(deep abdominal breathing). Several modified respiratory
movements force foreign objects from the respiratory
system (coughing, sneezing) or express emotions (laughing,
crying, sobbing).
3
Gases Are Exchanged at the Blood
Capillaries 382
• As shown, external respiration is the
exchange of gases between the air in
the alveoli and the blood in the pulmo-
nary capillaries. Also shown is internal
respiration, the exchange of gases
between blood in the systemic capillar-
ies and the tissue cells.The diffusion of
gases (O2, CO2) is driven by differences
in the partial pressures of a given gas.
(Gases move from high partial pressure
to low partial pressure.) Oxygen partial
pressure (PO
) is highest in the alveoli
and lowest in the tissues, while carbon
dioxide partial pressure (PCO2) is highest
in the tissues and lowest in the alveoli.
• Oxygen is transported throughout the body bound mainly to
hemoglobin in red blood cells (98.5%); only a small percent-
age travels in solution. Local changes in PO
PCO
pH, and
temperature affect the capacity of hemoglobin to carry oxy-
gen and may facilitate the loading or unloading of oxygen in
various places.
• Carbon dioxide is transported in the body in three forms:
bound to hemoglobin (23%) in red blood cells, as bicarbon-
ate ions in the plasma (70%), and dissolved in solution (7%).
The enzyme carbonic anhydrase within the red blood cell
plays an important role in the conversion between carbon
dioxide and bicarbonate.
Transport o f o x y g e n and carbon d io x id e in th e b lo o d • Figure 1 3 .6
Transport of
CO 2
7% dissolved in plasma
23% as Hb-CO2
70% as HCO3-
Transport of
O 2
1.5% dissolved in plasma
98.5% as Hb-O2
■ Alveoli
To right atrium
To tissue cells
Summary 393
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