Chapter 10Chapter 10

Blood Vessels and Blood PressureBlood Vessels and Blood Pressure

V edit. Pag 343-385

VI edit Pag 337-383

Function of the Cardiovascular System: Transport of O2, nutrients and waste

Cardiac Output at Rest Pulmonary circulation

Parallel arrangement of blood vessels supplying different organs assure fresh blood supply to every organ

Some organs received an excess of blood supply that can be adjusted according to the body’s needs

Systemic circulation

Flow through Blood VesselsFlow Rate= Pressure Gradient / Vessel’s Resistance

F = dP / R

Pressure Gradient (dP)Force that push blood through blood vessels

Resistance (R)Easiness by which blood flow through a blood

vessel

Resistance is determined by:1) Vessels’ length2) Vessel’s diameter (opening)3) Blood’s viscosity

R ~ (viscosity x length) / (diameter)4

Organization of the Vascular System

Vascular System consist of:1) Arteries2) Arterioles3) Capillaries4) Venules5) Veins

Organization of the Vascular System

Vascular System

Arteries Pressure reservoir

Arterioles Resistance Vessels

Capillaries Substance Exchange,Volume reservoir

Venules

Veins Resistance Vessels

Arteries

Function:PassagewayPressure reservoir

Structure:EndotheliumSmooth MuscleConnective tissue- elastin &

collagen

Arteries as a Pressure Reservoir

Blood Pressure

Blood Pressure Gradient

Mean Arterial Pressure (MAP) is the average pressure driving blood forward into

the tissueMAP = DP + (1/3) Pulse pressure

Pulse Pressure= SP – DP

Systemic ArteriolesFunction:PassagewayResistance control via

regulation of diameter

Structure:EndotheliumSmooth Muscle-

innervated by sympathetic NS

Connective tissue- with little elastin

Control of Blood Flow through Arterioles

Changes in the diameter of the arterioles can be used to regulate:

1) Arterial blood pressure

2) Distribution of blood between different organs (distribution of cardiac output)

Functional Consequences of Blood Flow Control in Arterioles

Factors that Regulate Blood Flow in Arterioles

Vasoconstriction(increased contraction of circular smoothmuscle in the arteriolarwall, which leads toincreased resistanceand decreasedflow through the vessel) Caused by:

Myogenic activityOxygen 2Carbon dioxide •2and other metabolitesEndothelinSympathetic stimulationVasopressin; angiotensin IICold

Intrinsic Factors

Extrinsic Factors

Factors that Regulate Blood Flow in Arterioles

Vasodilation(decreased contractionof circular smoothmuscle in the arteriolarwall, which leads todecreased resistanceand increased flowthrough the vessel)

Caused by:Myogenic activityOxygen (O2)Carbon dioxide (CO2)and other metabolitesNitric oxideSympathetic stimulationHistamine releaseHeat

Regulation of Blood Pressure in Arterioles by Metabolic Activity

Metabolic activity of skeletal muscle cells( oxygen need)

Adenosine

Vasodilation of arterioles

Blood flow to skeletal muscle cells

Oxygen available to meet oxygen need

Sympathetic Regulation of Blood Pressure in Arterioles

Alpha1 receptors: present in arterioles throughout the body (except brain). Mediate vasoconstriction

Beta2 receptors: present in in arterioles of heart and skeletal muscles. Mediate vasodilation

Functional Consequences?

Kidney Regulation of Blood Pressure

Vasopressin-regulate amount of water that is reabsorbed in kidneys

Increase blood volume

Vasoconstriction

Angiotensin II-regulate amount of sodium that is reabsorbed in kidneys

Increase blood volume

Vasoconstriction

Control of arteriolar diameter is important for blood pressure regulation

Total peripheral resistance

Arteriolar diameter

Blood viscosity

Number ofred blood cells

Concentrationof plasma proteins

Local (intrinsic) control

Extrinsic control

Myogenic responses to stretch Vasopressin

Heat, cold application(therapeutic use) Angiotensin II

Histamine release(involved with injuriesand allergic responses)

Epinephrine and norepinephrine

Local metabolic changes in O•2CO2, other metabolites

Sympathetic activity(exerts generalizedvasoconstrictor effect)

Major factors affecting arteriolar radius

Blood Pressure

© Brooks/Cole - Thomson Learning

Blood flows through a series of blood vessels in the systemic circulation due to the development of a pressure gradient. The mean arterial pressure is the average driving force, propelling blood through the vessels of the cardiovascular system.

Influence of Total Peripheral Resistance on Mean Arterial Pressure

MAP = Cardiac Output X Total Peripheral Resistance

Control of arteriolar diameter is important for blood pressure regulation

Total peripheral resistance

Arteriolar diameter

Blood viscosity

Number ofred blood cells

Concentrationof plasma proteins

Local (intrinsic) control

Extrinsic control

Myogenic responses to stretch Vasopressin

Heat, cold application(therapeutic use) Angiotensin II

Histamine release(involved with injuriesand allergic responses)

Epinephrine and norepinephrine

Local metabolic changes in O•2CO2, other metabolites

Sympathetic activity(exerts generalizedvasoconstrictor effect)

Major factors affecting arteriolar radius

© Brooks/Cole - Thomson Learning

Capillary

Function:PassagewaySubstance exchange

Structure:Endothelium

Diffusion in Capillaries Diffusion: movement of a

substance down its concentration gradient. There is no carrier-mediated transport in capillaries.

D ~ concentration/distance

Diffusion is facilitated in capillaries because:

1) Thin walls2) Narrow diameter3) Extensive branching4) Slow flow

The Blood-Brain BarrierConsist of tightly packed endothelial cells that prevent

unregulated flow of substances into the CSF and the brain

Velocity of Blood Flow

Substance exchange is facilitated in capillaries because of the slow velocity of the blood flow

Velocity = Flow rate / Total of flow cross-sectional

area

Notice flow rate = cardiac output= 5 L/min

Large cross-sectional area of the capillary system result in a significant reduction in Flow Velocity to keep up with a constant

Flow Rate

Substance Exchange Across Capillary Wall

Capillary pores allow substance exchange between blood and tissue

In some tissue capillaries don’t have pores (brain), whereas in others the pores are very large (liver)

Histamine can regulate pore size

Capillaries and Metarterioles

Capillary sphincters consist of smooth muscle rings

Intrinsic factors regulate the capillary sphincters, such as:

1) O2

2) CO2

3) Adenosine

Regulation of Capillary Sphincter

Interstitial FluidFluid surrounding every cell

Substance Exchange

Substance exchange takes place between:

1) Blood and interstitial fluid

2) Interstitial fluid and intracellular space

Substance ExchangeMechanism of substance

exchange across the capillary wall:

1) Simple diffusion: allow the passage of single substances (independent of each other)

2) Bulk flow: allow the passage of substances in bulk (filtration)

DiffusionAllow the movement of a single substance down its

concentration gradient

Bulk Flow or FiltrationAllows the movement of several substances in bulk and

requires some external force

In the capillaries, blood pressure acts as the driving force that push bulk substances into the interstitial fluid

Forces Influencing Bulk FlowCapillary blood pressure (PC)-fluid or hydrostatic

pressureCapillary osmotic pressure (=25 mm Hg)

Interstitial fluid hydrostatic pressure (PIF= 1 mm Hg)Interstitial fluid osmotic pressure (=0 mm Hg)

Osmotic PressureTendency of water to move down its concentration

gradient

•= Water molecule•= Solute molecule

Membrane (permeable to H2O but impermeable to solute)

Pure water Lower H•2O concentration,higher solute concentration

H2O moves from side 1 to side 2down its concentration gradient

Solute unable to move from side 2 toside 1 down its concentration gradient

Side 1 Side 2

Side 1 Side 2

Originallevel ofsolutions

H2O

•• Water concentrations not equal•• Solute concentrations not equal•• Tendency for water to diffuse by

•osmosis into side 2 is exactly•balanced by opposing tendency for•hydrostatic pressure difference to•push water into side 1

•• Osmosis ceases•• Opposing pressure necessary to

•completely stop osmosis is equal•to osmotic pressure of solution

Hydrostatic(fluid)pressuredifference

Osmosis

Hydrostatic pressure

Osmotic PressureThe larger the amount of solute,

the less water, the larger the tendency of water to move in,

the greater the osmotic pressure

Side 1 Side 2

Originallevel ofsolutions

Hydrostatic(fluid)pressuredifference

Osmosis

Hydrostatic pressure

From arteriole To venuleBlood capillary

Interstitial Fluid

Net Flowis determined by the interplay of all forces acting

on plasma and interstitial fluid

Net Pressure= (Capillary Blood P + IT osmotic P) –

(IF hydrostatic P + Plasma osmotic P)

Differences in MAP Determines Direction of Bulk Flow (Ultrafiltration and Reabsorption)

11 mm Hg(ultrafiltration)

Interstitial fluid

From arteriole To venule

-9 mm Hg(reabsorption)

Initial lymphaticvessel

Blood capillary

Bulk FlowUltrafiltration occurs at the beginning of the capillaries

Reabsorption occurs at the end of the capillaries

Forces at arteriolar endof capillary

Net outward pressureof 11 mm Hg =Ultrafiltration pressure

• Inward pressure

• Outward pressure

From arteriole

Forces at venular endof capillary

Net inward pressureof 9 mm Hg =Reabsorption pressure

• Inward pressure

• Outward pressure

Net Filtration and Net Absorption along the capillary

Functional role of bulk flow(Regulation of extracellular fluid volume distribution

between interstitial and plasma fluids)

Bulk flow plays an important role in regulating blood volume and blood pressure in cases of blood loss (hemorrhage, increased blood plasma volume)

Total peripheral resistance

Arteriolar diameter

Blood viscosity

Number ofred blood cells

Concentrationof plasma proteins

Local (intrinsic) control

Extrinsic control

Myogenic responses to stretch Vasopressin

Heat, cold application(therapeutic use) Angiotensin II

Histamine release(involved with injuriesand allergic responses)

Epinephrine and norepinephrine

Local metabolic changes in O•2CO2, other metabolites

Sympathetic activity(exerts generalizedvasoconstrictor effect)

Major factors affecting arteriolar radius

Functional role of bulk flow(Regulation of extracellular fluid volume distribution

between interstitial and plasma fluids)

Lymphatic SystemAllow return of fluid from interstitial fluid to the blood

Lymphatic SystemRun in parallel to circulatory system

The lymphatic system transport interstitial fluid as lymph back into the circulatory system

Function of Lymphatic System

1) Return excess filtered fluid (interstitial fluid)

2) Defense against disease3) Transport of absorbed fat4) Return of filtered proteins

Lymphatic Vessels

Movement of Lymph

Movement of lymph in lymphatic vessels is facilitated by:

1) Contraction of smooth muscles (intrinsic myogenic activity)

2) Squeezing effect of skeletal muscles

EdemaDisruption of lymphatic movements resulting in

increased interstitial fluidEdema is caused by:1) Reduced

concentration of plasma proteins

2) Increased protein permeability of capillaries

3) Increased venous pressure

4) Blockade of lymph vessels

Interstitial fluid

Lymph

Net exchange pressure=(Cap. blood Pressure+IF osmotic Pressure) -(IF hydrostatic Pressure+Plasma Osmotic Pressure)

VeinsFunction:PassagewayBlood reservoir

(capacitance vessels)

Structure:EndotheliumThin layer of smooth

muscles with little myogenic activity and elasticity

Veins as Capacitance VesselsVeins are highly stretchable

with no elastic recoil:• Thin walls with less

smooth muscles than arteries (less myogenic activity)

• Little elastin, more collagen

Veins control venous return by slowing down the flow of blood to the heart

Velocity of flow = Flow rate / Total cross-sectional area

Regulation of Venous ReturnVenous return to heart is

facilitated by differential pressure b/ atria and venous system

Extrinsic factors:• Sympathetic activity• Skeletal muscle, skeletal

muscle pump• Venous valves• Respiratory activity,

respiratory pump• Cardiac suction

17 mm Hg 0 mm Hg

Cardiac output

Stroke volume

End-diastolic volume

Venous valves(mechanically preventbackflow of blood)

Venous return

Passive bulk-flow shift of fluid from interstitial fluid into plasma

Salt and waterretention

Blood volume( venous pressure

pressure gradient)

Cardiac suction effect( pressure in heart

pressure gradient)

Respiratory pump( pressure in chest veins

pressure gradient•)

Pressure imparted to bloodby cardiac contraction( venous pressure

pressure gradient)

Sympatheticvasoconstrictor activity( venous pressure

pressure gradient;venous capacity)

Skeletal muscle pump( venous pressure

pressure gradient)

= Short -term control measures = Long -term control measures

Regulation of Venous Return

Sympathetic Regulation of Venous Return

Alpha1 receptors: present in arterioles/veins. Mediate vasoconstriction=increase resistance (in arterioles) and decrease venous capacity (blood volume that veins can accommodate)

Sympathetic stimulation produce venous vasoconstriction, decrease venous capacity

Venous Valves Prevent Backflow of Venous Blood

Malfunction of venous valves generate varicose veins

Skeletal Muscle Contractions Squeeze Venous Blood Toward Heart

Negative Thoracic Pressure (due to Respiration) Increase Flow of Venous

Blood Toward Heart

Venous Pressure and Gravity

Tendency of blood to accumulate in lower veins is counteract by:

1) Activation of sympathetic nervous system (vasoconstriction)

2) Squeezing effect of skeletal muscles

Blood Pooling in Lower Parts of the Body due to Gravity Induces Swelling

of Ankles and Feet

Ventricular contractionAV valves move downward, pressure in atria low

Decreased Atrial Pressure Increases Flow of Venous Blood Toward Heart

Blood Pressure RegulationMAP = CO x TPR

CO = HR x SV, TPF~viscosity/radiusBlood pressure need

to be regulated to maintain proper blood flow to all organs, prevent extra work by the heart

Blood pressure is regulated by controlling cardiac output, total peripheral resistance and blood volume

Control of Cardiac OutputCardiac output

Heart rate Stroke volume

Parasympatheticactivity

Sympatheticactivity (andepinephrine)

End-diastolicvolume

Venous return

Extrinsiccontrol

Intrinsic control

Intrinsic control

Factors that Regulate Total Peripheral Resistance

Total peripheral resistance

Arteriolar diameter

Blood viscosity

Number ofred blood cells

Concentrationof plasma proteins

Local (intrinsic) control

Extrinsic control

Myogenic responses to stretch Vasopressin

Heat, cold application(therapeutic use) Angiotensin II

Histamine release(involved with injuriesand allergic responses)

Epinephrine and norepinephrine

Local metabolic changes in O•2CO2, other metabolites

Sympathetic activity(exerts generalizedvasoconstrictor effect)

Major factors affecting arteriolar radius

Blood Pressure Regulation

Short term regulationBaroreceptor reflex - alter cardiac output and total

peripheral resistance

Long term regulationInvolve control of blood volume by regulating salt

and water balance

MAP = CO x TPRCO = HR x SV

Baroreceptor ReflexInvolve activation of carotid sinus and aortic arch

baroreceptors, transmission of information to cardiovascular center and stimulation of ANS

Changes in MAP Regulate Firing Rate of Baroreceptors

Figure 10.38 Page 378

Parasympatheticstimulation Heart Heart

rateCardiacoutput

Bloodpressure

Sympatheticstimulation Heart

Heartrate

Contractilestrengthof heart

Strokevolume

Cardiacoutput

Cardiacoutput

Bloodpressure

Bloodpressure

Bloodpressure

Total peripheralresistance

Strokevolume

VenousreturnVeins Vasoconstriction

VasoconstrictionArterioles

Baroreceptor Reflex

Homeostatic Regulation of Blood Pressure by Baroreceptor Reflex

When blood pressure becomeselevated above normal

Carotid sinusand aortic archreceptor potential

Rate of firingin afferent nerves

Cardiovascularcenter

Sympathetic cardiac nerve activity

sympathetic vasoconstrictor nerve activity

parasympathetic nerve activity

and

and

Heart rate

stroke volume

arteriolar andvenous vasodilation

Cardiac output

total peripheralresistance

Bloodpressuredecreasedtowardnormal

and

andand

Homeostatic Regulation of Blood Pressure by Baroreceptor Reflex

•Slide 52

When blood falls bellow normal

Carotid sinusand aortic archreceptor potential

Rate of firingin afferent nerves

Cardiovascularcenter

Sympathetic cardiac nerve activity

sympathetic vasoconstrictor nerve activity

parasympathetic nerve activity

and

and

Heart rate

stroke volume

arteriolar andvenous vasodilation

Cardiac output

total peripheralresistance

Bloodpressureincreasetowardnormal

and

andand

•Fig. 10.39 (2)•Page 379

Other Factors that Modulate BP1) Osmoreceptors in

hypothalamus

2) Chemoreceptors in aortic and carotid bodies

3) Increased load as a result of exercise

4) Hypothalamic control of cutaneous blood vessels

5) Emotions

Changes in Blood-Pressure Control Mechanisms

1) Hypotension (Blood pressure less than 100/60)

2) Hypertension (Blood pressure more than 140/90)

Disruption of Blood Pressure Control Causes Hypertension

1) Primary hypertensionCaused by a variety of factors like defect in salt regulation, drugs, local vasoactive substances

2) Secondary hypertensionCaused by renal dysfunctions, atherosclerotic plaques and lack of elasticity in arteries, abnormal release of adrenaline