Hemodynamic monitoring principles — MCQs

10 questions

What is the primary mechanism for maintaining constant cerebral blood flow despite changes in systemic blood pressure?

Which of the following physiologic changes decreases pulmonary vascular resistance (PVR)?

A 57-year-old man is admitted to the burn unit after he was brought to the emergency room following an accidental fire in his house. His past medical history is unknown due to his current clinical condition. Currently, his blood pressure is 75/40 mmHg, pulse rate is 140/min, and respiratory rate is 17/min. The patient is subsequently intubated and started on aggressive fluid resuscitation. A Swan-Ganz catheter is inserted to clarify his volume status. Which of the following hemodynamic parameters would you expect to see in this patient?

An 83-year-old male presents with dyspnea, orthopnea, and a chest radiograph demonstrating pulmonary edema. A diagnosis of congestive heart failure is considered. The following clinical measurements are obtained: 100 bpm heart rate, 0.2 mL O2/mL systemic blood arterial oxygen content, 0.1 mL O2/mL pulmonary arterial oxygen content, and 400 mL O2/min oxygen consumption. Using the above information, which of the following values represents this patient's cardiac stroke volume?

A 46-year-old male was found unconscious in the field and brought to the emergency department by EMS. The patient was intubated in transit and given a 2 liter bolus of normal saline. On arrival, the patient's blood pressure is 80/60 mmHg and temperature is 37.5°C. Jugular veins are flat and capillary refill time is 4 seconds. Vascular parameters are measured and are as follows: Cardiac index - Low Pulmonary capillary wedge pressure (PCWP) - Low Systemic vascular resistance - High Which of the following is the most likely diagnosis?

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?

A 69-year-old woman is admitted to the hospital with substernal, crushing chest pain. She is emergently moved to the cardiac catheterization lab where she undergoes cardiac angiography. Angiography reveals that the diameter of her left anterior descending artery (LAD) is 50% of normal. If her blood pressure, LAD length, and blood viscosity have not changed, which of the following represents the most likely change in LAD flow from baseline?

A 34-year-old male is brought to the emergency department by fire and rescue following a motor vehicle accident in which the patient was an unrestrained driver. The paramedics report that the patient was struck from behind by a drunk driver. He was mentating well at the scene but complained of pain in his abdomen. The patient has no known past medical history. In the trauma bay, his temperature is 98.9°F (37.2°C), blood pressure is 86/51 mmHg, pulse is 138/min, and respirations are 18/min. The patient is somnolent but arousable to voice and pain. His lungs are clear to auscultation bilaterally. He is diffusely tender to palpation on abdominal exam with bruising over the left upper abdomen. His distal pulses are thready, and capillary refill is delayed bilaterally. Two large-bore peripheral intravenous lines are placed to bolus him with intravenous 0.9% saline. Chest radiograph shows multiple left lower rib fractures. Which of the following parameters is most likely to be seen in this patient?

A 34-year-old woman comes to a physician for a routine health maintenance examination. She moved to Denver 1 week ago after having lived in New York City all her life. She has no history of serious illness and takes no medications. Which of the following sets of changes is most likely on analysis of a blood sample obtained now compared to prior to her move? Erythropoietin level | O2 saturation | Plasma volume

Q10

A 21-year-old lacrosse player comes to the doctor for an annual health assessment. She does not smoke or drink alcohol. She is 160 cm (5 ft 3 in) tall and weighs 57 kg (125 lb); BMI is 22 kg/m2. Pulmonary function tests show an FEV1 of 90% and an FVC of 3600 mL. Whole body plethysmography is performed to measure airway resistance. Which of the following structures of the respiratory tree is likely to have the highest contribution to total airway resistance?

Hemodynamic monitoring principles US Medical PG Practice Questions and MCQs

Question 1: What is the primary mechanism for maintaining constant cerebral blood flow despite changes in systemic blood pressure?

A. Endothelial factors
B. Baroreceptor reflex
C. Myogenic autoregulation (Correct Answer)
D. Metabolic control

Explanation: ***Myogenic autoregulation*** - **Myogenic autoregulation** is the intrinsic ability of vascular smooth muscle to contract when stretched by increased blood pressure, thereby maintaining a constant cerebral blood flow. - This mechanism operates within a specific range of mean arterial pressures (typically **60-150 mmHg**) to prevent both hypoperfusion and hyperperfusion of the brain. *Endothelial factors* - Endothelial cells release various vasoactive substances like **nitric oxide** and **endothelin**, which influence vascular tone. - While important for local blood flow regulation, these factors play a secondary role to myogenic autoregulation in maintaining constant cerebral blood flow against systemic pressure changes. *Baroreceptor reflex* - The **baroreceptor reflex** primarily controls systemic blood pressure by regulating heart rate and peripheral vascular resistance. - It does not directly regulate cerebral blood flow stability in response to systemic pressure changes; its main role is to stabilize the overall systemic arterial pressure. *Metabolic control* - **Metabolic control** regulates cerebral blood flow in response to the brain's metabolic demands, primarily by sensing local concentrations of **CO2**, **pH**, and **oxygen**. - While essential for matching blood supply to neuronal activity, it is not the primary mechanism for maintaining cerebral blood flow despite changes in systemic blood pressure.

Question 2: Which of the following physiologic changes decreases pulmonary vascular resistance (PVR)?

A. Inhaling the inspiratory reserve volume (IRV)
B. Exhaling the entire vital capacity (VC)
C. Exhaling the expiratory reserve volume (ERV)
D. Breath holding maneuver at functional residual capacity (FRC)
E. Inhaling the entire vital capacity (VC) (Correct Answer)

Explanation: ***Inhaling the entire vital capacity (VC)*** - As lung volume increases from FRC to TLC (which includes inhaling the entire VC), alveolar vessels are **stretched open**, and extra-alveolar vessels are **pulled open** by the increased radial traction, leading to a decrease in PVR. - This **maximizes the cross-sectional area** of the pulmonary vascular bed, lowering resistance. *Inhaling the inspiratory reserve volume (IRV)* - While inhaling IRV increases lung volume, it's not the maximal inspiration of the entire VC where **PVR is typically at its lowest**. - PVR continues to decrease as lung volume approaches total lung capacity (TLC). *Exhaling the entire vital capacity (VC)* - Exhaling the entire vital capacity leads to very low lung volumes, where PVR significantly **increases**. - At low lung volumes, **alveolar vessels become compressed** and extra-alveolar vessels **narrow**, increasing resistance. *Exhaling the expiratory reserve volume (ERV)* - Exhaling the ERV results in a lung volume below FRC, which causes a **marked increase in PVR**. - This is due to the **compression of alveolar vessels** and decreased radial traction on extra-alveolar vessels. *Breath holding maneuver at functional residual capacity (FRC)* - At FRC, the PVR is at an **intermediate level**, not its lowest. - This is the point where the opposing forces affecting alveolar and extra-alveolar vessels are somewhat balanced, but not optimized for minimal resistance.

Question 3: A 57-year-old man is admitted to the burn unit after he was brought to the emergency room following an accidental fire in his house. His past medical history is unknown due to his current clinical condition. Currently, his blood pressure is 75/40 mmHg, pulse rate is 140/min, and respiratory rate is 17/min. The patient is subsequently intubated and started on aggressive fluid resuscitation. A Swan-Ganz catheter is inserted to clarify his volume status. Which of the following hemodynamic parameters would you expect to see in this patient?

A. Cardiac output: ↓, systemic vascular resistance: ↔, pulmonary artery wedge pressure: ↔
B. Cardiac output: ↑, systemic vascular resistance: ↑, pulmonary artery wedge pressure: ↔
C. Cardiac output: ↑, systemic vascular resistance: ↓, pulmonary artery wedge pressure: ↔
D. Cardiac output: ↓, systemic vascular resistance: ↑, pulmonary artery wedge pressure: ↓ (Correct Answer)
E. Cardiac output: ↔, systemic vascular resistance: ↔, pulmonary artery wedge pressure: ↔

Explanation: ***Cardiac output: ↓, systemic vascular resistance: ↑, pulmonary artery wedge pressure: ↓*** - The patient's **hypotension (75/40 mmHg)** and **tachycardia (140/min)**, combined with severe burns, indicate **hypovolemic shock** due to massive fluid loss from damaged capillaries. - In response to decreased cardiac output and hypovolemia, the body compensates by increasing **systemic vascular resistance (SVR)** to maintain perfusion to vital organs, and **pulmonary artery wedge pressure (PAWP)** will be low due to reduced intravascular volume. *Cardiac output: ↓, systemic vascular resistance: ↔, pulmonary artery wedge pressure: ↔* - This option incorrectly suggests that systemic vascular resistance and pulmonary artery wedge pressure would be normal, which is inconsistent with **hypovolemic shock**. - In shock, the body's compensatory mechanisms would lead to significant changes in SVR and PAWP, not maintain them at baseline. *Cardiac output: ↑, systemic vascular resistance: ↑, pulmonary artery wedge pressure: ↔* - Increased cardiac output is usually seen in **distributive shock** (e.g., septic shock) where vasodilation leads to reduced SVR, not increased SVR as suggested here. - An elevated SVR coupled with an increased cardiac output would typically result in a higher blood pressure unless there is a compensatory drop in other parameters. *Cardiac output: ↑, systemic vascular resistance: ↓, pulmonary artery wedge pressure: ↔* - This pattern (high cardiac output, low SVR) is characteristic of **distributive shock**, such as **septic shock** or anaphylactic shock, rather than the hypovolemic shock expected in a burn patient. - Severe burns primarily cause massive fluid shifts, leading to hypovolemia and a reduced cardiac output, not an elevated one. *Cardiac output: ↔, systemic vascular resistance: ↔, pulmonary artery wedge pressure: ↔* - This scenario represents **normal hemodynamic parameters**, which would not be expected in a patient experiencing severe shock from extensive burns. - The patient's clinical presentation (hypotension, tachycardia) clearly indicates a state of hemodynamic instability.

Question 4: An 83-year-old male presents with dyspnea, orthopnea, and a chest radiograph demonstrating pulmonary edema. A diagnosis of congestive heart failure is considered. The following clinical measurements are obtained: 100 bpm heart rate, 0.2 mL O2/mL systemic blood arterial oxygen content, 0.1 mL O2/mL pulmonary arterial oxygen content, and 400 mL O2/min oxygen consumption. Using the above information, which of the following values represents this patient's cardiac stroke volume?

A. 30 mL/beat
B. 70 mL/beat
C. 40 mL/beat (Correct Answer)
D. 60 mL/beat
E. 50 mL/beat

Explanation: ***40 mL/beat*** - First, calculate cardiac output (CO) using the **Fick principle**: CO = Oxygen Consumption / (Arterial O2 content - Venous O2 content). Here, CO = 400 mL O2/min / (0.2 mL O2/mL - 0.1 mL O2/mL) = 400 mL O2/min / 0.1 mL O2/mL = **4000 mL/min**. - Next, calculate stroke volume (SV) using the formula: SV = CO / Heart Rate. Given a heart rate of 100 bpm, SV = 4000 mL/min / 100 beats/min = **40 mL/beat**. *30 mL/beat* - This answer would result if there was an error in calculating either the **cardiac output** or if the **arteriovenous oxygen difference** was overestimated. - A stroke volume of 30 mL/beat with a heart rate of 100 bpm would yield a cardiac output of 3 L/min, which is sub-physiologic for an oxygen consumption of 400 mL/min given the provided oxygen content values. *70 mL/beat* - This stroke volume is higher than calculated and would imply either a significantly **lower heart rate** or a much **higher cardiac output** than derived from the Fick principle with the given values. - A stroke volume of 70 mL/beat at a heart rate of 100 bpm would mean a cardiac output of 7 L/min, which is inconsistent with the provided oxygen consumption and arteriovenous oxygen difference. *60 mL/beat* - This value is higher than the correct calculation, suggesting an error in the initial calculation of **cardiac output** or the **avO2 difference**. - To get 60 mL/beat, the cardiac output would need to be 6000 mL/min, which would mean an avO2 difference of 0.067 mL O2/mL, not 0.1 mL O2/mL. *50 mL/beat* - This stroke volume would result from an incorrect calculation of the **cardiac output**, potentially from a slight miscalculation of the **arteriovenous oxygen difference**. - A stroke volume of 50 mL/beat at 100 bpm would mean a cardiac output of 5 L/min, requiring an avO2 difference of 0.08 mL O2/mL, which is not consistent with the given values.

Question 5: A 46-year-old male was found unconscious in the field and brought to the emergency department by EMS. The patient was intubated in transit and given a 2 liter bolus of normal saline. On arrival, the patient's blood pressure is 80/60 mmHg and temperature is 37.5°C. Jugular veins are flat and capillary refill time is 4 seconds. Vascular parameters are measured and are as follows: Cardiac index - Low Pulmonary capillary wedge pressure (PCWP) - Low Systemic vascular resistance - High Which of the following is the most likely diagnosis?

A. Septic shock
B. Anaphylactic shock
C. Cardiogenic shock
D. Hypovolemic shock (Correct Answer)
E. Neurogenic shock

Explanation: ***Hypovolemic shock*** - The patient presents with **hypotension**, **flat jugular veins**, **prolonged capillary refill**, and a **low cardiac index** and **low pulmonary capillary wedge pressure (PCWP)**, all indicative of inadequate intravascular volume. - The **high systemic vascular resistance** is a compensatory mechanism to maintain blood pressure in the setting of decreased circulating volume. *Septic shock* - Septic shock typically presents with **vasodilation**, leading to a **low systemic vascular resistance**, which contradicts the findings in this patient. - While patients can be hypotensive, the vascular parameters, especially SVR, do not align with septic shock. *Anaphylactic shock* - This type of shock is characterized by widespread **vasodilation** and increased capillary permeability, leading to a **low systemic vascular resistance** and often significant **edema** or **urticaria**, none of which are suggested here. - While it can cause hypotension and low PCWP due to fluid shifts, the high SVR makes it less likely. *Cardiogenic shock* - Cardiogenic shock is characterized by **pump failure**, leading to a **low cardiac index** but a **high PCWP** due to fluid backup in the pulmonary circulation. - This directly contrasts the patient's low PCWP. *Neurogenic shock* - Neurogenic shock involves a loss of **sympathetic tone**, resulting in widespread **vasodilation** and a **low systemic vascular resistance**, often accompanied by **bradycardia**. - The high SVR in this patient rules out neurogenic shock.

Question 6: 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?

A. Probe A: -6 mm Hg; Probe B: 0 mm Hg (Correct Answer)
B. Probe A: 0 mm Hg; Probe B: -1 mm Hg
C. Probe A: -4 mm Hg; Probe B: 0 mm Hg
D. Probe A: -4 mm Hg; Probe B: -1 mm Hg
E. Probe A: -6 mm Hg; Probe B: -1 mm Hg

Explanation: ***Probe A: -6 mm Hg; Probe B: 0 mm Hg*** - At the **end of inspiration**, the **intrapleural pressure (Probe A)** is at its most negative, typically around -6 to -8 cm H2O (equivalent to -4 to -6 mmHg), reflecting the maximum expansion of the thoracic cavity. - At the **end of inspiration**, just before exhalation begins, there is **no airflow**, so the **intrapulmonary pressure (Probe B)** equalizes with atmospheric pressure, resulting in a 0 mm Hg reading. *Probe A: 0 mm Hg; Probe B: -1 mm Hg* - An **intrapleural pressure of 0 mm Hg** would indicate a **pneumothorax** since it should always be negative to prevent lung collapse. - An **intrapulmonary pressure of -1 mm Hg** would indicate that **inspiration is still ongoing**, as air would be flowing into the lungs. *Probe A: -4 mm Hg; Probe B: 0 mm Hg* - While an **intrapulmonary pressure of 0 mm Hg** is correct at the end of inspiration, an **intrapleural pressure of -4 mm Hg** is typical for the **end of expiration (Functional Residual Capacity)** during quiet breathing, not the end of inspiration. - The **intrapleural pressure becomes more negative** during inspiration due to increased thoracic volume, so -4 mm Hg would be insufficient. *Probe A: -4 mm Hg; Probe B: -1 mm Hg* - An **intrapleural pressure of -4 mm Hg** is the normal pressure at the **end of expiration**, not the end of inspiration, where it becomes more negative. - An **intrapulmonary pressure of -1 mm Hg** indicates that **inspiration is still in progress**, not at its end, as air would still be flowing into the lungs. *Probe A: -6 mm Hg; Probe B: -1 mm Hg* - While an **intrapleural pressure of -6 mm Hg** is consistent with the end of inspiration, an **intrapulmonary pressure of -1 mm Hg** means that **airflow is still occurring into the lungs**. - At the **very end of inspiration**, just before the start of exhalation, airflow momentarily ceases, and intrapulmonary pressure becomes zero relative to the atmosphere.

Question 7: A 69-year-old woman is admitted to the hospital with substernal, crushing chest pain. She is emergently moved to the cardiac catheterization lab where she undergoes cardiac angiography. Angiography reveals that the diameter of her left anterior descending artery (LAD) is 50% of normal. If her blood pressure, LAD length, and blood viscosity have not changed, which of the following represents the most likely change in LAD flow from baseline?

A. Decreased by 93.75% (Correct Answer)
B. Increased by 6.25%
C. Decreased by 25%
D. Decreased by 87.5%
E. Increased by 25%

Explanation: ***Decreased by 93.75%*** - This option is correct based on Poiseuille's Law, which states that flow is proportional to the **fourth power of the radius (r^4)**. A 50% decrease in diameter means a 50% decrease in radius (0.5r). - The new flow would be (0.5)^4 = 0.0625 times the original flow. Therefore, the decrease in flow is 1 - 0.0625 = 0.9375, or **93.75%**. *Increased by 6.25%* - This answer incorrectly suggests an **increase** in flow, which is contrary to the effect of a narrowed artery. - While 6.25% represents the new flow as a percentage of baseline (since 0.0625 = 6.25%), the vessel stenosis causes a **decrease**, not an increase in flow. *Decreased by 25%* - This calculation might arise from considering a linear relationship (e.g., radius decreases by 50%, so flow decreases by 50% of 50%, which is incorrect). - It does not account for the **fourth power relationship** between radius and flow according to Poiseuille's Law. *Decreased by 87.5%* - This percentage represents a calculation error, likely from misapplying the fourth power relationship or confusing the calculation steps. - It does not accurately reflect the dramatic reduction in flow caused by a 50% reduction in vessel diameter. *Increased by 25%* - This option implies a significant increase in blood flow, which would not happen with a **stenosed artery**. - It completely contradicts the physiological response to a **narrowed vessel**.

Question 8: A 34-year-old male is brought to the emergency department by fire and rescue following a motor vehicle accident in which the patient was an unrestrained driver. The paramedics report that the patient was struck from behind by a drunk driver. He was mentating well at the scene but complained of pain in his abdomen. The patient has no known past medical history. In the trauma bay, his temperature is 98.9°F (37.2°C), blood pressure is 86/51 mmHg, pulse is 138/min, and respirations are 18/min. The patient is somnolent but arousable to voice and pain. His lungs are clear to auscultation bilaterally. He is diffusely tender to palpation on abdominal exam with bruising over the left upper abdomen. His distal pulses are thready, and capillary refill is delayed bilaterally. Two large-bore peripheral intravenous lines are placed to bolus him with intravenous 0.9% saline. Chest radiograph shows multiple left lower rib fractures. Which of the following parameters is most likely to be seen in this patient?

A. Increased cardiac output
B. Increased mixed venous oxygen saturation
C. Decreased pulmonary capillary wedge pressure (Correct Answer)
D. Decreased systemic vascular resistance
E. Increased right atrial pressure

Explanation: ***Decreased pulmonary capillary wedge pressure*** - The patient presents with classic signs of **hemorrhagic shock** (hypotension, tachycardia, somnolence, abdominal bruising, thready pulses) due to trauma, likely involving the spleen or kidney given the left upper abdominal bruising and rib fractures. - **Decreased pulmonary capillary wedge pressure (PCWP)** is expected in hypovolemic shock because it reflects left atrial and left ventricular end-diastolic pressure, which will be low due to reduced venous return and intravascular volume. *Increased cardiac output* - In **hemorrhagic shock**, the body attempts to compensate by increasing heart rate, but overall **cardiac output is typically decreased** due to profound reduction in preload (venous return) from blood loss. - While heart rate is elevated, the stroke volume is severely diminished, leading to a net decrease in cardiac output despite compensatory efforts. *Increased mixed venous oxygen saturation* - **Mixed venous oxygen saturation (SvO2)** is generally **decreased in hemorrhagic shock** due to increased oxygen extraction by tissues. - Inadequate oxygen delivery to the tissues forces them to extract more oxygen from the blood, leading to a lower SvO2. *Decreased systemic vascular resistance* - In **hemorrhagic shock**, the body activates compensatory mechanisms, including generalized **vasoconstriction**, to maintain blood pressure and prioritize blood flow to vital organs. - This leads to an **increased systemic vascular resistance (SVR)**, not decreased, as reflected by the thready distal pulses and delayed capillary refill. *Increased right atrial pressure* - **Right atrial pressure (RAP)**, representing CVP, is typically **decreased in hemorrhagic shock** due to reduced circulating blood volume. - A lower RAP indicates decreased venous return to the heart, a hallmark of hypovolemia.

Question 9: A 34-year-old woman comes to a physician for a routine health maintenance examination. She moved to Denver 1 week ago after having lived in New York City all her life. She has no history of serious illness and takes no medications. Which of the following sets of changes is most likely on analysis of a blood sample obtained now compared to prior to her move? Erythropoietin level | O2 saturation | Plasma volume

A. ↑ unchanged unchanged
B. ↑ ↓ ↓ (Correct Answer)
C. Unchanged ↓ unchanged
D. ↓ unchanged ↑
E. Unchanged unchanged ↓

Explanation: ***↑ ↓ ↓*** - Moving to a high altitude like Denver (from sea level NYC) leads to **hypoxia**, which triggers increased **erythropoietin (EPO)** production to stimulate red blood cell formation. - The immediate physiological response to high altitude is a **decrease in arterial PO2** and thus **oxygen saturation**, along with a **reduction in plasma volume** due to increased diuresis and fluid shifts. *↑ unchanged unchanged* - While **erythropoietin** would increase due to hypoxia at higher altitudes, **oxygen saturation** would decrease, not remain unchanged. - **Plasma volume** also tends to decrease acutely at high altitudes, rather than staying unchanged. *Unchanged ↓ unchanged* - **Erythropoietin** would be expected to increase, not remain unchanged, as a compensatory mechanism to hypoxia. - While **oxygen saturation** would decrease, **plasma volume** typically decreases acutely, not remaining unchanged. *↓ unchanged ↑* - **Erythropoietin** would increase, not decrease, in response to the lower atmospheric oxygen. - Both **oxygen saturation** and **plasma volume** would decrease, not remain unchanged or increase, respectively. *Unchanged unchanged ↓* - **Erythropoietin** would increase, not remain unchanged, to stimulate red blood cell production in response to hypoxia. - **Oxygen saturation** would decrease, not remain unchanged, at higher altitudes.

Question 10: A 21-year-old lacrosse player comes to the doctor for an annual health assessment. She does not smoke or drink alcohol. She is 160 cm (5 ft 3 in) tall and weighs 57 kg (125 lb); BMI is 22 kg/m2. Pulmonary function tests show an FEV1 of 90% and an FVC of 3600 mL. Whole body plethysmography is performed to measure airway resistance. Which of the following structures of the respiratory tree is likely to have the highest contribution to total airway resistance?

A. Conducting bronchioles
B. Terminal bronchioles
C. Segmental bronchi (Correct Answer)
D. Respiratory bronchioles
E. Mainstem bronchi

Explanation: ***Segmental bronchi*** - In healthy individuals, **medium-sized bronchi** (including segmental and subsegmental bronchi, approximately generations 4-8) contribute approximately **80% of total airway resistance**. - While **Poiseuille's Law** states resistance is inversely proportional to radius to the fourth power (R ∝ 1/r⁴), the key factor is the **total cross-sectional area** and **degree of branching**. - Medium-sized bronchi have moderate individual resistance and **limited parallel branching**, making them the dominant site of resistance. - This is why diseases affecting medium-sized airways (e.g., asthma, bronchitis) cause significant increases in airway resistance. *Terminal bronchioles* - Although individual terminal bronchioles have small radii and high individual resistance, there are **millions of them arranged in parallel**. - With parallel resistances, total resistance decreases: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃... - The **massive number** of small airways means their collective resistance is actually quite **low** (~10-20% of total). - This is why small airways disease is called the "**silent zone**" - significant pathology can occur before detection. *Conducting bronchioles* - These airways also benefit from extensive **parallel branching**, reducing their contribution to total resistance. - They contribute less than medium-sized bronchi due to their large cumulative cross-sectional area. *Respiratory bronchioles* - Part of the **respiratory zone** with the largest total cross-sectional area in the lungs. - Minimal contribution to airway resistance due to enormous parallel arrangement. - Primary function is **gas exchange**, not air conduction. *Mainstem bronchi* - These large airways have **low individual resistance** due to large diameter. - Together with the trachea, they contribute approximately **20% of total airway resistance**. - Not the primary site despite being early in the airway tree.

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