Gas exchange The major function of the<unk> s is gas exchange between the<unk> s and the blood. The alveolar and pulmonary capillary gases equilibrate across the thin blood–air barrier. This thin membrane (about 0.5 –2 μm thick) is folded into about 300 million alveoli, providing an extremely large surface area (estimates varying between 70 and 145 m2) for gas exchange to occur. respiratory muscles in expanding the rib cage. The<unk> s are not capable of expanding to breathe on their own, and will only do so when there is an increase in the volume of the thoracic cavity. This is achieved by the muscles of respiration, through the contraction of the diaphragm, and the intercostal muscles which pull the rib cage upwards as shown in the diagram. During breathing out the muscles relax, returning the<unk> s to their resting position. At this point the<unk> s contain the functional residual capacity (FRC) of air, which, in the adult human, has a volume of about 2.5–3.0 litres. During heavy breathing as in exertion, a large number of accessory muscles in the neck and abdomen are recruited, that during exhalation pull the ribcage down, decreasing the volume of the thoracic cavity. The FRC is now decreased, but since the<unk> s cannot be emptied completely there is still about a litre of residual air left.<unk>  function testing is carried out to evaluate<unk>  volumes and capacities. Protection The<unk> s possess several characteristics which protect against infection. The respiratory tract is lined by respiratory epithelium or respiratory mucosa, with hair-like projections called cilia that beat rhythmically and carry mucus. This mucociliary clearance is an important defence system against air-borne infection. The dust particles and bacteria in the inhaled air are caught in the mucosal surface of the airways, and are moved up towards the pharynx by the rhythmic upward beating action of the cilia. The lining of the<unk>  also secretes immunoglobulin A which protects against respiratory infections; goblet cells secrete mucus which also contains several antimicrobial compounds such as defensins, antiproteases, and antioxidants. A rare type of specialised cell called a pulmonary ionocyte that is suggested may regulate mucus viscosity has been described. In addition, the lining of the<unk>  also contains macrophages, immune cells which engulf and destroy debris and microbes that enter the<unk>  in a process known as phagocytosis; and dendritic cells which present antigens to activate components of the adaptive immune system such as T cells and B cells. The size of the respiratory tract and the flow of air also protect the<unk> s from larger particles. Smaller particles deposit in the mouth and behind the mouth in the oropharynx, and larger particles are trapped in nasal hair after inhalation. Other In addition to their function in respiration, the<unk> s have a number of other functions. They are involved in maintaining homeostasis, helping in the regulation of blood pressure as part of the renin–angiotensin system. The inner lining of the blood vessels secretes angiotensin-converting enzyme (ACE) an enzyme that catalyses the conversion of angiotensin I to angiotensin II. The<unk> s are involved in the blood's acid–base homeostasis by expelling carbon dioxide when breathing. The<unk> s also serve a protective role. Several blood-borne substances, such as a few types of prostaglandins, leukotrienes, serotonin and bradykinin, are excreted through the<unk> s. Drugs and other substances can be absorbed, modified or excreted in the<unk> s. The<unk> s filter out small blood clots from veins and prevent them from entering arteries and causing strokes. The<unk> s also play a pivotal role in speech by providing air and airflow for the creation of vocal sounds, and other paralanguage communications such as sighs and gasps. Research suggests a role of the<unk> s in the production of blood platelets.
Lung
Kidney
Liver
Heart
Brain
Breast
Central nervous system
Peripheral nervous system