alveolar sacs contain the primitive alveoli that form at the end of the alveolar ducts, and their appearance around the seventh month marks the point at which limited respiration would be possible, and the premature baby could survive. vitamin a deficiency main article: vitamin a deficiency the developing lung is particularly vulnerable to changes in the levels of vitamin a. vitamin a deficiency has been linked to changes in the epithelial lining of the lung and in the lung parenchyma. this can disrupt the normal physiology of the lung and predispose to respiratory diseases. severe nutritional deficiency in vitamin a results in a reduction in the formation of the alveolar walls (septa) and to notable changes in the respiratory epithelium; alterations are noted in the extracellular matrix and in the protein content of the basement membrane. the extracellular matrix maintains lung elasticity; the basement membrane is associated with alveolar epithelium and is important in the blood-air barrier. the deficiency is associated with functional defects and disease states. vitamin a is crucial in the development of the alveoli which continues for several years after birth. after birth at birth, the baby's lungs are filled with fluid secreted by the lungs and are not inflated. after birth the infant's central nervous system reacts to the sudden change in temperature and environment. this triggers the first breath, within about 10 seconds after delivery. before birth, the lungs are filled with fetal lung fluid. after the first breath, the fluid is quickly absorbed into the body or exhaled. the resistance in the lung's blood vessels decreases giving an increased surface area for gas exchange, and the lungs begin to breathe spontaneously. this accompanies other changes which result in an increased amount of blood entering the lung tissues. at birth the lungs are very undeveloped with only around one sixth of the alveoli of the adult lung present. the alveoli continue to form into early adulthood, and their ability to form when necessary is seen in the regeneration of the lung. alveolar septa have a double capillary network instead of the single network of the developed lung. only after the maturation of the capillary network can the lung enter a normal phase of growth. following the early growth in numbers of alveoli there is another stage of the alveoli being enlarged. function main articles: respiratory system, breathing, and gas exchange gas exchange the major function of the lungs is gas exchange between the lungs 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. the effect of the respiratory muscles in expanding the rib cage the lungs 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 lungs to their resting position. at this point the lungs 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 lungs cannot be emptied completely there is still about a litre of residual air left. lung function testing is carried out to evaluate lung volumes and capacities. protection the lungs 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.: 661–730 the lining of the lung 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 lung also contains macrophages, immune cells which engulf and destroy debris and microbes that enter the lung 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 lungs 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 lungs 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 lungs are involved in the blood's acid–base homeostasis by expelling carbon dioxide when breathing. the lungs also serve a protective role. several blood-borne substances, such as a few types of prostaglandins, leukotrienes, serotonin and bradykinin, are excreted through the lungs. drugs and other substances can be absorbed, modified or excreted in the lungs. the lungs filter out small blood clots from veins and prevent them from entering arteries and causing strokes. the lungs 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 lungs in the production of blood platelets. gene and protein expression further information: bioinformatics § gene and protein expression about 20,000 protein coding genes are expressed in human cells and almost 75% of these genes are expressed in the normal lung. a little less than 200 of these genes are more specifically expressed in the lung with less than 20 genes being highly lung specific. the highest expression of lung specific proteins are different surfactant proteins, such as sftpa1, sftpb and sftpc, and napsin, expressed in type ii pneumocytes. other proteins with elevated expression in the lung are the dynein protein dnah5 in ciliated cells, and the secreted scgb1a1 protein in mucus-secreting goblet cells of the airway mucosa. clinical significance main articles: respiratory disease and pulmonology lungs can be affected by a number of diseases and disorders. pulmonology is the medical speciality that deals with respiratory diseases involving the lungs and respiratory system. cardiothoracic surgery deals with surgery of the lungs including lung volume reduction surgery, lobectomy, pneumectomy and lung transplantation. inflammation and infection