Biology - Body Fluids and Circulation Question with Solution | TestHub

Blood pressure is determined by the product of cardiac output and peripheral resistance ($BP=CO\times PR$).

According to Poiseuille's law, peripheral resistance is directly proportional to the length of the blood vessel ($L$) and the viscosity of the blood ($\eta$), and inversely proportional to the fourth power of the vessel radius ($r^4$). Thus, blood pressure is directly proportional to the length of the blood vessel and the viscosity of blood.

Cardiac output is directly proportional to venous return. Since $BP$ is directly proportional to cardiac output, it is also directly proportional to venous return.

The partial pressures of ${\text{O}}_2$ and ${\text{CO}}_2$ influence blood pressure through complex chemoreceptor reflexes and local autoregulation (causing vasodilation or vasoconstriction), but blood pressure is not directly proportional to them.

Answer: Partial Pressure of ${\text{O}}_2$ and ${\text{CO}}_2$.