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Hemodynamics Lecture by Dr.Mohammed Sharique Ahmed Quadri

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

2 بسم الله الرحمن الرحيم

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

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.

5 Distribution of Cardiac Output at Rest

6 Blood Flow 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 Brain can least tolerate disrupted supply

Digestive organ, kidney, skin receive blood in excess of their own needs, therefore, can withstand better main 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.

8 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 F = ΔP R F = flow rate of blood through a vessel ΔP = pressure gradient R = resistance of blood vessels

9 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.

10 Relationship of flow to Pressure Gradient

11 RESISTANCE 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.

12 APPLIED 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.

13 RESISTANCE Resistance to blood flow depends on three factors:
1. Viscosity of blood 2. Vessel length 3. Radius of the vessel – this is most important.

14 RESISTANCE 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.

15 RESISTANCE 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.

16 RESISTANCE 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/r4

17 RESISTANCE 3. Radius of the vessel
Therefore, doubling the radius, reduces the resistance to 1/16 its original value. [r4 = 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.

18 Relationship of Resistance and Flow to Vessel Radius

19 APPLIED Clinically radius of arteriole can be regulated and is the most important factor in controlling resistance to blood flow throughout the vascular system.

20 Vascular Tree Closed system of vessels Consists of Arteries Arterioles
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

21 Basic Organization of the Cardiovascular System

22 Arteries 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


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

25 Blood Pressure 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

26 Arterioles 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.


28 ARTERIOLES 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

29 Arterioles 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

30 Arteriolar Vasoconstriction and Vasodilation

31 Capillaries 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

32 Veins 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

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

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