Respiratory physiology explains how the lungs bring oxygen into the body and remove carbon dioxide from the blood. This process matters because every tissue depends on a steady oxygen supply for aerobic metabolism and on carbon dioxide removal to help maintain acid-base balance. Understanding gas exchange also helps students connect normal anatomy to common diseases such as asthma, pneumonia, pulmonary embolism, and emphysema.
Lung volumes and ventilation-perfusion relationships are core ideas used in physiology, medicine, and critical care.
Gas exchange occurs across the thin alveolar-capillary membrane, where oxygen diffuses from alveoli into blood and carbon dioxide diffuses in the opposite direction. Lung volumes describe how much air moves in and out during breathing and how much remains after exhalation, which helps assess mechanics and reserve. Ventilation-perfusion matching compares airflow to blood flow in different lung regions and determines how efficiently oxygen enters the blood.
When diffusion, ventilation, or perfusion is disrupted, arterial oxygenation falls and characteristic clinical patterns appear.
Understanding Respiratory Physiology
Breathing moves air, but airflow alone does not guarantee that oxygen reaches cells. The chest wall and diaphragm create pressure changes that draw air into the lungs. During a quiet breath in, the diaphragm contracts and moves downward.
The chest becomes larger, pressure inside the lungs falls, and air enters. Most quiet breathing out is passive because elastic lung tissue recoils. Surfactant, a substance made by cells lining the alveoli, reduces surface tension.
Without it, small alveoli would tend to collapse, especially during exhalation. Premature babies can have too little surfactant, which makes breathing extremely difficult.
Diffusion depends on differences in partial pressure, not simply on the total amount of air. Oxygen moves into blood because its partial pressure is higher in alveolar air than in incoming venous blood. Hemoglobin then carries most of that oxygen.
It binds oxygen efficiently in the lungs and releases it more readily in active tissues. Carbon dioxide travels partly dissolved in blood, partly attached to proteins, and mostly as bicarbonate. This matters because carbon dioxide affects blood acidity.
Faster or deeper breathing removes more carbon dioxide and can raise blood pH. Slow or inadequate breathing retains carbon dioxide and can lower blood pH.
Lung volume measurements reveal different problems than oxygen measurements. Spirometry records air moved during forced breathing. In obstructive disease such as asthma, narrowed airways make it hard to push air out quickly.
Air can become trapped, leaving an unusually large amount after exhalation. In restrictive disease such as pulmonary fibrosis, stiff lungs cannot expand fully, so total capacity falls. Spirometry cannot directly measure the air left after a maximal exhalation.
Other methods, such as gas dilution or body plethysmography, are needed for that value. Students should separate the idea of air movement from the idea of total lung size.
Ventilation and blood flow are not evenly distributed through an upright lung. Gravity causes relatively more blood flow near the lung bases. Airflow is greater there too, though blood flow changes more strongly.
This creates regional differences in ventilation-perfusion matching. A blocked airway produces a low ventilation-perfusion area because blood arrives without enough fresh air. Pneumonia can fill alveoli with fluid or inflammatory material and create a similar pattern.
A pulmonary embolism blocks blood flow, leaving ventilated alveoli with little perfusion. That air becomes part of wasted ventilation, often called dead space.
Supplemental oxygen often helps low ventilation-perfusion regions, but it may help less when blood passes through completely unventilated alveoli. These distinctions explain why patients with different lung diseases can have similar low oxygen readings but need different treatment.
Key Facts
- Dalton's law: Ptotal = P1 + P2 + P3 + ...
- Fick's law: Gas diffusion is proportional to (A x D x deltaP) / T
- Minute ventilation = tidal volume x respiratory rate
- Alveolar ventilation = (tidal volume - dead space) x respiratory rate
- Vital capacity = IRV + TV + ERV
- V/Q ratio = alveolar ventilation / pulmonary blood flow, normal whole-lung value is about 0.8
Vocabulary
- Alveolus
- An alveolus is a tiny air sac in the lung where oxygen and carbon dioxide are exchanged with capillary blood.
- Tidal volume
- Tidal volume is the amount of air inhaled or exhaled during a normal quiet breath.
- Residual volume
- Residual volume is the air left in the lungs after a maximal exhalation and cannot be measured by simple spirometry.
- Dead space
- Dead space is the portion of inspired air that does not participate in gas exchange, either because it stays in conducting airways or reaches unperfused alveoli.
- Ventilation-perfusion matching
- Ventilation-perfusion matching is the relationship between airflow and blood flow in the lungs that determines how efficiently gas exchange occurs.
Common Mistakes to Avoid
- Confusing ventilation with perfusion, which is wrong because ventilation refers to airflow into alveoli while perfusion refers to blood flow through pulmonary capillaries.
- Assuming all lung volumes can be measured by spirometry, which is wrong because residual volume, functional residual capacity, and total lung capacity require methods beyond simple spirometry.
- Thinking oxygen and carbon dioxide move by active transport across the alveolar membrane, which is wrong because both gases normally move by passive diffusion down partial pressure gradients.
- Assuming a normal total ventilation always means normal gas exchange, which is wrong because poor V/Q matching or increased dead space can reduce oxygenation even when breathing rate looks adequate.
Practice Questions
- 1 A student has a tidal volume of 500 mL, an anatomic dead space of 150 mL, and a respiratory rate of 12 breaths per minute. What is the alveolar ventilation in mL/min?
- 2 A patient breathes with a tidal volume of 450 mL at 16 breaths per minute. Calculate the minute ventilation in L/min.
- 3 A region of lung is well ventilated but receives almost no blood flow because of a pulmonary embolism. Describe how the V/Q ratio changes in that region and explain why gas exchange becomes inefficient.