Thoracic Cavity and Respiratory System: Gross Anatomy and Functional Organization

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Thoracic Cavity: Gross Anatomy

Orientation and Structure

The thoracic cavity houses vital organs such as the heart and lungs, and is bounded by the rib cage, vertebral column, and diaphragm. Understanding its cross-sectional anatomy is essential for clinical and anatomical studies.

Spinous process projects posteriorly, serving as a landmark in transverse sections.
The heart is slightly left of the midline, making the left lung slightly smaller than the right.
The position of vertebrae, bronchi, and arteries varies with the level of the thoracic cavity.
Bronchi contain cartilaginous plates for structural support.

Transverse section through the thorax at T6, showing lungs, pleural membranes, and major organs Cross section through the thorax at T6, showing anatomical relationships of lungs, heart, and vessels

Arteries have elastic tissue and smooth muscle; veins have larger diameters and thinner walls.
Systemic and pulmonary vessels intermingle within the thoracic cavity.
Paired and unpaired vessels are distinguished by their anatomical positions.

Bronchopulmonary Segments

Segmental Organization of the Lungs

Each lung is divided into bronchopulmonary segments, which are functionally and anatomically discrete units. This segmentation is crucial for surgical resection and understanding pulmonary pathology.

There are approximately 10 segments per lung.
Each segment is supplied by a single tertiary (segmental) bronchus.
Segments are separated by dense connective tissue and contain multiple lung lobules.

Diagram of segmental bronchi supplying bronchopulmonary segments Distribution of bronchopulmonary segments on the lateral surface of the lungs

Segments are relatively independent, separated by connective tissue, with few small veins between them.

Pleural Membranes and Pleural Cavity

Structure and Clinical Importance

The lungs are enveloped by a double-layered serous membrane called the pleura. The pleural cavity, filled with pleural fluid, reduces friction during respiration and maintains negative pressure for lung expansion.

Visceral pleura adheres to the lung surface.
Parietal pleura lines the inner thoracic wall, diaphragm, and mediastinum.
The pleural cavity is the space between these layers, containing pleural fluid.
The inferior location of the parietal pleura is important for maintaining intrapleural pressure, especially during clinical procedures such as thoracentesis.

Anterior and posterior views of the thoracic wall showing the parietal pleura and rib landmarks

Respiratory Centers of the Brainstem

Neural Control of Breathing

Respiratory rhythm and pattern are regulated by centers in the brainstem, which integrate sensory input and coordinate muscle activity for ventilation.

Ventral Respiratory Group (VRG): Located in the medulla; generates the basic rhythm of breathing.
Pontine Respiratory Group: Located in the pons; smooths and modulates the respiratory pattern.
Dorsal Respiratory Group (DRG): Located in the medulla; integrates peripheral sensory input and modifies VRG output.

Diagram of respiratory centers in the brainstem and their connections to respiratory muscles

Peripheral and Central Chemoreceptors

Regulation of Ventilation by Blood Chemistry

Ventilation rate is adjusted based on blood chemistry, primarily the levels of carbon dioxide, oxygen, and pH. Chemoreceptors relay this information to the brainstem respiratory centers.

Central chemoreceptors: Located in the medulla; sensitive to changes in CO2 and pH in cerebrospinal fluid.
Peripheral chemoreceptors: Located in the carotid bodies (at the bifurcation of the common carotid artery, innervated by the glossopharyngeal nerve) and aortic bodies (in the aortic arch, innervated by the vagus nerve).

Diagram showing the location and innervation of peripheral chemoreceptors in the carotid and aortic bodies

These receptors provide rapid feedback to adjust ventilation in response to hypoxemia, hypercapnia, or acidosis.

Summary Table: Key Features of the Thoracic Cavity and Respiratory System

Structure	Function	Clinical Relevance
Thoracic cavity	Houses heart, lungs, major vessels	Landmarks for imaging, surgery
Bronchopulmonary segments	Functional lung units	Segmental resection, pathology
Pleural membranes	Reduce friction, maintain pressure	Pleural effusion, pneumothorax
Respiratory centers	Control breathing rhythm	Central respiratory disorders
Chemoreceptors	Monitor blood gases	Ventilatory response to hypoxia