Eating: Anatomy and Physiology of Eating (Encyclopedia of Food & Culture)
EATING: ANATOMY AND PHYSIOLOGY OF EATING. "Eating is the action of taking solid foods in the mouth in order to nourish oneself: this action is carried out by insertion [of the foodstuff] in the mouth, followed by mastication, swallowing, and digestion." This is the definition of "eating" proposed by Diderot in his famous Encyclopedia. He goes on to say that eating is specifically not the ingestion of non-food substances such as clay, chalk, stones, and charcoal, but only the ingestion of materials that can be conceived of as foods proper for nourishment. As a result, they also exclude such potential foodstuffs as blood and urine. In the present article, we follow the lead of our predecessor in adopting this definition.
Taking in nourishment is necessary for survival, and this usually involves eating. The following chapter provides an overview of the anatomy and physiology of eating, including the major nutritional processes that take place during digestion.
Eating can be divided into the following processes: eating proper, or ingestion, whereby food enters into the body; and digestion, the process through which nutrients from food are extracted in the gastrointestinal tract. Digestion is followed by absorption, the process through which nutrients are passed through into the blood stream; and by excretion, through which indigestible and unabsorbable products from food are eliminated.
The ability to eat and digest food hinges on an intricate, complex, and coordinated system known as the digestive system, all under control both of the central nervous system (brain and spinal cord) and of digestive system's own intrinsic nervous system, which is sometimes called the body's "second brain." The digestive system comprises two main groups of organs: the organs of the alimentary canal, also known as the gastrointestinal (GI) tract, and the accessory digestive organs.
The GI tract is a continuous tube that runs from the mouth to the anus. The organs of the gastrointestinal tract include the mouth, pharynx, esophagus, stomach, small intestine (consisting of duodenum, jejunum, and ileum), and large intestine. It is within the GI tract that food is chewed or masticated, then broken down into still smaller fragments, and absorbed into the blood.
The accessory organs of the digestive system are the teeth, tongue, salivary glands, liver, gallbladder, and pancreas. The teeth and tongue allow for chewing, tasting, and rasping of food. The other accessory organs of the digestive system produce secretions that aid in digestion. In embryonic life, these organs develop as outpouchings from the primitive GI tract, and their secretions travel into the GI tract via ducts.
In order to understand eating and digestion, it is important to imagine what the body needs to do when you think about food, eat food, swallow food, when food lands in your stomach, and when food makes it way through the small and large intestines. The digestive system is designed to prepare the body for eating and digestion before the first piece of food passes our lips. Once food is ingested, this system is designed to efficiently extract and absorb nutrients while it rids the body of waste products.
Preparation for Eating
In order to understand how the body prepares for eating, it is important to realize that eating and digestion require that our body maximizes blood flow to the digestive organs, in order to both supply oxygen and energy to these organs, and to carry away the nutrients that have been absorbed.
Blood flow to the digestive system is controlled primarily by the autonomic nervous system (ANS). The ANS has two anatomically and functionally different subdivisions, the sympathetic nervous system and the parasympathetic nervous system. The sympathetic nervous system is designed to stimulate the body to prepare for and engage in activities and behaviors that are highly arousing, for example, "fight or flight reactions," while the parasympathetic nervous system is designed to prepare the body to engage in activities and behaviors that are relaxing.
Eating and digestion require the body to be relaxed, to allow for blood to be shunted away from the muscles to the digestive system. In fact, from an evolutionary point of view, the process of eating requires us to stand still or (preferably) to sit or lie down, and concentrate on taking apart the food item and ingesting it, rather than running around. Thus the processes involved in eating are antithetical to moving about, either to get somewhere or to escape danger.
As a consequence of this organization, the body cannot appropriately engage in relaxing behaviors if the sympathetic nervous system is activated, and it cannot engage in arousing behaviors if the parasympathetic nervous system is activated. In other words, if you feel stressed, or you are engaged in physical activity, or you must flee from danger, you will not be able to eat and digest food, and vice versa.
Both mental stress and aerobic exercise involve activation of the sympathetic nervous system. You may have noticed that if you try to eat while you have been highly stressed, or while you are "on the go," or after you have exercised aerobically, your mouth may have been dry, making it difficult to moisten, taste, and swallow food. You may have also experienced stomach cramping and pain upon ingesting food. These responses occur because your sympathetic nervous system is stimulated. Your body is worried about maximizing its ability to fight or run; it is not ready to eat a meal.
If, however, you are in a relaxed state, the thought of food, the sight of food, or simply making a mental association with food, sets into motion a series of events that prepares the GI tract for incoming food. Upon sensing that eating is imminent, the parasympathetic nervous system prepares the GI tract via signals sent though three cranial nerves that exit from the brainstem: the vagus nerve (cranial nerve X), the trigeminal nerve (cranial nerve V), and the glossopharyngeal nerve (cranial nerve IX).
Preparation in the mouth. In the mouth, food must be wetted by saliva, so you can taste it, chew it, begin to break it down into smaller particles, and swallow it. When your parasympathetic nervous system is stimulated, cranial nerves V and IX stimulate the salivary glands to release saliva into the mouth. The average person produces about 1500 ml of saliva each day.
Once food is wet, it can be tasted. Tasting food is critical for identifying it, receiving pleasure if it tastes good, rejecting it if it tastes bad, and for signaling to the body that food is indeed about to be ingested. There are five primary tastants: sweet, sour, salt, bitter, and umami, which is the taste of the amino acid glutamate, a major ingredient of monosodium glutamate (MSG). Tasting is accomplished by taste receptors, which are located in structures called taste buds. Taste buds are housed in small bumps on the tongue and the roof of the palate called papillae. Fungiform papillae house the taste buds located on the anterior surface of the tongue, foliate papillae house taste buds that are located toward the back and side of the tongue. Circumvallate papillae, which form the chevron-like pattern in the back of the tongue, house very large numbers of taste buds. In addition, there are a scattering of taste buds in the palate, the posterior oropharynx, and the esophagus. It is interesting to note that the ideas that only sweet can be tasted at the tip of the tongue, only sour and salt can be tasted on the sides of the tongue, and only bitter can be tasted at the back of the tongue are myths. In fact, all taste buds are capable of tasting all tastants; however, they do so at varying levels of sensitivity.
Another reason for wetting food in the mouth is that it makes food easier to masticate and swallow. Furthermore, once food is chewed, it can be acted upon by salivary enzymes, which are compounds that work to chemically break down food into its molecular components. For example, saliva contains the enzyme salivary amylase, which begins the breakdown of carbohydrates, and lingual lipase, which begins the digestion of fat.
Preparation in the stomach. Food needs to be mixed with fluid in the stomach as well. Therefore, upon the initial cue that the arrival of food is imminent, cranial nerve X (the vagus nerve) stimulates the stomach to release the gastric juices that will be necessary for digestion. Contained in the gastric juices are water, hydrochloric acid (HCl), and pepsin, an enzyme that breaks down proteins in the stomach's acid environment. Water is necessary to keep food liquified. Hydrochloric acid serves several purposes: it signals further digestive events to occur; it dissolves food into smaller particles; it kills many microorganisms that may have been ingested; and it denatures proteins, causing the proteins to lose their structure so that they can be further digested.
In response to parasympathetic activation, the stomach also produces a protective coat of mucus. There are two types of mucus, visible mucus and soluble mucus, consisting of highly glycosylated proteins, called mucins. Mucous neck cells that surround the openings of the stomach's acid-producing glands (called gastric or oxyntic glands) produce visible mucus, which continuously forms a protective coat on the surface of the stomach. As the gastric gland sends out hydrochloric acid and enzymes, soluble mucus, formed by the cells that line the upper part of the gastric pits, is secreted ahead of it, and also covers the surface of the stomach. Together, the two types of mucus form a protective coat, called the glycocalyx. This coat is soluble in alcohol, while aspirin prevents the formation and secretion of mucin. Therefore, alcohol abuse, or excessive use of aspirin, can permit hydrochloric acid to attack the stomach lining, which leads to bleeding. In addition, individuals who are highly anxious do not experience proper parasympathetic activation when eating, and consequently do not produce sufficient mucus. Therefore, given the presence of other triggers for ulcer formation (and specifically infection with bacteria of the Helicobacter species), anxious individuals are often susceptible to peptic ulcers.
Preparation by the pancreas. Cranial nerve X stimulates the pancreas to release insulin from its beta cells. Insulin is the hormone responsible for moving glucose and amino acids out of the blood and into cells so that they can be used for fuel and for forming new or renewed tissue. Insulin is needed once food is digested and absorbed, because the concentration of glucose and amino acids in the blood then increases. Cranial nerve X also stimulates the pancreas to release bicarbonate into the small intestine, which will be used to neutralize the acid coming from the stomach. This process will be discussed in more detail in a later section.
Ingestion: The Mouth and Esophagus
Once the digestive system has properly prepared itself for its incoming meal, food is ingested. Food is wetted by saliva, tasted by the tongue and palate, and chewed or masticated by the teeth. Once the food is mechanically broken down into small enough pieces, it is ready for swallowing, or deglutition. Swallowing is actually a complicated process that requires coordinated activity of the tongue, palate, pharynx, and esophagus, over twenty-two muscle groups in all, controlled by separate regions of the brain. This activity requires strict control to guarantee that the food makes its way into the esophagus and not the trachea. If food is swallowed into the trachea, breathing is blocked and choking ensues. Once food enters the esophagus, the tongue blocks the mouth, the palate rises to close off the nasopharynx, and the larynx rises so that the epiglottis, a muscular flap, covers its opening into the respiratory passageways. Food is subsequently squeezed through the pharynx and into the esophagus by wavelike peristaltic contractions.
At the bottom of the esophagus is the cardiac or lower esophageal sphincter. The esophageal sphincter acts like a valve controlling the entry of food into the stomach. With each peristaltic contraction, the esophageal sphincter opens and a bolus of food lands in the stomach. The esophageal sphincter is also designed to stay closed while the stomach contracts, so that the acid from the stomach cannot reflux into the esophagus. The burning a person feels when acid refluxes into the esophagus is commonly known as heartburn.
Breakdown and Digestion: The StomachThe stomach is responsible for a large proportion of both the mechanical and chemical breakdown of food. Once
The stomach is anatomically designed to stretch to accommodate the entry of food, and to churn and mix food thoroughly so it may be acted upon by stomach secretions and enzymes. At the top of the stomach is the fundus, a dome shaped section where the majority of hydrochloric acid is secreted. Below the fundus is the body of the stomach. The body of the stomach and the antrum of the stomach are separated by the incisura. Below the incisura is the antrum, which narrows and terminates at the pylorus. The pylorus opens into the duodenum through the pyloric sphincter, a muscular valve that controls the emptying of chyme into the duodenum.
When food arrives in the stomach. When food arrives in the stomach, the esophageal sphincter closes; the stomach muscles relax, with the result that the pressure inside the stomach decreases. These two actions prevent food from refluxing back into the esophagus. The stomach's relaxation reflex is also designed to accommodate the increased volume and develops in response to the stretching of the stomach's walls. Note that this relaxation reflex can be "trained." When a person regularly eats large meals, relaxation is greater, and when a person eats small meals, the relaxation reflex is less vigorous. This less vigorous reflex is why people feel their stomach "shrinks" when they go on a diet.
Hydrochloric acid is secreted by parietal cells in glands of the fundus of the stomach. As a result, the pH in the fundus can be on the order of 1. With this low pH, pepsinogen, secreted by chief cells, also located in these glands, is converted to pepsin. Pepsin breaks down protein-containing foods in the fundus into peptides and amino acids, and calcium and vitamin B12 are released. These food components arrive in the antrum, where they induce the production of the hormone gastrin. Gastrin travels via the blood to the fundus to increase hydrochloric acid secretion. Incidentally, due to this signaling function of protein in the stomach, it is nutritionally important to consume some protein with each meal. Gastrin also enhances closure of the pylorus and esophageal sphincter. Local histamine secretion sensitizes parietal cells to the effects of activation the vagus nerve and of gastrin. These three processes form a carefully coordinated system that is designed to maximize hydrochloric acid secretion and retention of material in the stomach so that it is properly mixed with each wave of contraction.
To prevent dumping of the stomach's contents into the duodenum, the pylorus closes, so that only about one-tenth of the amount of food that entered the stomach actually reaches the duodenum immediately after ingestion. As the pylorus closes, the area around the incisura contracts; this provides a narrow round opening separating the antrum from the body and fundus of the stomach. The antrum then contracts, shooting the food back through this narrow opening into the fundus, a process known as retropulsion. As mentioned above, the fundus secretes hydrochloric acid, so the food, the hydrochloric acid, and the enzymes become thoroughly mixed. At the same time, fats are broken down into small globules through this churning process. The incisura then relaxes, and the food mixture is propelled forward into the antrum. This process repeats itself time and time again.
Digestion in the stomach. As noted earlier, proteins are first digested in the stomach by an enzyme called pepsin. Pepsin breaks down protein into smaller molecules called peptides or polypeptides, which make their way into the small intestine for further digestion and absorption.
The digestion of fat begins in the stomach as well, through emulsification by churning and retropulsion. Fats that we consume are mostly in the form of triglycerides. A triglyceride is a molecule that has a glycerol backbone and 3 fatty acid chains attached to it. Fats are not water soluble ("oil and water do not mix") and therefore need to be packaged for absorption into the blood. One process that aids in the digestion and packaging of fat is emulsification. Emulsification increases the surface area of the fat that is available for enzymatic action in the duodenum.
Finally, the stomach secretes intrinsic factor, which is required for vitamin B12 absorption in the intestine. Vitamin B12 is a large molecule, and requires intrinsic factor to protect it from destruction by the stomach, and to enable its absorption through a specific receptor in the ileum.
Reflexes designed to clear out the lower GI tract. Before chyme can adequately make its way into the small intestine, there is a series of reflexes that are designed to prepare the lower gut (small and large intestine) for digestion. The result of these reflexes is that the lower gut gets cleared of old material so that there will be room for the new material coming down.
One reflex is known as the gastrocolic reflex and is due to stretching of the stomach after food lands in it. This reflex moves fecal material into the rectum, so you have a desire to defecate. This is why you may need to go to the bathroom after eating, especially after breakfast.
Gastrin is involved in the gastro-ileal reflex. This reflex clears chyme from the ileum, the furthest point of the small intestine, moving the chyme into the colon. Finally there is a duodenocolic reflex that is brought about when chyme enters the duodenum. This reflex also brings fecal material into the rectum in preparation for defecation.
Digestion and Absorption: The Small Intestine
After the chyme is thoroughly mixed in the stomach, it moves into the small intestine. The small intestine is the primary site for the chemical digestion and the absorption of food. The small intestine has three subdivisions: the duodenum, the jejunum, and the ileum. The duodenum, which connects to the stomach, is the point of entry for the secretions from the pancreas and from the liver via the gall bladder. The jejunum is the longest section of the small intestine and is the site where the majority of nutrients are absorbed. The ileum is the third division and connects to the beginning of the large intestine.
The small intestine is anatomically designed for efficient nutrient absorption. Not only is it very long, but it also has three structural modifications which further amplify its absorptive area: plicae circulares, villi, and microvilli.
The plicae circulares are deep folds of the intestinal mucosa which force the chyme to spiral through the lumen of the intestine. This effect slows the movement and increases the mixing of the chyme, thereby creating time for maximal nutrient absorption.
Villi are fingerlike projections that lie on the surface of the plicae. These projections increase the amount of contact between the surface of the intestine and the chyme, and they make absorption more efficient because they each contain a dense capillary bed and a lymphatic capillary called a lacteal, which act to transport nutrients into circulation. Lacteals are essential for fat absorption.
Finally, at the end of each villus cell are tiny microscopic projections called microvilli, which form the intestine's brush border. The microvilli dramatically increase the absorptive surface area of the small intestine. Moreover, there are enzymes that reside on the brush border that complete the final stages of chemical digestion of carbohydrates and proteins.
When chyme arrives in the duodenum. Chyme must enter into the duodenum at a rate the duodenum can handle. Furthermore, the acidic mixture must be neutralized so that it does not damage the duodenum. Finally, the digestion of the chyme must continue, and the process of nutrient absorption must begin.
It is important to note that the hormones and nerve activities discussed below are usually responsible for more than one of these above processes, and that more than one hormone or nerve activity is involved with each of these processes. In other words, the digestive system has in place a number of checks and balances to ensure that each responsibility is met during intestinal digestion and absorption.
Gastric inhibitory peptide (GIP), released by the small intestine, is one such hormone. As its name suggests, GIP inhibits gastric motility, thereby slowing down the delivery of chyme to the duodenum. In addition, GIP is responsible for helping tissues prepare for an influx of glucose, by causing release of insulin from the beta cells of the endocrine pancreas.
Other ways in which the flow of chyme is slowed enough to accommodate digestion and absorption are through intrinsic nerve signals from the gut, through a decrease in the activity of the vagus nerve or an increase in sympathetic activity, through the hormones secretin and cholecystokinin-pancreozymin (CCK-PZ), which are secreted by the duodenum in the presence of chyme. Not surprisingly, these hormones also have several other functions, discussed below.
In order for the digestive enzymes of the small intestine to be activated, they have to be in an alkaline environment. Neutralization of stomach acid is accomplished by secretions from the pancreas and from Brunner's glands, which line the duodenum. Both secretions are rich in bicarbonate, a basic compound that neutralizes stomach acid. The process of secretion for both of these systems begins with activation of the vagus nerve, which itself begins with the thought of food, as noted earlier. Secretion is further enhanced by secretin and CCK-PZ release from the duodenal wall in response to fat and amino acids in chyme. Vitamins and minerals do not require digestion for absorption.
Fat digestion and absorption. In the small intestine, fat is further emulsified by bile, a fluid that is produced in the liver and stored in the gallbladder. The release of bile into the duodenum is induced by contraction of the gall-bladder, which propels the bile into the common bile duct, and from there into the pancreatic duct. The sphincter of Oddi relaxes, letting the bile and pancreatic juices flow into the duodenum. Contraction of the gall-bladder, relaxation of the sphincter of Oddi, and release of pancreatic juices are all induced by cholcystokininpancreozymin (CCK-PZ), produced, as noted above, by the duodenal wall in response to the presence of fat and amino acids.
Once fat is emulsified by bile, it can be acted upon by enzymes called lipases, which break down fat. Pancreatic lipase requires a high pH for activation, which is accomplished through secretion of pancreatic bicarbonate. Pancreatic lipase breaks off fatty acids from the triglyceride's glycerol backbone, leaving a monoglyceride and two fatty acids. Bile acids then join up with the monoglycerides and fatty acids to form mixed micelles. Bile salts need to be present at or above the proper concentrationhe critical micellar concentration, or CMCn order for micelles to form.
Mixed micelles are shaped like hockey pucks, with bile salts forming the outer ring of the micelles, and the hydrophilic (water-soluble) ends of the fatty acids and monoglycerides forming the circular surfaces. Because of this packaging, the micelles can cross the water layer at the surface of the cells of the small intestine. It should be noted that long-chain triglycerides (>12 carbons) require both breakdown by lipase and incorporation into mixed micelles in order to be absorbed, while medium-chain triglycerides (with 82 carbons in each fatty acid chain) only require breakdown by lipase, and not micelle formation, because they are more hydrophilic.
Bile acids are recovered from the chyme in the terminal ileum, and returned to the liver for re-use. Note that the terminal ileum is also where vitamin B12 is absorbed. No other part of the intestine can compensate for these functions should the terminal ileum be diseased or lost, although absorption of other nutrients can occur successfully with loss of a considerable length of jejunum.
Carbohydrate digestion and absorption. Carbohydrates are also digested and absorbed in the small intestine. They are hydrophilic compounds, and therefore do not require complex packaging to be absorbed. Carbohydrates in our diet exist as monosaccharides, which are one sugar unit long (simple sugars: e.g., glucose, fructose, and galactose); disaccharides, which are two sugar units long (e.g., sucrose, lactose, and maltose); and polysaccharides, which are long complex strings of sugar units (e.g., glycogen and starch).
The majority of carbohydrates are consumed as polysaccharides, either as starch or glycogen. It should be noted that fibers, either soluble or insoluble, are types of polysaccharides that cannot be digested because we lack enzymes capable of chemically breaking them down.
Carbohydrates that can be digested begin to be broken down into small sugars in the mouth by the enzyme salivary amylase (noted earlier). You may have noticed that a piece of bread that you keep in your mouth for some time begins to taste sweet because the amylase in your saliva breaks down complex sugars into sweeter-tasting simple sugars.
The majority of carbohydrate digestion takes place in the small intestine via enzymes that are secreted by the pancreas. Pancreatic enzymes break polysaccharides down into oligosaccharides (2 sugars long), which are then acted upon by enzymes of the intestinal brush border. Within the brush border, oligosaccharides are further broken down into glucose, fructose, or galactose, depending on their initial composition. Glucose is the primary source of fuel used by cells in the body, and fructose and galactose can be converted into glucose via biochemical mechanisms.
Protein digestion and absorption. Protein digestion in the small intestine is accomplished by pancreatic enzymes as well. The large polypeptides created by the action of pepsin in the stomach are broken down into small peptides and amino acids by the pancreatic enzymes trypsin, chymotrypsin, and carboxypeptidase, as well as by peptidases in the brush border. Amino acids, the smallest functional unit of proteins, and small peptides are then absorbed into the blood, where they can be transported to cells for protein synthesis or for fuel.
The Large Intestine: The End of the Journey
The undigested portion of food that does not get absorbed by the small intestine passes on into the large intestine. The large intestine is responsible for reabsorbing the water added by the stomach and the small intestine to keep the chyme fluid, for fermentation of undigested products by bacteria, and for packing waste products into feces for excretion. The subdivisions of the large intestine are the cecum, appendix, colon (ascending colon, transverse colon, and descending colon), rectum, and anal canal.
Bacteria normally live in the large intestine. These bacteria either make their way into the large intestine by surviving the journey through the stomach and small intestine, or via the anus. These bacteria break down the fiber that is consumed, releasing gases, as well as the very smelly short-chain fatty acids that provide the major source of fuel for the wall of the large intestine. In addition, these bacteria are capable of synthesizing some B vitamins and vitamin K.
Once the contents of the colon are moved along via contractions, they reach the rectum, which otherwise is usually empty. Stretching of the rectal wall initiates a defecation reflex. This reflex is mediated by the parasympathetic nervous system and causes the walls of the sigmoid colon and the rectum to contract and the anal sphincter to relax. Once feces are forced into the anal canal, the stretch sends a signal to the brain informing us of the need to defecate. Under normal circumstances, the defecation response is under voluntary control.
Such a complex process as eating, in which each step is predicated on the preceding one, is prone to large variations in normal function. Specifically, the timing and efficiency of each of the steps described here show wide variability both from person to person, and for different food choices and consumption patterns. Some people have rapid intestinal transit time, and some slow nutrients are more easily digested and absorbed from some diets than from others. However, the basic scheme outlined here holds true for all healthy people.
See also Appetite; Sensation and the Senses.
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Gershon, Michael D. The Second Brain: A Groundbreaking New Understanding of Nervous Disorders of the Stomach and Intestine. New York: HarperCollins, 1999.
Guyton, Arthur C., and John E. Hall. Textbook of Medical Physiology. 10th ed. Philadelphia: Saunders, 2000.
David R. Bauer Virginia Utermohlen