1) Blood flow is defined as the volume of blood flowing through an organ or vessel per unit time and is normally expressed in ml/min. The total blood flow in an adult is about 5.25 l/min, known as cardiac output. 2) Blood flow (Q) is calculated as the product of velocity (V) and cross-sectional area (A), or can be calculated using Darcy's law as the ratio of the pressure difference (ΔP) between two points and the resistance (R) of the vessel. 3) Blood pressure is usually measured in millimeters of mercury (mmHg) and refers to either systolic or diastolic pressure, which is the peak or minimum

1. Mariya Yeldhos
Assistant Professor
Faculty Of Engineeering
Avinashilingam Institute

2.  Blood flow is defined as the amount of blood
flowing through an organ, tissue or blood vessel at
a given time and is normally expressed in ml/min
 Overall blood flow in the total circulation of an
adult is about 5.25 l/min = cardiac output

3.  Since flow is a measure of volume per unit time
=> Q=VA,
where Q=Flow V=Velocity, A=Cross sectional area
 Since the vascular system obeys an adaptation of
Ohms law, known as Darcy’s law
=> Q=ΔP/R,
where ΔP is the pressure difference between
two points and R is the resistance

4.  Force that the blood exerts against the vessel wall.
 Blood pressure almost always is measured in
millimeters of mercury (mm Hg)
 Measuring BP ???
 Systolic Pressure – peak arterial BP attained during
ventricular systole
 Diastolic pressure – minimum arterial BP between
heartbeats

5.  Difference between the systolic and diastolic
pressure is called as Pulse Pressure
 Pulse pressure = 45 mmHg
MAP
 Sum of diastolic pressure and one-third of
the pulse pressure
 MAP=[ diastolic pressure+(Pulse pressure/3) ]
 Varies with the influence of gravity
 Standing adult-
 62 mmHg in the major arteries of the head
 180 mmHg – arteries in legs

6.  Blood flow can either be laminar or turbulent

7.  When blood flows through a long smooth vessel it flows in straight
lines, with each layer of blood remaining the same distance from the
walls of the vessel throughout its length
 When laminar flow occurs the different layers flow at different rates
creating a parabolic profile
 The parabolic profile arises because the fluid molecules touching the
walls barely move because of aherence to the vessel wall. The next
layer slips over these, the third layer slips over the second and so on.

8.  When the rate of blood flow becomes too great, when it
passes by an obstruction in a vessel, when it makes a sharp
turn, or when it passes over a rough surface, the flow may then
become turbulent
 Turbulent flow means that the blood flows crosswise in the
vessel as well as along the vessel.

9.  The tendency for turbulent flow increases in direct
proportion to the velocity of blood flow, the
diameter of the blood vessel, and the density of the
blood, and is inversely proportional to the viscosity of
the blood, in accordance with the following equation:
Re=(v.d.ρ)/ η
where Re is Reynolds' number and is the measure of the tendency for turbulence to occur,
ν is the mean velocity of blood flow (in centimeters/second), d is the vessel diameter (in
centimeters), ρ is density, and η is the viscosity (in poise)
 When Reynolds’ number increases above about 200
turbulent flow will result

10.  Resistance is the impediment to blood flow in a vessel
 Resistance to blood flow within a vascular network is determined
by the
 length of the vessel
 diameter of vessels,
 physical characteristics of the blood
 viscosity,
 density
 laminar flow vs turbulent flow, and
 extravascular mechanical forces acting upon the vasculature.

11.  Thickness of blood – RBC and Albumin
 Viscosity increases , blood flow ???
 Polycythemia, Dehydration etc
 Viscosity decreases , blood flow ???

13.  Conductance is a measure of the blood flow
through a vessel for a given pressure difference
 Conductance is equal to the reciprocal of
resistance.

14.  Vasomotion - Change in vessel radius
 Vasoconstriction – Narrowing of vessel
 Vasodilation – widening of vessel

15.  Slight changes in the diameter of a vessel cause
tremendous changes in the vessel's ability to conduct
blood when the blood flow is streamlined
 Although the diameters of these vessels increase only
fourfold, the respective flows are 1, 16, and 256
ml/mm, which is a 256-fold increase in flow.
 Thus, the conductance of the vessel increases in
proportion to the fourth power of the diameter

16.  The relationship between conductance and diameter can be explained by
considering the number of ‘layers’ of blood in a vessel.
 For a small vessel a large proportion of the blood is in contact with the
wall of the vessel.
 By integrating the velocities of all the concentric rings of flowing blood
and multiplying them by the areas of the rings, one can derive the
following formula, known as Poiseuille's law:
Q = (π ΔPr4)/8 ηl
where Q is the rate of blood flow, ΔP is the pressure difference between the ends of the vessel, r is the
radius of the vessel, l is length of the vessel, and η is viscosity of the blood.

20.  Turbulent and Laminar Flow
 Re = (v.d.ρ)
--------
η

21. Q = (π ΔPr4)
------------
8 ηL
•Blood flow is inversely proportional to the length of blood vessel
•Length of blood vessel increases, then blood flow decreases