Download CHAP 21a - Dr. Gerry Cronin

Chapter 21
Blood Vessels
and
Hemodynamics
Vessel Structure and Function
• The blood vessels of the body should not be
thought of as mere “pipes” carrying blood –
they are dynamic,
interactive, essential
components of the
cardiovascular
organ system.
Basic components of the CV organ system
Vessel Structure and Function
• Blood Vessel Types
• Arteries – carry blood away from the heart
• Large elastic arteries (>1 cm); medium muscular arteries
(0.1 – 10 mm); arterioles (< 0.1 mm)
• Capillaries – site of nutrient and
gas exchange
• Veins – carry blood towards
the heart
• Venules are small veins (< 0.1 mm)
Vessel Structure and Function
• All blood and lymph vessels in the body share
components of 3 basic layers or “tunics” which
comprise the vessel wall:
• Tunica interna
(intima)
• Tunica media
• Tunica externa
Vessel Structure and Function
• The tunica interna is the inner lining in direct
contact with blood.


– The epithelium of the intima is the same
endothelium that makes up the endocardial lining of
the heart.
– It has an active role in vessel-related activities.
The tunica media is chiefly composed of smooth
muscle that regulates the diameter of the vessel
lumen.
The tunica externa helps anchor vessel to surrounding
tissue through use of elastic and collagen fibers.
Vessel Structure and Function
• The largest arteries are the conducting arteries
(elastic arteries), best exemplified by the garden
hose-sized aorta.
• Their walls are thin compared to their overall size.
• Elastic arteries perform the important function of
storing mechanical energy during ventricular systole
and then transmitting that energy to keep blood
moving after the aortic and pulmonary valves close.
Vessel Structure and Function
Vessel Structure and Function
• Medium sized muscular (distributing) arteries
have more smooth muscle in their tunica media.
• Muscular arteries help maintain the
proper vascular tone to ensure efficient
blood flow to the distal tissue beds.
• Examples include the brachial artery in
the arm and radial artery in the forearm.
Vessel Structure and Function
• An anastomosis is a union of vessels supplying blood
to the same body tissue. Should a blood vessel
become occluded, a vascular anastomosis provides
collateral circulation (an alternative
route) for blood to reach a tissue.
• The shaded area in this graphic
shows overlapping blood
supply to the ascending colon.
Vessel Structure and Function
• Arterioles deliver blood to capillaries and have
the greatest collective influence on both local
blood flow and on overall blood pressure.
• They are the primary "adjustable nozzles” across
which the greatest drop
in pressure occurs.
Vessel Structure and Function
• Capillaries are the only sites in the entire
vasculature where gases, water and
other nutrients are
exchanged.
• Venules and veins have
much thinner walls than
corresponding arterioles
and arteries of similar size.
Vessel Structure and Function
• The terminal end of an arteriole tapers toward
the capillary junction to form a single
metarteriole.
• At the metarteriole-capillary junction, the distal most
muscle cell forms the
precapillary sphincter
which monitors and
regulates blood flow
into the capillary bed.
Vessel Structure and Function
• Capillaries are different from other vascular
structures in that they are made of only a single
endothelial cell sitting on a very thin basement
membrane - there are no other tunics, layers or
muscle.
• The minimalist nature of capillaries allows them to
be freely permeable to
many substances (gases,
fluids, and small ionic
molecules).
Vessel Structure and Function
• The body contains three types of capillaries:
• Continuous capillaries are the most common with
endothelial cells forming a continuous tube,
interrupted only by small intercellular clefts.
• Fenestrated capillaries (fenestra = windows), found
in the kidneys, villi of small intestines, and endocrine
glands are much more porous.
• Sinusoids form very porous channels through which
blood can percolate, e.g., in the liver and spleen.
Vessel Structure
and Function
3 Types of capillaries in the body
Vessel Structure and Function
• Veins have thinner walls, less muscle and elastic
tissue, and are designed to operate at much
lower pressures.
• Intravenous pressure in venules (16 mmHg) is less
than half that of arterioles (35 mmHg), and drops to
just 1-2 mmHg in some larger veins.
• Because intravenous pressure is so low, veins have
valves to keep blood flowing in only 1 direction.
• When exposed to higher than normal pressures, veins can
become incompetent (varicose veins).
Vessel Structure and Function
Fluid Exchange - Starling Forces
• As blood flows to the tissues of the body,
hydrostatic and osmotic forces at the capillaries
determine how much fluid leaves the arterial
end of the capillary and how much is then
reabsorbed at the venous end. These are called
Starling Forces.
• Filtration is the movement of fluid through the walls
of the capillary into the interstitial fluid.
• Reabsorption is the movement of fluid from the
interstitial fluid back into the capillary.
Fluid Exchange - Starling Forces
• Two pressures promote filtration:
• Blood hydrostatic pressure (BHP) generated by the
pumping action of the heart - decreases from 35 to
16 from the arterial to the venous end of the
capillary
• Interstitial fluid osmotic pressure (IFOP), which is
constant at about 1 mmHg
Fluid Exchange - Starling Forces
• Two pressures promote reabsorption:
– Blood colloid osmotic pressure (BCOP) is due to the
presence of plasma proteins too large to cross the
capillary - averages 36 mmHg on both ends.
– Interstitial fluid hydrostatic pressure (IFHP) is
normally close to zero and becomes a significant
factor only in
states of edema.
Fluid Exchange - Starling Forces
Fluid Exchange - Starling Forces
• Normally there is nearly as much fluid
reabsorbed as there is filtered.
• At the arterial end, net pressure is outward at 10
mmHg and fluid leaves the capillary (filtration).
• At the venous end, net pressure is inward at –9
mmHg (reabsorption).
• On average, about 85% of fluid filtered is
reabsorbed.
Fluid Exchange - Starling Forces
• Fluid that is not reabsorbed (about 3L/ day for
the entire body) enters the lymphatic vessels to
be eventually returned to
the blood.
Gas And Nutrient Exchange
• In contrast to the bulk flow of fluids at the
capillaries, the exchange of gases and small
particles (like certain nutrients and wastes) is a
purely passive diffusion process.
• Gases and these other
substances simply
move into or out of
the capillary down their
concentration gradient.
Venous Reserve
• Because systemic veins and venules contain a
large percentage of the blood volume (about
64% at rest), they function as blood reservoirs
from which blood can be diverted quickly if
needed.
• To counteract a drop in BP,
stimulation of the sympathetic
NS will cause venoconstriction,
allowing a greater volume of
blood to flow to skeletal muscles.
Venous Return
• The volume of blood returning through the veins to
the right atrium must be the same amount of blood
pumped into the arteries from the
left ventricle – this is
called the venous return.
• Besides pressure, venous
return is aided by the
presence of venous valves,
a skeletal muscle pump,
and the action of breathing.
Venous Return
• The skeletal muscle pump uses the action of
muscles to milk blood in 1 direction (due to
valves).
• The respiratory pump uses the negative
pressures in the thoracic and abdominal
cavities generated during
inspiration to pull
venous blood towards
the heart.
Proximal
valve
Distal
valve
1
2
3
Venous Return
• Although the venous circulation flows under
much lower pressures than the arterial side,
usually the small pressure differences
(venule 16 mmHg to
right atrium 0 mmHg),
plus the aid of muscle
and respiratory pumps
is sufficient.
Pressure, Flow, And Resistance
• Blood pressure is a measure of the force
(measured in mmHg) exerted in the lumen of
the blood vessels.
• Blood flow is the amount of blood which is
actually reaching the end organs
(tissues of the body).
• Resistance is the sum of
many factors which
oppose the flow of blood.
Pressure, Flow, And Resistance
• Cardiovascular homeostasis is mainly dependent
on blood flow… but blood flow is hard to
measure.
• Clinically, we check blood pressure because it is
easier to measure, and it is related to blood flow.
• The relationship between blood flow, blood
pressure, and peripheral resistance follows a simple
formula called Ohms Law.
BP = Flow x Resistance
Pressure, Flow, And Resistance
• In an effort to meet physiological demands, we
can increase blood flow by:
• Increasing BP
• Decreasing systemic vascular
resistance in the blood vessels
• Usually our body will do both –
when we exercise, for example.
figure adapted from
http://www.learnhemodynamics.com/hemo/basics.htm
Pressure, Flow, And Resistance
• As we have already seen, peripheral resistance
is itself dependent on other factors like the
viscosity of blood, the length of all the blood
vessels in the body (body size), and the
diameter of a vessel.
• The first two of these factors (viscosity and the
length of blood vessels) are unchangeable from
moment to moment.
• The diameter, however, is readily adjusted if the
body needs to change blood flow to a certain
capillary bed.
Pressure, Flow, And Resistance
Pressure, Flow, And Resistance
• Example: If the diameter of a blood vessel
decreases by one-half, its resistance to blood
flow increases 16 times!
• “Hardening of the arteries” (loss of elasticity)
seriously hampers the body’s
ability to increase
blood flow to meet
metabolic demands.
Pressure, Flow, And Resistance
(Interactions Animation)
• Vascular Regulation
You must be connected to the internet to run this animation
Autoregulation
• Homeostasis in the body tissues
requires the cardiovascular
system to adjust pressure and
resistance to maintain adequate
blood flow to vital organs at all
times – a process called
autoregulation.
• Autoregulation is controlled
through negative feedback
loops.
Autoregulation
• Autoregulation of blood pressure and blood flow
is a complex interplay between:
• The vascular system
• The nervous system
• The endocrine hormones and
organs like the adrenal gland
and the kidney
• The heart
Autoregulation
• The vascular system senses alterations of BP and
blood flow and signals the cardiovascular
centers in the brain.
– The heart then appropriately
modifies its rate and force
of contraction.
– Arterioles and the precapillary
sphincters of the metarterioles
adjust resistance at specific
tissue beds.
Autoregulation
• For example, during emergencies, the
autonomic nervous system will vasodilate the
precapillary sphincters of metarterioles in the
skeletal muscles, lungs, and brain, while
constricting the precapillary sphincters found in
tissues such as the skin, GI tract, and kidneys.
• This sends the majority of the cardiac output (blood
flow) to those organs important in a fight or flight
response, while temporarily depriving (through
vasoconstriction) the nonessential organs.