The Digestive System and the Role of Enzymes in the Breakdown and Release of Nutrients.

The Digestive System is the name given to the sequential process that our bodies perform when ingesting food, and the breaking down of macromolecules into micromolecules so that the body can absorb it’s nutrients into the bloodstream and it’s cellular system to obtain energy for cellular respiration, and the excretion all indigestible waste products. This happens within the digestive tract, which starts at the Mouth or Oral cavity and ends at the Anus. (Reference Appended image 1,’ The Human digestive system and associated organs). The molecules of food that we eat are generally Polymers, i.e., large, complex Molecules that are composed of long chains of Monomers. Polymers are insoluble and therefore cannot be absorbed into our bloodstream and need to be assimilated into different absorptive products. Polymers have to be hydrolysed into smaller, soluble molecules. This process happens during the process of digestion. Food is broken down by mechanical or chemical means (Hydrolysis) and this process is aided by Enzymes. Enzymes are biological, process catalysing Proteins which massively speed up the breaking down of compound molecules into micromolecules to allow nutritional absorption. All digestive Enzymes are Hydrolytic, i.e., a water molecule is added to allow compound molecular breakdown and separation. All Enzymes have a unique shape to their ‘active site’ allowing only the target substrate to bond for biological processing. Enzymes have optimum operating requirements and can denature if the temperature becomes too warm or the environment too acidic or alkaline. Digestive Enzyme secretion is regulated by both the nervous and the hormonal systems. Hormones are a chemical substance signalling system that communicates from one set of cells to another set, the target cells, which will then trigger enzyme secretion.
Upon ingesting food into the mouth, salivary glands secrete saliva which contains the digestive enzyme ‘Salivary Amylase’. This has a neutral pH and starts the breakdown of starch into Maltose. The mechanical actions of the teeth and the tongue break the food down into a swallowable Bolus. When ready, the bolus is swallowed through the Pharynx and travels down the Oesophagus by Peristalsis - the rhythmic, mechanical processing action achieved by the longitudinal and circular muscles of the Oesophagus lining. The bolus travels through the Gastroesophageal sphincter into the Stomach where digestion begins. The stomach is acidic, pH2, so as to kill off any food borne bacteria travelling with the warm, moist bolus. The stomach recognises the presence of food and releases gastric juices from the gastric glands within the stomach walls. These contain the enzyme Pepsin (for the breakdown of proteins and creation of Peptides), Hydrochloric acid and Mucus. Mucus protects the stomach wall from it’s acidic contents and is reproduced by Goblet cells within the stomach lining. The food is churned by mechanical processing by the contractions of the muscular, stomach wall. The processed food becomes a liquid called Chyme and is released through the Pyloric Sphincter into the first part of the Small Intestine – the Duodenum.
Duodenum intestinal wall lining is made up of thousands of Villi, which protrude like columns increasing the Duodenum’s surface area(Ref Appendix image 2. “Villus – a detailed view”). Each Villus contains blood capillaries, a thin lymphatic capillary, and has an outer wall of thousands of epithelial cells able to absorb nutritional, micro molecules. Most enzyme digestion occurs in the Duodenum. As Chyme travels through the Lumen of the small intestine, Bile, (produced by the Liver but stored in the Gall Bladder) is secreted via the Bile Duct and Pancreatic juices from the Pancreas are added. Bile, made from Bile salts and mineral salts, emulsifies the fats into fat droplets making it water soluble, whilst the pancreatic juices contain Sodium Bicarbonate which neutralises the acidic content of the Chyme from the stomach and the enzymes Pancreatic Amylase (for Starch digestion and reduction into Maltose), Trypsinogen-which is converted into Trypsin in the Duodenum (for Protein digestion and reduction to peptides) and Lipase (for Fat droplet digestion and reduction to monoglycerrides and fatty acids). The Duodenum secretes the enzymes Maltase and Peptidases into the Lumen. Peptidases complete the reduction of Protein into Amino acids whilst Maltase completes the reduction of maltose into Glucose which is then suitable for absorption through the Villi into the bloodstream. The enzymes produced from the pancreas and the Intestinal wall of the Duodenum therefore complete the body’s molecular digestion of protein and starch. The end product of Lipase digestion is Monoglycerride, which is absorbed through the Villi and then repackaged within the epithelial cells of the Villi into Lipoprotein droplets, called Chylomicrons. These then enter the Lacteal which is a small lymphatic capillary within the Villus, carrying sugars and amino acids in and around each Villus. The Lymph liquid then travels through the Lymphatic system directly to the cardiovascular system.
Control of digestive enzyme secretions is achieved via Hormones and the Hormonal System. When food is received into the stomach, the stomach produces the hormone Gastrin into the bloodstream, triggering the upper stomach to release more gastric juice, thus aiding the creation of Chyme. When the Chyme then enters the Duodenum, the hormones Secretin and Cholecystokinin (CCK) are released into the bloodstream to trigger the Pancreas to release its pancreatic juices and for the Gall bladder to release the Bile down the bile duct. (Ref Appendix image 3. ‘Hormonal control of digestive gland secretions.’). The Duodenum also secretes the hormone ‘gastric inhibitory peptide’ which will travel through the bloodstream to the stomach to start to inhibit Gastric gland secretion. Chyme will travel through the Jejunum and the Ileum by Peristalsis and continue into the Large Intestine.
The Large Intestine, which includes the Cecum, ascending, transverse and descending Colon, the Rectum and the Anal Canal, is wider in dimension but shorter than the Short Intestine. It’s functions include the absorption of water, salts, some vitamins, and the storage and eventual excretion of indigestible waste products at the Anus. The Cecum is found at the near end of the Large Intestine, just below the junction between both the Small and the Large Intestine and incorporates a small projection at it’s end, called the Vermiform Appendix. In the Human anatomy, this may also play a part in the fighting off of infections but it is also prone to inflammation. This inflammation is called Appendicitis and if it occurs the organ should be removed before subsequent bursting, which could lead to Peritonitis. Peritonitis is a general infection of the abdominal cavity which can, in some cases, become fatal.
The Colon has five sections. These are (sequentially) the Cecum, a small pouch which connects the Ileum to the Ascending Colon. Access into the Colon is controlled by the Ileocecal valve, or Bauhin’s valve; the Ascending Colon, which travels vertically up the right side of the body up to the level of the Liver; the Transverse Colon, which crosses the abdominal cavity just underneath the level of the Liver and the Stomach; the Descending Colon, which travels vertically down the left side of the body and the Sigmoid Colon, which enters into the Rectum. To defecate the indigestible waste, the Faeces are mechanically transported by Peristalsis from the descending Colon, into the Sigmoid Colon and then onwards into the Rectum. Faeces entering the Rectum and stretching the Rectum walls, initiates nerve impulses to the Spinal Cord, triggering a contraction of the rectal muscles and a relaxation of the Anal Sphincters. This combination of events results in the expulsion, or defecation of Faeces through the Anus. This expulsion of indigestible waste is a form of Homeostasis maintained by the digestive system.
Faeces are 75% water and 25% solids. The solid property of Faeces is due to the indigestible dietary fibre, bacteria and other indigestible products. The particular smell of faeces is due to the bacteria reacting with the indigestible material within the neutral pH environment, moisture and warmth of the human body at 37° Celsius. ‘Gas’ is also due to this bacterial reaction. The brown colour of faeces is due to the presence of oxidised iron and a breakdown product of Bilirubin. New culture methods have shown that over 99% of the bacteria present in the Colon are Obligate Anaerobes, i.e., bacteria that dies in the presence of oxygen. Bacteria therefore breaks down our indigestible material, but also provides us a service by providing B service vitamins and the majority of the vitamin K needed by our bodies.
After having been digested, the nutritious molecules of the digested products from the small intestine are transported via the Hepatic portal vein to the Liver, where it is detoxified and all poisonous substances are removed. The Liver also removes and stores Iron, the vitamins A,D,E,K and B₁₂, synthesises plasma proteins and regulates the amount of cholesterol in the blood.
A common medical issue concerning the digestive system is Gastroenteritis. Gastroenteritis can be the cause of inflammation and irritation of the stomach and intestines (the Gastrointestinal Tract), diarrhoea, abdominal pain, vomiting and nausea. This is most commonly caused by a viral infection but can also be caused by food borne bacteria or parasites. Gastroenteritis is particularly dangerous for infants and the elderly as it primarily causes dehydration through excessive vomiting. Viral cases can typically last for 1-2 days but bacterial cases may be longer. 50-70% of viral cases of Gastroenteritis are caused by Norovirus, which is highly contagious and can be transmitted by poor hand washing practises after using the toilet, or putting a finger to the mouth after touching an infected area. Other viruses causing gastroenteritis are Adenovirus, Parvovirus and Astrovirus. Bacterial infection causing gastroenteritis will infect and inflame both the stomach and the intestine, hence the name. A common bacterial strain that causes these symptoms by producing toxins within the body is Staphylococcus Aureus. Other common bacterial strains may be Salmonella, Campylobactor and Shigella.
Chlostridium Difficile can cause bacterial growth on the large intestine and is particularly susceptible after the body has endured a course of antibiotics for a period of time. Typical symptoms would include a low fever of 37.7° Celsius, nausea with or without vomiting, diarrhoea and cycling abdominal pain that can reduce immediately post bowel evacuation. More serious cases may include blood evident in vomit or stools, vomiting for more than 48 hours, a fever higher than 40°Celsius and considerable dehydration causing weakness, light headiness and reduced urination. Treatment is centralised around rehydration which should be dispensed in small, regular quantities. This may be difficult especially in infants as they may not want to drink when vomiting, however rehydration is key to recovery. Failure to rehydrate by drinking will result in hospital admission with intravenous fluid administration. Gastroenteritis can be avoided by general, regular washing of hands, maintaining of clinically clean food preparation areas and ensuring all foods are cooked thoroughly to a sufficient temperature to kill all internal food borne bacteria.
In summary, the Human Digestive System is a hugely complex and efficient process in which Enzymes play an integral role. The body is able to break down foodstuffs that enter via the Oral cavity into smaller molecular structures that can be absorbed into the bloodstream from within the small intestine. The blood is then filtered by the Liver before travelling through the Vena Inferior to the heart and being reoxygenated by the lungs before circulation around the rest of the body to all organs and tissues. This would not be possible without Enzymes.