Explain the structure of the respiratory tree, the mechanics of breathing and gas exchange, measure and interpret lung volumes, and compare obstructive and restrictive respiratory imbalances

Learning goal: Identify the parts of the respiratory system (the airways, lungs and diaphragm) and explain how the structure of each portion of the respiratory tree supports its function; explain respiration mechanics, including the movement of the diaphragm, the muscles of inspiration and expiration and the pressure-volume relationship; explain gas exchange across the alveolar epithelium and capillary endothelium; list the normal respiratory volumes, explain how spirometry measures them and what factors alter them; differentiate tidal volume from breathing rate; compare human respiration with other vertebrates; and describe homeostatic imbalances such as asthma, chronic obstructive pulmonary disease (COPD), tuberculosis, cystic fibrosis and the effects of smoking and pollution.

What the system does and its parts. The respiratory system transports and exchanges gases: it brings oxygen into the blood and removes carbon dioxide. It is made of the airways, the lungs and the diaphragm. The airways run from the nasal and oral cavities through the pharynx, the larynx (voice box), the trachea, the two main bronchi, branching smaller bronchi and bronchioles, to the alveoli, tiny air sacs where gas exchange happens. Because the branching looks like an upside-down tree, the whole set of tubes is called the respiratory tree.

Structure supports function along the tree. The nasal cavity is lined with a moist mucous membrane and a rich blood supply that warm and humidify incoming air and trap dust. The trachea and bronchi are held open by rings or plates of cartilage, so they do not collapse when pressure changes; the trachea's rings are C-shaped, open at the back where the esophagus lies. These larger airways are lined with pseudostratified ciliated columnar epithelium with mucus-secreting goblet cells: the mucus traps particles and the cilia beat it upward toward the throat, a mucus escalator that cleans the air. The bronchioles have no cartilage but have smooth muscle that can narrow or widen them to adjust airflow. The alveoli have walls of thin simple squamous epithelium wrapped in capillaries, so the barrier between air and blood is extremely thin, and there are hundreds of millions of alveoli, providing an enormous surface. Other alveolar cells secrete surfactant, which reduces surface tension so the tiny sacs do not collapse. Under a microscope, you can identify each section by these features: cartilage and cilia mean trachea or bronchus; smooth muscle without cartilage means bronchiole; thin-walled air spaces mean alveoli.

Respiration mechanics: breathing by changing volume. Air moves from high pressure to low pressure. For a fixed amount of gas at constant temperature, pressure and volume are inversely related (Boyle's law): if the volume of the chest increases, the pressure of the air inside the lungs falls. Inspiration (inhalation) is active. The diaphragm, a dome-shaped sheet of skeletal muscle under the lungs, contracts and flattens, and the external intercostal muscles between the ribs contract and lift the ribs up and out. The chest cavity gets larger, the lungs, which cling to the chest wall through the fluid-filled pleural membranes, are stretched, the pressure inside the alveoli drops slightly below atmospheric pressure, and air flows in. During deep or forced inhalation, accessory muscles in the neck such as the sternocleidomastoid and scalenes lift the upper ribs further. Quiet expiration (exhalation) is mostly passive: the diaphragm and external intercostals relax, the elastic lungs recoil, volume decreases, pressure rises above atmospheric and air flows out. Forced expiration, as in coughing or blowing out candles, uses the internal intercostal muscles to pull the ribs down and the abdominal muscles to push the diaphragm up.

Gas exchange. Gas exchange is the diffusion of gases across the alveolar epithelium and the capillary endothelium. Each gas diffuses down its own gradient of partial pressure. Air in the alveoli has a higher partial pressure of oxygen than the blood arriving from the pulmonary arteries, so oxygen diffuses into the blood, where most of it binds hemoglobin. Blood arriving at the lungs has a higher partial pressure of carbon dioxide than alveolar air, so carbon dioxide diffuses out. In the body's tissues, the gradients reverse: oxygen diffuses into cells that are using it, and carbon dioxide diffuses into the blood.

Control of breathing. Breathing rhythm is set by centers in the brainstem (medulla and pons). Chemoreceptors monitor the blood; a rise in carbon dioxide, which makes the blood more acidic, is the strongest signal to breathe faster and deeper, and a large fall in oxygen is an additional signal. This is negative feedback: extra ventilation removes carbon dioxide and restores pH.

Lung volumes, worked fully. Spirometry measures how much air moves in and out. Typical textbook values for a young adult male are: tidal volume (a normal quiet breath) about 500 mL; inspiratory reserve volume (extra air that can be forced in after a normal breath) about 3,100 mL; expiratory reserve volume (extra air that can be forced out after a normal breath) about 1,200 mL; and residual volume (air that always remains in the lungs) about 1,200 mL. Capacities are sums: vital capacity = 500 + 3,100 + 1,200 = 4,800 mL, the most that can be exhaled after the deepest breath; total lung capacity = 4,800 + 1,200 = 6,000 mL. A spirometer cannot measure residual volume, because that air never leaves. Values are typically smaller in females and vary with body size, age, fitness, altitude, posture and disease; they are compared with predicted values for a person's age, sex and height, not with one standard. Tidal volume differs from breathing rate: tidal volume is how much air moves per breath (depth), breathing rate is how many breaths per minute. Minute ventilation combines them: 500 mL x 12 breaths per minute = 6,000 mL, or 6 L, per minute. You measure breathing rate by counting chest rises for 30 seconds and doubling, and depth with a spirometer or, roughly, by chest expansion with a tape measure.

Comparing vertebrates. Lancelets and tunicates, the non-vertebrate chordates, exchange gases largely by diffusion across body surfaces and use their pharyngeal slits mainly for feeding. Fish use gills, where water flows past thin filaments in the opposite direction to blood, a countercurrent arrangement that keeps a diffusion gradient along the whole length. Amphibians use simple lungs filled by pushing air in with the mouth floor (positive pressure) plus their moist skin. Amniotes (reptiles, birds and mammals) fill their lungs by expanding the body cavity (negative pressure). Mammals use a diaphragm and dead-end alveoli, so fresh air mixes with used air; birds use air sacs that move air in one direction through the lungs, an especially efficient design.

Homeostatic imbalances. Respiratory diseases fall into two patterns. Obstructive diseases narrow the airways, making it hard to move air out quickly: in asthma, inflamed bronchioles constrict and fill with mucus in episodes; in COPD, long-term damage, usually from smoking, destroys alveolar walls (emphysema) and inflames the bronchi (chronic bronchitis). Restrictive conditions reduce how much the lungs can expand, lowering total volumes, as with scarring of lung tissue or a stiff chest wall. Tuberculosis is a bacterial infection that damages lung tissue. Cystic fibrosis is an inherited disorder in which thick mucus clogs airways and traps bacteria. Smoking paralyzes and destroys cilia, so mucus and particles accumulate. Pollution includes fine particles (PM2.5, 2.5 micrometers or smaller, smaller than a red blood cell, which is about 7 to 8 micrometers across) that pass the mucus escalator and reach the alveoli, causing inflammation.

Modeling a cold or flu. A cold or the flu inflames the lining of the nose and airways: vessels in the lining swell and extra mucus is secreted, narrowing the passages. A simple model uses drinking straws of two widths for open and swollen airways, with a little thick liquid inside to stand for mucus; timing how long it takes a squeeze bulb or syringe to push a fixed volume of air through each shows why narrowing makes breathing harder. You can then use the model to investigate how various remedies alleviate symptoms in principle: something that shrinks swollen vessels widens the straw, and humidified air or saline makes mucus thinner and easier to clear. The model shows airflow only; it cannot show infection, immune responses or whether any real product works.

Investigation: breathing rate and a lung-volume balloon. Each student uses their own new balloon. (Use a latex-free balloon for anyone with a latex allergy.) After a normal inhalation, exhale normally once into the balloon, pinch it, and measure its diameter; treat it as a sphere and calculate volume. Repeat for a maximal exhalation after the deepest breath (an estimate of vital capacity). Count breathing rate at rest and after one minute of steady stepping, and straight after the stepping also exhale one normal breath into a fresh balloon, so that depth (tidal volume) is measured in both conditions, not assumed. Anyone with asthma or a breathing condition records data instead. To model the two disease patterns without restricting anyone's breathing, use a bottle-and-balloon lung model or a large syringe: push air out through a narrow tube and then a wide one and time each (a narrowed airway, an obstructive model), and repeat with a rubber band wrapped around the balloon so it cannot fully inflate (limited expansion, a restrictive model). The balloon is not a true sphere, resists stretching, and cannot capture residual volume, so all values are estimates.

Worked example 1 (balloon volume): A normal exhalation gives a balloon 10 cm in diameter, so radius r = 5 cm. Volume of a sphere = (4/3) x pi x r cubed = (4/3) x 3.14 x 125 = 523 cubic centimeters, about 520 mL. Since 1 cubic centimeter = 1 mL, this estimates tidal volume at about 520 mL, close to the typical 500 mL. A maximal exhalation gives a diameter of 20 cm (r = 10): (4/3) x 3.14 x 1,000 = 4,187 mL, an estimate of vital capacity.

Worked example 2 (capacities): A spirometer records tidal volume 450 mL, inspiratory reserve volume 2,400 mL, expiratory reserve volume 900 mL. Vital capacity = 450 + 2,400 + 900 = 3,750 mL. Total lung capacity cannot be found from the spirometer alone, because residual volume never leaves the lungs; if residual volume is estimated at 1,000 mL by another method, total lung capacity = 3,750 + 1,000 = 4,750 mL.

Worked example 3 (minute ventilation): At rest a student breathes 14 times per minute with a tidal volume of 500 mL: 14 x 500 = 7,000 mL per minute, 7 L. After exercise, 24 breaths per minute with a tidal volume of 1,500 mL: 24 x 1,500 = 36,000 mL, 36 L per minute. Ventilation increased 36 / 7, about 5.1 times; both rate and depth increased, and depth contributed more (tidal volume tripled, rate less than doubled).

Worked example 4 (spirometry patient match): Hypothetical patient X can exhale a normal total volume but only slowly, with a small fraction in the first second; patient Y exhales quickly but has a small total volume. X shows an obstructive pattern, as in asthma or COPD, where narrowed airways slow outflow. Y shows a restrictive pattern, where the lungs cannot expand fully. An athlete would typically show normal or above-predicted volumes and fast flow.

Representation task: Draw the respiratory tree from nasal cavity to alveolus and label each section with its tissue type and one structural feature that supports its function. Then draw a spirogram (volume against time) showing three quiet breaths, a maximal inhalation and a maximal exhalation, and label tidal volume, inspiratory and expiratory reserve volumes, residual volume, vital capacity and total lung capacity using the typical values.

Independent performance: Carry out the balloon and breathing-rate investigation and write a report. Success criteria: (a) tidal volume and vital capacity are estimated from measured diameters with the sphere calculation shown and units stated; (b) breathing rate and tidal volume are both measured at rest and after exercise, and minute ventilation is calculated for each condition; (c) tidal volume and breathing rate are clearly distinguished; (d) the muscles of inspiration and expiration are named and the pressure-volume relationship is used to explain airflow; (e) the narrow-tube and banded-balloon models are identified as obstructive and restrictive and linked to a real disease; (f) at least three limitations of the balloon method are stated.