CHAPTER 21 BLOOD VESSELS
Blood vessels are a series of branching
tubes that carry blood from heart to tissues and back to the heart.
1. Arteries--carry blood AWAY from
heart to tissues
a. Large elastic conducting arteries
b. Medium muscular distributing arteries
c. Small arteries
2. Arterioles
3. Capillaries--in tissues, these are
where exchange takes place
4. Venules--smallest
veins, formed as capillaries unite
5. Veins--carry blood from tissues back
to the heart
The lumen is the hollow center of a
blood vessel (or any hollow organ).
A.
ARTERIES--the wall is
in 3 layers:
a. Lining of endothelium—lines all blood vessels and the heart. This is
the ONLY
tissue blood should come in contact
with
b. Basement membrane
c. Internal elastic lamina--layer of elastic tissue
2. Tunica media--middle coat--this is the thickest layer and consists of
elastic fibers and smooth muscle
3. Tunica adventitia or externa--outer
coat--composed of elastic and collagen fibers
Due mostly to the structure of the
tunica media, arteries have 2 essential properties:
1. Elasticity--as blood is forced from the heart into large arteries,
the elastic tissue in the wall first stretches without tearing and then snaps
back (elastic recoil) to push the blood onward. This helps keep the blood
moving while the ventricles are relaxed.
2. Contractility--smooth muscle in the wall can contract to narrow the
lumen (vasoconstriction). The opposite effect, vasodilation,
occurs when the smooth muscle relaxes. Sympathetic nerves control the muscle in the walls of blood
vessels, but these smooth muscle fibers can also change their degree of contraction
due to local factors.
The largest arteries are elastic
(conducting) arteries. Walls are relatively thin and tunica media contains
large numbers of elastic fibers and relatively little smooth muscle. Their
elastic fibers stretch & then recoil, pushing blood onwards while the
ventricles are relaxed. Includes:
Aorta
Subclavian Pulmonary
Brachiocephalic Vertebral
Common carotid
Common iliac
Medium-sized arteries branch off the elastic
conducting arteries and are muscular (distributing) arteries. Their walls are
relatively thick and contain more smooth muscle and less elastic fibers. They
are capable of greater vasoconstriction. Includes:
Axillary Mesenteric
Brachial
Femoral
Radial Popliteal
Intercostal Tibial
Splenic
B.
ARTERIOLES are very
small arteries that deliver blood to capillaries. As arterioles branch off an
artery, they have smooth muscle and a few elastic fibers in the tunica media.
These gradually taper away as the arteriole becomes smaller, leaving mostly the
endothelium and a few smooth muscle fibers by the time the arteriole connects
to the capillaries.
Arterioles play a key role in
regulating blood flow into capillaries. Vasoconstriction of arterioles
decreases blood flow into capillaries; vasodilation
increases flow. A change in the diameter of a large number of arterioles at
once will also affect blood pressure.
C.
CAPILLARIES--microscopic
vessels that connect arterioles and venules. They are
found near almost every cell in the body, but are more plentiful in tissues
with high metabolic activity (liver, kidneys, muscles, CNS). A few tissues lack
capillaries (sheets of covering and lining epithelium such as epidermis
and epithelial linings of viscera,
cornea, lens, cartilage).
Capillaries permit the exchange of
nutrients and wastes between blood and tissue cells. The walls consist of a
single layer of endothelial cells and a basement membrane, so substances must
cross only one cell layer. Walls of most other blood vessels are too thick to
allow exchange in the time available as the blood flows through them.
(Remember, blood is always moving.) Some exchange does occur across the walls
of the very first part of venules.
Most tissues contain many capillaries,
often in the form of extensive branching networks, to increase the surface area
for diffusion and allow the rapid exchange of materials. However, all
capillaries in the network (also known as the capillary bed) may not be filled
with flowing blood at all times. When
metabolic needs of the tissue are low, blood flows through only a portion of
the network. More of the network fills with blood as the tissue becomes active.
This is made possible by:
1. Metarterioles--special
vessels with smooth muscle fibers in their walls. They emerge from an arteriole
and enter the capillary network. The distal portion of the metarteriole,
known as a thoroughfare channel, will empty into a venule.
Metarterioles open when blood flow through the
capillaries of the network needs to be reduced. With the metarteriole
open, a relatively small amount of blood still flows through its capillaries
and most of the blood passes straight through to the venule.
When the tissue needs the maximum amount of blood, most of the metarterioles constrict so that more blood flows out into
the entire capillary network.
2. Precapillary
sphincters--capillaries in networks have rings of smooth muscle at their
origin. These sphincters open (relax) when the tissue is active to allow more
blood to enter and close (contract) when
less blood is needed.
Remember, precapillary
sphincters relax when metarteriole muscle contracts. Precapillary sphincters contract when metarteriole
muscle relaxes. In a process called vasomotion, there
is alternation between precapillary sphincter
contraction and metarteriole muscle contraction in an
area, so that blood flow through the capillary network is intermittent. Unless
there is great demand, blood is usually flowing through only about 25% of the
capillaries in an area at one time.
1. Continuous capillaries—plasma
membranes of endothelial cells form
tubes interrupted only by intercellular clefts—gaps between endothelial
cells.
Skeletal muscle
Smooth muscle
Connective tissue
Lungs
2. Fenestrated capillaries—plasma membranes
have tiny openings called fenestrations or pores. The size of these pores
varies.
Kidney
Small intestine
Choroid
plexuses of brain ventricles
Ciliary
processes of eyes
Endocrine glands
3. Sinusoids—these capillaries are
wider and more twisting than regular capillaries. Their endothelial cells
contain relatively large intercellular clefts and also very large fenestrations
(pores). The basement membrane may be incomplete
or absent. Walls of sinusoids may contain phagocytic cells which remove
bacteria, worn-out blood cells, and debris from the blood as it flows through.
Liver Spleen
Anterior pituitary Parathyroids
Red bone marrow
D.
VENULES are formed
when capillaries unite. The walls of the smallest venules
are thin with no tunica externa—this is the place
where phagocytic WBC often leave the blood to migrate to trouble spots. As venules become larger, they begin to develop a thicker wall
with all 3 layers. Larger venules unite to form
veins.
E.
VEINS are composed of
the same 3 coats as arteries but characteristics vary:
1. Tunica interna (intima)
is thinner with no internal elastic lamina
2. Tunica media is much thinner, with only very small numbers of smooth
muscle
and elastic fibers. Although the amount of smooth muscle present is
small, it
is enough to allow veins to alter their diameter to some extent.
3. Tunica externa (adventitia)
is thicker. This is the thickest layer of a vein wall
and contains more fibrous CT (collagen and elastic fibers).
Compared to arteries that carry similar
amounts of blood, vein walls are thinner and softer and the lumen is larger in
diameter.
In certain locations of the body, veins
are modified to form venous (vascular) sinuses. The walls consist of
endothelium supported by surrounding dense CT. There is no tunica media or
tunica externa and no smooth muscle. Venous sinuses
are found in the brain and heart.
Blood in veins has a much lower
pressure than blood in arteries. Most veins contain valves, which are flaps
(cusps) from the tunica intima, to maintain flow in
the proper direction. Varicose veins occur when valves leak. Blood pools in
that part of the vein and stretches the wall. This is most frequently seen in
leg veins, where gravity works against the flow of blood in veins, but can
occur in any vein.
Most tissues receive blood from more
than one artery. The union of the branches of 2 or more arteries supplying the
same body region is an anastomosis. Anastomoses provide alternate routes for blood to reach a
tissue or organ--this is called collateral circulation. Arteries that do not anastomose are called end arteries.
At rest the blood is distributed as
follows:
64% in systemic veins & venules
7% in systemic capillaries
13% in systemic arteries & arterioles
7% in heart
9% in pulmonary (lung) vessels
Systemic veins and venules
are known as blood reservoirs. A good bit of the blood contained in them can be
quickly diverted to other vessels if needed. The veins constrict and the blood
flows to where it is needed:
Exercise—skeletal muscle
Hemorrhage—to circulation
Major blood reservoirs are the veins of
the liver, spleen, and skin.
Only the blood in capillaries is
involved in exchanging material with tissue cells. Substances needed by cells
move through the endothelial walls, into interstitial fluid, and then into
tissue cells. Wastes move in the opposite direction. Two things normally do not
leave the blood—large proteins and formed elements. Movement in and out of
capillaries occurs by these methods:
1. Diffusion—always from greater concentration to lesser concentration.
a. Simple diffusion—this is the single most important method by which
substances enter and leave capillaries. Simple diffusion can occur across
endothelial cell plasma membranes, or through intercellular clefts and
fenestrations.
b. Facilitated diffusion—some substances also move through the
endothelial plasma membrane this way.
Substances which diffuse include:
Oxygen Amino acids
Carbon dioxide Steroid hormones
Glucose
In most capillaries, almost all plasma
solutes diffuse EXCEPT large proteins. In sinusoids, intercellular clefts are
very large so that even proteins & blood cells can enter the blood.
Remember the blood-brain barrier (BBB).
Brain capillaries do not have pores or intercellular clefts. Their endothelial
cells are connected by tight junctions. Special facilitated diffusion
mechanisms move some nutrients and other substances into brain tissue. Some
larger molecules can't enter at all.
2. Transcytosis—this is a form of pinocytosis (vesicular transport). It is uncommon (not may
things are moved this way). A few large nonlipid-soluble
molecules are taken in by endocytosis on one side of the endothelial cells,
pass through the cell, and leave on the other side by exocytosis.
Insulin enters blood
Antibodies crossing placenta
3. Bulk flow—passive process in which large numbers of ions, molecules,
or particles which are dissolved or suspended in fluid move together in the
same direction. Pressure causes the movement, which is from an area of higher
pressure to an area of lower pressure. The movement is faster than diffusion.
Bulk flow moves fluids and solutes both
ways, both in and out of capillaries.
filtration—movement out of
capillaries into tissue spaces
reabsorption—movement from
tissue spaces back into capillaries
Bulk flow:
1. Helps deliver oxygen, nutrients, ions, etc. to cells
2. Helps remove carbon dioxide, wastes, secretions, etc. from cells
3. Helps regulate the relative volumes of blood and interstitial fluid
(this is the most important function)
First, remember that wherever proteins
are found, they have a strong tendency to draw fluid to themselves and hold
it—this is called colloid osmotic pressure.
Forces encouraging filtration
(these forces predominate at the arterial end of a capillary):
**1. Blood
hydrostatic pressure (BHP) or just plain blood pressure
2. Interstitial fluid osmotic pressure—this is normally a very small
factor, since normal interstitial fluid contains very little protein
Forces encouraging reabsorption (these forces predominate at the venous end
of a capillary):
**1. Blood colloid osmotic pressure—as fluid and other solutes leave,
the remaining blood proteins become concentrated, giving a strong pull for
fluid to return to the capillary.
2. Interstitial fluid hydrostatic pressure—this would push fluid back
into the capillary, but since this pressure is normally very low (often zero)
this is a very minor factor.
volume of fluid filtered. This is known
as Starling’s Law of the Capillaries. In fact, reabsorption
does not quite equal filtration, so the remaining 10 - 15% of the filtered
fluid that is not reabsorbed into blood capillaries must be picked up and
returned to the bloodstream by the lymphatic system (about 3liters/day).
If filtration greatly exceeds reabsorption, the result is an accumulation of fluid in
tissue spaces, which is known as edema. Edema can be caused by:
v
Excess
filtration:
·
Increased
blood pressure causes greater volumes to be filtered
·
Increased
permeability of capillaries allows excess filtration of fluid and also the
escape of plasma proteins into tissue spaces
v
Inadequate
reabsorption:
·
Decreased
plasma protein levels due to malnutrition, burns, kidney disease, etc. lead to
decreased blood colloid osmotic pressure
v
Blockage
of normal lymphatic flow will also lead to edema
HEMODYNAMICS—FACTORS AFFECTING BLOOD FLOW TO TISSUES
VOLUME OF BLOOD FLOW (This
does not refer to the total blood volume---it refers to the amount of blood
moving through tissues.)
Cardiac output is the volume of blood
that circulates through the systemic or pulmonary blood vessels each minute.
Cardiac output equals stroke volume
times
rate. The
2 additional factors that influence how the cardiac output ecomes
distriuted to the various tissues are Blood pressure and Resistance.
BLOOD PRESSURE—force exerted by blood on blood vessel
walls. Blood pressure is generated by contraction of the ventricles and is
greatest in the aorta—about 110/70 in a healthy young adult at rest. Mean
arterial blood pressure is the average pressure and is calculated as follows:
Diastolic BP + 1/3
(Systolic BP - Diastolic BP) =
MABP
70 + 1/3 (110 – 70) =
83
If resistance does not change, increased cardiac output means
increased MABP, and decreased cardiac output
causes decreased MABP. Blood in the cardiovascular system always flows
from greater pressure to lesser pressure. Blood pressure falls steadily as
blood flows through the aorta to large arteries, medium arteries, small arteries, arterioles, capillaries, venules, and veins. The blood vessels with the lowest
pressure are the venae cavae,
and pressure is near zero in the right atrium.
There MUST be a pressure gradient for blood flow to continue.
RESISTANCE—opposition to blood flow, mainly due
to friction between blood and the walls of blood vessels. Factors:
1. Size of the lumen of blood vessels---this is the one that we adjust
and change most readily. Other factors in resistance usually are fairly
constant, or change gradually. Arterioles can change their size the
most---minimum size is about 1/3 of the maximum. The smaller the lumen of a
blood vessel, the greater the resistance to blood flow.
a. Vasoconstriction---smooth muscle in the blood vessel wall contracts
and the lumen becomes smaller. This increases resistance.
b. Vasodilation---smooth muscle relaxes and
lumen enlarges. This decreases resistance.
2. Blood viscosity—“thickness” of blood. Depends mostly on number of RBC
compared to the volume of plasma. To a smaller extent, it also depends on the
concentration of plasma proteins. The greater the viscosity, the greater the
resistance.
a. Causes of increased viscosity: Dehydration (which decreases volume of
plasma), production of unusually large numbers of RBC (polycythemia).
If viscosity increases, blood pressure must increase to maintain flow.
b. Causes of decreased viscosity: Decreased plasma protein levels,
decreased RBC due to anemia or hemorrhage. This “thin” blood flows more easily,
so blood pressure can drop and still deliver the same flow of blood to
tissues.
3. Total blood vessel length—the greater the length, the greater the
resistance, so the longer the total length of all the body’s blood vessels, the
higher the blood pressure must be to push the blood through the all the vessels
and back to the heart. Obesity causes increased resistance as the total length
of the body’s blood vessels increases to supply the adipose tissue (almost 200
extra miles of blood vessels for each pound of fat).
SVR—Systemic vascular resistance OR
TPR—Total peripheral resistance
Both of these terms refer to all the
vascular resistance offered by all the systemic blood vessels. By vasodilation and vasoconstriction, arterioles (and other
vessels to a lesser extent) change the SVR and therefore the blood pressure.
The center for regulation of SVR is the vasomotor center of the medulla.
Blood flow in systemic veins is due to
a pressure differential, with steadily lower pressures as the blood gets nearer
the heart. The movement of skeletal muscles and respiratory muscles aids in
venous return, by alternately squeezing and relaxing around the veins. Once
blood has moved forward in a vein, valves prevent backflow.
VELOCITY (SPEED) OF
BLOOD FLOW—measured in centimeters per second.
Velocity in blood vessels is faster where the total cross-sectional area is
smaller and slower where the total cross-sectional area is greater. The
cross-sectional area in the above statement refers to the total cross-sectional
area of ALL blood vessels of the type added together, not the size of any one
single vessel. See chart (you do NOT have to memorize these values!)
|
TYPE OF VESSEL |
CROSS-SECTIONAL AREA (SQ
CM) |
AVERAGE VELOCITY (CM/SEC) |
|
AORTA |
3 - 5 |
40 |
|
CAPILLARIES |
4500 - 6000 |
0.1 |
|
14 |
As seen above, velocity is fastest in
the aorta and slowest in the capillaries, which allows the maximum time for
capillary exchange to occur. The total time in a capillary is still not
great—about 1-2 seconds on average. Speed picks up in veins as the blood flows
back toward the heart, but never reaches maximum velocity.
Circulation time is the time required
for a drop of blood to pass from the right atrium, through the pulmonary
circulation, back to the left atrium, through the systemic circulation to the
foot, and back to the right atrium. At rest, this should be about one minute.
Negative feedback systems regulating
several functions work together to maintain blood pressure:
Heart rate
Stroke volume
Systemic vascular resistance (diameter of blood vessels)
Blood volume
Keep in mind:
Increased blood volume, increased cardiac output & vasoconstriction
cause INCREASED BP
Decreased blood volume, decreased cardiac output & vasodilation cause DECREASED BP
1. Cardiovascular center--groups of
neurons in the medulla oblongata regulate heart rate, force of contraction and
blood vessel diameter. The CV center receives input from sensory receptors and
higher brain regions. This includes input from:
Baroreceptors which monitor pressure in
certain blood vessels and the atria
Chemoreceptors which monitor blood pH, CO2
level, and O2 level
Proprioceptors which monitor movement of
joints & muscles
Cerebral cortex, limbic system, hypothalamus
After evaluating all this information,
one or more of these areas within the CV center become active:
a. Cardiostimulatory center—cardiac
accelerator nerves carry sympathetic impulses to the heart, causing an increase
in rate and force
b. Cardioinhibitory center--sends
parasympathetic impulses to the heart on the vagus
nerve to decrease heart rate
c. Vasomotor center--vasomotor nerves carry sympathetic impulses only
(no parasympathetic) to smooth muscle in the walls of blood vessels. These
affect all larger vessels but arterioles most of all. A moderate amount of
vasoconstriction is maintained at all times (vasomotor tone). Increased
sympathetic impulses cause vasoconstriction in most small arteries and
arterioles, raising the BP. In skeletal muscle and the heart these same
impulses cause vasodilation (remember
fight-or-flight). In veins, sympathetic stimulation causes constriction that
moves blood from the reservoirs into the circulation. Even though some vessels
dilate, more constrict, so the net response is a rise in BP.
|
|
Blood vessels in: Skeletal muscle Heart Lungs |
Blood vessels in: Skin Viscera (such as digestive organs) |
Overall effect |
Blood pressure |
|
Medium sympathetic stimulation |
Medium diameter |
Medium diameter |
Vasomotor tone |
Normal |
|
Increased sympathetic stimulation (such as
in physical activity) |
Dilate |
Constrict |
Vasoconstriction |
Rises |
|
Decreased sympathetic stimulation
(“Housekeeping”) |
Constrict |
Dilate |
Vasodilation |
Drops |
2. Neural regulation
a. Baroreceptor reflexes—baroreceptors
are nerve cells that detect changes in pressure, located in the walls of
arteries, veins and the right atrium. They act as receptors for these negative
feedback systems:
1) Carotid sinus reflex--purpose is to maintain sufficient blood flow to
the brain. Baroreceptors located in the carotid
sinuses, widenings of the proximal internal carotid
arteries, detect a drop in BP to the brain and send impulses to the CV center
of the medulla. These result in a quick increase in sympathetic impulses to
raise BP by increasing both rate and force of heartbeat and by
vasoconstriction. Although this reflex may be needed at various times, it is
especially important when we suddenly sit or stand up from a reclining
position. If the reflex does not operate smoothly, we have a brief feeling of
"blacking out."
Baroreceptors can also report an increase in
pressure. In this case, the cardiovascular center responds by causing a
decrease in heart rate and force and vasodilation.
2) Aortic reflex--baroreceptors in the aorta
are concerned with general systemic blood pressure.
b. Chemoreceptor reflexes—chemoreceptors
are nerve cells that monitor the chemical content of the blood. They are
located in the carotid bodies and the aortic bodies and detect changes in the
blood levels of O2, CO2, and blood pH. If O2
or pH drop or if CO2 rises, these chemoreceptors
send impulses that result in sympathetic stimulation to arterioles and veins.
(Rate of breathing is also adjusted.)
3. Hormonal regulation
a. Renin-angiotensin-aldosterone (RAA)
system—when blood flow through the kidney decreases, special cells there
secrete an enzyme, renin. This begins a series of
steps that result in the production of the hormone angiotensin
II, which raises BP in two ways:
1) Causes vasoconstriction (fast)
2) Stimulates secretion of aldosterone, which
increases reabsorption of sodium and water in the
kidney and therefore raises blood volume (slower)
b. Epinephrine and norepinephrine (adrenal
medulla) increase cardiac output and cause vasoconstriction of abdominal and cutaneous vessels; vasodilation
of vessels of heart and skeletal muscle.
c. Antidiuretic hormone (ADH)--causes
vasoconstriction (also known as vasopressin). If ADH
release is inhibited, vasodilation results.
d. Atrial natriuretic
peptide--produced by cells in the atria. It lowers BP by causing vasodilation and promoting loss of salt and water in the
urine, lowering blood volume.
In addition to changes in the systemic
blood pressure, blood flow in a particular area of the body can also be regulated.
This is known as autoregulation.
a. Physical changes such as warming or cooling a particular body
part--warming causes vasodilation, cooling causes
vasoconstriction
b. Chemical changes such as low oxygen levels, high CO2
levels, stretch of tissue and others can cause cells in the area to release
chemicals called vasoactive factors. These influence
diameter of blood vessels just in the affected area.
The pulse is a pressure wave that
travels through arteries. It is created by the alternate expansion and recoil
of the elastic arteries after each left ventricular systole. It is best felt in
arteries located near the surface of the body over a bone or other firm tissue.
Radial artery at wrist
Temporal artery lateral to orbit
Common carotid artery lateral to larynx
Normal resting pulse 70 - 80 beats per
minute.
Tachycardia—resting rate over 100
Bradycardia—rate under 60
Blood pressure is measured with a
sphygmomanometer, usually in the brachial artery of the arm. Remember the
definition, BP is the force exerted by the blood on the inner walls of the
heart and blood vessels.
Systolic BP (higher)--force following ventricular systole
Diastolic BP (lower)--force during ventricular relaxation
In an average healthy young adult at
rest, the BP might be 110/70.
Pulse pressure is the difference
between systolic and diastolic pressures—for the 110/70 person it would be 40.
Systolic/diastolic/pulse pressures should have approximately a 3 : 2 : 1 ratio.
SHOCK
Shock
is the failure of the cardiovascular system to deliver adequate amounts of
oxygen and nutrients to the tissues (inadequate tissue perfusion). Cells try to
go on, using anaerobic means of energy production. This results in accumulation
of lactic acid and further damage.
1.
Hypovolemic shock---decreased blood volume due to
loss of blood
(hemorrhage) or fluid loss from vomiting/diarrhea, dehydration, etc.
2.
Cardiogenic shock---inadequate heart function---myocardial
infarction, heart failure, severe dysfuntion of
valves
3.
Vascular shock--widespread inappropriate vasodilation
a. Anaphylactic shock—severe allergic
reaction in which very large amounts of histamine are released
b. Neurogenic
shock—damage to cardiovascular center of the brain
c. Septic shock—response to bacterial
toxins
4.
Obstructive shock---blockage of blood flow at any site in the CV
system---pulmonary embolism, cardiac tamponade, etc.
I.
COMPENSATED SHOCK (NONPROGRESSIVE
SHOCK)---minimal symptoms and the CV
system is able to compensate---loss of up to 10% total blood volume.
Negative feedback systems restore homeostasis:
1. Decreased blood flow to the kidneys
causes activation of the renin-angiotensin-aldosterone
system. Angiotensin II, a powerful natural vasopressor substance, causes vasoconstriction and the
secretion of aldosterone
2. Secretion of ADH, which also causes
vasoconstriction and increases reabsorption of water
by the kidneys.
3. Baroreceptors
report lowered BP and cause activation of sympathetic impulses which cause:
a. Vasoconstriction of arterioles, esp.
to skin and abdominal viscera
b. Increased in heart rate and force of
contraction
c. Secretion of epinephrine/norepinephrine, which intensify the effects of a & b
4. In response to hypoxia, affected cells
produce local vasodilation and help restore oxygen to
affected tissues. This may help a certain tissue, but overall it may interfere
with efforts to raise systemic BP.
II.
DECOMPENSATED SHOCK (PROGRESSIVE SHOCK )---shock becomes steadily worse as
compensatory mechanisms cannot keep up---occurs with loss of 10-20% blood
volume---positive feedback cycles arise:
1. Depression of cardiac activity due to
hypoxia of cardiac muscle---if systolic BP falls below 60, insufficient oxygen
reaches the heart muscle---this causes the heart to pump with less force. This
causes further hypoxia
2. Depression of vasoconstriction occurs as
the vasomotor center becomes depressed by lack of oxygen
3. Increased permeability of capillaries
causes a further drop in blood volume as fluid leaks out
4. Intravascular
(inappropriate) clotting due to sluggish circulation
5. Cellular destruction, including the heart
muscle cells
6. Acidosis---dysfunctional cells release
lactic acid
In
this stage, medical intervention can reverse the changes.
III.
IRREVERSIBLE SHOCK---rapid deterioration of the CV system that cannot be helped
by compensatory mechanisms or medial intervention. ATP of the heart muscle is
depleted and the heart cannot make more, so contractions cease.
SIGNS OF SHOCK
1.
Hypotension—systolic BP below 90 due to vasodilation and
decreased cardiac output
2.
Clammy,
cool, pale skin due to vasoconstriction of skin vessels
3.
Sweating
due to increased sympathetic stimulation
4.
Reduced
urine formation due to hypotension and increased aldosterone
and ADH secretion
5.
Cerebral ischemia causes altered mental status
6.
Acidosis
due to buildup of lactic acid
7.
Tachycardia due to sympathetic stimulation and
epinephrine release
8.
Weak
rapid pulse due to generalized vasodilation and
reduced cardiac output
9.
Thirst
due to loss of extracellular fluid
10.
Nausea
due to impaired circulation to the digestive system
From birth onward, blood follows 2
basic routes:
1. Systemic circulation--includes all blood vessels carrying blood from
the left ventricle to body tissues except for lungs and back to the right
atrium
2. Pulmonary circulation--from right ventricle through the lungs and
back to left atrium
All systemic arteries branch off the
aorta:
1. Ascending aorta--emerges from the left ventricle and passes upward
behind the pulmonary trunk, gives off 2 coronary arteries that penetrate into
heart muscle
2. Arch of the aorta--gives off vessels that supply the head and arms
3. Descending aorta--lies close to the backbone
a. Thoracic aorta--travels through the thorax and penetrates diaphragm
b. Abdominal aorta--continues through the abdominal cavity to the level
of the 4th lumbar vertebra where it bifurcates to form the 2 common iliac
arteries
Each section of the aorta gives off
branches that supply the systemic tissues.