Blood Flow, Pressure & Resistance Vocabulary

Key vocabulary covering how pressure, resistance, and vessel properties govern blood flow in systemic and pulmonary circuits.

Pressure Gradient (ΔP)

The difference in blood pressure between two points that drives blood to flow from higher to lower pressure.

Resistance (R)

Frictional opposition to blood flow within vessels, determined by viscosity, vessel length, and vessel radius.

Fusue’s (Flow) Law

Relationship stating Flow (Q) = Pressure Gradient (ΔP) ÷ Resistance (R).

Blood Flow (Q)

The volume of blood moving through a vessel or circuit per unit time.

Blood Viscosity

The thickness of blood; rises with dehydration or high red-cell count, increasing resistance.

Vessel Length (L)

Total distance blood travels; longer pathways raise resistance because of more cumulative friction.

Vessel (Lumen) Radius (r)

Internal half-diameter of a vessel; larger radius lowers resistance, smaller radius raises it.

Vasoconstriction

Smooth-muscle contraction that narrows vessel radius, raising resistance and lowering distal flow.

Vasodilation

Smooth-muscle relaxation that widens vessel radius, lowering resistance and increasing flow.

Smooth Muscle (in vessels)

Contractile tissue layer that enables vasoconstriction and vasodilation.

Systemic Circuit

Long, high-resistance vascular loop that carries oxygenated blood from left heart to body and back to right heart.

Pulmonary Circuit

Short, low-resistance loop that sends deoxygenated blood from right heart to lungs and back to left heart.

Aorta

First systemic artery; average pressure ≈ 90 mm Hg—sets the systemic pressure gradient.

Pulmonary Trunk

First pulmonary artery; average pressure ≈ 15 mm Hg—sets the pulmonary pressure gradient.

Arteriole

Microscopic artery leading to capillaries; major site of adjustable resistance via radius changes.

Capillary

Smallest blood vessel where exchange of gases, nutrients, and wastes occurs.

Venule

Microscopic vein that collects blood from capillaries and merges into larger veins.

Blood Pressure (BP)

Force per unit area exerted by blood on vessel walls; highest in arteries, lowest in veins.

Hydration Status

Relative body-water level; dehydration raises viscosity and resistance, slowing blood flow.

Neural/Hormonal/Local Signals

Stimuli that direct vascular smooth muscle to constrict or dilate, adjusting radius and resistance.