22.2 The Lungs

Lung Anatomy and Function

The lungs are the primary organs of respiration. They bring oxygen into the body, remove carbon dioxide, and play supporting roles in blood pH regulation, speech, and even filtration of the blood. Understanding their anatomy helps you see how structure drives function throughout the respiratory system.

Functions of the Lungs in Respiration

The lungs do more than just "breathe." They serve several functions you should know:

• Gas exchange: Oxygen diffuses from the alveoli into the pulmonary capillaries, while carbon dioxide diffuses in the opposite direction. This is the lungs' primary job.
• Blood pH regulation: By removing excess $CO_2$ from the blood, the lungs help control pH. $CO_2$ combines with water (via the enzyme carbonic anhydrase) to form carbonic acid, so exhaling $CO_2$ prevents the blood from becoming too acidic.
• Vocalization: Air moving past the vocal cords in the larynx produces sound, which is then shaped into speech by the mouth and tongue.
• Olfaction: Inhaling through the nose draws odorant molecules up to the olfactory receptors in the nasal cavity, enabling your sense of smell.
• Filtration: The pulmonary capillary beds trap and remove small blood clots and air bubbles before they can reach systemic circulation. A large clot that lodges here causes a pulmonary embolism.

Blood Flow in Pulmonary Circulation

Pulmonary circulation is a short loop between the heart and lungs. Its sole purpose is gas exchange. Here's the pathway:

1. The right ventricle pumps deoxygenated blood into the pulmonary trunk.
2. The pulmonary trunk splits into the left and right pulmonary arteries, one heading to each lung.
3. These arteries branch into smaller arterioles and then into pulmonary capillaries that wrap tightly around the alveoli.
4. Gas exchange occurs across the thin walls of the alveoli and capillaries: $O_2$ moves into the blood, $CO_2$ moves out.
5. Now-oxygenated blood collects into pulmonary venules, which merge into pulmonary veins.
6. Four pulmonary veins (two from each lung) deliver oxygenated blood to the left atrium of the heart.

Blood Vessels of the Lungs

This overlaps with pulmonary circulation, but make sure you can identify each vessel and what it carries:

• Pulmonary trunk — carries deoxygenated blood from the right ventricle; splits into the left and right pulmonary arteries.
• Pulmonary arteries (left and right) — deliver deoxygenated blood from the pulmonary trunk to each lung.
• Pulmonary capillaries — surround the alveoli; this is where gas exchange happens by diffusion across extremely thin walls.
• Pulmonary venules — collect oxygenated blood from the capillaries and merge into larger veins.
• Pulmonary veins (four total, two per lung) — carry oxygenated blood from the lungs back to the left atrium.

Structure of Pleural Membranes

Each lung is enclosed by a double-layered serous membrane called the pleura. These membranes are essential for normal breathing mechanics.

The two layers:

1. Visceral pleura — adheres directly to the outer surface of the lung.
2. Parietal pleura — lines the inner wall of the thoracic cavity.

Between these two layers is the pleural cavity, a thin space filled with a small amount of pleural fluid (serous fluid). This fluid serves two purposes: it reduces friction as the lungs expand and contract, and it creates surface tension that couples the lungs to the thoracic wall.

That surface tension is critical. It means that when the thoracic wall expands during inhalation, the lungs expand with it. The intrapleural pressure inside the pleural cavity is slightly negative compared to atmospheric pressure, and this negative pressure keeps the lungs inflated. If air enters the pleural cavity (from a puncture wound, for example), intrapleural pressure equalizes with atmospheric pressure, the surface tension is lost, and the lung collapses. This condition is called a pneumothorax.

Respiratory Physiology and Gas Exchange

A few key physiological concepts tie lung structure to function:

• Respiratory rate is the number of breaths per minute. It increases with exercise, stress, fever, and certain diseases, and is regulated by respiratory centers in the brainstem.
• Lung compliance describes how easily the lungs stretch during inhalation. High compliance means the lungs expand easily. Compliance depends on the elastic tissue of the lungs and on surfactant, which reduces surface tension in the alveoli.
• Partial pressure of gases drives diffusion. $O_2$ moves from areas of higher partial pressure (alveoli) to lower partial pressure (pulmonary capillary blood), and $CO_2$ moves in the reverse direction. The steeper the pressure gradient, the faster the exchange.
• Hypoxia is inadequate oxygen delivery to the tissues. It can result from problems at any point in the respiratory chain: blocked airways, damaged alveoli, impaired gas exchange, or insufficient blood flow.