Chapter 20 - Section 20.2 - Study Guide - Assess Your Learning Outcomes - Page 798: 9

Peripheral resistance refers to the resistance encountered by blood flow as it travels through the peripheral (smaller) arteries and arterioles. Several variables influence peripheral resistance, and their relationships can be categorized as directly proportional or inversely proportional to resistance. Additionally, some variables are more changeable from moment to moment than others. Here's a breakdown: **1. Vessel Diameter:** Directly Proportional - The diameter of blood vessels has a significant impact on resistance. Smaller vessel diameters result in higher resistance, as there is less space for blood to flow through. Conversely, larger vessel diameters lead to lower resistance, allowing for smoother blood flow. **2. Vessel Length:** Directly Proportional - Longer blood vessels offer more surface area for friction, increasing resistance. Shorter vessel lengths result in lower resistance. **3. Blood Viscosity:** Directly Proportional - Blood viscosity refers to the thickness or stickiness of blood. Higher viscosity leads to increased resistance, as thicker blood experiences more friction against vessel walls. Lower viscosity reduces resistance. **4. Blood Density:** Directly Proportional - Blood density, which is relatively constant, doesn't have a significant impact on resistance under normal physiological conditions. **5. Vessel Compliance (Elasticity):** Inversely Proportional - More compliant (elastic) vessels can expand and accommodate blood flow with less resistance. Stiffer vessels have higher resistance. **6. Sympathetic Nervous System Activity:** Directly Proportional - Activation of the sympathetic nervous system causes vasoconstriction, narrowing blood vessels and increasing resistance. **7. Local Factors (e.g., Metabolites):** Inversely Proportional - Some local factors released by tissues during metabolism cause vasodilation, reducing resistance. For instance, increased metabolic activity releases vasodilators like nitric oxide. **8. Hormones (e.g., Angiotensin II):** Directly Proportional - Certain hormones like angiotensin II can cause vasoconstriction, elevating resistance. **9. Autoregulation:** Inversely Proportional - Autoregulation mechanisms allow tissues to adjust their own blood supply by altering vessel diameter. Vasodilation reduces resistance and increases blood flow to meet tissue needs. **10. Temperature:** Inversely Proportional - Higher temperatures often cause vasodilation, reducing resistance and increasing blood flow. **Changeability from Moment to Moment:** Vessel diameter is the most changeable variable from moment to moment. It's dynamically regulated by the nervous system and local factors. For example, the sympathetic nervous system can rapidly adjust vessel diameter, leading to vasoconstriction or vasodilation. This provides a rapid and powerful mechanism for controlling peripheral resistance based on the body's immediate needs. In summary, peripheral resistance is influenced by multiple variables, some of which are directly proportional (e.g., vessel diameter, sympathetic activity) and some inversely proportional (e.g., vessel compliance, local factors). Vessel diameter is the most changeable variable and is tightly regulated to adapt to varying physiological demands.

Peripheral resistance refers to the resistance encountered by blood flow as it travels through the peripheral (smaller) arteries and arterioles. Several variables influence peripheral resistance, and their relationships can be categorized as directly proportional or inversely proportional to resistance. Additionally, some variables are more changeable from moment to moment than others. Here's a breakdown: **1. Vessel Diameter:** Directly Proportional - The diameter of blood vessels has a significant impact on resistance. Smaller vessel diameters result in higher resistance, as there is less space for blood to flow through. Conversely, larger vessel diameters lead to lower resistance, allowing for smoother blood flow. **2. Vessel Length:** Directly Proportional - Longer blood vessels offer more surface area for friction, increasing resistance. Shorter vessel lengths result in lower resistance. **3. Blood Viscosity:** Directly Proportional - Blood viscosity refers to the thickness or stickiness of blood. Higher viscosity leads to increased resistance, as thicker blood experiences more friction against vessel walls. Lower viscosity reduces resistance. **4. Blood Density:** Directly Proportional - Blood density, which is relatively constant, doesn't have a significant impact on resistance under normal physiological conditions. **5. Vessel Compliance (Elasticity):** Inversely Proportional - More compliant (elastic) vessels can expand and accommodate blood flow with less resistance. Stiffer vessels have higher resistance. **6. Sympathetic Nervous System Activity:** Directly Proportional - Activation of the sympathetic nervous system causes vasoconstriction, narrowing blood vessels and increasing resistance. **7. Local Factors (e.g., Metabolites):** Inversely Proportional - Some local factors released by tissues during metabolism cause vasodilation, reducing resistance. For instance, increased metabolic activity releases vasodilators like nitric oxide. **8. Hormones (e.g., Angiotensin II):** Directly Proportional - Certain hormones like angiotensin II can cause vasoconstriction, elevating resistance. **9. Autoregulation:** Inversely Proportional - Autoregulation mechanisms allow tissues to adjust their own blood supply by altering vessel diameter. Vasodilation reduces resistance and increases blood flow to meet tissue needs. **10. Temperature:** Inversely Proportional - Higher temperatures often cause vasodilation, reducing resistance and increasing blood flow. **Changeability from Moment to Moment:** Vessel diameter is the most changeable variable from moment to moment. It's dynamically regulated by the nervous system and local factors. For example, the sympathetic nervous system can rapidly adjust vessel diameter, leading to vasoconstriction or vasodilation. This provides a rapid and powerful mechanism for controlling peripheral resistance based on the body's immediate needs. In summary, peripheral resistance is influenced by multiple variables, some of which are directly proportional (e.g., vessel diameter, sympathetic activity) and some inversely proportional (e.g., vessel compliance, local factors). Vessel diameter is the most changeable variable and is tightly regulated to adapt to varying physiological demands.