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PowerPoint® Lecture Slides

prepared by

Karen Dunbar Kareiva

Ivy Tech Community College© Annie Leibovitz/Contact Press Images

Chapter 18 Part A

The

Cardiovascular

System:

Blood Vessels

© 2017 Pearson Education, Inc.

Why This Matters

• Understanding how the body controls blood

pressure helps to measure a patient’s blood

pressure accurately


Video: Why This Matters


Part 1 Blood Vessel Structure and Function

• Blood vessels: delivery system of dynamic

structures that begins and ends at heart

– Work with lymphatic system to circulate fluids

• Arteries: carry blood away from heart;

oxygenated except for pulmonary circulation

and umbilical vessels of fetus

• Capillaries: direct contact with tissue cells;

directly serve cellular needs

• Veins: carry blood toward heart; deoxygenated

except for pulmonary circulation and umbilical

vessels of fetus © 2017 Pearson Education, Inc.

Figure 18.1 The relationship of blood vessels to each other and to lymphatic vessels.


Venous system

Large veins(capacitancevessels)

Largelymphaticvessels

Arterial system

Arteriovenousanastomosis

Lymphaticsystem

Lymphaticcapillaries

Postcapillaryvenule

Sinusoid

Metarteriole

Terminalarteriole

Arterioles(resistancevessels)

Musculararteries(distributingarteries)

Elasticarteries(conductingarteries)

Heart

Small veins(capacitancevessels)

Lymphnode

Capillaries(exchangevessels)

Precapillarysphincter

Thoroughfarechannel

18.1 Structure of Blood Vessel Wall

• All vessels consist of a lumen, central blood-

containing space, surrounded by a wall

• Walls of all vessels, except capillaries, have

three layers, or tunics:

1. Tunica intima

2. Tunica media

3. Tunica externa

• Capillaries

– Endothelium with sparse basal lamina



1. Tunica intima

• Innermost layer that is in “intimate” contact with blood

• Endothelium: simple squamous epithelium that lines

lumen of all vessels

– Continuous with endocardium

– Slick surface reduces friction

• Subendothelial layer: connective tissue basement

membrane

– Found only in vessels larger than 1 mm



2. Tunica media

• Middle layer composed mostly of smooth muscle and

sheets of elastin

• Sympathetic vasomotor nerve fibers innervate this

layer, controlling:

– Vasoconstriction: decreased lumen diameter

– Vasodilation: increased lumen diameter

• Bulkiest layer responsible for maintaining blood flow

and blood pressure



3. Tunica externa

• Outermost layer of wall

• Also called tunica adventitia

• Composed mostly of loose collagen fibers that protect

and reinforce wall and anchor it to surrounding

structures

• Infiltrated with nerve fibers, lymphatic vessels

– Large veins also contain elastic fibers in this layer

• Vasa vasorum: system of tiny blood vessels found in

larger vessels

– Function to nourish outermost external layer


Figure 18.2b Generalized structure of arteries, veins, and capillaries.


Tunica media

(smooth muscle and elastic fibers)

• External elastic membrane

Tunica externa

(collagen fibers)

• Vasa vasorum

Artery Vein

LumenLumen

Valve

Endothelial cells

Basement membrane

Capillarynetwork

Capillary

Tunica intima

• Endothelium

• Internal elastic membrane

• Subendothelial layer

Table 18.1-1 Summary of Blood Vessel Anatomy


18.2 Arteries

• Arteries divided into three groups, based on size

and function

– Elastic arteries

– Muscular arteries

– Arterioles


Elastic Arteries

• Elastic arteries: thick-walled with large,

low-resistance lumen

– Aorta and its major branches: also called

conducting arteries because they conduct blood

from heart to medium sized vessels

• Elastin found in all three tunics, mostly tunica

media

• Contain substantial smooth muscle, but inactive

in vasoconstriction


Elastic Arteries (cont.)

• Act as pressure reservoirs that expand and

recoil as blood is ejected from heart

– Allows for continuous blood flow downstream

even between heartbeats


Muscular Arteries

• Elastic arteries give rise to muscular arteries

• Also called distributing arteries because they deliver blood to body organs

– Diameters range from pinky-finger size to

pencil-lead size

• Account for most of named arteries

• Have thickest tunica media with more smooth muscle, but less elastic tissue

– Tunica media sandwiched between elastic

membranes

• Active in vasoconstriction© 2017 Pearson Education, Inc.

Arterioles

• Arterioles: smallest of all arteries

– Larger arterioles contain all three tunics

– Smaller arterioles are mostly single layer of

smooth muscle surrounding endothelial cells

• Control flow into capillary beds via vasodilation

and vasoconstriction of smooth muscle

• Also called resistance arteries because

changing diameters change resistance to blood

flow

• Lead to capillary beds


18.3 Capillaries

• Microscopic vessels; diameters so small only

single RBC can pass through at a time

• Walls just thin tunica intima; in smallest vessels,

one cell forms entire circumference

• Pericytes: spider-shaped stem cells help

stabilize capillary walls, control permeability,

and play a role in vessel repair

• Supply almost every cell, except for cartilage,

epithelia, cornea, and lens of eye


18.3 Capillaries

• Functions: exchange of gases, nutrients,

wastes, hormones, etc., between blood and

interstitial fluid


Types of Capillaries

• All capillary endothelial cells are joined by tight

junctions with gaps called intercellular clefts

– Allow passage of fluids and small solutes

• Three types of capillaries

1. Continuous capillaries

• Abundant in skin, muscles, lungs, and CNS

– Continuous capillaries of brain are unique

» Form blood brain barrier, totally enclosed with tight

junctions and no intercellular clefts


Figure 18.3a Capillary structure.


Red bloodcell in lumen

Intercellularcleft

Endothelialcell

Endothelial nucleus

Tight junction Pinocytoticvesicles

Pericyte

Basementmembrane

Continuous capillaries are the least permeable and most common.

• Abundant in skin, muscles, lungs, and CNS.

• Often have associated pericytes.

• Pinocytotic vesicles ferry fluid across the endothelial cell.

• Brain capillary endothelial cells lack intercellular clefts and have

tight junctions around their entire perimeter. (This is the

structural basis of the blood brain barrier described in

Chapter 12.)

Continuous capillary

Types of Capillaries (cont.)

2. Fenestrated capillary

• Found in areas involved in active filtration (kidneys),

absorption (intestines), or endocrine hormone

secretion

• Endothelial cells contain Swiss cheese–like pores

called fenestrations

– Allow for increased permeability

– Fenestrations usually covered with thin glycoprotein

diaphragm


Figure 18.3b Capillary structure.



Intercellularcleft

Fenestrations(pores)

Endothelialcell

Endothelialnucleus

Basement membrane Tight junction

Pinocytoticvesicles

Fenestrated capillary

Fenestrated capillaries have large fenestrations (pores)

that increase permeability.

• Occur in areas of active filtration (e.g., kidney) or absorption

(e.g., small intestine), and areas of endocrine hormone secretion.

• Fenestrations are Swiss cheese–like holes that tunnel through

endothelial cells.

• Fenestrations are usually covered by a very thin diaphragm made

of extracellular glycoproteins. This diaphragm has little effect on

solute and fluid movement.

• In some digestive tract organs, the number of fenestrations in

capillaries increases during active absorption of nutrients.

Types of Capillaries (cont.)

3. Sinusoidal capillaries

• Fewer tight junctions; usually fenestrated with larger

intercellular clefts; incomplete basement membranes

– Usually have larger lumens

• Found only in the liver, bone marrow, spleen, and

adrenal medulla

• Blood flow is sluggish—allows time for modification of

large molecules and blood cells that pass between

blood and tissue

• Contain macrophages in lining to capture and destroy

foreign invaders


Figure 18.3c Capillary structure.


Nucleus ofendothelialcell


Endothelialcell

Tight junction

Largeintercellularcleft

Incompletebasement membrane

Sinusoid capillary

Sinusoid capillaries are the most permeable and

occur in limited locations.

• Occur in liver, bone marrow, spleen, and adrenal medulla.

• Have large intercellular clefts as well as fenestrations;

few tight junctions.

• Have incomplete basement membranes.

• Are irregularly shaped and have larger lumens than other capillaries.

• Allow large molecules and even cells to pass across their walls.

• Blood flows slowly through their tortuous channels.

• Macrophages may extend processes through the clefts to catch

“prey” or, in liver, form part of the sinusoid wall.

Capillary Beds

• Capillary bed: interwoven network of capillaries

between arterioles and venules

• Microcirculation: flow of blood through bed

• Capillary beds consist of two types of vessels

1. Vascular shunt: channel that connects

arteriole directly with venule (metarteriole–

thoroughfare channel)

2. True capillaries: actual vessels involved in

exchange


Capillary Beds (cont.)

1. Vascular shunt: metarteriole–thoroughfare

channel starts with:

• Terminal arteriole that feeds into

• Metarteriole (intermediate between arteriole and

capillary) that is continuous with

• Thoroughfare channel (intermediate between

capillary and venule) that feeds into

• Postcapillary venule that drains bed


Figure 18.4a Anatomy of a capillary bed.


Sphincters open—blood flows through true capillaries.

Precapillary

sphincters Metarteriole

Vascular shunt

Terminal arteriole Postcapillary venule

Thoroughfare

channel

True

capillaries

Capillary Beds (cont.)

2. True capillaries: 10 to 100 exchange vessels

per capillary bed

• Branch off metarteriole or terminal arteriole

• True capillaries normally branch from metarteriole and

return to thoroughfare channel

• Precapillary sphincters regulate blood flow into true

capillaries

– Blood may go into true capillaries or to shunt

• Regulated by local chemical conditions and

vasomotor nerves


Figure 18.4 Anatomy of a capillary bed.


Sphincters open—blood flows through true capillaries.

Precapillary

sphincters Metarteriole

Vascular shunt



Thoroughfare

channel

True

capillaries

Sphincters closed—blood flows through metarteriole –

thoroughfare channel and bypasses true capillaries.

18.4 Veins

• Veins: carry blood toward the heart

• Formation begins when capillary beds unite in

postcapillary venules and merge into larger and

larger veins


Venules

• Capillaries unite to form postcapillary venules

– Consist of endothelium and a few pericytes

– Very porous; allow fluids and WBCs into tissues

• Larger venules have one or two layers of smooth

muscle cells

Veins

• Formed when venules converge

• Have all tunics, but thinner walls with large

lumens compared with corresponding arteries

• Tunica media is thin, but tunica externa is thick

– Contain collagen fibers and elastic networks

• Large lumen and thin walls make veins good

storage vessels

– Called capacitance vessels (blood reservoirs)

because they contain up to 65% of blood supply


Figure 18.2a Generalized structure of arteries, veins, and capillaries.


Artery Vein

Figure 18.5 Relative proportion of blood volume throughout the cardiovascular system.


Pulmonary bloodvessels 12%

Heart 8%

Capillaries 5%

Systemic arteriesand arterioles 15%

Systemic veinsand venules 60%

Veins (cont.)

• Blood pressure lower than in arteries, so adaptations ensure return of blood to heart

– Large-diameter lumens offer little resistance

• Other adaptations

– Venous valves

• Prevent backflow of blood

• Most abundant in veins of limbs

– Venous sinuses

• Flattened veins with extremely thin walls

• Composed only of endothelium

• Examples: coronary sinus of the heart and dural

sinuses of the brain© 2017 Pearson Education, Inc.

Figure 18.2b Generalized structure of arteries, veins, and capillaries.


Tunica media

(smooth muscle and elastic fibers)

• External elastic membrane

Tunica externa

(collagen fibers)

• Vasa vasorum

Artery Vein

LumenLumen

Valve

Endothelial cells

Basement membrane

Capillarynetwork

Capillary

Tunica intima

• Endothelium

• Internal elastic membrane

• Subendothelial layer

Clinical – Homeostatic Imbalance 18.1

• Varicose veins: dilated and painful veins due to

incompetent (leaky) valves

• Factors that contribute include heredity and

conditions that hinder venous return

– Example: prolonged standing in one position,

obesity, or pregnancy; blood pools in lower

limbs, weakening valves; affects more than

15% of adults


Clinical – Homeostatic Imbalance 18.1

• Elevated venous pressure can cause varicose

veins

– Example: straining to deliver a baby or have a

bowel movement raises intra-abdominal

pressure, resulting in varicosities in anal veins

called hemorrhoids


18.5 Anastomoses

• Vascular anastomoses: interconnections of

blood vessels

• Arterial anastomoses: provide alternate

pathways (collateral channels) to ensure

continuous flow, even if one artery is blocked

– Common in joints, abdominal organs, brain, and

heart; none in retina, kidneys, spleen


18.5 Anastomoses

• Arteriovenous anastomoses: shunts in

capillaries; example: metarteriole–thoroughfare

channel

• Venous anastomoses: so abundant that

occluded veins rarely block blood flow


Part 2 Physiology of Circulation

Definition of Terms

• Blood flow: volume of blood flowing through

vessel, organ, or entire circulation in given

period

– Measured in ml/min, it is equivalent to cardiac

output (CO) for entire vascular system

– Overall is relatively constant when at rest, but at

any given moment, varies at individual organ

level, based on needs


18.6 Flow, Pressure, and Resistance

• Blood pressure (BP): force per unit area

exerted on wall of blood vessel by blood

– Expressed in mm Hg

– Measured as systemic arterial BP in large

arteries near heart

– Pressure gradient provides driving force that

keeps blood moving from higher- to

lower-pressure areas


Definition of Terms (cont.)


• Resistance (peripheral resistance): opposition

to flow

– Measurement of amount of friction blood

encounters with vessel walls, generally in

peripheral (systemic) circulation

– Three important sources of resistance

• Blood viscosity

• Total blood vessel length

• Blood vessel diameter



– Blood viscosity

• The thickness or “stickiness” of blood due to formed

elements and plasma proteins

– The greater the viscosity, the less easily molecules are

able to slide past each other

• Increased viscosity equals increased resistance

– Total blood vessel length

• The longer the vessel, the greater the resistance

encountered



– Blood vessel diameter

• Has greatest influence on resistance

• Frequent changes alter peripheral resistance

– Viscosity and blood vessel length are relatively

constant

• Fluid close to walls moves more slowly than in middle

of tube (called laminar flow)

• Resistance varies inversely with fourth power of

vessel radius

– If radius increases, resistance decreases, and

vice-versa

– Example: if radius is doubled, resistance drops to 1/16

as much



– Blood vessel diameter (cont.)

• Small-diameter arterioles are major determinants of

peripheral resistance

– Radius changes frequently, in contrast to larger arteries

that do not change often

• Abrupt changes in vessel diameter or obstacles such

as fatty plaques from atherosclerosis dramatically

increase resistance

– Laminar flow is disrupted and becomes turbulent flow,

irregular flow that causes increased resistance


Relationship between Flow, Pressure, and

Resistance

• Blood flow (F) is directly proportional to blood

pressure gradient (P)

– If P increases, blood flow speeds up

• Blood flow is inversely proportional to peripheral

resistance (R)

– If R increases, blood flow decreases, so

F = P/R

• R is more important in influencing local blood

flow because it is easily changed by altering

blood vessel diameter


18.7 Systemic Blood Pressure

• Pumping action of heart generates blood flow

• Pressure results when flow is opposed by

resistance

• Systemic pressure is highest in aorta and

declines throughout pathway

– Steepest drop occurs in arterioles


Figure 18.6 Blood pressure in various blood vessels of the systemic circulation.


Systolic pressure

Mean pressure

Diastolicpressure

0

20

40

60

80

100

120

Blo

od

p

re

ssure (m

m H

g)

Arterial Blood Pressure

• Determined by two factors:

1. Elasticity (compliance or distensibility) of

arteries close to heart

2. Volume of blood forced into them at any time

• Blood pressure near heart is pulsatile

– Rises and falls with each heartbeat


Arterial Blood Pressure (cont.)

• Systolic pressure: pressure exerted in aorta

during ventricular contraction

– Left ventricle pumps blood into aorta, imparting

kinetic energy that stretches aorta

– Averages 120 mm Hg in normal adult

• Diastolic pressure: lowest level of aortic

pressure when heart is at rest

• Pulse pressure: difference between systolic

and diastolic pressure

• Pulse: throbbing of arteries due to difference in

pulse pressures, which can be felt under skin© 2017 Pearson Education, Inc.


• Mean arterial pressure (MAP): pressure that

propels blood to tissues

– Pulse pressure phases out near end of arterial

tree

– Flow is nonpulsatile with a steady MAP pressure

• Heart spends more time in diastole, so not just a

simple average of diastole and systole



• MAP is calculated by adding diastolic pressure +

1/3 pulse pressure

– Example: BP = 120/80; Pulse Pressure =

120 − 80 = 40; so MAP = 80 + (1/3)40 = 80 +

13 = 93 mm Hg

• Pulse pressure and MAP both decline with

increasing distance from heart



• Clinical monitoring of circulatory efficiency

– Vital signs: pulse and blood pressure, along

with respiratory rate and body temperature

– Taking a pulse

• Radial pulse (taken at the wrist): most routinely used,

but there are other clinically important pulse points

• Pressure points: areas where arteries are close to

body surface

– Can be compressed to stop blood flow in event of

hemorrhaging


Figure 18.7 Body sites where the pulse is most easily palpated.


Common carotid artery

Brachial artery

Radial artery

Femoral artery

Popliteal artery

Posterior tibialartery

Superficial temporal artery

Facial artery

Dorsalis pedisartery


– Measuring blood pressure

• Systemic arterial BP is measured indirectly by

auscultatory methods using a sphygmomanometer

1. Wrap cuff around arm superior to elbow

2. Increase pressure in cuff until it exceeds systolic

pressure in brachial artery

3. Pressure is released slowly, and examiner listens for

sounds of Korotkoff with a stethoscope



– Measuring blood pressure (cont.)

• Systolic pressure: normally less than 120 mm Hg

– Pressure when sounds first occur as blood starts to

spurt through artery

• Diastolic pressure: normally less than 80 mm Hg

– Pressure when sounds disappear because artery no

longer constricted; blood flowing freely


Capillary Blood Pressure

• Ranges from 35 mm Hg at beginning of capillary

bed to ∼17 mm Hg at the end of the bed

• Low capillary pressure is desirable because:

1. High BP would rupture fragile, thin-walled

capillaries

2. Most capillaries are very permeable, so low

pressure forces filtrate into interstitial spaces


Venous Blood Pressure

• Changes little during cardiac cycle

• Small pressure gradient, only about 15 mm Hg

– If vein is cut, low pressure of venous system causes

blood to flow out smoothly

– If artery cut, blood spurts out because pressure is

higher

• Low pressure is due to cumulative effects of

peripheral resistance

– Energy of blood pressure is lost as heat during each

circuit

• Low pressure of venous side requires adaptations

to help with venous return© 2017 Pearson Education, Inc.

Venous Blood Pressure (cont.)

• Factors aiding venous return

1. Muscular pump: contraction of skeletal

muscles “milks” blood back toward heart;

valves prevent backflow

2. Respiratory pump: pressure changes during

breathing move blood toward heart by

squeezing abdominal veins as thoracic veins

expand

3. Sympathetic venoconstriction: under

sympathetic control, smooth muscles constrict,

pushing blood back toward heart


Figure 18.8 The muscular pump.


Venous valve(open)

Contractedskeletalmuscle

Vein

Venous valve(closed)

Direction ofblood flow

18.8 Regulation of Blood Pressure

• Maintaining blood pressure requires cooperation

of heart, blood vessels, and kidneys

– All supervised by brain

• Three main factors regulating blood pressure

– Cardiac output (CO)

– Peripheral resistance (PR)

– Blood volume

• Blood pressure varies directly with CO, PR, and

blood volume



• Remember:

F = P/R

and that F = CO, so substituting gives

CO = P/R

and rearranging,

P = CO R

• Shows that blood pressure (MAP) is directly

proportional to CO and PR

– Changes in one variable are quickly

compensated for by changes in other variables



• Recall that CO = SV HR, so if MAP = CO R,

then

MAP = SV HR R

• Anything that increases SV, HR, or R will also

increase MAP

– SV is effected by venous return (EDV)

– HR is maintained by medullary centers

– R is effected mostly by vessel diameter



• Factors can be affected by:

– Short-term regulation: neural controls

– Short-term regulation: hormonal controls

– Long-term regulation: renal controls


Figure 18.9 Major factors determining MAP.


Peripheral resistance

Diameter ofblood vessels

Bloodviscosity

Heartrate

Strokevolume

Bloodvessellength

Cardiac output

Mean arterial pressure (MAP)

Short-Term Regulation: Neural Controls

• Two main neural mechanisms control peripheral

resistance

1. MAP is maintained by altering blood vessel

diameter, which alters resistance

• Example: If blood volume drops, all vessels constrict

(except those to heart and brain)

2. Can alter blood distribution to organs in

response to specific demands



(cont.)

• Neural controls operate via reflex arcs that

involve:

– Cardiovascular center of medulla

– Baroreceptors

– Chemoreceptors

– Higher brain centers



(cont.)

• Role of the cardiovascular center

– Cardiovascular center: composed of clusters of sympathetic neurons in medulla

– Consists of:

• Cardiac centers: cardioinhibitory and

cardioacceleratory centers

• Vasomotor center: sends steady impulses via

sympathetic efferents called vasomotor fibers to

blood vessels

– Cause continuous moderate constriction called

vasomotor tone

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



(cont.)

• Baroreceptor reflexes

– Located in carotid sinuses, aortic arch, and walls

of large arteries of neck and thorax

– If MAP is high:

• Increased blood pressure stimulates baroreceptors to

increase input to vasomotor center

• Inhibits vasomotor and cardioacceleratory centers

• Stimulates cardioinhibitory center

• Results in decreased blood pressure



(cont.)

• Baroreceptor reflexes (cont.)

– Resulting decrease in blood pressure due to two

mechanisms:

1. Vasodilation: decreased output from vasomotor

center causes dilation

– Arteriolar vasodilation: reduces peripheral resistance,

MAP falls

– Venodilation: shifts blood to venous reservoirs,

decreasing venous return and CO



(cont.)


2. Decreased cardiac output: impulses to cardiac

centers inhibit sympathetic activity and stimulate

parasympathetic

– Reduces heart rate and contractility; CO decrease

causes decrease in MAP



(cont.)


– If MAP is low:

• Reflex vasoconstriction is initiated that increases CO

and blood pressure

• Example: when a person stands, BP falls and triggers:

– Carotid sinus reflex: baroreceptors that monitor BP to

ensure enough blood to brain

– Aortic reflex maintains BP in systemic circuit

• Baroreceptors are ineffective if altered blood pressure

is sustained

– Become adapted to hypertension, so not triggered by

elevated BP levels


Figure 18.10 Baroreceptor reflexes that help maintain blood pressure homeostasis.


Stimulus:

Blood pressure(arterial blood

pressure rises

above normalrange). Stimulus:


pressure falls below

normal range).

Homeostasis: Blood pressure in normal range

1

1

Slide 2



Baroreceptors

in carotid sinusesand aortic arch

are stimulated.

Stimulus:


pressure rises




normal range).

Baroreceptors


are inhibited


2

1

1

2

Slide 3



Baroreceptors


are stimulated.

Impulses from baroreceptors

stimulate cardioinhibitory center(and inhibit cardioacceleratory

center) and inhibit vasomotor center.

Stimulus:


pressure rises




normal range).

Baroreceptors


are inhibited


activate cardioacceleratory center(and inhibit cardioinhibitory center)

and stimulate vasomotor center.


2

3

1

1

2

3

Slide 4



Baroreceptors


are stimulated.

Rate of

vasomotor impulsesallows vasodilation,

causing R.

Sympathetic

impulses to heartcause HR,

contractility, and

CO.




Stimulus:


pressure rises

above normalrange).

Vasomotor

fibers stimulatevasoconstriction,

causing R.

Stimulus:



normal range).

Baroreceptors


are inhibitedSympathetic

impulses to heartCause HR,

contractility, and

CO.





2

3

4b

4a

1

4b

1

2

3

4a

Slide 5



Baroreceptors


are stimulated.

Rate of

vasomotor impulsesallows vasodilation,

causing R.CO and R

return bloodpressure to

homeostatic range.

Sympathetic

impulses to heartcause HR,

contractility, and

CO.




Stimulus:


pressure rises

above normalrange).

CO and R

return blood pressure to

homeostatic

range.

Vasomotor

fibers stimulatevasoconstriction,

causing R.

Stimulus:



normal range).

Baroreceptors


are inhibitedSympathetic

impulses to heartCause HR,

contractility, and

CO.





2

3

4b

5

4a

1

54b

1

2

3

4a

Slide 6


(cont.)

• Chemoreceptor reflexes

– Aortic arch and large arteries of neck detect

increase in CO2, or drop in pH or O2

– Cause increased blood pressure by:

• Signaling cardioacceleratory center to increase CO

• Signaling vasomotor center to increase

vasoconstriction



(cont.)

• Influence of higher brain centers

– Reflexes that regulate BP are found in medulla

– Hypothalamus and cerebral cortex can modify

arterial pressure via relays to medulla

– Hypothalamus increases blood pressure during

stress

– Hypothalamus mediates redistribution of blood

flow during exercise and changes in body

temperature


Short-Term Mechanisms: Hormonal Controls

• Hormones regulate BP in short term via

changes in peripheral resistance or long term

via changes in blood volume

• Adrenal medulla hormones

– Epinephrine and norepinephrine from adrenal

gland increase CO and vasoconstriction

• Angiotensin II stimulates vasoconstriction

• ADH: high levels can cause vasoconstriction

• Atrial natriuretic peptide decreases BP by

antagonizing aldosterone, causing decreased

blood volume© 2017 Pearson Education, Inc.

Table 18.2 Effects of Selected Hormones on Blood Pressure


Long-Term Mechanisms: Renal Regulation

• Baroreceptors quickly adapt to chronic high or

low BP so are ineffective for long-term

regulation

• Long-term mechanisms control BP by altering

blood volume via kidneys

• Kidneys regulate arterial blood pressure by:

1. Direct renal mechanism

2. Indirect renal mechanism (renin-angiotensin-

aldosterone)



(cont.)

• Direct renal mechanism

– Alters blood volume independently of hormones

• Increased BP or blood volume causes elimination of

more urine, thus reducing BP

• Decreased BP or blood volume causes kidneys to

conserve water, and BP rises


Figure 18.11 Direct and indirect (hormonal) mechanisms for renal control of blood pressure.


Arterial pressure

Blood volume

Aldosterone

Mean arterial pressure

Blood volume


Filtration by kidneys

Urine formation

Arterial pressure

Inhibits baroreceptors

Sympathetic nervoussystem activity

Water intakeWater reabsorption

by kidneys

Sodium reabsorption

by kidneys

ADH release by

posterior pituitary

Vasoconstriction;

peripheral resistanceThirst via

hypothalamusAdrenal cortex

Angiotensin II

Angiotensin converting

enzyme (ACE)

Secretes

Initial stimulus

Physiological response

Result

Direct renal mechanism

Angiotensin I

Angiotensinogen

Renin releasefrom kidneys

Indirect renal mechanism (renin-angiotensin-aldosterone)


(cont.)

• Indirect mechanism

– The renin-angiotensin-aldosterone mechanism

• Decreased arterial blood pressure causes release of

renin from kidneys

• Renin enters blood and catalyzes conversion of

angiotensinogen from liver to angiotensin I

• Angiotensin-converting enzyme, especially from lungs,

converts angiotensin I to angiotensin II



(cont.)

• Indirect mechanism (cont.)

– Angiotensin II acts in four ways to stabilize

arterial BP and ECF:

• Stimulates aldosterone secretion

• Causes ADH release from posterior pituitary

• Triggers hypothalamic thirst center to drink more

water

• Acts as a potent vasoconstrictor, directly increasing

blood pressure


Figure 18.11 Direct and indirect (hormonal) mechanisms for renal control of blood pressure.


Arterial pressure

Blood volume

Aldosterone


Blood volume


Filtration by kidneys

Urine formation

Arterial pressure

Inhibits baroreceptors

Sympathetic nervoussystem activity

Water intakeWater reabsorption

by kidneys

Sodium reabsorption

by kidneys

ADH release by

posterior pituitary

Vasoconstriction;

peripheral resistanceThirst via

hypothalamusAdrenal cortex

Angiotensin II

Angiotensin converting

enzyme (ACE)

Secretes

Initial stimulus


Result

Direct renal mechanism

Angiotensin I

Angiotensinogen

Renin releasefrom kidneys

Indirect renal mechanism (renin-angiotensin-aldosterone)

Summary of Blood Pressure Regulation

• Goal of blood pressure regulation is to keep

blood pressure high enough to provide

adequate tissue perfusion, but not so high that

blood vessels are damaged

– Example: If BP to brain is too low, perfusion is

inadequate, and person loses consciousness

– If BP to brain is too high, person could have

stroke


Figure 18.12 Factors that increase MAP.


Activity ofmuscularpump andrespiratory

pump

Fluid loss fromhemorrhage,

excessivesweating

Crisis stressors:exercise, trauma,

bodytemperature

Baroreceptors

Release of ANPP

Vasomotor tone;bloodbornechemicals

(epinephrine,NE, ADH,

angiotensin II)

Dehydration,high hematocrit

Body size

Conservationof Na+ and

water by kidneys

Blood volumeBlood pressure

Blood pHO2

CO2

ChemoreceptorsBloodvolume

Venousreturn

Activation of vasomotor and cardio-acceleratory centers in brain stem

Strokevolume

Heartrate

Diameter ofblood vessels

Bloodviscosity

Blood vessellength

Peripheral resistanceCardiac output

Mean arterial pressure (MAP)

Initial stimulus


Result

the cardiovascular system: blood vesselsdrjerrycronin.weebly.com/uploads/5/9/7/4/5974564/... ·...

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