Transport, exchange & defense · Topic 15 of 18

Respiratory System

Airway & the Mechanics of Breathing

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Anatomy · airway & breathing mechanics

Airway & the Mechanics of Breathing

Air travels nose to alveoli through a conducting zone that cleans it; contracting the diaphragm enlarges the chest, so air flows in.

01 · Air pathNose→pharynx→larynx→trachea→bronchi→alveoli

02 · InhaleInhale: diaphragm contracts and moves down

03 · LobesRight lung 3 lobes · left lung 2 lobes

Parts & jobs

Nasal cavity cleans, warms and moistens air coming in

Goblet-cell mucus traps debris; cilia sweep it up to the throat

Larynx = voice box; the epiglottis covers it when swallowing

Trachea: C-shaped cartilage rings; splits into right and left bronchi

Bronchioles: no cartilage; smooth muscle changes their diameter

Alveoli: tiny air sacs with thin type I cell walls for gas exchange

Surfactant (from type II cells) keeps alveoli from collapsing

Right lung: 3 lobes · left lung: 2 lobes (cardiac notch)

Pleura: visceral on lung, parietal on chest wall; fluid between them

Step by step

Air path: nose → pharynx → larynx → trachea → bronchi → bronchioles → alveoli

Quiet breathing

Step by step

Inhale: diaphragm contracts and moves down; ribs lift up and out

Exhale at rest is passive: muscles relax and the lungs recoil

Know by heart

Tidal volume (one quiet breath) ≈ 500 mL

Vital capacity = TV + IRV + ERV; residual volume always stays behind

The rules

Boyle's law: volume up → pressure down; air flows from high to low pressure must know

Don't confuse

Conducting zone moves and cleans air; respiratory zone (alveoli) exchanges gas

Breathing in or breathing out (at rest)?

Commonly confused

	Inhalation	Exhalation
Diaphragm	Contracts and flattens downward	Relaxes back into its dome
External intercostals	Contract: ribs move up and out	Relax
Chest volume	Increases	Decreases
Lung pressure vs outside air	Lower, so air flows in	Higher, so air flows out
Energy	Active (muscle work)	Passive (elastic recoil)

More detail

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Pharynx: nasopharynx (air only), oropharynx + laryngopharynx (air and food)

Total lung capacity = all four volumes (~6,000 mL males, ~4,200 mL females)

Forced breathing: scalenes help inhale; abdominal muscles force exhale

Lungs are passive: they follow the chest wall, not their own muscle

Dead space: air in the airways that never reaches the alveoli

Pearl

Diaphragm down, air in; diaphragm back up, air out.

Sources · OpenStax Anatomy & Physiology 2e §22.1 Organs and Structures of the Respiratory System · OpenStax Anatomy & Physiology 2e §22.2 The Lungs · OpenStax Anatomy & Physiology 2e §22.3 The Process of Breathing · Union Test Prep — HESI A2 Anatomy & Physiology study guide, page 5 (respiratory system) · NurseHub — HESI A2 Anatomy & Physiology review course (lesson list: Respiratory System Anatomy, Respiratory Physiology) · Built from the verified NurseSavvy HESI A2 fact map (facts reviewed 2026-10-16). HESI is a trademark of Elsevier; NurseSavvy is not affiliated with Elsevier.

Gas Exchange, Gas Transport & Control of Breathing

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Anatomy · gas exchange & breathing control

Gas Exchange, Gas Transport & Control of Breathing

Gases diffuse down partial-pressure gradients in lungs and tissues; hemoglobin carries most O₂, bicarbonate most CO₂, and rising CO₂ drives the medulla to breathe.

01 · Oxygen~98.5% of O₂ rides on hemoglobin

02 · Carbon dioxideMost CO₂ travels as bicarbonate (~70%)

03 · ControlMain breathing trigger: rising CO₂

What it is

Air ≈ 79% N₂, 21% O₂; partial pressure = one gas's share of air pressure

More CO₂ → more acid (lower pH); faster breathing blows off CO₂, raising pH

Parts & jobs

Respiratory membrane (alveolus + capillary wall) is only ~0.5 µm thick

Pons fine-tunes the medulla's rhythm (rate and depth)

Chemoreceptors: central (brainstem) and peripheral (carotid, aortic bodies)

Step by step

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ (carbonic anhydrase, in RBCs)

Know by heart

~98.5% of O₂ rides on hemoglobin; ~1.5% dissolves in plasma must know

CO₂: ~70% as bicarbonate, ~20% on hemoglobin, ~7–10% dissolved must know

Adult resting rate ≈ 12–18 breaths/min (some use 12–20); infants 30–60

The rules

Gases diffuse from higher to lower partial pressure (no energy used) must know

Medulla sets the rhythm; rising blood CO₂ is the main trigger to breathe must know

Don't confuse

External = lungs ↔ blood · internal = blood ↔ tissue cells must know

External or internal respiration?

Commonly confused

	External respiration	Internal respiration
Where	Alveoli and lung capillaries	Body capillaries and tissue cells
O₂ moves	Alveolus → blood	Blood → tissue cells
CO₂ moves	Blood → alveolus (exhaled)	Tissue cells → blood
How	Simple diffusion, high → low partial pressure	Simple diffusion, high → low partial pressure

Mix-ups to avoid

Ventilation = moving air; respiration = gas exchange; cellular respiration = cells use O₂

More detail

Show 6 more facts

Lungs: PO₂ 104 (alveoli) vs 40 (blood); PCO₂ 45 (blood) vs 40 (alveoli)

Acidic or warm blood → hemoglobin releases more O₂ (Bohr effect)

Low O₂ drives breathing only below about 60 mm Hg

Cortex = voluntary control; hypothalamus speeds breathing (heat, emotion)

CO₂ is ~20× more soluble than O₂, so a small gradient is enough

Ventilation (air) must match perfusion (blood flow) in the lungs

Pearl

The urge to breathe comes from too much CO₂, not too little O₂.

Sources · OpenStax Anatomy & Physiology 2e §22.4 Gas Exchange · OpenStax Anatomy & Physiology 2e §22.5 Transport of Gases · OpenStax Anatomy & Physiology 2e §22.3 The Process of Breathing (respiratory rate and control of ventilation) · OpenStax Anatomy & Physiology 2e §22.1 Organs and Structures of the Respiratory System (respiratory membrane) · OpenStax Biology 2e §39.4 Transport of Gases in Human Bodily Fluids (98.5% of O₂ on hemoglobin) · Healthline — What Is a Normal Respiratory Rate? (adults about 12–20 breaths/min) · Union Test Prep — HESI A2 Anatomy & Physiology study guide, page 5 (respiratory control, types of respiration) · NurseHub — HESI A2 Anatomy & Physiology review course (lesson list: Respiratory Physiology) · Built from the verified NurseSavvy HESI A2 fact map (facts reviewed 2026-10-16). HESI is a trademark of Elsevier; NurseSavvy is not affiliated with Elsevier.

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