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.

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

Marta 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.

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

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

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.

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.

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.

Carlos had his gallbladder surgically removed. His friend Marcos 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 Marcos'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 Carlos 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.

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.

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.