The lungs are the body’s primary respiratory organs. They are responsible for extracting oxygen from the atmosphere and delivering it to the bloodstream, while simultaneously removing carbon dioxide, a waste product. The unique, light texture of these organs—often described as spongy—is a direct reflection of the microscopic structures that perform this life-sustaining exchange. Understanding this texture requires examining the anatomical components that allow the lungs to hold air and move gases efficiently.
What Makes Lung Tissue Spongy
The characteristic spongy texture of the lungs is caused by millions of tiny, balloon-shaped air sacs called alveoli. These microscopic pouches form the bulk of the lung tissue, known as the parenchyma. A typical pair of human lungs contains an estimated 480 million alveoli, creating an enormous internal surface area for respiration.
If these air sacs were flattened out, they would cover a surface area between 70 and 80 square meters, roughly the size of a tennis court. The walls of these air sacs are extremely thin, composed primarily of a single layer of flattened cells.
A network of specialized proteins, predominantly elastin fibers, contributes to the tissue’s resilience. These fibers act like tiny rubber bands, allowing the air sacs to stretch open easily during inhalation. Their tendency to recoil helps passively push the air back out during exhalation, preventing the lungs from remaining overinflated.
How Gas Exchange Works
The actual work of respiration occurs across the alveolar-capillary membrane, the physical barrier separating the air inside the alveoli from the blood circulating through the lung. This barrier is incredibly thin, measuring only about 0.1 to 1.5 micrometers in thickness, which facilitates the rapid movement of gases. The alveoli are enveloped by a dense mesh of capillaries.
Gas exchange is driven by diffusion, where gases naturally move from an area of high concentration to an area of low concentration. The fresh air entering the alveoli has a high concentration of oxygen, while the blood arriving from the body has a low concentration of oxygen and a high concentration of carbon dioxide.
Oxygen molecules diffuse across the membrane from the alveolar air space into the passing red blood cells within the capillaries. Simultaneously, carbon dioxide, a metabolic waste product, leaves the blood and diffuses into the alveoli to be expelled during exhalation. This continuous, passive exchange allows the body to maintain the necessary balance of oxygen and carbon dioxide.
The Air Delivery System
The spongy, air-filled tissue requires a robust system of tubes to ensure a constant supply of fresh air reaches the millions of air sacs. This system begins with the trachea, or windpipe. The trachea then divides into the right and left bronchi, which enter the respective lungs.
These main bronchi branch repeatedly, forming a complex network of progressively smaller airways, often referred to as the bronchial tree. The smallest passageways are called bronchioles, which have diameters less than one millimeter. These narrow tubes finally terminate in the alveolar ducts, which lead directly to the clusters of alveoli. This extensive, branching pathway ensures that atmospheric air is efficiently conducted deep into the lung tissue.
When the Spongy Tissue Is Damaged
Damage to the spongy tissue directly impairs its primary function of gas exchange, leading to a loss of respiratory efficiency. In a condition like emphysema, the delicate walls of the alveoli are progressively destroyed, permanently reducing the surface area available for gas transfer. The loss of these walls also diminishes the lung’s ability to recoil, often leading to trapped air.
Pulmonary fibrosis involves the scarring and thickening of the tissue between the air sacs, an area called the interstitium. This scarring makes the lung tissue stiff and less compliant, hindering the ability of the alveoli to expand and recoil. The thickened tissue also increases the distance gases must travel to diffuse into the bloodstream, making oxygen uptake significantly harder.
Pneumonia, an infection, causes the air sacs to fill with fluid and inflammatory cells instead of air. This fluid buildup reduces the functional air space and severely compromises the diffusion of oxygen across the alveolar-capillary membrane.