THE DIGESTIVE SYSTEM © PDST Home Economics

 Without food, water and oxygen, human beings could not survive.  The digestive system is a set of organs which change what we eat into substances that can be used in the body.  These substances can be used for energy, growth and repair.

THE DIGESTIVE SYSTEM  The alimentary canal is a tube that runs from the mouth to the anus  It is composed of the mouth, oesophagus, stomach, small intestine and the large intestine  As food passes through the alimentary canal it is changed and the nourishment is taken into the blood  Waste passes out the end of the canal  Certain organs and glands add juices to the canal at various points

Mouth 1. Chemical digestion (amylase converts starch to maltose ) 2. Physical digestion (teeth break food down into smaller pieces) Oesophagus Connects mouth to stomach Stomach Liver Produces bile for the digestion of fats Gall bladder Stores bile 1.Holds the food for a while 2.Physical digestion (food is churned and mixed) 3.Chemical digestion (assisted by HCl) Pancreas Produces digestive juices Small intestine 1.Chemical digestion 2.Absorption of nutrients into blood Appendix Large intestine (colon) 1.Elimination of waste 2.Absorption of water Rectum Stores faeces Anus

Food can be broken down (digested) in one of two ways: 1.Physical Digestion This is where large pieces of food are broken down into smaller pieces of the same food 2.Chemical Digestion This is where food is broken down into a different substance that can easily pass into the blood

Mouth  The food is broken down by the teeth and mixed with saliva.  Saliva is excreted by three pairs of glands: The parotid gland (below the ear)The parotid gland (below the ear) The submandibular (under the tongue)The submandibular (under the tongue) The sublingual (under the tongue)The sublingual (under the tongue)  Saliva contains water, mucus and the enzyme salivary amylase.

Functions of Saliva  It lubricates food with mucus, making it easier to swallow.  It contains the enzyme salivary amylase, which acts on cooked starch turning some of it into maltose.  It keeps the mouth and teeth clean.  The ball of food that leaves the mouth is known as a bolus.

Functions of the Tongue  Taste: it is covered with thousands of taste buds. These are sensitive to salt, sweet, sour and bitter chemicals in food and drink. They help us enjoy food and drink and warn us when food, drink are off or inedible. They help us enjoy food and drink and warn us when food, drink are off or inedible.  Chewing: the tongue aids chewing by moving the food around the mouth, pushing it between the teeth and covering it with saliva, which contains enzymes that start the digestive process. The food is turned into a partially digested mass known as a bolus. The food is turned into a partially digested mass known as a bolus.  Swallowing: when the food is ready to travel to the stomach, the tongue pushes it to the back of the mouth.

Tongue Taste Centres Epiglottis Bitter Sour Salt Sweet Taste Buds Papillae

Oesophagus  The food passes into the pharynx (a muscular tube behind the mouth) and down the oesophagus.  The epiglottis a small flap of cartilage blocks the entrance to the larynx, this stops the food going down the wrong way and prevents choking.

Structure of the Oesophagus & Functions  It is a muscular tube.  It leads from the pharynx to the stomach. Function  To carry chewed food from the pharynx to the stomach.  Food moves along it by a muscular contraction known as peristalsis.  The muscle fibres contract and relax which acts like a wave on the tube, pushing the bolus forward.  It’s lining secretes mucus to lubricate the passage of food.

The Stomach Oesophagus Pyloric Sphincter Duodenum Body of Stomach Cardiac Sphincter

Stomach Structure  It is a J-shaped, elastic organ.  Food enters it from the oesophagus through the cardiac sphincter.  The cardiac sphincter, is a valve that stops back flow of the stomach`s contents.  Food leaves the stomach through the pyloric sphincter into the duodenum (first part of the small intestine).

 The walls of the stomach is made up of layers of muscle.  It has an inner mucous membrane.  This membrane has lots of folds.  When the stomach is full these folds stretch out, enabling it to expand, then they contract when the stomach empties.

Functions of The Stomach  It digests protein through the action of enzymes.  It churns food with the gastric juices.  It helps lubricate the food by producing mucus.  It absorbs alcohol.  It kills bacteria by producing hydrochloric acid.

Gastric Juices   Hydrochloric acid neutralises bacteria and activates pepsin.   Rennin is an enzyme that curdles milk protein in infants.   Pepsin is an enzyme that breaks down proteins into peptones.

Structure of the Small Intestine   It is seven metres long.   It is divided into three parts:   The duodenum   The jejunum   The ileum   The walls has four layers:   A muscular layer   A layer containing blood vessels, lymph vessels, and nerves.   A submucous layer,   A mucous layer.

  The inner wall is covered in villi, tiny hair like projections which increase the surface area for absorption.   Each villi contain blood vessels and lymph vessels.

Functions of The Small Intestine Digestion   Pancreatic juice is secreted into the duodenum and contains the following enzymes:   Trypsin: converts proteins into shorter chains.   Lipase: converts fats into fatty acids and glycerol.   Amylase: converts starch into disaccharides.   Bile: emulsifies fats (breaks them into smaller droplets).   Intestinal Juices have the following enzymes:   Maltase, sucrase, lactase: change disaccharides into monosaccharides.   Peptidase: changes polypeptides into amino acids.

Absorption   Digested food is absorbed through the villi walls.   Fats, fatty acids and glycerol are passed into the lymph system.   Amino acids and sugars pass along the portal vein to the liver.

Caecum Transverse Colon Descending Colon Ascending Colon RectumAnus Apendix

What is the Large Intestine   It deals with waste.   It is about 1.5m long.   It consists of the following:   The caecum: a small pouch; the ileum empties its contents into the caecum through the ileo-caecal valve.   The colon: ascending, transverse, descending colon.   The appendix: narrow tube attached to the caecum.   The rectum.   The anus

Functions   Whatever remains of the food, is passed into the large intestine   To reabsorb water and vitamins left in digestive waste.   It secretes mucus to help the movement of faeces.   Short term storage of faeces in the rectum.   Many bacteria live in the large intestine, they are harmless in the colon and may be useful e.g. produce Vitamin K.   Defecation: peristalsis pushes waste along the colon and then it is passed out of the body.

ENZYMES  An enzyme is a biological catalyst  A catalyst speeds up chemical reactions  Enzymes speed up biological reactions  All chemical reactions that take place in living systems require the action of an enzyme

ENZYMES  Digestive enzymes break food down into smaller, more soluble substances  This allows the food to be absorbed into the blood

ENZYMES  An example of a digestive enzyme is amylase  Amylase is present in saliva  Amylase chemically breaks down starch

ENZYMES Amylase converts starch into a sugar called maltose STARCH  MALTOSE

ENZYMES STARCH AMYLASE MALTOSE

ENZYMES  The substance that an enzyme works on is known as its SUBSTRATE  The substance formed by the enzyme is known as its PRODUCT  Therefore starch is the substrate for amylase and maltose is its product

STAGES OF NUTRITION There are four stages in human nutrition: 1.Eating (also called “ingestion”) 2.Digestion 3.Absorption of digested food into the blood 4.Elimination of undigested food (also called “egestion”)

STAGES OF NUTRITION  Digestion changes food into a form that can enter the blood  Physical (mechanical) digestion breaks food down into smaller pieces  Chemical digestion breaks food down into different, more soluble substances

 The contents of the alimentary canal are pushed along by a rhythmic pulsing of the muscles of the intestines  This is known as peristalsis

TEETH  An adult human has 32 teeth, 16 in either jaw  The shape and size of the tooth varies with the job it has to do  There are four different types of tooth

TEETH Incisors These cut and bite food Canines These grasp and tear food Pre-molars These grind and chew food Molars These also grind and chew food

TEETH Incisor Pre-molar Canine Molar

TEETH  The type of teeth that an animal has depends on what it eats  A herbivore eats plant material and has very large incisors and molars  A carnivore eats other animals and needs very large canines  An omnivore (e.g. humans) eats all kinds of food and needs and use all of the types of tooth equally