Lecture by
Dr.Mohammed Sharique Ahmed
Quadri
Assistant professor ,Physiology

• Discuss the pattern and physics of blood flow in vascular system.
• Define the term resístanse& pressure in the blood vessels.
• Describe laminar and turbulent blood flow.
• Explain factors affecting the peripheral resistance.
• Appreciate the typical functional features of different types of vessels in vascular tree.

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• Blood is transported to all parts of body through blood vessels.
• Blood vessel bring Oxygen and nutrition.
• They remove waste product.
We will study general principles regarding blood flow, Physics of blood flow, Role of blood vessels, then blood pressure regulation.
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Distribution of Cardiac
Output at Rest

• Blood is constantly reconditioned so composition remains relatively constant
• Reconditioning organs receive more blood than needed for metabolic needs
– Digestive organs, kidneys, skin
– Adjust extra blood to achieve homeostasis
• Blood flow to other organs can be adjusted according to metabolic needs

• Digestive organ, kidney, skin receive blood in excess of their own needs, therefore, can withstand better when blood flow is reduced.
• Brain can least tolerate disrupted supply
• Brain suffers irreparable damage when blood supply is not there for more than 4mins. If oxygen is not supplied to brain, permanent damage occurs.
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• Flow rate through a vessel (volume of blood passing through per unit of time):
– Directly proportional to the pressure gradient
– Inversely proportional to vascular resistance
F = Δ P
R
F = flow rate of blood through a vessel
Δ P = pressure gradient
R = resistance of blood vessels

• Pressure Gradient is difference in pressure
between the beginning and end of vessel.
• Blood flows from area of high pressure to an area of low pressure, down the pressure gradient.
• When heart contracts, it gives pressure to the blood, which is main driving force for flow through a vessel.
• Due to resistance in the vessel, the pressure drops as blood flows.
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What is Resistance?
• It is measure of hindrance or opposition to
blood flow through a vessel, caused by friction between the blood in the vessel wall.
• If resistance to flow increases, it is difficult for blood to pass through a vessel, therefore, flow rate decreases.
• When resistance increases, the pressure gradient must increase to maintain the same flow rate.
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• If vessels offer more resistance to flow e.g. increased peripheral resistance, which occurs in HIGH BLOOD PRESSURE then heart must work harder to maintain adequate circulation.
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• Resistance to blood flow depends on three factors:
1. Viscosity of blood
2. Vessel length
3. Radius of the vessel – this is most important.
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1. Viscosity of blood
• We write η for Viscosity.
• Viscosity refers to the friction, which is developed between the molecules of fluid as they slide over each other during flow of fluid.
• Greater the viscosity, Greater the resistance to flow.
• Blood viscosity is determined by number of circulating RBC, blood viscosity is increased in
Polycythemia and decreased in Anaemia.
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2. Vessel length
• Greater the length of a vessel, more will be the resistance.
How length of a vessel affects the resistance?
• When blood flows through a vessel, blood rubs against the vessel wall, greater the vessel surface area in contact with the blood ,
greater will be the resistance to the flow.
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3. Radius of the vessel
• It is the most important factor to determine the resistance to flow.
• Fluid passes more readily through a large vessel.
• Slight change in radius of a vessel brings great change to flow because Resistance is inversely proportional to the fourth power of the Radius
[multiplying the radius by itself four times].
R α 1/r 4
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3. Radius of the vessel
• Therefore, doubling the radius, reduces the resistance to 1/16 its original value.
[r 4 = 2×2×2×2 = 16 or R α 1/16 ] and there is increased flow through a vessel
16 fold.
• On the other hand, when we decrease the radius to the half, blood flow will be decreased 16 times.
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Relationship of
Resistance and Flow to Vessel Radius

• Clinically radius of arterioles can be regulated and is the most important factor in controlling resistance to blood flow throughout the vascular system.
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• Closed system of vessels
• Consists of
– Arteries
• Carry blood away from heart to tissues
– Arterioles
• Smaller branches of arteries
– Capillaries
• Smaller branches of arterioles
• Smallest of vessels across which all exchanges are made with surrounding cells
– Venules
• Formed when capillaries rejoin
• Return blood to heart
– Veins
• Formed when venules merge
• Return blood to heart

Basic Organization of the
Cardiovascular System

• Specialized to
– Serve as rapid-transit passageways for blood from heart to organs
• Due to large radius, arteries offer little resistance to blood flow
– Act as pressure reservoir to provide driving force for blood when heart is relaxing
• Arterial connective tissue contains
– Collagen fibers
» Provide tensile strength
– Elastin fibers
» Provide elasticity to arterial walls

Chapter 10 The Blood Vessels and Blood Pressure
Human Physiology by Lauralee
Sherwood ©2010 Brooks/Cole,
Cengage Learning

• Lateral force exerted by blood against a vessel wall
– Depends on
• Volume of blood contained within vessel
• Compliance of vessel walls
• Systolic pressure
– Peak pressure exerted by ejected blood against vessel walls during cardiac systole
– Averages 120 mm Hg
• Diastolic pressure
– Minimum pressure in arteries when blood is draining off into vessels downstream
– Averages 80 mm Hg

• Major resistance vessels
WHY?
• Because their radius is small.
• As arteriolar resistance is high, it causes marked drop in mean pressure as blood flows through arteriole.
• Mean Arterial Blood Pressure [ABP] of 93mm Hg in arteries falls to mean ABP of 37mm Hg as blood leaves the arteriole and enters the capillaries.

• Arteriolar Resistance converts the pulsatile systolic to diastolic pressure swings in the arteries into the non-fluctuating pressure present in the capillaries.
• Radius of the arteriole can be adjusted to achieve 2 functions:
– Distribute cardiac output among systemic organs, depending on body’s momentary needs
– Help regulate arterial blood pressure
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• Mechanisms involved in adjusting arteriolar resistance
– Vasoconstriction
• Refers to narrowing of a vessel
– Vasodilation
• Refers to enlargement in circumference and radius of vessel
• Results from relaxation of smooth muscle layer
• Leads to decreased resistance and increased flow through that vessel

Arteriolar Vasoconstriction and Vasodilation

• Thin-walled, small-radius, extensively branched
• Sites of exchange between blood and surrounding tissue cells
– Maximized surface area and minimized diffusion distance
– Velocity of blood flow through capillaries is relatively slow
• Provides adequate exchange time

• Venous system transports blood back to heart
• Capillaries drain into venules
• Venules converge to form small veins that exit organs
• Smaller veins merge to form larger vessels
• Veins
– Large radius offers little resistance to blood flow
– ALSO SERVE AS BLOOD RESERVOIR

• Human physiology by Lauralee Sherwood, seventh edition
• Text book physiology by Guyton &Hall,11 th edition
• Text book of physiology by Linda .s contanzo,third edition