1. Vascular resistance is affected by intrinsic and extrinsic control mechanisms. 2. Metabolic control of blood flow is a local mechanism that matches the blood flow of a tissue to its metabolic rate. 3. Autoregulation is a relative constancy of blood flow in an organ despite changes in perfusion pressure. 4. Many chemical signals act locally (as paracrines) to exert important control on vascular resistance. 5. Regardless of the status of arterioles, mechanical compression can reduce blood flow to a tissue. TABLE 24-1 Chemical Signals Important in Local Control of Systemic Arterioles* *Some of these chemical signals have different effects on pulmonary blood vessels than on systemic vessels. A high level of oxygen, for example, causes dilation of pulmonary vessels, whereas the effect in systemic vessels is vasoconstriction. See Chapter 46 for more details. Figure 24-3 summarizes an experiment that demonstrates autoregulation in the brain. Initially, the perfusion pressure (arterial pressure minus venous pressure) in this animal is 100 mm Hg, and the blood flow to the brain is 100 milliliters per minute (mL/min) (point A). When perfusion pressure is increased suddenly to 140 mm Hg, brain blood flow rises initially to 140 mL/min but returns toward its initial level over the next 20 to 30 seconds. Eventually, blood flow reaches a stable level of about 110 mL/min (point B). Conversely, if the perfusion pressure is decreased suddenly from 100 to 60 mm Hg, blood flow in the brain decreases initially to 60 mL/min but returns toward its initial level over the next 20 to 30 seconds (see dashed lines in the top and middle graphs of Figure 24-3). Eventually, blood flow reaches a stable level of about 90 mL/min (point C). These stable responses are plotted in the bottom graph. The remainder of the bottom graph is obtained in a similar way; that is, perfusion pressure is set artificially to various levels, ranging from 40 to 220 mm Hg, and the resulting steady-state levels of blood flow are plotted.
Local Control of Blood Flow
Metabolic Control of Blood Flow Is a Local Mechanism That Matches the Blood Flow of a Tissue to Its Metabolic Rate
Chemical Signal
Source
Effect
Signals Related to Metabolism
Oxygen
Delivered by arterial blood; consumed in aerobic metabolism
Vasoconstriction. (Rapid metabolism depletes O2, which causes vasodilation.)
Carbon dioxide
Produced by aerobic metabolism
Vasodilation
Potassium ions (K+)
Released from rapidly metabolizing cells
Vasodilation
Adenosine
Released from rapidly metabolizing cells
Vasodilation
Metabolic acids (e.g., lactic acid)
Produced by anaerobic metabolism
Vasodilation
Other Local Chemical Signals (Paracrines)
Endothelin-1 (ET1)
Endothelial cells
Vasoconstriction
Nitric oxide (NO)
Endothelial cells and some parasympathetic nerve endings
Vasodilation
Thromboxane A2 (TXA2)
Platelets
Vasoconstriction (also increases platelet aggregation)
Prostacyclin (PGI2)
Endothelial cells
Vasodilation (also decreases platelet aggregation)
Histamine
Mast cells
Vasodilation (also increases capillary permeability)
Bradykinin
Globulins in blood or tissue fluid
Vasodilation (also increases capillary permeability)
Autoregulation Is a Relative Constancy of Blood Flow in an Organ Despite Changes in Perfusion Pressure
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