Medical brain notes 27

  DIVISIONS OF THE RESPIRATORY SYSTEM   The respiratory system may be divided into the upper respiratory tract and the lower respiratory tract. The  upper respiratory tract consists of the parts outside   the chest cavity: the air passages of the nose, nasal cav-ities, pharynx, larynx, and upper trachea. The lower   respiratory tract  consists of the parts found within   the chest cavity: the lower trachea and the lungs them-selves, which include the bronchial tubes and alveoli. Also part of the respiratory system are the pleural membranes and the respiratory muscles that form the chest cavity: the diaphragm and intercostal muscles.   NOSE AND NASAL CAVITIES   Air enters and leaves the respiratory system through the  nose , which is made of bone and cartilage covered with skin. Just inside the nostrils are hairs, which help block the entry of dust.   The two  nasal cavities  are within the skull, sepa-rated by the  na...

  MECHANISM OF BREATHING   Ventilation  is the term for the movement of air to and   from the alveoli. The two aspects of ventilation are inhalation and exhalation, which are brought about by  the nervous system and the respiratory muscles. The respiratory centers are located in the medulla and pons. Their specific functions will be covered in a later section, but it is the medulla that generates impulses to the respiratory muscles.   Figure 15–6.  Actions of the res-piratory muscles. ( A ) Inhalation: diaphragm contracts downward; external intercostal muscles pull rib cage upward and outward; lungs are expanded. ( B ) Normal exhala-tion: diaphragm relaxes upward; rib cage falls down and in as exter-nal intercostal muscles relax; lungs are compressed. QUESTION:  Why is a normal exha-lation a passive process? These muscles are the diaphragm and the external and internal intercostal muscles (Fig. 15–6). The  diaphragm  is a dome-shaped mus...

  PULMONARY VOLUMES   The capacity of the lungs varies with the size and age of the person. Taller people have larger lungs than do shorter people. Also, as we get older our lung capacity diminishes as lungs lose their elasticity and the respi-ratory muscles become less efficient. For the following pulmonary volumes, the values given are those for healthy young adults. These are also shown in Fig. 15–7 1.   Tidal volume —the amount of air involved in one   normal inhalation and exhalation. The average tidal volume is 500 mL, but many people often have lower tidal volumes because of shallow breathing.   2.   Minute respiratory volume  (MRV)—the amount   of air inhaled and exhaled in 1 minute. MRV is cal-culated by multiplying tidal volume by the number of respirations per minute (average range: 12 to 20 per minute). If tidal volume is 500 mL and the res-piratory rate is 12 breaths per minute, the MRV is 6000 mL, or 6 liters of air per minute, which...

  EXCHANGE OF GASES   There are two sites of exchange of oxygen and carbon dioxide: the lungs and the tissues of the body. The exchange of gases between the air in the alveoli and the blood in the pulmonary capillaries is called  exter-nal respiration . This term may be a bit confusing at first, because we often think of “external” as being outside the body. In this case, however, “external” means the exchange that involves air from the external environment, though the exchange takes place within the lungs.  Internal respiration  is the exchange of gases between the blood in the systemic capillaries and the tissue fluid (cells) of the body.   The air we inhale (the earth’s atmosphere) is approximately 21% oxygen and 0.04% carbon diox-ide. Although most (78%) of the atmosphere is nitro-gen, this gas is not physiologically available to us, and we simply exhale it. This exhaled air also contains about 16% oxygen and 4.5% carbon dioxide, so it is apparent that ...

  TRANSPORT OF GASES IN THE BLOOD   Although some oxygen is dissolved in blood plasma and does create the P O 2  values, it is only about 1.5% of the total oxygen transported, not enough to sustain life. As you already know, most oxygen is carried in the blood bonded to the  hemoglobin  in red blood cells (RBCs). The mineral iron is part of hemoglobin and gives this protein its oxygen-carrying ability. The oxygen–hemoglobin bond is formed in the lungs where PO 2  is high. This bond, however, is rela-tively unstable, and when blood passes through tissues with a low PO 2 , the bond breaks, and oxygen is released to the tissues. The lower the oxygen concen-tration in a tissue, the more oxygen the hemoglobin will release. This ensures that active tissues, such as exercising muscles, receive as much oxygen as possible to continue cell respiration. Other factors that increase the release of oxygen from hemoglobin are a high PCO 2 (actually a lower pH) and a high ...