Cardiovascular Physiology
393
Table 12–5
Reference Summary of Arteriolar Control in Specifi c Organs
Heart
High intrinsic tone; oxygen extraction is very high at rest, so fl ow must increase when oxygen consumption increases if adequate oxygen
supply is to be maintained.
Controlled mainly by local metabolic factors, particularly adenosine, and fl ow autoregulation; direct sympathetic infl uences are minor
and normally overridden by local factors.
During systole, aortic semilunar cusps block the entrances to the coronary arteries, and vessels within the muscle wall are compressed;
thus coronary fl ow occurs mainly during diastole.
Skeletal Muscle
Controlled by local metabolic factors during exercise.
Sympathetic nerves cause vasoconstriction (mediated by alpha-adrenergic receptors) in refl ex response to decreased arterial pressure.
Epinephrine causes vasodilation via beta-2 adrenergic receptors when present in low concentration, and vasoconstriction, via alpha-
adrenergic receptors, when present in high concentration.
GI Tract, Spleen, Pancreas, and Liver (“Splanchnic Organs”)
Actually two capillary beds partially in series with each other; blood from the capillaries of the GI tract, spleen, and pancreas fl ows via
the portal vein to the liver. In addition, the liver receives a separate arterial blood supply.
Sympathetic nerves cause vasoconstriction, mediated by alpha-adrenergic receptors, in refl ex response to decreased arterial pressure and
during stress. In addition, venous constriction causes displacement of a large volume of blood from the liver to the veins of the thorax.
Increased blood fl ow occurs following ingestion of a meal and is mediated by local metabolic factors, neurons, and hormones secreted by
the GI tract.
Kidneys
Flow autoregulation is a major factor.
Sympathetic nerves cause vasoconstriction, mediated by alpha-adrenergic receptors, in refl ex response to decreased arterial pressure and
during stress. Angiotensin II is also a major vasoconstrictor. These refl exes help conserve sodium and water.
Brain
Excellent fl ow autoregulation.
Distribution of blood within the brain is controlled by local metabolic factors.
Vasodilation occurs in response to increased concentration of carbon dioxide in arterial blood.
Infl uenced relatively little by the autonomic nervous system.
Skin
Controlled mainly by sympathetic nerves, mediated by alpha-adrenergic receptors; refl ex vasoconstriction occurs in response to decreased
arterial pressure and cold, whereas vasodilation occurs in response to heat.
Substances released from sweat glands and noncholinergic, nonadrenergic neurons also cause vasodilation.
Venous plexus contains large volumes of blood, which contributes to skin color.
Lungs
Very low resistance compared to systemic circulation.
Controlled mainly by gravitational forces and passive physical forces within the lung.
Constriction, mediated by local factors, occurs in response to low oxygen concentration—just opposite that which occurs in the systemic
circulation.
previous page 421 Vander's Human Physiology The Mechanisms of Body Function read online next page 423 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off