nore digestive system labeling Flashcards
Large Intestine
The large intestine is much larger in diameter than the small intestine (thus its name) but shorter in length. About 1.5 m (5 feet) long, it extends from the ileocecal valve to the anus (Figure 14.8). Its major functions are to dry out the indigestible food residue by absorbing water and to eliminate these residues from the body as feces (fe′sēz). It frames the small intestine on three sides and has these subdivisions: cecum (se′kum), appendix, colon, rectum, and anal canal.The saclike cecum is the first part of the large intestine. Hanging from the cecum is the wormlike appendix, a potential trouble spot. Because it can easily become obstructed, it is an ideal location for bacteria to accumulate and multiply. Inflammation of the appendix, appendicitis, is the usual result.
The colon is divided into several distinct regions. The ascending colon travels up the right side of the abdominal cavity and makes a turn, the right colic (or hepatic) flexure, to travel across the abdominal cavity as the transverse colon. It then turns again at the left colic (or splenic) flexure and continues down the left side as the descending colon to enter the pelvis, where it becomes the S-shaped sigmoid (sig′moyd) colon. The sigmoid colon, rectum, and anal canal lie in the pelvis.
The anal canal ends at the anus (a′nus), which opens to the exterior. The anal canal has two valves: the external anal sphincter, composed of skeletal muscle, is voluntary, and the internal anal sphincter, formed by smooth muscle, is involuntary. These sphincters, which act rather like purse strings to open and close the anus, are ordinarily closed except during defecation, when feces are eliminated from the body.
Because most nutrients have been absorbed before the large intestine is reached, no villi are present in the large intestine, but there are tremendous numbers of goblet cells in its mucosa that produce alkaline (bicarbonate-rich) mucus. The mucus lubricates the passage of feces to the end of the digestive tract.
In the large intestine, the longitudinal layer of the muscularis externa is reduced to three bands of muscle called teniae coli (ten′ne-e ko′li; “ribbons of the colon”). Because these muscle bands usually display some degree of tone (are partially contracted), they cause the wall to pucker into small pocketlike sacs called haustra (haws′trah).
teeth
The role teeth play in processing food needs little introduction. We masticate, or chew, by opening and closing our jaws and moving them from side to side while continuously using our tongue and cheek muscles to keep the food between our teeth. In the process, the teeth tear and grind the food, breaking it down into smaller fragments.
Ordinarily, by the age of 21, two sets of teeth have been formed (Figure 14.9). The first set are deciduous (de-sid′u-us) teeth, also called baby teeth or milk teeth. The deciduous teeth begin to erupt around 6 months; the first teeth to appear are the lower central incisors. A baby has a full set (20 teeth) by the age of 2 years.As the second set of teeth, the deeper permanent teeth, enlarge and develop, the roots of the milk teeth are reabsorbed, and between the ages of 6 and 12 years they loosen and fall out. All of the permanent teeth but the third molars have erupted by the end of adolescence. The third molars, also called wisdom teeth, emerge between the ages of 17 and 25. Although there are 32 permanent teeth in a full set, the wisdom teeth often fail to erupt; sometimes they are completely absent.
when teeth are embeded
When teeth remain embedded in the jawbone, they are said to be impacted. Impacted teeth can exert pressure and cause a good deal of pain, so they usually must be removed surgically. Wisdom teeth are the most commonly impacted.
more abouth teeth
We classify the teeth according to shape and function as incisors, canines, premolars, and molars (see Figure 14.9). The chisel-shaped incisors are adapted for cutting; the fanglike canines (eyeteeth) are for tearing or piercing. The premolars (bicuspids) and molars have broad crowns with rounded cusps (tips) and are best suited for crushing and grinding.
A tooth consists of two major regions, the crown and the root (Figure 14.10). The enamel-covered crown is the exposed part of the tooth above the gingiva (jin′-jĭvah), or gum. The crown is covered with enamel, a ceramic-like substance as thick as a dime, that directly bears the force of chewing. It is the hardest substance in the body and is fairly brittle because it is heavily mineralized with calcium salts.The portion of the tooth embedded in the jawbone is the root; the root and crown are connected by a region called the neck. The outer surface of the root is covered by a substance called cement, which attaches the tooth to the periodontal (per″e-o-don′tal) ligament. This ligament holds the tooth in place in the bony jaw. Dentin, a bonelike material, underlies the enamel and forms the bulk of the tooth. It surrounds a central pulp cavity, which contains a number of structures (connective tissue, blood vessels, and nerve fibers) collectively called pulp. Pulp supplies nutrients to the tooth tissues and provides for tooth sensations. Where the pulp cavity extends into the root, it becomes the root canal, which provides a route for blood vessels, nerves, and other pulp structures to enter the pulp cavity of the tooth.
Salivary Glands
Three pairs of salivary glands empty their secretions into the mouth. The large parotid (pah-rot′id) glands lie anterior to the ears. Mumps, a common childhood disease, is an inflammation of the parotid glands. If you look at the location of the parotid glands (see Figure 14.1), you can readily understand why people with mumps complain that it hurts to open their mouth or chew.
The submandibular glands and the small sublingual (sub-ling′gwal) glands empty their secretions into the floor of the mouth through tiny ducts. The product of the salivary glands, saliva, is a mixture of mucus and serous fluids. The mucus moistens and helps to bind food together into a mass called a bolus (bo′lus), which makes chewing and swallowing easier. The clear serous portion contains an enzyme, salivary amylase (am′ĭ-lās), in a bicarbonate-rich (alkaline) juice that begins the process of starch digestion in the mouth. Saliva also contains substances such as lysozyme and antibodies (IgA) that inhibit bacteria; therefore, it has a protective function as well. Last but not least, saliva dissolves food chemicals so they can be tasted.
Pancreas
The pancreas is a soft, pink, triangular gland that extends across the abdomen from the spleen to the duodenum (look back at Figure 14.6). Most of the pancreas lies posterior to the parietal peritoneum; hence its location is referred to as retroperitoneal.
Only the pancreas produces enzymes (described later) that break down all categories of digestible foods. The pancreatic enzymes are secreted into the duodenum in an alkaline fluid that neutralizes the acidic chyme coming in from the stomach. The pancreas also has an endocrine function; it produces the hormones insulin and glucagon (as explained in Chapter 9).
Liver and Gallbladder
If bile is stored in the gallbladder for too long or too much water is removed, the cholesterol it contains may crystallize, forming gallstones. Gallstones can become stuck in one of the ducts leading from the gallbladder to the duodenum, causing agonizing pain when the gallbladder contracts (an event called a gallbladder attack).
Blockage of the common hepatic or bile ducts (for example, by wedged gallstones) prevents bile from entering the small intestine, and it begins to accumulate and eventually backs up into the liver. This exerts pressure on the liver cells, and bile salts and bile pigments begin to enter the bloodstream. As the bile pigments circulate through the body, the tissues become yellow, or jaundiced. Blockage of the ducts is just one cause of jaundice. More often it results from actual liver problems such as hepatitis, an inflammation of the liver, or cirrhosis (sir-ro′sis), a chronic inflammatory condition in which the liver is severely damaged and becomes hard and fibrous. Hepatitis is most often due to viral infection resulting from drinking contaminated water or transmitted in blood via transfusion or contaminated needles. Cirrhosis is almost guaranteed when someone drinks alcoholic beverages in excess for many years, and it is a common consequence of severe hepatitis.
