Transcript Chapter_21_Blood_Vessels

Blood Vessels
and
Circulation
Chapter 21
Introduction
• Structure and function of blood vessels
• Blood Circulation
• Factors controlling the flow of blood
• Major circulatory routes
STRUCTURE AND FUNCTION OF
BLOOD VESSELS
• Blood vessels form a
network in our body taking
the blood from the heart to
the tissues of the body and
returning it back to the
heart.
• The direction of the flow of
the blood in different
vessels of the body is:
Heart  Arteries 
Arterioles  Capillaries 
Venules Veins  Heart
Arteries
•
•
Are blood vessels that take the
blood away from the heart.
The wall of an artery consists of
three major layers
– Tunica interna (intima) –
innermost layer
• simple squamous
epithelium known as
endothelium
• basement membrane
• internal elastic lamina
– Tunica media – middle layer
• smooth muscle
• Capable of vasoconstriction
and vasodilation
– Tunica externa – outermost
layer
• elastic & collagen fibers
Arteries – Elastic and Muscular arteries
• Elastic (conducting) arteries
– Found closer to the heart
– Have more elastic fibers
and less smooth muscle,
are able to receive blood
under pressure
– Help to conduct blood
from heart to the
muscular arteries
• Muscular (distributing)
arteries
– Found closer to the organs
and tissues.
– Have more smooth muscle
tissue
– Capable of vasoconstriction
and vasodilation, control
flow of blood
Aging of the Arteries
Arteriosclerosis: is defined as thickening and toughening
of the arterial walls. Complications associated with
arteriosclerosis can lead to heart attack and stroke. Two
main types:
Focal calcification: deposition of calcium salts, replacing the
smooth muscle tissue of the arterial wall. Can be due to
aging, or a consequence of diabetes mellitus.
Atherosclerosis: deposition of lipids (plaques) in the tunica
media.
Arterioles
• small microscopic arteries delivering blood to
capillaries.
• an arteriole has tunica interna, tunica media and
very thin tunica externa.
• through vasoconstriction and vasodilation regulate
the amount of blood entering into the capillaries of
an organ or tissue.
Capillaries
• microscopic vessels which connect
arterioles and venules.
• Flow of blood from arterioles --- > to
capillaries --- > to venules, is termed
microcirculation.
• Capillaries are made up of an
endothelial and basement layer only.
• Primary function - permit the
exchange of nutrients and gases
between the blood and the tissues.
• Distribution of capillaries in tissues of
the body varies depending on the
metabolic activity of the body.
Types of Capillaries
•
•
•
Continuous capillaries
– plasma membrane of the
endothelial cells forms a
continuous ring around
the capillary. Have no
pores
– e.g. in skeletal & smooth
muscles, & lungs
Fenestrated capillaries
– plasma membranes have
many holes
– e.g. in kidneys, small
intestine, choroid
plexuses,
Sinusoids
– very large fenestrations
(pores)
– incomplete basement
membrane
– e.g. liver, bone marrow,
spleen
Venules and Veins
Venules:
• formed by joining of small
capillaries.
• collect blood from capillaries and
drain it into veins.
Veins:
• Veins consist of the same three tunics as arteries but differ
in the following:
– contain less smooth muscle and
elastic tissue
– thinner walled in comparison to arteries
– contain valves to prevent the backflow of blood.
– Lumen is larger than arteries.
Varicose veins: are due to weakening of the valves in
the veins of the thighs and legs. Blood pools in the
veins and become swollen.
Anastomoses and Blood Distribution
Anastomoses
– are the union of the branches of two or more
arteries supplying the same region.
– provide alternate routes for blood to reach a tissue
or organ.
Blood Distribution
• 60% of blood volume at rest
is in systemic veins and venules
– function as blood reservoir
– blood is diverted from it in
times of need
• 15% of blood volume in
arteries & arterioles
REVIEW!!!
• Now that we have completed the section on blood vessels
try to answer the following questions:
List the five types of blood vessels through which blood
flows in the body.
• List the three layers typically seen in the arteries and
veins.
• Describe the differences between the arteries and veins.
• Describe the structure of the capillary and explain how this
structure assists the capillaries in carrying out their
functions.
• List the two main types of arteries.
• List the three types of capillaries.
• Why are valves found in the arteries and not in the veins.
• Define varicose veins.
Factors Affecting Blood Flow
• Blood Flow is the volume of blood that flows
through any tissue in a given time period.
– Under normal conditions, blood flow is equal to cardiac
output, when cardiac output goes up, blood flow
through the capillary beds increases.
– It depends on two main factors - Pressure (P) and
Resistance (R).
Flow is directly proportional to pressure and inversely
proportional to resistance.
Factors affecting blood flow
Cardiovascular pressure is measured in terms of
three values:
• Blood Pressure (BP): is the hydrostatic pressure
exerted by the blood on the walls of arteries.
Measured in mmHg. Ranges from 100 mmHg
(near the aorta) to 35 mmHg (near the capillaries).
• Capillary hydrostatic pressure (CHP): is the
pressure exerted by the fluids on the capillary
walls. Ranges from 35 mmH to 18 mm Hg.
• Venous pressure: is the pressure within the veins.
Lowest of the three pressure, approx 18 mmHg.
For circulation to happen circulatory pressure must
overcome the total peripheral resistance.
FACTORS AFFECTING BLOOD FLOW
Blood Flow is the volume of blood that flows
through any tissue in a given time period. It
depends on the following factors:
Blood Pressure: is the pressure exerted on the
walls of a blood vessel.
• caused by contraction of the ventricles
• highest in aorta
– 120 mm Hg during systole & 80
during diastole
• The pressure falls as the distance from the
heart increases.
• Pulse pressure is the difference between the
systolic and the diastolic pressure.
Mean arterial pressure (MAP) is calculated
as follows:
MAP = diastolic pressure +pulse pressure/3
• Factors that affect blood pressure include
cardiac output, blood volume, viscosity,
resistance, and elasticity of arteries.
HEMODYNAMICS: FACTORS AFFECTING
BLOOD FLOW (CONTD)
Venous return: the volume of blood flowing
back to the heart from the systemic veins,
depends on the pressure difference from
venules to right atrium. Two mechanisms act
to return venous blood.
– Skeletal muscle pump: Skeletal muscles
surrounding the veins contract --> this
exerts pressure on the walls of the veins -> forces the valves open --> blood is
pumped up --> skeletal muscle relaxed -->
valve close preventing the backward
flow of blood.
– Respiratory pump : during inspiration
diaphragm moves inferiorly. This causes a
decrease in pressure in the thoracic cavity
and an increase in the pressure of
abdominopelvic cavity. As a result blood
flows from the veins in abdominopelvic
region to veins in thoracic region. During
expiration, valves close preventing back
flow.
Factors affecting blood flow
Resistance: is the friction between the blood and the walls
of the vessels which increases BP. Resistance depends
on the following factors.
• Vascular resistance: is the force that resists the flow of
blood within the blood vessels. it in turn depends on two
parameters:
– Size of the lumen: smaller the lumen, greater is the
resistance to blood flow.
– Total blood vessel length: Resistance is directly proportional
to the length of the blood vessel.
• Blood Viscosity: Thickness of the Blood is due to ratio of
RBCs to plasma and concentration of plasma proteins.
Anemia, polycethmia and other disorders change blood
viscosity and thus peripheral resistance.
• Turbulence: is due to high flow rates, irregular surfaces
and sudden changes in vessel diameter (due to
blockages). It increases resistance an thereby slows the
flow of blood.
Relationship among vessel diameter, cross sectional
area, blood pressure and blood velocity within the
systemic circuit.
Capillary Exchange
Substances enter and leave capillaries in three basic
ways described below:
• Diffusion : Substances like oxygen, carbon dioxide, glucose,
amino acids, hormones diffuse through the capillaries down
their concentration gradient. Lipid soluble molecules like gases
and certain hormones, pass directly through the phospholipid
layer. Water soluble compounds like glucose and amino acids
pass through the intercellular clefts.
• Filtration: Driven by hydrostatic pressure.
Water and small solutes forced through capillary wall,
Leaves larger solutes in bloodstream.
• Reabsorption: The result of osmotic pressure (OP).
Blood colloid osmotic pressure (BCOP)
Equals pressure required to prevent osmosis
Caused by suspended blood proteins that are too large to cross
capillary walls.
Dynamics of Capillary Exchange
• Two pressures promote this movement: hydrostatic pressure
(HP) and osmotic pressure (OP).
Net Filtration Pressure: Net Hydrostatic Pressure – Osmotic
Pressure
• NFP at the arterial end is 10 mm Hg and at the venous end it is 9 mm Hg.
• Thus at the arterial end there is net movement of molecules out
of the capillary, and at the venous end fluid moves into the
capillary from the surrounding tissue.
• On average only 85% of the fluid that comes out of the
capillaries reenters the capillaries at the venous end.
• The extra fluid is returned back to the circulatory system via the
lymphatic system.
• Edema: When the filtration exceeds the re absorption, it results
in an increase in the interstitial fluid volume, which is termed
edema.
Dynamics of Capillary Exchange
CONTROL OF BLOOD PRESSURE AND
BLOOD FLOW
Blood volume and blood flow are constantly regulated as
conditions change in the body. Blood pressure and blood flow
to specific areas of the body are under the control of the
following factors:
1. Cardiovascular center: is a group of neurons in
the medulla of the brain and it regulates heart
rate, contractility, and blood vessel diameter.
– Receives input from sensory receptors
(baroreceptors and chemoreceptors) .
– Cardiovascular center sends output via
sympathetic and parasympathetic fibers.
– Sympathetic activation results in increased
heart rate and vasoconstriction, whereas
parasympathetic activation causes decreased
heart rate.
CARDIOVASCULAR REGULATORY MECHANISM
2. Hormonal Regulation of Blood Pressure
•
•
•
•
Renin-angiotensin-aldosterone system
– decrease in BP or decreased blood flow to kidney 
release of renin by kidney  angiotensinogen
converted to angiotensin in liver  release of
aldosterone by adrenal cortex  increased water
reabsorption (increase in Blood volume) and
vasoconstriction (increase in BP)
Epinephrine & norepinephrine
– increases heart rate & force of contraction
– causes vasoconstriction
ADH causes vasoconstriction and increased water
reabsorption resulting in increase in BP and blood volume.
ANP (atrial natriuretic peptide) lowers BP and blood volume
– causes vasodilation & loss of salt and water in the urine
CARDIOVASCULAR REGULATORY MECHANISM
3. Local Regulation of Blood Pressure:
– The ability of a tissue to automatically adjust
its own blood flow to match its metabolic
demand for supply of O2 and nutrients and
removal of wastes is called autoregulation.
– Important for tissues that show major increase
in activity such as brain and skeletal muscles.
CARDIOVASCULAR RESPONSE TO EXERCISE
Blood, Heart, and Cardiovascular System
- Work together as unit
- Respond to physical and physiological changes (for example,
exercise and blood loss)
- Maintain homeostasis
The Cardiovascular Response to Exercise
Light Exercise
Extensive vasodilation
occurs increasing circulation
Venous return increases with
muscle contractions
Cardiac output rises
Venous return (Frank–
Starling principle)
Atrial stretching
Heavy Exercise
Activates sympathetic nervous
system
Cardiac output increases to maximum
About four times resting level
Restricts blood flow to “nonessential”
organs (e.g., digestive system)
Redirects blood flow to skeletal
muscles, lungs, and heart
Blood supply to brain is unaffected
of blood flow.
REVIEW!!
Now that we have completed the section on the
factors controlling the blood flow, lets try to
answer the following questions:
Define blood flow, and describe its relationship to
blood pressure and peripheral resistance.
• Which blood vessel has the lowest blood
pressure?
• Which blood vessel has the lowest blood flow?
• List the factors that contribute to total peripheral
resistance.
• List the hormones involved with the regulation of
cardiovascular physiology.
Circulatory Routes
• Most common route
– heart  arteries  arterioles
 capillaries  venules 
veins
– E.g. pulmonary and systemic
circulation
• Portal system
– blood flows through two
consecutive capillary networks
before returning to heart
• hypothalamus - anterior
pituitary
• found in kidneys
• between intestines – liver
• Anastomoses - Point where 2
blood
vessels merge
– E.g. coronary circulation
Pulmonary Circulation
• The pulmonary circulation takes deoxygenated blood
from the right ventricle to the air sacs of the lungs
and returns oxygenated blood from the lungs to the
left atrium. Pulmonary and systemic circulation are
different in several ways:
– Distance traveled by the blood is smaller
– Pulmonary arteries are larger in diameter, thinner
walls and hence resistance to blood flow is low.
– Hydrostatic pressure in pulmonary capillary is low,
tends to prevent pulmonary edema.
Pulmonary Circulation
Systemic Circulation
• The systemic circulation takes oxygenated blood from
the left ventricle through the aorta to all parts of the
body, including some lung tissue (but does not supply
the air sacs of the lungs) and returns the
deoxygenated blood to the right atrium.
• The aorta is divided into the ascending aorta, arch of
the aorta, and the descending aorta (thoracic and
abdominal aorta). Each section gives off arteries that
branch to supply the whole body.
• Blood returns to the heart through the systemic veins.
All the veins of the systemic circulation flow into the
superior or inferior vena cavae or the coronary sinus,
which in turn empty into the right atrium.
• The principal arteries and veins of the systemic
circulation are described in the next few slides.
Major arteries of the body
Aorta and Its Superior Branches
Common Carotid Branches
Subclavian Branches
Thoracic Aorta - Branches
Abdominal Aorta and Its Branches
Veins of the
Systemic
Circulation
Drain blood from
entire body &
return it to
right side of heart
Veins of the Head and Neck
Veins of Shoulder and Upper Limb
Veins draining blood to the superior vena cava
Fetal Circulation
• The fetal circulation involves the exchange of
materials between fetus and mother.
• The fetus derives its oxygen and nutrients and
eliminates its carbon dioxide and wastes through the
maternal blood supply by means of a structure called
the placenta.
• Blood passes from the fetus to the placenta via two
umbilical arteries and returns from the placenta via a
single umbilical vein.
• At birth, when pulmonary, digestive, and liver
functions are established, the special structures of
fetal circulation are no longer needed.
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–
–
–
The ductus arteriosus becomes the ligamentum arteriosum.
The foramen ovale becomes the fossa ovalis.
The ductus venosus becomes the ligamentum venosum.
The umbilical arteries become the medial umbilical
ligaments.
– The umbilical vein becomes the ligamentum teres (round
ligament).
Fetal Circulation
Age Related Changes in Cardiovascular System
• Three Age-Related Changes in Blood
– Decreased hematocrit
Peripheral blockage by blood clot (thrombus)
Pooling of blood in legs
Due to venous valve deterioration
• Five Age-Related Changes in the Heart
– Reduced maximum cardiac output
Changes in nodal and conducting cells
Reduced elasticity of cardiac (fibrous) skeleton
Progressive atherosclerosis
Replacement of damaged cardiac muscle cells by scar tissue
• Three Age-Related Changes in Blood Vessels
– Arteries become less elastic
Pressure change can cause aneurysm
Calcium deposits on vessel walls
Can cause stroke or infarction
Thrombi can form
At atherosclerotic plaques