BiophysicsLecture 4

Hemodynamics and Factors Affecting Blood Flow

By

Mohamed Ahmed Saleh, Ph.D.

11/1/2012

Faculty of PharmacyFaculty of PharmacyClinical Pharmacy ProgramClinical Pharmacy Program

Blood Flow:•Blood flow: Amount of blood that flows through any tissue

in a given period of time (mL/min)

•Total blood flow: Volume of blood that circulates through

the systemic and pulmonary blood vessels each minute →

Cardiac Output (CO)

•Cardiac output (CO) = heart rate (HR) x stroke volume

(SV)

•SV = the volume of blood pumped with each beat

•Distribution of CO into different body tissues:

1.Pressure difference of different parts of the body

Pressure ↑ → Blood Flow ↑

2.Resistance of specific blood vessels to blood flow

Resistance ↑ → Blood Flow ↓

• Blood flow or ‘‘F‘‘ = Blood flow means simply the quantity of blood that passes a given point in the circulation in a given period of time (mL/Sec).

Movie 1

1. Blood Pressure (BP):

•Blood flow: ↑ Pressure → ↓ Pressure

•BP: Contraction of the ventricles → hydrostatic pressure

exerted by blood on the walls of a blood vessel.

•Normal BP in a young adult at rest:–120 mmHg during ventricular contraction, systole

–80 mmHg during ventricular relaxation, diastole

–Systemic Circulation:

Blood Pressure (cont.):

•Mean Arterial Pressure (MAP): The average blood

pressure in arteries → closer to diastolic pressure during the

greater portion of the cardiac cycle

MAP = diastolic BP + 1/3(systolic BP – diastolic BP)

(80 + 1/3(120 – 80)) = 93 mmHg

•CO = HR x SV

•CO = MAP/R where R is the vascular resistance

•MAP = CO x R

•SV ↑ or HR ↑ → CO ↑

•and if R stays constant → MAP ↑

2. Vascular Resistance:

•Friction between blood and the walls of blood

vessels: resists to blood flow

•Vascular resistance depends on three factors:

(1) Size/Diameter of the blood vessels

(2) Blood viscosity

(3) Total blood vessels length

Size/Diameter of the blood vesselSize/Diameter of the blood vesselSmall changes in diameter result in big changes in flowSmall changes in diameter result in big changes in flow

–Large arteries have little resistance while small arterioles have a large resistance.–During exercise, muscular blood flow markedly increases as a result of metabolic arteriolar vasodilatation (= decrease in the peripheral resistance ).

Viscosity:Viscosity:Greater viscosity = greater resistance

Vessel Length:Vessel Length:

Poiseuille’s LawPoiseuille’s Law(relates pressure, flow and resistance)(relates pressure, flow and resistance)

The (R) is directly proportionate to the length of the vessel and the blood viscosity. while inversely proportionate to the 4th power of the radius of the vessel.

2.1. Size/Diameter of Blood Vessels:

•Size of the lumen of a blood vessel ↓ → Resistance to

blood flow ↑

•“forth power law”: the resistance increases in proportion to

the inverse of the forth power of the radius:

•Vasoconstriction: Narrowing of the vessels resulting from

contraction of the muscular wall of the vessels

•Vasodilation: Widening of the vessels resulting from

relaxation of the muscular wall of the vessels

•constant fluctuations → Resistance → BP

4

1

rR

Movie 2

2.2. Blood Viscosity:

•Blood: –Plasma: 0.015 Poise

–Hematocrit (Ht): normally about 45% for men and 40% for women

–If hematocrit =46% so viscosity =

•Factors: –Size of blood vessels ↓ → Velocity (shear rate) ↑ → Viscosity ↓

–Temperature ↓ → Viscosity ↑

1ºC ↓ → Viscosity ↑ by 2%

Ht.plasma 521

Poise.... 03204605210150

Viscosity, Viscosity, refers to the friction refers to the friction that developed between the that developed between the molecules of a fluid during the flow molecules of a fluid during the flow of that fluid.of that fluid.

It is determined by:It is determined by:

1) Hematocrit:1) Hematocrit:is the percentage (%) of the concentration of red blood cells in blood. It is normally about 45% for men and 40% for women.

Increased in polycythemia and decreased in anemia.

2) Plasma proteins.2) Plasma proteins.Increased in

hypergammaglobulinemia.

Hematocrits in a healthy (normal) person and in patients withanemia and polycythemia.

2.3. total length of blood vessel:

•Resistance to blood flow through a vessel ≈ total length of

blood vessel

•Hypertension → additional blood vessels in the adipose

tissue (fat/obese) → total blood vessel length is longer →

resistance to blood flow ↑ → blood pressure ↑

•For each extra kilogram of fat, an estimated 650 km (about

400 miles) of additional blood vessels can develop in our

body!!!

Rate of Blood Flow

Poiseuille’s Law:

–F: rate of blood flow

–∆P: pressure difference between two ends of the

vessel

–r: radius of the vessel

–η: blood viscosity

–l: length of the vessel

l

rPF

8

4 4

8tanRe

r

lRcesis

• Velocity (V ) is inversely related to the cross sectional area of the blood vessel (A):

• V=F/A

Blood does not flow as a plug in Blood does not flow as a plug in largelarge vessels. vessels.

How does it flow ?How does it flow ?

Movie 3

The fluid nearest the vessel wall flows the slowest, and fluid in the center of the tube moves the most rapidly.

This produces layers (‘laminae’).

It is silent.

Flow is laminar up to certain critical velocity. If the flow rate is increased then the trend for turbulence will increase .

Flo

w

Pressure Gradient

criticalvelocity Turbulence especially

occurs when critical velocity is exceeded mostly in areas of

stenosis

‘Eddy’ currents, unpredictable

velocities

It is disturbed blood flow in form of eddy currents in various directions

Thank You!