A 21-year-old man presents to his physician because he has been feeling increasingly tired and short of breath at work. He has previously had these symptoms but cannot recall the diagnosis he was given. Chart review reveals the following results: Oxygen tension in inspired air = 150 mmHg Alveolar carbon dioxide tension = 50 mmHg Arterial oxygen tension = 71 mmHg Respiratory exchange ratio = 0.80 Diffusion studies reveal normal diffusion distance. The patient is administered 100% oxygen but the patient's blood oxygen concentration does not improve. Which of the following conditions would best explain this patient's findings?

Vacation at the top of a mountain

A 54-year-old man comes to the physician because of excessive daytime sleepiness for 5 months. He wakes up frequently at night, and his wife says his snoring has become louder. He is 180 cm (5 ft 10 in) tall and weighs 104 kg (230 lb); his BMI is 33 kg/m2. His pulse is 80/min and his respiratory rate is 11/min. His jugular venous pressure is 7 cm H2O. He has 2+ pitting edema of the lower legs and ankles. Arterial blood gas analysis on room air shows a pH of 7.42 and a PCO2 of 41 mm Hg. An x-ray of the chest shows normal findings. Which of the following is the most likely underlying cause of this patient's condition?

Intermittent collapse of the oropharynx

Daytime alveolar hypoventilation

Decreased levels of hypocretin-1

Increased medullary ventilatory responsiveness

A 3-year-old boy is brought to the emergency department with a history of unintentional ingestion of seawater while swimming in the sea. The amount of seawater ingested is not known. There is no history of vomiting. On physical examination, the boy appears confused and is asking for more water to drink. His serum sodium is 152 mmol/L (152 mEq/L). Which of the following changes in volumes and osmolality of body fluids are most likely to be present in this boy?

Increased ECF volume, decreased ICF volume, increased body osmolality

Decreased ECF volume, unaltered ICF volume, unaltered body osmolality

Increased ECF volume, unaltered ICF volume, unaltered body osmolality

Increased ECF volume, increased ICF volume, decreased body osmolality

Decreased ECF volume, decreased ICF volume, increased body osmolality

A 17-year-old previously healthy, athletic male suddenly falls unconscious while playing soccer. His athletic trainer comes to his aid and notes that he is pulseless. He begins performing CPR on the patient until the ambulance arrives but the teenager is pronounced dead when the paramedics arrived. Upon investigation of his primary care physician's office notes, it was found that the child had a recognized murmur that was ruled to be "benign." Which of the following conditions would have increased the intensity of the murmur?

Placing the patient in a squatting position

An investigator is studying muscle tissue in high-performance athletes. He obtains blood samples from athletes before and after a workout session consisting of short, fast sprints. Which of the following findings is most likely upon evaluation of blood obtained after the workout session?

Decreased concentration of NADH

Decreased concentration of lactate

Increased concentration of insulin

A 27-year-old man is running on the treadmill at his gym. His blood pressure prior to beginning his workout was 110/72. Which of the following changes in his cardiovascular system may be seen in this man now that he is exercising?

Decreased systemic vascular resistance

Increased systemic vascular resistance

A 60-year-old woman with a history of emphysema has been referred by her pulmonologist for follow-up pulmonary function testing. During the test, the patient reaches a point where her airway pressure is equal to the atmospheric pressure. Which of the following is most likely to be found during this respiratory state?

Pulmonary vascular resistance is at a minimum

Pulmonary vascular resistance is at a maximum

Transmural pressure of the lung-chest wall system is at a maximum

Transmural pressure of the chest wall is at a minimum

Transmural pressure of the lung-chest wall system is at a minimum

A 35-year-old woman volunteers for a study on respiratory physiology. Pressure probes A and B are placed as follows: Probe A: between the parietal and visceral pleura Probe B: within the cavity of an alveolus The probes provide a pressure reading relative to atmospheric pressure. To obtain a baseline reading, she is asked to sit comfortably and breathe normally. Which of the following sets of values will most likely be seen at the end of inspiration?

Probe A: -6 mm Hg; Probe B: 0 mm Hg

Probe A: 0 mm Hg; Probe B: -1 mm Hg

Probe A: -4 mm Hg; Probe B: 0 mm Hg

Probe A: -4 mm Hg; Probe B: -1 mm Hg

Probe A: -6 mm Hg; Probe B: -1 mm Hg

Integrated respiratory responses for USMLE Step 3: High-Yield Physiology Lessons

Response to Exercise - The Body's Hustle

Metabolic Demand: ↑ O₂ consumption, ↑ CO₂ production, and ↑ ventilation rate.
Arterial Gas Homeostasis:
- $P_aO_2$ and $P_aCO_2$ remain near normal during moderate exercise due to tightly matched ventilation.
- Mixed venous $PCO_2$ ↑ due to increased tissue metabolism.
Right Shift: ↑ Temperature and ↓ pH (Bohr effect) enhance O₂ unloading in muscles.

Ventilation and arterial gases during graded exercise

⭐ During strenuous exercise, lactic acidosis can stimulate peripheral chemoreceptors, causing hyperventilation that drives $P_aCO_2$ below 40 mmHg.

High Altitude - Thin Air, Big Changes

Initial exposure to ↓ barometric pressure causes alveolar hypoxia (↓$P_iO_2$).

Acclimatization (Days to Weeks):
- Renal: ↑ Bicarbonate ($HCO_3^-$) excretion to correct alkalosis (~2-3 days).
- Hematologic: ↑ Erythropoietin (EPO) → ↑ RBC mass & Hct; ↑ 2,3-BPG shifts O₂-dissociation curve right, enhancing O₂ unloading.
- Cellular: ↑ Mitochondria & myoglobin.

⭐ Chronic hypoxia leads to pulmonary vasoconstriction, which can cause pulmonary hypertension and right ventricular hypertrophy.

Physiological responses to hypoxia

Diving Physiology - Deep Sea Pressure Cooker

Boyle's Law ($P \propto 1/V$): As pressure ↑, gas volume in body cavities ↓. Can cause barotrauma (e.g., middle ear, sinus squeeze).
Henry's Law: ↑ ambient pressure → ↑ gas dissolved in blood (N₂, O₂).
- Nitrogen Narcosis: At depths >100 ft, high $P_{N2}$ causes euphoria & disorientation.
  - 📌 Martini's Law: Effects similar to one martini every 50 ft.
- Decompression Sickness ("The Bends"): On rapid ascent, dissolved N₂ bubbles out of solution, causing joint pain, neurological deficits, and respiratory distress ("chokes").

⭐ Treatment for Decompression Sickness: Immediate 100% oxygen and recompression in a hyperbaric chamber.

Special States - Sleep, Cough & Sneeze

Sleep Physiology:
- NREM Sleep: Regular, slow-wave breathing. Ventilation is stable, but chemosensitivity to $CO_2$ is ↓.
- REM Sleep: Breathing becomes irregular and shallow. Marked ↓ in skeletal muscle tone (including upper airways) and ↓ response to hypoxia/hypercapnia, predisposing to obstructive events.
Cough & Sneeze Reflexes:

*   **Cough:** Afferent limb is primarily the **Vagus nerve (CN X)**.
*   **Sneeze:** Afferent limb is the **Trigeminal nerve (CN V)**.

⭐ The afferent limb for the cough reflex can be stimulated in the external auditory canal (Arnold's nerve, a branch of CN X), causing a cough when cleaning the ear.

Cough Reflex Arc

High‑Yield Points - ⚡ Biggest Takeaways

During exercise, ventilation matches metabolic demand, keeping PaO2 and PaCO2 normal; venous PCO2 increases.

High altitude causes hypoxic hyperventilation leading to respiratory alkalosis, compensated by renal HCO3− excretion.

Chronically, altitude stimulates erythropoietin (EPO) and 2,3-BPG production.

In CO poisoning and anemia, PaO2 is normal, so ventilation is not stimulated despite decreased O2 content.

Ventilation decreases during sleep, causing a mild physiological increase in PaCO2.

Continue reading on Oncourse

CONTINUE READING — FREE

or get the app

App Store|Google Play