Mammals_Concept_4

The mouth

The mouth is where the digestion (breaking down) of food begins. In mammals, two distinct digestive processes take place in the mouth. The first is the use of teeth to break food down physically into smaller pieces (whereas many reptiles and birds, particularly carnivorous ones, swallow food whole). Most teeth can be described as cutters (incisors), rippers (canines) or grinders (molars). Carnivores will usually have rippers: pointed fang-like teeth for ripping flesh off the bone. Herbivores will have cutters at the front and grinders at the back Omnivores (such as humans and bears) will have a full mix. The food is cut into manageable pieces by the front cutting teeth; then the tongue transfers it to the back of the mouth, where it is ground up and mixed with saliva into a paste before being swallowed.

The other digestive process going on in the mouth involves saliva.This mildly acidic, lubricating liquid begins to break down starch-based foods, into sugars, which can be digested more easily Saliva aiso adds moisture to the food allowing it to be swallowed.

Down the hatch...

It has probably not escaped your notice that the mouth can be used for both food and air. There are two tubes leading out of the back of the mouth: the trachea and the oesophagustThe action of swallowing closes the trachea (the air passage) and opens the oesophagus (which leads to the stomach), The oesophagus is ringed with muscles that constrict above the food ball, forcing it downwards — a process known as peristalsis, Gravity also helps the food to descend: eating while upside down is diffcult!

In some mammals, known as ruminants (such as the cow), these muscles also work in reverse: they bring the food back up for a second chew These mammals have more than one stomach chamber, because the food they live on requires a lot of work to digest: they often obtain most of their energy from plant fbre or cellulose (for example, in grass). which requires a iot of enzyme action before digestion can be completed

Into the stomach...

The stomach is a holding bay for further digestion, allowing the digested food to be released into the duodenum (the top end of the small intestine) in a regulated way. In the stomach, the food is sprayed with hydrochloric acid. This tenderises the food and kills most of the germs on it, preparing it for the digestive enzymes (complex biological molecules that stimulate chemical changes) to get to work on.The mucus coating of the stomach lining normally prevents the stomach lining from being attacked by the acid. (ffthis protection breaks down, stomach ulcers will develop) At this point, an enzyme called pepsin begins to break protein down into amino acids, and other enzymes begin to break down other food types.Water and some simple sugars are absorbed into the body through the stomach lining,

A long and winding road...

The small intestine (so called because of the diameter of the tube, not the length) is called the duodenum where it leaves the stomach and the ileum further down. On leaving the stomach the food, which is now a thick, soupy liquid known as chyme, is sprayed with bile. Bile is a liquid made in the liver and stored in the gall bladder; it breaks down fats into tiny droplets, giving them a larger surface area for the enzymes to act on. Bile also neutralises the stomach acids, allowing the enzymes to work more effectively These enzymes are released into the duodenum from the pancreas, and include lipase (digests fat into fatty acids), carbohydrase (digests starch into glucose) and protease (completes the digestion of proteins).

The remaining length of the small intestine, the ileum, follows up digestion with absorption,The food, reduced to very simple forms as a result of the enzyme action, can now be absorbed into the blood supply through the blood capillaries in the lining of the ileum.

And finally, down the waste chute...

The colon (or large intestine) collects all ofthe undigested food most of which will be indigestible.This slurry is reduced to a more solid state by the extraction of most of the moisture content, which helps to prevent dehydration. A notable addition at this stage is a vast quantity of dead red blood cells. Indeed, almost a third of the total mass of faeces produced is used red blood cells — giving the waste its unmistakeable colour.

The chemical factory

Once the food has been absorbed into the blood, most of it is transported to the liver for further processing. The liver performs many essential functions. From the digested food, it manufactures proteins (including anti-infection and clotting agents) and produces oxygen- and fat-carrying compounds. It produces bile (see page 74), which is stored next to it in the gall bladder. It is also able to counter the effects of some poisons by filtering them from the bloodstream, then neutralising and excreting them, The human liver is positioned just under the diaphragm, on the right-hand side of the body.

Why you need to know these facts

Children are fascinated to discover what happens to food once it has been swallowed, and how it can be turned into something that is useful to mammals (in terms of growth or movement). The chemical processes that form the basis of digestion are quite detailed; but simplified models can help the children to begin to understand them.

Vocabulary

Canines — ripping teeth.

Colon — the large intestine, where the undigested food has moisture removed from it and dead red blood cells are added prior to excretion.

Digestion — the process of breaking food down into simple chemicals for absorption by the body.

Duodenum — the upper part of the small intestine, where further enzymes and bile are added to the chyme (see above) to break down the more complex foods.

Ileum — the lower end of the small intestine where the digested food is absorbed into the body via the blood supply

Incisors — cutting teeth.

Molars — grinding teeth,

Mouth — where food is physically broken down (chewed) into smaller pieces and saliva is added.

Saliva — a lubricating digestive juice produced in the mouth. Stomach — a rounded vessel in the body where acidic digestive juices are added to the food to reduce it to a 'thick soup' called chymes

Amazing facts

Food takes between 10 and 20 hours to pass through the human digestive tract, spending up to three hours in the stomach alone.

The adult human intestine is up to 8m long.

Ruminants are able to snatch a lot of food quickly and then ^feat' it properly later. This is very useful if you are likely to be attacked by a predator at any moment! Of the ruminants, camels and llamas have three stomachs; sheep, cattle and goats have four.

The digestive system of the giant panda is based on digesting meat rather than the bamboo that is its staple diet. This helps to explain why it is in danger of becoming extinct: it is too reliant on a poor source of nutrition — and too specialized to go back to eating meat,

The liver can lose up to 75% of its tissue (to disease or surgery) and still keep working. This is why transplants usually only involve a section of the donor's liver.

Common Misconceptions

All food is digested in the stomach.

Simpler foods, such as sugars, are digested and absorbed in the stomach; but most foods will travel on into the small intestine for further digestion before being absorbed into the blood.

Questions

What happens when you are sick?

Vomiting is the result of the body reacting to impulses from the brain's 'vomit centre' (yes, there really is one!)This centre may receive signals from the stomach (that it is overloaded, or contains something disagreeable or poisonous), from the inner ear (the balance centre which is disturbed in motion sickness), or from sense organs (particularly taste and smell). Whatever the stimulus that provokes it, the vomiting reflex is essentially the same: the diaphragm presses down on the stomach at the same time that the valve between the stomach and the oesophagus relaxes, allowing the stomach contents to be pushed upwards. The sour taste of vomit is due to the gastric juices from the stomach — but the presence of diced carrot is still a scientific mystery!

Why do you burp?

Burping (or belching) is caused by the release of gas trapped in the stomach. This gas may be air swallowed while eating too quickly, carbon dioxide from fizzy drinks or gases released as a result of digestion. Any gases released at the other end tend to be the result of the digestive processes acting on a high-fibre diet.

Teaching ideas

As with the heart and lungs, i would not recommend obtaining a mammalian digestive tract from a butcher for inspection or dissection! VVork on this topic is best done using reference materials.

Teeth and diet (observing, comparing)

Looking at pictures of the skulls of mammals (or even of faces showing teeth) will allow the children to make deductions about the kind of diet each animal lives on. They can check their deductions by referring to books on pets, wildlife videos and the internet.

Journey in the gut (explaining, describing)

Ask the children to take on the role of a piece of food and to describe their journey as they travel through the digestive tractThis could be presented as a story a newspaper report, a cartoon strip, a series of pictures or a piece of drama The BBC series The Human Body provides some excellent images as a stimulus,

Digestive navigation (exploring, observing) The children can work in small groups to navigate themselves through the digestive system, using appropriate video clips or animations from internet sites such as www.bbc.co.uk]schools or YouTube.They could then be encouraged to write a diary entry or news report on their journey.