39.3 Breathing

Respiratory System Anatomy and Physiology

Breathing depends on pressure differences between the air outside your body and the air inside your lungs. The respiratory system creates these pressure changes through muscular contractions that alter the volume of the thoracic cavity. This section covers the structures involved, the physics behind each breath, and how gas exchange actually happens at the alveolar level.

Lung and Chest Cavity Mechanics

The lungs are paired, spongy organs located in the thoracic cavity. Each lung is surrounded by two layers of pleural membranes:

• The visceral pleura directly covers the lung surface
• The parietal pleura lines the inside of the thoracic cavity wall
• A thin layer of pleural fluid sits between the two membranes, reducing friction as the lungs expand and contract during breathing

The thoracic cavity itself is bounded by the ribs, sternum, and vertebral column. It houses the lungs, heart, and major blood vessels. At the bottom of this cavity sits the diaphragm, a dome-shaped skeletal muscle that separates the thoracic cavity from the abdominal cavity. The diaphragm is the primary muscle of respiration.

Breathing works because of Boyle's Law: when volume increases, pressure decreases (and vice versa). The two phases of breathing apply this principle:

1. Inhalation (inspiration): The diaphragm contracts and flattens downward. At the same time, the external intercostal muscles contract, lifting the ribs and sternum outward. Together, these actions increase thoracic volume. This increase in volume lowers the intrapulmonary pressure (the pressure inside the lungs) below atmospheric pressure, so air flows in.
2. Exhalation (expiration): The diaphragm and external intercostal muscles relax. The thoracic cavity returns to its smaller resting volume, which increases intrapulmonary pressure above atmospheric pressure, pushing air out. During quiet breathing, exhalation is mostly passive.

Tidal volume is the amount of air moved in and out of the lungs during a single normal breath, typically about 500 mL in an average adult.

Lung Compliance and Resistance

Compliance and resistance are two physical properties that determine how easily you can move air in and out of your lungs. They work in opposition: high compliance makes breathing easier, while high resistance makes it harder.

Compliance measures how easily the lungs stretch and expand. It depends on:

• Elastic fibers (elastin and collagen) in lung tissue, which allow the lungs to stretch during inhalation and recoil during exhalation
• Surfactant, a substance produced by type II alveolar cells that reduces surface tension in the alveoli. Without surfactant, the high surface tension of the thin fluid lining the alveoli would cause them to collapse. Higher compliance means the lungs inflate more easily for a given pressure change.

Resistance measures the opposition to airflow through the respiratory passages. It depends on:

• Airway diameter: Narrower airways create more resistance. Bronchoconstriction (smooth muscle contraction around the airways) significantly increases resistance.
• Airway obstruction: Excess mucus or inflammation narrows the airway lumen, further increasing resistance. Think of it like breathing through a narrow straw versus a wide tube.

These properties are central to understanding respiratory disorders:

Ventilation-Perfusion Mismatches

Efficient gas exchange requires that air reaching the alveoli (ventilation) and blood flowing through the pulmonary capillaries (perfusion) are well matched. The ventilation-perfusion (V/Q) ratio describes this relationship. When ventilation and perfusion are balanced, oxygen uptake and carbon dioxide removal are maximized.

V/Q mismatches occur when ventilation and perfusion are not properly matched in a region of the lung. There are two main types:

• Dead space (high V/Q): Alveoli receive air but little or no blood flow. The ventilation is "wasted" because there's no blood to pick up the oxygen.
  • Anatomical dead space refers to the conducting airways (trachea, bronchi, bronchioles) that move air but have no alveoli for gas exchange. This is normal and accounts for about 150 mL of each breath.
  • Alveolar dead space refers to alveoli that are ventilated but not perfused, for example due to a pulmonary embolism blocking blood flow to that region.
• Shunt (low V/Q): Blood flows past alveoli that are poorly ventilated or not ventilated at all. The blood passes through without picking up oxygen, which leads to hypoxemia (low blood oxygen levels). A collapsed or fluid-filled alveolus is a common cause.

Consequences of significant V/Q mismatches include:

• Hypoxemia (low blood $O_2$)
• Hypercapnia (elevated blood $CO_2$)
• Increased work of breathing as the body tries to compensate
• Cardiovascular strain from pulmonary vasoconstriction (blood vessels constrict in poorly ventilated areas to redirect blood toward better-ventilated regions)

Gas Exchange and Respiratory Gases

Gas exchange occurs at the alveoli, the tiny air sacs at the end of the respiratory tree. Oxygen diffuses from the alveolar air into the pulmonary capillary blood, while carbon dioxide diffuses in the opposite direction. This movement is driven entirely by partial pressure gradients.

Partial pressure is the pressure exerted by a single gas within a mixture of gases. Each gas in a mixture contributes to the total pressure in proportion to its concentration. Gases always diffuse from regions of higher partial pressure to regions of lower partial pressure.

For example, the partial pressure of oxygen ($P_{O_2}$) in the alveoli is about 104 mmHg, while in the deoxygenated blood arriving at the pulmonary capillaries it's about 40 mmHg. This gradient drives oxygen into the blood. For carbon dioxide, the gradient is reversed: $P_{CO_2}$ in the blood is about 45 mmHg versus 40 mmHg in the alveoli, so $CO_2$ diffuses out into the alveolar air to be exhaled.

Minute ventilation is the total volume of air moved in and out of the lungs per minute. You can calculate it as:

$\text{Minute Ventilation} = \text{Tidal Volume} \times \text{Respiratory Rate}$

For a typical adult breathing 12 times per minute with a tidal volume of 500 mL, minute ventilation would be $500 \text{ mL} \times 12 = 6{,}000 \text{ mL/min}$ (or 6 L/min). Higher minute ventilation increases the rate of gas exchange by bringing more fresh air into the alveoli per unit time.