Exercises: The digestive system

Order the following organs according to the path followed by the bolus of food from the moment it enters the body until faeces are expelled: large intestine, oesophagus, anus, mouth, small intestine, stomach.

Briefly explain what happens to food in the stomach and in the small intestine.

Explain the difference between mechanical digestion and chemical digestion, and give an example of each, indicating the organ where it takes place.

Match each organ of the digestive tract with the function it performs: mouth, oesophagus, stomach, small intestine and large intestine, choosing from grinding and mixing the food with saliva, transporting the bolus without digesting it, churning the food with acidic juices, completing digestion and absorbing the nutrients, and absorbing water to form faeces.

Explain what saliva is, where it is produced, and what two functions it performs on food while we chew.

Explain what faeces are, what they contain, in which organ they are formed, and why it is necessary to expel them.

State whether these statements are true or false and justify your answer: a) the teeth carry out chemical digestion of food; b) the oesophagus carries the bolus to the stomach; c) most nutrients are absorbed in the large intestine.

Explain what digestive enzymes are and why it is said that each one acts on a specific type of nutrient.

Aoife has eaten a ham sandwich. Describe the path the food will take through her digestive system, naming in order the organs it will pass through until the unabsorbed waste leaves the body.

In the image of the digestive system, locate the liver and the pancreas. These two organs are not part of the digestive tract, but are accessory glands. Explain what this means and state the function of each in digestion.

A person suffers from a stomach ulcer and the doctor recommends avoiding very fatty foods. Knowing that the stomach mainly digests proteins, explain where fats will be mainly digested and why reducing the workload of the stomach can help its recovery.

Describe what happens to a piece of bread, rich in starch (a carbohydrate), from the moment it enters the mouth until its nutrients reach the blood, naming the organs and digestive juices involved.

The stomach contains acids and enzymes capable of breaking down proteins, and yet it does not digest itself. Explain what protects it and reason what can happen if this protection fails.

A glass of milk provides proteins, fats and a carbohydrate called lactose. State in which organ the digestion of each of these nutrients begins and in which one the final products are absorbed.

It is often said that "you must chew your food well". Explain, relating mechanical digestion to chemical digestion, what advantages chewing slowly has for the work of the rest of the digestive system.

The liver and the pancreas pour their juices into the digestive tract even though food does not pass through them. Explain how these juices reach the intestine and which nutrient bile mainly helps to digest.

In a patient with undiagnosed celiac disease, the immune reaction to gluten has destroyed and flattened the villi of their small intestine, reducing their internal absorption surface area from about $250\ \text{m}^2$ to less than $15\ \text{m}^2$. Even though this person eats an adequate amount of food daily ($2400\ \text{kcal}$), they are rapidly losing weight and suffer from anemia and constant fatigue.

1. a) Justify why weight loss and symptoms of malnutrition occur despite the patient eating properly, linking it to the function of intestinal villi.
2. b) A family member suggests: "If the problem is that you absorb very little, you just need to eat three times as much food a day to make up for it." Scientifically reason whether this solution would work or what problems it would cause to their digestive system.

Seán had his gallbladder surgically removed. His friend Darragh tells him: "From now on, your body won't be able to digest fats anymore because you've lost the organ that makes bile."

1. a) Identify the two scientific errors in Darragh's statement and correct them, explaining precisely which organ actually produces bile and what the exact function of the gallbladder was.
2. b) The doctor has warned Seán to avoid high-fat meals. Explain at a physical and chemical level what happens to fats in the duodenum when a concentrated discharge of bile is not available, and why pancreatic lipase enzymes have difficulty acting if bile only arrives in a slow, continuous trickle.

For prolonged space missions, it is proposed to feed astronauts with liquid preparations where proteins are already broken down into free amino acids, complex carbohydrates into glucose, and lipids into fatty acids and glycerol. An astronaut claims: "With this shake, the stomach and pancreas will have to work twice as hard to digest everything very quickly."

1. a) Refute the astronaut's claim by explaining which mechanical and chemical stages of digestion are completely spared and which digestive secretions will see their activity drastically reduced.
2. b) Deduce in which section of the digestive tract the nutrients from this preparation will mainly be absorbed, and predict what consequences the prolonged absence of fiber and solid waste in the diet will have on the motility and health of the large intestine.

In a nutritional study, the digestive transit of two groups of volunteers who consumed the same amount of calories and water, but with different diets, was compared:

• Group A (diet rich in ultra-processed foods and meats, low in fiber): Mean digestive transit time = $68\ \text{h}$; mean fecal mass = $75\ \text{g}$ per day (hard and dry feces).

• Group B (diet rich in vegetables, legumes, and whole grains): Mean digestive transit time = $28\ \text{h}$; mean fecal mass = $180\ \text{g}$ per day (soft and bulky feces).

1. a) Based on the data, explain the physical mechanism by which plant fiber modifies stool consistency and the speed of transit through the large intestine.
2. b) A student states: "Since plant fiber cannot be digested and provides neither energy nor nutrients to our cells, it is a useless substance that we should eliminate from the diet." Refute their statement by deducing what health problems Group A is exposed to and explaining the role that fiber plays for the intestinal microbiota.

A medical test is performed on two patients (A and B) after fasting and drinking a glass of milk containing $25\ \text{g}$ of lactose. Their blood glucose is measured over the following two hours, as well as the concentration of hydrogen gas ($\text{H}_2$) in their breath:

• Patient A: Their blood glucose increased by $35\ \text{mg/dL}$ at $45\ \text{minutes}$; $\text{H}_2$ in their breath remained at virtually zero levels. They showed no discomfort.

• Patient B: Their blood glucose barely changed; however, at $90\ \text{minutes}$ the $\text{H}_2$ in their breath increased sharply, and they began to experience abdominal pain, bloating, and diarrhea.

1. a) Interpret the results: identify which of the two patients has lactose intolerance, justify why their blood glucose does not rise, and explain which microorganisms generate the $\text{H}_2$ gas in the large intestine and through which process.
2. b) Predict what would happen to patient B's blood glucose levels and symptoms if they repeated the test drinking "lactose-free" milk (industrially treated with lactase), and justify why this milk has a noticeably sweeter taste than regular milk.

During bacterial gastroenteritis, toxins irritate the walls of the digestive tract, causing severely accelerated peristalsis (the muscular contractions that push the contents along the tract). An affected child experiences frequent liquid diarrhea.

1. a) Taking into account the usual function of the colon, justify why excessively rapid digestive transit prevents the normal absorption of water and mineral salts, leading to watery stools.
2. b) A relative of the child proposes: "To prevent dehydration and help them regain strength, the best thing is to give them all at once a large glass of water with three tablespoons of sugar and a cola soft drink." Scientifically justify why this remedy will worsen the diarrhea and water loss (apply the concept of osmosis across the intestinal wall: water crosses membranes from the area with fewer dissolved solutes towards the area with more) and explain why oral rehydration solution should be administered in very small, frequent amounts.

There is a popular belief that jumping into cold water or exercising intensely after eating causes a "digestion shutdown" (corte de digestión) because the stomach becomes paralyzed upon suddenly cooling down. In reality, it is a circulatory conflict due to the redistribution of blood flow:

1. a) After a heavy meal, the digestive system experiences a large increase in blood flow (postprandial hyperemia). Justify which two metabolic needs the digestive organs (stomach and intestine) have to demand that extra blood flow.
2. b) If a person suddenly starts swimming intensely right after eating, their skeletal muscles also urgently demand a massive supply of oxygenated blood. Predict what circulatory conflict is triggered when the heart cannot fully supply both systems at the same time, and explain what symptoms (dizziness, muscle cramps, syncope) may appear.

In a physiology laboratory, a model of the small intestine is set up using a bag made of a semipermeable membrane whose pores only allow molecules smaller than $1\ \text{nm}$ to pass through (a size similar to that of the pores that allow passage into blood capillaries). A mixture containing water, mineral salts, glucose, starch, amino acids, and proteins is placed inside the bag. Then, the bag is submerged in a container of pure water.

1. a) After three hours, the water in the outer container is analyzed. Reasonably predict which of these substances will be detected in the outer water and which will have remained trapped inside the bag.
2. b) Based on the result, biologically justify why complex foods such as meat or bread must undergo chemical digestion with enzymes (such as proteases and amylases) before being absorbed, whereas water, salts, and vitamins can be absorbed directly without prior digestion.