Sensory impulses to cardiac center
Parasympathetic impulses to heart
This mechanism helps regulate blood pressure by inhibiting the S-A node.
Blood pressure returns toward normal
Rising blood pressure
Decreased peripheral resistance
Stimulation of baroreceptors in aortic arch and carotid sinuses
Vasodilation of arterioles
Sensory impulses to vasomotor center
Less frequent sympathetic impulses to arteriole walls
Vasomotor center inhibited
This mechanism helps regulate blood pressure by dilating arterioles.
outflow. The resulting release of epinephrine and norepinephrine vasoconstricts most systemic vessels, increasing peripheral resistance. This helps return blood pressure toward normal.
The vasomotor center's control of vasoconstriction and vasodilation is especially important in the arterioles of the abdominal viscera (splanchnic region). These vessels, if fully dilated, could accept nearly all the blood of the body and send the arterial pressure toward zero. Thus, control of their diameters is essential in regulating normal peripheral resistance.
Certain chemicals, including carbon dioxide, oxygen, and hydrogen ions, also influence peripheral resistance by affecting precapillary sphincters and smooth muscles in arteriole and metarteriole walls. For example, increasing blood carbon dioxide, decreasing blood oxygen, and lowering of the blood's pH relaxes these muscles in the systemic circulation. This increases local blood flow to tissues with high metabolic rates, such as exercising skeletal muscles.
Other chemicals also influence peripheral resistance and thus blood pressure. Nitric oxide, produced by endothelial cells, and bradykinin, formed in the blood, are both vasodilators. The hormone angiotensin plays a role in vasoconstriction; and endothelin, released by cells of the endothelium, is a powerful vasoconstrictor. Clinical Application 15.6 discusses high blood pressure.
U What factors affect cardiac output?
^9 Explain Starling's law of the heart.
^9 What is the function of the baroreceptors in the walls of the aortic arch and carotid sinuses?
□ How does the vasomotor center control peripheral resistance?
Blood pressure decreases as the blood moves through the arterial system and into the capillary networks, so little pressure remains at the venular ends of capillaries (see fig. 15G). Instead, blood flow through the venous system is only partly the direct result of heart action and depends on other factors, such as skeletal muscle contraction, breathing movements, and vasoconstriction of veins.
For example, contracting skeletal muscles press on veins, moving blood from one valve section to another. This massaging action of contracting skeletal muscles helps push the blood through the venous system toward the heart (fig. 15.40).
Contracted skeletal muscle
Contracted skeletal muscle
Valve closed ure 15.40
The massaging action of skeletal muscles helps move blood through the venous system toward the heart.
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