Unit 8: Physiology of Human Body Systems.
Explore the physiology of the digestive system and the use of the corrective treatments for dietary related diseases.
Nutrition and health.
The function of the digestive system is to break down the foods you eat, release their nutrients, and absorb those nutrients into the body. Although the small intestine is the workhorse of the system, where the majority of digestion occurs, and where most of the released nutrients are absorbed into the blood or lymph, each of the digestive system organs makes a vital contribution to this process.
The digestive system consists of:
The mouth is the beginning of the digestive tract; and, in fact, digestion starts here when taking the first bite of food. Chewing breaks the food into pieces that are more easily digested, while saliva mixes with food to begin the process of breaking it down into a form your body can absorb and use.
The cheeks, tongue, and palate frame the mouth, which is also called the oral cavity (or buccal cavity).
At the entrance to the mouth are the lips, or labia which regulate what comes in and goes out of the mouth. The cheeks make up the oral cavity’s sidewalls while their outer covering is skin, their inner covering is mucous membrane, which is made up of non-keratinized, stratified squamous epithelium.
The pharynx, or throat, is a funnel-shaped tube connected to the posterior end of the mouth. The pharynx is responsible for the passing of masses of chewed food from the mouth to the oesophagus. The pharynx also plays an important role in the respiratory system, as air from the nasal cavity passes through the pharynx on its way to the larynx and eventually the lungs. Because the pharynx serves two different functions, it contains a flap of tissue known as the epiglottis that acts as a switch to route food to the oesophagus and air to the larynx.
The oesophagus is a muscular tube connecting the pharynx to the stomach that is part of the upper gastrointestinal tract. It has a wall made of four layers: mucous membrane to secrete mucus enabling smooth passage of food; submucosa holding mucous membrane in place; a relatively thick layer of muscle consisting of circular and longitudinal smooth muscle fibres, and outer protective covering.
Additionally, it also carries swallowed masses of chewed food along its length. At the inferior end of the oesophagus is a muscular ring called the lower oesophageal sphincter or cardiac sphincter.
The function of this sphincter is to close of the end of the oesophagus and trap food in the stomach by peristalsis.
The stomach is a muscular sac that is located on the left side of the abdominal cavity, just below the diaphragm. In an average person, the stomach is about the size of their two fists placed next to each other. Wall consists of a thick layer of muscle, consisting of longitudinal, circular and oblique smooth muscle fibres, lined with epithelial cells.
This major organ acts as a storage tank for food so that the body has time to digest large meals properly. The stomach also contains hydrochloric acid and digestive enzymes that continue the digestion of food that began in the mouth.
The small intestine
The small intestine is a long, thin tube about 1 inch in diameter and about 10 feet long that is part of the lower gastrointestinal tract. It is located just inferior to the stomach and takes up most of the space in the abdominal cavity. The entire small intestine is coiled like a hose and the inside surface is full of many ridges and folds. These folds are used to maximise the digestion of food and absorption of nutrients. By the time food leaves the small intestine, around 90% of all nutrients have been extracted from the food that entered it.
This thin tube is made up of three parts, in order from the stomach:
1. The duodenum is about 25 cm long, has a diameter 2.5 cm and is fixed to the dorsal abdominal wall, it consist of layers of smooth muscle cells lined with epithelium. Receives pancreatic juice with hydrolytic enzymes; receives bile from liver. These secretions enter the duodenum at the sphincter of Oddi.
2. The jejunum is about 2.5 m long, has a diameter 3.8 cm, and extends from duodenum to ileum.
3. The ileum is about 3.6 m long. Walls are thinner than those of jejunum and highly folded; epithelium contains vili (finger-like projections). These features greatly increase the surface area for absorption of the products of digestion. Jejunum and ileum are supported on a membrane called mesentery.
The pancreas is a large gland located just inferior and posterior to the stomach. It is about 6 inches long and shaped like short, lumpy snake with its “head” connected to the duodenum and its “tail” pointing to the left wall of the abdominal cavity. The pancreas secretes digestive enzymes into the small intestine to complete the chemical digestion of foods. Clusters of acini (secretory cells), surrounds ducts. Inside the acinar cells are large amounts of rough endoplasmic reticulum and vesicles containing the newly made enzyme molecules. Epithelial cells lining the pancreatic ducts secrete hydrogencarbonate ions that make pancreatic juice alkaline (pH 8).
The pancreas also has an endocrine function as beta cells in the islets secrete hormone insulin, in response to increased blood glucose level. Insulin helps liver, muscle and many other cells take up more glucose, thereby lowering blood glucose level. In liver and muscle cells, the glucose is converted to glycogen. Glucagon is secreted by alpha cells in the islets, in response to lowered blood glucose level; it causes stored glycogen in the liver to be broken down to glucose and released into the blood.
Thin-walled green muscular sac, about 10 cm long, settled into a depression (fossa) on the ventral surface of the liver. It stores bile made in the liver and releases the bile, via bile duct, into the duodenum at the sphincter of Oddi, when food enters the duodenum from the stomach.
Large gland in the abdomen, ventral to (in front of) the stomach; consists of hexagonal-shaped lobules, inside which are hepatocytes. Oxygenated blood enters the liver from the hepatic artery and nutrient-rich blood from the ileum enters the liver via the hepatic portal vein. Deoxygenated blood leaves the liver in the hepatic vein. Hepatocytes make bile that enters caniliculi and passes to the gall bladder. It is stored in the gall bladder until needed.
-salt that emulsify fats to increase their surface area for digestion;
-hydrogencarbonate ions to neutralise acidic chyme;
-products (bilirubin and biliverdin) of broken-down red blood cells and cholesterol.
It also does not contain enzymes; however the liver stores glycogen, helps regulate blood glucose level, makes plasma proteins, stores fat-soluble vitamins (for example, vitamin A) and metabolises (chemically alters by reactions inside cells) alcohol, drugs and other toxins.
It breaks down excess amino acids to make urea for removal at the kidneys.
The large intestine
The large intestine is the final section of the gastrointestinal tract that performs the vital task of absorbing water and vitamins while converting digested food into excrements. Although shorter than the small intestine in length, the large intestine is considerably thicker in diameter as it is about 5 feet (1.5 m) in length and 2.5 inches (6-7 cm) in diameter in the living body, but becomes much large as the smooth muscle tissue of the intestinal wall relaxes.
It consists of five parts:
1. Caecum is the sac like first part of the large intestine, branching from it is the appendix that contains much lymphoid tissue and bacteria that may help recolonise the gut microbiata.
2. The colon consists of four parts: ascending colon, transverse colon, descending colon and sigmoid colon. Colon mucosa consists of columnar epithelial cells, no vili or folds and very few/no digestive enzymes-secreting cells’ many globet cells secrete mucus that protects wall from acids and gasses produced by bacteria that live there. The mucus lubricates passage of faeces to rectum and anal canal. The gut bacteria are essential for our wellbeing and make vitamins B and K as well as certain appetite-regulating hormones. Water is absorbed from undigested food in the colon. Faeces containing undigested fibre, bacteria, gut epithelial cells and excretory products such as bilirubin and biliverdin, pass into the rectum. More water is then absorbed and the faeces pass into the anal canal to be expelled.
The anus is the opening where the gastrointestinal tract ends and exits the body. The anus starts at the bottom of the rectum, the last portion of the colon (large intestine) and the anorectal line separates the anus from the rectum.
Stretching of the rectum wall initiates the defaecation reflex. As faeces are forced into anal canal, impulses reach the brain and we can make voluntary decisions as to whether or not to open the external anal sphincter.
The digestive system is responsible for taking whole foods and turning them into energy and nutrients to allow the body to function, grow, and repair itself. The five primary processes of the digestive system include:
The first function of the digestive system is ingestion, or the intake of food. The mouth is responsible for this function, as it is the opening through which all food enters the body. The mouth and stomach are also responsible for the storage of food as it is waiting to be digested. This storage capacity allows the body to eat only a few times each day and to ingest more food than it can process at one time.
Secretion is also necessary as in the course of a day, the digestive system secretes a large amount of fluids. These fluids include saliva, mucus, hydrochloric acid, enzymes, and bile. Saliva moistens dry food and contains salivary amylase, a digestive enzyme that begins the digestion of carbohydrates. Mucus serves as a protective barrier and lubricant inside of the gastrointestinal tract. Hydrochloric acid helps to digest food chemically and protects the body by killing bacteria present in our food. Enzymes are like tiny biochemical machines that deconstruct large macromolecules like proteins, carbohydrates, and lipids into their smaller components. Finally, bile is used to emulsify large masses of lipids into tiny globules for easy digestion.
Mechanical digestion is the breakdown of large pieces of food into smaller pieces. This mode of digestion begins with the chewing of food by the teeth and is continued through the muscular mixing of food by the stomach and intestines. Bile produced by the liver is also used to mechanically break fats into smaller globules. While food is being mechanically digested it is also being chemically digested as larger and more complex molecules are being broken down into smaller molecules that are easier to absorb.
Chemical digestion begins in the mouth with salivary amylase in saliva splitting complex carbohydrates into simple carbohydrates. The enzymes and acid in the stomach continue chemical digestion, but the bulk of chemical digestion takes place in the small intestine due to the action of the pancreas. The pancreas secretes the pancreatic juice, which is capable of digesting lipids, carbohydrates, proteins and nucleic acids. By the time food has left the duodenum, it has been reduced to its chemical building blocks which consist of fatty acids, amino acids, mono-saccharides, and nucleotides.
Once food has been reduced to its building blocks, it is ready for the body to absorb.
The last fundamental process is absorption which begins in the stomach with simple molecules being absorbed directly into the bloodstream. Most absorption takes place in the walls of the small intestine, which are densely folded to maximise the surface area in contact with digested food. Small blood and lymphatic vessels in the intestinal wall pick up the molecules and carry them to the rest of the body. The large intestine is also involved in the absorption of water and vitamins B and K before excrements leave the body.
Enzymes, as we know, are protein molecules referred as biological catalyst; if enzymes are not present in the digestion of the food, the energy of the chain reaction will encounter different challenges for instance acidity, diarrhoea, bloating and burning sensation and it helps strengthens the immune system of the body.
Linked to the functions of the digestive system, digestive enzymes play a really important role on the chemical digestion of food, absorption and the delivery of various nutrients throughout our body.
The main enzyme-producing structures of the human digestive system are the salivary glands, stomach, pancreas, liver and small intestine.
Where it is made Where it works Enzyme Substrate Products pH
Salivary glands Mouth Amylase Starch Maltose 7
Stomach cells Stomach Protease Protein Amino acids 2-3
Liver Small intestine Bile salts Fat Fat droplets 8-9
Pancreas Small intestine Amylase
Glycerol & fatty acids 8-9
Small intestine Small intestine Maltose
Amino acids 8-9
Test for nutrients
1) Starch (iodine test), to approximately 2 cm3 of test solution, added two drops of iodine. A blue-black colour indicated the presence of starch as starch-polyiodide complex is formed. Starch is only slightly soluble in water, but the test works well in a suspension or as a solid.
2) Reducing sugars (Benedict’s test). All monosaccharides and most disaccharides (except sucrose) will reduce copper (II) sulphate, producing a precipitate of copper (I) oxide on heating, so they are called reducing sugars. Benedict’s reagent is an aqueous solution of copper (II) sulphate, sodium carbonate and sodium citrate. To approximately 2 cm3 of test of solution, added an equal quantity of Benedict’s reagent. Shacked, and heated for a few minutes at 95°C in a water bath. A precipitate indicates reducing sugars; the colour and density of the precipitate gives an indication of the amount of reducing sugar present, so this test is semi-quantitative. The original pale blue colour mans no-reducing sugar, a green precipitate means relatively little sugar; a brown or red precipitate means progressively more sugar is present.
3) Non-reducing sugars (Benedict’s test). Sucrose is a non-reducing sugar because it does not reduce copper sulphate, so there is no direct test for sucrose. However, if it was first hydrolysed (broken down) to its constituent monosaccharides (glucose and fructose), and gave then give a positive Benedict’s test. Therefore, sucrose was the only sugar that gave a negative Benedict’s test before hydrolysis and positive test afterwards. First, tested a sample for reducing sugars, to see if there were any present before hydrolysis. Then using a separate sample, boiled the test solution with diluted hydrochloric acid for a few minutes to hydrolyse the glycosidic bond. Neutralised the solution by gently adding small amounts of solid sodium hydrogen carbonate until t stopped fizzing, and then tested as before for reducing sugars.
4) Lipids (emulsion test). Lipids don’t dissolve in water, but do dissolve in ethanol. This is characteristics is used in the emulsion test; didn’t start by dissolving the sample in water but instead shake dome of the test sample with about 4 cm3 of ethanol. Decanted the liquid into a test tube of water, left any undissolved substance behind. The presence of lipids dissolved in ethanol, precipitated in the eater forming a cloudy white emulsion.
5) Protein (biuret test). To about 2 cm3 of test solution, added an equal volume of biuret solution, down the side of the test tube. A blue ring formed at the surface of the solution, which disappeared on shaking, and the solution turned lilac-purple, indicating protein. The colour was due to a complex between nitrogen atoms in the peptide chain and Cu+2 ions, so this was really a test for peptide bonds.
Humans need to eat a balanced diet to maintain health. Macronutrients include carbohydrates, fats and proteins. These may all provide energy