Exercises: Structures of nutrition

List the four processes of animal nutrition and explain in one sentence the function of each, indicating what would happen to a cell if any of them failed.

Order the path of a mouthful of food through the human digestive tract, from the mouth to the anus, naming all the organs it passes through.

Differentiate between mechanical digestion and chemical digestion, and give an example of each in the mouth and another in the stomach.

Name the accessory glands of the human digestive system (salivary glands, liver and pancreas) and indicate which substance each one pours into the digestive tract and what it is for.

Explain the two functions of the stomach (mechanical and chemical) and discuss why its acidic juice does not digest the stomach's own wall (the mucus layer, cell renewal).

Indicate in which section of the digestive tract most nutrients are absorbed and describe the adaptations that multiply its surface (length, folds, villi and microvilli).

Distinguish between pulmonary ventilation, gas exchange (external and internal respiration) and cellular respiration, indicating where each process occurs and what role the blood plays between them.

Explain the function of the kidney in the formation of urine and which substances it eliminates, and distinguish excretion (urea, salts, water) from egestion (faeces), discussing why faeces are not a product of cellular metabolism.

Compare branchial (gill), tracheal, and pulmonary respiration: in which animal group each occurs, what exchange surface it uses, and to which environment (aquatic or terrestrial) it is adapted.

Explain the difference between an open and a closed circulatory system, and between single and double circulation, citing a representative animal group for each case and one advantage of double circulation.

Observe the image of the human digestive system. a) Describe what happens to food in structures 1, 4, and 8. b) Indicate which numbered structures are accessory glands, that is, those that are not part of the digestive tract, and what each secretes into the tract. c) Order structures 1, 3, 4, 8, 9, and 11 according to the path of the bolus.

Describe the path of air from the nose to the alveoli (pharynx, larynx, trachea, bronchi, bronchioles) and explain which adaptations make the alveolus an ideal place for gas exchange (an extremely thin wall, a capillary network, moisture, total surface area).

Inhaled air contains $21\ \%$ $\text{O}_2$ and $0.04\ \%$ $\text{CO}_2$; exhaled air, $16\ \%$ $\text{O}_2$ and $4\ \%$ $\text{CO}_2$. Interpret these figures: what they demonstrate about the efficiency of the exchange, why not all the oxygen is extracted, and why mouth-to-mouth ventilation can oxygenate another person.

Describe the path of the blood through the pulmonary and systemic circuits starting from the right ventricle, naming the heart chambers, the valves and the great vessels it passes through, and explain what change the blood undergoes in the lungs and in the tissues.

Compare the structure of arteries, veins and capillaries (thickness and composition of the wall, diameter, presence of valves) and relate it to their function and to the pressure of the blood they carry.

The liver is the body's "metabolic hub": describe three of its functions (glycogen storage, urea production, bile production, detoxification) and discuss why its failure affects digestion, excretion and the composition of the blood all at once.

A paleontology team analyzes the remains of two extinct mammals of similar size (~50 kg). Species A possesses worn molars with pronounced enamel ridges (lophodont) and an abdominal cavity volume that suggests a large-capacity digestive system; species B presents molars with sharp cutting cusps (secodont) and a remarkably reduced abdominal cavity.

1. a) Predict and justify in a comparative manner the relative length of the digestive tract and the food retention time in both species based on the chemical constraints of degrading cellulose versus proteins.
2. b) If a live animal with a digestive anatomy analogous to species A is given prolonged treatment with broad-spectrum antibiotics, it suffers drastic loss of body mass despite continuing to consume abundant forage. Justify this phenomenon at the biochemical and physiological levels.
3. c) Why has natural selection not favored a hypertrophied digestive tract with microbial fermentation in obligate carnivores if this would allow them to extract nutrients from plant material when prey is scarce?

Analyze the following two physiological scenarios regarding nitrogen excretion:

• Case 1: Amphibian tadpoles are obligate ammonotelic organisms, but during metamorphosis into terrestrial adults they activate the genes for the enzymes of the ornithine cycle and become ureotelic.

• Case 2: The placental mammal embryo eliminates urea across the placenta, whereas the embryo of birds and reptiles encloses its nitrogenous wastes inside the shelled (cleidoic) egg.

1. a) Justify the switch in excretion during amphibian metamorphosis by evaluating the trade-off among cellular toxicity, environmental water availability, and energy cost in ATP molecules.
2. b) Predict the physiological and osmotic consequences for the development of a bird embryo if it were to carry a mutation preventing it from synthesizing uric acid and causing it to excrete only urea inside the egg. Compare its viability with that of a mammal embryo.
3. c) A student states: "Uric acid is the best evolutionary strategy for any animal because it allows water conservation at no biological cost." Refute this statement using thermodynamic and metabolic arguments.

A volunteer is orally administered glucose labeled with a radioactive isotope ($^{14}\text{C}$) and is made to inhale air enriched with heavy oxygen ($^{18}\text{O}$). During an intense muscular exercise session involving the arms, the metabolism of their tissues is analyzed.

1. a) Trace chronologically the pathway of the radioactive glucose from the lumen of the small intestine to the sarcoplasm of a biceps muscle fiber, specifying the four organ systems involved in nutrition, the biological membranes or barriers it crosses, and the type of transport it uses.
2. b) A classmate claims: "The $\text{CO}_2$ expelled by the athlete through the lungs directly contains the inhaled $^{18}\text{O}$, and the nitrogen from muscle protein catabolism is converted into urea inside the muscle itself before passing into the blood." Refute both statements by justifying the biochemical fate of $^{18}\text{O}$ in the respiratory chain and the role of the glucose-alanine cycle and the liver in nitrogen excretion.
3. c) Predict what homeostatic alterations would occur in blood pH and composition if a transient failure of renal perfusion occurred during exertion, and what compensatory respiratory mechanism will be immediately activated.

In 1984, Barry Marshall challenged the medical paradigm that "no bacterium can colonize the human stomach" by ingesting a culture of Helicobacter pylori.

1. a) In a laboratory, H. pylori is inoculated into two media at $\text{pH} = 2.0$: Medium 1 (standard nutrient broth) and Medium 2 (nutrient broth supplemented with 0.5 % urea). The bacteria die rapidly in Medium 1, but proliferate in Medium 2, locally raising the pH around the colonies and releasing ammonium ($\text{NH}_4^+$). Interpret this experiment by explaining the adaptive mechanism of the bacterium and the direct chemical damage it causes to the mucosal barrier of the gastric epithelium.
2. b) A patient with a gastric ulcer takes proton-pump inhibitors (antacids) for one month. The symptoms resolve completely, but one month after discontinuing the medication, the ulcer recurs with greater severity. Justify why this symptomatic treatment is not curative and why combined therapy with antibiotics is required at the tissue and bacterial population levels.
3. c) Evaluate Marshall's self-experimentation by applying Koch's postulates: indicate which ones he managed to satisfy on his own person and point out what methodological and ethical limitations this procedure presents compared to a modern clinical trial.

The cardiorespiratory function of two 60-year-old individuals with the same body mass is evaluated:

• Subject A (non-smoker): Normal total lung capacity, forced expiratory volume in one second ($\text{FEV}_1$) of 3.4 L, and hematocrit of 44%.

• Subject B (smoker of 25 cigarettes/day for 35 years): Hyperinflated lungs, $\text{FEV}_1$ of 1.1 L, carbon monoxide diffusing capacity of the lung ($\text{DLCO}$) reduced to 45% of the predicted value, and hematocrit of 55%.

1. a) Apply Fick's law of gas diffusion across biological membranes to justify why the breakdown of interalveolar septa (emphysema) and the loss of lung elasticity cause the collapse of $\text{DLCO}$ and severe dyspnea during moderate exertion.
2. b) Justify at a physiological level why Subject B presents with polycythemia (hematocrit of 55%), relating the impairment in gas exchange to the renal endocrine response.
3. c) A commercial product claims to "regenerate alveolar walls and clean up destroyed elastin in smokers in just three months." Refute this claim using histological arguments regarding the mitotic and regenerative capacity of the alveolar parenchyma.

A patient at rest exhibits sustained blood pressure readings of $160/105\ \text{mm Hg}$. Their lab tests show a decreased glomerular filtration rate and microalbuminuria (leakage of proteins into the urine), along with concentric left ventricular hypertrophy on the echocardiogram.

1. a) Justify what each reading ($160\ \text{mm Hg}$ systolic and $105\ \text{mm Hg}$ diastolic) biophysically reflects in terms of cardiac stroke volume, the elasticity of the large arteries, and the peripheral vascular resistance offered by the arterioles.
2. b) Explain the vicious cycle that develops between hypertension, renal microvasculature damage, and the dysregulated activation of the renin-angiotensin-aldosterone system, deducing how this affects blood volume and blood pressure.
3. c) Justify why the left ventricle hypertrophies in response to chronic hypertension while the right ventricle initially tends to maintain its original thickness, and predict why silent vascular damage increases the risk of stroke and capillary rupture.

A patient with advanced chronic kidney disease undergoes hemodialysis. In the machine, their blood flows in a countercurrent manner separated by a semipermeable membrane from a dialysis fluid formulated with: urea concentration = $0\ \text{mmol/L}$, $\text{K}^+ = 2.0\ \text{mmol/L}$ (compared to $5.8\ \text{mmol/L}$ in the patient's plasma), glucose = $5.5\ \text{mmol/L}$, and bicarbonate = $35\ \text{mmol/L}$.

1. a) Explain, using concentration gradients and passive transport processes, why the dialysate must have precisely these relative solute concentrations, and predict what cellular catastrophe would occur if the dialysis fluid were accidentally replaced with pure distilled water.
2. b) Although dialysis normalizes nitrogenous waste and electrolytes on a weekly basis, the patient exhibits severe anemia and abnormal vascular calcifications linked to bone demineralization (osteodystrophy). Justify why mechanical dialysis is unable to reverse these two problems, indicating which endocrine functions of the healthy nephron are lost.
3. c) Compare hemodialysis and kidney transplantation by evaluating the hemodynamic and endocrine advantages of the grafted kidney versus the compromise entailed by immunosuppressive drugs to prevent rejection.

During a treadmill stress test, an athlete's cardiovascular parameters are monitored, showing the following changes between resting state and strenuous exercise:

• Cardiac output increases from $5.0\ \text{L/min}$ to $24.0\ \text{L/min}$.

• The percentage of blood flow directed to skeletal muscle rises from 20% to 84%.

• Perfusion of the splanchnic territory (stomach and intestines) drops from 24% to 2.5%.

• Plasma lactate concentration increases from $1.1\ \text{mmol/L}$ to $12.5\ \text{mmol/L}$.

1. a) Explain the mechanisms of sympathetic autonomic control and local metabolic autoregulation by tissue metabolites through which visceral vasoconstriction and vasodilation in active muscle are simultaneously coordinated.
2. b) If the athlete had consumed a heavy meal rich in lipids and proteins one hour before the test, predict what hemodynamic conflict of blood distribution would arise and what consequences it would have both on her performance and at the digestive level.
3. c) Upon abruptly stopping the run, ventilation and oxygen consumption remain well above baseline values for more than 15 minutes (oxygen debt or EPOC). Biochemically justify which metabolic and cellular processes in the liver and muscle this additional $\text{O}_2$ consumption is allocated to.