Page 143 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

ample, to decreased atmospheric P O 2 at high
Internal (Tissue) Respiration, Hypoxia
altitudes (! p. 136), reduced alveolar ventila-
O 2 diffuses from peripheral blood to adjacent tion, or impaired alveolar gas exchange.
tissues and CO 2 in the opposite direction 2. Anemic hypoxia (! B2): reduced O 2-car-
(! pp. 20ff. and 106). Since CO 2 diffuses much rying capacity of blood (! p. 128), e.g., due to
faster (! p. 120), O 2 diffusion is the limiting decreased total Hb in iron deficiency anemia
factor. Sufficient O 2 delivery is ensured by a (! p. 90).
dense capillary network with a gas exchange 3. Stagnant or ischemic hypoxia (! B3): in-
2
area of about 1000 m . The diffusion distance sufficient O 2 reaches the tissue due to reduced
.
(! R in A) is only 10–25µm. The driving force blood flow (Q"). The cause can be systemic
for diffusion is the difference in partial pres- (e.g., heart failure) or local (e.g., obstructed
sures of oxygen (∆P O 2 ) in capillary blood and artery). The reduction of blood flow must be
mitochondria, where the P O 2 must not fall compensated for by a rise in E O 2 to maintain an
below 0.1 kPa ! 1 mmHg. Since P O 2 decreases adequate O 2 delivery (see Eq. 5.7). This is not
with distance parallel and perpendicular to the the case in hypoxic and anemic hypoxia. The
course of capillaries, the O 2 supply to cells at influx and efflux of substrates and metabolites
the venous end far away from the capillaries is also impaired in stagnant hypoxia. Anaero-
bic glycolysis (! p. 72) is therefore of little
Respiration (hypoxia), this is sometimes called the “lethal help because neither the uptake of glucose nor
(large R) is lowest, as shown using Krogh’s cyl-
inder model (! A1). Since these cells are also
the discharge of H ions dissociated from lactic
+
the first to be affected by oxygen deficiency
acid is possible.
4. Hypoxia can also occur when the diffusion dis-
5 corner” (! A2). tance is increased due to tissue thickening without a
Using Fick’s principle (! p. 106), oxygen
.
consumption of a given organ, VO 2 (in L/min), is corresponding increase in the number of blood capil-
calculated as the difference between the arte- laries. This results in an insufficient blood supply to
.
cells lying outside the O 2 supply radius (R) of the
rial supply (Q ! [O 2]a) and non-utilized venous Krogh cylinder (! A).
.
.
O 2 volume/time (Q ! [O 2]v), where Q is rate of 5. Histotoxic or cytotoxic hypoxia occurs due to im-
blood flow in the organ (L/min) and [O 2] is the paired utilization of O 2 by the tissues despite a suffi-
oxygen fraction (L O 2/L blood): cient supply of O 2 in the mitochondria, as observed in
.
.
V O 2 ! Q ([O 2] a – [O 2] v) . [5.7] cyanide poisoning. Cyanide (HCN) blocks oxidative
To meet increased O 2 demands, Q can therefore cellular metabolism by inhibiting cytochromoxidase.
be increased by vasodilatation in the organ in Brain tissue is extremely susceptible to hyp-
question and/or by raising the oxygen extrac- oxia, which can cause critical damage since
tion (E O 2 ). E O 2 describes the O 2 consumption in dead nerve cells generally cannot be replaced.
.
the organ (= Q ([O 2]a – [O 2]v); see Eq. 5.7) rela- . Anoxia, or a total lack of oxygen, can occur due
.
tive to the arterial O 2 supply (Q ! [O 2]a). Since Q to heart or respiratory failure. The cerebral sur-
can be simplified, vival time is thus the limiting factor for overall
E O 2 ! ([O 2] a – [O 2] v)/ [O 2] a [5.8] survival. Unconsciousness occurs after only
E O 2 varies according to the type and function of 15 s of anoxia, and irreparable brain damage
the organ under resting conditions: skin 0.04 occurs if anoxia lasts for more than 3 min or so.
(4%), kidney 0.07; brain, liver and resting Cyanosis is a bluish discoloration of the skin,
skeletal muscle ca. 0.3, myocardium 0.6. The lips, nails, etc. due to excessive arterial deoxy-
E O 2 of muscle during strenuous exercise can hemoglobin (" 50 g/L). Cyanosis is a sign of
rise to 0.9. Skeletal muscle can therefore meet hypoxia in individuals with normal or only
increased O 2 demands by raising the E O 2 (0.3 " moderately reduced total Hb levels. When
0.9), as can myocardial tissue to a much total Hb is extremely low, O 2 deficiencies
smaller extent (! p. 210). (anemic hypoxia) can be life-threatening, even
Hypoxia. An abnormally reduced O 2 supply in the absence of cyanosis. Cyanosis can occur
to tissue is classified as follows: in absence of significant hypoxia when the Hb
1. Hypoxic hypoxia (! A2, B1): an insuffi-
130 cient O 2 supply reaches the blood due, for ex- level is elevated.
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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