Difficulty: Intermediate | Prerequisites: Basic anatomy, circulatory system, diffusion
The respiratory system handles gas exchange: getting oxygen into the blood and carbon dioxide out. This section covers the anatomy of the airways, the mechanics of breathing, how gases move between the lungs and tissues, and how factors like smoking, obesity, exercise, and air pollution affect lung function. You should already understand diffusion, partial pressures, and the basics of the circulatory system. This material connects directly to the cardiovascular unit (oxygen delivery to tissues) and the immune system unit (pathogens entering through the airways).
Air travels from the nasal cavity through the pharynx, larynx, trachea, bronchi, and bronchioles to reach the alveoli, where gas exchange occurs by diffusion. Breathing rate is driven primarily by CO2 levels (high PCO2 increases breathing). Hemoglobin releases more oxygen at higher temperatures. Smoking, obesity, and air pollution all reduce lung function, while regular exercise maintains alveolar elasticity and bronchiole diameter.
Nasal cavity
The first section of the airway. Warms, humidifies, and filters incoming air. The correct inhalation pathway begins here.
Pharynx
The throat, shared by the respiratory and digestive systems. Air passes through the pharynx after the nasal cavity.
Larynx
The voice box, sitting below the pharynx and above the trachea. Contains the vocal cords.
Trachea
The windpipe, reinforced with C-shaped cartilage rings to prevent collapse. Connects the larynx to the bronchi.
Bronchi
The two main branches of the trachea that enter the lungs. Supported by cartilage.
Bronchioles
Smaller airways branching from the bronchi. They lack cartilage, which means they can collapse. Smooth muscle around bronchioles contracts during asthma attacks or exposure to cold air.
Alveoli
Tiny air sacs at the end of the bronchioles where gas exchange occurs. Lined with surfactant to reduce surface tension. Think of them as clusters of tiny balloons where oxygen crosses into the blood and CO2 crosses out.
Surfactant
A substance that reduces surface tension inside alveoli, making it easier for the lungs to inflate and deflate. Without surfactant, the alveoli would collapse.
Vital capacity
The maximum volume of air a person can exhale after a maximum inhalation. Affected by smoking (both personal and maternal smoking during pregnancy), age, sex, and diaphragm thickness.
Tidal volume (TV)
The volume of air inhaled or exhaled in a normal breath. A breathing pattern of 10 breaths/min with 800 ml tidal volume delivers the most oxygen per minute (because dead space is subtracted from each breath).
Peak expiratory flow
The maximum speed at which a person can exhale. Higher in men than women, decreases from age 30 to 70, and decreases as BMI increases (line V on Fig. A).
PCO2 (partial pressure of carbon dioxide)
The pressure exerted by CO2 in a gas mixture or dissolved in blood. High PCO2 is the primary stimulus that increases the rate and depth of breathing. CO2 moves down its concentration gradient from tissues (high PCO2) into the blood.
PO2 (partial pressure of oxygen)
The pressure exerted by O2. Oxygen diffuses from areas of high PO2 (lungs) to areas of low PO2 (tissues).
Hemoglobin dissociation
The release of oxygen from hemoglobin. At a tissue PO2 of 40 mmHg, hemoglobin releases the most oxygen at 43 degrees C (higher temperatures shift the curve rightward, releasing more O2 to active tissues).
COPD (chronic obstructive pulmonary disease)
A group of lung diseases (including emphysema and chronic bronchitis) characterised by higher resistance to airflow and lower surface area/volume in the alveoli. India has the highest COPD-related DALY rate, largely due to poorly regulated air pollution.
DALYs (disability-adjusted life years)
A measure of overall disease burden, combining years of life lost due to premature death and years lived with disability. Used to compare health impacts across countries.
Diaphragm
The primary muscle of breathing. A thicker diaphragm generates greater lung expansion, increasing vital capacity (line M on Fig. A: positive correlation between diaphragm thickness and vital capacity).
Air pressure gradient
The difference in air pressure between two points. Bulk flow of air is highest when the air pressure gradient is high, not when atmospheric pressure alone is high.
Correct order: nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles
Bronchi have cartilage for structural support; bronchioles lack cartilage and can collapse
Smooth muscle around bronchioles contracts during asthma attacks or cold air exposure, narrowing the airways
Alveoli are the site of gas exchange, lined with surfactant to reduce surface tension
Bulk flow of air depends on the air pressure gradient (the difference between atmospheric and intrapulmonary pressure), not just the absolute atmospheric pressure
The diaphragm is the main breathing muscle; a thicker diaphragm produces greater vital capacity (line M on Fig. A: positive correlation)
Peak expiratory flow is higher in men than women, and decreases with age (30 to 70 years)
Increasing BMI (from 20 to 40) decreases peak expiratory flow (line V on Fig. A: negative correlation)
A breathing pattern of RR = 10 breaths/min with TV = 800 ml/breath delivers the most oxygen per minute
Why: anatomical dead space (about 150 ml) is subtracted from each breath; fewer, deeper breaths lose less volume to dead space than many shallow breaths
Compare: 40 breaths/min at 200 ml = 8,000 ml/min total, but 40 x 150 = 6,000 ml lost to dead space, leaving only 2,000 ml of fresh air
10 breaths/min at 800 ml = 8,000 ml/min total, with only 10 x 150 = 1,500 ml lost to dead space, leaving 6,500 ml of fresh air
CO2 diffuses from tissues (high PCO2) into the blood, and from blood into the alveoli (low PCO2)
If arterial blood PCO2 = 30 mmHg and muscle tissue PCO2 = 50 mmHg, the blood leaving the tissue (vena cava) will have a PCO2 of approximately 50 mmHg (equilibrates with the tissue)
High PCO2 is the primary driver of increased breathing rate and depth (not low PO2 or high pH)
Hemoglobin releases more oxygen at higher temperatures (Fig. E: at PO2 = 40 mmHg in muscle tissue, the most O2 is released at 43 degrees C)
Smoking reduces vital capacity; maternal smoking during pregnancy also reduces the child's vital capacity
A 20-year-old female who is a current smoker and whose mother smoked during pregnancy will have the smallest vital capacity (all else being equal)
Women who quit smoking between ages 30 and 50 have significantly lower lung cancer death risk than current smokers, but their risk remains higher than lifelong non-smokers (Fig. C)
Regular aerobic exercise maintains alveolar elasticity and increases the resting diameter of bronchioles
Smaller particles penetrate deeper into the airways
Particles of 2.5 micrometres (PM2.5) are the most likely to reach the alveoli
Larger particles (10, 20, 30 micrometres) are filtered out in the upper airways
COPD patients have higher resistance to airflow and lower alveolar surface area/volume
India has the highest DALY rate from COPD (Fig. G), largely because of poorly regulated air pollution
An odds ratio (OR) below 1.0 suggests a protective effect; above 1.0 suggests increased risk
A result is statistically significant only if the 95% confidence interval does not cross 1.0
Example: Sheepdog OR = 0.80 (0.65 to 0.99) is significant because the CI does not cross 1.0, so children growing up with a sheepdog have a significantly reduced risk of developing asthma
Students often think high atmospheric pressure drives bulk airflow. What matters is the air pressure gradient (the difference between pressures), not the absolute pressure.
Students confuse what drives breathing rate. The primary stimulus is high PCO2, not low PO2. The body monitors CO2 levels far more sensitively than oxygen levels.
Students assume temperature has no effect on hemoglobin. Higher temperatures cause hemoglobin to release more oxygen, which is why active (warm) muscles get more O2.
Students mix up bronchi and bronchioles when asked about cartilage. Bronchi have cartilage; bronchioles do not, which is why bronchioles can collapse.
⚠️ Know the correct inhalation pathway in order: nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles.
⚠️ Understand why 10 breaths/min at 800 ml delivers the most oxygen (dead space calculation).
⚠️ High PCO2 drives breathing, not low PO2. This is tested repeatedly.
⚠️ Fig. E: hemoglobin releases the most O2 at 43 degrees C when tissue PO2 is 40 mmHg.
⚠️ Know how to read confidence intervals: if the 95% CI crosses 1.0, the result is not statistically significant (applies to the dog breed/asthma question and the antibiotic/obesity question).
⚠️ Fig. A interpretation is tested multiple ways: line V = negative correlation (peak expiratory flow vs. BMI, thymus size vs. age); line M = positive correlation (vital capacity vs. diaphragm thickness, cancer vs. telomerase).
⚠️ PM2.5 particles reach the alveoli. Larger particles are filtered out higher in the airways.
⚠️ Maternal smoking during pregnancy reduces the child's vital capacity, even if the child never smokes.
Fill in the blank: The airways that lack cartilage and can collapse are the ______. (Bronchioles.)
True or false: Low PO2 is the primary stimulus for increasing breathing rate. (False, high PCO2 is the primary stimulus.)
Fill in the blank: The substance that reduces surface tension in the alveoli is called ______. (Surfactant.)
True or false: Peak expiratory flow increases with age from 30 to 70. (False, it decreases.)
Fill in the blank: Particles of ______ micrometres are most likely to reach the alveoli. (2.5.)
Q: What is the correct pathway of air during inhalation?
A: Nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles.
Q: If arterial blood has a PCO2 of 30 mmHg and skeletal muscle tissue has a PCO2 of 50 mmHg, what will the PCO2 in the vena cava be?
A: Approximately 50 mmHg. The blood equilibrates with the tissue as it passes through the capillary bed, picking up CO2 from the metabolically active muscle.
Q: A breathing pattern of 10 breaths/min at 800 ml tidal volume and one of 40 breaths/min at 200 ml tidal volume both move 8,000 ml/min. Why does the first deliver more oxygen?
A: Anatomical dead space (about 150 ml) is subtracted per breath. At 10 breaths/min, dead space = 1,500 ml, leaving 6,500 ml of fresh air. At 40 breaths/min, dead space = 6,000 ml, leaving only 2,000 ml of fresh air.
Q: At what temperature does hemoglobin release the most oxygen to tissues when PO2 is 40 mmHg?
A: 43 degrees C. Higher temperatures shift the oxygen-hemoglobin dissociation curve to the right, increasing oxygen release to active tissues.
Q: Which 20-year-old female will have the smallest vital capacity, if all are the same height, weight, and exercise level?
A: A current smoker whose mother also smoked during pregnancy. Both personal smoking and prenatal exposure to smoke reduce vital capacity.
Q: Based on the odds ratios and confidence intervals, which dog breed is associated with a significantly reduced risk of childhood asthma: Terriers 0.95 (0.73 to 1.25), Dachshund 0.82 (0.56 to 1.20), Sheepdog 0.80 (0.65 to 0.99), or Sighthound 0.77 (0.42 to 1.42)?
A: Sheepdog, because its 95% confidence interval (0.65 to 0.99) does not cross 1.0, making it the only statistically significant result.
Q: What happens to smooth muscle around the airways during an asthma attack or cold air exposure?
A: The smooth muscle contracts, narrowing the airways and making breathing more difficult.
The respiratory system depends on the circulatory system to transport gases: oxygen delivery to tissues is governed by hemoglobin saturation, which ties back to cardiovascular physiology. COPD and lung cancer connect to the immune system material (alveolar macrophages, inflammation). The effect of obesity on peak expiratory flow and NK cell function bridges this section with the cancer biology notes.
Respiratory system, gas exchange, lungs, alveoli, bronchi, bronchioles, trachea, larynx, pharynx, nasal cavity, surfactant, vital capacity, tidal volume, peak expiratory flow, respiration rate, dead space, diaphragm, PCO2, PO2, partial pressure, hemoglobin, oxygen-hemoglobin dissociation curve, Bohr effect, COPD, chronic obstructive pulmonary disease, emphysema, chronic bronchitis, asthma, smoking, air pollution, PM2.5, DALYs, odds ratio, confidence interval, BMI, obesity, exercise, BIO 118, biology 118, University of Washington