Exam 3, L6 & Other Important Concept Connections

Question 1

How much oxygen is stored in the lungs at FRC in a healthy adult?

Answer 1

Approximately 395 mL (based on 13% O₂ in 3 L of FRC).

Question 2

How much oxygen does a resting adult typically consume per minute?

Answer 2

Around 250 mL/min.

Question 3

Why is the oxygen stored in FRC referred to as a buffer?

Answer 3

It acts as a reserve to maintain oxygenation between breaths or during apnea.

Question 4

What is the alveolar PO₂ between breaths?

Answer 4

About 100 mmHg.

Question 5

What determines the rate at which alveolar PO₂ falls during apnea?

Answer 5

The patient’s oxygen consumption rate and lung volume (FRC).

Question 6

What factors reduce FRC and compromise oxygen buffering?

Answer 6

Anesthesia, supine position, muscle paralysis, abdominal pressure, and obesity.

Question 7

How does anesthesia reduce FRC?

Answer 7

It causes loss of muscle tone, diaphragmatic elevation, and reduced chest wall compliance.

Question 8

Why is preoxygenation important before induction of anesthesia?

Answer 8

It replaces nitrogen in the lungs with oxygen, maximizing the O₂ reservoir to delay desaturation during apnea.

Question 9

How does preoxygenation affect alveolar composition?

Answer 9

It increases the fraction of O₂ in the alveoli to near 100%, eliminating nitrogen.

Question 10

What happens to alveolar PO₂ if FRC drops and O₂ consumption remains constant?

Answer 10

Alveolar PO₂ falls more rapidly, increasing the risk of hypoxemia.

Question 11

What is the primary stimulus for central respiratory drive under normal conditions?

Answer 11

Arterial CO₂ (PaCO₂) — detected by central chemoreceptors in the medulla.

Question 12

What does the CO₂ response curve represent?

Answer 12

The relationship between PaCO₂ and minute ventilation.

Question 13

What is the apneic threshold?

Answer 13

The PaCO₂ level below which spontaneous breathing ceases — usually around 32–34 mmHg.

Question 14

How do opioids affect the CO₂ response curve?

Answer 14

They cause a right shift and decreased slope, meaning more CO₂ is required to stimulate ventilation, and the response is blunted.

Question 15

How does hypoxemia affect the CO₂ response curve?

Answer 15

In moderate hypoxia, ventilation increases for a given PaCO₂ — steeper slope. In severe hypoxia, it may depress ventilation.

Question 16

What causes a left shift in the CO₂ response curve?

Answer 16

Conditions like pregnancy, metabolic acidosis, or anxiety, which increase ventilatory drive.

Question 17

How does gravity affect pulmonary perfusion?

Answer 17

Blood flow is highest at the base of the lung due to gravitational effects on hydrostatic pressure.

Question 18

What are West’s Zones of the Lung based on?

Answer 18

The relationships between alveolar pressure, arterial pressure, and venous pressure.

Question 19

Describe West Zone 1 physiology.

Answer 19

Alveolar pressure > arterial > venous → no perfusion (physiologic dead space).

Question 20

Describe West Zone 2 physiology.

Answer 20

Arterial > alveolar > venous → intermittent perfusion (‘waterfall’ effect).

Question 21

Describe West Zone 3 physiology.

Answer 21

Arterial > venous > alveolar → continuous perfusion — most normal gas exchange occurs here.

Question 22

What is hypoxic pulmonary vasoconstriction (HPV)?

Answer 22

A protective mechanism where pulmonary arterioles constrict in response to low alveolar PO₂ to redirect blood to better-ventilated areas.

Question 23

How do volatile anesthetics affect HPV?

Answer 23

They blunt HPV, leading to worsened V/Q mismatch under general anesthesia.

Question 24

Why do patients with fever desaturate more quickly?

Answer 24

Increased temperature causes a right shift of the O₂ curve → more unloading in tissues but faster depletion of O₂ stores.

Question 25

Why do hypothermic patients retain O₂ in their blood longer?

Answer 25

Left shift from low temperature → increased affinity, less tissue unloading.

Question 26

Why does a patient with carbon monoxide poisoning have normal SpO₂ but tissue hypoxia?

Answer 26

CO binds Hb with high affinity → falsely elevates pulse ox readings, reduces actual O₂ delivery.

Question 27

What is the danger of anemia despite a normal PO₂ and O₂ sat?

Answer 27

Low hemoglobin concentration = reduced O₂ carrying capacity → low total O₂ content (CaO₂).

Question 28

What causes a right shift of the O₂ dissociation curve?

Answer 28

↑CO₂, ↓pH, ↑Temperature, ↑2,3-BPG, exercise, acidosis — improves tissue unloading.

Question 29

What causes a left shift?

Answer 29

↓CO₂, ↑pH, ↓Temp, ↓2,3-BPG, CO poisoning, fetal Hb — reduces unloading.

Question 30

What is the equal pressure point (EPP)?

Answer 30

The point in the airway during forced expiration where airway pressure equals pleural pressure — beyond this, the airway collapses.

Question 31

Where is the EPP located in healthy individuals?

Answer 31

In the larger, cartilaginous airways — which resist collapse.

Question 32

What happens to the EPP in emphysema?

Answer 32

It shifts distally (toward smaller airways), which are not supported by cartilage → early airway collapse during expiration.

Question 33

How does dynamic compression limit expiratory flow?

Answer 33

Once pleural pressure exceeds airway pressure, flow becomes effort-independent — pushing harder doesn't increase flow.

Question 34

Why is FEV1 effort-independent in late expiration?

Answer 34

Because increased effort raises both alveolar and pleural pressure equally, not improving flow beyond the EPP.

Question 35

How does this concept explain the scooped-out flow-volume loop in COPD?

Answer 35

Loss of elastic recoil shifts the EPP distally → small airway collapse → reduced flow in effort-independent phase.

Question 36

How is Functional Residual Capacity (FRC) related to preoxygenation time during anesthesia?

Answer 36

FRC is the oxygen reservoir between breaths; higher FRC = longer time before desaturation during apnea.

Question 37

How is flow-volume loop shape related to the equal pressure point (EPP) and elastic recoil?

Answer 37

In disease (e.g., emphysema), the EPP shifts distally due to lost recoil, causing early airway collapse and a scooped loop.

Question 38

How does the CO₂ dissociation curve and Haldane effect connect to V/Q mismatch and ETCO₂ readings?

Answer 38

In V/Q mismatch or increased dead space, less CO₂ reaches alveoli, lowering ETCO₂ despite normal or high PaCO₂.

Question 39

How do 2,3-BPG, pH, and temperature relate to oxygen delivery in exercise or anemia?

Answer 39

These cause a right shift of the O₂ dissociation curve → more O₂ released to tissues when metabolic demand is high or hemoglobin is low.

Question 40

What is the connection between surfactant function and lung compliance?

Answer 40

Surfactant reduces surface tension, maintaining alveolar stability and preventing collapse → increases compliance, especially at low lung volumes.

Question 41

How does diffusion-limited O₂ exchange connect to exercise, altitude, and pulmonary fibrosis?

Answer 41

In these conditions, gas exchange becomes limited by membrane thickness or time, not perfusion → O₂ fails to equilibrate.

Question 42

What is the relationship between pleural pressure and alveolar stability during ventilation?

Answer 42

Negative pleural pressure keeps alveoli open; loss (e.g., pneumothorax) or inadequate positive pressure can lead to alveolar collapse.

Question 43

How does understanding pulmonary compliance affect ventilator settings?

Answer 43

Low compliance (e.g., in ARDS) requires higher pressures to deliver volume → PEEP is needed to prevent atelectasis and improve V/Q matching.

Question 44

How are Bohr and Haldane effects complementary in gas exchange?

Answer 44

Bohr effect: CO₂/H⁺ promotes O₂ unloading in tissues; Haldane effect: Deoxygenated Hb promotes CO₂ uptake in tissues and CO₂ unloading in lungs.

Question 45

Why does ETCO₂ underestimate PaCO₂ in dead space ventilation?

Answer 45

Because non-perfused alveoli dilute exhaled CO₂, lowering ETCO₂ — the wider the gradient, the more dead space or V/Q mismatch is present.