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

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: Right vs. Left 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


• 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: 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


• 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”


– 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)


• Where is velocity of blood flow highest?

• A: Where area is lowest…………..


• 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)


• 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)


• 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


• Question 2: What pressure drop would one measure to calculate the systemic vascular resistance (SVR)?

• Answer:

mean arterial pressure

minus

mean right atrial pressure


• 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


• 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


–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)?

14 08 12.Intro to the Circulation Slides

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