Comprehensive Study Notes on the Pleurae, Bronchial Tree, and Pulmonary Alveoli, and Pulmonary Circulation

The Pleurae

The pleura ($\text{PLOOR-uh}$) is a serous membrane that serves two primary anatomical functions: it lines the thoracic wall and forms the surface of the lung. This membrane is composed of two distinct layers: the visceral pleura and the parietal pleura. The visceral pleura forms the outermost surface of the lung and extends deep into the fissures located between the lung lobes. At the hilum (the region where the lung is attached to its roots), this membrane turns back on itself to form the parietal pleura. The parietal pleura adheres to the mediastinum, the inner surface of the rib cage, and the superior surface of the diaphragm. An extension of the parietal pleura, known as the pulmonary ligament, serves to connect the pleura to the diaphragm.

The space existing between the parietal and visceral pleurae is designated as the pleural cavity. It is important to note that the pleural cavity does not contain the lung itself; instead, it wraps around the lung in a manner similar to how the pericardium wraps around the heart. Under normal conditions, the pleural cavity is only a potential space, containing nothing but a thin film of lubricating pleural fluid. This means there is normally no actual room between the two membranes. However, under pathological conditions, such as chest wounds or the seepage of fluid into the space (a condition known as pleural effusion), the pleural cavity can fill with air or liquid.

Pulmonary Circulation and Lung Blood Supply

The lungs receive blood via two distinct supplies that serving different purposes: the pulmonary circuit and the systemic bronchial arteries. The pulmonary circuit is responsible for gas exchange. It begins with the pulmonary trunk, which arises from the right ventricle of the heart. After a short ascent, the trunk divides into the right and left pulmonary arteries. These then divide into major branches called lobar arteries, which supply the three lobes of the right lung and the two lobes of the left lung. Within the lungs, the finer branches of these arteries travel alongside the bronchi and eventually give rise to a dense web of blood capillaries in the wall of every alveolus. This circuit's sole purpose is to unload carbon dioxide ($CO_2$) picked up from the body and to pick up a fresh load of oxygen ($O_2$) from inhaled air, returning this oxygenated blood to the heart via the pulmonary veins.

In addition to the pulmonary circuit, the lungs receive a systemic blood supply from the bronchial arteries, which arise from the thoracic aorta. These arteries carry freshly oxygenated blood into the lungs to nourish the lung tissue itself, specifically the pleura, bronchi, bronchioles, and larger pulmonary blood vessels. Notably, bronchial arteries do not lead to the alveoli. Bronchial veins drain this systemic blood from the lungs into the azygos vein of the thorax.

Oxygen Saturation and Physiological Shunting

Most pulmonary blood leaving well-ventilated alveoli is $100\%$ saturated with oxygen. However, the small pulmonary veins anastomose (join) with the bronchial veins along their path, causing their blood streams to mix. A significant portion of the bronchial venous blood enters the pulmonary veins—a process called a right-to-left shunt. This is so named because this blood would normally have flowed into the right atrium of the heart but is instead diverted to the left heart. Consequently, the oxygen in the pulmonary veins is slightly diluted by the less-oxygenated blood of the bronchial veins.

Another reason oxygen levels are lower than a perfect $100\%$ in the systemic circulation is that some blood in the pulmonary circuit passes through collapsed or unventilated alveoli that offer no oxygen. Due to these factors, the measurement of arterial blood oxygen saturation using a fingertip pulse oximeter typically shows values in the $98\%$ to $99\%$ range rather than $100\%$. Furthermore, it is critical to prevent fluid accumulation in the alveoli because gases diffuse too slowly through liquid to aerate the blood sufficiently. The alveoli are kept dry by the absorption of fluid, leaving only a thin film of moisture on the alveolar walls.

The Bronchial Tree and Airway Structure

Each lung contains a branching system of air tubes known as the bronchial tree, which extends from the main bronchus to approximately $65,000$ terminal bronchioles. Starting at the tracheal fork, the right main (primary) bronchus is $2$ to $3\,\text{cm}$ long and is slightly wider and more vertical than the left bronchus. Due to this orientation, aspirated (inhaled) foreign objects lodge in the right bronchus more frequently than in the left. The right main bronchus gives off three branches: the superior, middle, and inferior lobar (secondary) bronchi—one for each lobe. The left main bronchus is approximately $5\,\text{cm}$ long, narrower, and less vertical than the right, giving off superior and inferior lobar bronchi to the two lobes of the left lung.

In both lungs, the lobar bronchi branch into segmental (tertiary) bronchi. There are $10$ segmental bronchi in the right lung and $8$ in the left. Each segmental bronchus ventilates a functionally independent unit of lung tissue termed a bronchopulmonary segment. Main bronchi are supported by rings of hyaline cartilage, while the cartilage in lobar and segmental bronchi transitions to overlapping crescent-shaped plates. All bronchi are lined with ciliated pseudostratified columnar epithelium, which becomes thinner and shorter as the airway progresses distally. The lamina propria contains many mucous glands and lymphoid nodules (mucosa-associated lymphoid tissue, or MALT) to intercept pathogens. Additionally, elastic connective tissue provides recoil for air expulsion, and the muscularis mucosae (a layer of smooth muscle) regulates airflow by constricting or dilating the airway.

Bronchioles and the Terminal Airway

Bronchioles ($\text{BRON-kee-olz}$) are continuations of the airway that are $1\,\text{mm}$ or less in diameter and lack supportive cartilage. The specific portion of the lung ventilated by a single bronchiole is called a pulmonary lobule. Bronchioles feature a ciliated cuboidal epithelium and a well-developed layer of smooth muscle. At the time of death, spasmodic contractions of this muscle cause bronchioles to appear with a wavy lumen in histological sections. Each bronchiole divides into $50$ to $80$ terminal bronchioles, which are the final branches of the conducting zone. Terminal bronchioles measure $0.5\,\text{mm}$ or less in diameter and contain no mucous glands or goblet cells. However, they do possess cilia, allowing the mucociliary escalator to drive mucus back toward the proximal passages to prevent congestion in the alveoli.

Alveolar Structure and Comparison

Human lung efficiency is a result of complex alveolar architecture. While amphibians like frogs have simple, balloon-like hollow lungs, mammals have evolved human lungs that are spongy masses composed of approximately $480\text{ million}$ little sacs called alveoli. These provide a massive gas-exchange surface of roughly $70\,\text{m}^2$ per lung, which is equivalent to the floor area of an American indoor handball court or a room $8.4\,\text{m}$ ($25\,\text{ft}$) square.

An individual alveolus ($\text{AL-vee-OH-lus}$) is a pouch measuring roughly $0.2$ to $0.5\,\text{mm}$ in diameter. They are polygonal in shape and flat where they interface with others, resembling clustered soap bubbles rather than grapes. Alveoli have pores in their walls to exchange air with one another. When viewing lung tissue microscopically, it is important to remember that the tissue is often shriveled or distorted by histological fixatives.

Alveolar Cell Types and the Respiratory Membrane

There are three primary cell types within the alveoli. First are the squamous (type I) alveolar cells, which are thin, broad cells covering approximately $95\%$ of the alveolar surface area. Their extreme thinness is specialized for rapid gas diffusion. Second are the great (type II) alveolar cells, which are round to cuboidal and cover the remaining $5\%$ of the surface area. Despite covering less area, they outnumber squamous cells. Great alveolar cells have two primary functions: (1) repairing the alveolar epithelium when squamous cells are damaged, and (2) secreting pulmonary surfactant. Pulmonary surfactant is a mixture of phospholipids and protein that coats the alveoli and smallest bronchioles to prevent them from collapsing during exhalation. To visualize the difference, a ball of dough represents a great alveolar cell, while that same dough rolled out into a thin pizza crust represents a squamous alveolar cell.

Third are the alveolar macrophages (dust cells), the most numerous of all lung cells. These wander the lumens and connective tissues, phagocytizing dust particles, bacteria, and blood cells to keep the lungs clear of debris. Approximately $100\text{ million}$ of these macrophages perish daily as they are carried up the mucociliary escalator to be swallowed and digested.

The barrier between alveolar air and the blood is the respiratory membrane. It is remarkably thin, with a total thickness of only $0.5\,\mu\text{m}$ (about $1/15$ the diameter of a single erythrocyte). This membrane consists of the squamous alveolar cell, the squamous endothelial cell of the capillary, and their shared basement membrane.

The Path of Airflow

The movement of air through the respiratory system is divided into the conducting zone (no gas exchange) and the respiratory zone (gas exchange occurs). The sequence is as follows:

1. Conducting Zone: Nasal cavity (\rightarrow) Pharynx (\rightarrow) Trachea (\rightarrow) Main bronchus (\rightarrow) Lobar bronchus (\rightarrow) Segmental bronchus (\rightarrow) Bronchiole (\rightarrow) Terminal bronchiole.

2. Respiratory Zone: Respiratory bronchiole (which have alveoli budding from their walls and scanty smooth muscle) (\rightarrow) Alveolar duct (elongated passages with alveoli along the walls) (\rightarrow) Atrium (a central space with equal length and width) (\rightarrow) Alveolus.