14 08 12.Intro to the Circulation Slides
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Transcript of 14 08 12.Intro to the Circulation Slides
Introduction to the Circulation
Bio-Med 3662
August 2019
Douglas Burtt, MD, FACC
Functions of the Cardiovascular System
• Deliver oxygen-carrying blood to the tissues
• Provide nutrients to the cells
• Remove waste products from cells
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
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
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
The “Circuitry” of the Cardiovascular System
The “Circuitry” of the Cardiovascular System
• Path of blood flow through Heart and Lungs
Cardiac Blood Flow
Cardiac Blood Flow
Cardiac Blood Flow
Cardiac Blood Flow
Cardiac Blood Flow
Cardiac Blood Flow
Cardiac Blood Flow
Cardiac Blood Flow: Right vs. Left Heart
Cardiac Blood Flow: Right vs. Left Heart
Cardiac Blood Flow: Right vs. Left Heart
Cardiac Blood Flow: Right vs. Left Heart
Cardiac Blood Flow: Entire Heart
Cardiac Blood Flow: Entire Heart
Circulation through the body: “Circuitry”
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
Circulation through the body: Arteries and Veins
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
Circulation through the body: Arteries and Veins
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
The “microcirculation”: arterioles through venules
Circulation through the body: Capillaries
Microcirculation: Capillaries
–Intestines
–Glomeruli
–Most common capillary
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”
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)
Area and Volume of blood vessels
The physics of blood flow
Sir Isaac Newton:
Formulator of law of gravitation
Inventor of Calculus
Established study of optics
“Father” of physics
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)
Velocity of Blood Flow
Velocity of Blood Flow
• Where is velocity of blood flow highest?
• A: Where area is lowest…………..
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
Area and Volume of the
Cardiovascular System
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)
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)
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)
Relationship of Pressure, Flow and Resistance
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
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
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
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
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
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
Resistance in Series and in Parallel
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?
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
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”
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
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
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
Compliance of blood vessels
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
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
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
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)?