Blood Vessels

• Arteries – rapid distribution, pressure reservoir

• Arterioles – RESISTANCE VESSELS

• Capillaries

• Venules

• Veins

Pulse Pressure

• Pressure difference between systolic and diastolic pressure

• Example

– If blood pressure is 120/80, pulse pressure is 40 mm Hg (120mm Hg –80mm Hg)

• Pulse that can be felt in artery lying close to surface of skin is due to pulse pressure

• Pulse pressure reflects the amount of blood entering aorta and the rapidity that it runs off into the vessels of the peripheral circulation

– Increase systolic

1. Bigger stroke volume into a set of large distribution arteries = : ( 

2. Same stroke volume into a smaller, less elastic, calcified distribution arteries = : {

– Increased diastolic

1. Harder run off due to smaller or constricted arterial field

– Isometric skeletal muscle contraction = normal

– Calcified, non elastic arteries vessels = not normal

Mean Arterial Pressure

• Average pressure driving blood forward into tissues throughout cardiac cycle

• Formula for approximating mean arterial pressure:

Mean arterial pressure = 

diastolic pressure + ⅓ pulse pressure

At 120/80, mean arterial pressure = 

80 mm Hg + ⅓ (40 mm Hg) = 93 mm Hg

During Rest, heart spends 2/3 time in disatole

1/3 time in systole, with pressure decreasing towards diastolic pressure

Fig. 10-1, p. 262

21%

100% Lungs

Left side of heartRight side of heart

Digestive system

(Hepatic portal system)

Liver

Kidneys

Skin

Brain

Heart muscle

Skeletal muscle

Bone

Other8%

5%

15%

3%

9%

13%

20%

6%

Flow = DP/R

= MAP/resistance of system’s arterioles           ………………= 9393 mmHg/Resistance in “systemic circuit”

What is resting cardiac output ?   5 liters 

What % of resting cardiac output goes to regional circuit? __

What is actual liters of blood flow to any regional circuit, perminute ? __%__   x   5 liters/min CO  =  ______ liters per min

Flow  liter/minute = P/ Resistance

Flow liter/minute = 93mmHg/  Resistance Units

Resistance Units = Liters/min/93mmHg

In general, lower flow = increase resistance when pressure is constant

Fig. 10-8, p. 269

Pre

ssu

re (

mm

Hg

)

Systolic pressure

Diastolicpressure

Mean pressure

Leftventricle

Largearteries

Arterioles Capillaries Venules and veins

120

110

100

90

80

70

60

50

40

30

20

10

0

Blood Vessels

• Arteries – rapid distribution, pressure reservoir

• Arterioles – RESISTANCE VESSELS• Capillaries

• Venules

• Veins

Arterioles• Major resistance vessels of vascular tree

– 500,000 arterioles dictate flow to 10 billion capillaries– Typically can alter radius from 0 units to 4 units, thereby altering 

resistance and flow by 44 or 256 times

• Regulate the distribution of systemic cardiac output among systemic organ “regional” circulations

– For most regions of systemic circulation local metabolic rate dictates flow requirements

• Increase metabolic rate, increase oxygen requirement, increase blood flow

– For skin,  whole body  temperature regulation– For kidney, overall fluid and electrolyte balance

• Maintain Mean Arterial Pressure at homeostatic set point– MAP = CO x TsPR

Arterioles • Altering arteriole radius alters arteriolar resistance and thereby flow through downstream capillaries

– Normal, innate, inherent diameter of arteriole exists when no extrinsic factor is altering natural contractile activity of smooth muscle in arterial wall

– Factors that alter smooth muscle contractile activity are numerous

(a) Scanning electron micrograph of an arteriole showing howthe smooth muscle cells run circularly around the vessel wall

Smoothmuscle cells

Fig. 10-9a, p. 270

(b) Normal arteriolar tone due to smooth musclecontraction activity with no outside factors

Fig. 10-9b, p. 270

(c) Vasoconstriction (increased contraction of circularsmooth muscle in the arteriolar wall, which leads toincreased resistance and decreased flow through the vessel)

Fig. 10-9c, p. 270

Cross sectionof arteriole atnormal arterioletone in a humanin homeostasisMAP = 93mmHgCO = 5 literTPR = 18.6 r.u.’s

Major causes of additionalvasoconstriction:

Fig. 10-9c, p. 270

(c) Vasoconstriction: when a person is a normal rest, most systemic arterioles are somewhat vasoconstricted and blood is distributed as in figure 10-1

Major causes of additionalvasoconstriction:

ExtrinsicA. An increase in the normal resting 

amount of:1. Norepinephrine from 

sympathetic postganglionic efferent neurons or from adrenal medulla

2. Epinephrine from adrenal medulla

3. Arginine Vasopressin (ADH) from posterior pituitary

4. Angiotensin II from Angiotensin I from Renin

IntrinsicMyogenic Mechanisms – stretch 

induced contraction

(d) Vasodilation (decreased contraction of circular smoothmuscle in the arteriolar wall, which leads to decreasedresistance and increased flow through the vessel)

Major causes:Local control: O2 and other local chemical changes indicative of increased metabolic rate

ACTIVE HYPEREMIA results from metabolically induced increase in local metabolites (locally active chemical messengers)

Extrinsic control:LESS NE, E, AVP, ANGIO II

IF COMPLETELY REMOVE THESE CIRCULATORY SHOCK OCCURS

Fig. 10-9d, p. 270

Arterioles

• Local vasoactive mediators

– Endothelial cells

•Release chemical mediators that play key role in locally regulating arteriolar caliber

•Release locally acting chemical messengers in response to chemical changes in their environment

•Among best studied local vasoactive mediators is nitric oxide (NO)

Fig. 10-11, p. 275

Arteriolar radius

Blood Vessels

• Arteries – rapid distribution, pressure reservoir

• Arterioles – resistance vessels

• Capillaries – EXCHANGE VESSELS

• Venules

• Veins

Capillaries• Thin‐walled, small‐radius, extensively branched

• Sites of exchange between blood and surrounding tissue cells

– Maximized surface area and minimized diffusion distance

– Velocity of blood flow through capillaries is relatively slow

• Provides adequate exchange time

– 2 types of passive exchanges: Diffusion, bulk flow

Capillaries • Narrow, water‐filled gaps (pores) lie at junctions between cells

• Permit passage of water‐soluble substances 

• Lipid soluble substances readily pass through endothelial cells by dissolving in lipid bilayer barrier

• Size of pores in capillary walls varies from organ to organ

CapillaryRed blood cell

Fig. 10-12, p. 276

Capillaries• Under resting conditions many capillaries are not open

• Capillaries surrounded by precapillarysphincters

– Contraction of sphincters reduces blood flowing into capillaries in an organ

– Relaxation of sphincters increases blood flow

• Metarteriole

– Runs between an arteriole and a venule

– Bypasses capillaries

Fig. 10-13, p. 276

Velocity of flow(mm/sec)

Anatomicaldistribution

Total cross-sectional area (cm2)

Blood flow rate(liters/min)

Aorta Venaecavae

VeinsVenules

CapillariesArterioles

Arteries

200

0.3

4.0

3000

5

Blood flow is slow through capillary beds as they have

large total area

Na+, K+, glucose,amino acids

Lipid-solublesubstancespass throughtheendothelialcells

Plasma proteinsgenerally cannotcross the capillarywall

Exchangeableproteins aremoved acrossby vesiculartransport

(b) Transport across a typical capillary wall

Water-filled poreallows water movement

Plasma

Smallwater-solublesubstances passthrough the pores

Plasmamembrane

Cytoplasm

Endothelial cellInterstitial fluid

Exchangeableproteins

Plasmaproteins

O2, CO2

typical

Fig. 10-15b, p. 278

Independent exchange of individual solutes down their own concentration gradients across the capillary wall.

Capillary Exchange

• Osmotic Pressure favors fluid movement into capillaries along entire length of capillary

– Due to presence of plasma proteins that(1) Exist in high concentration inside capillary

(2) Cannot “diffuse” to equilibrium, so are “non-penetrating”

(3) Create constant osmotic pressure that favors movement of H2O from ISF into capillary

Osmotic Pressure (fluid into capillary) constant

Blood Pressure (fluid out of capillary) decreasing arterial to venous

Reabsorption of fluid into capillary

Arterial End: Filtration of fluid out of capillary

Arterial end of capillary venous end

= plasma protein

Overall Overall Filtration Filtration

from from capillarycapillary

Capillary ExchangeCapillary Exchange

Inward pressure( πP + PIF)

25 + 1

Outward pressure(PC + π IF)

37 + 0

Inward pressure = 26( πP + PIF)

25 + 1

Outward pressure = 17(PC + π IF)

17 + 0

=26

=37

Overall Overall capillary = 11 capillary = 11

out out –– 9 in9 in= 2 out to = 2 out to

interstitial fluidinterstitial fluid

All values are given in mm Hg.

Capillary ExchangeCapillary Exchange

Fig. 10-18, p. 280

Transitionpoint

Inward pressure( πP + PIF)

Capillary lengthBeginning

Outward pressure(PC + π IF)

In

0

Out

End

37

26

17

Flu

id m

ove

men

t

Cap

illar

y p

ress

ure

(m

m H

g)

KEY

= Ultrafiltration = Reabsorption

Capillary ExchangeCapillary Exchange

2 mmHg outward pressure during capillary exchange forms

Lymph.

Lymph is a fluid which once formed fills LYMPHATIC

CIRCULATION

• Extensive network of one‐way vessels

• Provides accessory route by which fluid can be returned from interstitial to the blood

• Initial lymphatics– Small, blind‐ended terminal lymph vessels

– Permeate almost every tissue of the body

• Lymph– Interstitial fluid that enters a lymphatic vessel

• Lymph vessels– Formed from convergence of initial lymphatics

– Eventually empty into venous system near where blood enters right atrium

– One way valves spaced at intervals direct flow of lymph toward venous outlet in chest

Lymphatic Vessels collect lymph formed by net capillary filtration of fluid into interstitial space

(a) Relationship between initial lymphatics and blood capillaries

To venoussystem

Interstitialfluid

Arteriole

Tissuecells

Venule

Blood capillary

Initiallymphatic

Fig. 10-19a, p. 281

Plasmafiltered

Lymphcollected

Overlappingendothelial cells

(b) Arrangement of endothelial cells in an initial lymphatic

Fluid pressure on the outside of the vesselpushes the endothelial cell's free edge inward, permitting entrance ofinterstitial fluid(now lymph).

Fluid pressure on the inside of the vesselforces the overlapping edges together sothat lymph cannot escape.

BecomesLymph

Fig. 10-19b, p. 281

PlasmaFilteredFrom

Capillaryinto

Interstitialfluid

HeartVeins

Valve

Lymphnode

Bloodcapillaries

Initiallymphatics

Pulmonarycirculation

Lymph node

Lymph vessel

Systemiccirculation

Initiallymphatics

Arteries

(a) Relationship of lymphatic system to circulatory system

Bloodcapillaries

Fig. 10-20a, p. 282

(b) Comparison of blood flow and lymph flow per day

17 L /day

20 L /day 3 L /day

7200 L /day

Lymph

Blood

Fig. 10-20b, p. 282

Lymphatic System

• Functions

– Return of excess filtered fluid

– Defense against disease

• Lymph nodes have phagocytes which destroy bacteria filtered from interstitial fluid

– Transport of absorbed fat

– Return of filtered protein

Edema 

• Swelling of tissues

• Occurs when too much interstitial fluid accumulates

• Causes of edema

– Reduced concentration of plasma proteins

– Increased permeability of capillary wall

– Increased venous pressure

– Blockage of lymph vessels