Copyright © 2010 Pearson Education, Inc. Marieb Chapter 19 Part A: Blood Vessels.

Copyright © 2010 Pearson Education, Inc.

Marieb Chapter 19 Part A: Blood Vessels

Copyright © 2010 Pearson Education, Inc. Figure 19.2

Large veins(capacitancevessels)

Largelymphaticvessels

Arteriovenousanastomosis

Lymphaticcapillary

Postcapillaryvenule

Sinusoid

MetarterioleTerminal arteriole

Arterioles(resistance vessels)

Muscular arteries(distributingvessels)

Elastic arteries(conductingvessels)

Small veins(capacitancevessels)

Lymphnode

Capillaries(exchange vessels)

Precapillary sphincterThoroughfarechannel

Lymphaticsystem

Venous system Arterial systemHeart

Copyright © 2010 Pearson Education, Inc. Figure 19.1b

Tunica media(smooth muscle andelastic fibers)

Tunica externa(collagen fibers)

LumenArtery

LumenVein

Internal elastic lamina

External elastic lamina

Valve

(b)

Endothelial cellsBasement membrane

Capillarynetwork

Capillary

Tunica intima• Endothelium• Subendothelial layer


Elastic (Conducting) Arteries

• Large thick-walled arteries with elastin in all three tunics

• Aorta and its major branches

• Large lumen offers low-resistance

• Act as pressure reservoirs—expand and recoil as blood is ejected from the heart

• The recoil keeps blood flowing when the entire heart is in diastole.

• What would happen if recoil couldn’t occur?


Muscular (Distributing) Arteries and Arterioles

• Distal to elastic arteries; deliver blood to body organs

• Have thick tunica media with more smooth muscle

• Arterioles control flow into capillary beds via vasodilation and vasoconstriction


Capillaries

• Microscopic blood vessels

•Walls of thin tunica intima, one cell thick

• Size allows only a single RBC to pass at a time (blood flow slows and RBCs need to flex to get through)

• Diffusion of nutrients and wastes


Capillary Beds• Networks of microscopic capillaries link arterioles

and venules

• Two types of vessels

1. Vascular shunt (metarteriole — thoroughfare channel):

• Directly connects a terminal arteriole and a postcapillary venule

• Always remain open so tissue receives oxygen and nutrients

2. True capillaries

• 10 to 100 vessels per bed

• Branch off the metarteriole or terminal arteriole

• Precapillary sphincters can shut off flow


Large veins(capacitancevessels)

Largelymphaticvessels

Arteriovenousanastomosis

Lymphaticcapillary

Postcapillaryvenule

Sinusoid

MetarterioleTerminal arteriole

Arterioles(resistance vessels)

Muscular arteries(distributingvessels)

Elastic arteries(conductingvessels)

Small veins(capacitancevessels)

Lymphnode

Capillaries(exchange vessels)

Precapillary sphincterThoroughfarechannel

Lymphaticsystem

Venous system Arterial systemHeart


Blood Flow Through Capillary Beds

• Precapillary sphincters regulate blood flow into true capillaries

• Primarily regulated by local chemical conditions

( , and )


(a) Sphincters open—blood flows through true capillaries.

(b) Sphincters closed—blood flows through metarteriole thoroughfare channel and bypasses true capillaries.

Precapillarysphincters Metarteriole

Vascular shunt

Terminal arteriole Postcapillary venule

Terminal arteriole Postcapillary venule

Thoroughfare channel

True capillaries


Veins• Formed when venules converge

• Have thinner walls, larger lumens than arteries

• Blood pressure is much lower than in arteries

• Called capacitance vessels (blood reservoirs); contain up to ______ of the blood supply

• Veins stretch (like )

• In what body regions do they stretch and why?

Copyright © 2010 Pearson Education, Inc. Figure 19.1a

Artery

Vein

(a)


Heart 8%

Capillaries 5%

Systemic arteriesand arterioles 15%

Pulmonary bloodvessels 12%

Systemic veinsand venules 60%

Where is the blood, at rest?


Physiology of Circulation: Definition of Terms

• Blood flow

• Volume of blood flowing through a vessel, an organ, or the entire circulation in a given period

• Measured as ml/min or L/min

• Equivalent to cardiac output (CO) for entire vascular system

• Relatively constant when at rest

• Varies widely through individual organs, based on needs



• Blood pressure (BP)

• Force per unit area exerted on the wall of a blood vessel by the blood

• Expressed in mm Hg (systolic/diastolic)

• Can calculate an average value (MAP)

• Measured as systemic arterial BP in large arteries near the heart

• A pressure gradient keeps blood moving from higher to lower pressure areas



• Resistance (peripheral resistance)

• Opposition to flow

• Measure of the amount of friction the blood encounters

• Mostly in the peripheral systemic circulation

• Three important sources of resistance:

• Blood viscosity

• Total blood vessel length

• Blood vessel diameter


Resistance• Factors that remain relatively constant:

• Blood viscosity

• The “stickiness” of the blood due to formed elements and plasma proteins

remember the karo syrup?

• Blood vessel length

• The longer the vessel, the greater the resistance encountered

• Need more pressure to send water through a longer hose!


Resistance

• Changing vessel diameter significantly alters peripheral resistance

• Varies inversely with the fourth power of vessel radius

• E.g., if the radius is doubled, the resistance is 1/16 as much (not 1/2 as you would expect)


Resistance

• Small-diameter arterioles are the site of change in peripheral resistance

• Abrupt changes in diameter or fatty plaques from atherosclerosis dramatically increase resistance

• Disrupt laminar flow and cause turbulence


Relationship Between Blood Flow and Resistance

• Blood flow is inversely proportional to peripheral resistance (R)

• If R increases, blood flow decreases

• R is more important in influencing local blood flow because it is easily changed by altering blood vessel diameter


Systemic Blood Pressure

• The pumping action of the heart generates blood flow

• Pressure results when the flow is opposed by resistance

• Systemic pressure is highest in the aorta and then declines throughout the pathway

• The steepest drop occurs in arterioles


Systolic pressure

Mean pressure

Diastolic pressure


Arterial Blood Pressure

• Systolic pressure: pressure exerted during ventricular contraction

• Diastolic pressure: lowest arterial pressure when the heart is completely at rest


Arterial Blood Pressure

• Mean arterial pressure (MAP): average pressure that propels the blood to the tissues

MAP = diastolic pressure + 1/3 (Systolic - diastolic)

• MAP declines with increasing distance from the heart


Let’s Calculate Some MAP Values

• BP is 115/85

• BP is 145/95

• BP is 105/60


Capillary Blood Pressure

• Ranges from only 15 to 35 mm Hg

• Low capillary pressure is desirable

• High BP would rupture fragile, thin-walled capillaries

• Most are very permeable, so low pressure forces filtrate into interstitial spaces


Maintaining Blood Pressure

• Requires the heart, blood vessels, and kidneys to work together

• Supervision by the brain



• The main factors influencing blood pressure:

• Cardiac output (CO)

• Peripheral resistance (TPR)

• Blood volume (BV)



• Blood pressure = CO x PR (and CO depends on blood volume)

• MAP = CO X TPR = (SV X HR) X TPR

• Blood pressure varies directly with CO, TPR, and blood volume

• Changes in one variable are quickly compensated for by changes in the other variables


Cardiac Output (CO)

• Determined by venous return and neural and hormonal controls

• Resting heart rate is maintained by the CIC via the parasympathetic vagus nerves

• Stroke volume is controlled by preload (EDV)


Cardiac Output (CO)

• During stress, the CAC increases heart rate and stroke volume via sympathetic stimulation

• ESV decreases and MAP increases


Venous return

Exercise

Contractility of cardiac muscle

Sympathetic activity Parasympathetic activity

Epinephrine in blood

EDV ESV

Stroke volume (SV) Heart rate (HR)

Cardiac output (CO = SV x HR

Activity of respiratory pump(ventral body cavity pressure)

Activity of muscular pump(skeletal muscles)

Sympathetic venoconstriction

BP activates cardiac centers in medulla

Initial stimulus

Result

Physiological response


Control of Blood Pressure

• Short-term neural and hormonal controls

• Oppose blood pressure changes by altering peripheral resistance (VMC action)

• Also regulate HR and force of contraction

• Long-term renal regulation

• Counteracts fluctuations in blood pressure by altering blood volume (renin-angiotensin)


Short-Term Mechanisms: Neural Controls

• Neural controls operate via reflex arcs that use:

• Baroreceptors

• Vasomotor centers and vasomotor fibers

• Vascular smooth muscle


The Vasomotor Center

• A cluster of sympathetic neurons in the medulla that send a message to change blood vessel diameter

• Part of the cardiovascular center, along with the cardiac centers

• Maintains vasomotor tone (moderate constriction of arterioles by symp NS)

• Receives inputs from baroreceptors, chemoreceptors, and higher brain centers


Short-Term Mechanisms: Baroreceptor-Initiated Reflexes

• Baroreceptors are located in the:

• Carotid sinuses

• Aortic arch

• Walls of large arteries of the neck and thorax



• Increased blood pressure stretches the baroreceptors so they fire faster

• Inhibits the vasomotor center (VMC), causing arteriole dilation and venodilation

• Inhibits the cardioacceleratory center (CAC)

• Stimulates the cardioinhibitory center (CIC)


Baroreceptors in carotid sinusesand aortic archare stimulated.

Baroreceptorsin carotid sinusesand aortic archare inhibited.

Impulses from baroreceptorsstimulate cardioinhibitory center(and inhibit cardioacceleratorycenter) and inhibit vasomotorcenter.

Impulses from baroreceptors stimulatecardioacceleratory center (and inhibit cardioinhibitorycenter) and stimulate vasomotor center.

CO and Rreturn bloodpressure tohomeostatic range.

CO and Rreturn blood pressureto homeostatic range.

Rate ofvasomotor impulsesallows vasodilation,causing R

Vasomotorfibers stimulatevasoconstriction,causing R

Sympatheticimpulses to heartcause HR, contractility, and CO.


Stimulus: Blood pressure(arterial bloodpressure falls belownormal range).

Stimulus: Blood pressure(arterial bloodpressure rises abovenormal range).

3

2

1

5

4a

4b

Homeostasis: Blood pressure in normal range

4b

3

2

1

5

4a

Copyright © 2010 Pearson Education, Inc. Figure 19.9 step 5


CO and R returnblood pressureto homeostaticrange.

Rate ofvasomotor impulsesallows vasodilation,causing R

Baroreceptorsin carotidsinuses and aortic arch are stimulated.

Impulses from baroreceptorsstimulate cardioinhibitory center(and inhibit cardioacceleratory center) and inhibit vasomotor center.

Stimulus: Blood pressure(arterial bloodpressure risesabove normalrange).

2

3

1

4b

4a

5


What happens when the blood pressure rises?

Copyright © 2010 Pearson Education, Inc. Figure 19.9 step 5

Baroreceptorsin carotid sinusesand aortic archare inhibited.

Impulses from baroreceptorsstimulate cardioacceleratory center(and inhibit cardioinhibitory center)and stimulate vasomotor center.

CO and Rreturn bloodpressure tohomeostaticrange.

Vasomotorfibers stimulatevasoconstriction,causing R


Stimulus: Blood pressure(arterial bloodpressure fallsbelow normalrange).

2

3

1

4b

4a

5


What happens when the blood pressure falls?



• Baroreceptors taking part in the carotid sinus reflex protect the blood supply to the brain

• Respond to MAP of 40 - 180 mm Hg

• Baroreceptors taking part in the aortic reflex help maintain adequate blood pressure in the systemic circuit

• Respond to MAP > 100 mm Hg


What Happens to BP When You Get Out Of Bed?


Short-Term Mechanisms: Chemoreceptor-Initiated Reflexes

• Chemoreceptors are located in the

• Carotid sinus

• Aortic arch

• Large arteries of the neck

• Pretty much in the same places as the baroreceptors!


Short-Term Mechanisms: Chemoreceptor-Initiated Reflexes

• Chemoreceptors respond to rise in CO2, drop in pH or O2

• Increase blood pressure via the vasomotor center and the cardioacceleratory center

• Are MUCH more important in the regulation of respiratory rate and depth (Chapter 22)


Short-Term Mechanisms: Hormonal Controls

• Adrenal medulla hormones norepinephrine (NE) and epinephrine cause generalized vasoconstriction and increase cardiac output

• Angiotensin II, generated by kidney release of renin, causes vasoconstriction


Short-Term Mechanisms: Hormonal Controls

• Atrial natriuretic peptide causes blood volume and blood pressure to decline, causes generalized vasodilation

• Antidiuretic hormone (ADH)(vasopressin) causes intense vasoconstriction in cases of extremely low BP


Long-Term Mechanisms: Renal Regulation

• Baroreceptors adapt ( ) to chronic high or low BP

• Since they stop working, what does our body then do?

• Let the kidneys spring into action to regulate arterial blood pressure

1. Direct renal mechanism via blood volume

2. Indirect renal (renin-angiotensin) mechanism


Indirect Mechanism Involves Renin-Angiotensin

• The renin-angiotensin mechanism

• Arterial blood pressure release of renin

• Renin production of angiotensin II

• Angiotensin II is a potent vasoconstrictor

• Angiotensin II aldosterone secretion

• Aldosterone renal reabsorption of Na+ and urine formation

• Angiotensin II stimulates ADH release


Arterial pressure

Baroreceptors

Indirect renalmechanism (hormonal)

Direct renalmechanism

Sympathetic stimulationpromotes renin release

Kidney

Renin release

catalyzes cascade,resulting in formation of

ADH releaseby posterior

pituitary

Aldosteronesecretion by

adrenal cortex

Waterreabsorptionby kidneys

Blood volume

Filtration

Arterial pressure

Angiotensin II

Vasoconstriction( diameter of blood vessels)

Sodiumreabsorptionby kidneys

Initial stimulus


Result


Activity ofmuscularpump andrespiratory

pump

Releaseof ANP

Fluid loss fromhemorrhage,

excessivesweating

Crisis stressors:exercise, trauma,

bodytemperature

Bloodbornechemicals:

epinephrine,NE, ADH,

angiotensin II; ANP release

Body size

Conservationof Na+ and

water by kidney

Blood volumeBlood pressure

Blood pH, O2, CO2

Dehydration,high hematocrit

Bloodvolume

Baroreceptors Chemoreceptors

Venousreturn

Activation of vasomotor and cardiacacceleration centers in brain stem

Heartrate

Strokevolume

Diameter ofblood vessels

Cardiac output

Initial stimulus

Result


Mean systemic arterial blood pressure

Bloodviscosity

Peripheral resistance

Blood vessellength


Control of Hypertension

Copyright © 2010 Pearson Education, Inc. Marieb Chapter 19 Part A: Blood Vessels.

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Transcript of Copyright © 2010 Pearson Education, Inc. Marieb Chapter 19 Part A: Blood Vessels.