4.2 Pulmonary Ventilation and Lung Volumes

Pulmonary Ventilation and Gas Exchange

Pulmonary ventilation is the mechanical process of moving air into and out of the lungs. This airflow is what makes gas exchange possible: oxygen enters the bloodstream, and carbon dioxide is removed. Without continuous ventilation, cells can't perform aerobic respiration and blood pH shifts dangerously toward acidosis.

Your body adjusts both the rate and depth of ventilation to match metabolic demand. At rest, you might breathe 12–20 times per minute. During intense exercise, that rate climbs and each breath gets deeper, pulling in more oxygen and flushing out more $CO_2$.

Gas Exchange in the Lungs

Gas exchange happens at the alveoli, the tiny air sacs clustered at the ends of the respiratory bronchioles. There are roughly 300 million alveoli in each lung, creating an enormous surface area (about 70 $m^2$) for diffusion.

The process is driven by concentration gradients:

• Oxygen is at a higher partial pressure in the alveoli than in the deoxygenated blood arriving via pulmonary capillaries, so it diffuses into the blood.
• Carbon dioxide is at a higher partial pressure in the blood than in the alveolar air, so it diffuses out into the alveoli to be exhaled.

Three factors determine how efficiently this exchange occurs:

1. Surface area of the alveoli (reduced in emphysema, for example)
2. Thickness of the respiratory membrane (the alveolar-capillary barrier is normally only ~0.5 µm thick; edema or fibrosis increases this distance and slows diffusion)
3. Ventilation-perfusion matching (airflow and blood flow need to be proportional; a mismatch means some blood passes through without picking up adequate oxygen)

Inspiration and Expiration Process

Mechanics of Inspiration (Inhalation)

Inspiration is an active process, meaning it requires muscle contraction and energy expenditure. Here's the sequence:

1. The diaphragm contracts and flattens downward, while the external intercostal muscles contract and lift the ribs upward and outward.
2. These movements increase the volume of the thoracic cavity.
3. As thoracic volume increases, intrapulmonary pressure (pressure inside the lungs) drops below atmospheric pressure. This is sometimes called "negative pressure."
4. Air flows down the pressure gradient, from the higher-pressure atmosphere into the lower-pressure lungs.

During heavy exercise or respiratory distress, accessory muscles kick in to help. The sternocleidomastoid elevates the sternum, and the scalenes lift the upper ribs, further expanding the thoracic cavity.

Mechanics of Expiration (Exhalation)

Quiet expiration is a passive process. No muscular effort is needed under normal resting conditions.

1. The diaphragm and external intercostals relax, allowing the thoracic cavity to decrease in volume.
2. The elastic recoil of the lungs and chest wall pulls everything back to resting position, much like a stretched rubber band returning to its original shape.
3. As thoracic volume decreases, intrapulmonary pressure rises above atmospheric pressure.
4. Air flows out of the lungs, again following the pressure gradient.

During forced expiration (heavy breathing, coughing, blowing out candles), the internal intercostals and abdominal muscles actively contract to push air out faster and more completely.

Lung Volumes and Capacities

Static Lung Volumes

These are the four non-overlapping volumes that together account for all the air the lungs can hold. Approximate adult values are listed, but they vary with age, sex, height, and fitness level.

• Tidal volume (TV): ~500 mL. The amount of air moved in or out during a normal, relaxed breath.
• Inspiratory reserve volume (IRV): ~3,000 mL. The extra air you can inhale beyond a normal breath if you try.
• Expiratory reserve volume (ERV): ~1,100 mL. The extra air you can force out beyond a normal exhale.
• Residual volume (RV): ~1,200 mL. The air that stays in your lungs even after you exhale as hard as possible. This prevents the alveoli from collapsing and keeps gas exchange going between breaths.

Lung Capacities (Combinations of Lung Volumes)

Capacities combine two or more volumes. The formulas are worth memorizing:

• Inspiratory capacity (IC) = TV + IRV ≈ 3,500 mL. The maximum air you can inhale starting from a normal end-expiratory position.
• Functional residual capacity (FRC) = ERV + RV ≈ 2,300 mL. The air remaining in the lungs after a normal, passive exhale.
• Vital capacity (VC) = TV + IRV + ERV ≈ 4,600 mL. The maximum air you can move in a single breath, from deepest inhale to most forceful exhale.
• Total lung capacity (TLC) = TV + IRV + ERV + RV ≈ 5,800 mL. All the air the lungs can hold after a maximal inhalation.

Note that RV cannot be measured by spirometry because you can never fully empty your lungs. This means FRC and TLC also can't be measured by spirometry alone; they require techniques like helium dilution or body plethysmography.

Spirometry Results and Significance

Spirometry Parameters and Interpretation

A spirometer measures lung volumes, capacities, and airflow rates as a patient breathes into the device. The key values it produces:

• Forced vital capacity (FVC): The total volume of air a patient can forcibly exhale after a maximal inhalation. A reduced FVC suggests restrictive disease (the lungs can't fully expand).
• Forced expiratory volume in one second ($FEV_1$): How much air is expelled in the first second of that forced exhale. A reduced $FEV_1$ points toward obstructive disease (the airways are narrowed).
• $FEV_1$/FVC ratio: This is the critical number for distinguishing obstructive from restrictive patterns.
  • Obstructive disorders (asthma, COPD): ratio is decreased (typically below 0.70) because airflow is limited even though lung volume may be normal or increased.
  • Restrictive disorders (pulmonary fibrosis, chest wall deformity): ratio is normal or increased because both $FEV_1$ and FVC are reduced proportionally, or FVC drops more.
• Peak expiratory flow rate (PEFR): The maximum speed of airflow during a forced exhale. Particularly useful for monitoring asthma day-to-day; a drop in PEFR signals worsening airway constriction.

Clinical Significance of Spirometry

Spirometry is one of the most common pulmonary function tests in clinical practice. Its uses include:

• Diagnosing respiratory conditions like asthma, COPD, and interstitial lung disease
• Grading severity of obstruction (mild, moderate, severe) based on how much $FEV_1$ is reduced from the predicted value
• Guiding treatment decisions, such as whether a patient with COPD needs bronchodilator therapy or inhaled corticosteroids
• Monitoring disease progression over time (a declining $FEV_1$ trend in COPD indicates worsening obstruction)
• Assessing bronchodilator responsiveness: if $FEV_1$ improves by ≥12% and ≥200 mL after inhaling a bronchodilator, the obstruction is considered reversible, which is characteristic of asthma rather than COPD
• Preoperative screening to evaluate whether a patient's lungs can tolerate surgery, especially thoracic procedures