Chapter 10

The Blood Vessels and 

Blood Pressure

Fig. 10-1, p. 262

20%

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%

10%

13%

20%

6%

Regulate a constant mean arterial pressure to distribute the blood flow by alterations in resistance to flow in each branch of the systemic circulation.

Distribute flow given to a circuit by altering resistance in smaller metarterioles that distribute to specific capillary beds

Tissues receive blood in proportion to their oxygen requirement as opposed to their weight

Flow is directly proportional to pressure difference and inversely proportional to resistance. Resistance is proportional to difference in radius4.  If radius double from 1 to 2, resistance changes 24. If radius 

triples from 1 to 3, resistance changes 34 

Blood Flow 

• Flow rate through a vessel:

– Directly proportional to the pressure gradient

– Inversely proportional to vascular resistance

F = ΔPR

F = flow rate of blood through a vessel

ΔP = pressure gradient

R = resistance of blood vessels

F = ΔPR

F = flow rate of blood through a vessel    Cardiac Output     (Qc)

ΔP = pressure gradient Mean Arterial Pressure (MAP)

R = resistance of blood vessels Total Systemic Peripheral Resistance (TsPR)

MAP =  CO                           x    TsPR

MAP =  HR x SV                  x     TsPR

MAP =  HR x (EDV‐ESV)    x     TsPR

EDV is dependent upon adequate VENOUS RETURN

ESV is dependent upon level of catecholamines arriving from  sympathetic neurons and adrenal medulla

CO = VR because heart cannot pump out blood that is not there

Flow of fluids through tubes is fluid dynamics. Flow of blood through arterial vessels is hemodynamics. Same equations and principles govern each

ΔΔPPRR

F =F =

ΔP is Pressure gradient is pressure difference between beginning and end of a vessel

Blood flows from area of higher pressure to area of lower pressure

50 mm Hgpressure

90 mm Hgpressure

(a) Comparison of flow rate in vessels with a different ∆P

10 mm Hgpressure

10 mm Hgpressure

∆P = 40 mm HgVessel A

∆P = 80 mm HgVessel B

∆P in vessel B = 2 times that of vessel A

Flow in vessel B = 2 times that of vessel A

Flow ∆P

Fig. 10-2a, p. 263

90 mm Hgpressure

180 mm Hgpressure

(b) Comparison of flow rate in vessels withthe same ∆P

10 mm Hgpressure

100 mm Hgpressure

∆P = 80 mm HgVessel B

∆P = 80 mm HgVessel C

∆P in vessel C = the same as that of vessel B, despite the larger absolute values

Flow in vessel C = the same as that of vessel B

Flow ∆P

Fig. 10-2b, p. 263

Flow of fluids through tubes is fluid dynamics. Flow of blood through arterial vessels is hemodynamics. Same equations and principles govern each

•• Resistance Resistance is measure of opposition of blood flow through a vessel

– Depends on 3 things:

• Blood viscosity

• Vessel length

• Vessel radium– Major determinant of resistance to flow is vessel’s radiusradius

– Slight change in radius produces significant change in blood flow

• R is proportional to  1r4

ΔΔPPRR

F =F =

Fig. 10-3b, p. 263

•Resistance ∝ 1/r 4 so at any P Flow ∝ r 4

Vessel A

Vessel B

Samepressuregradient

Radius in vessel B = 2 times that ofvessel 1

•Resistance in vessel B = 1/24 = 1/16 that of vessel A

•Flow in vessel B = 16 times that of vessel A at same P

(b) Influence of vessel radius on resistance and flow

Radius may quadruple

Increase 4x

= 44 =1/256 resistance or 256x more flow

AT SAME P

Fig. 10-1, p. 262

20%

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%

10%

13%

20%

6%

Flow = P/R

= MAP/resistance of system’s arterioles           ………= 100mmHg/Resistance in “systemic circuit”

RR

What is resting cardiac output ?   ______

What % of resting cardiac output goes to kidneys? __

What is actual liters of blood flow to kidney, per minute ? _____   x   _______  =  ______ liters per min

Kidney Flow 1 liter/minute = P/ Kidney Resistance

Kidney Flow 1 liter/minute = 100mmHg/ ___    Kidney Resistance Units

Kidney Resistance = ______ Resistance Units

55

20%20%

20%20%55 11

100100

100100

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 = P/R

= MAP/resistance of system’s  arterioles

MAP = 100mmHg, what is resistance in Bone’s arterioles ?

5% of CO = 5% of 5 liters = 0.25 liter

0.25liters = 100mmHg/resistance

Resistance in renal arterioles = 400 resistance units

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 = P/R

= MAP/resistance of system’s arterioles           ………………= 93mmHg/Resistance in “systemic circuit”

What is resting cardiac output ?   ______

What % of resting cardiac output goes to brain? __

What is actual liters of blood flow to brain, per minute ? _____   x   _______  =  ______ liters per min

Brain Flow .65 liter/minute = P/ Brain Resistance

Brain Flow .65 liter/minute = 93mmHg/ _____Brain Resistance Unit

Brain Resistance = _              Resistance Units

55

13%13%

13%13%55 .65.65

143143

143143

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

Most of cardiac and renal regulation is designed to keep MAP constant so that each part of the systemic circulation can have access to the 

flow required to support its metabolic rate

Vascular Tree

• Closed system of vessels• Consists of 

– Arteries• Carry blood away from heart to tissues

– Arterioles• Smaller branches of arteries that serve as RESISTANCE VESSELS

– Capillaries• Smaller branches of arterioles• Smallest of vessels across which all exchanges are made with 

surrounding cells– Venules

• Formed when capillaries rejoin• Return blood to heart

– Veins• Formed when venules merge• Serve as CAPACITANCE VESSELS• Return blood to heart

Fig. 10-4, p. 264

Smaller arteries branching off to supply various tissues

Lungs

Airway

Air sac

Tissues

PULMONARYCIRCULATION

Pulmonarycapillaries

Pulmonaryartery

Systemicveins

Arterioles

Venules

Venules

Arterioles

Pulmonaryveins

Aorta(majorsystemicartery)

SYSTEMICCIRCULATION

Systemiccapillaries

For simplicity, onlytwo capillary beds withintwo organs are illustrated.

Arteries 

• Specialized to

– Serve as rapid‐transit passageways for blood from heart to organs

• Due to large radius, arteries offer little resistance to blood flow

– Act as pressure reservoir to provide driving force for blood when heart is relaxing

• Arterial connective tissue contains

– Collagen fibers

» Provide tensile strength

– Elastin fibers

» Provide elasticity to arterial walls

Arteries

Arterioles

To capillaries

To capillaries

Arterioles

(a) Heart contracting and emptying

(b) Heart relaxing and filling

From veins

From veins

Arteries

Fig. 10-5, p. 268

Blood Pressure

• Force exerted by blood against a vessel wall– Depends on

• Volume of blood contained within vessel• Compliance of vessel walls

• Systolic pressure – Peak pressure exerted by ejected blood against vessel walls 

during cardiac systole– Averages 120 mm Hg

• Diastolic pressure– Minimum pressure in arteries when blood is draining off into 

vessels downstream– Averages 80 mm Hg– Falls to 0 mmHg if the heart stops contracting

Blood Pressure

• Can be measured indirectly using sphygmomanometer

• Korotkoff sounds

– Sounds heard when determining blood pressure

– Sounds are distinct from heart sounds associated with valve closure

Fig. 10-6, p. 267

Art

eria

l pre

ss

ure

(m

m H

g)

120

80

93

Pulsepressure

Meanpressure

Systolic pressure

Time (msec)

Diastolic pressure

Notch inpressurecurve causedby closureof aortic valve

Stethoscope

(a) Use of a sphygmomanometer in determining blood pressure

Inflatablecuff

Mercury sphygmanomometer

Fig. 10-7a, p. 268

The inflating bulb is used to inflate the cuff. It contains two one‐ way valves. Valve A allows air to enter the back of the bulb. When the bulb is squeezed this valve closes and the air is propelled through valve B to the cuff. Valve B stops the air going back into the bulb.  After the cuff has been inflated and the blood pressure taken, the cufy may be deflated by opening valve C. The reservoir contains the supply of mercury which rises up the measurement tube. As the pressure within the cuff increases the mercury is displaced from the reservoir into the graduated tube. The two leather discs (D and E) allow air to pass in and out of the column, but prevent mercury escaping from the sphygmomanometer. 

(b) Blood flow through the brachial artery in relation to cuff pressure and sounds

When blood pressure is 120/80:

When cuff pressure is greater than120 mm Hg and exceeds bloodpressure throughout the cardiac cycle:

No blood flows through the vessel.

No sound is heard because no bloodis flowing.

When cuff pressure is between120 and 80 mm Hg:Blood flow through the vessel is turbulent whenever blood pressure exceeds cuff pressure.

2 The first sound is heard at peak systolic pressure.

3 Intermittent sounds are producedby turbulent spurts of flow as bloodpressure cyclically exceeds cuff pressure.

When cuff pressure is less than 80 mm Hg and is below blood pressure throughout the cardiac cycle:

Blood flows through the vessel insmooth, laminar fashion.

4 The last sound is heard at minimumdiastolic pressure.

5 No sound is heard thereafter becauseof uninterrupted, smooth, laminar flow.

Fig. 10-7b, p. 268

1

2

3

4

5

Cuff pressure Blood pressure

Fig. 10-7c, p. 268

Time

Pre

ssu

re (

mm

Hg

)

140

120

100

80

12

4 5

3

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