Alveolar sac

Fig. 7-2. Particle deposition as a function of particle diameter in various regions of the lung. The nasopharyngeal region consists of the nose and throat the tracheobronchial region consists of the windpipe and large airways and the pulmonary region consists of the small bronchi and the alveolar sacs. Source Task Group on Lung Dynamics, Health Phys. 12, 173 (1966).

Alveolar duct Airway distal to respiratory bronchiole leading to individual alveoli and alveolar sacs. 

Alveolar sac Group of alveoli originating from an expansion of the alveolar... 

Branching from the terminal bronchioles are the respiratory bronchioles. This is the first generation of airways to have alveoli in their walls. Finally, there are the alveolar ducts which are completely lined with alveolar sacs. This region, from the respiratory bronchioles through the alveoli, is referred to as the respiratory zone, which comprises most of the lungs and has a volume of about 3000 ml at the end of a normal expiration. 

Millien G, Spira A, Hinds A, Wang J, Williams MC, Ramirez MI (2006) Alterations in gene expression in T1 alpha null lung a model of deficient alveolar sac development. BMC Dev Biol 6 35... 

Figure 11.23 (a) Structure of dipalinitoyl phosphatidylcholine. (b) Aggregates c)f alveoli. The advantage of dipalmitoyl phos-phab dylcholine is that it spreads at physiological temperatures to form a thin fat film over the whole inner surface of the alveolar sacs, which reduces surface tension. There are 300-400 million alveoli in each human lung. 

Fig. 3.1 Schematic diagram of the human respiratory system. The gross anatomy of the lung, the covering membranes (pleura), airways and air sacs (alveoli) are shown. The average diameter of portions of the air flow system are indicated trachea, 20 mm bronchus, 8 mm terminal and respiratory bronchioles, 0.5 mnn alveolar duct, 0.2 mm alveolar sacs, 0.3 mm.

The lower respiratory tract (pulmonary region or alveolar ducts and sacs) is the area where gas exchange occurs. Alveolar sacs, clusters of two or more alveoli, branch from alveolar ducts. It is generally considered that there is a total of approximately 300 million alveoli in the lungs of adult humans. The total alveolar surface area in the lungs of adult humans is... 

Atrium. The upper chamber of each half of the heart the portion of nasal cavity in the lung, a subdivision of the alveolar duct from which alveolar sacs open. 

FIGURE 10.1 Schematic representation of the lower respiratory system. The trachea leads into the bronchioles that branch and finally reach the alveolar sacs only on one side of the bronchial tree. Inset represents the alveolar tubules and alveolar sacs present at the terminal portions of the bronchial tree. 

Despite the fact that most of the alveolar surface is composited of alveolar epithelium, three primary types of cells are present in the alveoli type I alveolar cells, type II alveolar cells, and alveolar macrophages. Type I alveolar cells are also referred to as squamous pulmonary epithelial cells and are the continuous lining of the alveolar sac. Type II alveolar cells are also referred to as septal cells. Type II alveolar cells secrete the alveolar fluid that is necessary to keep the surface moist and to maintain surface tension of the alveolar fluid surface tension is necessary to keep the alveoli from collapsing. Alveolar fluid is a suitable environment for proteins when compared to the low pH and high protease levels associated with the intestine... 

Figure 3.10 The structure of the mammalian respiratory system (A) trachea, (B) bronchiole, (C) alveolar sac with blood supply, (D) arrangement of blood vessels around alveoli, (E) arrangement of cells and airspaces in alveoli showing the large surface area available for absorption, (F) cellular structure of alveolus showing the close association between (G) the endothelial cell of the capillary (H) with erythrocytes and (I) the epithelial cell of the alveolar sac. The luminal side of the epithelial cell is bathed in fluid, which also facilitates absorption and gaseous exchange. Source From Ref. 1.

Inhaled particles of 20 [im diameter or more tend to be retained in upper parts of the respiratory tract (trachea, pulmonary bronchi and terminal bronchioles) and then removed by ciliary action. In contrast, smaller particles of around 6 pm diameter reach all parts of the respiratory system including the alveolar sacs. Smaller (2 pm) and very small particles (0.2 pm) may not reach the alveolar sacs, only the terminal bronchioles and alveolar ducts. Particles of diameter 1 pm or less may be absorbed if they reach the alveolar sacs. 

Those submicron particulates which enter the alveolar sacs may undergo various degrees of absorption, depending upon the solubility of their components, or are transported to the base of the ciliated bronchiolar epithelium (54). Alveolar absorptive efficiency for most trace elements is 50-80% (50). Retention or absorption is not necessarily a simple function of solubility. Silver iodide, for example, is rapidly absorbed from the lungs even though it is weakly soluble in water (56). Likewise, insoluble elemental lead deposited in the respiratory passages is absorbed, but the mechanism involved remains to be elucidated (49). Vanadium probably accumulates in human lungs in insoluble forms... 

The transition zone consists of the respiratory bronchioles (generations 17 to 19), which contain alveoli. At the terminal end, the respiratory zone is composed of parenchyma that contains the alveolar ducts and about 300 million alveoli (alveolar sacs) to provide the gas-exchange surface. Since the surface area expands to such a large extent within the very last generations of bifurcations, the inhalation airflow rapidly slows down to zero velocity so that the movement of gas molecules and the exchange occurs entirely by diffusion (Stocks and Hisloop 2002). 

If it is inhaled deeply into the lungs, the vapors will come in contact with over 300 million alveolar sacs representing 171 yd2 (143 m2) of surface area. Within three seconds, cocaine is absorbed into the blood and pumped... 

The lung. The middle diagram is a close-up view of the end of a bronchiole. Each bronchiole terminates in many alveolar sacs. The diagram on the right is a close-up view of an alveolus. The cell membrane of each alveolus is very thin and surrounded by many blood capillaries, thereby allowing for rapid exchange of oxygen for carbon dioxide, as well as absorption of other substances. 

The respiratory tract starts at the nose and terminates deep in the lung at an alveolar sac. There are a number of schemes for categorizing the different regions of the respiratory tract. With respect to pulmonary drag delivery, division into the following three regions is useful (Figure 10.1) ... 

Type-I pneumocytes thin cells offering a very short airways-blood path length for the diffusion of gases and drug molecules. Type-I pneumocytes occupy about 93% of the surface area of the alveolar sacs, despite being only half as abundant as type-II cells. 

When a substance is inhaled, it is exposed to membranes of the mouth or nose, pharynx, trachea, bronchi, bronchioles, alveolar sacs, and alveoli (Fig. 1). The lung has a potential absorption surface of some 70 m, a much larger surface than the small intestine. However, the lungs and their associated airways are... 

Lipid-soluble contaminants diffuse passively through the thin alveolar-vascular cell barrier of the alveolar sac and then dissolve into the blood according to the ability of the contaminant to partition between alveolar air and circulating blood. Substances that are very soluble in blood are rapidly transported into the bloodstream. For these substances, like styrene and xylene, the amount absorbed will be greatly enhanced by increasing the rate and the depth of respiration, as is likely to happen when doing strenuous physical work. On the other hand, substances that are poorly soluble in blood have limited capacity for absorption due to rapid saturation of blood. For these substances, like the solvents cyclohexane and methyl chloroform, the amount absorbed may be increased only by... 

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