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Chapter 8: Respiratory System
Learning Tip Take several deep, slow
Box 8-1 The Diaphragm The diaphragm is a thin, dome-shaped muscle that controls breathing. It separates the thoracic cavity from the abdominal cavity. When the diaphragm contracts, it moves down- ward, which is why you see your abdomen rise every time you breathe in. While it moves downward, other muscles pull the rib cage upward and outward. Inhalation is caused by this thoracic cavity expansion. The increased size decreases the thoracic pressure, forcing air into the lungs. When the diaphragm and rib cage muscles relax, exhalation occurs. The dia- phragm moves upward, and the thoracic cavity returns to its smaller size. This increases the thoracic pressure, forcing air out of the lungs. Those with pulmonary diseases may find it easier to breathe when sitting, as gravity is able to assist the diaphragm during contraction. The diaphragm has other functions unrelated to breathing. It also increases pressure in the abdominal cavity, which aids in vomiting, urination, and defecation.
breaths. While you do so, visualize the path that oxy- gen is taking while it moves from your external environ- ment into your respiratory system and finally into your bloodstream. Next, visualize the reverse path that carbon dioxide follows while it leaves your body. Now verbally describe both pathways to a real or imaginary partner.
Inspiration
Expiration
Air inhaled
Air exhaled
Chest relaxes
Chest expands
Ribs
Ribs
Lung
Lung
Diaphragm relaxes
Diaphragm contracts
for measuring the acidity or alkalinity of a substance (Figure 8-2). While blood becomes more acidic, its pH level decreases, triggering the urge to breathe. When we inhale, we bring fresh, oxygen-rich air into our lungs, where it can be absorbed into our blood. The act of exhalation , or breathing out (or expiration), allows our bodies to eliminate excess CO 2 , thus restoring a healthy blood pH level. Contrary to what most people think, the drive to breathe is not triggered by lower oxygen levels in the blood but by the lowered pH level caused by CO 2 buildup.
7.35–7.45
6.9 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8
Death
Acidosis
Alkalosis
Death
0 1 2 3 4
5 6 7 8 9 1011121314
Acid
Base
Neutral
FIGURE 8-2 Human blood must stay within the narrow range of 7.35 to 7.45 on the pH scale.
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