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Hemodynamics. Dr. Amel Eassawi. OBJECTIVES. The student should be able to : Know and understand the structure and basic function of different parts of the vascular system. Describe laminar and turbulent blood flow . Explain factors affecting peripheral resistance . Hemodynamics.

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Dr. AmelEassawi



  • Know and understand the structure and basic function of different parts of the vascular system.

  • Describe laminar and turbulentbloodflow.

  • Explainfactors affecting peripheralresistance.


  • Hemodynamics refers to the principle that govern the blood flow in the cardiovascular system.

  • Blood is transported to all parts of the body through blood vessels.

  • Blood vessel bring Oxygen and nutrition and remove waste product.


Distribution of Cardiac Output:

  • Blood is constantly reconditioned so composition remains relatively constant

  • Digestive organ, kidney, skin receive blood in excess of their own needs, therefore, can survive better when blood flow is reduced.

  • Brain suffers irreparable damage when blood supply is not there for more than 4mins. If oxygen is not supplied to brain, permanent damage occurs.


Blood Flow:

  • 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


Pressure Gradient:

  • 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.

Relationship of flow to p ressure gradient
Relationship of flow to Pressure Gradient

Vascular resistance

It is the resistance offered by the blood vessels to the flow of the blood.

Factors affecting the resistance to blood flow:

1- Directly proportional to the length of the blood vessel.

The greater the length of the B.V. the greater will be the resistance.

2- Directly proportional to the viscosity of the blood.

The greater the viscosity of the blood the greater will be the resistance.

3- Inversely proportional to fourth power of the radius of the blood vessel.

Vascular resistance1

1.Viscosity of Blood

  • Viscosity refers to the friction, which is developed between the molecules of fluid as they slide over each other during flow of fluid.

  • Blood viscosity is determined by number of circulating RBC, blood viscosity is increased in Polycythemia and decreased in Anaemia.

    2. Length of a Blood Vessel

  • 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.

    3.Radius of the vessel

  • 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

Vascular tree
Vascular Tree

  • 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


  • 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


Major Resistance vessels:

  • 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.

  • Mean Arterial Pressure

    Average pressure driving blood forward into tissues throughout cardiac cycle


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


Arteries take blood from the heart

Veins return blood to the heart


1.Connect arteries & veins

2. Site where gasses, nutrients & wastes are exchanged between blood and tissues


  • 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, Lauralee Sherwood, seventh edition.

  • Text book physiology by Guyton &Hall,11th edition.

  • Physiology by Berne and Levy, sixth edition.