14 08 12.Intro to the Circulation Slides
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Transcript of 14 08 12.Intro to the Circulation Slides
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Introduction to the Circulation
Bio-Med 3662
August 2019
Douglas Burtt, MD, FACC
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Functions of the Cardiovascular System
• Deliver oxygen-carrying blood to the tissues
• Provide nutrients to the cells
• Remove waste products from cells
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Arteries, Veins & Capillaries
• Arteries carry blood from the heart to the tissues
• Veins carry blood from tissues back to the heart
• Thin-walled capillaries, interposed between arteries & veins allow exchange of nutrients, wastes and fluid
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Arteries, Veins & Capillaries
• Arteries carry blood from the heart to the tissues
• Veins carry blood from tissues back to the heart
• Thin-walled capillaries, interposed between arteries & veins allow exchange of nutrients, wastes and fluid
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Other Cardiovascular functions
• “Homeostatic” functions
– Regulation of blood pressure
– Regulation of body temperature
– Facilitates adjustments to altered physiologic states • Exercise
• Change in posture
• Hemorrhage
• Delivery of endocrine hormones to sites of action in tissues
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The “Circuitry” of the Cardiovascular System
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The “Circuitry” of the Cardiovascular System
• Path of blood flow through Heart and Lungs
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Cardiac Blood Flow
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Cardiac Blood Flow
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Cardiac Blood Flow
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Cardiac Blood Flow
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Cardiac Blood Flow
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Cardiac Blood Flow
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Cardiac Blood Flow
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Cardiac Blood Flow: Right vs. Left Heart
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Cardiac Blood Flow: Right vs. Left Heart
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Cardiac Blood Flow: Right vs. Left Heart
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Cardiac Blood Flow: Right vs. Left Heart
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Cardiac Blood Flow: Entire Heart
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Cardiac Blood Flow: Entire Heart
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Circulation through the body: “Circuitry”
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Circulation through the body: “Circuitry”
• Cardiac output distributed to organs in parallel
• Distribution is variable, regulated by arterioles
• Approximate distribution: – 15% brain
– 5% heart
– 25% kidneys
– 25% GI
– 25% muscle
– 5% skin
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Circulation through the body: Arteries and Veins
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Circulation through the body: Arteries and Veins
• Arteries – Thick-walled
– High-pressure system
– Become progressively smaller, branching into “arterioles”
– Site of arteriolar resistance
– Alpha-1 and Beta-2 receptors
• Veins – Thin-walled
– Low-pressure system
– Large capacitance
– Small branches are called “venules”
– Also innervated by sympathetic nervous system
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Circulation through the body: Arteries and Veins
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Circulation through the body: Capillaries
• Capillaries:
• Lined with a single layer of endothelial cells
• Surrounded by basal lamina
• Are the site of exchange of: – Nutrients
– Gases
– Water
– Solutes
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The “microcirculation”: arterioles through venules
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Circulation through the body: Capillaries
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Microcirculation: Capillaries
–Intestines
–Glomeruli
–Most common capillary
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Microcirculation: Capillaries
• Capillaries:
• Lipid-soluble substances (e.g. oxygen and CO2) cross
the endothelial cell membranes
• Water-soluble substances (e.g. ions) cross either through: – Water-filled clefts between cells
– Large pores in walls of “fenestrated” capillaries
– Pinocytotic vesicles via “transcytosis”
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Microcirculation: Capillaries
– Not all capillaries are perfused at all times (e.g.
during exercise)
– Perfusion is governed by dilation or constriction of arterioles and pre-capillary sphincters
– Regulated by sympathetic innervation of blood vessels and vasoactive metabolites which act locally (at tissue level)
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Area and Volume of blood vessels
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The physics of blood flow
Sir Isaac Newton:
Formulator of law of gravitation
Inventor of Calculus
Established study of optics
“Father” of physics
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Velocity of Blood Flow
• Velocity = rate of displacement of blood per unit time
Defined as: v = Q/A
where v = velocity in cm/sec
Q = flow in mL/sec
A = cross sectional area (cm2)
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Velocity of Blood Flow
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Velocity of Blood Flow
• Where is velocity of blood flow highest?
• A: Where area is lowest…………..
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Velocity of Blood Flow
• Conversely, velocity of blood flow is lowest in the………………
– Capillary bed – allowing for more
time for exchange of nutrients, etc.
Velocity in aorta ~ 800 x velocity in
capillaries
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Area and Volume of the
Cardiovascular System
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Regulation of Blood flow: Relationship of Pressure, Flow and Resistance
• Blood flow through a vessel is determined by: • Pressure difference at each end of the vessel
• Resistance of the vessel to blood flow
This is a re-formulation of Ohm’s Law
(where ΔV = i x R)
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Relationship of Pressure, Flow and Resistance
In a blood vessel:
ΔP = Q x R
Where: Q = flow (mL/min)
ΔP = Pressure difference (mm Hg)
R = Resistance (mm Hg/mL/min)
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Relationship of Pressure, Flow and Resistance
• Likewise, resistance can be calculated, if one knows the change in pressure and the blood flow (using the equation: R = ΔP / Q)
• Resistance can be calculated in an organ, or in a system of organs
• The resistance of the entire systemic vasculature is called the “Total Peripheral Resistance” (TPR) or the “Systemic Vascular Resistance” (SVR)
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Relationship of Pressure, Flow and Resistance
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Relationship of Pressure, Flow and Resistance
• Example 1: • The blood flow to the left kidney is measured at 500 mL/min
• The pressure in the renal artery is 100 mm Hg
• The pressure in the renal vein is 10 mm Hg
• The vascular resistance of the left kidney is:
R = ΔP / Q
or
R = (100 – 10) / 500 = 0.18 mm Hg/mL/min
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Relationship of Pressure, Flow and Resistance
• Question 2: What pressure drop would one measure to calculate the systemic vascular resistance (SVR)?
• Answer:
mean arterial pressure
minus
mean right atrial pressure
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Relationship of Pressure, Flow and Resistance
• Question 2: What pressure drop would one measure to calculate the pulmonary vascular resistance (PVR)?
• Answer:
mean pulmonary arterial pressure
minus
mean left atrial pressure
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Determinants of Vascular Resistance
• Determinants of vascular resistance include: • Blood vessel diameter and length
• Viscosity of the blood
As described by the Poiseuille equation:
R = 8ηl / πr4
Where η = blood viscosity
l = length of blood vessel
r = vessel radius
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Vascular Resistance
Since
R = 8ηl / πr4
Where η = blood viscosity
l = length of blood vessel
r = vessel radius
Therefore:
Resistance increases as viscosity increases
Resistance increases as length increases
Resistance increases as radius decreases
to the fourth power
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Resistance in Series and in Parallel
• Series resistance is calculated by simple additon of each segment’s resistance
• Rtotal = R1 + R2 + R3
• Parallel resistance is calculated as follows: • Rtotal = 1/R1 + 1/R2 + 1/R3
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Resistance in Series and in Parallel
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Series Resistance: e.g. arrangement of blood vessels within an organ
–As blood flows through the series – pressure decreases
–Where is the resistance the highest?
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Laminar Flow
• Ideally, blood flow in the cardiovascular system is laminar
• Laminar flow implies a parabolic profile of velocity
• Irregularities in the vessel cause turbulent flow
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Turbulent flow
• In turbulent flow, streams are propelled radially and axially
• More energy is required • Turbulent flow in the heart can cause
a “murmur” • Turbulent flow in blood vessels can
cause a “bruit”
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Reynold’s number
• Reynold’s number is a dimensionless number used to predict whether blood flow is laminar or turbulent
NR < 2000
Predicts laminar flow
NR > 2000
Predicts turbulent flow
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Reynold’s number
• Major influences on Reynold’s number:
• Blood viscosity (decreased viscosity increases turbulence – e.g. anemia)
• Velocity of flow (increased velocity increases turbulence)
• How does blood vessel narrowing affect turbulence? – Decreased radius occurs, but
velocity increases by square of radius
– Therefore, narrower vessels (or stenotic vessels) have higher turbulence
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Compliance of Blood vessels • Compliance in a blood vessel is similar to
compliance of the heart:
– Compliance is proportional to ΔV / ΔP
– Compliance of veins is high – (veins can hold large volume of blood at low pressure)
– Compliance of arteries is lower – (they hold a lower volume at a higher pressure)
– Older arteries are least compliant
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Compliance of blood vessels
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Pressures in the Cardiovascular System
• There is a progressive drop in mean pressure as blood flows from: – The aorta: to the arteries, to
the arterioles, to the capillaries, to the venules and to the great veins
– The largest pressure drop occurs at the arteriolar level
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Pressures in the Cardiovascular System
• Arterial pressure is pulsatile, due to the cardiac cycle – Systolic pressure represents the highest pressure
in the pressure tracing
– Diastolic pressure represents the lowest pressure in the pressure tracing
– Mean pressure is the driving pressure, and is calulcated as: Diastolic pressure + 1/3 of the pulse pressure
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Pressures in the Cardiovascular System
–Mean pressures: – Aorta: 90 - 100 mm Hg
– Arterioles: 50 mm Hg
– Capillaries: 20 mm Hg
– Vena Cava: 4 mm Hg
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Pressures in the Cardiovascular System Examples of “Learning Objectives”
• What happens to blood pressure with:
• The aging process?
• Stenosis of the subclavian artery?
• Aortic stenosis?
• Aortic insufficiency (regurgitation)?