The Respiratory System: Gas Exchange

The Respiratory System: Gas Exchange

The primary function of the respiratory system is to deliver oxygen (O2) to the cells of the body's tissues and remove carbon dioxide (CO2) as a cell waste product. This process is known as gas exchange. The main structures of the human respiratory system are the nasal cavity, the trachea, and lungs.

Direct Diffusion

In small multicellular organisms, diffusion across the outer membrane is sufficient to meet their oxygen needs. Gas exchange by direct diffusion across surface membranes is efficient for organisms less than 1 mm in diameter. In simple organisms, such as cnidarians and flatworms, every cell in the body is close to the external environment. Their cells are kept moist, and gases diffuse quickly via direct diffusion. Flatworms are small, literally flat worms, which 'breathe' through diffusion across the outer membrane.

Mammalian Systems

In mammals, pulmonary ventilation occurs via inhalation (breathing). During inhalation, air enters the body through the nasal cavity, which is located just inside the nose. As air passes through the nasal cavity, the air is warmed to body temperature and humidified. The respiratory tract is coated with mucus to seal the tissues from direct contact with air. Mucus is high in water. As air crosses these surfaces of the mucous membranes, it picks up water. These processes help equilibrate the air to the body conditions, reducing any damage that cold, dry air can cause.

Gas Exchange in the Lungs

Gas exchange between the lung and blood takes place in the alveolus, where capillaries in the lungs of the farm animal species and horses also possess intravascular macrophages, which are important as a reticuloendothelial organ in the processing of antigens.

Insect Tracheal Systems

Insects have specialized tracheal systems that enable rapid recovery from hypoxia in a manner matched to respiratory and metabolic needs. Tracheae transport O2 directly to organs without a blood or hemolymph system. Oxygen diffuses rapidly from tracheal endings to the mitochondria of end cells because of a high diffusivity of O2 in air compared to fluids and because most insects actively ventilate their tracheal system.

Discontinuous Respiration in Tracheated Arthropods

Discontinuous gas-exchange cycles (DGC) occur during rest or pupal development, dependent on the ability to actively close spiracles. DGC was first described in lepidopterous pupae. Typical DGC consists of three phases: (i) closed spiracle (C), (ii) flutter (F), and (iii) open (O) or ventilation (V). In C-phase, external gas exchange is negligible; hence internal O2 concentration decreases. Simultaneously, CO2 is released and buffered in the hemolymph and tissues, generating a negative pressure in the tracheal system.

In summary, the respiratory system plays a crucial role in gas exchange, delivering oxygen to the body's tissues and removing carbon dioxide. This process occurs through various mechanisms, including direct diffusion, mammalian pulmonary ventilation, and specialized tracheal systems in insects and arthropods. The efficiency of these processes is essential for maintaining the health and survival of organisms.