BIOLOGY F R O M M O U T H T O R E C T U M

FROM MOUTH TO RECTUM BIOLOGY The organs in the digestive system are presented in the order food passes through them. In the mindmaps I have chosen to...
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FROM MOUTH TO RECTUM

BIOLOGY The organs in the digestive system are presented in the order food passes through them. In the mindmaps I have chosen to concentrate my description of the different organs to include their different parts, sections and occasionally cells, motility processes, the exocrine and in some cases endocrine secretions, elements that trigger secretions and the visual characteristics and microscopic structures of the lumen. But first, an explanation of some expressions used in this part of my research. The digestive system refers to the gastrointestinal tract/digestive tract and the accessory digestive organs.1 The gastrointestinal tract consists of the following organs as shown in the illustration on the next page; the oral cavity (1), pharynx (2), esophagus (3), stomach(12), small intestine with its three parts; duodenum (5), jejunum (13) and ileum (9) and the large intestine with its four colons (7,8,14,15), cecum (10,11), rectum (16) and anus. The accessory digestive organs refers to the teeth, tongue, salivary glands, liver (4), gallbladder and pancreas (6). The lumen refers to the area inside the gastrointestinal tract. Epithelium refers to a type of animal tissue, 26

generally described as the outermost layer of cells that line the walls of cavities and surfaces in our body (e.g. the inner walls of the digestive tract).2 Mucosa and sub-mucosa refers to the different layers of epithelial cells that the inner walls of the digestive tract consists of.3 Digestion refers to the breakdown of large food molecules into their smallest subunits (monomers). Absorption refers to the passage of monomers through the walls of the lumen of the small and large intestine. Motility refers to the movement of food through the digestive tract which is driven by a number of different processes including ingestion (putting food into our mouth) mastication (chewing), deglutition (swallowing) and peristalsis (a type of muscle contractions).4 Secretions is the general expression for all substances that are secreted into the lumen of the gastrointestinal tract (exocrine secretions) and all the substances that are secreted from the gastrointestinal tract (endocrine secretions).5

MIRJAM MITTERNACHT

SEROSA SUBMUCOSA

LUMEN

MUCOSA EPITHELIUM

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FROM MOUTH TO RECTUM

ORAL CAVITY AND PHARYNX The oral cavity and its accessory digestive organs; teeth, tongue and salivary glands is the first organ in the digestive system. The oral cavity is covered by a mucous membrane. An adult human has 32 permanent teeth (including the wisdom teeth) which breaks down food into smaller parts by mastication.6 Our tongue is our primary taste organ and is a mucus covered muscular organ. In certain glands on the tongue a fat digesting enzyme (lingual lipase) is produced, an enzyme especially important for small children with a not yet fully functioning pancreas. We have 6 major salivary glands in our mouth, three on each side. The secretion of saliva into the oral cavity is a direct reaction to mastication but also a central mechanism triggered by the thought of something tasty.7 Saliva consists of 98% water and 2% mixed substances, among them the salivary enzyme amylase that breaks down starch (carbohydrate) in our food into sugars.8 Deglutition is a voluntary activity in which the larynx is raised so that the epiglottis covers the entrance to the respiratory tract and prevents food from entering. 25 muscles in the pharynx are involved in the act of swallowing.9 The soft pallet in the “roof” of the oral cavity closes the nasal passages while we swallow. From the oral cavity and pharynx, food, now called bolus, continues its journey into the esophagus (gullet).

ESOPHAGUS The esophagus is a 25 cm long muscular tube through which food is transported by peristalsis, a type of muscular movement in which the esophagus’s smooth muscles contracts behind 28

the bolus while the muscles in front relaxes. This type of muscular contraction is also found in other parts of the digestive tract. The esophagus ends in the gastroesophageal sphincter through which food enters the stomach. The gastroesophageal sphincter is closed until food is pushed through by peristalsis.10

STOMACH The stomach is the most extendible organ of the digestive tract. It has two main parts, the cardiac region (consisting of the fundus and the lower body) and the pyloric region (consisting of the antrum and the pyloric sphincter). The main functions of our stomach are to store food (now called chyme), break it down by mixing it with gastric juice through muscle contractions, kill ingested bacteria and start digesting proteins with the help from the enzyme pepsin. The inner structure of the stomach is characterized by long folds called rugae. The gastric mucosa is however also folded and forms gastric pits. In the bottom of these pits you find the gastric glands.11 In the gastric glands groups of secreting cells are positioned. These cells secrete mucus, hydrochloric acid, intrinsic factor and pepsinogen into the lumen of the stomach. Through a reaction between pepsinogen and hydrochloric acid, the active enzyme pepsin is produced. Together with water these secretions form the acidic solution: gastric juice. Cells in the stomach also produce a small amount of the fat digesting enzyme gastric lipase.12 Since our body is built up by different proteins, protein-digesting enzymes that are secreted into the digestive tract form a significant risk to our tissues. Our body has two main defenses against this. First of all, these enzymes are secreted in an inactive form (as pro-enzymes) and react with other substances in the lumen to become active

MIRJAM MITTERNACHT enzymes. Secondly, epithelial cells along the digestive tract produces a thick layer of mucus that prevents the activated enzymes from injuring the inner walls. In the stomach, as well as in the duodenum, the mucus produced by the epithelial cells also contains bicarbonate that neutralizes the hydrochloric acid in the gastric juice.13

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THE SMALL INTESTINE Duodenum The duodenum stretches from the pyloric sphincter, which connects it to the stomach, into the jejunum which is the next part of the small intestine. Through the pyloric sphincter food is portioned out into the duodenum by the muscle contractions in the lower part of the stomach. This, so that sufficient amounts of chyme can be mixed with the secretions in duodenum. The mucosa in duodenum, as well as in the rest of the small intestine is folded into fingerlike forms called villi. Each villus is covered by a brush border called microvilli that consists of folded epithelial cell membranes. Stuck on the microvilli you find brush border enzymes and through the villi nutrients are absorbed into the capillary network (blood) and lymph system (via the lacteal). Thanks to the folds the area of absorption in the small intestine increases considerably.14 Even though you find villi and microvilli in the duodenum, very little absorption takes place here. Instead the duodenum together with the accessory digestive organs; the liver, gallbladder and pancreas is responsible for most of the enzyme driven digestion in the gastrointestinal tract.15 The chyme that reaches duodenum is strongly acidic and needs to be neutralized to protect the mucosa. The duodenum has, apart from its bicarbonate and mucus producing epithelial cells also a type of exocrine glands (Brunner’s glands) that produce bicarbonate to add protection against the acidic chyme. In the pancreatic juice (explained further down) you also find bicarbonate that adds to the neutralization of chyme.16 When chyme enters the duodenum a chain of reactions starts. Cells in the epithelium detect 30

the chyme and thereby free the hormone cholecystokinin (among others). This hormone gives signals to the pancreas and liver (gallbladder) to secrete bile and pancreatic juice into the duodenum through the common bile duct which has its opening on the right middle part of the duodenum.17 Bile, a yellowish/green secretion concentrated in the gallbladder and originating from the liver has many components, e.g. bile salt that works as a emulsifier and splits fat molecules in the chyme into smaller fat drops so that enzymes later can break them down. Bile also contains the pigment bilirubin that is responsible for the colour of our feces. The gallbladder is a saclike organ attached to the liver. When the liver produces bile it is stored and concentrated in the gallbladder and later ejected through the cystic duct and common bile duct into the duodenum.18 Pancreatic juice, produced in the pancreas, contains a mix of bicarbonate, enzymes and proenzymes that can break down carbohydrates, fats and proteins into smaller parts. One of the protein-digesting pro-enzymes in pancreatic juice (trypsinogen) reacts with the brush border enzyme enteropeptidase, and becomes the active enzyme trypsin. This enzyme then activates other proenzymes in the pancreatic juice so that proteins can be digested.19 The pancreas is a soft glandular organ with both endocrine and exocrine functions. While its endocrine secretions; the hormones insulin and glucagon; are produced by the pancreatic islets (the islets of Langerhans), its exocrine secretions; the components in pancreatic juice; are produced by cells in the exocrine secretory units, the acini.20

MIRJAM MITTERNACHT

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Jejunum and ileum Jejunum and Ileum stretch from the duodenum to the ileocecal valve and is the longest part of the digestive tracts, approximately 3-5 meters.21 In the jejunum and ileum most nutrients are absorbed. Proteins as well as carbohydrates do however need to go through one last step of digestion before they can be absorbed into the capillary network in the villi. The brush border enzymes, stuck on the inner walls of the small intestine are responsible for breaking apart the final bounds. Fat has been broken down by bile salts and pancreatic lipase and it’s now absorbed into the lymphatic system through the lacteal.22 All along the jejunum and ileum nutrients are absorbed through the villi. The activity however decreases in ileum. When chyme enters the large intestine through the ileocecal valve only a few components are left.

THE LARGE INTESTINE The large intestine is the last part of the digestive tract, ending in the anus. The mucosa differs from the one in the small intestine. The epithelial cells still produce mucus but instead of fingerlike folds resulting in villi, you find folds that form crypts (the crypts of Lieberkühn).23 The large intestine has little or no digestive function but through the epithelial cells’ membranes, water is absorbed from the food (now called chyme or feces) as a result of the osmoregulation in our body. This absorption is closely linked to the absorption of salts (electrolytes) that also occurs in the large intestine. It should be mentioned that water and salt also are absorbed in ileum, yet the large intestine gets rid of the last water leaving less than approx 200 ml to be excreted in the feces.24

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The large intestine is also home to a large amount of bacteria, called the intestinal microbiota. Over 400 different species populate our intestines and many of them are found in the large intestine. The relationship between us and our intestinal microbiota can be described as either mutual; we both benefit, and/or as commensal; bacteria benefit and we are neither harmed or benefit. The bacteria in our intestines are among other things responsible for the production of vitamin K and B vitamins and for the production of short chain fatty acids (which give energy to the epithelial cells) and ferment indigestible molecules in our food, i.e. different types of fibres (complex carbohydrates). While fibres are fermented the gas methane is also produced.25 It can also be noted that nearly a third of the dry weight of our feces consists of bacteria.26 The cecum and the appendix are two parts of the large intestine that you find in the beginning of the ascending colon below its connection to ileum. Humans do not necessarily need the appendix but this small part of the large intestine has functions in the immune system.27 The chyme/feces travels slowly through the large i ntestine’s four colons driven forward by peristalsis and periodic mass movements and in the end, what’s left of the food we once ate, exits through our anus.

TIME FRAME Foods journey through our body takes about 3648 hours. The stomach portions out food into the Duodenum continuously during 3-4 hours after a meal and this food will reach the ileocecal valve after approximately 8 hours. The time frame for the journey through the large intestine varies depending on the human but should normally take between 24-48 hours.28

MIRJAM MITTERNACHT

AFTER ABSORPTION After being absorbed through the walls of the small and large intestine, monomers from carbohydrates and proteins go from the capillary networks into the portal vein and continues into the liver. The liver is a complex organ responsible for many regulating processes29 and can e.g. remove the carbohydrate monomer glucose from the blood in case of a surplus and store it as glycogen. Later when needed, the liver can release it as glucose or triglycerides. Many of the processes in the liver are driven by hormones.30

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NUTRIENTS All nutrients we eat serve different purposes in our body and are all needed. Here follows a short description of the different nutrients.

Proteins The smallest building unit (monomer) in protein molecules is the amino acid. There are 20 different amino acids and their characteristics and the order in which they are placed forms the functions and characteristics of a protein. In the human body you find about 100 000 different proteins. Eleven types of amino acids can be built by our body but the remaining nine are essential and need to be a part of our diet. Especially babies need many of these essential amino acids since they are important for the growth of tissue.31 Proteins have many essential functions in our body and are used: - For transportation of nutrients (oxygen, lipids, iron etc) and digesting products in our blood and tissues - For storage and release of oxygen in our muscle cells - As receptors on cell membranes to, e.g. give cells signals from hormones - As structure builders e.g. as collagen (unit in bone tissue and connective tissue) and/or keratin (unit in hair, nails and skin) - For contractions in muscles - As enzymes - As defenders in our immune system - As regulators of activities in our cells.32 Proteins are found in most food types except in pure carbohydrate or fat products. Proteins and amino acids are not stored in our body, an excess of proteins will be used as energy or turned into fat, why we must have proteins in our diet.33 34

Carbohydrates Carbohydrates is one of our body’s most important sources of energy and the most common organic compounds on earth.34 The smallest building units in carbohydrates are monosaccharides. Glucose is the most common monosaccharide and the monomer in starch and maltose. Glucose is transported by our blood to all cells in our body and the hormone insulin is responsible for the consumption level of glucose in our muscle- and fat- cells. Glucose is first of all an energy source but can also be transformed into glycogen or triglyceride. Glycogen can be stored in our body (e.g. in our liver, as earlier mentioned). Our brain needs 100-150 g of glucose each day to function and has no own storage. Thereby it is crucial that the liver can free glucose from its glycogen storage between meals to provide energy to our brain.35

Health The amount of glucose in our blood is regulated by three hormones, insulin - that lowers the levels, and glucagon and adrenaline that both rase the level. A disturbance in the hormone levels can result in hyperglycemia (high blood sugar) or hypoglycemia (low blood sugar) depending on which hormone that is disturbed. The disease diabetes is the result of low insulin levels and causes a state of hyperglycemia with its following effects.36 There is a huge difference in the speed, in which different carbohydrate products are digested and absorbed after a meal. To use the glycemic index (GI) is a way to measure how much the blood glucose level raises and thereby measure how fast the carbohydrates are absorbed. Low GI indicates a slow absorption resulting in a low blood glucose level while high GI indicates fast absorption and a high blood glucose level. The type of product as well as how it is prepared has an effect on the glycemic index.37

MIRJAM MITTERNACHT

Fibres and dietary fibres Fibres are complex carbohydrates from plants, which can’t be digested by the enzymes in the digestive tract. Instead some of them (soluble) are fermented by the microbiota in our large intestine while others (insoluble) travels through our digestive system unaffected. The most important types are cellulose, hemicellulose, pectin and lignin.38 Soluble and insoluble fibres have different effects on the large intestine. Insoluble fibres can absorb water and thereby ease the feces’ passage through the large intestine (defecation). Because the feces then travels faster, constipation is prevented. Soluble fibres also ease the defecation because the microbiotic mass increases.39 A diet rich of carbohydrates, especially dietary fibres automatically lowers the amount of fat absorbed. This because, the fermenting microbiota in our intestines will be more active and thereby produce more acids which will change the pH-level in the large intestine. This, in the next step, lowers the levels of bile salts secreted from the liver into the duodenum and the amount of fat that can be absorbed (there are, in other words, hormones secreted by the cells in the large intestine that affect the functions in the liver).40

Fats (lipids) Lipids are important for our body for a number of reasons and they come in different forms; Triglycerides are rich energy providers for our cells and can also be stored in our fat cells (lipocytes). Triglycerides consist of the alcohol glycerol and three fatty acids. Fatty acids Fatty acids have a number of different characteristics; they can be saturated, polyunsaturated or just unsaturated. Some of them are also essential and must be a part of our diet. For humans linoleic acid and alpha-linoleic acid are essential.

Fatty acids are structural parts of our cell membranes and are important in the production of arachidonic acid and different eicosanoids. Eicosanoids are hormone like substances that play an important role in different regulating processes in our body e.g. blood pressure and the secretion of gastric juice. Phospholipids, another type of fat is a building unit in our cell membranes.41 Cholesterol is not only a part of the food we eat but also produced by our liver by digestive products such as glucose, other lipids and amino acids. The livers production is regulated by the levels of cholesterol in the food we eat. High value equals lower production and vice versa. Cholesterol is also an important component in some hormones, in our cell membranes, in lipoproteins and helps in the production of vitamin D and bile salts.42 The digestion, absorption and transport of lipids in our body is a complex matter. The main digestion takes place in duodenum where bile salts and different pancreatic lipases split lipids into smaller parts. However lingual lipase and gastric lipase; active enzymes in earlier parts of the digestive tract, helps in this process in an perhaps unusual way. To understand how lipids are digested it is important to first of all understand that lipids’ chemical structure makes them insoluble in water. Since they latter must travel through blood and lymph, both water based fluids, their chemical structure must be changed so that this transport is possible. This is done through emulsion and the production of micelles; a type of molecules with a hydrophobic (water rejecting) center and a hydrophilic (water friendly) surface. With the help from different emulsifiers e.g. bile salts and free fatty acids, the digested lipids are rearranged into micelles. Free fatty acids that are involved in this process are digested products broken down by the lingual and gastric lipase enzymes. To create fatty acids that are part of this process is the main purpose of lingual and gastric lipase.43 35

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Micelles are molecules that can be absorbed into the lymphatic system through the cell membranes in the villi and glands because of their water friendly surface. They travel through the lymphatic system and are later transferred to the blood where they continue their journey as lipoproteins (a molecule consisting of lipids and proteins).44

Sodium chloride (NaCl) or salt is an ionic compound. The sodium ion is essential and has several important functions in our body. Sodium ions does e.g. play an important role in the osmoregulation. To eat too much salt can raise the blood pressure and thereby our heartbeat.48

Permeability describes the rate in which molecules and ions can pass through the membranes of the epithelial cells. A number of factors effect permeability and an increased permeablilty can lead to an increased risk for different diseases.

Our body consists of 50 - 75% water and most of it is found in our cells. Water is essential for all metabolic processes in our body and helps maintaining the physiological balance, transporting nutrients to our cells, transporting waste, producing digestive fluids, by being the medium in which biochemical reactions can happen as well as act as a reactive substance in them, prevent over-heating and contribute to the form and structure of an organism. 49

It has been shown that eating fat increases the permeability of LPS (lipo-polysaccharides found in the cell membranes of gram-negative cells). LPS can cause inflammations that are behind several metabolic diseases and the insulin-resistance syndrome.45

Vitamins Vitamins are organic substances important for our body that must be added to our diet since we can’t synthesize them ourselves (apart from at certain conditions). They are a heterogenic group divided into two main subgroups; water soluble vitamins (B-vitamins and vitamin C) and fat soluble vitamins (A, D, E and K).46

Minerals Minerals have three main functions in our body. First of all they have structural functions, secondly, regulating functions and thirdly, they are involved in the transmission of signals between cells and tissues. The range in which they are needed in our body divides minerals into different groups. While calcium, magnesium, potassium, sodium, sulphur, phosphate and chlorine occurs frequently and are highly needed; iron, zinc, copper, cobalt, manganese, iodine, selenium and molybdenum are essential but only needed in utterly small concentrations.47 36

Water

DIGESTION & ABSORPTION Nutrients and water are absorbed in the small and large intestine once they have reached their absorbable form. In the map on the right page, the digestion and absorption of proteins, carbohydrates, lipids, salt and fermentable fibers can be seen. Water is a component in most nutrients, separated from them as they split and is absorbed in the small and large intestine.

MIRJAM MITTERNACHT

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SUMMARY Nutrients are digested and mechanically broken down in a sequence of processes during their journey through our body. Carbohydrates are digested in three steps, first by salivary amylase in the mouth, secondly by pancreatic enzymes in duodenum and thirdly by brush border enzymes in jejunum and ileum. When they have become monosaccharides they can be absorbed into the blood through the walls of jejunum and ileum. Proteins are digested in three steps as well, first by pepsin in our stomach, secondly by four different pancreatic enzymes in duodenum and thirdly by brush border enzymes in jejunum and ileum. They can be absorbed as free amino acids, dipeptides or as tripeptides into the blood through the walls of jejunum and ileum. Fats are digested in four steps (the first two are of less relevance). First by lingual lipase in the oral cavity, secondly by gastric lipase in our stomach, thirdly by bile salts in duodenum and finally by pancreatic enzymes in duodenum. They are rearranged into micelles and absorbed into the lymphatic system through the walls of jejunum and ileum. Water and salt (NaCl) is primarily absorbed into the blood through the epithellial cell menbranes in the walls of the large intestine. Food is mechanically broken down in two main areas of the digestive system. In the oral cavity through chewing and in the stomach through muscle contractions. There are, in other words, two types of processes in which food is broken down into smaller parts by our body; mechanical processes and biochemical processes. 38

TEXTURES, STRUCTURES AND COLOURS To investigate the microscopic structures of the walls and lumen in the digestive tract I used the Science Photo Library Database.50 To investigate the colours and structures I used an endoscopic journey through the digestive system made by Bengt Jeppsson. To get an idea of the visual textures of the different organs, I primarily used images from Henry Gray’s Anatomy of the human body.51