Digestive System

• The digestive system processes food for energy, breaking it down into tiny pieces so it can be absorbed into body cells.
• Humans need to eat loads of food, mostly for energy, to move about and keep their metabolism running.
• The three main groups of nutrients humans need are carbohydrates like starch and glucose, proteins, and fats.
• Unfortunately, these nutrients are locked up inside large molecules which themselves are inside big pieces of food.
• The role of digestion is to break all this food down into tiny pieces so it can be absorbed into body cells.
• Digestion relies on both physical breakdown and chemical breakdown by enzymes.
• Once food is placed in the mouth, it is broken down physically by chewing and saliva is released from the salivary glands which helps make the mixture more liquid and contains salivary amylase which helps break down carbohydrates.
• The food is swallowed and passes down the gullet, also known as the esophagus, and almost straight away it passes through to the stomach.
• Bile is not made in the gallbladder, it's made in the liver and stored in the gallbladder until it's released into the small intestine.
• Once the food has been digested into lots of tiny pieces, it can be absorbed across the lining of the intestine straight into the bloodstream.
• The lining of the small intestine is an example of an exchange surface, and it's adapted to its function by having villi which increase the surface area of the small intestine, allowing digested food to be absorbed into the bloodstream more quickly.
• The villi only have a single layer of surface cells, which means that the nutrients only have to diffuse a very short distance.
• The villi have a very good blood supply, which maintains the concentration gradient.
• Once all the useful nutrients have been absorbed into the bloodstream, there's still a lot of material left in the small intestine because lots of the food that we eat isn't actually digestible and because of all the secretions from the pancreas, stomach, and gallbladder, the leftover material is going to be really watery.
• The leftover material from the small intestine passes into the large intestine, which absorbs all of this excess water, leaving behind feces that are stored in the rectum until they're removed.
• The stomach is a muscular sac that contracts its muscular walls to push the food around and mix it, produces pepsin which is a protease enzyme that breaks down proteins, and produces hydrochloric acid which kills bacteria and provides the right environment for the pepsin enzyme to work.
• After an hour or so in the stomach, the food is squeezed out into the small intestine where it will be absorbed into the bloodstream and most of the digestion will take place.
• The pancreas, an organ that secretes things into the small intestine, makes most of the digestive enzymes and pushes them into the small intestine in the form of pancreatic juices.
• The gallbladder, another organ that secretes things into the small intestine, releases bile rather than digestive enzymes but its role is to neutralize the acid from the stomach making the pH more ideal for the digestive enzymes to function and it can do this because the bile itself is alkaline.
• The gallbladder's other role is to emulsify fats, which means it helps break big blobs of fat into tiny droplets, and this helps because it gives the fat a much bigger surface area for the digestive enzymes to work on.