On a moment to moment basis, vascular resistance and blood flow to our tissues are regulated by a variety of mechanisms, including the release of vasoactive substances, changes in vessel diameter, and alterations in cardiac output.
For example, the sympathetic nervous system can release norepinephrine, which causes vasoconstriction and increases vascular resistance. Conversely, the parasympathetic nervous system can release acetylcholine, which causes vasodilation and decreases vascular resistance.
In addition, local factors such as changes in oxygen and carbon dioxide levels, pH, and temperature can also affect vascular resistance and blood flow. For example, decreased oxygen levels can cause vasodilation and increased blood flow to tissues, while increased carbon dioxide levels can cause vasoconstriction and decreased blood flow.
Changes in vessel diameter are also an important mechanism for regulating vascular resistance and blood flow. Vascular smooth muscle cells can contract or relax in response to various stimuli, such as changes in blood pressure, hormone levels, or local factors. This allows for precise control of blood flow to different tissues and organs.
Finally, alterations in cardiac output, such as changes in heart rate and stroke volume, can also affect vascular resistance and blood flow. For example, increased sympathetic activity can increase heart rate and contractility, leading to increased cardiac output and blood pressure.
Overall, vascular resistance and blood flow are regulated by a complex interplay of neural, hormonal, and local factors that allow for precise control of blood flow to meet the changing metabolic demands of our tissues.