Chapter 10
The Blood Vessels and Blood Pressure
100%
Lungs
Right side of heart Left side of heart
Digestive
system
(Hepatic portal
system)
Liver
Kidneys
Skin
Brain
20%
6%
20%
10%
13%
Heart
muscle
3%
Skeletal
muscle
15%
Bone
Other
Regulate a constant mean arterial pressure to distribute the blood flow by alterations in resistance to flow in each branch of the systemic circulation.
5%
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
8%
Fig. 10-1, p. 262
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 = ΔP
R
F = flow rate of blood through a vessel
ΔP = pressure gradient
R = resistance of blood vessels
F = ΔP
R
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
F = ΔP
R
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 Hg
pressure
10 mm Hg
pressure
∆P = 40 mm Hg
Vessel A
90 mm Hg
pressure
10 mm Hg
pressure
∆P = 80 mm Hg
Vessel B
∆P in vessel B = 2 times that of vessel A
Flow in vessel B = 2 times that of vessel A
Flow  ∆P
(a) Comparison of flow rate in vessels with a different ∆P
Fig. 10-2a, p. 263
90 mm Hg
pressure
10 mm Hg
pressure
∆P = 80 mm Hg
Vessel B
180 mm Hg
pressure
100 mm Hg
pressure
∆P = 80 mm Hg
Vessel 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
(b) Comparison of flow rate in vessels with
the same ∆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
ΔP
F =
R
• Resistance is measure of opposition of blood flow through a vessel
Resistance – Depends on 3 things:
• Blood viscosity
• Vessel length
• Vessel radium
– Major determinant of resistance to flow is vessel’s radius
radius
– Slight change in radius produces significant change in blood flow
• R is proportional to 1
r4
Vessel A
Same
pressure
gradient
Vessel B
Radius in vessel B = 2 times that of
vessel 1
•Resistance in vessel B = 1/24 = 1/16 that of vessel A
Radius may
quadruple
Increase 4x
= 44 =1/256
resistance or
256x more flow
•Flow in vessel B = 16 times that of vessel A at same P
•Resistance ∝ 1/r 4
so at any P
Flow ∝ r 4
AT SAME P
(b) Influence of vessel radius on resistance and flow
Fig. 10-3b, p. 263
100%
Lungs
Flow = P/R
Right side of heart Left side of heart
Digestive
system
(Hepatic portal
system)
Liver
Kidneys
Skin
Brain
R
6%
What % of resting cardiac output goes to kidneys? __
20%
10%
What is actual liters of blood flow to kidney, per minute ? 5
20%
1
_____ x _______ = ______ liters per min
13%
Kidney Flow 1 liter/minute = P/ Kidney Resistance
3%
Skeletal
muscle
15%
Other
5
What is resting cardiac output ? ______
20%
Heart
muscle
Bone
100%
20%
= MAP/resistance of system’s arterioles ………= 100mmHg/Resistance in “systemic circuit”
100 Kidney Resistance Units
Kidney Flow 1 liter/minute = 100mmHg/ ___ 100
Kidney Resistance = ______ Resistance Units
5%
8%
Fig. 10-1, p. 262
Lungs
Flow = P/R
Right side of heart Left side of heart
Digestive
system
(Hepatic portal
system)
Liver
Kidneys
Skin
Brain
21%
6%
20%
5% of CO = 5% of 5 liters = 0.25 liter
0.25liters = 100mmHg/resistance
13%
3%
Skeletal
muscle
15%
Other
MAP = 100mmHg, what is resistance in Bone’s arterioles ?
9%
Heart
muscle
Bone
= MAP/resistance of system’s arterioles
Resistance in renal arterioles = 400 resistance units
5%
8%
Fig. 10-1, p. 262
100%
Lungs
Flow = P/R
Right side of heart Left side of heart
Digestive
system
(Hepatic portal
system)
Liver
Kidneys
Skin
Brain
21%
6%
20%
9%
13%
Heart
muscle
3%
Skeletal
muscle
15%
Bone
Other
= MAP/resistance of system’s arterioles ………
………= 93mmHg/Resistance in “systemic circuit”
5
What is resting cardiac output ? ______
13%
What % of resting cardiac output goes to brain? __
What is actual liters of blood flow to brain, per minute ? _____ x _______ = ______ liters per min
5
13%
.65
Brain Flow .65 liter/minute = P/ Brain Resistance
143
Brain Flow .65 liter/minute = 93mmHg/ _____Brain
Resistance Unit
Brain Resistance = _ Resistance Units
143
5%
In general, lower flow = increase resistance when pressure is constant
8%
Fig. 10-1, p. 262
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
Airway
Lungs
Air sac
Pulmonary
capillaries
Arterioles
Venules
PULMONARY
CIRCULATION
Pulmonary
artery
Pulmonary
veins
Aorta
(major
systemic
artery)
Systemic
veins
SYSTEMIC
CIRCULATION
Tissues
Venules
For simplicity, only
two capillary beds within
two organs are illustrated.
Systemic
capillaries
Arterioles
Smaller arteries
branching off to
supply various tissues
Fig. 10-4, p. 264
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
From veins
(a) Heart contracting and emptying
Arteries
Arterioles
From veins
To capillaries
(b) Heart relaxing and filling
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
Arterial pressure (mm Hg)
120
Systolic pressure
Pulse
pressure
Notch in
pressure
curve caused
by closure
of aortic valve
Mean
pressure
93
80
Diastolic pressure
Time (msec)
Fig. 10-6, p. 267
Fig. 10-7a, p. 268
(a) Use of a sphygmomanometer in determining blood pressure
Mercury sphygmanomometer
Inflatable
cuff
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. Stethoscope
When blood pressure is 120/80:
When cuff pressure is greater than
120 mm Hg and exceeds blood
pressure throughout the cardiac cycle:
No blood flows through the vessel.
1 No sound is heard because no blood
is flowing.
When cuff pressure is between
120 and 80 mm Hg:
Blood flow through the vessel is turbulent
whenever blood pressure exceeds cuff pressure.
22
The first sound is heard at peak systolic pressure.
33
Intermittent sounds are produced
by turbulent spurts of flow as blood
pressure 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 in
smooth, laminar fashion.
44 The last sound is heard at minimum
diastolic pressure.
55 No sound is heard thereafter because
of uninterrupted, smooth, laminar flow.
(b) Blood flow through the brachial artery in relation to cuff pressure and sounds
Fig. 10-7b, p. 268
Cuff pressure
Pressure (mm Hg)
140
1
2
Blood pressure
3
120
100
4
5
80
Time
Fig. 10-7c, p. 268
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
100%
Lungs
Flow = DP/R
= MAP/resistance of system’s arterioles ………
………= 93
93 mmHg/Resistance in “systemic circuit”
Right side of heartLeft side of heart
Digestive
system
(Hepatic portal
system)
Liver
Kidneys
Skin
Brain
21%
6%
20%
9%
13%
Heart
muscle
3%
Skeletal
muscle
15%
Bone
Other
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, per
minute ? __%__ x 5 liters/min CO = ______ liters per min
Flow liter/minute = P/ Resistance
Flow liter/minute = 93mmHg/ Resistance Units
Resistance Units = Liters/min/93mmHg
5%
8%
In general, lower flow = increase resistance when pressure is constant
Fig. 10-1, p. 262