Figure 19.2

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

Copyright © 2010 Pearson Education, Inc. Figure 19.3a

Red bloodcell in lumenIntercellularcleftEndothelialcell

Endothelialnucleus

Tight junction Pinocytoticvesicles

Pericyte

Basementmembrane

(a) Continuous capillary. Least permeable, and most common (e.g., skin, muscle).

Copyright © 2010 Pearson Education, Inc. Figure 19.3b

Red bloodcell in lumen

Intercellularcleft

Fenestrations(pores)

Endothelialcell

EndothelialnucleusBasement membrane

Tight junction

Pinocytoticvesicles

(b) Fenestrated capillary. Large fenestrations (pores) increase permeability. Occurs in special locations (e.g., kidney, small intestine).

Copyright © 2010 Pearson Education, Inc. Figure 19.3c

Nucleus ofendothelialcell

Red bloodcell in lumen

Endothelialcell

Tight junctionIncompletebasementmembrane

Largeintercellularcleft

(c) Sinusoidal capillary. Most permeable. Occurs in special locations (e.g., liver, bone marrow, spleen).


(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

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%

Copyright © 2010 Pearson Education, Inc.

Physiology of Circulation: Definition of Terms

• Blood pressure (BP)• Force per unit area exerted on the wall of a

blood vessel by the blood • Expressed in mm Hg• Measured as systemic arterial BP in large

arteries near the heart• The pressure gradient provides the driving

force that keeps blood moving from higher to lower pressure areas


Physiology of Circulation: Definition of Terms

• Peripheral resistance• Opposition to flow • Measure of amount of friction blood encounters• Generally encountered in peripheral circulation

• Three important sources of resistance• Blood viscosity• Total blood vessel length• Blood vessel diameter


Arterial Blood Pressure

• Systolic pressure: pressure exerted during ventricular contraction

• Diastolic pressure: lowest level of arterial pressure

• Pulse pressure = difference between systolic and diastolic pressure


Factors Aiding Venous Return

1. Respiratory “pump”: pressure changes created during breathing move blood toward the heart by squeezing abdominal veins as thoracic veins expand

2. Muscular “pump”: contraction of skeletal muscles “milk” blood toward the heart and valves prevent backflow

3. Vasoconstriction of veins under sympathetic control


Valve (open)

Contractedskeletalmuscle

Valve (closed)

Vein

Direction ofblood flow


Maintaining Blood Pressure

• The main factors influencing blood pressure:• Cardiac output (CO)

• Peripheral resistance (PR)

• Blood volume


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

ResultPhysiological response


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.

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


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 stimulusPhysiological responseResult


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

ResultPhysiological response

Mean systemic arterial blood pressure

Bloodviscosity

Peripheral resistance

Blood vessellength



Blood Flow Through Body Tissues

• Blood flow (tissue perfusion) is involved in

• Delivery of O2 and nutrients to, and removal of wastes from, tissue cells

• Gas exchange (lungs)

• Absorption of nutrients (digestive tract)

• Urine formation (kidneys)

• Rate of flow is precisely the right amount to provide for proper function


BrainHeart

Skeletalmuscles

Skin

Kidney

Abdomen

Other

Total blood flow during strenuousexercise 17,500 ml/min

Total bloodflow at rest5800 ml/min


Metabolic Controls

• Vasodilation of arterioles and relaxation of precapillary sphincters occur in response to

• Declining tissue O2

• Substances from metabolically active tissues (H+, K+, adenosine, and prostaglandins) and inflammatory chemicals


Metabolic Controls

• Effects

• NO is the major factor causing vasodilation

• Vasoconstriction is due to sympathetic stimulation and endothelins


Myogenic Controls

• Myogenic responses of vascular smooth muscle keep tissue perfusion constant despite most fluctuations in systemic pressure

• Passive stretch (increased intravascular pressure) promotes increased tone and vasoconstriction

• Reduced stretch promotes vasodilation and increases blood flow to the tissue


Metaboliccontrols

pH Sympathetic

a Receptors

b Receptors Epinephrine,norepinephrine

Angiotensin II

Antidiuretichormone (ADH)

Atrialnatriureticpeptide (ANP)

DilatesConstricts

ProstaglandinsAdenosine

Nitric oxideEndothelins

Stretch

O2

CO2

K+

Amounts of:

Amounts of:

Nerves

Hormones

Myogeniccontrols

Intrinsic mechanisms(autoregulation)

• Distribute blood flow to individual organs and tissues as needed

Extrinsic mechanisms• Maintain mean arterial pressure (MAP)• Redistribute blood during exercise and thermoregulation


Long-Term Autoregulation

• Angiogenesis• Occurs when short-term autoregulation cannot

meet tissue nutrient requirements

• The number of vessels to a region increases and existing vessels enlarge

• Common in the heart when a coronary vessel is occluded, or throughout the body in people in high-altitude areas


Blood Flow: Skeletal Muscles

• At rest, myogenic and general neural mechanisms predominate

• During muscle activity• Blood flow increases in direct proportion to the

metabolic activity (active or exercise hyperemia)

• Local controls override sympathetic vasoconstriction

• Muscle blood flow can increase 10 or more during physical activity


Blood Flow: Skin

• Blood flow to venous plexuses below the skin surface• Varies from 50 ml/min to 2500 ml/min,

depending on body temperature

• Is controlled by sympathetic nervous system reflexes initiated by temperature receptors and the central nervous system


HP = hydrostatic pressure• Due to fluid pressing against a wall• “Pushes”• In capillary (HPc) • Pushes fluid out of capillary • 35 mm Hg at arterial end and 17 mm Hg at venous end of capillary in this example• In interstitial fluid (HPif) • Pushes fluid into capillary • 0 mm Hg in this example

OP = osmotic pressure• Due to presence of nondiffusible solutes (e.g., plasma proteins)• “Sucks”• In capillary (OPc) • Pulls fluid into capillary • 26 mm Hg in this example• In interstitial fluid (OPif) • Pulls fluid out of capillary • 1 mm Hg in this example

Arteriole

Capillary

Interstitial fluid

Net HP—Net OP(35—0)—(26—1)

Net HP—Net OP(17—0)—(26—1)

Venule

NFP (net filtration pressure)is 10 mm Hg; fluid moves out

NFP is ~8 mm Hg;fluid moves in

NetHP35mm

NetOP25mm

NetHP17mm

NetOP25mm


Signs and symptoms

Acute bleeding (or other events that causeblood volume loss) leads to:

1. Inadequate tissue perfusion resulting in O2 and nutrients to cells2. Anaerobic metabolism by cells, so lactic acid accumulates3. Movement of interstitial fluid into blood, so tissues dehydrate

Initial stimulus

Result

Physiological response

Chemoreceptors activated(by in blood pH)

Baroreceptor firing reduced(by blood volume and pressure)

Hypothalamus activated(by pH and blood pressure)

Major effect Minor effect

Brain

Activation ofrespiratory centers

Cardioacceleratory andvasomotor centers activated

Sympathetic nervoussystem activated

ADHreleased

Neuronsdepressed

by pH

Intense vasoconstriction(only heart and brain spared)Heart rate Central

nervous systemdepressed

Adrenalcortex

Kidney

Renin released

Renal blood flow

Aldosteronereleased

Kidneys retainsalt and water

Angiotensin IIproduced in blood

Waterretention

Urine outputRate anddepth of

breathing

Tachycardia,weak, thready

pulse

Skin becomescold, clammy,and cyanotic

Thirst Restlessness(early sign)

Coma(late sign)

CO2 blownoff; bloodpH rises

Blood pressure maintained;if fluid volume continues to

decrease, BP ultimatelydrops. BP is a late sign.

Documents

Figure 19.2