Page 225 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

other hand, a low PO 2 in the surrounding alve-
Regulation of the Circulation
oli causes the vessels to contract (hypoxic vaso-
The blood flow must be regulated to ensure an constriction; ! p. 122).
adequate blood supply, even under changing ! Local metabolic (chemical) effects: An in-
environmental conditions and stress (cf. p.74). crease in local concentrations of metabolic
This implies (a) optimal regulation of cardiac products such as CO 2, H , ADP, AMP, adenosine,
+
activity and blood pressure (homeostasis), (b) and K in the interstitium has a vasodilatory ef-
+
adequate perfusion of all organ systems, and fect, especially in precapillary arterioles. The
(c) shunting of blood to active organ systems resulting rise in blood flow not only improves
(e.g., muscles) at the expense of the resting or- the supply of substrates and O 2, but also accel-
gans (e.g., gastrointestinal tract) to keep from erates the efflux of these metabolic products
overtaxing the heart (! A). from the tissue. The blood flow to the brain and
Regulation of blood flow to the organs is myocardium (! p. 210) is almost entirely sub-
Cardiovascular System sponse to (1) local stimuli (! B2a/b), (2) hor- to 5-fold increase in blood flow to an affected
mainly achieved by changing the diameter of
ject to local metabolic control. Both local meta-
bolic effects and O 2 deficiencies lead to an up
blood vessels. The muscle tone (tonus) of the
vascular smooth musculature changes in re-
region in response to the decreased blood flow
(reactive hyperemia).
monal signals (! B3 a/b) and (3) neuronal sig-
nals (! B1 a/b). Most blood vessels have an in-
! Vasoactive substances: A number of vasoac-
tive substances such as prostaglandins play a
termediary muscle tone at rest (resting tone).
tion, resulting in a basal tone. This occurs due
Hormonal Control of Circulation
8 Many vessels dilate in response to denerva- role in autoregulation (see below).
to spontaneous depolarization of smooth
muscle in the vessels (see also p. 70). Vasoactive substances. Vasoactive hormones
either have a direct effect on the vascular
Local Regulation of Blood Flow (Autoregulation) musculature (e.g., epinephrine) or lead to the
Autoregulation has two functions: local release of vasoactive substances (e.g.,
! Autoregulatory mechanisms help to main- nitric oxide, endothelin) that exert local para-
tain a constant blood flow to certain organs crine effects (! B).
when the blood pressure changes (e.g., renal ! Nitric (mon)oxide (NO) acts as a vasodila-
vessels constrict in response to rises in blood tory agent. NO is released from the en-
pressure; ! p. 150). dothelium when acetylcholine (M receptors),
! Autoregulation also functions to adjust the ATP, endothelin (ET B receptors), or histamine
blood flow according to changes in metabolic (H 1 receptors) binds with an endothelial cell
activity of an organ (metabolic autoregulation); (! p. 278). NO then diffuses to and relaxes
the amount of blood flow to the organ (e.g., vascular myocytes in the vicinity.
cardiac and skeletal muscle; ! A and p. 201) ! Endothelin-1 can lead to vasodilatation by
can thereby increase many times higher than inducing the release of NO from the en-
the resting level. dothelium by way of ET B receptors (see above),
Types of autoregulatory mechanism: or can cause vasoconstriction via ET A receptors
! Myogenic effects arising from the vascular in the vascular musculature. When substances
musculature of the lesser arteries and arteri- such as angiotensin II or ADH (= vasopressin;
oles (Bayliss effect) ensure that these vessels V 1 receptor) bind to an endothelial cell, they
contract in response to blood pressure-related release endothelin-1, which diffuses to and
dilatation (! B2a) in certain organs (e.g., kid- constricts the adjacent vascular muscles with
neys, gastrointestinal tract and brain), but not the aid of ET A receptors.
in others (e.g., skin and lungs). ! Epinephrine (E): High concentrations of E
! Oxygen deficiencies generally cause the from the adrenal medulla (! p. 86) have a
blood vessels to dilate. Hence, the degree of vasoconstrictive effect (α 1-adrenoceptors),
blood flow and O 2 uptake increase with in- whereas low concentrations exert vasodilatory
212 creasing O 2 consumption. In the lungs, on the effects by way of " 2 adrenoceptors in the myo-
!
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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