Hemodynamics

Hemodynamics

Purpose of control mechanisms of blood flow?

Maintain homeostasisPurpose of blood flow?

Nutrient and waste exchange

Blood flow to brain and heart must be maintained

Insufficient blood volume to perfuse all tissuesimultaneously

Blood flow must match metabolic needs of tissue

AJ Davidoff

MAP = CO x TPR

Important to maintainadequate perfusion pressure in order to control blood flow

Sherwood Fig 10-1

HR x SV

MAP = mean arterial pressureTPR = total peripheral resistanceCO = cardiac output

Sherwood

Capillary exchange is the sole purpose of the circulatory system

Blood flow depends on pressure gradients and vascular resistance

Relationship between blood flow, pressure and resistance

Ohm's Law: V = I*R or I = V/R

V = voltage (potential difference)I = currentR = resistance

Blood Flow: P = Q*R or Q = P/R

Q = flow (mL/min) P = pressure gradient (mm Hg)R = resistance (mm Hg/mL/min)

The major mechanism for changing blood flow is by changing arterial resistance (e.g., TPR or in a single

organ)

Pressure gradients

Pressure difference affects flownot absolute pressure

Sherwood Fig 10-3

Resistance to Blood Flow

Poiseuille equationR = 8L r4

R = resistance = viscosity of bloodL = length of blood vesselr4 = radius of blood vessel raised to the fourth power

If radius decreases by one half, resistance increases by 16-fold (= 24)!!!

(r4 = area)

Radius profoundly affects blood flow

Sherwood Fig 10-4

R~ 1/r4

Q = P/R

Flow ~ r4

Costanzo Fig 4-5

Q = P/R

Total resistance equals the sum of the individual resistances

Total flow is the same at each level, but pressure decreases progressively

(93 mm Hg) (4 mm Hg)

Why?

Series Resistance

Parallel Resistance

Flow in aorta is equal to the flow in the vena cave (steady state)Flow to each organ is a fraction of the total blood flowTotal resistance is less then any of the individual resistances,therefore no significant loss of arterial pressure to each organ

5 L/min 5 L/min

Needs work

Velocity of Blood Flowv = Q/A

v = velocity of flow (cm/sec)Q = flow (ml/sec)A = cross-sectional area (cm2)Costanzo Fig 4-4

Costanzo Fig 4-3

Total cross sectional area of systemic blood vessels

v = Q/A

Laminar flow and Turbulence

Laminar flow is parabolic, highest velocity in center (least resistance), lowest adjacent to vessel walls

Turbulent flow is disoriented, no longer parabolic, energy wasted, thus more pressure required to drive blood flow.

quiet

noisy

Costanzo Fig 4-6

Ganong Fig 30-8

Turbulence is velocity of blood flowdiameter of blood vessel1/ viscosity of blood

Mohrman and Heller Fig 6-6

Bernouilles Principle (in a single vessel)Total energy = distending pressure (PD) + kinetic energy (KE)

Higher velocity through a constrictionPDKE

Bad for plaque regionsWhy?

Total energy is actually not conserved completely because of heat loss

Bad for aneurysmsWhy?

KE

PD

Bernouilles Principle

Cardiovascular Physiology Conceptshttp://www.cvphysiology.com/Blood%20Pressure/BP004.htm

Compliance of blood vessels

C = compliance (mL/mm Hg)V = volume (mL)P = pressure (mm Hg)

C = V/ P

• Compliance is a slope

• At low pressures, veins have a greater compliance than arteries

• At high pressures, compliance is similar in veins and arteries (but volume is much greater in veins)

Compliance changes related to vasocontraction or aging

With vasocontraction:• Venous volume

decreases and pressure increases

• Venous compliance decreases

Similar effects in arteries with aging

Martini Fig 21-2

ArteriesConduits

BloodVessels

Pressure reservoir

Sherwood Fig 10-6 & -7

Elastic recoil continues to drive blood toward arterioles during diastole

B&B Fig 17-11

MAP = diastolic pressure + 1/3 pulse pressure(at rest)

2/3 time in diastole1/3 time in systole

80 mph for 40 min120 mph for 20 min

Sherwood Fig 10-7

G&H Fig 15-6

Dampening pulse pressures

Arterial pulse pressureinfluenced by:

elasticityrigidityresistance

resistance, pulse pressure

What does systolic pressure tell you?What does diastolic pressure tell you?

CO & TPRTPR

Cardiac Output (CO) = MAP TPR

Sherwood Fig 10-9

G&H Fig 15-4 and B&B

Aortic pressure changes

rigid

G&H Fig 15-4

Aortic pressure changes

G&H Fig 23-4

Mean arterial pressure (MAP) is the main driving force for blood flow through capillaries

G&H Fig 14-2

Basis of auscultatory method for measuring BP(Sounds of Korotkoff)

Mohrman and Heller Fig 6-9Turbulent flow is noisy

Why should cuff be placed at heart level?

What effects on BP measurement wouldthe presence of obesity cause?