Anatomy
Humans have a complex and intricate respiratory system. The human respiratory system is comprised of two main parts- the upper respiratory tract and the lower respiratory tract. The upper respiratory tract consist of the mouth, nasal cavity, pharynx and trachea. The lower is made up of the bronchi and lungs. Humans have a right and left lung. The right lung is slightly larger than the left because it contains three lobes where the left contains just two. Both lungs are surrounded by a pleural sac. Much of the respiratory system is comprised of branching airways within the lungs, along which ventilation occurs. These airways can be separated into two zones- the conducting zone and the respiratory zone.
The conducting zone, comprised of the trachea, bronchi, bronchioles and terminal bronchioles, is the site of bulk air flow. The relaxation and contraction of smooth muscle in these airways is responsible for the ease with which air can flow through these passages. When the smooth muscle is contraction, the bronchioles constrict and resistance in the airways increases. Contrary, when the smooth muscle is relaxed, bronchodilation occurs and air moves with more ease through the airways.
The respiratory zone, including the respiratory bronchioles, alveolar ducts and alveolar sacs, is the region in which carbon dioxide and oxygen molecules are exchanged between the lungs and the blood, in order to deliver oxygen to tissues where it is required, and to expel carbon dioxide from the body. This process is referred to as ventilation.
Negative-Pressure Breathers
During inhalation, the floor of the chest cavity is lowered causing the volume of the lungs to increase. This increase in lung volume causes a decrease in pressure, drawing air into the lungs. For this reason humans are referred to as negative pressure breathers. On the contrary, during expiration, the floor of the chest cavity rises and decreases lung volume, thus increasing the pressure. This will push the air out of the lungs.
Site of Gas Exchange
In humans, gas exchange occurs in the respiratory zone. More specifically, the alveolar sacs are responsible for the release and uptake of molecules such as O2 and CO2. The process of exchange of these molecules occurs by the way of passive diffusion. For this reason, the respiratory system branches into millions (approximately 8,000,000) alveolar sacs which are surrounded by thin-walled capillaries, creating a favoured environment for gas exchange.
