Download Figure 15-1

Chapter 15
The Blood Vessels
Circulatory System
Heart: pumps blood
– Right side: pumps blood to lungs and back
• Pulmonary circuit
Left side: pumps blood to rest of body
• Systemic circuit
Aorta
Arteries
Arterioles
Capillaries
Venules
Veins
Major veins (venae cavae)
Figure 15-1
Copyright © 2010 Pearson Education, Inc.
Figure 15-2
Copyright © 2010 Pearson Education, Inc.
Blood Vessels (Systemic Circulation)
Arteries
– Pressure reservoir
• The aorta and other large systemic arteries have
very elastic walls
• The pressure produced by the force of left
ventricle contraction is stored in the elastic walls
and slowly released through elastic recoil
• During ventricular relaxation, elastic recoil of the
artery walls maintains a continuous driving
pressure for blood flow
Blood Vessels (Systemic Circulation)
Arterioles
– Downstream from the arteries
– Arterioles have variable resistance
• Selectively constrict or dilate, directing blood
flow to certain tissues
– Arteriole diameter is regulated by
• Local factors (tissue oxygen concentration, etc.)
• Autonomic nervous system
• Hormones
Figure 15-1
Copyright © 2010 Pearson Education, Inc.
Capillaries
– Smallest diameter
– Has “leaky” epithelium
• Allows for exchange of materials between
plasma, interstitial fluid, and cells
• Fresh blood coming from the heart brings
oxygen and nutrients to the cells
• These materials “leak” out of the capillaries and
move across to the cells (by diffusion)
• Cells send carbon dioxide and other waste
products out into the IF and from there it enters
the capillaries
Blood Vessels (Systemic Circulation)
Venules
– Located at distal end of the capillary beds
– Venules start out small and progressively get
bigger and bigger, eventually becoming veins
– Venules and veins collect blood to return to the
heart
Blood Vessels (Systemic Circulation)
Veins
– Return blood to the heart
– Act as a volume reservoir
• Hold more than half of the total amount of blood
• If BP falls too low, venous side of circulation can
add more blood to the arterial side
Blood Vessels (Anatomy)
Blood Vessels (Anatomy)
3 layers:
– Tunica intima (inside layer)
• Made of endothelium (a type of epithelium)
• In larger arteries, an elastic tissue is also
present
– Tunica media (middle layer)
• Made of smooth muscle
• Thickness of this layer varies
–Larger vessels, more muscle, smaller ones,
less muscle
– Tunica adventitia or Tunica externa (outside layer)
• Made of fibrous connective tissue
Blood Vessels (Anatomy)
Figure 15-2
Copyright © 2010 Pearson Education, Inc.
Endothelium
– Inner lining of blood vessels
– Capillaries consist only of this layer
– Once thought to be just a passive barrier
– Now known to be a very complex structure (not
just a simple epithelium)
– Endothelium does the following:
• Secretes lots of different paracrines
• Helps regulate blood pressure, blood vessel
growth, absorption of materials
Vascular Smooth Muscle
Tunica media consists of this
Can be arranged in either a circular or a spiral layer
Vasoconstriction
– Narrows the diameter of the blood vessel lumen
Vasodilation
– Increases the diameter of the blood vessel lumen
Muscle tone:
– Smooth muscle cells are partially contracted at all
times
Vascular Smooth Muscle
Muscle tone in vascular smooth muscle can be
influenced by:
– Neurotransmitters
– Hormones
– Paracrines
• Secreted by endothelium or by cells surrounding
the blood vessels
Contraction of vascular smooth muscle requires entry
of Ca++ ions from the ECF (like in cardiac muscle)
More on Arteries and Arterioles (p.515)
Aorta and major arteries have “stiff and springy” walls
– They have thick layers if smooth muscle
– and
– Large amounts of elastic and fibrous connective
tissues
Arteries and arterioles have a divergent branching
pattern (one branch becoming many)
Veins and venules have a convergent branching
pattern (many branches becoming one)
More on Arteries and Arterioles (p.515)
As major arteries branch into smaller and smaller
arteries, the artery wall changes dramatically
As each artery branches into a smaller one, the walls
become less elastic and more muscular
Arterioles (really small arteries) have several layers of
smooth muscle, allowing them to contract or relax
when influenced by chemical signals
Microcirculation: arterioles, capillaries, venules
Metarterioles
– Even smaller arteries
– Some arterioles branch into these
– These have precapillary sphincters:
• Rings of smooth muscle that can be tightened to
shut off blood flow to a capillary bed
• When the sphincter is constricted, blood is
shunted around the capillary bed and into the
venules
• When open, blood flows into the capillary bed
Figure 15-3
Copyright © 2010 Pearson Education, Inc.
Capillaries
Smallest of the blood vessels
Lack smooth muscle, elastic tissue, fibrous tissue
Consist only of one cell layer of endothelium supported
by a basal lamina or basement membrane
Pericytes
– Highly branched contractile cells which can
surround the capillaries
– Can contribute to “tightness” of capillary
• Like blood-brain barrier
Capillaries
Pericytes
– Secrete capillary growth factors
– Can differentiate into new endothelium or into new
smooth muscle cells
– Diabetic retinopathy is characterized by loss of
pericytes in the capillaries in the retina
• Cause: Unknown
Veins and Venules
Smallest venules are similar to capillaries in structure
As venules get bigger, smooth muscle begins to
appear in their walls
As veins progress towards the heart, they become
bigger and bigger in diameter
Largest veins are the superior and inferior vena cava
Veins are more numerous than arteries and are usually
larger in diameter
Veins are closer to the surface than arteries
Angiogenesis
The process by which new blood vessels develop
In children, this is part of normal growth
In adults (see next slide)
Angiogenesis
In adults, angiogenesis occurs
– during wound healing
– during the monthly regrowing of the uterine lining
(after menstruation)
– During endurance training
– Also occurs as part of the growth of a malignant
tumor
– In coronary artery disease, new blood vessels can
sometimes develop spontaneously and go around
a blocked artery
Blood Pressure
Highest in the arteries, lowest in the veins
By the time it gets to the capillary beds, the pressure is
mostly dissipated
Driving force: contraction of left ventricle
– Creates a pressure wave (pulse) which travels
through the blood vessels
Pressure decreases as blood moves through the
circulatory system
– Due to resistance to flow
– And to friction between the blood cells
Figure 15-4, overview
Copyright © 2010 Pearson Education, Inc.
Figure 15-5
Copyright © 2010 Pearson Education, Inc.
Blood Pressure
Systolic pressure
– During ventricular systole (contraction)
– Pressure in aorta averages 120 mm Hg during this
period
Diastolic pressure
– During ventricular diastole (relaxation)
– Pressure in aorta falls to a low of 80 mm Hg during
this period
Pressure units: mm Hg
Millimeters of mercury (Hg)
Blood Pressure
Pulse or pressure wave
– Caused by L. ventricle contraction
– Moves 10 times faster than the blood is moving
– A pulse felt in the arm is the pressure wave slightly
after ventricular contraction (takes a short time to
move over to the arm)
Pulse Pressure
A measure of the strength of the pressure wave
Systolic pressure – Diastolic pressure = Pulse
pressure
In the aorta:
120 mm Hg – 80 mm Hg = 40 mm Hg (pulse pressure)
Figure 15-5
Copyright © 2010 Pearson Education, Inc.
Veins and Blood Flow Back to the Heart
By the time blood reaches the veins, no pressure wave
left
Venous blood flow is steady not pulsatile (like arteries)
Venous blood flow is pushed along by the continuous
movement of blood out of the capillaries and into the
venules
Veins and Blood Flow Back to the Heart
Blood in veins above the heart use gravity to flow
“downhill” to the heart
Blood in veins below the heart must flow “uphill” to get
back to the heart
Venous return to the heart is aided by
– One-way valves in the larger veins (fig. 15-6)
– Skeletal muscle pump (p. 501)
– Respiratory pump (p. 501)
Figure 15-6
Leg veins
with one-way
valves
Copyright © 2010 Pearson Education, Inc.
Skeletal Muscle Pump
Skeletal muscle contractions that squeeze veins,
compressing them and pushing blood closer towards
the heart
In the larger leg veins with one-way valves, once the
blood has been squeezed upward by muscle
contraction, then the valve closes, keeping the blood
from flowing backward due to gravity
During walking, running, etc. this helps move blood
back to the heart
When sitting or standing still, the pump isn't working
Respiratory Pump
Another mechanism for moving blood through the
veins
Respiratory pump is created by breathing in
During inspiration, the chest expands and the
diaphragm moves downward towards the abdomen
This expands the chest (thoracic) cavity and also
creates a subatmospheric pressure within the cavity
Respiratory Pump
This lower pressure in the chest cavity decreases
pressure in the inferior vena cava as it passes through
the thorax
The lower pressure helps draw more blood into the
vena cava from the abdominal veins
During this time period (inspiration), the abdominal
veins are experiencing higher pressure around them
Summary: During inspiration, higher pressure on
abdominal veins and lower pressure on thoracic veins
Arterial Blood Pressure
Since ventricular pressure is difficult to measure, it is
customary to assume that arterial pressure reflects
ventricular pressure
Arterial pressure is usually measured in the brachial
artery
Mean Arterial Pressure (MAP) (p. 517)
– Diastolic pressure plus 1/3 of pulse pressure
– Gives a single value for the driving (pulsatile)
arterial pressure
Mean Arterial Pressure (MAP)
Diastolic pressure plus 1/3 of pulse pressure
MAP = diastolic P + 1/3 (systolic P – diastolic P)
Example: systolic =120 and diastolic = 80
MAP = 80 mm Hg + 1/3 (120-80 mm Hg)
MAP = 93 mm Hg
MAP is always closer to diastolic P since diastole lasts
twice as long as systole
Hypotension
– BP is too low
– The driving force for blood flow isn't dtrong enough
to overcome gravity
– Blood and O2 supply to brain may become
impaired, can cause dizziness
Hypertension
– BP is too high, can become chronically elevated
Hypertension
– BP is too high, can become chronically elevated
– Continued high pressure on the walls of the blood
vessels can cause weakened areas to rupture and
bleed into the surrounding tissue
– Cerebral hemorrhage
• A rupture in the brain
• May cause a stroke
– Rupture in a major artery (e.g. aorta)
• Can cause rapid loss of BP, may quickly be fatal
Measuring BP
Usually measured in the brachial artery
Sphygmomanometer
– Used to measure BP
– Consists of an inflatable cuff and a pressure
gauge. A stethoscope is used to hear the sounds
– The cuff is placed around the upper arm and is
then inflated until it exerts a pressure higher than
the patient's systolic pressure and blood flow is
temporarily cut off
Figure 15-7
Copyright © 2010 Pearson Education, Inc.
Measuring BP
Next, the pressure in the cuff is gradually released
When cuff pressure falls below systolic pressure, then
blood begins to flow again
Korotkoff Sound
– The thumping noise, heard through the
stethoscope
– The noise is caused by the blood squeezing
through the still partially compressed artery
– The sound diasppears once the artery is fully open
again
Figure 15-7
Copyright © 2010 Pearson Education, Inc.
Measuring BP
Systolic Pressure
– Highest pressure in the artery
– The pressure at which the first Korotkoff sound is
heard
Diastolic Pressure
– Lowest pressure in the artery
– The pressure at which the Korotkoff sounds
disappear
Hypertension (High BP)
Average value for BP: 120/80
– Subject to wide variability
• Within a single individual, moment to moment
• Varies from person to person
Hypertension
– At rest, a systolic pressure consistently over 140
– At rest, a diastolic pressure consistently over 90
Current guidelines (as of 2003) recommend that
individuals maintain their BP below 120/80
Prehypertension
Individuals are considered to be prehypertensive if:
Systolic pressure
– Is consistently 120-139
And/or
Diastolic pressure
– Is consistently 80-89
Prehypertension can be treated by providing
counseling on lifestyle modifications and also by
certain medications (diuretics, etc.)