Digestive System Part 2

Two Types of Digestion Digestion is the process of breaking down nutrients so they can be absorbed – Undigested nutrients cannot be absorbed and pass through the small intestine to the large intestine – For more information, ask someone who is lactose-intolerant about drinking milk Mechanical (physical) digestion is the chewing and churning of food Chemical digestion is the breaking of chemical bonds in food

Mechanical Digestion The mouth (chewing) and stomach (churning) perform mechanical digestion of food The mouth accomplishes this with hard calcium structures called teeth and heavy muscles attached to the mandible chewing The stomach has layers of smooth muscles that churn the food before it moves to the small intestine

Chemical Digestion The mouth and stomach also initiate the process of chemical digestion – The mouth has salivary glands that produce saliva, which begins chemical digestion – The stomach has pepsin, an enzyme that denatures and breaks down protein Note chemical digestion of fats does not begin until the small intestine

Small Intestine Activity When food enters the stomach it still consists of starches, proteins and whole fats – These cannot be absorbed since they are too large! The small intestine can only absorb the small molecules that make these up (simple sugars, amino acids, and fat molecules) – Large molecules that are not digested cannot be absorbed! – Pancreas to the rescue!

Pancreatic Digestion The pancreas secretes a huge battery of chemicals to complete the breakdown of nutrients for absorption – Pancreatic lipase: for lipids – Pancreatic amylase: for starches – Trypsin (and others): for proteins – Pancreatic nucleases: for DNA and RNA digestion – Bicarbonate (a base that cancels out the acid of the stomach) The small intestine itself also expresses many enzymes that break two-part molecules apart

Small Intestine Absorption Since nutrients can only be absorbed through cell membrane, the small intestine epithelial tissue has adaptations for increasing surface area – The surface has many circular folds – The tissue is arranged into fingerlike villi (singular villus) which create more surface area – The individual cells have a brush border, or microvilli (singular microvillus) These cells expend ATP to actively transport nutrients into the blood

Small Intestine Microanatomy

Digestion Review Starches broken down to sugars by salivary amylase and pancreatic amylase Proteins broken down to amino acids by pepsin and trypsin Fat drops split apart with bile and broken down with pancreatic lipase DNA and RNA are broken apart and absorbed by pancreatic nucleases

Liver Function Blood flows directly from the small intestine to the liver – This is called hepatic portal circulation – Protects the rest of the body from whatever you just ate The liver has metabolic enzymes to process many nutrients after they are absorbed – Production of fat and glycogen – Storage of vitamins – Breakdown of toxins such as alcohol

Large Intestine The large intestine is filled with colonies of bacteria that feast on our leftovers (often producing gas as a waste product) The large intestine is capable of absorbing water and vitamins, including those produced by bacteria – The large intestine is not actually necessary for life and can be removed if needed

Normal Flora Redux Remember the concept of normal flora from immunology? – If you wipe out your normal flora in your large intestine you are at risk for infection by a much nastier type of bacterium! – Probiotics are foods that restock your large intestine with healthy bacteria This is still a poorly understood field of science

Lactose Intolerance Lactose Intolerance is one of many digestive disorders caused by a failure to absorb nutrients – Lactose (milk sugar) is made of a molecule of glucose and galactose stuck together (too large to absorb) – Most people express a small intestine enzyme called lactase that breaks them apart Failure to digest lactose (or other nutrients) results in the nutrients ending up in the large intestine – Bacteria metabolize it and release gas and waste acids

Gastrointestinal Distress Inflammation of the intestines can also cause failure to absorb nutrients – These nutrients get to the large intestine and bacteria call dibs at that point – This results in diarrhea Celiac disease is when inflammation results from gluten (a wheat protein) exposure to the small intestine – If you do not have celiac disease, gluten is NOT bad for you