HEMODYNAMICS
(BLOOD FLOW, PRESSURE AND RESISTANCE)
LECTURE OBJECTIVES
• Understand the term hemodynamics
• Describe the principles governing the flow of blood in the human
body
• Know and explain the factors effecting resistance to flow and how
it effects the normal physiological environment of the body
HEMODYNAMICS
• The part of cardiovascular physiology dealing with the forces the heart (pump) has to
develop to circulate blood through the cardiovascular system
• Cardio vascular system consists of two pumps connected in series
– Right ventricle- pulmonary circuit
– Left ventricle systemic circuit
• Cardiac output or flow through each of the circuit is equal
PATH OF BLOOD FLOW IN THE CIRCULATORY SYSTEM
Systemic
Heart (left ventricle)
Pulmonary
Heart (left atrium)
aorta
arteries
arterioles
capillaries
venules
veins
vena cava
pulmonary veins
capillaries
pulmonary arteries
Heart (right atrium)
Heart (right ventricle)
PHYSICAL CHARACTERISTICS OF CIRCULATION
• Functional organization of vessels
• Blood volume in different segments
• Cross sectional area
• Velocity of blood flow in vessels
FUNCTIONAL ORGANIZATION OF CIRCULATION
• ARTERIES—high pressure vessels
• ARTERIOLES---strong muscular wall< can change
radius
• CAPILLARIES—smallest vessel for exchange of
nutrients
• VENULES—coalesce to form larger veins
• VEINS– low pressure system, major reservoir of blood
CROSS SECTIONAL AREA OF VESSELS
Aorta 2.5 cm2
Small arteries 20cm2
Arterioles 40cm2
Capillaries 2500 cm2
Venules 250 cm2
Small veins 80 cm2
Venae cavae 8 cm2
Largest diameter vessel aorta but smallest
cross sectional area
Capillaries all together represent largest
cross section area
VELOCITY OF BLOOD FLOW
• Inversely related to total cross sectional area
• Velocity greatest in aorta
• Minimum in capillaries favors nutrient exchange
BLOOD VOLUME IN DIFFERENT SEGMENT
• 84% in systemic circulation
• 16% In heart and lungs
• Out of 84%, more than 2/3rd is in systemic veins
PRESSURE GRADIENT IN CIRCULATION
• Blood flows from higher pressure to lower pressure gradient,
i.e., from ventricles to the rest of the body and the pulmonary
circuit, and back into atria
Systemic
Pulmonary
Heart (left ventricle)
Heart (left atrium)
aorta
arteries
arterioles
capillarie
venules
veins
vena cava
pulmonary veins
capillaries
pulmonary arteries
Heart (right atrium)
Heart (right ventricle)
IMPORTANT PRESSURE GRADIENTS IN THE BODY
Left ventricle: 120/0 mmHg
Aorta: 120/80 mmHg
Systemic arteries: Mean Arterial pressure: 93mmHg
Peripheral veins: 15 mmHg
Right atrium: 0mmHg
Pressure gradient: 93-0 = 93mmHg
PULMONARY CIRCUIT
Right ventricle:25/0 mmHg
Pulmonary circuit: 25/8 mmHg
Mean Pulmonary Arterial pressure: 15 mmHg
Pulmonary venous pressure: 5 mmHg
Left atrium: 5-10 mmHg
Pressure gradient in pulmonary circuit: 15-5 =10 mmHg
PRINCIPLES GOVERNING HEMODYNAMICS IN THE BODY
• Flow of blood
• Mean Arterial Pressure
• Resistance
The most important factor governing flow is resistance
RELATIONSHIP BETWEEN PRESSURE,
FLOW AND RESISTANCE
Change in Pressure
Flow =
Resistance
Q
=
P
R
P = QR
Change in Pressure = Flow x Resistance
Similar to Ohm’s Law
for electricity
I=
V
R
or
V = IR
Change in pressure is the difference between input (upstream) and output
(downstream) pressure
•When describing the flow of blood for an organ, the pressure difference is generally
expressed as the difference between the arterial pressure (PA) and venous
pressure (PV).
RESISTANCE TO FLUID FLOW
•
•
•
As fluid passes through a resistance pressure drops.
A resistance dissipates energy, so as the fluid works its way through the resistance it must
give up energy.
It gives up potential energy in the form of a drop in pressure
P1
P2
resistance
Fluid flow
DETERMINANTS OF RESISTANCE IN LAMINAR FLOW
The three major determinants of Resistance are:
•
•
•
•
•
•
length
viscosity
Radius
Therefore, R α L / r4
Or;
R =   L / r4
TURBULENT FLOW
• Non layered flow
•
•
•
•
Creates murmur
Heard as bruits
Produce more resistance than flow
Reynolds no.= diameter x velocity xdensity
viscosity
• > 2000 =turbulent flow
• <2000= laminar flow
POISEUILLE’S LAW
• Since,
Q=P
R
And;
R α  L / r4
Q =  P x r4
L
Therefore, the most important factor effecting resistance is the radius
RESISTANCE IN SERIES
• If resistors are connected in series:
• The total resistance is the sum of all the individual resistances
i.e., R1 + R2 + R3 …..
Therefore,
• Adding a resistor in series will increase the resistance of the system
• The flow of blood in an individual organ e.g, a nephron is in series, so
adding resistance at any point in this circuit will :
– Increase pressure upstream
– Decrease pressure downstream
– Flow decreased at all points equally through the series system
• This is demonstrated by the constriction of blood vessels in Hypertension
RESISTANCE IN PARALLEL
• If resistors are connected in parallel:
• The reciprocal of the total resistance is the sum of reciprocals of all the
individual resistances
i.e., 1 = 1 + 1 + 1
R
R1 R2 R3 …..
Therefore,
• Adding a resistor in series will decrease the resistance of the system
• The flow of blood in the systemic circuit is in parallel, so altering the
resistance at any INDIVIDUAL point in this circuit will :
– Decrease total peripheral resistance
– Not change the flow in the remaining circuits
• This is demonstrated by the obesity, in which higher pressure than normal
is required to maintain blood flow in the systemic circuit
REFERENCES
• Guyton and Hall’s text book of physiology
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