Human Digestive System: Alimentary Canal

Human Digestive System

• Digestion is important because it breaks down food into small molecules that can be absorbed into the bloodstream.
• There are physical and chemical digestion processes.

Alimentary Canal

• The alimentary canal is where food is broken down into very small molecules and absorbed.
• Digestion is breaking food down into smaller molecules that the body can absorb.
• Even after chewing, food particles are still too large to be absorbed.
• Digestion involves chemicals called enzymes.
• Enzymes in digestive juices mix with food.
• Once food is broken down, blood transports the products of digestion to other parts of the body, and undigested products are egested/defecated.
• The human digestive system consists of the alimentary canal, a long tube from the mouth to the anus, and other organs.

Physical Digestion

• Physical digestion includes the use of teeth and the tongue.
• Food enters the mouth and is cut by incisors.
• Premolar and molar teeth grind food during chewing.
• Saliva in the mouth helps with chewing.
• Saliva acts as a lubricant, making it easier to swallow food.
• Saliva includes salivary amylase, which digests starch into maltose.
• The tongue rolls chewed food into a bolus to be swallowed into the oesophagus (gullet).
• Amylase begins the digestion of starch for the first time in the mouth

Chemical Digestion

• Chemical digestion uses the alimentary canal and includes the relaxation of muscles to create wave like contractions.
• The walls of the alimentary canal have muscles that squeeze and relax which creates a squeezing action called peristalsis to push food.
• The oesophagus is a short tube connecting the mouth to the stomach.
• Balls of food are pushed into the stomach by peristalsis.
• No digestion happens in the oesophagus.
• The stomach is a muscular sac that controls the flow of food in and out.
• In the stomach, food is churned up by peristalsis and mixed with gastric juice.
• Gastric juice contains protease enzymes, which start the digestion of proteins.
• Hydrochloric acid in gastric juice kills unwanted bacteria.
• A creamy liquid is created by continuous mixing that passes into the small intestine at regular intervals for one to six hours.

Small Intestine

• The first part of the small intestine, is called the duodenum.
• Bile is added to the food here via the bile duct and is produced in the liver and stored in the gallbladder.
• Bile neutralises the stomach acid and emulsifies fats (breaks them into tiny droplets).
• Emulsifying increases the surface area of fats for enzymes to work on.
• Pancreatic juice from the pancreas is deposited into the duodenum.
• The walls of the small intestine produce intestinal juice, which contains enzymes to digest different food materials.
• Enzymatic action helps the intestines digest food

Enzymes

• Enzymes include amylase, protease, lipase, and carbohydrase
• Amylase digests starch (not already digested in the mouth) to maltose.
• Protease digests protein into amino acids.
• Lipase digests fat into fatty acids and glycerol.
• Carbohydrase the enzymes remaining carbohydrates to glucose.
• After the enzymes assist in digestion. this brings that part of the digestive process to an end. The remainder of the process is absorption into the blood.
• The end products of digestion are glucose, amino acids, fatty acids, and glycerol.
• These molecules are absorbed into the bloodstream through the wall of the small intestine in the form of a solution in the blood plasma.
• The wall of the small intestine is folded and covered with tiny projections called villi.
• Villi have very thin walls to increase the surface area of the intestine for better absorption of food molecules into the bloodstream (glucose is a type of sugar).
• Food stays in the small intestine between 5-6 hours.
• Undigested material passes into the large intestine.

Appendix and Large Intestine

• The appendix is an organ where the small and large intestines join.
• In humans, the appendix has no function.
• In herbivores, the appendix is larger and helps digest cellulose in plant cell walls.
• Material passing into the large intestine mainly includes water, vegetable fibres, and cellulose, is called roughage.
• Water is absorbed into the blood through the walls of the large intestine, leaving behind semi-solid faeces.
• Faeces are stored in the rectum for between twelve and twenty-four hours until the muscles in the wall of the rectum control the release of faeces through the anus, a process known as egestion or defecation.

Enzymes As Catalysts

• Enzymes are biological catalysts which means they speed up chemical reactions.
• Enzymes speed up chemical reactions but are not broken down or changed by it.
• They lower the amount of every required for a chemical reaction to take place.
• Enzymes break down starch, protein, and fat molecules into smaller ones such as Glucose, amino acids, fatty acids and glycerol for absorption into the bloodstream.
• Enzymes have strange names but usually, an enzyme's name gives you a clue about what it does.
• For example, carbohydrases (e.g. amylase) break down starch into simple sugars like glucose.
• Proteases break down proteins into amino acid molecules
• Lipase, an enzyme that breaks down fats (lipids) into fatty acid and glycerol molecules.
• Throughout these breaking down processes, the enzymes are working hard to digest the food molecules.

Alimentary canal

• It’s about 10m long and food takes about 24 hours to pass from one end to the other.