Snakes possess a highly specialized respiratory system adapted to their elongated body shape. The most notable feature is the asymmetry of their lungs. Most species have only one fully functional lung, with the right lung serving as the primary organ of respiration. The left lung, if present, is significantly reduced or exists as a non-functional remnant. This unique arrangement allows internal organs to be distributed linearly, which is necessary for their narrow body plan.
The Primary Breathing Apparatus: The Vascular Lung
The single, functional right lung in a snake is highly elongated and divided into two distinct regions to accommodate its slender body. The anterior section, located near the head, is the vascular lung, which serves as the primary site for gas exchange. This region is structurally similar to the lungs of other reptiles, featuring a honeycomb-like network of capillary-bearing partitions known as faveoli, which maximize the surface area for oxygen absorption.
This vascularized portion is rich in blood vessels, allowing oxygen to diffuse into the bloodstream efficiently. The respiratory tissue is often confined to the most forward section of the lung, sometimes only making up about 20% of its total volume. This anatomical placement near the head ensures that fresh air reaches the gas exchange surface quickly and effectively.
The posterior region of the main lung is known as the saccular lung or caudal air sac. This section is thin-walled, lacks significant vascularization, and does not participate in gas exchange. Instead, it acts as a large, membranous reservoir to move air through the functional anterior portion. For example, the air sac can account for approximately 76% of the total lung volume in a one-kilogram snake, highlighting its role as a substantial air holder.
Addressing the “Second Lung” Question: The Vestigial Structure
The left lung is almost always vestigial in the majority of species. This reduction is an evolutionary adaptation that frees up space within the narrow body cavity. The degree of reduction varies, ranging from being completely absent to existing as a small, rudimentary structure.
In more advanced snake families, such as the Colubrids, the left lung is typically only a small, non-functional stub. In some primitive snakes, like boas and pythons (Boidae), the left lung may be slightly larger, though it remains significantly smaller than the right. Even when a remnant is present, it lacks the necessary vascularity and structure for meaningful respiration.
Developmental studies indicate that this asymmetry begins early in the embryonic stage. The left lung bud either fails to develop fully or its growth is significantly slowed compared to the right.
Specialized Breathing Mechanics and Air Storage
Snakes do not possess a diaphragm, so they rely on the movement of their ribs and associated intercostal muscles to draw air in and out of their lungs. Air is inhaled when the rib muscles contract, expanding the rib cage and creating negative pressure within the body cavity. This mechanism is highly flexible, allowing the snake to shift which sections of the rib cage it uses for breathing.
This respiratory flexibility is particularly important when a snake is consuming large prey, a process that can take hours and severely compress the front portion of the body. A snake can immobilize its front ribs and instead activate a set of ribs further down the body to maintain air flow. The air sac portion of the lung is used during these periods to store a large volume of air, ensuring a continuous supply of oxygen.
In some species, a specialized structure called the tracheal lung is also present, which is a vascularized extension of the windpipe itself. The tracheal lung provides a limited gas exchange surface even before the air reaches the main lung. Furthermore, the snake can extend the glottis, the opening to the windpipe, forward and out the side of its mouth, allowing it to breathe around the bulk of a swallowed meal.