Ch. 21 –
Blood Vessels
and Circulation
The main types of blood vessels
• Arteries – carry blood away from the
heart
– They are higher pressure vessels
– They typically have a rounder lumen on
microscope slides
– They have a relatively thick wall compared to
their lumen size
• Veins – carry blood towards the heart
– They are lower pressure vessels
– They are typically flattened on microscope
slides
– They have a relatively thin wall compared to
their lumen size
• Capillaries – are the microscopic vessels
that are the site of the exchange of gases,
nutrients, wastes, etc. between blood and
interstitial fluid
Fig. 21-2, p. 727
1
The structure of blood vessel walls
•
The walls of most BVs have 3 main layers:
– 1. Tunica intima (tunica interna) = a smooth endothelial lining that reduces
friction for blood flow
– 2. Tunica media = mostly smooth muscle (for vasoconstriction and
vasodilation); it’s thicker in arteries
– 3. Tunica externa (tunica adventitia) = a (mostly) connective tissue sheath
that blends into adjacent tissues, supporting/stabilizing and anchoring the blood
vessel; in veins, it’s often thicker than the tunica media
•
Elastic fibers may be found throughout; they are especially prevalent in
arteries
Fig. 21-1, p. 725
Comparing
the wall
structures
of various
types of
vessels
Fig. 21-2, p. 727
2
•
Are elastic and contractile
– Contractility affects blood flow
through the capillaries, blood pressure (BP), and afterload on the heart
Arteries
• Vasoconstriction = ↓ vessel
diameter
• Vasodilation = ↑ vessel diameter
•
Types:
– Elastic arteries – a.k.a.
conducting arteries – dampen the
dramatic pressure changes that
occur during the cardiac cycle as
they conduct a large volume of
blood away from the heart
– Muscular arteries – a.k.a.
medium-sized or distribution
arteries – distribute blood to skeletal
muscles and internal organs via
vasoconstriction and -dilation
• Most arteries are this type
– Arterioles – a.k.a. resistance
vessels – regulate blood flow to
capillaries in response to local
conditions (e.g. changes in CO2, O2,
and/or pH), sympathetic innervation,
and certain hormones
Fig. 21-2, p. 727
Capillaries
• Connect arterioles to
venules
• Have no tunica media or
tunica externa
Fig. 21-2, p. 727
– They have an
endothelium and a thin
underlying basement
membrane only
• Function: exchange (via
diffusion or active
transport) of solutes
between the blood and
interstitial fluid
– They have thin walls, low
blood velocity, and a large
total surface area, all of
which ↑ the rate of the
exchange
Fig. 21-4b, p. 731
3
Types of
capillaries
•
Fig. 21-3, p. 730
Continuous
capillaries – have
a complete (i.e.
continuous) endothelial lining
– They have openings
between the endothelial cells only (so
they have relatively
↓ permeability)
– They are found in
most regions of the
body
•
Fenestrated
capillaries
(fenestra = “window”)
– They have fenestrations (pores) through the endothelial cells (so they have ↑ permeability
than continuous capillaries)
– They are found in choroid plexuses, some endocrine glands, the intestinal tract, and
kidneys (glomeruli)
•
Sinusoids
– Also have fenestrations, but in addition, their basement membrane is thin or absent, with
gaps between endothelial cells (so they have even more ↑ permeability than fenestrated
capillaries)
– They are found in the liver, bone marrow, spleen, and some endocrine glands
Capillary beds (plexuses or networks)
•
•
Their entrances are regulated
by precapillary sphincters
Anastomosis = a connection
between two vessels
– Arterial anastomosis = an
alternative route to the same
capillary bed
• It is often formed by the
fusion of collateral arteries
– Arteriovenous
anastomosis – bypasses a
particular capillary bed
•
Capillary blood flow is
variable due to vasomotion:
– The constriction and dilation
of precapillary sphincters,
metarterioles, and
arteriovenous anastomoses
•
Autoregulation
– Local conditions (e.g. O2,
pH) mainly determine which
and how many capillaries are
open within a capillary bed
– More on this later
Fig. 21-4a, p. 731
4
•
Types:
– Venules – collect blood from capillary beds
– Medium-sized veins
– Large veins – e.g. the SVC and IVC
•
Veins
Fig. 21-2, p. 727
Because BP is very low in veins, and often
blood flow through veins must fight gravity,
venous return to the heart is assisted by:
– Muscular compression – blood is forced
towards the heart, and can’t flow
backwards due to the presence of one-way
valves (folds of tunica intima) in the limbs
– The
respiratory
pump –
inhalation
↓ pressure
in the
thoracic
cavity,
pulling
blood from
the abdomen into
the IVC
and R. atrium
Fig. 21-5 p. 733
Where is
your blood?
•
•
Veins are
a.k.a. capacitance vessels, because
they act as a
blood reservoir
If needed (e.g.
during serious
hemorrhaging),
venoconstriction
can shift some of
the blood from the
venous system
(called the venous
reserve) to the arteries
and capillaries (and thus
vital organs)
Fig. 21-6, p. 733
5
= total F
Cardiovascular
physiology
• The goal: to
maintain adequate
blood flow (F)
through the
capillaries of
tissues/organs in
order for exchange
to take place
F  ΔP
R
ΔP = the difference in
pressure at the opposite
ends of a vessel
P must > R!
R = resistance to flow
Resistance
•
Total peripheral resistance = the
resistance to blood flow across the entire
CV system, which is influenced by…
– Vascular resistance is due to the friction
between the blood and the vessel walls
• It is  vessel length
• It is inversely  vessel diameter (R  1/r4)
– Differences in vessel diameter have a much
more significant effect on resistance than
differences in vessel length
– Vessel diameter is highly variable due to
vasoconstriction and vasodilation
– Viscosity (not shown here) – blood
viscosity is > water viscosity due to the
presence of plasma proteins and blood cells
– Turbulence = eddies and swirls due to high
flow rates or rough endothelial surfaces
(e.g.
due to
endothelial
injury
and/or
plaques)
Fig. 21-7 p. 735
6
Pressure
• Blood pressure is a hydrostatic pressure (= the force exerted
by a fluid pressing against the wall of its container)
– E.g. the heart pumps blood, applying hydrostatic pressure on the walls
of blood vessels
• Remember: fluids (such as blood) move from areas of
higher pressure to areas of lower pressure
• 3 important CV pressures:
– 1. Blood pressure (BP) = systemic arterial pressure (average range
≈ 120-35 mm Hg)
– 2. Capillary hydrostatic pressure (CHP) – a.k.a. capillary pressure
(range ≈ 35-18 mm Hg)
– 3. Venous pressure = the pressure difference between the systemic
venules and the R. atrium (≈ 18-2 mm Hg)
• Overall total circulatory pressure = the ΔP across the entire
systemic circuit = the difference between the pressure at the
base of the ascending aorta and the pressure in the R. atrium
– Circulatory pressure must > total peripheral resistance for blood flow
to occur
Relationships among vessel diameter, total
cross-sectional area, BP, and blood velocity
Fig. 21-8, p. 737
7
•
•
•
•
Maintains blood flow through
the capillary beds
Systolic pressure = the
maximum BP generated during
ventricular contraction
Diastolic pressure = the
minimum BP at the end of
ventricular relaxation
Pulse pressure =
systolic - diastolic
Arterial blood pressure
Fig. 21-8 p. 737
– This disappears
due to diminishing
elastic rebound as blood
travels through the arteries
•
Mean arterial pressure
(MAP) = a weighted average
BP
– It is calculated as diastolic
pressure + 1/3 pulse
pressure
•
BP is typically reported as:
systolic pressure
diastolic pressure
e.g. 120 mm Hg
80 mm Hg
Capillary exchange
• = the movement of fluids and solutes across capillary walls
• Depends upon the processes of:
– Diffusion = the net movement of ions/molecules from an area where their
concentration is ↑ to an area where their concentration is ↓
• Diffusion may occur through the gaps between endothelial cells (e.g. water,
ions, glucose, amino acids), through membrane channels in the endothelial
plasma (cell) membranes (e.g. water,
ions), through fenestrations (e.g. peptides, larger carbohydrates), via simple diffusion through the endothelial
plasma membranes (e.g. fatty acids,
steroids, O2, CO2), and/or through the
larger gaps between the endothelial
cells of sinusoids (e.g. plasma proteins made by the liver)
– Filtration = the movement of water and
solutes across a porous membrane in response to hydrostatic pressure (shown here)
• Which solutes pass through and which
are “filtered out” of the solution depends
on the solute size compared to the gap
(or fenestration) size
– Reabsorption – see the next slide
Fig. 21-10, p. 739
8
Reabsorption (and how it interacts with filtration)
•
•
Reabsorption is due to osmosis (remember, osmosis is the movement of water
across a selectively permeable membrane in response to osmotic pressure (OP),
which is proportional to the number of particles in solution, or solute concentration)
So…
– Filtration (due to hydrostatic pressure) = water and solutes leave the capillary (OUT)
– Reabsorption (due to osmotic pressure) = water re-enters the capillary (IN)
•
Forces that affect capillary filtration and reabsorption:
– Capillary Hydrostatic Pressure (CHP) – i.e., the BP in the capillaries
• It’s ~ 35 mm Hg at the arterial end of the capillary, and ~ 18 mm Hg at the venous end of the
capillary
– Interstitial fluid Hydrostatic Pressure (IHP)
• It’s small: assume 0 mm Hg (it is ignored)
– Blood Colloid Osmotic Pressure (BCOP) – is due mostly to the plasma proteins
(mainly albumins) that are too big to leave the capillary
• It’s ~ 25 mm Hg
– Interstitial fluid Colloid Osmotic Pressure (ICOP) – is due to the small number of
proteins in the interstitial fluid that are too big to enter the capillary
• It’s small: assume 0 mm Hg (it is ignored)
NFP (net filtration pressure) = (net hydrostatic pressure) - (net colloid osmotic
pressure), so…
At the arterial end: NFP = (CHP - IHP) - (BCOP - ICOP)
= (35 - 0) - (25 - 0) = 10 mm Hg (i.e. OUT)
At the venous end: NFP = (CHP - IHP) - (BCOP - ICOP)
= (18 - 0) - (25 - 0) = -7 mm Hg (i.e. IN)
Filtration and reabsorption
•
Note that
slightly more
fluid leaves
the capillaries
and escapes
into the
tissues than
re-enters the
capillaries
from the
tissues
–
The lost
fluid is
returned to
the blood
circulation
by the
lymphatic
system
Fig. 21-11, p. 740
9
The importance of the continuous
movement of fluid out of and back into the
capillaries, and lymphatic reabsorption
• 1. It allows constant exchange and communication between
the blood plasma and interstitial fluid
• 2. It speeds the distribution of nutrients, hormones, and
gases into the tissues
• 3. It helps with the transport of insoluble lipids and tissue
proteins into the bloodstream (via the lymphatic system)
• 4. It flushes the tissues (carrying bacteria, toxins, and
chemicals to the lymph nodes), assisting the body’s defense
systems
• Edema = an abnormal accumulation of interstitial fluid
– Some causes: ↑ CHP (e.g. hypertension), ↓ BCOP (e.g. capillary
damage or ↑ permeability, causing a loss of plasma proteins to the
interstitial fluid), lymphatic blockage
Cardiovascular regulation
• Tissue perfusion (= blood flow through the
tissues) depends upon 3 interrelated factors:
– 1. Cardiac output
– 2. Peripheral resistance
– 3. Blood pressure
• How are they related? BP = CO x PR
• There are 3 control mechanisms that help
maintain sufficient tissue perfusion:
– 1. Autoregulation
– 2. Neural mechanisms
– 3. Endocrine (hormonal) mechanisms
10
Overview: responses to
↓ BP and blood flow
Fig. 21-12, p. 743
Autoregulation and neural mechanisms
• Autoregulation acts locally on precapillary sphincters…
– Local vasodilators include ↓ O2, ↑ CO2, ↓ pH, nitric oxide (NO),
histamine (inflammation), and ↑ local temperature
– Local vasoconstrictors include (e.g. during hemostasis) endothelins,
prostaglandins, and thromboxanes
• Neural mechanisms
– Neural control of cardiovascular function is mainly via the cardiovascular (CV) center in the medulla oblongata, which includes the:
• Cardiac centers – see Ch. 20
• Vasomotor center – which includes separate groups of neurons that cause
vasoconstriction and vasodilation
– The vasoconstrictors are always active, producing a background level of
vasomotor tone
» Remember, small adjustments in vessel diameter can cause large changes in
peripheral resistance and thus BP and blood flow
– There is significant reflex control of cardiovascular function:
• Baroreceptor reflexes respond to Δ BP, which is detected in the carotid
sinuses, aortic sinuses, and R. atrial wall
• Chemoreceptor reflexes respond to Δ pH, Δ O2, and Δ CO2 in the blood
and CSF, which are detected in the carotid bodies, aortic bodies, and
medulla oblongata (for CSF)
11
↓ HR
↓ PR
Baroreceptor
reflexes
Remember: BP = CO x PR
and CO = HR x SV
↑ PR
↑ HR
Fig. 21-13, p. 745
Chemoreceptor
reflexes
↑ HR
↑ PR
Remember: BP = CO x PR
and CO = HR x SV
Fig. 21-14, p. 747
12
Hormonal responses to ↑ BP
and ↑ blood volume
Fig. 21-15a, p. 748
Fig. 21-15b, p. 748
13
Hormonal responses to ↓ BP
and ↓ blood volume
CV responses to hemorrhaging and blood loss
•
Also, ↓ CHP → ↑ capillary
reabsorption (“recall of
fluids”)
•
•
These responses
occur if hemostasis
(see Ch. 19) is not
completely
successful
A body can usually
cope with a loss of
up to 20% of total
blood volume
If these responses
fail, hypovolemic
shock
(= inadequate
tissue perfusion)
occurs
Fig. 21-16, p. 751
Table 21-2, p. 749
FYI
Exercise and
the CV
system
Table 21-3, p. 750
14