The Gastrointestinal System


The Gastrointestinal System

3.1a The physiology of the gastrointestinal system

In this chapter we examine the physiology and diseases of a particular physiological system, in this case, the digestive or gastrointestinal system. This section is concerned with the function of the digestive system in health, and provides a foundation for the study of the investigation and treatment of digestive diseases that is described in the successive four sections.

Because of its length, this section is divided into two parts. If you are using this book as a study course, you are advised to work through the two parts of this section in separate study sessions.

Part I: From our food to the stomach1

The digestive system and digestion

The function of the digestive system is to ensure that the water and nutrients present in the diet are absorbed adequately by the body and extraneous waste products are eliminated effectively. It also has a protective function in that it minimizes damage from toxins and microbes in the diet. Digestion takes place along the length of a tube known variously as the gastrointestinal or digestive tract, the gut or the alimentary canal. All these terms are used in medical language and are interchangeable.

Through the process of digestion, the complex contents of food are broken down to release nutrients in a form that can pass easily through the lining of the digestive tract and thence into the bloodstream. Water taken in with the diet is absorbed into the bloodstream at the same time. Not all of our food has value as a source of nutrients, and not all the water within it is absorbed. Some is left within the digestive tract to become waste material. This waste food material and water travels down the length of the digestive tract to be expelled in the form of feces.

The nutrients in our diet

A nutrient is any substance that can be used by the body either to produce energy or to enable other vital chemical processes to take place. There are six classes of nutrients: carbohydrates, fats, proteins, mineral salts, vitamins and water. A healthy diet should contain an ideal balance of these. In addition to these nutrients, a healthy diet should also provide fiber. Fiber is a medical term used to describe indigestible cellulose from plant-derived foods. Fiber enables healthy transit and elimination of waste, as it provides bulk and water-retaining properties to the feces. Dietary fiber may also encourage a healthy balance of bacteria in the colon that are associated with beneficial anti-inflammatory benefits.

A healthy diet will consist largely of starchy foods such as bread, rice, pasta and cereals, and fruit and vegetables. Current UK guidelines for the public are that over one-third of our diet should consist of starchy foods, and that additional nutrients should come from at least five portions of fruit or vegetables a day. In addition, there should be some food containing protein, either from the meat/eggs/pulses food group or from the milk/cheese food group. Foods containing fats and sugars should be kept to a minimum. The UK Food Standards Agency “eat well plate” summarizes this advice in pictorial form, as shown in Figure 3.1a-I.2


Figure 3.1a-I The balance of good health


Starchy foods and fruit and vegetables together provide the carbohydrates that the body needs. Carbohydrates contain the elements carbon, hydrogen and oxygen. These are the foods that, once broken down, provide the basic fuel for the energy-producing process of internal respiration (see Section 1.1b). Internal respiration takes place in the mitochondria of all living cells, where simple sugars and other small molecules derived from fats and proteins react with oxygen to produce cellular energy. Carbon dioxide and water are the waste products. The energy produced is stored partly in the form of the charged molecule adenosine triphosphate (ATP), and is partly released as heat.

The simplest carbohydrates are the monosaccharides, or simple sugars, such as glucose and fructose. These are in a perfect form to be utilized by the mitochondria, and so provide the most rapidly accessible form of energy. Monosaccharides taste sweet.

The molecules of the monosaccharides are also found linked together in pairs in our food in the form of sweet-tasting disaccharides, such as sucrose and lactose (the sugar in milk). Monosaccharides are also found in the form of chains known as polysaccharides, of which starch is an example. These do not immediately taste sweet, but can be broken down in the digestive tract to release the monosaccharides. Starches are therefore a form of slow-release energy for the body. Cellulose, which forms the fibrous parts of fruit and vegetables, is also a polysaccharide, but cannot be readily utilized by the human digestive tract. This is why it can perform its role as fiber. Grass-eating animals, however, have the capability of breaking down cellulose and extracting its valuable food energy.

If carbohydrates are in excess in the diet, superfluous monosaccharides can be converted within the liver to a polysaccharide called glycogen. Glycogen is held in the liver so that when blood sugar levels are low it can readily be converted back to accessible glucose. Any excess carbohydrate after this process has taken place is converted to fat for long-term energy storage.


Fats (lipids) are non-soluble oily substances found in diverse foods in our diet, including meat, nuts, eggs, milk and some vegetables. Most fats consist of two components joined together, a fatty acid and glycerol. When broken down by the body, the fatty acids and glycerol are released to provide energy for the bodily processes.

Saturated fat is so called because of the nature of the chemical bonds in the fat molecules. Saturated fat molecules contain more hydrogen than unsaturated fat molecules and this property causes saturated fat to tend to form into a solid more readily than unsaturated fat. Lard from meat and butter are both largely saturated fats. Saturated fat is a good source of energy for the body, but an excess in the diet is known to cause the lipids in the bloodstream to be carried in the more unhealthy form of low-density lipoprotein and triglycerides (LDL and TGs). It is believed that LDL is more likely to contribute to the formation of atheroma that leads to cardiovascular disease than another main category of lipoprotein, HDL (high density lipoprotein), which is considered to have a more protective role in this regard.

Unsaturated fat is generally derived from vegetable matter, and tends to form clear oil rather than a solid fat. It consists of molecules containing an unsaturated fatty acid and a glycerol. The omega-3, omega-6 and omega-9 fatty acids are all unsaturated fatty acids derived from unsaturated fats. These polyunsaturated fatty acids are essential components for cellular and intercellular communication processes and the building of cellular structures.

Vegetable, nut and olive oils contain unsaturated fat that provides largely omega-6 and omega-9 fatty acids. Fish oils contain unsaturated fat that provides omega-3 fatty acids. Most vegetable oils are made of polyunsaturated fats, but olive oil, canola oil and avocados contain a particular form of unsaturated fat known as a monounsaturated fat (this contains omega-9 fatty acids).

Unsaturated fats and monosaturated fats in particular seem to have the property of reducing the amount of more unhealthy LDL in the bloodstream. For this reason, it is now considered important for a healthy diet that these forms of fats predominate over saturated fats, and that monounsaturated fats predominate over polyunsaturated fats (as is found in the Mediterranean diet).

Unsaturated fats may protect against cardiovascular disease by providing more membrane fluidity than the more solid saturated fats.

Moreover, a current dietary theory is that some oils and fats are less likely to contribute to unhealthy inflammation in the body. The omega-3 fatty acid-containing oils that are present in fish oils are meant to be most beneficial in this regard. Whilst all vegetable oils contain omega-3 fatty acids, rapeseed oil and flax seed oil carry the highest percentage of these potentially beneficial components (found in the form of EPA and DHA in fish and ALA in nuts and vegetable seeds).

It is possible that the Western diet which contains much more omega-6 oils than omega-3-containing oils (over ten times omega-6 to omega-3 on average) is not presenting these vital molecules in a healthy ratio. Current evidence suggests that this is one of the many factors in the Western diet that promotes disease, in this case, by promoting inflammatory processes.3

Cholesterol is another oily substance that is particularly important as a component of cell membranes, supporting nerve cells and also for making hormones and other essential molecules, including the steroids and vitamin D. Cholesterol is found in dairy products, meat and eggs, but for use in the body it is largely synthesized in the body itself, particularly in the liver, where it is built up from more simple molecules. Cholesterol is carried in the blood in the form of the complex compound molecules of LDL, very low-density lipoprotein (VLDL) and HDL. These structures package the cholesterol so that it is readily accessible to the cells. As mentioned, LDL (and VLDL) is associated with vascular ill health, but HDL appears to be protective against vascular damage. All these components of the blood are measured in the frequently performed fasting lipids blood test, and the figures are often described in terms of high or low cholesterol. However, the balance of these lipid structures in the blood seems to be less affected by the amount of cholesterol in the diet than by the relative amounts of saturated and unsaturated fats, and so cholesterol in the diet is no longer considered to be unhealthy. This is in direct contrast to the health advice given freely by doctors at the end of the 20th century. Nevertheless, dietary cholesterol’s falsely attributed bad reputation persists in the public consciousness. In fact, it is now saturated fats and trans fatty acids that are thought to be most deleterious to health.

Trans fatty acids are only found in traces in a natural diet. However, they have made up a significant proportion of many Western diets because they are found in vegetable oils that have been processed forming partially hydrogenated oils (PHOs) to preserve their longevity for foods such as margarines, cakes and biscuits. The United States Food and Drug Administration (FDA) stated that, according to recent estimates, the average American was eating 5.8g of trans fatty acids a day. It is known that trans fatty acids alter the balance of cholesterol in the blood to an unhealthy state (they increase levels of LDL at the expense of the more healthy HDL). For this reason, in some European states (e.g. Denmark and Switzerland) the use of trans fatty acids in food manufacturing has been banned. In 2015 the FDA passed legislation limiting the manufacture of foods containing PHOs, which will be enforceable by 2018. This is predicted to have a large health benefit in terms of reduction of deaths from cardiovascular disease,4 although this would be even more marked if the foods that currently contain trans fats (processed cakes, biscuits and pastries) were a far less prominent component of most Western people’s diet altogether.


Proteins are complex chain-like structures that are made up of simple molecules called amino acids. All amino acids contain the element nitrogen as well as carbon, hydrogen and oxygen. Some also contain trace minerals such as zinc, iron and copper. There are just over 20 different amino acids, which can be combined in potentially infinite permutations to form protein chains. Proteins are made in animal and plant cells in the location of the ribosomes as a result of decoding instructions held on the cellular DNA (see Section 1.1b). The ribosome is the site at which amino acids are linked together to form a unique and characteristic protein chain, after which the particular makeup of the protein chain encourages the structure to fold in a characteristic way. Proteins are large molecules that have a unique three-dimensional shape. The shape and form of a particular protein might give it special structural properties (e.g. the flexible protein collagen found in skin and hair) or make it soluble in water (e.g. the albumen found in blood and in egg white). In other proteins the shape is essential for communication, and this is the basis of how cell membrane proteins recognize the external stimulants, such as hormones, that bring about internal change in the cell itself. Antibody-antigen recognition also depends on the protein antibody having a unique shape that fits the shape of the antigen.

Protein in food is found in meat, eggs, fish, cereals, nuts and pulses. All the essential amino acids can be derived from the complete proteins in meat, fish, eggs, milk and soya. Without soya products, a vegetarian will need to eat combinations of nuts, cereals and pulses to ensure that all amino acids are obtained from the diet.

Protein is broken down to soluble amino acids in the body, and these circulate dissolved in the blood so that they reach all the tissues. The ribosomes of the cells utilize amino acids to produce their own proteins so that they can reproduce and continue performing their unique functions. Proteins are used to make cellular and extracellular structures (such as the contractile fibers in muscle cells and the connective tissue fibers in cartilage), plasma proteins (such as the clotting factors and albumen), antibodies, enzymes and also some hormones.

In a state of starvation, proteins are broken down by the cellular mitochondria to produce energy. The by-products of this process include chemicals called ketones. When excess ketones are produced, the body is said to be in a state of ketosis.


Mineral salts are simple inorganic substances that are required in small quantities to maintain bodily processes. Unlike carbohydrates, fats and proteins, they are not a source of energy. Essential minerals include calcium, sodium, potassium, iodine, phosphate, iron, chromium, zinc, selenium, magnesium and manganese. Certain minerals, such as calcium and phosphate (in bone) and sodium (as dissolved salt in the body fluids), constitute a significant proportion of the body mass, whereas others are required only in trace amounts.


Vitamins are more complex compounds than minerals, and are also essential for bodily processes. They are essential dietary components because either they cannot be made by the body at all, or they cannot be made in sufficient quantities to sustain health. Vitamins are now broadly grouped into the four fat-soluble vitamins A, D, E and K, and the nine water-soluble vitamins B1–B8 and C. The lettering system approximately relates to the order in which the vitamins were discovered (beginning with vitamin A in 1909). Those substances initially called F–J were either reclassified as B vitamins or were subsequently deemed not to qualify as vitamins at all. The B vitamins are now grouped together because they are all vital in the production of energy from nutrients by the mitochondria. The food sources and the functions of the 13 vitamins are summarized in Table 3.1a-I.

Table 3.1a-I The 13 vitamins, their main food sources and their functions in the body


Food sources

Use in the body

A (retinol)

Green vegetables, milk, liver

Pigments in the eye and health of the skin

D (calciferol)

Milk, eggs, cod liver oil; ultraviolet light

Calcium absorption and bone formation

E (tocopherol)

Margarine, seeds, green leafy vegetables

Antioxidant: protects fatty acids and cell membranes from damaging oxidation

K (phylloquinone)

Green leafy vegetables

Formation of certain clotting factors

B1 (thiamine)

Meats (pork), grains, legumes

Carbohydrate metabolism; also nerve and heart function

B2 (riboflavin)

Milk, liver, eggs, grains, legumes

Energy metabolism

B3 (niacin or nicotinic acid)

Liver, lean meats, grains, legumes

Energy metabolism

B5 (pantothenic acid)

Milk, liver, eggs, grains, legumes

Energy metabolism

B6 (pyridoxine)

Wholegrain cereals, vegetables, meats

Amino acid metabolism

B7 (biotin)

Meats, vegetables, legumes

Fat synthesis and amino acid metabolism

B9 (folic acid)

Whole wheat foods, green vegetables, legumes

Nucleic acid metabolism

B12 (cobalamin)

Red meats, eggs, dairy products

Nucleic acid production

C (ascorbic acid)

Citrus fruits, green leafy vegetables, tomatoes

Collagen formation in the teeth, bone, and connective tissue of blood vessels; may help in resisting infection


Water is an essential part of the diet because the body is continually losing water through urine, feces, sweat and in exhaled air. About 60 percent of the adult body mass is water, and all the essential bodily processes take place in a watery milieu. Water has to be lost as it is the vehicle through which waste chemicals (in urine and sweat) and also waste food materials (in the feces) are carried for expulsion from the body. It is also lost through the breath and sweat to aid with cooling of the body.

On average, about 2.5 liters of water need to be replaced by the diet every day. Much of this is contained in food, and so a human being can survive on about 1.5 liters of water in drinks per day. Any water that is excess to requirements is passed out of the body in the form of dilute urine. If insufficient water is drunk, the kidneys make dark concentrated urine from the fluid in the plasma. In this case the water required for excretion of wastes is drawn from the tissues, which then become dehydrated. A significant state of dehydration will be reflected in a loss of body weight of more than 2–3 percent. Because the sensitive tissues of the body are not able to withstand the state of dehydration for more than a few days, water is essential for life. Loss of more than 10 percent of the body weight through dehydration is usually not compatible with life.

images Information box 3.1a-I

The nutrients: comments from a Chinese medicine perspective

In Chinese medicine, each food has a characteristic energetic property and also a tendency to affect a particular bodily substance or organ. For this reason, food can be used as medicine, and a diet can be recommended to suit the needs of the patient. The taste and consistency of the food bear an important relationship to its energetic properties. Moreover, the freshness and the method of cooking can have a bearing on the value of the food.

For example, fresh grapes are recognized to Tonify the Qi and Blood, and affect the Lungs, Spleen and Kidneys. Beef is recognized to Tonify Yin and Qi and Blood, and affect the Large Intestine, Stomach and Spleen. Some foods, such as pears, are Cool, and so may be useful in moderation in Hot conditions, and some are Warm in nature, such as chicken liver, and so are used to treat Cold conditions.

Foods contain a complex array and unique balance of nutrients that contribute to their characteristic taste and consistency as recognized in Chinese medicine. For example, a wide range of aromatic substances gives herbs and spices their characteristic flavors, and would result in a Pungent or Bitter nature as recognized by Chinese medicine. Acids in foods such as fruit and vinegar cause the Sour flavor, and salts the Salty flavor. The Sweet flavor is more likely to come from starch, fat or protein. There is an increasingly large body of knowledge about what chemical constituents of individual foods (and herbs) give them their characteristic health-giving properties, so providing a scientific foundation for the centuries-old dietary wisdom of Chinese medicine. For example, cardamom, widely used in Chinese formulas for relief of congested Qi and Blood, contains 11 different aromatic terpenoids that give it its unique flavor.5

However, the distinctions on the basis of flavor and organ and direction are not usually made in conventional medicine when dietary advice is given. Instead, rather broad statements are made about the six types of nutrients found in food groups and how to aim for a balance of these.6 Furthermore, very little emphasis is placed on how the food is prepared, apart from avoidance of overcooking, as this might inactivate the vitamin C in fruits and vegetables. There is also little recognition that cold or raw food might be deleterious for the digestion.

The preparation of dietary supplements (such as vitamin pills, minerals such as zinc and isolated food components such as curcumin from turmeric), which are chemically derived key ingredients, makes total sense to a scientific nutritionist. However, these supplements would not be recognized by a Chinese medicine practitioner to have the complex beneficial properties of the foods in which these nutrients can be naturally found. Instead, it is fresh food, carefully prepared and combined with other foods, which would be considered to be alive and most nourishing for the Qi, Blood, Yin and Yang.7,8

The structure of the digestive tract

The digestive tract is a tube that begins at the mouth and ends at the anus. At both these orifices there is a junction between the tough and dry keratinized epithelium of the skin and the soft and moist mucous epithelium that lines the whole of the digestive tract. This junction can be easily examined in the mouth where the two lips meet. At the junction, the dry, slightly ridged keratinized skin of the external lip becomes moist and smooth mucous epithelium on the inside of the lip.

The tube of the digestive tract has a basic structure that can be found in slightly different forms along its length. The tube has four distinct layers (see Figure 3.1a-II):

The adventitia is the outer layer of the digestive tract. Below the diaphragm, the adventitia forms the peritoneum that is a serous membrane (i.e. it secretes watery fluid). This protects and lubricates the digestive tract so that it can move easily within the abdominal cavity.


Figure 3.1a-II The general structure of the digestive tract

The muscle layer consists of two layers of smooth muscle. The longitudinal and circular fibers contract in coordinated waves to propel the contents of the food down the length of the tract. This motion is called peristalsis.

The submucous layer is a layer of connective tissue that contains blood vessels, collections of lymph nodes, lymphatic vessels and networks (plexuses) of nerves.

The mucosa consists of a mucous (epithelial) membrane (see Section 1.1d), with its combined functions of protection, secretion and absorption, and two supportive connective tissue layers beneath this. The mucous membrane secretes mucus for further protection and lubrication. Other specialized cells in the mucous epithelium are sensitive to changes in the contents of the digestive tract. These secrete hormones into the bloodstream within the submucous layer.

The mouth is the only section of the digestive tract that does not have this general structure.

The tube of the digestive tract has accessory organs, which open out or project into the hollow of the tube. These organs are the tongue, salivary glands, liver, gallbladder, pancreas and appendix.

The salivary glands open via tubes (ducts) into the mouth; the pancreas, liver and gallbladder all open via ducts into the duodenum; and the appendix opens directly into the beginning of the colon. Figure 3.1a-III illustrates the position of the organs associated with the digestive tract, and how, with the exception of the mouth and the esophagus, the main part of the digestive tract is situated below the diaphragm.


Figure 3.1a-III The organs of the digestive system


The mouth is where the process of digestion begins. The teeth, tongue and the muscles of chewing are adapted to grind up food into a paste with saliva.

Saliva is produced by six exocrine salivary glands that open into the mouth. The size and extent of these important glands is illustrated in Figure 3.1a-IV.


Figure 3.1a-IV The position of the salivary glands

Saliva is produced in response to the action of eating, and also in response to the anticipation of food. Certain flavors, such as the sourness of lemons, are particularly powerful in inducing a good flow of saliva. About 1.5 liters of saliva is produced a day, and the moisture is very important to help maintain the health of the mouth and to assist with chewing. The moisture is also essential for enjoyment of food, as it enables food to be tasted. Saliva also contains protective substances such as mucus and antibodies, together with the protein lysozyme, all of which protect the digestive tract from damage by microorganisms and toxins.

The salivary glands also produce the first digestive enzyme that the eaten food will encounter, and thus begins the breakdown of the nutrients into the building blocks that are absorbed into the bloodstream. An enzyme is a biological substance that encourages a chemical reaction. In this case the enzyme is called salivary amylase, and in its presence the chains of complex polysaccharides in carbohydrate come apart to form disaccharides. This reaction only occurs in the mouth, as the acid contents of the stomach stop the amylase from working. This is one reason why it is important to chew thoroughly.

The tongue is a muscular structure adapted for two distinct functions: eating (taste, chewing and swallowing) and speech. It is covered with little bumps called papillae, which are sensitive to the four main tastes: salt, sweet, sour and bitter. The subtleties of taste are also dependent on a healthy sense of smell, as anyone who has a heavy cold will recognize. Figure 3.1a-V illustrates the position of the different sorts of papillae.


Figure 3.1a-V The papillae of the tongue


The esophagus runs from the pharynx at the back of the mouth to the opening of the stomach. As it descends the thorax it runs posterior to the airways that lead to the lungs. A muscle ring (sphincter) at the end of the pharynx prevents air entering the esophagus during breathing in. The opening of the stomach coincides with the position where the esophagus passes through the diaphragm. The muscles of the diaphragm act as a sphincter to prevent stomach contents from returning into the esophagus. This muscle ring is also known as the cardiac sphincter, because it lies just below the heart. The cardiac sphincter is situated at the level of the xiphisternal joint (the location of acupoint REN-16, Zhong Ting9).


The stomach is a stretchy expansion of the tube of the digestive tract, but still retains the four layers of mucosa, submucosa, muscle and adventitia. Figure 3.1a-VI illustrates that the stomach is bounded by the cardiac sphincter above and the pyloric sphincter below. These muscle rings keep food contents within the stomach for up to six hours to enable the beginning of a thorough digestion process. The stomach muscles enable the muscle to churn the food contents stored within it to aid digestion.


Figure 3.1a-VI The location of the stomach

Gastric (the term coming from the Greek for stomach) juices are secreted by specialized glands that open into the stomach. These juices consist of water, acid and an enzyme called pepsin, which enables the breakdown of protein into amino acids. The acid is generated by specialized proteins in the cell walls of the gastric glandular cells known as proton pumps. The stomach lining also secretes copious mucus to protect its own cells from the acid produced by the proton pump cells.

In the stomach, the food is churned with the fluid of the gastric juice to form a liquid soup known as chyme. Pepsin and acid work together, breaking down the protein in the food into amino acids. The smell of this process is the familiar smell of vomit. Very little of the diet is absorbed through the lining of the stomach, but water and alcohol are two exceptions. This explains why drinking alcohol on an empty stomach can have such a rapid effect.

The stomach is also the source of a protein called intrinsic factor, which has an attraction for vitamin B12 (cobalamin). This is important, as each molecule of this essential vitamin can only be later absorbed if it is bound to a molecule of intrinsic factor.

Part II: From the duodenum to the outside world

The duodenum (the first part of the small intestine)

The duodenum is the first 12 inches of small intestine that receives the partly digested food that descends from the stomach. It is into the duodenum that bile from the gallbladder and pancreatic juices from the pancreas enter the digestive tract and mix with the food. Both these fluids are extremely important in the final stages of the digestion of food.

images Information box 3.1a-II

The duodenum: comments from a Chinese medicine perspective

It is interesting to note that the duodenum, the entry point for the pancreatic digestive juices as well as the bile duct leading from the liver and gallbladder, is situated at the level of the origin of 12th thoracic vertebra, the level of the Back-Shu point of the Spleen (BL-20, Pi Shu). This point function has the profound role of regulation and harmonization of the Qi of the Middle Jiao.10 This first 12 inches of small intestine is linked by the ligament of Treitz to the aorta. This is the major vessel leading down from the heart at this level in the abdomen that is responsible for blood flow to the digestive organs and kidneys as well as to the rest of the body. In this way, Keown (2014)11 points out there is an anatomical link which mirrors the link recognized in Chinese medicine between Small Intestine and Heart. At this level the spleen and the pancreas are anatomically linked with both the duodenum and also the splenic artery that branches away from the aorta at this level.

The pancreas

The pancreas is a gland with two distinct roles – one is endocrine and the other is exocrine. (For a reminder of the difference between these two categories of epithelial structure, see Section 1.1d.)

One of the endocrine roles of the pancreas is to secrete the hormones insulin and glucagon directly into the bloodstream. Both these hormones are important for ensuring that the concentration of glucose in the blood remains at the optimum level for the function of the cells of the body.

The exocrine role of the pancreas is to secrete a diverse array of digestive enzymes into the duodenum via the pancreatic duct. The enzymes can digest all three of the complex food components (carbohydrates, proteins and fats), and therefore complete the work started by the salivary glands and the gastric juices.

Figure 3.1a-VII illustrates how the C-shaped loop of the duodenum hugs the pancreas. The spleen, kidneys, liver and transverse colon are all situated at this level of the abdomen. This diagram also illustrates the closely located aorta descending posteriorly, and some of the blood vessels that lead away from the aorta to nourish all the aforementioned organs.


Figure 3.1a-VII The duodenum and associated structures

The gallbladder

This hollow organ is a store for the dark green and bitter tasting fluid called bile, which is manufactured in the liver. Bile contains substances called bile salts that aid the digestion of fats. It also contains some of the body’s waste products that have been processed by the liver. These will pass eventually into the feces. These by-products of the liver’s detoxification include a deeply hued chemical called stercobilin (derived from the breakdown of hemoglobin), which gives the feces their characteristic brown color.

When food enters the duodenum, the cells of its lining respond to the change by releasing hormones, including a substance called cholecystokinin (CCK) into the bloodstream. These messengers travel to the gallbladder and stimulate it to contract and release bile when it is most needed. Similarly, other duodenal hormones stimulate the pancreas to release pancreatic juices.

The small intestine

The small intestine is simply a continuation of the duodenum. It receives the fluid mixture of partly digested food and digestive enzymes from the duodenum, powered by the wave-like muscular contractions of peristalsis.

The small intestine is about 5 meters long, and its lining is thrown into folds. Microscopic peaks of mucosa project from these folds, and appear, when greatly magnified, like a vast mountain range. These peaks are called villi. Each cell of the mucosa has tiny projections called microvilli. This unique structure greatly increases the area of the mucosa that is in contact with the fluid contents of the digestive tract, an adaptation that maximizes absorption. Figure 3.1a-VIII illustrates the microscopic structure of a villus, including the tiny microvilli on each cell, and also the way the blood and lymph supply of the digestive tract lining project into the center of the villus.


Figure 3.1a-VIII One complete villus in the small intestine

As the digestive process continues along the length of the small intestine, the saccharides, amino acids, glycerol, water-soluble vitamins and salts resulting from digestion are taken up (usually by facilitated diffusion) into the mucosal cells. From here they pass into the bloodstream. Numerous collections of tiny lymph nodes in the submucosal tissues protect the bloodstream from any infectious agents that may have been present in the diet.

Fatty acids and the vitamin B12 intrinsic factor complex are absorbed by the very end segment of the small intestine (known as the terminal ileum). The bile acids, which aid the digestion of fats, are also reabsorbed here and circulate in the bloodstream back to the liver, from where they can be used again in the bile. Vitamins A, D, E and K are soluble in fats, and so are also absorbed at this end of the small intestine. Fatty acids, glycerol and these vitamins pass directly into the lymphatic system rather than the bloodstream. The small intestine also reabsorbs over 8 liters of water a day from the fluid contents that initially enter the duodenum. This leaves a semi-solid residue to enter the large intestine. This residue consists of fiber, waste from the bile, and many thousands of dead mucosal cells and intestinal bacteria that have been shed along the digestive journey.

images Information box 3.1a-IV

The anatomy of the organs associated with the duodenum: comments from a Chinese medicine perspective

Note from Figure 3.1a-VII that the spleen lies at the same level as the pancreas. As mentioned previously, these organs share a common embryological root and blood supply. The spleen is conventionally considered to have its main role in the lymphatic system, with absolutely no part to play in the digestion of food. However, in keeping with its embryological roots, it has been found to contain pancreatic stem cells, and its absence is associated with an increased risk of developing diabetes, thus demonstrating it may have more of a directly digestive role than previously understood.13 It is interesting to see that the kidneys are also at this level in the body, and very close by are the liver, gallbladder and loops of the large and small intestines, all clustering around the C-shaped loop of the duodenum.

Keown (2014) also explains that the duodenum shares a fascial (connective tissue) plane with the right kidney, so speculating that, again, there is a direct anatomical correspondence in this powerful region of the body that mirrors the link recognized between Ming Men and Spleen and Stomach, in that one function of the Ming Men is to provide warmth for the Spleen and Stomach in the function of digestion.14

As well as its function in digestion, Yuan Qi from the Kidneys in Chinese medicine is also the motive force for all the Organs. In light of this, it is interesting that the physiological kidneys, recognized to be influenced by the deep branch of the Kidney meridian in Chinese medicine, sit at the center of a cluster of so many other organs, and in particular the kidneys are located very close to those that deal with digestion.

The large intestine

The large intestine, or colon, receives the residue of digestion from the small intestine. The first few centimeters of the colon is called the cecum, which literally means blind ending. This is because it is above the cecum that the end of the small intestine opens into the colon, with the result that the cecum is like a cul-de-sac opening out from the passage. At the end of this cul-de-sac is the narrow tube of the appendix. The parts of the large intestine are illustrated in Figure 3.1a-IX. Note how the horizontal stretch of the transverse colon overlies the duodenum and pancreas at the level of the T12 thoracic vertebra. This is one more organ to traverse this remarkable part of the body, described by the ancient Chinese as the Middle Jiao.


Figure 3.1a-IX The parts of the large intestine

The large intestine is home for most of the healthy commensal bacteria and yeasts described in Section 2.4b. The food residue that enters the large intestine is digested further by these organisms. The by-product of this digestion is gas, which is eventually passed out of the anus. Amongst the by-products of this process are some necessary vitamins, which are absorbed from the large intestine into the bloodstream.

In animals that live on a diet of grass, such as sheep, the cecum and appendix are much longer than those in humans and contain many bacteria. Here, the bacteria can further digest the cellulose fiber in grass to release more saccharides, which can be used by the sheep. It is believed that the appendix in humans is simply a vestige of this larger organ found in vegetarian animals. Cellulose cannot be digested by humans, and instead provides an important role as dietary fiber.

The muscular contractions of the colon are slow and infrequent. The contractions cause the food residue to move gradually along the length of the colon before entering a storage region called the rectum. The rectum is closed at its bottom end by the muscular sphincter of the anus. When the rectum is filled, there is the sensation of needing to open the bowels. However, the act of emptying the rectum is voluntary, and can usually be delayed to a convenient time. About two-thirds of the fluid that enters the large intestine is reabsorbed, so that the substance that enters the rectum has the familiar consistency of feces. Mucus secreted along the length of the colon lubricates the passage of the semi-solid feces.

The liver

The structure and function of this important organ have been left to last because many of the functions of the liver are less directly related to the process of breakdown of food into basic nutrients. Instead, most of the important functions of the liver are about the processing of these nutrients to make them useful to the cells of the body.

The liver is the largest organ in the body and sits under the diaphragm on the right-hand side. If the palm of the right hand is rested on the lower section of the front of the right-hand half of the rib cage so that the little finger runs along the bottom edge of the rib cage, the hand will be resting over the liver.

The liver receives all the blood that has passed through the digestive tract, so that it is the first stop for all the nutrients, and also toxins, that have been absorbed during the process of digestion. This nutrient-rich blood passes through tiny channels in the liver called sinusoids, so that all the liver cells can come into close contact with it. This means that at any one time the liver is holding a large volume of blood within its tissue. This link between the blood leaving the digestive tract and the tissue of the liver is the physical basis for the liver’s two distinct roles in the digestive process.

The first role is to act as a filter for toxins and other harmful substances such as drugs and alcohol. The liver cells are able to first absorb and then destroy or change some of these substances that have entered the bloodstream from the diet before they can affect other tissues of the body. This is why many drugs and toxins, if in excess, may damage the liver (e.g. paracetamol and alcohol). There are also many phagocytic white cells (macrophages) lining the sinusoids. One of their functions is to clear away any microbes that may have entered the bloodstream from the diet.

The second role is to begin processing some of the nutrients into a useful form for the cells, or into a form in which they can be stored for later use. Glucose, for example, is stored in the stable form of glycogen within the liver, from which it can be obtained at a later time if sugar supplies are needed quickly. Amino acids are processed so that they can be used by the cells to build new proteins. A similar process occurs for fats. Many vitamins and iron are also stored for later use in the liver. This is why animal liver is such a nutritious source of food.

Other roles of the liver include:

The use of amino acids to make the proteins carried within the plasma (including the clotting factors).

The removal of worn-out red blood cells. The spleen has the main responsibility for this function (see Section 3.4a), but is assisted in this task by the liver. Phagocytic white blood cells in the spleen and the liver digest the old red blood cells and then release many of the by-products of digestion into the blood for reuse by the body. The waste product of this process is bilirubin, the green-yellow colored substance that is excreted into the bile.

To be a source of heat. The liver is such an active organ that it is one of the most important sources of heat for the body.

To secrete bile. All the liver cells are able to produce bile that contains bile salts and bilirubin. This is secreted into tiny tubules in the liver. From here, the bile drains into a duct that leaves the liver and enters the duodenum. The gallbladder also opens into this duct, so that the bile collects here rather than passing directly into the duodenum.

Figure 3.1a-X helps to illustrate how the gallbladder can be surgically removed without disturbing the free flow of bile from the liver. The function of the gallbladder is simply to delay the entry of bile into the duodenum until the best time for digestion of fats. Removal of the gallbladder results in a constant trickle of bile into the duodenum. This is not ideal for perfect digestion of fats, but is not conventionally considered to be a problem for over-nourished Western people.


Figure 3.1a-X The direction of the flow of bile from the liver to the duodenum

images Information box 3.1a-V

The liver: comments from a Chinese medicine perspective

The vascular spaces formed by the liver sinusoids are receptacles for all the blood that drains from the digestive tract. Here, the blood is cleansed and refreshed before traveling on to nourish other tissues. This is in keeping with the Chinese medicine observation that the Liver stores Blood, and that this function is necessary for replenishing the Qi.

The liver is also responsible for receiving nutrients via the digestive venous system, and ensuring that a steady clean supply of useful nutrients is available to the cells. This would very much be in accord with the Chinese medicine function of the Liver of ensuring the smooth flow of Qi. It would also suggest that the liver plays a part in the processes attributed in Chinese medicine to the Spleen: the Transformation and Transportation of Gu (food) Qi.

Keown (2014) postulates that the liver’s role in the metabolism of the allergy hormone histamine may provide an explanation for the Chinese association of the Liver with irritability, allergies and menstrual problems, all of which can be associated with raised levels of histamine.15

However, there is no obvious conventional link between the function of the liver and the function of tendons and the sinews (although Keown cites a study that links tendon rupture with high cholesterol levels), or indeed in the ability to plan, which are other functions of the Liver Organ in Chinese medicine. The concept of the Hun, or the ethereal soul, has no counterpart in conventional medicine.

Interestingly, however, chronic liver disease can be conventionally recognized to cause changes in the nails, as the deficiency of plasma protein that results leads to characteristically white nails. Similarly, although there is no conventional link between the liver and the eyes in conventional medicine, in chronic liver disease the white of the eyes is the first place in which jaundice becomes apparent.

For more details on the correspondences between the functions of the liver and gallbladder as described in conventional medicine and the Liver and Gallbladder Organs in Chinese medicine, see Appendix I.

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Feb 5, 2018 | Posted by in MANUAL THERAPIST | Comments Off on The Gastrointestinal System
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