Respiratory System

Respiratory System

Part 1 of 4 — Airway Structure & the Mechanics of Ventilation

Conducting vs. Respiratory Zones

• Airway defenses: nasal hairs, the mucociliary escalator (goblet-cell mucus swept upward by cilia), and alveolar macrophages.
• Alveoli are one cell thick (type I pneumocytes); type II pneumocytes secrete surfactant.

Breathing Is Boyle's Law

At constant temperature, $P_1 V_1 = P_2 V_2$ — changing thoracic volume changes alveolar pressure, and air flows down the pressure gradient.

Inspiration (active):

1. Diaphragm contracts (flattens downward) and external intercostals lift the ribs → thoracic volume ↑.
2. Alveolar pressure drops below atmospheric → air flows in.

Expiration (passive at rest):

1. Muscles relax; elastic recoil of lungs and chest wall shrinks thoracic volume.
2. Alveolar pressure rises above atmospheric → air flows out.
3. Forced expiration recruits abdominal muscles and internal intercostals (active).

The Pleural Seal — Why Lungs Follow the Chest Wall

• The intrapleural pressure between the visceral and parietal pleurae is negative relative to the alveoli (about −4 to −6 cmH2O at rest) because the lungs recoil inward while the chest wall springs outward.
• This negative pressure couples lung to chest wall like suction. Puncture the pleura (pneumothorax) → air rushes in, intrapleural pressure equalizes with atmosphere → the lung collapses from its own elastic recoil.

Surface Tension, Surfactant & Laplace

The fluid lining each alveolus creates surface tension that resists expansion. For a bubble-like alveolus:

$P = \frac{2T}{r}$

• Smaller radius → higher collapsing pressure: without correction, small alveoli would empty into big ones.
• Surfactant (a phospholipid mixture from type II cells) lowers surface tension — and lowers it MORE where the surfactant molecules are crowded (small alveoli) — stabilizing alveoli of different sizes and increasing compliance.

• The main surfactant phospholipid is dipalmitoylphosphatidylcholine (DPPC).

• Premature infants lacking surfactant develop neonatal respiratory distress syndrome: stiff lungs, collapsing alveoli.

Lung Volumes & Capacities (capacities = sums of volumes)

Spirometry cannot measure RV (or any capacity containing it: FRC, TLC) — you cannot exhale it into the machine.

Alveolar Ventilation Beats Total Ventilation

$\dot{V}_A = (TV - V_{dead}) \times f$

Rapid shallow breathing wastes a larger fraction of each breath on dead space: 250 mL × 24 breaths moves 6 L/min total but only $(250-150)\times 24 = 2.4$ L/min reaches alveoli, versus $(500-150)\times 12 = 4.2$ L/min at normal depth — same total ventilation, very different gas exchange.

Ventilation Mechanics 🎯

Worked Examples — Mechanics & Volumes

Question: Patient A breathes 400 mL × 20 breaths/min; Patient B breathes 800 mL × 10 breaths/min. Dead space is 150 mL in both. Compare total and alveolar ventilation.

Solution:

1. Total ventilation: A = 400 × 20 = 8.0 L/min; B = 800 × 10 = 8.0 L/min — identical.
2. Alveolar: A = (400 − 150) × 20 = 5.0 L/min; B = (800 − 150) × 10 = 6.5 L/min. ✓
3. Deep slow breathing wins because the fixed dead-space "tax" is paid fewer times per minute.

MCAT note: this is why panting (very shallow, very fast) exchanges little gas — a favorite experimental-passage manipulation.

Question: Two connected alveoli have radii $r$ and $2r$ with EQUAL surface tension $T$. Which way does air flow, and how does surfactant fix this?

Solution:

1. $P = 2T/r$: the small alveolus has TWICE the collapsing pressure of the large one.
2. Air flows from high to low pressure → the small alveolus empties into the large one and collapses.
3. Surfactant's concentration rises as an alveolus shrinks (same molecules, less area), so it lowers $T$ MORE in small alveoli — equalizing pressures and stabilizing both. ✓

Question: Given TV = 500 mL, IRV = 2900 mL, ERV = 1100 mL, RV = 1200 mL, find VC, FRC, and TLC.

Solution:

1. $VC = IRV + TV + ERV = 2900 + 500 + 1100 = 4500$ mL ✓
2. $FRC = ERV + RV = 1100 + 1200 = 2300$ mL ✓
3. $TLC = VC + RV = 4500 + 1200 = 5700$ mL ✓

Pattern: every "capacity" is a sum of two or more non-overlapping "volumes"; only the four volumes are primitive.

Key Takeaways — Part 1

• Conducting zone = dead space (~150 mL), no exchange; respiratory zone = alveoli; type II pneumocytes make surfactant.
• Breathing is Boyle's law: muscles change thoracic volume → alveolar pressure → passive airflow. Quiet expiration is passive elastic recoil.
• Negative intrapleural pressure couples lung to chest wall; pneumothorax breaks the seal and recoil collapses the lung.
• Laplace ($P = 2T/r$) predicts small-alveolus collapse; surfactant lowers surface tension preferentially in small alveoli.
• Alveolar ventilation $= (TV - V_{dead}) \times f$; deep slow breathing beats rapid shallow breathing. Spirometry cannot measure RV, FRC, or TLC.

• Low-yield extras: the main surfactant phospholipid is dipalmitoylphosphatidylcholine (DPPC).

Respiratory System

Part 2 of 4 — Gas Exchange & Transport (O2, CO2 & Hemoglobin)

Partial Pressures Drive Everything

Dalton's law: each gas in a mixture exerts its own partial pressure, $P_x = F_x \times P_{total}$. Henry's law: the amount of gas dissolved in liquid is proportional to its partial pressure. Gases always diffuse down partial-pressure gradients — never "toward where they're needed."

• Alveolar $P_{O_2}$ is below atmospheric because inspired air is humidified (water vapor dilutes it) and mixes with CO2-rich residual gas.
• At the alveolus: O2 moves alveolus → blood; CO2 moves blood → alveolus. At the tissues, both gradients reverse.

Diffusion Across the Alveolar Membrane

Fick's law of diffusion (for the membrane):

$\text{Rate} \propto \frac{A \cdot D \cdot \Delta P}{T}$

• $A$ = surface area (emphysema destroys alveolar walls → ↓A), $T$ = membrane thickness (fibrosis, edema → ↑T), $D$ = diffusivity, $\Delta P$ = partial-pressure gradient.
• CO2 diffuses ~20× faster than O2 (much higher solubility), so diffusion disease impairs O2 exchange first.

Hemoglobin and the Oxygen Dissociation Curve

Dissolved O2 is tiny (~1.5%); ~98.5% rides on hemoglobin (4 subunits, each with a heme Fe$^{2+}$; up to 4 O2).

• Cooperativity: binding one O2 shifts Hb toward the high-affinity R state, making the next binding easier → a sigmoidal curve. Myoglobin (single subunit, muscle storage) is hyperbolic, with higher affinity — it takes O2 FROM Hb.
• The flat top (loading plateau) means arterial saturation stays ~97% even if alveolar $P_{O_2}$ dips modestly; the steep middle means large unloading at tissue pressures (~75% saturation at $P_{O_2} = 40$).

Right Shift = Easier Unloading (the Bohr Effect)

A right shift lowers Hb's O2 affinity (higher $P_{50}$), releasing more O2 at the same tissue $P_{O_2}$. Right-shifters — all signatures of active tissue:

• ↑ $P_{CO_2}$, ↑ H$^+$ (lower pH) — the Bohr effect
• ↑ temperature
• ↑ 2,3-BPG (chronic hypoxia, altitude, anemia)

Left shift (higher affinity, harder unloading): the opposites, plus fetal hemoglobin (HbF binds 2,3-BPG poorly → pulls O2 across the placenta from maternal HbA) and carbon monoxide.

• CO poisoning is a double hit: CO binds heme ~250× more tightly than O2 (↓ carrying capacity) AND shifts the remaining sites left (↓ unloading). Arterial $P_{O_2}$ — dissolved gas — reads normal.

CO2 Transport — Three Forms

The bicarbonate reaction is the hinge of blood pH:

$CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$

• Haldane effect (Bohr's mirror): deoxygenated Hb is a better buffer and carbamino carrier, so venous blood carries extra CO2; oxygenation in the lungs pushes CO2 off for exhalation.
• Everything reverses in the lungs: HCO3$^-$ re-enters RBCs, recombines with H$^+$, and carbonic anhydrase regenerates CO2 for exhalation.

Gas Exchange & Transport 🎯

Worked Examples — Curves & Transport

Question: A passage shows Hb saturation curves at pH 7.2 and pH 7.4. At a tissue $P_{O_2}$ of 40 mmHg, the pH 7.2 curve reads 60% saturation versus 75% at pH 7.4. What does this mean physiologically?

Solution:

1. Lower pH → curve shifted right (Bohr effect): at the same $P_{O_2}$, Hb holds LESS O2.
2. Saturation dropping from 75% to 60% means an extra 15% of carried O2 is released to the tissue.
3. Acidic microenvironments are exactly where metabolism is fastest (CO2 and lactic acid production) — the shift automatically delivers more O2 where demand is highest. ✓

Trap: a right shift is "bad" for loading only in the lungs — and the plateau is so flat there that loading barely suffers.

Question: Follow a CO2 molecule produced in exercising muscle until it is exhaled, naming each conversion.

Solution:

1. CO2 diffuses muscle → interstitium → plasma → into an RBC (all down pressure gradients).
2. Carbonic anhydrase: CO2 + H2O → H2CO3 → H$^+$ + HCO3$^-$. HCO3$^-$ swaps out for Cl$^-$ (chloride shift); deoxy-Hb buffers the H$^+$ (Haldane).
3. In pulmonary capillaries everything reverses: HCO3$^-$ re-enters, joins H$^+$, carbonic anhydrase regenerates CO2 → diffuses into the alveolus ($P_{CO_2}$ 46 → 40) → exhaled. ✓

Question: A patient's hemoglobin falls from 15 to 7.5 g/dL (severe anemia), but arterial saturation remains 98%. Explain why the patient is still hypoxic, using $O_2 \text{ content} = 1.34 \times [Hb] \times S_{a}$ (ignoring dissolved gas).

Solution:

1. Normal: $1.34 \times 15 \times 0.98 \approx 19.7$ mL O2/dL.
2. Anemic: $1.34 \times 7.5 \times 0.98 \approx 9.9$ mL O2/dL — half the content at perfect saturation.
3. Saturation is a percentage of AVAILABLE seats; anemia removes seats. Delivery = CO x content, so the heart compensates with tachycardia, and 2,3-BPG rises to right-shift the curve. ✓

Passage skill: saturation, partial pressure, and content are three different axes — identify which one the figure actually plots before answering.

Key Takeaways — Part 2

• Gases move down partial-pressure gradients (Dalton + Henry); memorize the anchor values: alveolar 100/40, venous 40/46 mmHg for O2/CO2.
• Diffusion rate ∝ area × gradient / thickness — emphysema kills area, fibrosis/edema add thickness; CO2 diffuses ~20× faster than O2.
• Hb: sigmoidal (cooperativity, T→R); myoglobin: hyperbolic, higher affinity. Right shift (↑CO2, ↑H⁺, ↑temp, ↑2,3-BPG) = easier unloading; HbF and CO shift left.
• CO poisoning: normal arterial $P_{O_2}$, low content, impaired unloading.
• CO2 travels ~70% as bicarbonate (carbonic anhydrase + chloride shift), ~20% carbamino, ~7-10% dissolved; Haldane effect couples O2 loading to CO2 release.

Respiratory System

Part 3 of 4 — Control of Breathing, Acid–Base & Ventilation–Perfusion Matching

The Respiratory Control Center

• Rhythm is generated in the medulla oblongata; pontine centers fine-tune the pattern. Breathing is automatic but can be voluntarily overridden (cortex) — up to the point where chemoreceptor drive wins (you cannot hold your breath to death).
• Motor output travels via the phrenic nerve to the diaphragm.

• The medullary rhythm generators are the dorsal and ventral respiratory groups; the phrenic nerve arises from spinal roots C3–C5.

Chemoreceptors — CO2 Is the Boss

• The carotid bodies signal via the glossopharyngeal nerve (CN IX), the aortic bodies via the vagus (CN X).

• Blood H$^+$ crosses the blood–brain barrier poorly, but CO2 crosses freely and is hydrated to H$^+$ in the CSF — so central chemoreceptors read arterial CO2 with an H$^+$ electrode.
• Normal drive: a rise in arterial $P_{CO_2}$ of just a few mmHg sharply increases ventilation. Hypoxic drive matters at altitude and in chronic CO2 retainers.

Ventilation Sets Blood pH — The Respiratory Handle

Because $CO_2 + H_2O \rightleftharpoons H^+ + HCO_3^-$, ventilation directly controls acid:

• The system also compensates metabolic disorders: metabolic acidosis (e.g., diabetic ketoacidosis, diarrhea) → chemoreceptors drive deep, rapid Kussmaul breathing → CO2 falls → pH partially corrected. Metabolic alkalosis → mild hypoventilation.
• Compensation moves pH toward normal but never overshoots; full correction of metabolic problems belongs to the kidneys (days).

Ventilation–Perfusion (V/Q) Matching

Gas exchange needs air AND blood at the same alveolus. $V/Q$ = alveolar ventilation / capillary perfusion (ideal ≈ 0.8 overall).

• Upright lung: gravity sends more blood AND more ventilation to the base, but perfusion increases faster → V/Q is high at the apex, low at the base.

• Typical upright values: V/Q about 3 at the apex and about 0.6 at the base.

• Shunt (V/Q → 0): perfused but not ventilated (mucus plug, pneumonia fluid). Blood passes unoxygenated; giving 100% O2 helps little because the gas never reaches those alveoli.
• Dead space (V/Q → ∞): ventilated but not perfused (pulmonary embolism). Air moves but no blood collects it.

Hypoxic Pulmonary Vasoconstriction — The Opposite Rule

• Systemic arterioles dilate in hypoxia (feed the starving tissue).
• Pulmonary arterioles constrict where alveolar O2 is low — diverting blood AWAY from poorly ventilated alveoli toward well-ventilated ones. This is self-correction of V/Q mismatch.
• Cost: global alveolar hypoxia (altitude, COPD) constricts the whole pulmonary tree → pulmonary hypertension → right-ventricular strain.

• Right-heart failure caused by lung disease is called cor pulmonale.

Control & V/Q 🎯

Worked Examples — Control & Matching

Question: A diabetic patient presents with pH 7.25, low bicarbonate, $P_{CO_2}$ = 28 mmHg (normal 40), and deep rapid breathing. Identify the primary disorder and the role of the low CO2.

Solution:

1. pH 7.25 = acidemia. Low HCO3$^-$ points to a metabolic acidosis (ketoacids consume bicarbonate buffer).
2. If low CO2 were the PRIMARY problem, pH would be HIGH (respiratory alkalosis). It isn't — so the low CO2 is respiratory compensation: chemoreceptors sense H$^+$ and drive Kussmaul hyperventilation.
3. Compensation is partial — pH approaches but does not reach 7.4; definitive correction requires treating the ketoacidosis (and renal acid excretion). ✓

Rule of thumb: find the abnormality that EXPLAINS the pH direction; the other value moving the "wrong way" for its own disorder is compensation.

Question: A lobe filled with pneumonia exudate is perfused but unventilated. Supplemental 100% O2 barely raises the patient's arterial O2 content. Why?

Solution:

1. Shunted blood never contacts alveolar gas — its O2 content is fixed at venous levels regardless of inspired O2.
2. Blood from HEALTHY alveoli is already on the flat plateau (~98% saturated); extra O2 adds only a little dissolved gas (Henry's law), not bound O2.
3. Mixing near-saturated blood with fixed venous shunt blood yields low arterial content that O2 therapy cannot fix — the hallmark distinguishing shunt from other causes of hypoxemia. ✓

Question: In an experiment, a subject breathes air containing 5% CO2 for two minutes. Predict ventilation, arterial pH, and which chemoreceptors respond.

Solution:

1. Inspired CO2 raises alveolar and arterial $P_{CO_2}$ → CO2 crosses into CSF → central chemoreceptors fire; peripheral bodies add to the response.
2. Ventilation rises steeply (CO2 is the most potent respiratory stimulus) — but cannot fully clear CO2 that keeps arriving in inspired air.
3. Arterial pH falls slightly (respiratory acidosis) until the exposure ends. ✓

Contrast: breathing 10% O2 (hypoxia, normal CO2) stimulates ONLY peripheral chemoreceptors, and only once $P_{O_2}$ nears ~60 mmHg — the resulting hyperventilation then LOWERS CO2, partially braking its own drive.

Key Takeaways — Part 3

• Medulla generates rhythm (phrenic nerve → diaphragm); CO2 (read as CSF H⁺ by central chemoreceptors) is the dominant drive; only peripheral carotid/aortic bodies sense low O2 (< ~60 mmHg).
• Hypoventilation → respiratory acidosis; hyperventilation → respiratory alkalosis; ventilation compensates metabolic disorders (Kussmaul in DKA) but never overshoots.
• V/Q ≈ 0.8 ideally; apex high V/Q, base low V/Q upright. Shunt = perfused/unventilated (O2-refractory hypoxemia); dead space = ventilated/unperfused (embolism).
• Pulmonary vessels CONSTRICT in hypoxia (opposite of systemic) to self-correct V/Q; global hypoxia → pulmonary hypertension → right-heart strain.

• Low-yield extras: the dorsal and ventral respiratory groups (medullary rhythm generators); phrenic nerve roots C3–C5; carotid bodies on CN IX, aortic bodies on CN X; typical V/Q values of ~3 at the apex and ~0.6 at the base; the term cor pulmonale for lung-driven right-heart failure.

Respiratory System

Part 4 of 4 — MCAT-Style Integration: Altitude, Exercise, Pathology & Passage Skills

High Altitude — A Complete Integration Story

At altitude, total pressure falls, so inspired $P_{O_2}$ falls (same 21% of a smaller number). The response unfolds on three timescales:

1. Minutes: low arterial $P_{O_2}$ (< ~60 mmHg) fires peripheral chemoreceptors → hyperventilation → CO2 blown off → respiratory alkalosis (which initially brakes the hypoxic drive).
2. Days: kidneys excrete HCO3$^-$ to normalize pH (renal compensation), unleashing fuller hyperventilation; 2,3-BPG rises, right-shifting the Hb curve to aid tissue unloading.
3. Weeks: hypoxia-driven EPO from the kidney raises hematocrit (more O2 content per liter); chronic pulmonary vasoconstriction can produce pulmonary hypertension.

Exercise vs. Altitude — A Classic Discrimination

Exercise hyperpnea is NOT driven by abnormal arterial gases — ventilation rises in lockstep with metabolism, keeping arterial values constant. A passage showing normal blood gases during heavy exercise is testing exactly this.

Obstructive vs. Restrictive Disease — Read the Spirometry

$\text{Key ratio:} \quad FEV_1 / FVC$

($FEV_1$ = volume expelled in the first second of a forced exhale; $FVC$ = total forced vital capacity.)

• Emphysema logic: destruction of alveolar walls (often elastase excess — smoking, or alpha-1-antitrypsin deficiency) → loss of elastic recoil → ↑ compliance, floppy airways collapse on exhalation, ↓ surface area for diffusion.
• Fibrosis logic: scarred interstitium → ↓ compliance (stiff), thickened membrane → ↓ diffusion; small but fast-emptying lungs (recoil is strong), hence the normal-to-high ratio.

Passage-Skill Checklist

• Identify the axis first: saturation vs. content vs. partial pressure; curves shifted vs. curves depressed (anemia lowers content, not saturation).
• Timescale sorting: chemoreflexes (seconds) → renal pH compensation (days) → EPO/hematocrit (weeks).
• Perturbation propagation: any change routes through $\dot{V}_A = (TV - V_{dead}) \times f$, Fick diffusion, Hb affinity, and V/Q — name the step that broke.
• Opposite-rules table: pulmonary vessels constrict in hypoxia (systemic dilate); CO2 mostly travels as HCO3$^-$ (O2 mostly on Hb); central chemoreceptors ignore O2.

Respiratory Integration 🎯

Worked Examples — Integration Passages

Question: A passage describes a young nonsmoker with emphysema and a deficiency of alpha-1-antitrypsin, a protease inhibitor. Connect the molecular defect to the spirometry pattern.

Solution:

1. Alpha-1-antitrypsin normally inhibits neutrophil elastase; without it, elastase digests alveolar elastin unchecked (smoking accelerates this by recruiting neutrophils and oxidizing the inhibitor).
2. Elastin loss → ↓ elastic recoil → ↑ compliance; alveolar walls coalesce → ↓ diffusion area (Fick).
3. Exhalation depends on recoil, so airflow out is slow and airways collapse: obstructive pattern — low $FEV_1/FVC$, air trapping (↑RV, ↑TLC). ✓

Question: A figure plots O2 CONTENT (mL/dL) versus $P_{O_2}$ for: (a) 50% anemia, and (b) 50% carboxyhemoglobin. Both plateau at half-normal content. How do the curves — and the patients — differ?

Solution:

1. Anemia: half the Hb, but each Hb behaves normally → half-height curve with normal shape and P50; unloading at tissues is proportionally normal, and 2,3-BPG helps further.
2. CO: half the sites occupied by CO AND the remaining sites left-shifted → half-height curve that also hugs its O2 longer (low P50); tissue unloading is doubly crippled.
3. Same content at the plateau, very different delivery — CO poisoning is worse than equivalent anemia. ✓

Skill: the plateau height reads capacity; the curve's position (P50) reads affinity. Grade figures on both.

Question: A passage reports a drug that selectively paralyzes the diaphragm by 50% in an awake subject. Predict tidal volume, respiratory rate, arterial CO2, and pH immediately after, before any compensation plateau.

Solution:

1. Weaker inspiratory pump → tidal volume falls.
2. Alveolar ventilation $(TV - V_{dead}) \times f$ falls disproportionately (dead space is fixed per breath) → CO2 accumulates: respiratory acidosis (↑$P_{CO_2}$, ↓pH).
3. Rising CO2 fires central chemoreceptors → rate increases, partially offsetting — but rapid shallow breathing pays the dead-space tax more often, limiting the rescue.
4. Net: low TV, high f, elevated CO2, depressed pH. ✓

Skill: always let the reflex respond, then ask whether the compensation is efficient given dead-space arithmetic.

Key Takeaways — Part 4

• Altitude timeline: peripheral chemoreceptor hyperventilation + respiratory alkalosis (minutes) → renal HCO3⁻ dumping + ↑2,3-BPG (days) → EPO/hematocrit (weeks).
• Exercise keeps arterial gases constant (feedforward ventilation); altitude does not — a favorite discrimination.
• $FEV_1/FVC$ low = obstructive (asthma, bronchitis, emphysema; air trapping raises RV/TLC); normal-high ratio with small volumes = restrictive (fibrosis, weakness).
• Emphysema = recoil/area loss (elastase vs. alpha-1-antitrypsin); fibrosis = stiffness + thick membrane.
• Figure skills: plateau height = capacity, P50 = affinity; sort responses by timescale; propagate any perturbation through ventilation → diffusion → transport → V/Q.