Chapter 21: Blood Vessels and Circulation

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Chapter 21Blood Vessels and Circulation
Primary sources for figures and content Marieb,
E. N. Human Anatomy Physiology. 6th ed. San
Francisco Pearson Benjamin Cummings,
2004. Martini, F. H. Fundamentals of Anatomy
Physiology. 6th ed. San Francisco Pearson
Benjamin Cummings, 2004.
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The types of blood vessels, their structures and
functions.
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6 Classes of Blood Vessels

• Arteries
• carry blood away from heart
• Branch and decrease in diameter
• Arterioles
• Are smallest branches of arteries
• Connect to capillaries
• Capillaries
• are smallest blood vessels
• location of exchange between blood and
  interstitial fluid

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6 Classes of Blood Vessels

• Venules
• Smallest veins
• collect blood from capillaries
• Veins
• return blood to heart
• Converge and increase in diameter
• Anastomoses
• - Bypass connection between vessels

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The Largest Blood Vessels

• Attach to heart
• Pulmonary trunk
• carries blood from right ventricle
• to pulmonary circulation
• Aorta
• carries blood from left ventricle
• to systemic circulation

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The Smallest Blood Vessels

• Capillaries
• Have small diameter and thin walls
• Chemicals and gases diffuse across walls

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The Anatomy of Blood Vessels

• Arteries, veins, and capillaries
• Have different functions
• Have different structures

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Arteries and Veins

• Walls have 3 layers
• 1. tunica intima
• 2. tunica media
• 3. tunica externa

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The Tunica Intima/Tunica interna

• Is the innermost layer
• Includes
• the endothelial cell lining
• Endothelium simple squamous epithelial-like
  cells connected by tight junctions
• With basal lamina of loose connective tissue
  containing elastic fibers (elastin)
• Arteries have internal elastic membrane
• extra layer of elastic fibers on the outer edge

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Tunica Media

• Is the middle layer
• Contains smooth muscle cells in loose connective
  tissue with sheets of elastin
• Binds to inner and outer layers
• Arteries have external elastic membrane
• extra layer of elastic fibers on the outer edge

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Tunica Externa/Tunica adventitia

• Is outer layer
• Contains collagen rich external connective tissue
  sheath
• Infiltrated with nerve fibers and lymphatic
  vessels
• Large vessels contain vasa vasorum
• Arteries more collagen, scattered elastic fiber
  bands
• Veins extensive fiber networks, bundles of
  smooth muscle cells

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Vasa Vasorum

• Small arteries and veins
• Found
• in walls of large arteries and veins
• Function
• Supply cells of tunica media and tunica externa

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Structure of Vessel Walls
Figure 21-1
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Structure of Blood Vessels
Figure 21-2
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1. Arteries

• Designed to change diameter, elastic and
  muscular, thick walls
• Tunica externa contains collagen
• Pressure
• Elasticity allows arteries to absorb pressure
  waves that come with each heartbeat
• Contractility
• Arteries change diameter
• Controlled by sympathetic division of ANS

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Vasoconstriction and Vasodilation

• Vasoconstriction
• The contraction of arterial smooth muscle by the
  ANS
• Vasodilatation
• The relaxation of arterial smooth muscle
• Enlarging the lumen
• Affect
• afterload on heart
• peripheral blood pressure
• capillary blood flow

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Artery Characteristics

• From heart to capillaries, arteries change
• from elastic arteries
• to muscular arteries
• to arterioles

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Elastic Arteries

• Also called conducting arteries
• Diameter up to 2.5cm
• Elastin in all three tunics
• Elasticity evens out pulse force
• Stretch (ventricular systole) and rebound
  (ventricular diastole)
• Not involved in systemic vasoconstriction

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Muscular Arteries

• Also called distribution arteries
• Are medium-sized (most arteries)
• Transport blood to organs and tissues
• Diameter 10mm 0.3mm
• More smooth muscle and less elastin in tunica
  media than elastic arteries
• Involved in systemic vasoconstriction via
  sympathetic stimulation

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2. Arterioles

• Also known as resistance vessels
• Connect blood supply to capillary beds
• Are small diameters 300µm 10µm
• All three tunics thin with few elastic fibers
• Involved in local vasoconstriction via endocrine
  or sympathetic stimulation

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Health Problems with Arteries

• 1. Aneurysm
• Pressure of blood exceeds elastic capacity of
  wall
• Causes bulge or weak spot prone to rupture
• Caused by chronic high blood pressure or
  arteriosclerosis

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Health Problems with Arteries

• 2. Arteriosclerosis
• Variety of pathological conditions causing
  changes in walls that decrease elasticity
  (thickenings)
• Focal calcification smooth muscle degenerates,
  replaced by calcium salts
• Atherosclerosis
• 3. Atherosclerosis lipid deposits

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Health Problems with Arteries

• 4. Stroke cerebrovascular accident (CVA)
• Interruption of arterial supply to portion of
  brain due to embolism or atherosclerosis
• Brain tissue dies and function is lost

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3. Capillaries

• Only vessels with thin enough wall structure to
  allow complete diffusion
• Designed to allow diffusion to/from the tissue
• Diameter 8 µm
• Consists of tunica intima only
• endothelium basal lamina
• Human body contains 25,000 miles of capillaries

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Capillary Structure
Figure 21-4
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Capillary Function

• Location of all exchange functions of
  cardiovascular system
• Materials diffuse between blood and interstitial
  fluid

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Capillary Structure

• Endothelial tube, inside thin basal lamina
• No tunica media
• No tunica externa
• Diameter is similar to red blood cell

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Capillaries
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Types of Capillaries

• Continuous capillaries
• Normal diffusion to all tissues except epithelium
  and cartilage
• Complete endothelium, tight junctions
• Functions
• Permit diffusion of water, small solutes,
  lipid-soluble materials
• Block blood cells and plasma proteins
• e.g., the bloodbrain barrier

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Types of Capillaries

• 2. Fenestrated capillaries
• High volume fluids or large solute transfer
• Pores/fenestrations span endothelium
• Permit rapid exchange of water and larger
  solutes between plasma and interstitial fluid

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Fenestrated Capillaries

• Are found in
• choroid plexus
• endocrine organs
• kidneys
• intestinal tract

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Sinusoids

• Areas in
• liver
• spleen
• bone marrow
• endocrine organs
• Have gaps between adjacent endothelial cells

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Types of CapillariesSinusoids

• 3. Sinusoids
• Cell or large protein exchange
• Gaps between endothelial cells
• Permit free exchange of water and large plasma
  proteins between blood and interstitial fluid
• Phagocytic cells monitor blood at sinusoids
• Found liver, bone marrow, lymphoid tissues

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Capillary Networks
Figure 21-5
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Capillaries Networks

• Organized into Capillary bed or capillary plexus
• Connect 1 arteriole and 1 venule
• Not enough total blood to fill all capillaries at
  once
• Flow through capillary bed must be controlled
  based on need via precapillary spincters

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Capillary Sphincter

• Guards entrance to each capillary
• Opens and closes, causing capillary blood to flow
  in pulses

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Vasomotion

• Contraction and relaxation cycle of capillary
  sphincters
• Spincter relaxed flow in capillary bed
• Spincter constricted capillary bed empty, flow
  through anastomoses
• Causes blood flow in capillary beds to constantly
  change routes

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Structure of Blood Vessels
Figure 21-2
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Veins vs. Arteries

• Are larger in diameter
• Have thinner walls
• Carry lower blood pressure

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4. Veins

• Collect blood from capillaries in tissues and
  organs
• Return blood to heart
• Can serve as blood reservoir
• Thin walls but large lumens
• Thin tunica media little smooth muscle or
  elastin
• Tunica externa elastin and smooth muscle
• Tunica intima valves to prevent back-flow

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3 Vein Categories

• Venules (5th type of vessel)
• very small veins
• Average diameter 20 µm
• collect blood from capillary beds
• Small venules lack tunica media
• Medium-sized veins
• - Diameter 2-9 mm
• Large Veins
• - Diameters up to 3 cm

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Valves in the Venous System
Valves in tunica intima insure one way movement
Figure 21-6
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Vein Valves

• Valves Folds of tunica intima
• Prevent blood from flowing backward
• Pressure from heart drives blood flow in
  arteries, but pressure in veins often too low to
  oppose gravity
• Compression pushes blood toward heart
• Skeletal muscle movement required to squish
  blood through veins

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Health Problems with Veins

• Resistance to flow (gravity, obesity) causes
  pooling above valves, veins stretch out
• Varicose veins
• Hemorrhoids

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Blood Reservoirs in Venous System

• Venous system contains 65-70 total blood volume
• Can constrict during hemorrhage to keep volume in
  capillaries and arteries near normal

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6. Anastomoses

• Bypass routes between vessels
• Bypass the capillary bed
• Not present in retina, kidney, or spleen
• More common in veins

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A cross section of tissue shows several small,
thin-walled vessels with very little smooth
muscle tissue in the tunica media. Which type of
vessels are these?

1. arteries
2. capillaries
3. arterioles
4. veins

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Why are valves located in veins, but not in
arteries?

1. venous blood pressure is lower
2. venous blood pressure is higher
3. venous walls are more muscular
4. venous lumens are larger

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Where in the body would you find fenestrated
capillaries?

1. absorptive areas of intestine
2. filtration areas of kidney
3. choroid plexus of brain
4. all of the above are correct

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Blood Distribution
Figure 21-7
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The mechanisms that regulate blood flow through
arteries, capillaries, and veins.
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Physiology of Circulation
Figure 21-8
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Physiology of Circulation

• Blood flow volume of blood flowing through a
  vessel in given period
• Total body flow Cardiac output
• Blood Pressure force per unit area exerted on
  vessel by blood (mmHg)
• Blood flows from high pressure ? low
• Resistance opposition to blood flow, friction
• Incr. blood viscosity incr. resistance
• Incr. vessel length incr. resistance
• Decr. Vessel diameter incr. resistance

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Factors that influence blood pressure and its
regulation.
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Pressure

• Pressure (P)
• The heart generates P to overcome resistance
• Absolute pressure is less important than pressure
  gradient
• The Pressure Gradient (?P)
• The difference between pressure at the heart and
  pressure at peripheral capillary beds

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Force (F)

• Is proportional to the pressure difference (?P)
• Divided by R

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Vascular Resistance

• Adult vessel length is constant
• Vessel diameter varies by vasodilation and
  vasoconstriction
• R increases exponentially as vessel diameter
  decreases

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Vasoconstriction andVasodilation

• Vasoconstriction
• Decr. Flow
• Incr. Blood Pressure
• Incr. Resistance
• Vasodilation
• Incr. Flow
• Decr. Blood Pressure
• Decr. Resistance

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Pressure

• Blood pressure changes throughout body
• Greatest in arteries leaving heart, lowest in
  veins returning to heart
• Persons BP measured at arteries near heart
• Systolic pressure/diastolic pressure (from
  ventricles, squeeze/rest)
• Normal 110/70 mmHg

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Vessel Diameter and Cardiac Pressure
Figure 21-9a
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Pressures in the Systemic Circuit
Figure 21-10
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Pressures in the Systemic Circuit

• Systolic pressure
• peak arterial pressure during ventricular systole
• Diastolic pressure
• minimum arterial pressure during diastole
• Pulse pressure
• difference between systolic pressure and
  diastolic pressure

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Abnormal Blood Pressure

• Hypertension
• Arterial pressure gt 150/90 mmHg
• abnormally high blood pressure
• Causes incr. workload for heart
• Untreated enlarged left ventricle requires more
  O2 heart can fail
• Hypotension
• abnormally low blood pressure

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Blood Pressure

• As arteries branch, area for blood increases,
  pressure decreases and becomes constant
• Blood at arterioles 35mmHg ? capillaries ? Blood
  at venules 18mmHg
• Pressure continues to decline as veins increase
  diameter

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In a healthy individual, where would the blood
pressure be greater, at the aorta or at the
inferior vena cava?

1. aorta
2. inferior vena cava

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While standing in the hot sun, Sally begins to
feel light headed and faints. Explain.

1. Blood has pooled in her lower limbs.
2. Cardiac output has decreased, sending less blood
   to the brain.
3. Sweating has reduced blood volume.
4. All of the above have occurred.

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The mechanisms and pressures involved in the
movement of fluids between capillaries and
interstitial spaces.
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Capillary Exchange

• Vital to homeostasis
• Functions to feed tissues and remove wastes
• Due to filtration and diffusion
• Dependent on good blood flow and pressure
• Moves materials across capillary walls by
• 1. Diffusion
• 2. Filtration
• 3. Reabsorption

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1. Diffusion

• Movement of ions or molecules
• from high concentration to lower concentration
• 1. Small ions transit through endothelial cells
• e.g. Na
• 2. Large ions small organics pass between
  endothelial cells
• E.g. glucose, amino acids
• 3. Lipids pass through endothelial membrane
• e.g. steroid hormones

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1. Diffusion

• 4. Large water soluble compounds diffuse at
  fenestrated capillaries
• e.g. in intestine
• 5. Large plasma proteins diffuse only at
  sinusoids
• e.g. in liver

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2. Filtration

• The removal of large solutes through a porous
  membrane
• Pressure forces substances through membrane
• Blood hydrostatic pressure in capillaries drives
  water and solutes out of plasma to tissues
• 24L/day
• Most recollected by osmosis (plasma proteins)
  back into capillary
• filtered at arteriole end
• absorbed at venule end

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2. Filtration

• 3.6 L/day flows through interstitial spaces,
  recollected by lymphatic system
• Accelerates distribution of nutrients
• Flushes out toxins and pathogens
• Will be removed/detoxified by immune cells in
  lymphatic system

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3. Reabsorption

• The result of osmosis
• Hydrostatic pressure
• forces water out of solution
• Osmotic pressure
• forces water into solution
• Both control filtration and reabsorption
  through capillaries

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Forces Across Capillary Walls
Figure 21-12
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Net Hydrostatic Pressure

• The difference between
• capillary hydrostatic pressure (CHP)
• and interstitial fluid hydrostatic pressure (IHP)
  
• Pushes water and solutes
• out of capillaries
• into interstitial fluid

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Net Colloid Osmotic Pressure

• The difference between
• blood colloid osmotic pressure (BCOP)
• and interstitial fluid colloid osmotic pressure
  (ICOP)
• Pulls water and solutes
• into capillary
• from interstitial fluid

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Capillary Exchange

• At arterial end of capillary
• fluid moves out of capillary
• into interstitial fluid
• At venous end of capillary
• fluid moves into capillary
• out of interstitial fluid

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Edema

• Buildup of fluid in the tissues, due to too much
  diffusion or filtration, not enough osmosis, or
  blocked lymphatics

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KEY CONCEPT

• Total peripheral blood flow equals cardiac output
• Blood pressure overcomes friction and elastic
  forces to sustain blood flow
• If blood pressure is too low
• vessels collapse
• blood flow stops
• tissues die
• If blood pressure is too high
• vessel walls stiffen
• capillary beds may rupture

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Cardiovascular Regulation
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Cardiovascular Regulation

• Flow, BP, and resistance must be controlled to
  insure delivery of nutrients and removal of
  wastes in tissues
• Changes blood flow to a specific area
• at an appropriate time and area
• without changing blood flow to vital organs
• 3 Regulatory Mechanisms
• 1. Autoregulation
• 2. Neural Mechanism
• 3. Hormonal Regulation

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1. Autoregulation

• Autoregulation
• causes immediate, localized homeostatic
  adjustments
• Single capillary bed action at a precapillary
  sphincter

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1. Autoregulation

• Autoregulation
• Local vasodilators (increase blood flow)
• Incr. CO2 or decr. O2
• Lactic acid, Incr. K or H
• Inflammation histamine, NO
• Elevated temperature
• Local vasoconstrictors (decrease blood flow)
• Prostaglandins
• Thromboxanes
• Endothelins

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2. Neural Mechanisms

• 1. Cardiovascular (CV) centers
• cardiac and vasomotor centers of medulla
  oblongata
• adjust cardiac output and peripheral resistance
• Cardiac Center
• Cardioacceleratory center sympathetic incr. CO
• Cardioinhibitory center parasympathetic decr.
  CO
• Vasomotor Center
• Sympathetic NE vasoconstriction

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2. Neural Mechanisms

• 2. Baroreceptor reflexes
• Respond to changes in blood pressure
• Trigger cardiovascular center
• 3. Chemoreceptor reflexes
• Respond to changes in blood and CSF CO2 and O2,
  pH
• Trigger respiratory and cardiac center

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3. Hormonal Regulation

• 1. Antidiuretic Hormone (ADH)
• From pituitary gland in response to low blood
  volume
• Causes vasoconstriction and water conservation at
  kidney
• 2. Angiotensin II
• From kidney in response to low BP
• Causes
• Na retention and K loss at kidney
• Stimulates release of ADH, stimulates thirst,
  Stimulated CO
• Stimulates arteriole constriction

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3. Hormonal Regulation

• 3. Erythropoietin
• From kidney in response to low O2
• Stimulates production and maturation of RBCs
• 4. Atrial Natriuretic Peptides (ANP)
• From atria in response to stretching
• Causes
• Increase Na and H2O loss at kidney
• Reduced Thirst
• Blocks ADH release
• Stimulates vasodilation

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KEY CONCEPT

• Cardiac output cannot increase indefinitely
• Blood flow to active vs. inactive tissues must be
  differentially controlled
• This is accomplished by autoregulation, neural
  regulation, and hormone release

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Cardiovascular Response to Hemorrhages

• Short term (aimed at incr. BP and incr. Flow)
• Blood flow to brain kept constant while other
  systems adjust, can compensate for 20 blood
  loss
• Incr. cardiac output trigger peripheral
  vasoconstriction to incr. BP
• Venoconstrict to moblize venous reserve to incr.
  blood volume
• Release NE, ADH, Angiotensin II to incr. BP

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Cardiovascular Response to Hemorrhages

• Long term (aimed at restoring normal blood volume
  after hemorrhage)
• Recall fluid from interstitial spaces
• Release Incr. ADH for fluid retention at kidney
• Increase thirst
• Release EPO to Incr. RBCs

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Shock

• Low BP and inadequate blood flow
• Due to
• Loss of gt 30 blood volume
• Damage to heart
• External pressure on heart
• Extensive vasodilation
• Result in
• Hypotension, rapid weak pulse clammy skin,
  confusion
• Incr. heart rate
• Decr. urine production and blood pH
• Body focuses on supplying blood to brain at
  expense of other tissues

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Circulatory Collapse

• Blood flow stops completely as muscles in vessels
  no longer contract due to lack of oxygen
• Results in no blood flow death

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Aging and the Cardiovascular System

• Decreased hematocrit
• Increased blood clots (thrombus) formation
• Blood-pools in legs
• due to venous valve deterioration
• Reduction in max Cardiac output
• Increased arteriosclerosis

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A blockage of which branch from the aortic arch
would interfere with blood flow to the left arm?

1. left common carotid artery
2. left subclavian artery
3. brachiocephalic trunk
4. right common carotid artery

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Why would a compression of the common carotid
arteries cause a person to lose consciousness?

1. Because it would cause a reflexive decrease in
   heart rate and blood pressure.
2. Because cerebral arteries would dilate in
   response to pressure.
3. Because increased blood pressure would occur at
   the carotid sinus.
4. Because rapid fall in blood flow to the brain
   would occur.

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Whenever Tim gets angry, a large vein bulges in
the lateral region of his neck. Which vein is
this?

1. superior vena cava
2. brachiocephalic vein
3. internal jugular vein
4. external jugular vein

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A blood sample taken from the umbilical cord
contains a high concentration of oxygen and
nutrients and a low concentration of carbon
dioxide and waste products. Is this a sample
from an umbilical artery or from the umbilical
vein?

1. umbilical artery
2. umbilical vein

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SUMMARY

• 3 types of blood vessels
• arteries
• veins
• Capillaries
• Structure of vessel walls
• Differences between arteries and veins
• Atherosclerosis, arteriosclerosis, and plaques
• Structures of
• elastic arteries
• muscular arteries
• arterioles

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SUMMARY

• Structures of capillary walls
• continuous
• Fenestrated
• Structures of capillary beds
• precapillary sphincters
• Vasomotion
• Functions of the venous system and valves
• Distribution of blood and venous reserves
• Circulatory pressures
• blood pressure
• capillary hydrostatic pressure
• venous pressure

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SUMMARY

• Resistance in blood vessels
• viscosity
• turbulence
• Vasoconstriction
• The respiratory pump
• Capillary pressure and capillary exchange
• osmotic pressure
• net filtration pressure
• Physiological controls of cardiovascular system
• autoregulation
• neural controls
• hormonal controls