Hemodynamics: Blood Flow Factors

• Hemodynamics are factors affecting blood flow.

Blood Flow

• Volume of blood moving through any tissue in a given time period, measured in mL/min or L/min.
• Total blood flow is dictated by cardiac output (HR*SR)
• CO ≈ MAP / R: cardiac output is approximately equal to mean arterial pressure over total peripheral resistance.
• Pressure is the force per unit of service area; in blood, it's the force of blood pushing against vessel surfaces.
• Flow ∝ Δpressure: increased pressure causes increased flow.
• Fluids move from areas of higher pressure to areas of lower pressure (pressure gradient).
• Cardiac output determines pressure at the aorta, but pressure decreases throughout circulation.
• Mean Arterial Pressure (MAP): 1/3(SBP-DBP)+DBP; it is the driving pressure, determined by cardiac output into the aorta, but changes as it moves through vasculature.
• Pulse Pressure = SBP - DBP
• Resistance is the force opposing the normal flow of blood.
• Flow ∝ resistance^-1: increased resistance causes decreased flow.
• In healthy states, there is increased resistance to gut/kidneys during exercise to increase flow to active muscles
• Atherosclerosis is problematic, causing higher blood pressure

Pressure Changes

• Initial pressure comes from the heart.
• Systolic Pressure: highest pressure during contraction, around 120 mmHg.
• Diastolic Pressure: lowest pressure, around 80 mmHg.
• Pulsatile pressure goes up and down due to continuous heartbeats.
• There is not much drop in pressure from the aorta to arteries.
• Brachial artery is where BP is normally measured since the pressure is still pulsatile.
• Pressure drops and pulses are evened out in arterioles.
• Capillaries need lower pressure to avoid bursting.
• Veins have a larger diameter and are farther from the heart, so pressure continues dropping.

Sphygmomanometry

• Laminar flow: streamlined movement of blood through straight vessels; soundless; outer layers move slower (friction) but inner layer moves quickly.
• Turbulent (non-laminar) flow: fluid passing through constriction, sharp turns, or rough surfaces causes sounds.
• A blood pressure cuff (sphygmomanometer) is used to measure diastolic and systolic blood pressure.
• Inflation: A dial is used to inflate the cuff, causing pressure in the cuff to exceed pressure in vessels.
• Occlusion happens when cuff pressure > vessel pressure, the vessel is occluded and there is no flow, therefore no sound.
• Stethoscope is used to hear sounds, and deflation is when cuff pressure is slowly decreased, causing the appearance of Korotkoff sounds.
• Korotkoff sounds start once vessel pressure > cuff pressure; the first one is at SBP, and the last one is at DBP.

Resistance

• Resistance is equal to viscosity * length of tube
• Viscosity is the thickness of the blood.
• Resistance ∝ viscosity: increased viscosity causes increased resistance.
• Viscosity shouldn't change in a healthy individual, but may get more viscous under severe dehydration or conditions which cause increased cellular components.
• Length of tube does not generally change, so it should stay stable.
• Diameter: vasoconstriction increases resistance, while vasodilation decreases resistance.
• Resistance ∝ diameter^-4: increased diameter decreases the resistance by truly ridiculous amounts.

Velocity

• Cross-sectional area: the total cross-sectional area of all the branches of the vessel within that region.
• For example, capillaries are HUGE
• Velocity ∝ cross-section^-1: increased cross-sectional area decreases velocity.
• Cross-section ∝ surface area: increased cross-sectional area results from increased branching, which creates increased SA for gas exchange.
• Aorta: highest velocity.
• Capillaries: lowest velocity.
• Veins: medium velocity.