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Lecture 9

Lecture 9 Notes.pdf

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Biological Sciences
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Stephen Reid

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1    Lecture 9: Blood Pressure Continued and Lung Mechanics I See lecture 8 notes for blood pressure regulation. 1. Lung Anatomy Lung mechanics describe the physiological changes in pressures, pressure differences, lung volumes, muscle contraction and air flow that occur during a single breath. These can be thought of as the respiratory equivalent of the cardiac cycle. There are two lungs, a right and a left, situated in the chest (thoracic cavity) and protected by the chest wall. They are lobular - the left lung is made up of two lobes (a large superior lobe and a small inferior lobe) while the right lung is made up of three (the previous lobes, as well as a middle lobe). The left lung has a greater capacity than the right lung and there are slight differences in size. The major respiratory muscle in humans is the diaphragm, which is a curved, thin muscle located under the lungs. It separates the thoracic cavity from the abdominal cavity below. The diaphragm tends to curve slightly upward on the right-hand side, due to the position of the liver immediately beneath it. The consequence of this is that the right lung is about 5 centimeters shorter than the left lung. The cardiac notch is a slight indentation in the left lung caused by the position of the heart. This makes the left lung slightly narrower than the right lung. Overall, however, the left lung has a greater capacity than the right lung. When we look at a topic called ventilation-perfusion matching, we'll see that there is also a difference in the efficiency of gas exchange between the bottom of the lungs and the top. 2    2. The Respiratory Tract The respiratory tract is the pathway through which gases enter and exit the lungs. It opens at the top through the mouth and the nose and continues all the way to the alveoli. From the mouth or the nasal cavity, the respiratory tract converges in a tube it shares with the digestive system, the pharynx, located behind the mouth. Underneath this is the larynx (vocal cords) and then the trachea. The trachea is surrounded by rings of cartilage that make it rigid and prevent its collapse. This rigidity is termed airway patency (this is in contrast to the esophagus, which collapses when not in use). A lack of airway patency often results in apnea (a cessation of breathing), and generally occurs when the upper airway muscles surrounding the trachea are relaxed during sleep. These muscles, when active, help to “pull on” the trachea keeping it open (patent) and preventing collapse. Apnea is often caused by obesity. When the upper airway muscles relax, the excess weight in the throat and neck push on the trachea and cause it to collapse. This is called obstructive sleep apnea and accounts for approximately 90% of sleep apnea cases. The other 10% are referred to as central sleep apnea and result from abnormalities in the respiratory control centres in the brain. The opening of the trachea is referred to as the glottis, and a flap of tissue called the epiglottis acts as a barrier to prevent ingested food and drink from entering the trachea rather than the glottis. The glottis is controlled by a laryngeal branch of the vagus nerve. The epiglottis covers the glottis during the swallowing reflex. The trachea branches to form two primary bronchi and they branch into secondary bronchi, tertiary bronchi and so on - normally there are between twenty to twenty-five orders of bronchi branching depending on the person. Sometimes these bronchi are given different names; for example, primary bronchi going to each lobe may be referred to as lobular bronchi. All of these orders of bronchi end in what are called terminal bronchi, which have little offshoots called respiratory bronchioles that end in alveolar sacs. Everything from the trachea down to the terminal bronchioles is considered to be part of the “conducting zone”, whilst the respiratory bronchioles and the alveolar sacs (and associated alveoli) are considered the “respiratory 3    zone”. In the conducting zone, there is no gas exchange: the walls of cartilage and mucus are too thick for gases to move across. Thus, it is considered 'anatomical dead space' because it is not involved in gas exchange. However, it does play an important role by warming and humidifying air before it enters the lungs, and helping to conserve water which is lost during exhalation. As we proceed from the trachea to the alveolar sacs, the amount of cartilage is reduced whilst the amount of circular smooth muscle increases. This means that the bronchi become less rigid as they decrease in size and branch order. The smooth muscle can constrict and dilate, and the constriction of this muscle accounts for the feelings of tightness in the chest and difficulty breathing during an allergic reaction. A micrograph of alveoli reveals that they are covered by thin and delicate capillaries. The delicateness of these structures is the reason why the pulmonary circulation is under lower pressure than the systemic circuit. There is generally about 150 ml of air in the conducting zone. Later we'll see how the anatomical dead space leads to constraints of increasing overall breathing by increasing breathing rate. Anatomical dead space causes breath volume (actually alveolar ventilation volume) to decrease when breathing rate increases. Ultimately breathing can become so fast that there is no gas exchange because air only moves in and out of the conducting zone (more on this later). 4    The respiratory zone consists of the respiratory bronchi, the alveolar ducts and the alveoli themselves. The alveoli are very thin-walled structures of simple squamous epithelium, which connect to each other as an alveolar sac, and to the alveolar duct (which in turn connects to the respiratory bronchioles). The thin nature of the alveoli allows for gas exchange with blood and they are the primary location for oxygenation of the blood. Deoxygenated blood flows from arteries through capillaries that surround the alveoli, oxygen moves into the blood from the lung gas and carbon dioxide moves from the blood into the lung gas. The newly oxygenated blood returns to the heart. Oxygenation and carbon dioxide excretion are interconnected processes that are very closely linked (more on this in the lectures on gas transport). Alveolar sacs can be obstructed, for example by a pneumonia-induced buildup of fluid and pus as well as an inflammation of the alveoli that inhibits gas exchange. This can cause a serious reduction in gas exchange efficiency leading to health problems and possibly death. 5    3. The Thoracic Cavity The lungs are located within the thoracic cavity which is formed by the chest wall. The chest wall not only protects the lungs and heart but also contains the muscles that allow for lung expansion and therefore inspiration. The chest wall consists of the ribs, as well as the sternum, the thoracic vertebrae at the back, connective tissue between them, and intercostal muscles that lie between the ribs. There are two types of intercostal muscles, internal and external. The internal muscles lie deeper than t
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