22.1 Organs and Structures of the Respiratory System

The respiratory system is a network of organs and structures that bring oxygen into the body and remove carbon dioxide. From the nose to the lungs, each part plays a specific role in moving, conditioning, and exchanging gases. The system is divided into upper and lower tracts, with the alveoli of the lungs serving as the primary site of gas exchange.

Respiratory System Anatomy

Structures of Respiratory Tracts

The respiratory system is split into two main regions based on location and function.

Upper Respiratory Tract

• Nose and nasal cavity — Nasal hairs and mucus trap particles like dust and pollen. The nasal cavity also warms and humidifies incoming air, preparing it before it reaches the lungs.
• Pharynx (throat) — A shared passageway for both air and food/drink. It has three regions: the nasopharynx (behind the nasal cavity), the oropharynx (behind the mouth), and the laryngopharynx (just above the larynx).
• Larynx (voice box) — Contains the vocal cords, which produce sound for speech. The epiglottis, a flap of cartilage, covers the larynx during swallowing to prevent food or liquid from entering the airway (a dangerous event called aspiration).

Lower Respiratory Tract

• Trachea (windpipe) — Lined with ciliated pseudostratified columnar epithelium and mucus-producing goblet cells, which trap particles and sweep them upward and out. C-shaped cartilage rings keep the trachea open; the open side of the "C" faces posteriorly, allowing the esophagus to expand during swallowing.
• Bronchi — The trachea splits into the right and left primary bronchi, each entering its respective lung. These branch into secondary (lobar) bronchi, one for each lobe (3 on the right, 2 on the left). Secondary bronchi further divide into tertiary (segmental) bronchi, which continue branching into progressively smaller bronchioles.
• Lungs — The right lung has three lobes; the left lung has two lobes (the left is smaller to accommodate the heart). Pleural membranes cover the lungs (visceral pleura) and line the thoracic cavity wall (parietal pleura), with a thin layer of serous fluid between them that reduces friction during breathing. Within the lungs, the airways terminate in alveoli, tiny air sacs surrounded by capillaries where gas exchange occurs. Alveoli are coated with surfactant, a substance produced by type II alveolar cells that reduces surface tension and prevents the alveoli from collapsing.

Gas Exchange in Lungs

Ventilation is the movement of air in and out of the lungs. It depends on pressure changes in the thoracic cavity:

1. Inhalation — The diaphragm and external intercostal muscles contract, expanding the thoracic cavity. This increases volume and decreases pressure inside the lungs (following Boyle's Law: as volume increases, pressure decreases). Air flows in from the atmosphere down the pressure gradient.
2. Exhalation — The diaphragm and external intercostal muscles relax, decreasing thoracic volume and increasing pressure. Air is pushed out of the lungs. Quiet exhalation is a passive process that doesn't require muscle contraction (forced exhalation, like coughing, does use muscles such as the internal intercostals and abdominals).

Gas exchange occurs across the walls of the alveoli. Each lung contains roughly 300 million alveoli, providing an enormous surface area.

• Oxygen diffuses from the alveoli into the surrounding capillary blood, while carbon dioxide diffuses from the blood into the alveoli. Both gases move down their concentration gradients (from high to low concentration), driven by differences in partial pressure.
• Oxygen is picked up by hemoglobin in red blood cells, forming oxyhemoglobin, and transported to tissues throughout the body.
• Carbon dioxide travels back to the lungs in three forms: dissolved in plasma, bound to hemoglobin as carbaminohemoglobin, and (most commonly) converted to bicarbonate ions ($HCO_3^-$) in the blood.

This exchange is what fuels cellular respiration (ATP production) and removes carbon dioxide, a metabolic waste product.

Conducting vs. Respiratory Zones

The airway can be divided into two functional zones.

Conducting Zone

This zone includes the nose, pharynx, larynx, trachea, bronchi, and bronchioles (up to the terminal bronchioles). It serves three purposes:

1. Provides a passageway (conduit) for air to reach the respiratory zone
2. Warms, humidifies, and filters incoming air (sometimes called "air conditioning")
3. No gas exchange occurs here, so this space is called anatomical dead space

Respiratory Zone

This zone includes the respiratory bronchioles, alveolar ducts, and alveoli. It's where gas exchange actually happens.

1. The alveolar walls are extremely thin (0.2–0.6 μm), creating a very short diffusion distance between air and blood
2. A dense capillary network wraps around each alveolus, maximizing contact between blood and the gas exchange surface
3. Alveolar macrophages (also called dust cells) patrol the alveoli, removing debris and pathogens that make it past the conducting zone's defenses

Respiratory Mechanics

• The thoracic cavity houses the lungs, heart, and other mediastinal structures. Its ability to change volume is what drives ventilation.
• Compliance refers to how easily the lungs stretch and expand during inhalation. High compliance means the lungs expand easily; low compliance (as in pulmonary fibrosis) means more effort is needed to inflate them.
• Dead space is the volume of air in the respiratory system that does not participate in gas exchange. Anatomical dead space refers to air sitting in the conducting zone. Physiological dead space includes anatomical dead space plus any alveoli that are ventilated but not properly perfused with blood (in healthy lungs, these two values are nearly equal).