Lecture 6: Pressure, Force, Elasticity

How is blood flow rate controlled?

In response to tissue demand for nutrients and oxygen

What determines blood flow rate?

Pressure gradient of blood between two ends of a vessel and resistance

Flow rate equation

F = ∆P/R

∆P in pulmonary circuit

Pressure in pulmonary arteries - pressure in pulmonary veins

∆P in systemic circuit

Pressure in aorta - pressure in vena cava

Bulk flow

Movement of fluid/gases from higher to lower pressure

Function of blood pressure

Adequate arterial blood pressure is essential to maintain blood flow

Atherosclerosis

thickening/hardening of arteries caused by buildup of plaque in the inner lining of artery

Laminar blood flow

Smooth and streamlined, quiet, parabolic flow

Parabolic flow

Velocity of flow at center of vessel is greater

Turbulent blood flow

Turbulent and disordered, audible sounds

Where can turbulent blood flow occur?

stenotic cardiac/venous valves that are abnormally narrowed

Arteries

Carry oxygenated blood from heart to organs

Veins

Carry deoxygenated blood from organs to heart

Poiseuille’s Law

R: resistance
L: Length of blood vessel

n: fluid viscosity

r: radius

Effect of radius on resistance

Smaller radius → more resistance

Factors affecting length of blood vessel

Obesity increases number of blood vessels and length

Blood viscosity

Thickness depends on amount of RBCs and proteins

Total peripheral resistance

Combined resistance of all blood vessels within systemic circuit

Arterioles

Connect arteries with capillaries

Venules

connect capillaries with veins

Compliance

How easily a lumen of a blood vessel expands when it is filled with a volume of blood

Compliance equation

∆Volume / ∆Pressure

Elastance

Tendency of blood vessels to recoil towards its original dimensions as blood pressure falls

Elastance equation

∆Pressure/∆Volume

Arteries properties

Elastic but not compliant

Veins properties

Compliant but not elastic

Venous return

Return of blood flow to right atrium

Factors affecting venous return

Blood volume, venous pressure, skeletal muscle pumps, venoconstriction

Respiratory pump

Inhalation causes contraction of intercostal muscles and diaphragm

How does respiratory pump pull blood to heart?

Pressure increases in abdominal cavity and decreases in thoracic cavity → creates pressure gradient

Exhalation

Relaxation of intercostal muscles and diaphragm