancreatic ampulla is usually closed by the hepatopancreatic ampullar sphincter.
Given the following structures:
1.bile canaliculus
2.common bile duct
3.common hepatic duct
4.major duodenal papilla
5.hepatopancreatic ampulla
6.right and left hepatic ducts
7.small hepatic ducts of the portal triads
Make a list of the structures in the order bile passes through them, starting with the secretion of bile from hepatocytes and ending with movement of bile into the duodenum.
1, 7, 6, 3, 2, 5, 4Functions of the Liver
1.Bile Production. The liver produces about 700 ml of bile per day.
A.Bile contains bile salts. The bile salts emulsify fats, i.e., break fat globules into smaller pieces. This is important because it allows digestive enzymes to act on the fat molecules.
B.Bile contains bicarbonate ions, which neutralize acid from the stomach. It is necessary to raise pH in the small intestine so that enzymes from the pancreas and the wall of the digestive tract can function optimally.
C.Bile contains excretory products such as bile pigments (breakdown product from old red blood cells), cholesterol, and hormones. Bile pigments are responsible for the color of feces. Failure to secrete bile can result in bile movement into the blood, producing the condition of jaundice.
2.Storage. The liver stores a variety of substances.
A.The liver stores glycogen, fats, fat soluble vitamins (A, D, E, and K), copper, and iron.
B.The liver stores harmful substances that cannot be broken down or excreted (e.g., DDT).
C.The liver functions to regulate blood glucose (sugar levels) by storing glucose. When blood glucose levels are high, the liver removes glucose and stores it as glycogen. When blood glucose levels are low, the liver releases glucose into the blood.
3.Nutrient Interconversion.
A.The liver processes nutrients, helping to ensure that the body has the proper amounts of carbohydrates, fats and proteins. For example, excess proteins, resulting from a high protein diet, can be converted into carbohydrates and lipids.