24.11:
Vascular Resistance
When blood flows through the vessels, the friction between the vessel walls and blood creates a resistive force called peripheral or vascular resistance.
This resistance is affected by three main factors.
The first factor is blood viscosity or its thickness. It remains constant except in conditions like polycythemia, where an abnormally high RBC count increases the blood viscosity, leading to higher resistance and high blood pressure.
The second factor is the length of the blood vessels; the longer the vessels, the higher the resistance. It remains relatively constant in healthy individuals, except in obesity, where additional vessels in adipose tissue increase resistance and blood pressure.
The third factor, lumen diameter, is variable across the cardiovascular system. The resistance increases when the lumen diameter is small or when a blood vessel constricts.
All the blood vessels in the cardiovascular system have a cumulative resistance known as the total peripheral or systemic vascular resistance — SVR.
Due to their smaller diameter, the arterioles, capillaries, and venules contribute most to SVR and blood pressure.
24.11:
Vascular Resistance
Vascular resistance is a critical concept in understanding blood flow dynamics in the circulatory system. It refers to the resistance that blood encounters as it flows through the blood vessels. This resistance is a key factor in determining blood pressure and cardiac workload.
The primary determinants of vascular resistance are vessel diameter, blood viscosity, and vessel length. Among these, vessel diameter plays the most significant role due to the fourth power relationship described by Poiseuille's law: small changes in diameter lead to large changes in resistance. For instance, if a vessel's diameter halves, resistance increases by 16 times.
Blood viscosity, the thickness of blood, also affects resistance. Higher viscosity, as seen in conditions like polycythemia, increases resistance and hence blood pressure. Conversely, lower viscosity, as seen in anemia, reduces resistance.
Vessel length contributes to resistance, but this factor remains relatively constant in adults. Longer vessels naturally have higher resistance, but since vessel length doesn't change appreciably over short periods, its impact is more static compared to diameter and viscosity.
Vascular resistance is primarily regulated by arterioles, the small branches of arteries. These vessels contain smooth muscle that can constrict or dilate, thereby adjusting resistance. Vasoconstriction increases resistance and blood pressure, while vasodilation decreases them.
The systemic vascular resistance (SVR) and pulmonary vascular resistance (PVR) are specific measures used to assess resistance in systemic and pulmonary circulations, respectively. Elevated SVR can indicate conditions such as hypertension, while increased PVR may suggest pulmonary hypertension.
Understanding vascular resistance is crucial for diagnosing and managing cardiovascular diseases. It helps in assessing the efficiency of the circulatory system and the effectiveness of therapeutic interventions aimed at modulating blood pressure and flow.