Sistem Pencernaan Manusia
Gambar sistem pencernaan manusia
Digestion takes place almost continuously in a watery, slushy environment. The large intestine absorbs water from its inner contents and stores the rest until it is convenient to dispose of it. Attached to the first portion of the large intestine is a troublesome pouch called the (veriform) appendix. The appendix has no function in modern humans, however it is believed to have been part of the digestive system in our primitive ancestors.
The large intestine, or colon, consists of ascending, transverse, descending, and sigmoid portions. The ascending portion extends from the cecum superiorly along the right abdominal wall to the inferior surface of the liver and bends sharply at a right angle to the left at a curve called the hepatic flexure. From there, it crosses the abdominal cavity as the transverse colon to the left abdominal wall at the splenic flexure and begins the descending colon which traverses inferiorly along the left abdominal wall to the pelvic region. The colon then forms an angle medially from the pelvis to form an s-shaped curve called the sigmoid colon.
The last few inches of the colon is the rectum which is a storage site for solid waste which leaves the body by way of an external opening called the anus, controlled by muscles called sphincters. Substances which have not been absorbed in the small intestine enter the large intestine in the form of liquid and fiber. The large intestine or “bowel” is sometimes called the “garbage dump” of the body, because the materials that reach it are of very small use to the body and are sent on to be disposed of. The first half of the colon absorbs fluids and recycles them into the blood stream. The second half compacts the wastes into feces, secretes mucus which binds the substances, and lubricates it to protect the colon and ease its passage. Of the two to two and one-half gallons of food and liquids taken in by the average adult, only about twelve ounces of waste enters the large intestine. Feces are comprised of about three quarters water. The remainder is protein, fat, undigested food roughage, dried digestive juices, cells shed by the intestine, and dead bacteria. A common disorder of the large intestine is inflammation of the appendix, or appendicitis. Waste that accumulates in the appendix cannot be moved easily by peristalsis since the appendix has only one opening. The symptoms of appendicitis include muscular rigidity, localized pain in the right lower quarter of the abdomen, and vomiting. The chief danger of appendicitis is that is may rupture and empty its contents of fecal matter and waste into the abdominal cavity producing an extremely serious condition called peritonitis.
The epiglottis is the flap of cartilage lying behind the tongue and in front of the entrance to the larynx (voice box). At rest, the epiglottis is upright and allows air to pass through the larynx and into the rest of the respiratory system. During swallowing, it folds back to cover the entrance to the larynx, preventing food and drink from entering the windpipe. The throat contains both an air passage (the wind pipe) and a food passage (the esophagus). If these passages were both open when a person swallowed, air could enter the stomach and food could enter the lungs. Part of the safety hatch that seals off the windpipe is the “epiglottis,” a little valvelike cartilage, which works with the larynx to act as a lid every time we swallow. The larynx draws upward and forward to close the windpipe. This keeps solid food and liquid out of the respiratory tract. At the end of each swallow, the epiglottis moves up again, the larynx returns to rest, and the flow of air into the windpipe continues. The uvula (Latin for “little grape”) is a fleshy piece of muscle, tissue and mucous membrane that hangs down from the palate. It is the part that moves upward when we say, “Ah!” It flips up and helps close off the nasal passages when we swallow. Contrary to the depictions seen in cartoons, the uvula does not vibrate during singing and shouting and, in fact, has nothing to do with the voice.
The gallbladder is an active storage shed, which absorbs mineral salts and water received from the liver and converts it into a thick, mucus substance called “bile,” to be released when food is present in the stomach. The gallbladder is a small, pear-shaped sac which is situated just below the liver and is attached to it by tissues. It stores bile and then releases it when food passes from the stomach to the duodenum (the first part of the small intestine) to help in the process of digestion. It has a capacity of around one and one-half fluid ounces. When food leaves the stomach, a secretion causes the gallbladder to contract and expel its contents into the duodenum, where the bile disperses the fats in the food into liquid. Pythagoras, the 6th Century BC Greek mathematician, believed that life is based on the four elements of earth, air, fire and water which correspond to the body’s “humors”: blood (hot and moist), phlegm (cold and moist), yellow bile (hot and dry) and black bile (cold and dry). The perfect or imperfect balance of these humors supposedly determined one’s health and intelligence. We still speak in terms of “melancholia” (excess black bile, leading to depression) and “phlegmatic” (sluggish or impassive) and scientists have named the heavy mucus secreted in the respiratory passages – phlegm. Pythagoras was kind of a “square”. Oh, come on; where’s your sense of “humor”?
Thirty per cent of the blood pumped through the heart in one minute passes through the body’s chemical factory, which is called the liver. The liver cleanses the blood and processes nutritional molecules, which are distributed to the tissues. The liver also receives bright red blood from the lungs, filled with vital oxygen to be delivered to the heart. The only part of the body which receives more blood than the liver is the brain. The liver is located at the top of the abdomen, just below the diaphragm and has two main lobes. It is the largest gland in the body, weighing 2.5 to 3.3 pounds. When we eat, more blood is diverted to the intestines to deal with digestive processes; when not eating, three-fourths of the blood supply to the liver comes from the intestines. It also produces about two and one-half pints of bile in its ducts, which is delivered to the gallbladder through a small tube called the “cystic duct” for storage. “Liver” is probably an appropriate name for this gland, which makes the important decision as to whether incoming substances are useful to the body or whether they are waste. The liver is an extremely important organ and has multiple functions. The liver detoxifies blood cells by mixing them with bile and by chemical alteration to less toxic substances, such as the alteration of ammonia to urea. Many chemical compounds are inactivated by the liver through modification of chemical structures. The liver converts glucose to a storage form of energy called glycogen, and can also produce glucose from sugars, starches, and proteins. The liver also synthesizes triglycerides and cholesterol, breaks down fatty acids, and produces plasma proteins necessary for the clotting of blood, such as clotting factors I, III, V, VII, IX and XI. The liver also produces bile salts and excretes bilirubin. A “lily-livered coward” was someone whose liver contained no blood. The Greeks and Romans sacrificed animals to the gods before going into battle. When the liver was examined, if it was healthy and the blood was bright red, a victory was promised; if it was diseased or the blood was pale, defeat was predicted.
Mouth (An Overview)
The function of the mouth and its associated structures is to form a receptacle for food, to begin mechanical digestion through chewing (mastication), to swallow food, and to form words in speech. It can also assist the respiratory system in the passage of air.
The omentum is an apronlike double fold of fatty membrane that hangs down in front of the intestines. It contains blood vessels, nerves, lymph vessels and lymph nodes. It acts as a storage for fat and also may limit the spread of infection in the abdominal cavity.
The trachea begins immediately below the larynx (voicebox) and runs down the center of the front part of the neck ends behind the upper part of the sternum. Here it divides to form two branches which enter the lung cavities. The trachea (windpipe) forms the trunk of an upside-down tree and is flexible, like a vacuum tube, so that the head and neck may twist and bend during the process of breathing. The trachea, or windpipe, is made up of fibrous and elastic tissues and smooth muscle with about twenty rings of cartilage, which help keep the trachea open during extreme movement of the neck. The lining includes cells that secrete mucus along with other cells that bear very small hairlike fringes. This mucus traps tiny particles of debris, and the beating of the fringes moves the mucus up and out of the respiratory tract, keeping the lungs and air passages free. In Russian folk medicine, there is the thought that rubbing the chest with pork fat will cure a cold. Mustard plasters and boiled snails in barley water were thought to be effective by others, and nobody knows what the ingredients were for early “cure-all tonics” and “snake oil” kits. It is now believed that the best medicine is to rest, keep warm, drink plenty of fluids, and eat good, digestible meals. Sounds good to me…and certainly better smelling.
The mouth also contains the salivary glands which are accessory digestive glands that produce a fluid secretion called saliva. Saliva functions as a solvent in cleansing the teeth and dissolving food particles so that they may be tasted. Saliva also contains starch-digesting enzymes and mucus, which lubricates the pharynx to facilitate swallowing. There are three major pairs of salivary glands. The largest of which is the parotid gland and is located anteriorly and inferiorly to the ear between the skin and the muscle of chewing, the masseter. The parotid duct carries its contents and drains into the mouth. It is the parotid gland that becomes swollen and infected with the mumps or parotitis. The submandibular gland is located inferiorly to the mandible or jawbone midway along the inner side of the jaw. It has a muscular covering and empties its contents by way of the submandibular duct into the floor of the mouth on both sides. The sublingual gland, as its name implies, lies under the floor of the mouth and on the side of the tongue. Each sublingual gland possesses several small sublingual ducts that empty into the floor of the mouth in an area posterior to the submandibular duct.
The rectum is a short, muscular tube that forms the lowest portion of the large intestine and connects it to the anus. Feces collects here until pressure on the rectal walls cause nerve impulses to pass to the brain, which then sends messages to the voluntary muscles in the anus to relax, permitting expulsion.
If the small intestine were not looped back and forth upon itself, it could not fit into the abdominal space it occupies. It is held in place by tissues which are attached to the abdominal wall and measures eighteen to twenty-three feet in the average adult, which makes it about four times longer than the person is tall. It is a three-part tube of about one and one-half to two inches in diameter and is divided into three sections: (1) the duodenum, a receiving area for chemicals and partially digested food from the stomach; (2) the jejunum, where most of the nutrients are absorbed into the blood and (3) the ileum, where the remaining nutrients are absorbed before moving into the large intestine. The intestines process about 2.5 gallons of food, liquids and bodily waste every day. In order for enough nutrients to be absorbed into the body, it must come in contact with large numbers of intestinal cells which are folded like gathered skirts. Each of these cells contain thousands of tiny finger-like projections called “villi,” and each villus contains microscopic “microvilli”. In one square inch of small intestine, there are about 20,000 villi and ten billion microvilli. Each villus brings in fresh, oxygenated blood and sends out nutrient-enriched blood. The villi sway constantly to stir up liquefied food and remove the nutrients which can be absorbed and then passed through the membranes of the villi into the blood and lymph vessels. The fatty nutrients go to the lymph vessels, and glucose and amino acids go to the blood and on to the liver. The muscles which encircle this tube constrict about seven to twelve times a minute to move the food back and forth, to churn it, knead it, and to mix it with gastric juices. The small intestine also makes waves which move the food forward, but these are usually weak and infrequent to allow the food to stay in one place until the nutrients can be absorbed. If a toxic substance enters the small intestine, these movements may be strong and rapid to expel the poisons quickly.
The spleen is the largest of the lymphoid tissues. It is just about the size of the heart and is a spongy material which will hold up to .3 gallons of blood. It is located on the left side of the body, just behind the stomach. The spleen is a valuable organ which produces some of the white blood cells, filters the blood, destroys old, worn-out red blood cells and returns needed iron to the blood, disposing of the rest as waste. The spleen also stores excess blood for emergencies; for example, when oxygen in the circulatory system is short. We often hear that the victim of an auto accident has had a ruptured spleen which has been removed surgically. Because the spleen is so soft and spongy, it cannot be repaired by surgery, so it is removed to stop the loss of blood.
A hollow, sac-like organ connected to the esophagus and the duodenum (the first part of the small intestine), the stomach consists of layers of muscle and nerves that continue the breakdown of food which begins in the mouth. It is also a storage compartment, which enables us to eat only two or three meals a day. If this weren’t possible, we would have to eat about every twenty minutes. The average adult stomach stretches to hold from two to three pints and produces approximately the same amount of gastric juices every twenty-four hours. The stomach has several functions: (1) as a storage bin, holding a meal in the upper portion and releasing it a little at a time into the lower portion for processing; (2) as a food mixer, the strong muscles contract and mash the food into a sticky, slushy mass; (3) as a sterilizing system, where the cells in the stomach produce an acid which kills germs in “bad” food; (4) as a digestive tub, the stomach produces digestive fluid which splits and cracks the chemicals in food to be distributed as fuel for the body. The process of digestion is triggered by the sight, smell or taste of food, so that the stomach is prepared when the food arrives. Every time you pass a bakery shop or smell your mother’s good cooking, the body begins a digestive process. If the stomach is not filled, these gastric juices begin eroding the stomach lining itself, so fill ‘er up!
A tooth is a hard structure, set in the upper or lower jaw, that is used for chewing food. Teeth also give shape to the face and aid in the process of speaking clearly. The enamel that covers the crown (the part above the gum) in each tooth can be broken down by acids produced by the mouth for digestive purposes. This process is called “decay”. To prevent decay, good oral hygiene, consisting of daily brushing and flossing, is necessary. The hardest substance in the human body is one of the four kinds of tissue which make up the tooth. It is enamel and covers the crown (area above the gum line) of the tooth. A bony material called “cementum” covers the root, which fits into the jaw socket and is joined to it with membranes. “Dentin” is found under the enamel and the cementum, and this material forms the largest part of the tooth. At the heart of each tooth is living “pulp,” which contains nerves, connective tissues, blood vessels and lymphatics. When a person gets a toothache, the pulp is what hurts.