S2 Human Digestive System

Blood can only carry substances that are dissolved in it. A plate of rice, chicken and vegetables is neither dissolved nor small enough to pass into a blood vessel, so none of it could reach your cells in the form it arrives in. The digestive system exists to solve that problem: it reduces food, first by mechanical action and then by enzymes, until the molecules are small and soluble enough to cross the wall of the gut.

Everything eaten follows one continuous route. The food itself stays inside the lumen — the hollow centre of the tube — while the useful molecules are taken through the wall into the body. Whatever is never broken down is carried the whole length of the tube and leaves at the far end.

Nutrition in a human is usually described as a sequence of five processes. The order matters, because each one depends on the one before it.

Digestion is the step with two mechanisms. Physical digestion breaks food into smaller pieces by crushing, grinding and mixing it, and it changes only the size of the pieces. Chemical digestion uses enzymes to break large food molecules into simpler ones, so it changes what the substances are.

Several organs lie alongside the canal and empty their secretions into it. They are not part of the tube itself, and food never travels through them, but digestion depends on them.

The wall of the canal contains two layers of muscle. In the circular layer the fibres run around the tube, so when they contract the tube narrows; in the longitudinal layer the fibres run along the tube, so when they contract the tube shortens. The two layers never pull together — wherever one contracts, the other relaxes — and that alternation is what moves the food.

Picture a bolus in the oesophagus. The ring of muscle immediately behind it tightens and that stretch of tube becomes narrower; at the same moment the muscle immediately in front of the bolus slackens and the tube opens out. The food is therefore pushed from a tight section into a waiting space, and the pair of changes repeats further down a moment later, so a wave of movement chases the bolus all the way to the stomach.

The gullet is a narrow muscular tube running from the back of the mouth down to the stomach. No digestion takes place in it. Nothing is ground up there, and no enzyme is added — the only chemical digestion at this stage is the starch digestion already started in the mouth by salivary amylase, which the bolus has carried down with it.

Its work is transport. The muscular wall pushes the bolus steadily downwards by peristalsis (section 4), so the food arrives at the stomach whether you are upright, lying down or upside down.

The stomach is a muscular bag sitting below the diaphragm, on the left side of the abdominal cavity. Food that has left the gullet stays inside it for several hours — a large meal can take about four hours to be dealt with — and that time is what allows the churning and the protein digestion below to be completed.

The small intestine is one long tube with three sections, in this order: the duodenum, then the jejunum, then the ileum, which is the longest of the three. Digestion is completed in the first part of the tube and the products are taken into the body in the last part.

The large intestine continues from the small intestine, running up the right side of the abdominal cavity. It is considerably shorter than the small intestine — about 1.5 m on average — but it is wider, and that width is where its name comes from. Its two named sections are the short pouch called the caecum at the junction, and the colon, which makes up most of its length.

Very little digestion happens here. The tube holds material that the small intestine could not break down — plant fibre, for instance — together with bacteria, cells that have been shed from the lining, mucus, mineral salts and water. Its main job is to reclaim what is still useful: water and mineral salts are absorbed through the wall before the remaining waste leaves the body.

Each enzyme acts on one kind of substrate and no other. A carbohydrase will not touch protein, and a lipase will not touch starch. When an enzyme is said to catalyse a breakdown, it means that the reaction happens far faster with the enzyme present than it would on its own.

Enzymes themselves work only in the right conditions. Every enzyme has a temperature and a pH at which it acts fastest, and outside those conditions it works more slowly. If the temperature is raised too far or the pH is far from the value it needs, the enzyme is denatured: its structure loses the shape it needs, and it stops working altogether. Denaturing is permanent; cooling an enzyme back to its usual temperature restores its activity, but denaturing it by heat does not.

An enzyme acts fastest at one particular pH. Because the gut is not one uniform chemical environment, the secretions released into each region have been matched to the pH of that region, and each part of the canal therefore contains enzymes whose optimum lies close to the pH there.

Bile helps fat digestion by acting on the fat physically rather than chemically. Fat arriving from the stomach is in large globules, and an enzyme can only work at the surface of a globule. Bile lowers the surface tension of those globules and breaks them apart into a very large number of tiny droplets, a change known as emulsification.

An investigation into enzymes has to show that the enzyme is doing the work, and it has to keep everything else the same so that the condition being tested is the only thing that changes. Starch and amylase are convenient to investigate because both the substrate and its digestion can be detected easily: starch turns iodine solution blue-black, and it does not do so once it has all been digested into maltose, while a reducing sugar such as maltose gives a brick-red precipitate with Benedict's solution on heating.

Digestion is only useful if the products can get into the body quickly, and absorption happens across a surface. The small intestine is therefore built to pack as much absorbing surface as possible into the length of tube available.

The products of digestion do not all leave the small intestine by the same route, and a question that names a nutrient is usually testing which route it takes.

Once these molecules have been absorbed they are used in the body in the ways set out below. Notice that the uses fall into two kinds: some products are broken down further to release energy, and others are built into the body's own material.

• Feeling weak and tired.
• Losing weight without trying to.
• Passing urine very often, and feeling very thirsty.
• Wounds that are slow to heal, and a greater tendency to catch infections.
• Damage to nerves and blood vessels, which can lead to loss of sensation and to blindness.

There are three types in all: type 1 diabetes, type 2 diabetes, and a form that can arise during pregnancy. The first two are the ones examined.

A healthy diet supplies everything the body needs, in the amounts it needs them. The usual guide is the healthy diet pyramid. Foods shown at the base of the pyramid should make up the largest part of what is eaten each day, and foods towards the top should be eaten in steadily smaller amounts, with the small section at the peak used sparingly.

Undigested matter normally spends up to about 36 hours in the colon, which is the time needed for the remaining water and mineral salts to be absorbed from it. If it moves through more slowly than that, absorption carries on for too long: too much water is taken out, the waste becomes dry and hard, and it is then painful and difficult to pass. Not drinking enough, a sudden change in eating habits, too little fibre in the diet, anxiety, and some medicines can all slow the colon down. Drinking more, eating more fibre each day and getting more exercise usually put the problem right.

Gastritis is inflammation or swelling of the lining of the stomach. It causes pain in the stomach, indigestion, nausea and vomiting. It can be brought on by infection with H. pylori (section 7), by a viral infection, by too much alcohol, caffeine or oily food, by anti-inflammatory painkillers such as aspirin, or by stress.

An infection or other illness of the gut can make the bowel move its contents along too quickly. Water is then not in the colon long enough to be absorbed, and the result is diarrhoea. Someone who loses a great deal of water in this way can become dehydrated, so replacing fluids matters. Diarrhoea is often caused by eating food that has been contaminated by bacteria such as Salmonella, which is why food hygiene during preparation is so important.

The advice issued by the National Environment Agency rests on five habits:

• Separate raw and cooked food — raw food can carry dangerous microorganisms, so use different utensils, boards and plates for raw meat and for food that is ready to eat, and cross contamination is avoided.
• Keep food at the right temperature — store raw food below 5 °C and frozen food below −18 °C, since harmful bacteria multiply rapidly at room temperature.
• Select food carefully — check packaging for damage, look for mould, and never use food past its expiry date.
• Have good hygiene practices — keep food, hands, utensils and the kitchen clean.
• Cook food thoroughly — thorough cooking kills the harmful bacteria in food.

(a) Bread is largely starch. Trace what happens to that starch as the food passes from the mouth to the end of the small intestine, naming the class of enzyme involved at each stage and stating the substance that is finally absorbed into the blood. [4]

(b) Explain why very fatty meals may cause discomfort in a patient whose gall bladder has been removed, even though the liver still produces bile. [3]

(c) Explain how a bolus of food is moved down the oesophagus in a person lying flat on their back. [3]

(d) State and explain the change in pH of the food as it leaves the stomach and enters the duodenum. [2]

(e) A student investigates the effect of pH on the digestion of starch by amylase. Describe how the investigation should be carried out and what should be measured. [4]