The respiratory system in man and other air-breathing vertebrates includes the lungs and the tubes by which air reaches them. Normally air enters the human respiratory system by way of the external nares or nostrils, but it may also enter by way of the mouth. The nostrils, which contain small hairs to filter incoming air, lead into the nasal cavities, which are separated from the mouth below by the palate. The nasal cavities contain the sense organs of smell, and are lined with mucus-secreting epithelium which moistens the incoming air. Air passes from the nasal cavities via the internal nares into the pharynx, then through the glottis and into the larynx. The larynx is often called the “Adam’s apple”, and is more prominent in men than women. Stretched across the larynx are the vocal cords.
The opening to the larynx, called the glottis, is always open except when swallowing, when a flap-like structure (the epiglottis) covers it. Leading from the larynx to the chest region is a long cylindrical tube called the trachea, or windpipe. In a dissection, the trachea can be distinguished from the esophagus by its cartilaginous C-shaped rings which serve to hold the tracheal tube open. In the middle of the chest, the trachea bifurcates into bronchi which lead to the lungs. In the lungs, each bronchus branches, forming smaller and smaller tubes called bronchioles. The smaller bronchioles terminate in clusters of cup-shaped cavities, the air sacs. In the walls of the smaller bronchioles and the air sacs are the alveoli, which are moist structures supplied with a rich network of capillaries. Molecules of oxygen and carbon dioxide diffuse readily through the thin, moist walls of the alveoli. The total alveolar surface area across which gases may diffuse has been estimated to be greater than 100 square meters.
Each lung, as well as the cavity of the chest in which the lung rests, is covered by a thin sheet of smooth epithelium, the pleura. The pleura is kept moist, enabling the lungs to move without much friction during breathing. The pleura actually consists of two layers of membranes which are continuous with each other at the point at which the bronchus enters the lung, called the hilus (roof). Thus, the pleura is more correctly a sac than a single sheet covering the lungs.
The chest cavity is closed and has no communication with the outside. It is bounded by the chest wall, which contains the ribs on its top, sides, and back, and the sternum anteriorly. The bottom of the chest wall is covered by a strong, dome-shaped sheet of skeletal muscle, the diaphragm. The diaphragm separates the chest region (thorax) from the abdominal region, and plays a crucial role in breathing by contracting and relaxing, changing the intrathoracic pressure.
Just prior to inspiration, at the conclusion of the previous expiration, the respiratory muscles are relaxed and no air is flowing into or out of the lungs. Inspiration is initiated by the contraction of the dome-shaped diaphragm and the intercostal muscles. When the diaphragm contracts, it moves downward into the abdomen. Simultaneously, the intercostal muscles which insert on the ribs contract, leading to an upward and outward movement of the ribs. As a result of these two physical changes, the volume of the chest cavity increases and hence the pressure within the chest decreases. Then, the atmospheric pressure, which is now greater than the intrathoracic pressure, forces air to enter the lungs, and causes them to inflate or expand. During exhalation, the intercostal muscles relax and the ribs move downward and inward. At the same time, the diaphragm relaxes and resumes its original dome shape. Consequently, the thoracic volume returns to its pre-inhalation state, and the pressure within the chest increases. This increase in pressure, together with the elastic recoil of the lungs, forces air out of the lungs causing them to deflate.
2. COMPREHENSION CHECK.
1. Provide evidence to support or disprove the following statements:
I. The nasal cavities are not separated from the mouth.
II. The vocal cords are stretched across the larynx.
III. The opening to the larynx, called the glottis, is always closed.
IV. In the walls of the smaller bronchioles and the air sacs are the alveoli.
V. Molecules of oxygen and carbon dioxide can hardly diffuse through the thick walls of the alveoli.
VI. The chest cavity is bounded by the chest wall, which contains the ribs on its top, sides, back and front.
VII. The diaphragm separates the chest region from the abdominal region and doesn’t play any role in breathing.
VIII. A little air flows into or out of the lungs prior to inspiration.
IX. When the diaphragm contracts, it moves towards the abdomen.
2. Prepare extensive answers to the questions below:
a) What does the respiratory system in man include?
b) By what way does air enter the human respiratory system?
III. How does air pass from the nasal cavities into the larynx?
IV. What is the glottis?
V. What is the trachea?
VI. What is the structure of bronchi and lungs?
VII. What enables the lungs to move without much friction during breathing?
VIII. What does the pleura consist of?
IX. Does the chest cavity have communication with the outside?
X. What makes the volume of the chest cavity increase?
XI. What forces air to enter the lungs and makes it get out of the lungs?
3. LANGUAGE FOCUS.
a) Fill in the table with the derivatives of the following words (where possible):
b) Make sure you can supply the plural for each of the following nouns:
c) Looking through the lists of words below match the words which are: a) contrary or b) similar in meaning:
|to terminate||upward||nares||to include|
|to inflate||dry||to force||to name|
|to increase||to deflate||consequently||motion|
|moist||to begin||correct||to cause|
d) Say this in English:
а). Ноздри, содержащие маленькие волоски для очищения поступающего воздуха, ведут в носовые полости.
б). Трахея отличается от пищевода хрящевыми с-образными кольцами.
в). Каждый бронх разветвляется, образуя бронхиолы – трубочки все уменьшающегося размера.
г). Подсчитали, что общая площадь поверхности альвеол, через которую могут диффундировать газы, составляет более 100 м2.
д). Плевра скорее представляет собой мешок, чем пелену, покрывающую легкие.
е). Диафрагма представляет собой крепкий куполообразный слой скелетных мышц.
ж). Во время выдыхания межреберные мышцы расслабляются и ребра двигаются вниз и внутрь.
4. FOLLOW-UP ACTIVITIES.
1. Describe the process of breathing in man.
2. Write an essay about the respiratory system in man and other air-breathing vertebrates.
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