BLOOD CIRCULATIONBLOOD CIRCULATION

By

A.Arputha Selvaraj

APMP – IIM CALCUTTA

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Peripheral Circulation and Regulation

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Peripheral Circulatory System

• Systemic vessels– Transport blood through most all body parts

from left ventricle and back to right atrium

• Pulmonary vessels– Transport blood from right ventricle through

lungs and back to left atrium

• Blood vessels and heart regulated to ensure blood pressure is high enough for blood flow to meet metabolic needs of tissues

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Blood Vessel Structure

• Arteries– Elastic, muscular, arterioles

• Capillaries– Blood flows from arterioles to capillaries– Most of exchange between blood and

interstitial spaces occurs across the walls– Blood flows from capillaries to venous system

• Veins– Venules, small veins, medium or large veins

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Capillaries• Capillary wall consists

mostly of endothelial cells

• Types classified by diameter/permeability – Continuous

• Do not have fenestrae

– Fenestrated• Have pores

– Sinusoidal• Large diameter with

large fenestrae

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Capillary Network

• Blood flows from arterioles through metarterioles, then through capillary network

• Venules drain network• Smooth muscle in

arterioles, metarterioles, precapillary sphincters regulates blood flow

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Structure of Arteries and Veins

• Three layers except for capillaries and venules

• Tunica intima– Endothelium

• Tunica media– Vasoconstriction

– Vasodilation

• Tunica adventitia– Merges with connective

tissue surrounding blood vessels

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Structure of Arteries

• Elastic or conducting arteries– Largest diameters, pressure high and fluctuates

• Muscular or medium arteries– Smooth muscle allows vessels to regulate blood

supply by constricting or dilating

• Arterioles– Transport blood from small arteries to capillaries

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Structure of Veins

• Venules and small veins– Tubes of endothelium on delicate basement

membrane

• Medium and large veins

• Valves– Allow blood to flow toward heart but not in

opposite direction

• Atriovenous anastomoses– Allow blood to flow from arterioles to small

veins without passing through capillaries

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Blood Vessel Comparison

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Aging of the Arteries

• Arteriosclerosis– General term for

degeneration changes in arteries making them less elastic

• Atherosclerosis– Deposition of plaque

on walls

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Pulmonary Circulation

• Moves blood to and from the lungs

• Pulmonary trunk– Arises from right ventricle

• Pulmonary arteries– Branches of pulmonary trunk which project to

lungs

• Pulmonary veins– Exit each lung and enter left atrium

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Systemic Circulation: Arteries

• Aorta– From which all arteries are derived either

directly or indirectly– Parts

• Ascending, descending, thoracic, abdominal

• Coronary arteries– Supply the heart

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Branches of the Aorta

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Systemic Circulation: Veins

• Return blood from body to right atrium

• Major veins– Coronary sinus (heart)– Superior vena cava (head, neck, thorax, upper

limbs)– Inferior vena cava (abdomen, pelvis, lower

limbs)

• Types of veins– Superficial, deep, sinuses

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Major Veins

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Veins of Thorax

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Hepatic Portal System

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Dynamics of Blood Circulation

• Interrelationships between– Pressure– Flow– Resistance– Control mechanisms that regulate blood

pressure– Blood flow through vessels

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Laminar and Turbulent Flow• Laminar flow

– Streamlined

– Outermost layer moving slowest and center moving fastest

• Turbulent flow– Interrupted

– Rate of flow exceeds critical velocity

– Fluid passes a constriction, sharp turn, rough surface

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Blood Pressure

• Measure of force exerted by blood against the wall

• Blood moves through vessels because of blood pressure

• Measured by listening for Korotkoff sounds produced by turbulent flow in arteries as pressure released from blood pressure cuff

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Blood Pressure Measurement

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Blood Flow, Poiseuille’s Lawand Viscosity

• Blood flow– Amount of blood

moving through a vessel in a given time period

– Directly proportional to pressure differences, inversely proportional to resistance

• Poiseuille’s Law– Flow decreases when

resistance increases– Flow resistance

decreases when vessel diameter increases

• Viscosity– Measure of resistance

of liquid to flow– As viscosity increases,

pressure required to flow increases

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Critical Closing Pressure, Laplace’s Law and Compliance

Critical closing pressure– Pressure at which a blood

vessel collapses and blood flow stops

Laplace’s Law– Force acting on blood

vessel wall is proportional to diameter of the vessel times blood pressure

Vascular compliance– Tendency for blood

vessel volume to increase as blood pressure increases

– More easily the vessel wall stretches, the greater its compliance

– Venous system has a large compliance and acts as a blood reservoir

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Physiology of Systemic Circulation

• Determined by– Anatomy of circulatory system– Dynamics of blood flow– Regulatory mechanisms that control heart and

blood vessels

• Blood volume– Most in the veins– Smaller volumes in arteries and capillaries

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Cross-Sectional Area

• As diameter of vessels decreases, the total cross-sectional area increases and velocity of blood flow decreases

• Much like a stream that flows rapidly through a narrow gorge but flows slowly through a broad plane

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Pressure and Resistance

• Blood pressure averages 100 mm Hg in aorta and drops to 0 mm Hg in the right atrium

• Greatest drop in pressure occurs in arterioles which regulate blood flow through tissues

• No large fluctuations in capillaries and veins

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Pulse Pressure• Difference between

systolic and diastolic pressures

• Increases when stroke volume increases or vascular compliance decreases

• Pulse pressure can be used to take a pulse to determine heart rate and rhythmicity

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Capillary Exchange andInterstitial Fluid Volume Regulation

• Blood pressure, capillary permeability, and osmosis affect movement of fluid from capillaries

• A net movement of fluid occurs from blood into tissues. Fluid gained by tissues is removed by lymphatic system.

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Fluid Exchange Across Capillary Walls

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Vein Characteristics andEffect of Gravity on Blood

PressureVein Characteristics• Venous return to heart

increases due to increase in blood volume, venous tone, and arteriole dilation

Effect of Gravity• In a standing position,

hydrostatic pressure caused by gravity increases blood pressure below the heart and decreases pressure above the heart

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Control of Blood Flow by Tissues

• Local control– In most tissues, blood flow is proportional to

metabolic needs of tissues

• Nervous System– Responsible for routing blood flow and

maintaining blood pressure

• Hormonal Control– Sympathetic action potentials stimulate

epinephrine and norepinephrine

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Local Control of Blood Flow by Tissues

• Blood flow can increase 7-8 times as a result of vasodilation of metarterioles and precapillary sphincters in response to increased rate of metabolism– Vasodilator substances produced as metabolism increases– Vasomotion is periodic contraction and relaxation of precapillary

sphincters

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Nervous Regulation of Blood Vessels

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Short-Term Regulation ofBlood Pressure

• Baroreceptor reflexes– Change peripheral resistance, heart rate, and stroke

volume in response to changes in blood pressure

• Chemoreceptor reflexes– Sensory receptors sensitive to oxygen, carbon dioxide,

and pH levels of blood

• Central nervous system ischemic response– Results from high carbon dioxide or low pH levels in

medulla and increases peripheral resistance

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Baroreceptor Reflex Control

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Baroreceptor Effects

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Chemoreceptor Reflex Control

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Effects of pH and Gases

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Long-Term Regulation of Blood Pressure

• Renin-angiotensin-aldosterone mechanism

• Vasopressin (ADH) mechanism

• Atrial natriuretic mechanism

• Fluid shift mechanism

• Stress-relaxation response

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Renin-Angiotensin-AldosteroneMechanism

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Vasopressin (ADH) Mechanism

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Long Term Mechanisms

• Atrial natriuretic– Hormone released

from cardiac muscle cells when atrial blood pressure increases, simulating an increase in urinary production, causing a decrease in blood volume and blood pressure

• Fluid shift– Movement of fluid

from interstitial spaces into capillaries in response to decrease in blood pressure to maintain blood volume

• Stress-relaxation– Adjustment of blood

vessel smooth muscle to respond to change in blood volume

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Shock

• Inadequate blood flow throughout body

• Three stages– Compensated: Blood pressure decreases only a moderate

amount and mechanisms able to reestablish normal blood pressure and flow

– Progressive: Compensatory mechanisms inadequate and positive feedback cycle develops; cycle proceeds to next stage or medical treatment reestablishes adequate blood flow to tissues

– Irreversible: Leads to death, regardless of medical treatment