10.7: Factors Affecting Blood Flow

Blood volume, pressure, and flow are fundamental components of the circulatory system and are intricately interconnected. A decrease in blood volume typically leads to a corresponding reduction in both blood pressure and flow, while an increase in blood volume results in elevated pressure and flow. Under normal physiological conditions, blood volume is maintained within a relatively stable range.

Viscosity is a critical determinant of blood flow, directly influencing resistance. An increase in blood viscosity raises resistance and impedes flow, whereas a decrease in viscosity reduces resistance and enhances flow. Under normal conditions, blood viscosity remains relatively stable over short periods. Blood viscosity is primarily governed by two factors: the formed elements and plasma proteins. Erythrocytes, the predominant component of the formed elements, significantly impact viscosity; thus, conditions affecting erythropoiesis, such as polycythemia or anemia, alter blood viscosity. The liver, responsible for producing most plasma proteins, also plays a role, and any disruption in liver function can slightly affect viscosity, consequently influencing blood flow. Although leukocytes and platelets typically make up a small proportion of the formed elements, certain pathological conditions leading to their overproduction can also impact viscosity.

Vessel length is directly proportional to resistance; the longer the vessel, the greater the resistance and the lower the flow. Conversely, shorter vessels reduce resistance and enhance flow. In adults, vessel length typically remains constant under normal physiological conditions. However, the distribution of blood vessels varies across tissues. For instance, adipose tissue has a less extensive vascular network compared to skeletal muscle. Approximately one pound of adipose tissue contains about 200 miles of vessels, while skeletal muscle contains twice that amount. Vessel length generally decreases only in cases of significant mass reduction, such as amputation. 

In a 150-pound individual, the vascular system extends roughly 60,000 miles. Gaining 10 pounds can add 2,000 to 4,000 miles of vessels, depending on the type of tissue. Weight loss benefits the cardiovascular system by reducing the resistance the heart must overcome, relieving it of the additional burden posed by the increased vascular network.

The diameter of blood vessels is a highly dynamic parameter, varying based on vessel type and frequently changing throughout the day in response to neural and chemical signals that induce vasodilation or vasoconstriction. The vascular tone, determined by the contractile state of the vessel's smooth muscle, is the key factor influencing diameter, and consequently, resistance and blood flow. The relationship between vessel diameter and resistance is inversely proportional: with a constant blood volume, an increase in diameter leads to lower pressure against the vessel wall, reduced friction, decreased resistance, and enhanced flow. In contrast, a reduction in diameter increases the contact between blood and the vessel wall, elevating resistance and reducing flow.

The impact of lumen diameter on resistance is profound; even small changes in diameter can cause dramatic shifts in resistance. According to the principle that resistance is inversely proportional to the fourth power of the vessel radius (R ∝ 1/r^4), if an artery or arteriole constricts to half its original radius, resistance increases 16-fold. Conversely, if the vessel dilates to twice its initial radius, resistance decreases to 1/16 of its original value, resulting in a 16-fold increase in blood flow.