HMA - Blood flow and tissue perfusion phys

Static (lateral, potential) pressure

in Bernoulli's theorem, pressure that the blood exerts on the vessel wall, i.e. what we consider when we talk about blood pressure.

Dynamic (kinetic) pressure

in Bernoulli's theorem, pressure associated with the kinetic energy of the moving blood, i.e. higher blood velocity equates to higher dynamic pressure.

Blood flow (Q)

volume of blood passing a point per unit time (cm^3/s).

Velocity (v)

distance blood crosses per unit of time (cm/s).

Cross-sectional area (A)

relates to volume of a blood vessel (cm^2).

Poiseuille’s law

describes how blood flow (Q) through a vessel depends on the pressure gradient, vessel dimensions, and properties of blood.

Pressure gradient (ΔP)

the difference in pressure between the beginning and end of the vessel (not absolute pressure).

Resistance (R)

the opposition or force that blood must overcome to flow through blood vessels, affected by vessel radius, vessel length, and blood viscosity.

Vessel radius (r)

narrower vessel lumens increase resistance to flow, which can be mediated with contraction or relaxation of vascular smooth muscle.

Vessel length (l)

longer vessels have higher resistance to flow. Not typically a physiological factor but is affected in growth, systemic vs pulmonary circulation, and capillaries.

Blood viscosity (η)

more viscous blood causes higher resistance to flow. Not typically a physiological factor and is determined by haematocrit, which may vary with altitude, disease, or in blood doping.

Series circuit

single pathway for blood flow, meaning total resistance = sum of individual resistances (R total = R1 + R2 + R3), leading to a greater total resistance.

Parallel circuit

total blood flow divides into multiple, simultaneous pathway, meaning 1/total resistance = sum of 1/individual resistances (1/R total = 1/R1 + 1/R2 + 1/R3), leading to a total resistance less than that of its individual parts.

Resistance arterioles

blood vessels with thick smooth muscle walls capable of contracting or relaxing to control blood flow. Have high resistance due to a small cross-sectional area that cannot compensate for increase in resistance with the relatively small radius.

Compliance

ability of a blood vessel to expand and stretch as it fills with blood, contributing to high capcitance in veins.

Capacitance

volume of blood a portion of the circulation can store relative to increasing pressure.

Laminar flow

smooth, orderly movement of blood in parallel concentric layers through a blood vessel, where the centre moves fastest, experiencing less shearing forces from the vessel wall.

Axial streaming

cells concentrate in the centre of the vessel where flow is fastest.

Turbulent flow

irregular fluid motions within a vessel greatly reducing flow for a given pressure.

Apparent viscosity

the effective internal resistance to flow of blood related to haematocrit, decreasing when vessel diameter is less than 300 um.

Plasma skimming, spreading of RBCs

What contributes to reduced apparent viscosity in vessels with a diameter less than 300 um?