UNIT FOUR
558
Clinical Skills and Care
trapping carbon dioxide in your blood, causing a buildup of carbonic acid, which in turn prompts the medulla to make you breathe. REGULATION OF RESPIRATION IN CHRONIC LUNG DISEASE. People who have severe chronic lung disease grad- ually develop a different stimulus to breathe. Unlike the sit- uation in persons with healthy lung tissue, in people with severe chronic lung disease, air is trapped in the alveoli for long periods because oxygen and carbon dioxide are unable to diffuse freely across the stiff, nonelastic alveolar mem- brane. This raises the blood’s level of carbon dioxide and lowers the oxygen level. The body gradually acclimates to the higher carbon dioxide level because it cannot continue to increase the respiratory rate higher and higher to blow it off. Thus the high carbon dioxide level no longer stimulates res- piration, and the body begins to respond to the lower level of oxygen, which now becomes its stimulus to breathe. Then, in some situations, high levels of supplemental oxygen can impair the message from the medulla that instructs the body to breathe. It will be important to remember this later in the chapter when we discuss oxygen fl ow rates for patients with chronic lung disease. Internal and External Respiration External respiration occurs between the alveoli and the capillaries. The alveoli are air sacs encircled by tiny capil- laries. Oxygen moves via diffusion from an area of higher concentration to an area of lower concentration. When you inhale, the inspired air contains more oxygen, so it passes from the alveoli into the capillaries. Internal respiration occurs between the bloodstream and the body cells. When fresh oxygen enters the bloodstream, it is carried by the hemoglobin on the red blood cells through the arteries and arterioles to the capillaries. In the tiny cap- illaries, where the walls are just one cell thick, the oxygen diffuses to the tissue cells where the concentration is lower. At the same time, carbon dioxide and other waste products diffuse into the blood from the tissue cells for the same rea- son. The blood then returns to the heart and lungs through the venules and veins. Once the blood is pumped back to the lungs by the right side of the heart, the carbon dioxide and waste products leave the blood and enter the alveoli to leave the body with exhalation. Anatomy and Physiology Connection The Pleura The chest cavity is lined with a thin membrane that also covers each lung, similar to a sac within a sac. This membrane is called the pleura. The portion of the pleura that lines the chest cavity is called the parietal pleura. This continuous membrane then covers the lungs, and that portion is called the visceral pleura. There is a very narrow space between the two layers, referred to as the
pleural space, in which a negative pressure, or vacuum, must be maintained so that expansion of the chest wall will cause the lung to expand as the chest wall expands. If this vacuum is lost and air or fluid enters into the pleural space, the lung will collapse rather than expand. This space contains a small amount of fluid that helps hold the two layers together but also allows them to glide smoothly against one another with each inhalation and exhalation. A similar situation occurs when you add a drop of water to a microscope slide, then place the coverslip over it. The water forms a film that holds the coverslip on the slide but allows it to glide against the slide for correct placement.
KNOWLEDGE CONNECTION What role do muscles, nerves, and chemicals play in respi- ration? What are the similarities and differences between internal and external respiration?
IMPAIRED OXYGENATION If a blockage occurs in the airway or oxygen cannot pass into the blood in the alveolar capillaries, the patient is unable to obtain needed oxygen from room air. If oxygen cannot cross into the tissue cells in the peripheral capillaries, the patient is unable to provide oxygen to body cells. Each of these situations can lead to impaired oxygenation. It is very important to recognize the signs and symp- toms of decreased oxygen in the blood and tissues because oxygen is necessary for brain and organ function. Often the symptoms do not become noticeable until the oxygen level has dropped considerably. When oxygen levels in the blood drop below normal range, it is referred to as hypoxemia. In the event of hypox- emia, the blood cannot take adequate amounts of oxygen to the tissues during internal respiration, causing hypoxia. Without enough oxygen in the tissues, you will see changes in mental function, in the color of the skin and mucous mem- branes, and in respiratory rate and rhythm. Box 28.1 lists the signs and symptoms of hypoxia. • WORD • BUILDING • parietal pleura: parietal – forming wall of a cavity; pleura – pleural membrane visceral pleura: visceral – body organs; pleura – pleural membranes hypoxemia: hypo – deficient + ox – oxygen + em – blood + ia – condition hypoxia: hypo – deficient + ox – oxygen + ia – condition
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