Essentials of Anatomy and Physiology

• Physiological dead space—the volume of non-
  functional alveoli; decreases compliance.

Exchange of Gases

1. External respiration is the exchange of gases
   between the air in the alveoli and the blood in the
   pulmonary capillaries.


2. Internal respiration is the exchange of gases
   between blood in the systemic capillaries and tissue
   fluid (cells).


3. Inhaled air (atmosphere) is 21% O 2 and 0.04%
   CO 2. Exhaled air is 16% O 2 and 4.5% CO 2.


4. Diffusion of O 2 and CO 2 in the body occurs
   because of pressure gradients (see Table 15–1). A
   gas will diffuse from an area of higher partial pres-
   sure to an area of lower partial pressure.


5. External respiration: PO 2 in the alveoli is high, and
   PO 2 in the pulmonary capillaries is low, so O 2 dif-
   fuses from the air to the blood. PCO 2 in the alveoli
   is low, and PCO 2 in the pulmonary capillaries is
   high, so CO 2 diffuses from the blood to the air and
   is exhaled (see Fig. 15–8).


6. Internal respiration: PO 2 in the systemic capillaries
   is high, and PO 2 in the tissue fluid is low, so O 2 dif-
   fuses from the blood to the tissue fluid and cells.
   PCO 2 in the systemic capillaries is low, and PCO 2 in
   the tissue fluid is high, so CO 2 diffuses from the tis-
   sue fluid to the blood (see Fig. 15–8).

Transport of Gases in the Blood

1. Oxygen is carried by the iron of hemoglobin (Hb)
   in the RBCs. The O 2 –Hb bond is formed in the
   lungs where the PO 2 is high.


2. In tissues, Hb releases much of its O 2 ; the impor-
   tant factors are low PO 2 in tissues, high PCO 2 in tis-
   sues, and a high temperature in tissues.


3. Oxygen saturation of hemoglobin (SaO 2 ) is 95% to
   97% in systemic arteries and averages 70% to 75%
   in systemic veins.


4. Most CO 2 is carried as HCO 3 – ions in blood
   plasma. CO 2 enters the RBCs and reacts with H 2 O
   to form carbonic acid (H 2 CO 3 ). Carbonic anhy-
   drase is the enzyme that catalyzes this reaction.
   H 2 CO 3 dissociates to H+ ions and HCO 3 – ions.
   The HCO 3 – ions leave the RBCs and enter the
   plasma; Hb buffers the H+ions that remain in the
   RBCs. Cl–ions from the plasma enter the RBCs to
   maintain ionic equilibrium (the chloride shift).


5. When blood reaches the lungs, CO 2 is re-formed,
   diffuses into the alveoli, and is exhaled.

Nervous Regulation of Respiration

(see Fig. 15–9)

1. The medulla contains the inspiration center and
   expiration center.


2. Impulses from the inspiration center to the respira-
   tory muscles cause their contraction; the chest cav-
   ity is expanded.


3. Baroreceptors in lung tissue detect stretching and
   send impulses to the medulla to depress the inspi-
   ration center. This is the Hering-Breuer inflation
   reflex, which also prevents overinflation of the
   lungs.


4. The expiration center is stimulated by the inspi-
   ration center when forceful exhalations are needed.


5. In the pons: the apneustic center prolongs inhala-
   tion, and the pneumotaxic center helps bring about
   exhalation. These centers work with the inspiration
   center in the medulla to produce a normal breath-
   ing rhythm.


6. The hypothalamus influences changes in breathing
   in emotional situations. The cerebral cortex per-
   mits voluntary changes in breathing.


7. Coughing and sneezing remove irritants from the
   upper respiratory tract; the centers for these
   reflexes are in the medulla.

Chemical Regulation of Respiration

(see Fig. 15–10)

1. Decreased blood O 2 is detected by chemoreceptors
   in the carotid body and aortic body. Response:
   increased respiration to take more air into the
   lungs.


2. Increased blood CO 2 level is detected by chemo-
   receptors in the medulla. Response: increased res-
   piration to exhale more CO 2.


3. CO 2 is the major regulator of respiration because
   excess CO 2 decreases the pH of body fluids (CO 2 +
   H 2 O →H 2 CO 3 →H++ HCO 3 – ). Excess H+ions
   lower pH.


4. Oxygen becomes a major regulator of respiration
   when blood level is very low, as may occur with
   severe, chronic pulmonary disease.

Respiration and Acid–Base Balance

1. Respiratory acidosis: a decrease in the rate or effi-
   ciency of respiration permits excess CO 2 to accu-
   mulate in body fluids, resulting in the formation of
   excess H+ions, which lower pH. Occurs in severe
   pulmonary disease.


2. Respiratory alkalosis: an increase in the rate of res-

364 The Respiratory System