Respiratory system

The respiratory system, also called the respiratory apparatus or ventilatory system, is a set of organs and structures that animals and plants use to exchange gases. In land animals, the surface where this exchange happens is tucked inside the body, lining the lungs. Within the lungs, gas exchange takes place in millions of tiny air sacs. In mammals and reptiles, these sacs are called alveoli; in birds, they are known as atria.

These microscopic sacs are richly supplied with blood, which brings the air into close contact with the bloodstream. A network of hollow tubes—the airways—connects these sacs to the outside world. The largest of these tubes is the trachea, which splits in the middle of the chest into two main bronchi. These enter the lungs and divide into narrower secondary and tertiary bronchi, which then branch into even smaller tubes: bronchioles in mammals and reptiles, and parabronchi in birds.

The bronchioles or parabronchi generally open into the microscopic alveoli (in mammals) or atria (in birds). Air is moved from the environment into these sacs by breathing, a process driven by the muscles of respiration. In most fish and many other aquatic animals—both vertebrates and invertebrates—the respiratory system uses gills. These are partly or fully external organs, bathed in water.

Fish

Water flows over the gills either actively or passively. Gas exchange occurs in the gills, which are made of thin, flat filaments and lamellae that expose a very large surface area of highly vascularized tissue to the water. Other animals, such as insects, have much simpler respiratory systems. Plants have respiratory systems too, but the direction of gas exchange can be opposite to that in animals.

Anatomy

Plant respiratory systems include structures like stomata, found in various parts of the plant. In humans and other mammals, the typical respiratory system is organized as the respiratory tract. This tract is divided into an upper and a lower portion. The upper tract includes the nose, nasal cavities, sinuses, pharynx, and the part of the larynx above the vocal folds.

The lower tract includes the lower part of the larynx, the trachea, bronchi, bronchioles, and alveoli. The branching airways of the lower tract are often called the respiratory tree or tracheobronchial tree. The spaces between successive branch points along the tree are referred to as branching generations.

In an adult human, there are about 23 such generations. The earlier generations (roughly generations 0–16) include the trachea, bronchi, and larger bronchioles, which act simply as air conduits, delivering air to the respiratory bronchioles, alveolar ducts, and alveoli (roughly generations 17–23), where gas exchange actually occurs. Bronchioles are defined as small airways that lack any cartilaginous support. The first bronchi to branch from the trachea are the right and left main bronchi.

Second only in diameter to the trachea (1.8 cm), these bronchi (1–1.4 cm in diameter) enter the lungs at each hilum. There they branch into narrower secondary bronchi, called lobar bronchi, which then branch into narrower tertiary bronchi, called segmental bronchi. Further divisions of the segmental bronchi (1 to 6 mm in diameter) are known as 4th order, 5th order, and 6th order segmental bronchi, or are grouped together as subsegmental bronchi. Compared to the average 23 branchings in an adult human, a mouse has only about 13 such branchings.

The alveoli are the dead-end terminals of the tree, meaning any air that enters them must exit by the same route. This creates dead space—a volume of air (about 150 ml in an adult human) that fills the airways after exhalation and is breathed back into the alveoli before fresh environmental air reaches them. At the end of inhalation, the airways are filled with environmental air, which is then exhaled without ever contacting the gas exchanger. During the breathing cycle, the lungs expand and contract, drawing air in and out.

Ventilatory volumes

The volume of air moved in or out under normal resting conditions (the resting tidal volume, about 500 ml), as well as volumes moved during maximally forced inhalation and exhalation, are measured in humans by spirometry. Not all the air in the lungs can be expelled even during maximally forced exhalation (ERV). The air that remains is the residual volume (about 1.0–1.5 liters), which cannot be measured by spirometry. Volumes that include the residual volume—such as functional residual capacity (about 2.5–3.0 liters) and total lung capacity (about 6 liters)—also cannot be measured by spirometry and require special techniques.

The rates at which air is breathed in or out, through the mouth or nose or into or out of the alveoli, are tabulated along with how they are calculated. The number of breath cycles per minute is the respiratory rate. An average healthy human breathes 12–16 times per minute. The mechanics of breathing involve the muscles of respiration, which pump air from the environment into the alveoli or atria.