Digestive System

• Ingestion is the intake of food into the body.
• Digestion is the breakdown of food into soluble form that can be absorbed by cells.
• Absorption is the diffusion and uptake of soluble substances.
• Assimilation is the incorporation of nutrients into the body cells.
• Egestion is the removal of feces from the body.
• The Digestive System, also known as the Alimentary Canal, is a long tube that runs from the mouth to the anus.
• Digestion begins in the mouth, also known as the buccal cavity, and ends in the small intestine.
• There are 2 forms of digestion:

  • Mechanical/Physical Digestion
  • Chemical Digestion
• Mechanical/Physical Digestion, also called mastication, involves chewing in the mouth, churning of the food in the stomach, and back and forth movement in the stomach walls. It breaks down the pieces of food that are large into small pieces.
• Chemical Digestion occurs when enzymes break down complex molecules into simpler ones to easily digest them.
• Digestive System

  A) mouth

  B) salivary glands

  C) esophagus

  D) liver

  E) stomach

  F) gallbladder

  G) pancreas

  H) large intestine

  I) small intestine

  J) appendix

  K) rectum

  L) anus

  
• The mouth is where ingestion takes place where the teeth break down the food into smaller pieces by grinding, biting or slicing. The tongue helps to form the bolus and the saliva softens, moistens and lubricates the food.

  
• The mouth has both physical and chemical digestion.
• Salivary amylase is the enzyme found in the mouth to break down starch into maltose.
• The esophagus is a tube from the mouth to the stomach where the bolus goes to the stomach by peristalsis. Peristalsis is the rhythmic movements caused by the contraction and relaxation of the muscles in the walls.

  
• The stomach is an organ that churns food and secretes gastric juices from the gastric glands. The gastric juices contain mucus, hydrochloric acid and pepsin. This forms chyme which is the partly digested food mixed with stomach juices.

  
• Mucus in the stomach protects the lining of the wall from being damaged by strong acids and enzymes.
• Hydrochloric acid kills bacteria and also creates an environment for the enzymes to work in.
• Pepsin is an enzyme that breaks proteins into polypeptides.
• Rennin is an enzyme in the stomach that clots the protein in milk.
• The stomach has both physical and chemical digestion.
• Pancreatic juice contains enzymes, such as lipase, trypsin and pancreatic amylase, that break down the remaining partially digested food into soluble products. Sodium hydrogen carbonate neutralizes the acid from the stomach.

  
• The liver destroys old blood cells and poisons. It also produces and secretes bile which is stored in the gallbladder. The bile salts emulsify (break down) fats into small globules. The liver is also where food changes into energy.

  
• After being in the stomach, food enters the duodenum (the first part of the small intestine), where it is mixed with juices from the pancreas and liver.

  
• The walls of the small intestine secrete intestinal juices that contain enzymes such as maltase, sucrase, lactase and peptidase, that break down the remaining partially digested food into soluble products.
• Absorption mostly occurs by the villi of the small intestine where the soluble products are absorbed through the lining of the ileum (the last part of the small intestine).
• Adaptations of the small intestine

  • 7 meters long giving the food enough time to be absorbed
  • Contains millions of villi increasing the surface area, hence increasing the rate of absorption
  • The epithelium lining or cell membrane of each villus contains microvilli which increases the surface area of the cells
  • The epithelium lining is thin allowing quick absorption
  • Each villus has a dense network of blood and lacteal capillaries absorbing and transporting sugar, amino acids, vitamins and fats.
• Villus

  A) epithelium

  B) Lacteal

  C) Blood

  
• Nutrients are absorbed into the bloodstream by the villi due to diffusion along the concentration gradient. However, nutrients can be taken up by active transport which is why mitochondria are present in the epithelial cells.
• Materials that pass into the large intestine include undigested materials (cellulose and roughage), mucus and water. When water and electrolytes are reabsorbed leaving a semi-solid mass called faeces which passes into the rectum.

  
• Assimilation is the process of making use of the digested food in the body. Food is needed for growth repair, energy and development and excess is then stored in the body.
• Glucose is used for respiration and excess is converted to glycogen and stored in the liver, muscles and in adipose tissue as fat.
• Amino acid is used for growth and repair and to also make enzymes and hormones. They are deaminated in the liver where the toxic portion is converted to urea then transported to the kidney and excreted as urine. The useful portion is then converted to glucose and glycogen in the liver.
• Constipation occurs when a diet lacks fiber and can result in a blockage of the alimentary canal. Hemorrhoid is the swelling of the anal veins which results from force pushing.