Upper respiratory tract:
This includes the nose, mouth, and the beginning of the trachea (the section that takes air in and lets it out).
Lower respiratory tract:
This includes the trachea, the bronchi, broncheoli and the lungs (the act of breathing takes place in this part of the system).
The organs of the lower respiratory tract are located in the chest cavity. They are delineated and protected by the ribcage, the chest bone (sternum), and the muscles between the ribs and the diaphragm (that constitute a muscular partition between the chest and the abdominal cavity).
The trachea – the tube connecting the throat to the bronchi.
The bronchi – the trachea divides into two bronchi (tubes). One leads to the left lung, the other to the right lung. Inside the lungs each of the bronchi divides into smaller bronchi.
The broncheoli - the bronchi branches off into smaller tubes called broncheoli which end in the pulmonary alveolus.
Pulmonary alveoli – tiny sacs (air sacs) delineated by a single-layer membrane with blood capillaries at the other end.
The exchange of gases takes place through the membrane of the pulmonary alveolus, which always contains air: oxygen (O2) is absorbed from the air into the blood capillaries and the action of the heart circulates it through all the tissues in the body. At the same time, carbon dioxide (CO2) is transmitted from the blood capillaries into the alveoli and then expelled through the bronchi and the upper respiratory tract.
The inner surface of the lungs where the exchange of gases takes place is very large, due to the structure of the air sacs of the alveoli.
The lungs – a pair of organs found in all vertebrates.
The structure of the lungs includes the bronchial tree – air tubes branching off from the bronchi into smaller and smaller air tubes, each one ending in a pulmonary alveolus.
The act of breathing
The act of breathing has two stages – inhalation and exhalation
Inhalation – the intake of air into the lungs through expansion of chest volume.
Exhalation – the expulsion of air from the lungs through contraction of chest volume.
Inhalation and exhalation involves muscles:
Rib muscles = the muscles between the ribs in the chest.
Diaphragm muscle
Muscle movement – the diaphragm and rib muscles are constantly contracting and relaxing (approximately 16 times per minute), thus causing the chest cavity to increase and decrease.
During inhalation – the muscles contract:
Contraction of the diaphragm muscle – causes the diaphragm to flatten, thus enlarging the chest cavity.
Contraction of the rib muscles – causes the ribs to rise, thus increasing the chest volume.
The chest cavity expands, thus reducing air pressure and causing air to be passively drawn into the lungs. Air passes from the high pressure outside the lungs to the low pressure inside the lungs.
During exhalation – the muscles relax:
The muscles are no longer contracting, they are relaxed.
The diaphragm curves and rises, the ribs descend – and chest volume decreases.
The chest cavity contracts thus increasing air pressure and causing the air in the lungs to be expelled through the upper respiratory tract. Exhalation, too, is passive. Air passes from the high pressure in the lungs to the low pressure in the upper respiratory tract.
Inhalation and exhalation are involuntary and therefore their control requires an effort.
