Question

How is systemic vascular resistance determined?

Answer 1

If blood viscosity increases, then the total peripheral resistance (TPR) will necessarily increase, thereby reducing blood flow. Conversely, when viscosity decreases, blood flow and perfusion will increase. The viscosity of blood is important in maintaining maximum pressure during one heartbeat.

Complete answer:
Systemic vascular resistance reflects the left ventricular afterload. The SVR force is determined by the three factors : length, diameter, and viscosity of blood in blood vessels. SVR concepts are important to understand the vital role in the establishment and manipulation of the blood vessels.

Mathematically expressed as MAP= CO × TPR, where CO stands for cardiac output and MAP for mean arterial pressure.

Many pathogenic conditions can change the normal person’s SVR, may cause an increase or decrease or can damage the wall of blood vessels. Damage in the blood vessel wall, ability to dilate or constrict the hemodynamic changes adaptation become impaired, often leading to high resistance in that blood vessel causing further damage or preventing the blood flow in that vascular territory. For heart failure patients, lowering of SVR is very helpful to strain off of the heart and improve cardiac output.
The increased systemic vascular resistance is because of the following reasons:-
Increased number of RBCs as in polycythemia that increases viscosity.
Increased body size as in obesity, which increases total vessel blood length, or,
Decrease in blood vessel radius length as vasoconstriction.

Note:
Systemic vascular resistance (SVR) is also referred to as total peripheral resistance (TPR) is defined as the amount of force exerted on blood circulating vessels of the body. SVR is clinically important when a person’s blood pressure is too high or low.