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Hemodynamics and Factors Affecting Blood Flow
Amir GolnabiENGS 166
Spring 2008
Outline: •Blood Flow
•Blood Pressure–Facts
–Mean Arterial Pressure
–Total Blood Volume
•Vascular Resistance–Size Of The Lumen
–Blood Viscosity
–Total Length of Blood Vessel
•Conclusion–Rate of Blood Flow
•References
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)
•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 ↓
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:–110 mmHg during ventricular contraction, systole
–70 mmHg during ventricular relaxation, diastole
–Systemic Circulation:
Torto
ra, G
erard, and B
ryan D
errickson. P
rinciples of A
natom
y and P
hysiology. W
iley, 2006. 747.
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 ↑
Blood Pressure (cont.):
•Total volume of blood in the cardiovascular
system:
•5 liters of blood in total
–Modest decrease in BV → Homeostatic mechanism
–More than 10% → BP ↓
–Water retention → BP ↑
2. Vascular Resistance:
•Friction between blood and the walls of blood
vessels: resists to blood flow
•Vascular resistance depends on three factors:
(1) Size of the blood vessels
(2) Blood viscosity
(3) Total blood vessels length
2.1. Size 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 diameter:
•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
dR
2.2. Blood Viscosity:
•Shearing force F
•Different layers of the blood move with different velocities
causing a shearing action (friction) between them
•Rate of shear (γ) is the relative displacement of one fluid
layer with respect to the next–slope of the velocity profile: v/h
•Blood moves by the action of shear stress–the force per area, τ = F/A
•Viscosity:–η = shear stress / shear rate = τ / γ
2.2. Blood Viscosity (cont.):
•Blood: –Plasma: 0.015 Poise
–Hematocrit (Ht): normally about 46% for men and 38% for women
•Factors: –Size of blood vessels ↓ → Velocity (shear rate) ↑ → Viscosity ↓
–Temperature ↓ → Viscosity ↑
1ºC ↓ → Viscosity ↑ by 2%
Ht.plasma 521
Poise.... 03204605210150
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 → 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
References:
•"An Introduction to Blood Pressure". Maryland Virtual High School of Science and
Mathematics. 04/27/2008 <http://mvhs.shodor.org/edgrid/bloodpressure/bptoblt.pdf>.
•Bipin , Upadhyay. "BLOOD VISCOSITY FUNCTIONAL PARAMETERS". COLLEGE OF
BIOMEDICAL ENGINEERING AND APPLIED SCIENCES. 04/28/2008
<http://www.biomedical.edu.np/biomed_files/>.
•Freeman, Scott. Biological Science. Upper Saddle River, NJ: Pearson Prentice Hall,
2005.
•Guyton, Arthur, and John Hall. Textbook of Medical Physiology. PA: Elsevier, 2006.
•Purves, William K.; David Sadava, Gordon H. Orians, H. Craig Heller (2004). Life: The
Science of Biology, 7th, Sunderland, Mass: Sinauer Associates, 954.
•Tortora, Gerard, and Bryan Derrickson. Principles of Anatomy and Physiology. Wiley,
2006.