Digestion

• Digestive system

  The digestive system is made up of the gastrointestinal tract (GI) (tube from mouth to anus) and accessory organs that help the body break down and absorb food.
• Mouth and Salivary Glands

  Food is taken into the mouth (ingested).

  Mastication (chewing) allows the teeth to mechanically breakdown the food into smaller portions, increasing its surface area.

  Salivary Glands secrete saliva into the mouth, softening food, forming a bolus (food + saliva) and moistening the mouth for swallowing.

  Saliva contains the enzyme, salivary amylase, which begins to break long chains of starch (polysaccharides) into maltose (disaccharide).

  Saliva also contains lingual lipase, which initiates chemical digestion of triglycerides into shorter chain fatty acids
• Oesophagus

  • When you swallow, the bolus moves into the oesophagus and is pushed down due to muscle contractions called peristalsis.
  • It takes 5-10 seconds for the bolus to reach the stomach
  • Sphincter muscles are found at each end of the oesophagus (upper and lower oesophageal sphincters)
• Stomach

  • The stomach wall is thick and muscular. The muscles mechanically mix and churn the bolus and break it down into smaller pieces.
  • Gastric glands release gastric juices(hydrochloric acid and enzymes), via small openings of the wall and mix with the bolus to form a semiliquid called chyme.
• Duodenum

  • The duodenum links the stomach to the small intestine and receives pancreatic juices
  • Pancreatic amylase turns starch into maltose
  • Trypsin turns polypeptide chains into small peptide chains (a few AA)
  • Lipase turns lipids into a monoglyceride (fatty acid and glycerol) and two fatty acids
• Small intestine

  • Despite its name, the small intestine it is quite a long section of the digestive tract, (6m in length).
  • Small intestine secretes intestinal enzymes, which are responsible for breaking down all carbohydrates into their smallest units (monosaccharides – glucose, fructose & galactose), and polypeptides into amino acids.
  • Maltase breaks maltose into two glucose molecules
  • Sucrase breaks sucrose into one glucose and one fructose molecule
  • Lactose breaks lactose into one glucose and one galactose molecule
• Digestion in the large intestine

  • Some water and any material which is not absorbed (waste) travels through to the large intestine (colon). As material passes along the colon, water and salts are absorbed. The solid mass (faeces) is then stored in the rectum.
  • The faeces are excreted (egested) through the anus by contractions of the rectum wall, and the opening of the anal sphincter. This is called defecation
• Appendix

  • Connected to the large intestine the appendix has no function in humans.
  • In grass-eating mammals such as rabbits their appendix contains large numbers of bacteria, which can digest cellulose and break it down into glucose.
• Microbiome


  Living ecosystem of 10-100 trillion symbiotic bacteria in/on the body, more than human cells
• Microbiome changes quickly in first 1-2 years of life then stabilises
• Microbiome changes


  Affected by diet, antibiotics, etc. Can influence health conditions like asthma, diabetes, depression, obesity
• Microbiome functions

  Digest fibre, affect immune system, produce vitamins and short chain fatty acids
• Prebiotics


  • Soluble fibre, resistant starch, oligosaccharides in foods like apples, berries, garlic, legumes, etc.
• Probiotics


  • Live bacteria in foods like yoghurt, kefir, kombucha, kimchi, sauerkraut, miso
• Coeliac disease


  Genetic intolerance to gluten, damages villi and reduces nutrient absorption
• Coeliac disease can lead to nutrient deficiencies like anaemia and osteoporosis
• Hydrochloric Acid

  • helps to kill any germs
  • begins to denature proteins (uncoiling)
  • lowers the pH, creating the optimum conditions for the enzymes to work - causes pepsin to become active.
• Pepsin in the Stomach

  • The enzyme found in the stomach is called pepsin, which helps to break down large proteins, by breaking the peptide bonds, forming smaller polypeptide chains.
  • The semi-digested food (chyme) moves to the duodenum
• Liver - Accessory Organs

  • Glucose, which is absorbed by the small intestine passes into the liver, with any excess being converted and stored as glycogen (polysaccharide).
  • The liver also produces bile which helps to emulsify fats (breakdown fats into smaller droplets, increasing surface area).
  • Emulsification is a mechanical process and aids the enzyme lipase.
• Gall Bladder

  • stores and  releases bile into the duodenum
• Pancreas

  • Secretes insulin (hormone) when blood sugar is too high, causing glucose to be stored as glycogen in the liver (or glucagon if blood glucose levels are too low, breaking down glycogen down into glucose).
  • Produces pancreatic juice containing enzymes which are secreted into the duodenum to digest protein, starch and fat. The pancreatic secretions also contain sodium bicarbonate, an alkaline (base), which neutralises the acidic chyme.
• Chemical Digestion of Carbohydrates

  • Pancreatic amylase enters the small intestine via the pancreatic duct
  • This enzyme continues to break down polysaccharides into disaccharides which are eventually broken down to maltose
  • The final steps are completed by specific enzymes secreted by the intestinal cells, which break down disaccharides into monosaccharides and are then ready to be absorbed
  • Maltose breaks maltose into two glucose molecules
  • Sucrase breaks sucrose into one glucose and on fructose molecule
  • Lactase breaks lactose into one glucose and on galactose molecule
• Chemical digestion of proteins

  • The shorter polypeptide chains (produced from the chemical digestion in the stomach by pepsin) enter the small intestine where their digestion is completed
  • Pancreatic and intestinal protease enzymes, including trypsin and chymotrypsin, break them down into singular amino acids or shorter peptide chains consisting of two (dipeptides) or three amino acids
  • Peptidase, completes the digestion of these shorter peptides into singular amino acids
• Chemical digestion of lipids

  • Most of lipid digestion takes place in the small intestine
  • Once the small intestine detects the gallbladder releases its stores of bile via the common bile duct
  • Bile emulsifies larger fat molecules and converts them into small fat droplets, increasing the surface area
  • Pancreatic lipase either breaks down the triglyceride into a monoglyceride or all three fatty acids (leaving a free molecule of glycerol). 
  • These are then packaged as a lipoprotein, which are absorbed and transported through the lymph system before entering the bloodstream near the heart.
• Villi

  Finger-like projections in the wall of the small intestine that create greater surface area for the absorption of nutrients and water
• Structure of the wall of the small intestine


  • Very muscular
  • Contains thousands of villi
• Microvilli


  Structures that cover each villi, increasing the surface area even further
• Absorption of nutrients and water in the small intestine

  1. Monosaccharides and amino acids taken up by blood capillaries

  2. Lipids taken up by lacteal

  3. Contents of lacteal drain back into blood close to the heart
• Within the villi there are numerous blood capillaries and a lacteal for taking up the absorbed food
• Microbiome

  • It is believed that there are 10-100 trillion symbiotic bacteria in or on our body.
  • There is slightly more microbes than our actual cells (1.3:1),with an estimated 500-1000 different species. 
  • The first microbes are thought to colonise during the birthing process and lactation. Our gut microbiome (total of all microbe genes) changes quickly over the first year or two of life and then stabilises.
• Microbiome

  • The microbiome is a living ecosystem, with changes to their environment (diet, antibiotics, etc) affecting the number and type of microbes. 
  • Studies are suggesting that the type of microbes can influence an individual’s health, including the likelihood of developing; asthma, type 1 diabetes, depression, obesity.
  • The microbes in our large intestine are also responsible for digesting some of our fibre, affect our immune system and produce vitamins (B, folate, B12, H, K) and short chain fatty acids (SCFA) which can be used as an energy source for epithelial cells in the colon.
• Malabsorption


  relates to disorders which affect the absorption of nutrients during digestion.
• Coeliac Disease

  • Coeliac disease has a genetic link and is therefore found in families. Those with a specific gene are more susceptible to having the disease. 
  • Individuals with coeliac disease are intolerant to gluten, a protein found in wheat, rye, barley and oats.
• Coeliac Disease

  • If they consume gluten, their bodies immune system overreacts, causing their villi to be damaged.
  • Damaged villi, reduces the surface area available for the absorption of nutrients.  
  • Coeliac’s may be nutrient deficient in a variety of nutrients due to poor absorption. This can result in diet related disorders including anaemia (iron and/or B12)  & osteoporosis.
• Symptoms of Coeliac Disease

  • Bloating,
  • Flatulence
  • Diarrhoea
  • Nausea
  • Fatigue
  • Stomach cramping
• Lactose Intolerance

  • Individuals who are lactose intolerant do not make the lactase enzyme required to break down lactose (disaccharide) into glucose and galactose (monosaccharides).
  • Symptoms include gut pain, gas, bloating and diarrhoea.
• Lactose Intolerance

  • Individuals with lactose intolerance must remove all dairy foods as they contain lactose.  
  • Dairy foods are very high in calcium, and phosphorous.  Alternative food sources must be included in the diet.