As a common mammal, the mouse relies on a respiratory system to exchange gases with the environment. This system brings oxygen into the body for cellular energy production and expels the waste product, carbon dioxide. The architecture and operation of the lungs are central to the animal’s ability to sustain its high-energy lifestyle.
Anatomical Structure of Mouse Lungs
The mouse lung has a distinct, asymmetrical lobar arrangement. The left lung is a single, uniform lobe, while the right lung is larger and subdivided into four separate lobes: the cranial, middle, caudal, and accessory. The accessory lobe, also known as the post-caval lobe, is situated medially and wraps around major blood vessels.
Air enters the respiratory system through the trachea, which bifurcates into the main bronchi leading to each lung. In the mouse, airways with supportive cartilage rings end at these lobar bronchi. From there, the airways continue to branch into progressively smaller, non-cartilaginous tubes called bronchioles. This branching pattern is primarily monopodial, with a main axial airway running through each lobe and smaller branches emerging from its sides.
The pathway concludes as terminal bronchioles lead into the lung’s gas exchange region. This area is composed of alveolar ducts and sacs, which are populated with millions of tiny, thin-walled air sacs known as alveoli. It is within the alveoli that the transfer of gases between the air and the bloodstream occurs. The immense number of alveoli creates a vast surface area, maximizing the efficiency of this process.
Physiological Function and Respiration
The physiological operation of mouse lungs is linked to the animal’s high metabolic rate. Mice have a greater mass-specific oxygen consumption and carbon dioxide production compared to many other laboratory animals. To meet these metabolic demands, mice exhibit a rapid respiratory frequency, taking many more breaths per minute than larger mammals to ensure a sufficient supply of oxygen.
This rapid breathing facilitates gas exchange at the alveolar level. When air fills the alveoli, oxygen diffuses across the thin alveolar and capillary walls into the blood. Simultaneously, carbon dioxide transported from the body’s tissues moves from the blood into the alveoli to be exhaled.
The efficiency of this exchange is often assessed by measuring the respiratory exchange ratio (RER), which is the ratio of carbon dioxide produced to oxygen consumed. This value provides insights into which metabolic substrates, such as carbohydrates or fats, are being used for energy. The continuous ventilation of the lungs maintains the concentration gradients for these gases, allowing for their passive diffusion.
Comparison to Human Lungs
While the fundamental process of gas exchange is similar, there are significant architectural differences between mouse and human lungs. The human lung has a different lobar structure, with two lobes on the left and three on the right. Another structural variance is in the airway composition, as cartilage rings in humans extend much deeper into the bronchial tree to support smaller airways.
The branching pattern of the airways also differs, with human airways exhibiting a more symmetric, forked branching. A notable difference in the deeper lung is the near absence of respiratory bronchioles in mice. In humans, these structures form a transition zone where alveoli begin to appear on the walls of the smallest bronchioles.
At the cellular level, the distribution of certain cell types varies. Mouse airways contain a higher proportion of Club cells compared to humans, where they are more restricted to smaller airways. Conversely, mucus-producing goblet cells and submucosal glands are less abundant in mouse airways and are confined to the most proximal parts of the trachea.
Role in Biomedical Research
The anatomical and physiological traits of mouse lungs make them a frequently used tool in respiratory research. Scientists develop “mouse models” to study human diseases by recreating specific pathological conditions in the animals. For instance, asthma research uses models where mice are exposed to allergens like ovalbumin to trigger airway inflammation and hyperresponsiveness.
To study conditions like pulmonary fibrosis, a disease characterized by lung scarring, researchers may administer the chemical bleomycin to mice. This induces a fibrotic response that shares features with human idiopathic pulmonary fibrosis. In cancer research, specific lung tumors can be studied using mice genetically altered to develop cancers that resemble human malignancies, such as those driven by mutations in the Kras gene.
The ability to genetically modify mice has profoundly advanced lung research. Using techniques like CRISPR/Cas9, scientists can create “knockout” mice that lack a specific gene or “transgenic” mice that express a foreign gene. For example, knockout mice for the proteins ACE2 and TMPRSS2 were instrumental in demonstrating how the SARS virus enters cells. Building on this, transgenic mice expressing the human ACE2 receptor were developed as a model for studying COVID-19, as they can be infected with SARS-CoV-2 and develop lung disease similar to that seen in humans.