Page 119 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

5 Respiration

The human body contains around 300 mil-
Lung Function, Respiration
lion alveoli, or thin-walled air sacs (ca. 0.3 mm
The lung is mainly responsible for respiration, in diameter) located on the terminal branches
but it also has metabolic functions, e.g. con- of the bronchial tree. They are surrounded by a
version of angiotensin I to angiotensin II dense network of pulmonary capillaries and
2
(! p. 184). In addition, the pulmonary circula- have a total surface area of about 100 m . Be-
tion buffers the blood volume (! p. 204) and fil- cause of this and the small air/blood diffusion
ters out small blood clots from the venous distances of only a few µm (! p. 22, Eq. 1.7),
circulation before they obstruct the arterial sufficient quantities of O 2 can diffuse across the
circulation (heart, brain!). alveolar wall into the blood and CO 2 towards
External respiration is the exchange of the alveolar space (! p. 120ff.), even at a ten-
gases between the body and the environment. fold increased O 2 demand (! p. 74). The oxy-
(Internal or tissue respiration involves nutrient gen-deficient “venous” bloodofthe pulmonary
oxidation, ! p. 228). Convection (bulk flow) is artery is thus oxygenated (“arterialized”) and
the means by which the body transports gases pumped back by the left heart to the periphery.
over long distances (! p. 24) along with the The cardiac output (CO) is the volume of blood
flow of air and blood. Both flows are driven by a pumped through the pulmonary and systemic circu-
pressure difference. Diffusion is used to trans- lation per unit time (5–6 L/min at rest). CO times the
port gases over short distances of a few µm— arterial–venous O 2 difference (avDO 2)—i.e., the differ-
e.g., through cell membranes and other physi- ence between the arterial O 2 fraction in the aorta
ological barriers (! p. 20ff.). The exchange of and in mixed venous blood of the right atrium (ca.
gas between the atmosphere and alveoli is 0.05 L of O 2 per L of blood)—gives the O 2 volume
called ventilation. Oxygen (O 2) in the inspired transported per unit time from the lungs to the periph-
ery. At rest, it amounts to (6 ! 0.05 = ) 0.3 L/min, a
.
air is convected to the alveoli before diffusing value matching that of V O 2 (see above). Inversely, if
.
across the alveolar membrane into the blood- V O 2 and avDO 2 have been measured, CO can be cal-
stream. It is then transported via the blood- culated (Fick’s principle):
.
stream to the tissues, where it diffuses from CO " V O 2 /avD O 2 [5.1]
the blood into the cells and, ultimately, to the The stroke volume (SV) is obtained by dividing CO by
intracellular mitochondria. Carbon dioxide the heart rate (pulse rate).
(CO 2) produced in the mitochondria returns to According to Dalton’s law, the total pressure
the lung by the same route. . (P total) of a mixture of gases is the sum of the
The total ventilation per unit time, VT (also partial pressures (P) of the individual gases.
called minute volume) is the volume (V) of air The volume fraction (F, in L/L; ! p. 376), of the
inspired or expired per time. As the expiratory individual gas relative to the total volume
volume is usually measured, it is also abbre- times P total gives the partial pressure—in the
.
viated VE. (The dot means “per unit time”). At case of O 2, for example, PO 2 = FO 2 ! P total. The
.
rest, the body maintains a VE of about 8 L/min, atmospheric partial pressures in dry ambient
with a corresponding oxygen consumption air at sea level (P total = 101.3 kPa = 760 mmHg)
.
rate (VO 2 ) of about 0.3 L/min and a CO 2 elimina- are: FO 2 = 0.209, FCO 2 = 0.0004, and FN 2 + noble
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tion rate (VCO 2) of about 0.25 L/min. Thus, gases = 0.79 (! A, top right).
about 26 L of air have to be inspired and ex-
pired to supply 1 L of O 2 (respiratory equivalent If the mixture of gases is “wet”, the partial pressure
of water, PH 2 O has to be subtracted from P total (usu-
= 26). The tidal volume (VT) is the volume of air ally = atmospheric pressure). The other partial pres-
moved in and out during one respiratory cycle. sures will then be lower, since Px = Fx (P total – PH 2 O).
.
VE is the product of VT (ca. 0.5 L at rest) and res- When passing through the respiratory tract (37 #C),
piration rate f (about 16/min at rest) (see p. 74 inspired air is fully saturated with water. As a result,
for values during physical work). Only around PH 2 O rises to 6.27 kPa (47 mmHg), and PO 2 drops
.
5.6 L/min (at f = 16 min ) of the VE of 8 L/min 1.32 kPa lower than the dry atmospheric air
-1
reaches the alveoli; this is known as alveolar (! p. 112). The partial pressures in the alveoli, arter-
106 . ies, veins (mixed venous blood), tissues, and expira-
ventilation (VA). The rest fills airways space not tory air (all “wet”) are listed in A.
contributing to gas exchange (dead space ven-
.
tilation, VD; ! pp. 114 and 120).
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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