Blood Vessels
Chapter 19
Blood Vessels
Arteries – carry blood away
from the heart
• “branch” and “supply”
Veins – carry blood toward the
heart
• “merge” and “drain”
Lumen – the hollow space that
carries the blood
Figure 19.1
Blood Vessels
Vessel Histology:
• Arteries have a thicker wall
and a rounder lumen
• Veins have thinner walls
and a larger, slit-like lumen
• Arteries and veins often
run side-by-side
Figure 19.1a
Blood Vessels
Vessel Tunics:
-The 3 layers of a vessel wall
1.Tunica Intima – innermost layer
with simple squamous endothelium
and its underlying connective tissue
2.Tunica Media – middle layer of
smooth muscle (thickest in arteries)
Vasoconstriction – shrinks
lumen
Vasodilation – enlarges lumen
3.Tunica Externa – outer layer of
connective tissue for support, with
nerve fibers and small vessels
(thickest in veins)
Vasa vasorum – series of
small vessels bringing
blood to large vessel walls
Figure 19.1b
Blood Vessels
Types of Arteries:
Elastic arteries • aorta and its branches
• D = 1.0 - 2.5cm
• highly elastic, act as pressure
reservoirs
Muscular arteries • vessels supplying muscles/organs
• D = 0.3mm – 1.0cm
• thick media to regulate blood flow
Arterioles –
• smallest arteries
• D = 10µm – 0.3mm
• # of tunics varies with size
• regulate blood supply to capillary
beds
Figure 19.2
Blood Vessels
Types of Veins:
Venules –
• smallest of the veins
• D = 8 – 100 µm
• Drain blood from capillary beds
• # of tunics varies with size
Veins –
• D = 0.1mm – 2.5 cm
• have all three tunics
• act as blood reservoirs, holding
60-65% of blood at any time
• pressure is low, need valves
Venous Sinuses – large flat
veins that are not true vessels,
supported by the surrounding
tissues, not tunics
Figure 19.2
Capillaries
Capillaries:
• smallest vessels, allow
exchange of solutes with
tissues
• pericytes – muscle-like
cells for support
• only 1 tunic, intima
• D = 8 – 10 µm
• Length = 1mm
3 type of Capillaries:
1.Continuous
2.Fenestrated
3.Sinusoid
Figure 19.3
Capillaries
3 type of Capillaries:
1. Continuous:
• most common type,
found in muscle and skin
•
wall has continuous
sheet of endothelial cells
•
Intercellular clefts –
small gaps between
endothelial cells allowing
some fluid and solute
movement
Figure 19.3a
Capillaries
3 type of Capillaries:
2. Fenestrated:
• found in kidneys, small
intestine, and endocrine
glands
•
small holes, fenestrations,
make these vessels highly
permeable
•
used for filtration and
absorption
Figure 19.3b
Capillaries
3 type of Capillaries:
3. Sinusoid:
• found in the liver, bone
marrow and lymph tissues
•
large lumens, large
fenestrations, and large
intercellular clefts
•
Highly permeable to fluids,
solutes and even cells!
Figure 19.3c
Capillaries
Capillary bed – an interweaving network of 10-100 capillaries from arteriole
to venule, performing ‘microcirculation’
• Vascular shunt – allows blood to bypass a bed
• True capillaries – the many vessels for gas and nutrient exchange
• Precapillary sphincters – regulate blood flow into the true capillaries
based on local tissue needs
Figure 19.4
Physiology of Circulation
Note: Blood moves down its pressure gradient (high to low)
• Blood flow – the volume of blood passing through a vessel in a given
amount of time, expressed as mL/min
• Blood pressure – the force exerted by blood on a vessel wall, measured
in mmHg. Typically represents pressure in a large artery.
• Resistance – the friction encountered by the blood, reducing flow. Caused
by 3 main factors:
1. Blood viscosity – thicker blood, as in polycythemia, experiences
more resistance
2. Blood vessel length (total) – additional pounds of adipose adds
miles of vessels to the body, increasing resistance
3. Blood vessel diameter – blood in smaller vessels experiences
more resistance
• Formula :
Flow = ∆P/R
Blood Pressure
Arterial blood pressure – the
pulsatile pressure resulting from
heart contraction and relaxation
• Systolic pressure – the peak
pressure caused by heart
contractions (av. = 120 mmHg)
• Diastolic pressure – the low
pressure resulting from heart
relaxation (av. = 80 mmHg)
• Pulse pressure – the
difference between systolic and
diastolic pressures, causing a
pulse in an artery. This P
dissipates at the end of the
arterial system
Figure 19.5
Blood Pressure
Mean arterial pressure – the
average pressure in a vessel that
drives blood movement.
Decreases from aorta to R. atrium
Formula:
MAP = diastolic P + (pulse P/3)
If P = 120/80…
MAP = 80mmHg + (40mmHg/3)
MAP = 93mmHg
MAP in…
Arterioles - 80-35 mmHg
Capillaries – 35-15 mmHg
Veins – 15-0 mmHg
Figure 19.5
Blood Pressure in Veins
Low venous pressure requires the
following adaptations:
• Large lumens – decreases resistance
• Venous valves – catch blood similar to
semilunar valves
• Respiratory ‘pump’ – the rising and
falling of thoracic pressure squeezes
veins and propels blood
• Muscular ‘pump’ – skeletal muscle
movements squeeze veins and propels
blood
• Venous smooth muscle – contracts in
fight-or-flight responses to propel blood
Figure 19.6
Regulating Blood Pressure
Blood Pressure Regulation:
1. Short Term – done by the CNS and
hormones
•
Make changes in cardiac output or
peripheral resistance, via changes
in heart rate, stroke volume, and
vessel diameter (increased CO
and PR means higher pressure)
2. Long term – done by the kidneys
•
Done by making changes to
blood volume (increased volume
means higher pressure)
Figure 19.7
Regulating Blood
Pressure
Neural Regulation:
Baroreceptors – stretch receptors in
large arteries that measure
pressure
When pressure rises…
1. Stretch of baroreceptors sends
impulse to medulla
2. Medulla slows heart rate and dilates
arteries, lowering the pressure
When pressure falls…
1. Baroreceptor inactivity sends
impulse to medulla
2. Medulla increases heart rate and
constricts arteries, raisin the
pressure
Figure 19.8
Regulating Blood Pressure
Hormonal Regulation:
1. Adrenal medulla hormones – norepinephrine and epinephrine are
released to aide the fight-or-flight response. These trigger
vasoconstriction and increased CO, therefore increasing pressure
2. Atrial natriuretic peptide – hormone from heart’s atria that causes
vasodilation and a decrease in blood volume by increasing urine
output, thereby decreasing pressure
3. Antidiuretic Hormone – hormone from hypothalamus that
increases blood volume by decreasing urine output, therefore
increasing pressure
4. Angiotensin II – a kidney hormone that causes vasoconstriction,
thereby increasing pressure
Regulating Blood
Pressure
Renal Regulation (Long term):
1. Direct mechanism – does not use
hormones
•
Increased pressure means
increased kidney filtration and
urine output, lowering blood
volume and pressure
•
Lower pressure means
decreased filtration and urine
output, raising blood volume
and pressure
2. Indirect mechanism – uses
hormones to affect blood volume
•
Called the Renin-Angiotensin
mechanism
Figure 19.9
Regulating Blood
Pressure
Renin-Angiotensin mechanism
•
Decreased pressure causes kidneys to
release the hormone Renin, which triggers
Angiotensin II production
•
Angiotensin II causes vasoconstriction and
aldosterone release from the adrenal
glands
•
Aldosterone causes sodium retention which
causes water retention, increasing blood
volume
•
Increased blood volume increases blood
pressure
Figure 19.9
Regulating Blood Pressure
Figure 19.10
Pulse
Pulse – a wave of stretch and
recoil of an artery due to the rising
and falling of arterial pressure
• Pressure/Pulse points –
locations where major
arteries are superficial and
easily felt
Figure 19.11
Measuring Blood Pressure
Ausculatory method – measures blood
pressure by listening to sounds in the
vessel. Needs cuff to constrict the vessel,
pressure gauge to measure, and
stethoscope to listen.
1. Inflate cuff to a pressure greater than
systolic to collapse vessel = no flow
2. Slowly release air from cuff until sounds are
heard = turbulent flow. The reading on the
gauge is now the systolic pressure
3. Continue releasing air until sounds dissipate
to nothing = laminar flow. The reading on
the gauge is now the diastolic pressure
Measuring Blood Pressure
Healthy blood pressure = 120/80 mmHg
Hypotension
•
Low blood pressure, systolic P < 100
mmHg
•
Can be due to good fitness, or
malnutrition causing low blood viscosity
Hypertension
•
High blood pressure, 140/90 mmHg or
above
•
Causes include diet, obesity, age,
diabetes, heredity, stress, smoking
•
Can lead to heart failure, renal failure,
stroke, enlarged myocardium, etc.
Regulating Blood Flow
Tissue perfusion – blood
flow through tissues for
nutrient delivery, gas
exchange, absorption, or
filtration.
Blood distribution – at rest,
large volumes of blood are
sent to the kidneys and
abdominal organs. During
exercise, large volumes of
blood are sent to skeletal
muscles and skin
Figure 19.12
Regulating Blood Flow
Autoregulation – the automatic adjustment of blood flow through a
tissue based on that tissue’s need. Can be regulated metabolically or
myogenically.
• Metabolic control – decreased O2, increased CO2, and
decreased nutrient levels trigger local vasodilation of arterioles,
supplying more blood to local capillary beds
• Myogenic control – increased stretch of arterioles triggers their
vasoconstriction, thereby protecting the fragile capillaries.
Decreased stretch of arterioles triggers their vasodilation.
Autoregulation in specific organs:
• Skeletal muscle – major increase in blood flow during activity, called
exercise hyperemia. Largely triggered by low O2 and high waste levels
• Skin – besides normal metabolic and myogenic controls, increased
body temperature causes vasodilation here. WHY?
• Lungs – areas of lung tissue with low O2 levels triggers local
vasoconstriction, while lung tissue with high O2 levels triggers local
vasodilation. WHY?
Capillary Dynamics
Capillary Exchange – movement into
or out of capillaries occurs 4
different ways
1. Diffusion – direct movement of
lipid-soluble molecules through the
endothelial cells
2. Intercellular clefts – movement of
water-soluble molecules between
endothelial cells
3. Fenestrations – movement of
water-soluble molecules through
endothelial holes
4. Vesicular Transport – movement
of large molecules by vesicles
Figure 19.15
Capillary Dynamics
Bulk flow – the leakage of plasma out of, then back into a capillary, based
on the balance of pressures
Hydrostatic pressure – the pressure of a fluid on a vessel wall (same as
blood pressure)
Colloid osmotic pressure – the pressure generated by nondiffusable
solutes tendency to draw water towards themselves
Figure 19.16
Capillary Dynamics
Net Filtration Pressure (NFP) – the pressure generated by the balance
between hydrostatic and colloid osmotic pressures
• NFP is a positive pressure at the arterial end of a capillary bed, forcing
plasma to leak into the surrounding tissue
• NFP is a negative value at the venous end of a capillary bed, drawing
plasma back into the vessels
Figure 19.16