Ultrasound Physics Chapter 18 Hemodynamics

Hemodynamics

the study of blood moving through the circulatory system

flow (volume flow rate)

-volume of blood moving during a particular time

-"how much"

-units: volume/time; liters/min

velocity

-the speed or swiftness of a fluid moving from one location to another

-"how fast"

-units: distance/time; cm/s

Three basic forms of blood flow

-pulsatile

-phasic

-steady

pulsatile flow

-occurs when blood moves with a variable velocity

-blood accelerates and decelerates as a result of cardiac contraction

-commonly appears in arterial circulation

phasic flow

-occurs when blood moves at variable velocity

-blood accelerates and decelerates as a result of respiration

-often appears in venous circulation

steady flow

-occurs when a fluid moves at a constant speed or velocity

-present in venous circulation when individuals stop breathing for a brief moment

laminar flow

-occurs when the flow streamlines are aligned and parallel (layers of blood that travels at individual speed)

-commonly found in normal physiologic state

two forms of laminar flow

-plug flow

-parabolic flow

plug flow

occurs when all of the layers and blood cells travel at the same velocity

parabolic flow

-bullet shaped profile

-velocity is highest in the center of the lumen and gradually decreases to its minimum at the vessel wall

Reynolds number

-predicts whether flow is laminar or turbulent

-equals average flow speed X tube diameter X density/viscosity

Reynolds number for laminar flow

less than 1500

Reynolds number for turbulent flow

greater than 2000

turbulent flow

-characterized by chaotic flow patterns in many different directions and at many speeds

-streamlines are often onliterated

eddy current or vortex

small, hurricane like, swirling, rotational patterns that appear in turbulent flow

turbulence is often associated with

-cardiovascular pathology and elevated blood velocities

-may be seen downstream from a significant stenosis in a vessel

sound associated with turbulence is called

murmur or bruit

tissue associated with turbulence is called a

thrill (palpable murmur)

energy gradient

-blood moves from regions of higher energy to lower energy

what event provides energy to the circulating blood?

energy is imparted to blood by the contraction of the heart during systole

three forms of energy associated with blood

-kinetic

-pressure

-gravitational

total energy at a specific location within the circulation is

the sum of all three forms of energy

kinetic energy

-associated with a moving object

-determined by mass of object and speed at which it moves (higher mass/speed, the more energy)

pressure energy

-form of stored or potential energy

-has ability to perform work

-creates flow by overcoming resistance

gravitational energy

-form of stored or potential energy

-associated with elevated object

-elevated objects can perform work

three ways energy is loss

-viscous loss

-frictional loss

-inertial loss

viscosity

-thickness of a fluid

-more energy is loss with movement of high viscosity fluids

-units: Poise

-energy loss is determine by hematocrit

hematocrit

-is the percentage of blood made up of red blood cells

-normal value is 45%

frictional energy loss

-occurs when flow energy is converted to heat as one object rubs against another

inertial energy loss

-energy loss when the speed of a fluid changes, regardless of whether the fluid speeds up of slows down (resist changes in its velocity)

three events in which inertial energy loss occurs

-pulsatile flow

-phasic flow

-velocity changes at a stenosis

maximum velocity exists where the vessel is (widest/narrowest)

narrowest

stenosis

narrowing in the lumen of a vessel

affects of a stenosis

-change in flow direction

-increase velocity as vessel narrows

-turbulence downstream from stenosis

-pressure gradient across the stenosis

-loss of pulsatility

Bernoulli's principle

-describes the relationship between velocity and pressure in moving fluid

-states that with steady flow, the sum of all forms of energy is the same everywhere

Law of conservation of energy

explains why pressure energy is lowest wherever the kinetic energy is the highest, at the narrowest part of the lumen

pressure gradient

-equals flow X resistance

-increases when either flow increases or resistance increases

flow increases when either

-pressure gradient increases or resistance decreases

Ohm's Law

-voltage = current X resistance

-describes the movement of electricity through a wire

pressure in a fluid system is called _____ in an electrical system, flow in a fluid system is called _____ in an electrical one

voltage; current

electrical resistance is reported in units of

ohms

in the circulatory system, the resistance vessels are called

arterioles

normal venous and arterial resistance are

similar (low resistance)

hydrostatic pressure

-pressure related to the weight of blood pressing on a vessel measured at a height above or below the heart level

-units: mmHg

when a person is supine; all body parts are at the same level as the heart, and the hydrostatic pressure is

zero everywhere

hydrostatic pressure in a standing individual at locations below the heart level is

positive (measured pressure will be higher than the true circulatory pressure)

hydrostatic pressure in a standing individual at locations above the heart level is

negative (measured pressure will be lower than the true circulatory pressure)

measured pressure

equals circulatory pressure + hydrostatic pressure

venous flow during inspiration

-diaphragm moves downward toward the abdomen

-thoracic pressure decreases

-abdominal pressure increases

-venous return to the heart increases

-venous flow in the legs decreases

venous flow during expiration

-diaphragm moves upward into the thorax

-thoracic pressure increases

-abdominal pressure decreases

-venous return to the heart decreases

-venous flow in the legs increases