Page 227 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

!
cardium, skeletal muscle and liver (! C). The ef- the cerebral cortex to circulatory centers when
fect of E mainly depends on which type of a muscle group is activated, or (b) via neuronal
adrenoceptor is predominant in the organ. α 1- feedback from the organs whose activity level
adrenoceptors are predominant in the blood and metabolism have changed. If the neuronal
vessels of the kidney and skin. and local metabolic mechanisms are conflict-
! Eicosanoids (! p. 269): Prostaglandin (PG) ing (e.g., when sympathetic nervous stimula-
F 2α and thromboxane A 2 (released from plate- tion occurs during skeletal muscle activity),
lets, ! p. 102) and B 2 have vasoconstrictive ef- the metabolic factors will predominate. Va-
fects, while PGI 2 (= prostacyclin, e.g. released sodilatation therefore occurs in the active
from endothelium) and PGE 2 have vasodila- muscle while the sympathetic nervous system
tory effects. Another vasodilator released from reduces the blood flow to the inactive muscles.
the endothelium (e.g., by bradykinin; see Blood flow to the skin is mainly regulated by
+
below) opens K channels in vascular myo- neuronal mechanisms for the purpose of con-
Cardiovascular System endothelium-derived hyperpolarizing factor to centralization of blood flow, i.e., vasocon-
cytes and hyperpolarizes them, leading to a
trolling heat disposal (temperature control;
drop in the cytosolic Ca
! p. 224). Hypovolemia and hypotension lead
concentration. This
2+
(EDHF), has been identified as a 11,12-epoxy-
striction in the kidney (oliguria) and skin (pal-
lor) occurs to increase the supply of blood to
eicosatrienoic acid (11,12-EET).
! Bradykinin and kallidin are vasodilatory
vital organs such as the heart and central
agents cleaved from kininogens in blood
nervous system (! p. 218).
also acts as a vasodilator. All three substances
cold-induced vasoconstriction of cutaneous vessels
8 plasma by the enzyme kallikrein. Histamine During exposure to extremely low temperatures, the
influence also vessel permeability (e.g., during
is periodically interrupted to supply the skin with
infection) and blood clotting. blood to prevent tissue damage (Lewis response).
Axoaxonal reflexes in the periphery play a role in
Neuronal Regulation of Circulation this response, as afferent cutaneous nerve fibers
Neuronal regulation of blood flow (! B1a/b) transmit signals to efferent vasomotor axons. Skin
mainly involves the lesser arteries and greater reddening in response to scratching (dermato-
arterioles (! p. 188), while that of venous re- graphism) is also the result of axoaxonal reflexes.
turn to the heart (! p. 188) can be controlled Central regulation of blood flow (! C) is the
by dilating or constricting the veins (changes in responsibility of the CNS areas in the medulla
their blood storage capacity). Both mecha- oblongata and pons. They receive information
nisms are usually controlled by the sympa- from circulatory sensors (S) or receptors (a) in
thetic nervous system (! B1a and p. 78ff.), the high-pressure system (barosensors or
whereby norepinephrine (NE) serves as the pressure sensors, S P, in the aorta and carotid
postganglionic transmitter (except in the artery); (b) in the low-pressure system (stretch
sweat glands). NE binds with the α 1 adreno- sensors in the vena cava and atria, S A and S B);
ceptors on blood vessels, causing them to con- and (c) in the left ventricle (S V). The sensors
strict (! B). Vasodilatation is usually achieved measure arterial blood pressure (S P), pulse rate
by decreasing the tonus of the sympathetic (S P and S V) and filling pressure in the low pres-
system (! B1b). This does not apply to blood sure system (indirect measure of blood
vessels in salivary glands (increased secretion) volume). The A sensors (S A) mainly react to
or the genitals (erection), which dilate in re- atrial contraction, whereas the B sensors (S B)
sponse to parasympathetic stimuli. In this case, react to passive filling stretch (! C2). If the
vasoactive substances (bradykinin and NO, re- measured values differ from the set-point
spectively) act as the mediators. Some neurons value, the circulatory control centers of the CNS
release calcitonin gene-related peptide transmit regulatory impulses through efferent
(CGRP), a potent vasodilator. nerve fibers to the heart and blood vessels
Neuronal regulation of blood flow to organs (! D and p. 5 C2).
occurs mainly: (a) via central co-innervation Situated laterally in the circulatory “center”
214 (e.g., an impulse is simultaneously sent from is a pressor area (! C, reddish zone), the neu-
!
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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