Digestive System

Why is digestion important?

When you eat foods—such as bread, meat, and vegetables—they are not in a form that the body can use as nourishment. Food and drink must be changed into smaller molecules of nutrients before they can be absorbed into the blood and carried to cells throughout the body. Digestion is the process by which food and drink are broken down into their smallest parts so the body can use them to build and nourish cells and to provide energy.

Tests for Chemical Components of Food

CHEMICAL	TEST	POSITIVE RESULT
Protein	Add a few drops of copper sulphate solution to the food sample, then a few drops of sodium hydroxide solution and mix well.	Blue-purple
Starch	Add a few drops of iodine solution to the food sample.	Blue-black
Glucose	Add an equal volume of Benedict's solution to the food sample and warm in a very hot water bath for a few minutes.	Orange-red
Lipid (Fats and Oils)	Rub a food sample onto brown paper and allow it to dry.	Translucent spot

3 Processes in Digestion

1. Ingestion - the mechanical taking in of food, chewing and swallowing
2. Digestion - The breakdown of food into smaller particles by physical or chemical means
3. Absorption - the absorption of these smaller food particles from the digestive tract into the blood

2 Types of Food Breakdown

1. Physical or Mechanical Breakdown - e.g. chewing, mixing with bile
2. Chemical Breakdown - e.g. enzymes, hydrochloric acid

Parts and Functions of the Human Digestive System

Mouth

• Ingests food
• Teeth physically break down food by chewing
• Saliva lubricates food
• The enzyme, salivary amylase also called ptyalin, breaks down starch into simple sugar, glucose

The Tooth

A tooth is a hard structure, set in the upper or lower jaw, that is used for chewing food. Teeth also give shape to the face and aid in the process of speaking clearly. The enamel that covers the crown (the part above the gum) in each tooth can be broken down by acids produced by the mouth for digestive purposes. This process is called "decay". To prevent decay, good oral hygiene, consisting of daily brushing and flossing, is necessary. The hardest substance in the human body is one of the four kinds of tissue which make up the tooth. It is enamel and covers the crown (area above the gum line) of the tooth. A bony material called "cementum" covers the root, which fits into the jaw socket and is joined to it with membranes. "Dentin" is found under the enamel and the cementum, and this material forms the largest part of the tooth. At the heart of each tooth is living "pulp," which contains nerves, connective tissues, blood vessels and lymphatics. When a person gets a toothache, the pulp is what hurts.

Pharynx

• At the back of the mouth cavity
• Both food and air pass through here

Oesophagus

• Tube between mouth and stomach
• A flap called the epiglottis closes over the top of the windpipe or trachea when swallowing, so that food does not enter the respiratory tract
• The walls of the digestive tract from the oesophagus to the anus are muscular, and contract rhythmically to move food. The muscular contractions are called peristalsis.

Stomach

• 2 circular muscles called sphincters surround the entry and exit of the stomach to control the flow of food
• Food remains in the stomach for about 3 hours where it physically broken down by the churning muscular contractions of the stomach wall muscles
• Gastric juice contains hydrochloric acid and has a pH of 1 without food, and 3 with food.
• HCl helps to kill bacteria, and works in association with the enzyme, pepsin, to partially break down proteins.
• The stomach lining contains mucus to prevent damage from the acid
• Only alcohol and a few drugs can be absorbed through the stomach wall into the blood

Small Intestine

• Long tube that is about 7 metres long and 2.5 cm in diameter
• 3 parts of the small intestine are duodenum, jejunum and ileum
• Most of digestion occurs in the small intestine
• The enzymes of the small intestine do not function on the acidic stomach contents. First, an alkaline substance from the pancreas is secreted into the duodenum to neutralise the stomach acid.
• A green substance called bile that is made in the liver and stored in the gall bladder is released into the duodenum to break the large fat particles into smaller fat particles
• Enzymes from the pancreas also break down food chemically. Amylase breaks starch into simple sugars. Lipase breaks lipids into fatty acids and glycerol. Peptidases and Trypsin break down proteins into amino acids.
• After the food into broken into smaller particles, it is absorbed through finger-like projections called villion the walls of the small intestine into blood capillaries.

Large Intestine

• Thicker in diameter than the small intestine
• Water is absorbed from the remains of undigested food to make faeces
• Bacteria produce Vitamin B in the large intestine
• Mucus lubricates the faeces
• The appendix is attached to part of the large intestine. It may be part of the body's immune system.
• Diarrhoea is the condition of more liquid faeces when the large intestine is infected.

Rectum

• Storage area for faeces at the end of the large intestine
• There is a sphincter surrounding the anus, the hole through which faeces passes on defaecation.
• Constipation is the condition of dry hard faeces as a result of low fibre in the diet.

The Role of the Liver in Digestion

• Bile - The liver produces bile which is stored temporarily in the gall bladder, and then secreted into the duodenum (first part of the small intestine) for the emulsification of lipids. Bile is also alkaline and aids in the neutralisation of stomach acid in the small intestine.
• Sugar Conversion - After a meal, excess simple sugars in the bloodstream pass to the liver and are converted and stored as glycogen. However, between meals, the glycogen is converted back to simple sugars and released into the bloodstream. In this way, the blood sugar remains constant.

Appendix

Digestion takes place almost continuously in a watery, slushy environment. The large intestine absorbs water from its inner contents and stores the rest until it is convenient to dispose of it. Attached to the first portion of the large intestine is a troublesome pouch called the (veriform) appendix. The appendix has no function in modern humans, however it is believed to have been part of the digestive system in our primitive ancestors.

Additional hormones in the digestive system regulate appetite:

• Ghrelin is produced in the stomach and upper intestine in the absence of food in the digestive system and stimulates appetite.
• Peptide YY is produced in the digestive tract in response to a meal in the system and inhibits appetite.

Both of these hormones work on the brain to help regulate the intake of food for energy. Researchers are studying other hormones that may play a part in inhibiting appetite, including glucagon-like peptide-1 (GPL-1), oxyntomodulin (+ ), and pancreatic polypeptide.

Nerve Regulators

Two types of nerves help control the action of the digestive system.

Extrinsic, or outside, nerves come to the digestive organs from the brain or the spinal cord. They release two chemicals, acetylcholine and adrenaline. Acetylcholine causes the muscle layer of the digestive organs to squeeze with more force and increase the “push” of food and juice through the digestive tract. It also causes the stomach and pancreas to produce more digestive juice. Adrenaline has the opposite effect. It relaxes the muscle of the stomach and intestine and decreases the flow of blood to these organs, slowing or stopping digestion.

The intrinsic, or inside, nerves make up a very dense network embedded in the walls of the esophagus, stomach, small intestine, and colon. The intrinsic nerves are triggered to act when the walls of the hollow organs are stretched by food. They release many different substances that speed up or delay the movement of food and the production of juices by the digestive organs.

Together, nerves, hormones, the blood, and the organs of the digestive system conduct the complex tasks of digesting and absorbing nutrients from the foods and liquids you consume each day.

Comparison between the Digestive Tracts of
Herbivores and Carnivores

• Cellulose is a complex carbohydrate that is a major component of leaves and grasses. Humans refer to cellulose as fibre, as it stimulates peristalsis and passes out in the faeces. Humans cannot digest cellulose.
• Only a few organisms, such as fungi, protozoans and bacteria, can break down cellulose with anenzyme called cellulase.
• Herbivores have a symbiotic relationship with these organisms that live in the herbivores' digestive tracts. The micro-organisms receive food and shelter and, in return, digest cellulose anaerobically into smaller molecules that the host can absorb.
• Some herbivores have a caecum or hind-gut, where the small and large intestines join and where further breakdown of cellulose can occur. However, the faeces of these animals (e.g. horses, koalas, possums and rabbits) contain much cellulose.
• Other herbivores have a more effective rumen or fore-gut before the stomach, where partly digested plant matter can be regurgitated into the mouth for further chewing and swallowed again. Ruminants mammals include cattle, sheep, kangaroos and wallabies.