3.1 Gastrointestinal disease
This is the first chapter that will examine the physiology and diseases of a particular physiological system, in this case the digestive or gastrointestinal system. This chapter 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 next four chapters of this section.
Because of its length, this chapter is divided into Parts I and II. If you are using this book as a study course, then you are advised to work through the two sections of this chapter in separate study sessions.
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 minimises damage from toxins and microbes in the diet. Digestion takes place along the length of a tube known variously as the ‘gastrointestinal tract’ 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 blood stream. Water taken in with the diet is absorbed into the blood stream 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 faeces (see Q3.1a-1).
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 basic nutrients: carbohydrates, fats, proteins, mineral salts, vitamins and water. A healthy diet should contain an ideal balance of these nutrients. In addition to these nutrients, a healthy diet also should provide ‘fibre’. Fibre is indigestible cellulose from plant-derived foods. Fibre enables healthy transit and elimination of waste, as it provides bulk and water-retaining properties to the faeces.
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 about 60% of the energy value of food (calories) should come from starchy foods and that additional energy 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’ summarises this advice in pictorial form as shown in Figure 3.1a-I.
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 Chapter 1.1b). Internal respiration takes place in the mitochondria of all living cells, where simple sugars 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 most simple carbohydrates are the monosaccharides, or simple sugars, such as glucose and fructose. These are in a perfect form to be utilised by the mitochondria, and so provide the most rapidly accessible form of energy. Monosaccharides taste sweet.
The molecules of the monosaccharides also occur bound in pairs in our food in the form of sweet-tasting disaccharides. Sucrose and lactose (the sugar in milk) are examples of disaccharides. The 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 utilised by the human digestive tract. This is why it can perform its role as fibre. Grass-eating animals, however, do 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 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. It consists of molecules containing a saturated fatty acid and a glycerol. Saturated fat tends 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 will cause cholesterol to be carried in the blood in the more unhealthy form of low-density lipoprotein and triglycerides (LDL and TGs).
Unsaturated fat is generally derived from vegetable matter, and tends to form a 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 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 cholesterol that is carried in the blood in the more unhealthy LDL form. For this reason, it is now considered important for a healthy diet that these forms of fats predominate over the 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.
Cholesterol is another oily substance that is particularly important for making hormones, including the steroids. Cholesterol is found in dairy products, meat and eggs. It can also be synthesised in the body, particularly in the liver, from more simple molecules. Cholesterol is carried in the blood in the form of complex molecules called LDL, very low-density lipoprotein (VLDL) and high-density lipoprotein (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. The balance of these 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 (see above).
Trans fatty acids are only found in traces in a natural diet. However, they make up a significant proportion of many western diets because they are found in vegetable oils that have been processed (hydrogenated) to preserve their longevity for foods such as margarines, cakes and biscuits. The American Food and Drugs Administration (US FDA) has estimated that the average American eats 5.8 g 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). In some European states (e.g. Denmark and Switzerland) the use of trans fatty acids in food manufacturing has been banned. Many other countries now have policies whereby the proportion of trans fatty acids has to be described in food labelling.
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 about 20 different amino acids, which can be combined in potentially infinite permutations to form protein chains. Proteins are made in animal and plant cells at the ribosomes as a result of decoding instructions held on the cellular DNA (see Chapter 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. This means that proteins are large molecules that have a unique shape. The shape and form of a particular protein might give it special structural properties (e.g. the collagen 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 ‘recognise’ the external stimulants, such as hormones, that bring about internal change in the cell itself. Antibody–antigen recognition also depends on the 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 converted to amino acids in the body, and these circulate dissolved in the blood so that they reach all the tissues. The ribosomes of the cells utilise 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 fibres in muscle cells and the connective tissue fibres 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 recognised to be linked 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 summarised in Table 3.1a-I.
|Vitamin||Food sources||Use in the body|
|A (retinol)||Green vegetables, milk, liver||Pigments in eye and health of 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)||Wholewheat 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 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 the urine, faeces, sweat and in exhaled air. About 60% 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 faeces) are carried for expulsion from the body. It is also lost through the breath and sweat to aid with cooling of the body.
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, whereby a diet is recommended to suit the needs of the patient. The taste and the 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 recognised to Tonify the Qi and Blood, and affect the Lungs, Spleen and Kidneys. Beef is recognised 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 in Cold conditions.
Despite the fact that most foods contain a complex array and unique balance of nutrients, these distinctions are not made in conventional medicine, in which rather broad statements are made about the six types of nutrients found in food groups. Furthermore, very little emphasis is also placed on how the food is prepared, apart from avoidance of overcooking, in that this might inactivate the vitamin C in fruits and vegetables.
per day. Any water 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, and 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%. 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% of the body weight through dehydration is usually not compatible with life (see Q3.1a-2-Q3.1a-6).
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 keratinised 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 keratinised skin of the external lip becomes moist and smooth mucous epithelium on the inside of the lip.
• The adventitia is the outer layer of the digestive tract. Below the diaphragm, the adventitia forms the peritoneum which 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.
• The muscle layer consists of two layers of smooth muscle. The longitudinal and circular fibres contract in coordinated waves to propel the contents of the food down the length of the tract. This motion is called ‘peristalsis’.
• The mucosa consists of a mucous (epithelial) membrane (see Chapter 1.1d), with its combined functions of protection, secretion and absorption, and two supportive layers beneath this. The mucous membrane secretes mucus for further protection and lubrication. Other specialised cells in the mucous epithelium are sensitive to changes in the contents of the digestive tract. These secrete hormones into the blood stream within the submucous layer.
The structure of the tube itself changes throughout its length according to the function required. The mouth, oesophagus, stomach, duodenum, small intestine, large intestine (colon), rectum and anus are described in more detail below.
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 oesophagus, the main part of the digestive tract is situated below the diaphragm.
Saliva is produced is response to the action of eating, and also in response to the anticipation of food. Certain flavours, such as the sourness of lemons, are particularly powerful in inducing a good flow of saliva. About 1.5 litres 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 lyzozyme, 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 blood stream. 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 why it is important to chew thoroughly (see Q3.1a-7).
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 recognise. Figure 3.1a-V illustrates the position of the different sorts of papillae.
The oesophagus runs from the pharynx at the back of the mouth to the opening of the stomach. The oesophagus runs posterior to the airways which lead to the lungs. A muscle ring (sphincter) at the end of the pharynx prevents air entering the oesophagus during breathing in. The opening of the stomach coincides with the position where the oesophagus passes through the diaphragm. The muscles of the diaphragm act as a sphincter to prevent stomach contents from returning into the oesophagus. 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 Zhongting).
The stomach is a stretchy expansion of the tube of the digestive tract, but still retains the four layers. 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 6 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.
Gastric (the term coming from the Greek for ‘stomach’) juices are secreted by specialised 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 produced by specialised 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’. The pepsin and the acid work together to break 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 attraction is important, as each molecule of this essential vitamin can only be later absorbed if it is bound to a molecule of intrinsic factor (see Q3.1a-8).
The duodenum is the tubular receptacle for 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.
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, is situated at the level of the origin of 12th thoracic vertebra, the level of the Back-Shu point of the Spleen (Bl 20; Pishu).
The endocrine role of the pancreas is to secrete the hormones insulin and glucagon directly into the blood stream. 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.
Information Box 3.1a-III The endocrine pancreas: comments from a Chinese medicine perspective
The endocrine role of the pancreas is to allow the accessibility of glucose to the cells, an essential prerequisite for the production of cellular energy in the process of internal respiration. Although this function is not directly related to digestion, it could be interpreted as broadly parallel with the wider Transforming and Transporting function that the Spleen Organ has on Gu (food) Qi.
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 will complete the work started by the salivary glands and the gastric juices.
This hollow organ is a store for the fluid called ‘bile’, which is manufactured in the liver. Bile is dark green and very bitter tasting. It contains substances called ‘bile salts’, which aid the digestion of fats.
Note from Figure 3.1a-VII that the spleen lies at the same level as the pancreas. It is possible that the ancient Chinese assumed that the pancreas and spleen were one anatomical organ, as the pancreas is never directly referred to in the classics of Chinese medicine. Anatomically the two organs are very closely related and share a common blood supply. This may explain why digestive functions were attributed to the Spleen. However, 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. It is also 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.
include a deeply hued chemical called ‘stercobilin’ (derived from the breakdown of haemoglobin), which gives the faeces their characteristic brown colour.
When food enters the duodenum, the cells of its lining respond to the change by releasing hormones, including a substance called cholecystokin (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 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 metres 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’. Furthermore, 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 maximises 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 centre of the villus.
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 (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 litres 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 fibre, waste from the bile, and many thousands of dead mucosal cells and intestinal bacteria, which have been shed along the digestive journey (see Q3.1a-9 and Q3.1a-10).
The large intestine, or colon, receives the residue of digestion from the small intestine. The first few centimetres of the colon are called the ‘caecum’, which literally means ‘blind ending’. This is because it is above the caecum that the end of the small intestine opens into the colon, with the result that the caecum 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.
The large intestine is the home for most of the ‘healthy’ bacteria and yeasts which were first described in Chapter 2.4b. The food residue that enters the large intestine is digested further by these ‘healthy’ organisms. The by-product of this digestion is gas, which is eventually passed out of the anus. Some necessary vitamins are made as by-products of this process, which can then be absorbed into the bloodstream.
In animals that live on a diet of grass, such as sheep, the caecum and appendix are much longer than those in humans and contain many bacteria. Here, the bacteria can digest further the cellulose fibre 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 the bulking agent known as ‘dietary fibre’.
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 usually can 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 faeces. Mucus secreted along the length of the colon lubricates the passage of the semi-solid faeces (see Q3.1a-11).
The structure and function of this important organ have been left to last because many of the functions of the liver are not 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, then 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 which have entered the bloodstream from the diet before they can affect other tissues of the body. This is the reason 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 liver is considered to be so nutritious as a food.
• The removal of worn-out red blood cells. The spleen has the main responsibility for this function (see Chapter 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 coloured substance that is excreted into the bile.
The last role to be described here is that of the secretion of bile. In addition to the diverse functions already listed, all the liver cells are able to produce bile and secrete it 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).
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 travelling 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 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 described in Chinese medicine as the Transformation and Transportation of Gu (food) Qi.
However, there is no obvious conventional link between the function of the liver and the function of tendons and the sinews, 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 recognised 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 detail 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.
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 overnourished western people.
This last section of this chapter touches on the metabolism, which can be seen as the end result of the digestive process. ‘Metabolism’ simply means all the chemical processes that take place in the body. These processes require basic nutrients, oxygen and water to occur.
• Catabolism includes all those reactions that lead to the breakdown of the complex structures of the cell, and often leads to the release of energy (this might be described as a Yang process in Chinese medicine language).
In health, the anabolic and catabolic processes generally balance each other. This leads to a constant process of turnover of the old substances in the body for replacement with the new. It is this balance that allows for adaptation to change. In times of growth and development the anabolic processes are more prominent, and in times of illness and ageing the catabolic processes are more prominent.
Healthy metabolism is dependent on an adequate supply of ‘fuel’ – the basic nutrients and oxygen. If either of these is lacking, fuel has to be obtained from within the body. Initially, the glucose store in the liver is drawn upon, but very soon after this body fat and muscle protein is broken down. This is a state in which catabolism is in excess, and is not a healthy state if prolonged. Nevertheless, it is the state that people who diet ‘successfully’ put themselves into by choice. Conversely, if there is excess fuel, or if the metabolism is sluggish, then a state in which anabolism is in excess results. If there are no demands for the body to grow or develop, then the excess nutrients are laid down as fat.
Information Box 3.1a-VI The metabolism: comments from a Chinese medicine perspective
Signs and symptoms of disorders of metabolism suggest that the overall control of the metabolism might be broadly equated with the Spleen Organ, as Spleen Deficiency can lead to both weakness and inability to gain weight, and conversely to Accumulation of Damp and weight gain.
Self-test 3.1a The physiology of the gastrointestinal system
3. Carbohydrates are broken down to saccharides by the enzymes in saliva, and proteins are broken down to amino acids in the stomach. The chewing of food and the churning action of the stomach turn the solid diet into a fluid ready for further digestion in the small intestine.
4. Pancreatic juice contains enzymes, which further break down carbohydrates into saccharides and proteins into amino acids. These enzymes also digest some fats (not including cholesterol, which is absorbed unchanged) into fatty acids, and glycerol. Bile salts in the bile aid the digestion of fatty acids.
5. The small intestine is adapted to maximise absorption by being very long and in the structure of its mucosa, which has folds, villi and microvilli to increase the surface area further. The blood and lymphatic supply of the small intestine is very rich and is arranged so that a blood vessel and a lymphatic vessel project into each of the thousands of villi on the mucosa.
Patients with suspected disease of the gastrointestinal system may be referred for further investigation by their GP either to a gastroenterologist (a hospital physician who specialises in the medical management of digestive disease) or to a surgeon who specialises in surgery of the bowel.
Once referred to any hospital specialist, patients may be offered a series of tests chosen to exclude a wide range of possible diagnoses that may affect the gastrointestinal system. This may mean that the patient may undergo some tests that are not strictly necessary in their particular case, but it is considered good practice to be thorough in the investigation. Some tests might involve the patient in considerable inconvenience or discomfort, but are performed because it is believed that it is best to be as informed as possible about the health of the system before treatment is chosen.
The physical examination of the gastrointestinal involves the stages listed in Table 3.1b-I. For the purposes of physical examination of a supine patient, the accessible region of abdomen is considered to occupy the approximately hexagonal region that is bounded by the ribs above and the inguinal ligaments and pubic bone below.
As Figure 3.1b-I illustrates, many important organs are situated underneath the nine named regions of the abdomen. A lump felt in the suprapubic area, for example, might suggest a problem of the bladder or the uterus, whereas pain in the left iliac fossa would suggest a problem of the colon or left ureter. The names for these nine regions of the abdomen are frequently used in medical texts for the description of abdominal conditions, so it is useful to be familiar with them.
• Serum sample. Proteins in the serum of the blood are examined to look for features of liver disease (a test known as the ‘liver function test’ (LFT)). In addition to the LFT there is a wide range of other less commonly performed serum tests that are used to help diagnose particular digestive diseases.
Examination of the faeces may reveal the presence of blood. Culture of the stool is performed to look for infectious agents. The presence of undigested nutrients in the stool, such as fat or sugars, may indicate a problem with digestion known as ‘malabsorption’.
The most efficient way of visualising the oesophagus, stomach and duodenum is by means of a slim, flexible tubular telescope called an ‘endoscope’. In upper gastrointestinal endoscopy a heavily sedated patient is encouraged to swallow the tube. As the fibre-optic end of the endoscope descends, images of the lining of the upper digestive tract can be seen by the examining doctor on a screen. This procedure does not use X-rays.
X-ray studies are also used in the investigation of digestive disease. Because the soft tissue of the bowel is not clearly exposed by X-ray imaging, a liquid mixture containing the salt barium sulphate is used to provide ‘contrast’ between the hollow space within the bowel and the surrounding soft tissues (as the metal of barium shows up as white on X-ray images). In the procedure called ‘barium swallow’, the patient is requested to drink a liquid containing barium sulphate. As the liquid lines the upper digestive tract, X-ray images will reveal its outline and show up areas of muscle spasm, ulcers or tumours. Figure 3.1b-II is a clear barium-swallow X-ray image, in this case depicting an oesophageal cancer. The white area shows the internal shape of the oesophagus, and the arrows indicate the narrowing caused by the tumour.
The endoscope can also be used to pass a fine tube into the pancreatic duct and bile ducts. Barium can be injected up this tube so that X-ray images can show up the delicate internal structure of the pancreas and the gallbladder. This procedure is known as ‘endoscopic retrograde cholangiopancreatography’ (ERCP). Figure 3.1b-III illustrates an image taken during an ERCP. The thick white curved tube in this image is the end of the endoscope.
To investigate the lower digestive tract, a form of endoscope called a ‘colonoscope’ is directed into the anal canal and thence into the rectum and colon. To investigate the lower part of the colon (sigmoid colon) and the rectum, a rigid telescope-like instrument called a ‘sigmoidoscope’ may be used. This is slightly less invasive than a colonoscopy.
A ‘barium enema’ is a procedure for visualising the lining of the bowel. In this investigation, barium sulphate liquid is injected into the colon via the anus. This is an uncomfortable procedure and may require a night in hospital to ‘prepare the bowel’ by means of the administration of a strong laxative.
Ultrasound scans, magnetic resonance imaging (MRI) scans and computerised tomography (CT) scans are all useful investigations for diseases of the digestive tract, as these all can reveal the presence of soft-tissue masses, fluid and stones. Ultrasound is the least invasive of these investigations. MRI exposes the patient to strong magnetic and
Self-test 3.1b The investigation of the gastrointestinal system
radiofrequency fields, but these currently are considered to carry little risk. However, an abdominal CT scan exposes the patient to over 50 times the amount of radiation associated with a chest X-ray, and so its use has to be guided on the basis that the potential benefits of the information outweigh the risks of carcinogenesis (cancer formation) from radiation exposure.
‘Biopsy’ literally means ‘examination of a living sample’. In the investigation of digestive disorders, a sample of the bowel lining can be obtained by use of the endoscope tube. Delicate instruments at the end of the tube can be manipulated to pinch off a section of tissue, which can then be removed and examined under a microscope.
This chapter looks at the most important diseases of the upper digestive tract. The diseases described are those that are important either because they are common, or because they have serious consequences for the patient. These diseases are:
To help with progression along this journey of diseases of each part of the gastrointestinal tract, it may be useful to be reminded first of the physiological descriptions to be found in the relevant sections in Chapter 3.1a (see Q3.1c-1).
Mouth ulcers are usually isolated, small, painful, whitened breaches in the mouth lining, and often have no obvious precipitating cause. They tend to be more common in some people than others. In most cases the sufferer is otherwise well, although occasionally a vitamin deficiency might be found. False or sharp teeth occasionally trigger ulcers. A small mouth ulcer will usually heal within 1–3 days.
A crop of very painful, reddened ulcers points to a viral infection. The very first encounter with the cold sore virus (herpes simplex) can cause such a condition. This is most common in young children, and can be so severe that hospital admission is necessary to give fluids by intravenous drip, as severe inflammation of the mouth prevents eating and drinking for a few days.
Less commonly, mouth ulcers may be part of a more severe digestive condition. The lower gastrointestinal diseases of Crohn’s disease, ulcerative colitis and coeliac disease are all associated with mouth ulceration. These conditions are studied in more detail in Chapter 3.1e.
The nature of infection by the herpes simplex virus (HSV) in the causation of cold sores was introduced in Chapter 2.4a by means of an example of the pathology of infectious diseases. First infection by HSV may cause a crop of painful ulcers. After this the virus remains ‘latent’ in a nerve cell, and characteristic symptoms of the painful crusting and weeping cold sore may reoccur at times of stress or exposure to extreme sunlight.
The same virus may also cause genital herpes infections. Oral sex can lead to the transmission of an oral cold sore to the genitalia of the sexual partner (for more detail on genital herpes; see Chapter 5.2d).
Patients may be recommended by their doctor or pharmacist to apply a drug called aciclovir in the form of a cream to the affected area at the onset of a cold sore. Aciclovir prevents the replication of viruses and does seem to shorten or abort the attack, although it does not prevent future recurrences.
Information Box 3.1c-II Cold sores: comments from a Chinese medicine perspective
In Chinese medicine, cold sores are often described as an invasion of Wind Heat and Damp. Aciclovir, which reduces the inflammation and crusting, is therefore Cooling in its action. As Aciclovir does not treat any underlying Heat and Damp that may have been expressed in the form of the cold sore, it may be described as energetically suppressive in its action.
Overgrowth of the Candida yeast (thrush) is described in Chapter 2.4b. Overgrowth of the Candida yeast in the mouth (oral thrush) appears as white patches on the tongue and lining of the mouth, and may or may not be painful. Thrush of the oesophagus can result in difficult and painful swallowing. Thrush of the oesophagus is a common first feature of the progression of human immunodeficiency virus (HIV) infection to acquired immune deficiency syndrome (AIDS).
Except in the case of newborns, in whom oral thrush is common, thrush in the mouth should be taken as an indication of possible depletion of the immune system. It is seen most commonly in frail elderly people, diabetics and people with HIV infection.
Information Box 3.1c-III Oral thrush: comments from a Chinese medicine perspective
In Chinese medicine, thrush would be viewed as a sign of internal Damp or Damp Heat. Nystatin and amphotericin appear to clear Damp. Their action is likely to be suppressive in nature, dealing as they do with only the outward expression of a deeper internal weakness.
Leukoplakia is important as it may reflect early cancerous changes. It is more common in smokers and those who drink alcohol in excess. A patient with leukoplakia should be referred, because an early diagnosis of cancer of the tongue can be life-saving.
‘Gingivitis’ literally means ‘inflammation of the gums’, and is the common condition that causes bleeding of the gums on brushing. Gingivitis is due to inflammation of the gum in contact with plaque on the teeth, and is encouraged by poor dental hygiene. Gingivitis results from an overgrowth of ‘healthy bacteria’, which normally reside harmlessly in the mouth. It is very common in pregnancy and is more common in smokers.
Occasionally, gingivitis will flare up to lead to an acutely painful ulcerated gum and tenderness in the associated teeth. This is a common reason for an ‘urgent’ visit to the dentist. The treatment of this condition which may or may not involve a collection of pus around the roots of tooth (dental abscess) is a 3–7 day course of an antibiotic (usually metronidazole or amoxicilin) and antiseptic mouthwashes. This acute form of severe gingivitis is much more common in smokers. The conventional explanation for the vulnerability to gum disease in smokers is that the toxins in smoke impair the natural immune barriers to infection in the mouth (the mucosa and saliva).
There are three pairs of major salivary glands (see Chapter 3.1a). The parotid glands hug the posterior border of the masseter muscle on the face, the submandibular glands sit under the inferior edge of the lower jaw bone (mandible), and the sublingual glands sit under the tongue. All are exocrine glands and release their secretions via ducts into the mouth cavity.
These glands can become inflamed as a result of infection. The mumps virus is probably the most common cause of this in otherwise healthy people, although this disease is much less common since the introduction of widespread vaccination of children. The ‘hamster face’ seen in children with mumps is due to inflammation and swelling of the parotid glands.
Bacteria can cause infections in the salivary glands, although this should be seen, like thrush, as a sign of immune depletion. Underlying causes can include dehydration, alcoholism and diabetes. The symptoms include tender painful swollen salivary glands and fever. Eating, which causes salivation, is painful. The cause is an overgrowth of ‘healthy’ oral bacteria.
Information Box 3.1c-VI Salivary gland infection: comments from a Chinese medicine perspective
A ‘stone’ (calculus) forms in a salivary gland when secretions dry up and accumulate to form a hard solid mass. If a stone develops it may periodically block the flow of saliva, most commonly in a single parotid gland. The patient might complain of pain and swelling of the cheek after eating, and may be more at risk of bacterial infections of the salivary glands.
Tumours of the salivary glands are not uncommon. They are most frequently found in the parotid glands of elderly people. They usually give rise to a firm, irregular painless swelling of one cheek. 85% of these tumours are benign and can be removed by surgery. Features that suggest a malignant tumour include rapid irregular growth, lymph-node involvement and weakness of the muscles of the mouth on the same side as the swelling (a result of cancerous infiltration of the facial nerve) (see Q3.1c-2).
Some patients with diseases of the mouth will benefit from referral to a conventional doctor for assessment and/or treatment. Red flags are those symptoms and signs that indicate that referral is to be considered. The red flags of the diseases of the mouth are described in Table 3.1c-I. This table forms part of the summary on red flags given in Appendix III, which also gives advice on the degree of urgency of referral for each of the red flag conditions listed.
|4.1||Persistent oral thrush (candidiasis) (appearing as a thick, white coating on the tongue or palate)||Although common in the newborn, oral thrush in children and adults is not a normal finding, and merits referral to exclude an underlying cause. Common causes include corticosteroid use (including asthma inhalers), diabetes, immunodeficiency (including HIV/AIDS) and cancer. Dentures in elderly people can also predispose to oral thrush|
|4.2||Persistent painless white plaque (leukoplakia) (appearing as a coat that appears to sit on the surface of the sides of the tongue)|
Leukoplakia is a precancerous change that signifies an increased risk of mouth cancer. It is more common in smokers and in those with a high alcohol intake. A particular form of leukoplakia is also associated with HIV/AIDS
|4.3||Painless enlargement of a salivary gland over weeks to months||This needs referral to exclude salivary gland cancer, which is most common in people over the age of 60 years|
|4.4||Painful or painless enlargement of a salivary gland immediately after eating||This suggests a salivary gland stone or obstruction from dried secretions. Early treatment is to maximise hydration by encouraging drinking, and to encourage salivation (e.g. with lemon juice). If the problem is persistent, referral is recommended, as surgical removal of the stone may be necessary|
|4.5||Tender or inflamed gums or salivary glands which do not respond within days to your treatment||May be accompanied by fever or malaise. These symptoms suggest dental abscess or infection of a salivary gland, and if they persist indicate a need for referral for antibiotic treatment to prevent inflammatory damage to the dental roots or salivary glands|
|4.6||Ulceration of the mouth if persistent (for more than one week) or if preventing proper hydration|
The most common cause of painful ulceration of the mouth is herpes simplex. This can be so severe as to inhibit drinking in a child. If this is the case, the child may need to be hospitalised for rehydration
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