Anatomy of the Respiratory System

The [1] division of the respiratory system exchanges gases with the blood, whereas the [2] division serves only for airflow to and from 1. The respiratory system begins with the nose, which is partially shaped and supported by lateral and alar [3]. The nasal cavity is divided into right and left [4] by the nasal septum. Each of these spaces is largely filled by three folds of tissue called [5], which have narrow air passages called [6] between them. The respiratory and digestive tracts intersect in the [7], then diverge at the point where food enters the esophagus and air enters a cartilaginous chamber, the [8], containing the vocal cords. The large [9] cartilage of this chamber forms the anterior prominence nicknamed the Adam's apple. This chamber has [10] muscles that elevate it during swallowing and [11] muscles that control the vocal cords.

Below this, the airway continues as the "windpipe," or [12], which branches at its inferior end into a right and left [13]. These tubes enter a hilum on the [14] surface of each lung. The right tube branches into three [15] and the left tube branches into two, corresponding to the number of [16] in each lung. At a diameter of 1 mm and less, the air tubes have no [17] in their walls and are called [18]. Ultimately, the air passages end in thin microscopic sacs called [19]. The walls of these sacs are mostly squamous [20] cells, but there are also some rounded [21] cells that secrete a detergent called [22]. The most abundant cells in the lung, however, are phagocytes called [23]. Each lung is enclosed in a double-walled sac, the [24], with a potential space called the [25] between its two layers.

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Mechanics of Ventilation

According to Boyle's law, if the volume of a space increases, its [26] decreases. This principle applies to the expansion of the thoracic cavity during inspiration. As the respiratory muscles enlarge the thoracic cavity, the pressure between the pleural membranes, or [27] pressure, drops to about 6 mmHg below the atmospheric pressure outside the body (that is, to –6 mmHg). Pressure within the alveoli, called [28] pressure, drops to –3 mmHg. The pressure gradient from the outside air to the alveoli causes air to flow into the lungs. As it does so, the air is heated by the body, and according to [29] law, it expands. This thermal expansion contributes to the inflation of the lungs. Expiration requires no muscular effort. Because of the [30] of the lungs and thoracic cage, they recoil on their own when the muscular effort of inspiration ceases. Ventilation is controlled by several muscles, but especially the [31] inferior to the lungs and the [32] muscles between the ribs.

Airflow is reduced by degenerative diseases that reduce pulmonary [33], the ease with which the lungs expand, and by [34], the narrowing of the bronchioles. Airflow into the lungs is also resisted by the cohesion of water molecules in the alveoli. According to the law of [35], this cohesion becomes greater when the alveolae deflate and contract, bringing the water molecules closer together. However, the secretion noted at 22 above significantly reduces this cohesion and prevents the alveoli from collapsing during expiration.

Not all the air that you inhale ventilates the alveoli. If you inhale 500 mL of air in a typical breath, about 150 mL of it merely fills the conducting division of the respiratory tract and is thus called [36]. The other 350 mL makes it all the way to the alveoli. If you multiply that amount by the respiratory rate (breaths/min), you get a value called the [37], which is an important measure of the amount of air available for gas exchange with the blood. Pulmonary ventilation is measured with a device called a [38]. Among the many values that can be recorded with this device are the following: [39], the amount of air inhaled and exhaled in one normal, relaxed breath; [40], the amount in excess of 39 that you can inhale with a maximum effort; [41], the extra air you can exhale with a maximum effort; [42], which is the sum of 39 to 41; and [43], the amount of air remaining in the lungs after you have exhaled all you possibly can. [44] disorders of respiration, such as pulmonary fibrosis, reduce the amount by which the lungs can inflate, and thus reduce 42, among other values. [45] disorders, such as asthma, do not affect pulmonary volumes or capacities, but affect the speed of airflow.

The normal rhythm of breathing is called [46], whereas difficulty in breathing and shortness of breath is called [47]. A temporary cessation of breathing is [48] and a permanent cessation (unless there is successful intervention) is [49]. Abnormally rapid breathing, which expels CO₂ faster than the body produces it, is called [50] and is frequently due to anxiety attacks.

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Neural Control of Ventilation

Unlike the rhythmic heartbeat, the rhythm of respiration requires the coordinated action of numerous skeletal muscles, and thus has its pacemaker not in the chest but in the [51]. Here, there are [52] neurons that regulate normal inspiration and [53] neurons that are active during forced expiration. A pool of 52 neurons make up the inspiratory center, or [54]. As long as they are firing, you inhale; when they stop, you exhale. These neurons receive signals from the [55] center in the pons, which governs whether breathing is slow and deep or fast and shallow. The brainstem receives input from other centers of the brain; from [56] that monitor the O₂ and CO₂ levels and pH of the blood; and from stretch receptors in the lungs, which can activate an inhibitory [57] reflex to prevent overinflation of the lungs.

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Gas Exchange and Transport

The air we breathe is a mixture of gases, each of which contributes a [58] to the total gas pressure. Nearly 79% of the atmosphere is [59], nearly 21% is [60], and only 0.04% is [61]. In expired air, of course, the concentration of 60 is less because the body consumes this gas, and the concentration of 61 is higher because the body produces it. For any gas to enter the bloodstream, it must dissolve in the body fluids. Henry's law says that the amount of a gas that will dissolve in water is determined by its water solubility and by its [62] in the gas mixture. Therefore, the more oxygen there is in the alveolus, the more of it will diffuse into the blood. We take advantage of this when we give [63] oxygen therapy for disorders such as gangrene and carbon monoxide poisoning. Aside from the concentration gradient and gas solubility, gas exchange with the blood is also strongly influenced by the thickness and surface area of the [64] that separates the air from the blood. It is thus reduced by such conditions as pulmonary edema and emphysema, which increase membrane thickness and reduce alveolar surface area, respectively. To optimize gas exchange, the lungs direct air to areas with the best blood flow, and direct the most blood to the best-ventilated areas of the lung. This is called [65] coupling.

When oxygen binds to hemoglobin (Hb), it forms [66] (HbO₂). One Hb molecule can bind up to [67] molecules of O₂. The [68] curve graphically shows the relationship of P_O2 to the percent HbO₂ in the blood. It has an S shape because the binding of each O₂ promotes the binding of more by other [69] groups of the molecule. Most CO₂ (about 70%) in the blood reacts with water to form [70], which then dissociates into bicarbonate and hydrogen ions. The bicarbonate ions produced in the RBCs diffuse out of the cell in exchange for Cl^- ions from the blood plasma; this exchange is called the [71]. About 23% of the CO₂ in the blood binds to proteins in the plasma and RBCs, especially hemoglobin. This forms HbCO₂, called [72]. The small remaining percentage of CO₂ is carried as dissolved gas bubbles. Even though CO₂ does not bind to the same place on hemoglobin as oxygen does, it tends to cause dissociation of HbO₂ and the unloading of O₂ to the systemic tissues. The percentage of its O₂ that the systemic blood gives up on one pass through the systemic tissues is called the [73]. It is about 22% at rest. In the lungs, Hb binds to O₂, and this causes it to release [74] ions. Those ions react with bicarbonate ions to produce H₂CO₃, which then decomposes into [75] and water.

Hemoglobin does not release the same amount of oxygen to all tissues, but releases varying amounts depending on each tissue's metabolic need. One way of doing this is the [76] effect–active tissues produce more CO₂, this lowers the pH of the tissue fluids, and the low pH promotes faster HbO₂ dissociation. Hemoglobin also picks up more CO₂ from active tissues than from resting ones because a low level of HbO₂ allows it to bind more CO₂. This is called the [77] effect.

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Blood Gases and the Respiratory Rhythm

The rate and depth of breathing are adjusted for the pH, PO₂, and P_CO2 of the blood. These are monitored by chemoreceptors in the [78] of the aortic arch and [79] of the carotid arteries, and by [80], located near the ventral surface of the medulla oblongata. The 80s monitor the chemistry of the [81] fluid rather than of the blood itself. The strongest stimulus to respiration is the [82] of this fluid. This is influenced mainly by the diffusion of [83], not H⁺, from the blood. Normally, the blood has a pH of [84] (state this as a range). A pH above this range, called [85], can result from a CO₂ deficiency, called [86]. An abnormally low pH, called [87], usually results from a CO₂ excess, called [88]. However, it can also occur in such conditions as [89], a result of incomplete fat oxidation in diabetes mellitus.

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Respiratory Disorders

Respiratory diseases typically result in insufficient oxygenation of the tissues, called [90] hypoxia when due to inadequate oxygenation of the blood, [91] hypoxia when due to poor blood circulation, [92] hypoxia when due to a reduced oxygen transport capacity of the blood, and [93] when due to metabolic poisoning of the tissues and their inability to use the oxygen delivered to them. Asthma, emphysema, and chronic bronchitis are collectively called the [94] diseases. The last two are almost always caused by [95]. In chronic bronchitis, the bronchi become congested with a thick mixture of mucus and cellular debris called [96]. Emphysema is marked by a breakdown of the [97], thus a reduction of the surface area available for gas exchange. These diseases tend to lead to polycythemia, an elevated [98], because the blood is hypoxemic and the kidneys respond by secreting more [99] than normal. Smokers are highly susceptible to these conditions as well as to lung cancer. The most common form of lung cancer, called [100], originates with the basal cells of the bronchial epithelium. The most dangerous form, called [101], tends to metastasize quickly to the mediastinum and other thoracic organs. Some other lung diseases include the common cold, or [102]; [103], a streptococcus infection that leads to pulmonary edema and fluid-filled alveoli; and [104], in which fibrous nodules form around certain bacteria (Mycobacterium) in the lungs.

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