This document provides an overview of the circulatory system, including blood vessels and circulation. It describes the anatomy and layers of blood vessels, distinguishing between arteries, capillaries, and veins. Arteries are divided based on size into conducting, distributing, and resistance arteries. The roles of elastic tissue and smooth muscle in blood vessel walls are explained. Capillary types and their selective permeability is covered. Control of blood flow and factors influencing blood pressure such as resistance are summarized.

1. 20-1
Chapter 20
Circulatory
System
Blood Vessels
and Circulation

2. 20-2
Blood Vessels and Circulation
• General anatomy of blood vessels
• Blood pressure, resistance and flow
• Capillary exchange
• Venous return and circulatory shock
• Special circulatory routes
• Anatomy of
– pulmonary circuit
– systemic arteries and veins

3. 20-3
Anatomy of Blood Vessels
• Arteries carry blood away from heart (efferent)
• Veins carry blood back to heart (afferent)
• Capillaries connect smallest arteries to veins

4. 20-4
Anatomy of Blood Vessels

5. 20-5
Vessel Wall
• Composed of three layers:
1. Tunica interna (tunica intima)
2. Tunica media
3. Tunica externa (tunica adventitia)

6. 20-6
Vessel Wall
1. Tunica interna (intima)
– Acts as a selective permeable barrier to
materials entering or exiting the bloodstream.
– smooth inner layer that repels blood cells
and platelets
– simple squamous endothelium overlying a
basement membrane and layer of fibrous
tissue
– Secretes chemicals that stimulate dilation or
constriction of the vessel

7. 20-7
Vessel Wall
2. Tunica media
– middle layer
– usually thickest; smooth muscle, collagen,
some elastic tissue
– 2 functions:
1. * strengthen the vessel and prevent blood
pressure from rupturing them
2. Provides vasomotion

8. 20-8
Vessel Wall
3. Tunica externa (tunica adventitia)
– outermost layer
– loose connective tissue
– Function:
• Anchors vessel
• Provides passage for small nerves, lymphatic
vessels, and smaller blood vessels
• Vasa vasorum

9. 20-9
Large Vessels
Why are elastic laminae found in arteries but not veins?

10. 20-10
Arteries
Divided in to 3 catagories:
1. Conducting (elastic or large) artieries
2. Distributing (muscular or medium)
3. Resistance (small) arteries

11. 20-11
Arteries
• Conducting (elastic) arteries - largest
– pulmonary, aorta and common carotid,
subclavian, common iliac
– tunica media consists of perforated sheets of
elastic tissue, alternating with thin layers of
smooth muscle, collagen and elastic fibers
– expand during systole, recoil during diastole;
lessens fluctuations in BP
– Recoil between heartbeats prevents the blood
pressure from dropping to low during the
quiescent period.

12. 20-12
Medium Vessels

13. 20-13
Arteries
• Distributing (muscular) arteries
– distributes blood to specific organs; femoral
splenic, renal, and brachial arteries
– smooth muscle layers constitute 3/4 of wall
thickness

14. 20-14
Small Vessels

15. 20-15
Arteries and Metarterioles
• Resistance (small) arteries
– arterioles control amount of blood to various
organs
• Metarterioles
– short vessels connect arterioles to capillaries
– muscle cells form a precapillary sphincter
about entrance to capillary
• Precapillary sphincters control which beds
are well perfused
– only 1/4 of the capillaries are open at a given
time

16. 20-16

17. 20-17
Arterial Sense Organs
• Major arteries above heart, monitor
pressure and chemistry
• Information is transmitted to the
brainstem to regulate heartbeat,
vasomotion, and respiration.
• 3 types of sensory receptors:
1. Carotid sinuses
2. Carotid bodies
3. Aortic bodies

18. 20-18
Arterial Sense Organs
• Carotid sinuses
– These are baroreceptors
(pressure sensors)
– in walls of internal carotid
artery
– monitors BP – signaling
brainstem via CN IX
• HR ↓ and vessels dilate
thereby lowering BP

19. 20-19
Arterial Sense Organs
• Carotid bodies
(chemoreceptors)
– oval bodies near carotids
– Innervated by CN IX, X
– monitor Δ blood chemistry
• adjust respiratory rate to
stabilize pH, CO2, and O2
• Aortic bodies
– in walls of aorta
– same function as carotid
bodies

20. 20-20
Capillaries
• Allow passing of nutrient, waste, and hormones.
• AKA exchange vessels
• Composed of endothelium and basement
membrane.
• Scarce in tendons, ligaments, cornea, lens
• 3 types of cap. distinguished by the ease with
which they allow substances to pass.
– Continuous, Fenestrated, and Sinusoids

21. 20-21
Control of Capillary Bed Perfusion

22. 20-22
Control of Capillary Bed Perfusion

23. 20-23
Types of Capillaries
• Continuous - occur in most tissues
– endothelial cells have tight junctions (uninterrupted tube) with
intercellular clefts (allow passage of solutes)
– Plasma proteins, other large molecule are held back.
– BBB

24. 20-24
Fenestrated Capillary
Fenestrated - kidneys, small intestine
• organs that require rapid absorption or filtration
•filtration pores – spanned by very thin glycoprotein layer -
allows passage of only small molecules
• proteins, and other molecules, but retain RBC and
platelets in bloodstream

25. 20-25
Sinusoid
Sinusoids - liver, bone marrow,
spleen
• irregular blood-filled spaces;
some have extra large
fenestrations, allow proteins and
blood cells to enter
• This is how albumin, clotting
factors and proteins synthesized
by the liver enter the bloodstream
• newly formed RBC enter
circulation from bone marrow and
lymphatic organs

26. 20-26
Veins
• Veins
– thinner walls, flaccid
– less muscular and elastic tissue,
– expand easily, have high capacitance
– valves aid skeletal muscles in upward blood
flow
– Lower BP because further away from heart.

27. 20-27
Veins
• Follow the direction of blood flow, smallest to
largest veins
• Postcapillary venules
– Receive blood directly from capillaries
– more porous than capillaries, thus exchange
fluid with surrounding tissues
• Muscular venules
– Receive blood from the postcapillary venules

28. 20-28
Veins
• Medium veins
– Individual names; radial, ulnar, saphenous
– Infolding of tunica interna that meets in the
middle of lumen = venous valves
– Skeletal muscle pump (varicose veins)
• Venous sinuses
– veins with thin walls, large lumens, no smooth
muscle
– Coronary sinus, dural sinus of brain

29. 20-29
Veins
• Large veins
– Venae cavae
– Pulmonary veins
– Jugular vein
– Renal veins

30. 20-30
Circulatory Routes
• Most common route
– heart → arteries →
arterioles → capillaries →
venules → veins
• Portal system
– blood flows through
two consecutive
capillary networks
before returning to heart
• hypothalamus - anterior
pituitary
• found in kidneys
• between intestines - liver

31. 20-31
Anastomoses
• Point where 2 blood
vessels merge
• Arteriovenous shunt
– artery flows directly into
vein
• Venous anastomosis
– most common, blockage
less serious
– alternate drainage of
organs
• Arterial anastomosis
– collateral circulation
(coronary)

32. 20-32
Principles of Blood Flow
• Blood flow: amount of blood flowing
through a tissue in a given time (ml/min)
• Perfusion: rate of blood flow per given
mass of tissue (ml/min/g)
• Important for delivery of nutrients and
oxygen, and removal of metabolic
wastes
• Total flow = cardiac output (5.25 L/min)

33. 20-33
Principles of Blood Flow
• Hemodynamics
– physical principles of blood flow based on
pressure and resistance
• F ∝ ∆P/R, (F = flow, ∆P = difference in pressure,
R = resistance to flow)
• The greater the pressure difference between two
points, the greater the flow;
• The greater the resistance (R) the less the flow.

34. 20-34
Blood Pressure
• Force that blood exerts against a vessel
wall
• Measured at brachial artery of arm
• Systolic pressure: BP during ventricular
systole
• Diastolic pressure: BP during ventricular
diastole
• Normal value, young adult: 120/75 mm Hg
• Pulse pressure: systolic - diastolic
– important measure of stress exerted on small
arteries

35. 20-35
BP Changes With Distance

36. 20-36
Blood Pressure
• Importance of arterial elasticity
– expansion and recoil maintains steady flow of
blood throughout cardiac cycle, smoothes out
pressure fluctuations and ↓ stress on small
arteries
• BP rises with age: arteries less distensible
• BP determined by cardiac output, blood
volume and peripheral resistance
» Three variables: blood viscosity, vessel length, and
vessel radius.

37. 20-37
Abnormalities of Blood Pressure
• Hypertension
– chronic resting BP > 140/90
– consequences
• can weaken small arteries and cause aneurysms
• Hypotension
– chronic low resting BP
– caused by blood loss, dehydration, anemia

38. 20-38
Peripheral Resistance
• The opposition to flow that blood
encounters in vessels away from the
heart.
• Blood viscosity - by RBC’s and albumin
↓ viscosity with anemia, hypoproteinemia
↑ viscosity with polycythemia , dehydration
• Vessel length
– pressure and flow ↓ with distance (friction)
• Vessel radius - very powerful influence
over flow
– most adjustable variable, controls resistance
quickly
– vasomotion: change in vessel radius

39. 20-39

40. 20-40
Peripheral Resistance
• Vessel radius (cont.)
– laminar flow - flows in layers, faster in center
– blood flow (F) proportional to the fourth power
of radius (r), F ∝ r4
• arterioles can constrict to 1/3 of fully
relaxed radius (see fig. 20.12)

41. 20-41
Peripheral Resistance
• Example:
– Lets say a blood vessel has a 1-mm radius when fully constricted
and a 3-mm radius when fully relaxed. (F ∂ r 4
)
r = 1mm r 4
= 1 4
= 1 F = 1 mm/sec
r = 2mm r 4
= 2 4
= 16 F = 16 mm/sec
r = 3mm r 4
= 3 4
= 81 F = 81 mm/sec
– 3X ↑ in radius results in 81X ↑ in flow
– Demonstrates that vessel radius exerts a very powerful influence
over flow.

42. 20-42

43. 20-43
Flow at Different Points
• From aorta to capillaries, flow ↓ for 3
reasons
– greater distance, more friction to ↓ flow
– smaller radii of arterioles and capillaries
– farther from heart, greater total cross
sectional area
• From capillaries to vena cava, flow ↑ again
– large amount of blood forced into smaller
channels
– never regains velocity of large arteries

44. 20-44
Regulation of BP and Flow
• Local control
• Neural control
• Hormonal control

45. 20-45
Local Control of BP and Flow
• Metabolic theory of autoregulation
– tissue inadequately perfused, wastes
accumulate = vasodilation
• Vasoactive chemicals
– substances that stimulate vasomotion;
histamine, bradykinin (trauma, exercise)
• Reactive hyperemia
– blood supply cut off then restored
• Angiogenesis - growth of new vessels
– regrowth of uterine lining, around
obstructions, exercise, malignant tumors
– controlled by growth factors and inhibitors

46. 20-46
Neural Control of BP and Flow
• Vasomotor center of medulla oblongata:
– sympathetic control stimulates most vessels
to constrict, but dilates vessels in skeletal and
cardiac muscle
– integrates three autonomic reflexes
• baroreflexes
• chemoreflexes
• medullary ischemic reflex

47. 20-47
Neural Control: Baroreflex
• Changes in BP detected by stretch receptors
(baroreceptors), in large arteries above heart
– aortic arch
– aortic sinuses (behind aortic valve cusps)
– carotid sinus (base of each internal carotid artery)
• Autonomic negative feedback response
– baroreceptors send constant signals to brainstem
↑ BP causes rate of signals to rise, inhibits vasomotor
center, ↓ sympathetic tone, vasodilation causes BP ↓
↓ BP causes rate of signals to drop, excites vasomotor
center, ↑ sympathetic tone, vasoconstriction and BP ↑

48. 20-48
Baroreflex
Negative Feedback Response

49. 20-49
Neural Control: Chemoreflex
• Chemoreceptors in aortic bodies and
carotid bodies
– located in aortic arch, subclavian arteries,
external carotid arteries
• Autonomic response to changes in blood
chemistry
– pH, O2, CO2
– primary role: adjust respiration
– secondary role: vasomotion
• hypoxemia, chemoreceptors, instruct vasomotor
center to cause vasoconstriction, ↑ BP, ↑ lung
perfusion and gas exchange

50. 20-50
Other Inputs to Vasomotor Center
• Medullary ischemic reflex
– inadequate perfusion of brainstem
• cardiac and vasomotor centers send sympathetic
signals to heart and blood vessels
∀↑ cardiac output and causes widespread
vasoconstriction
∀↑ BP
• Other brain centers
– stress, anger, arousal can also ↑ BP

51. 20-51
• Angiotensinogen (prohormone produced by liver)
↓ Renin (kidney enzyme released by low BP)
• Angiotensin I
↓ ACE (angiotensin-converting enzyme in lungs)
ACE inhibitors block this enzyme lowering BP
• Angiotensin II
– very potent vasoconstrictor
Hormonal Control of BP and Flow

52. 20-52
Hormonal Control of BP and Flow
• Aldosterone
– promotes Na+
and water retention by kidneys
– increases blood volume and pressure
• Atrial natriuretic factor (↑ urinary sodium excretion)
– generalized vasodilation
• ADH (water retention)
– pathologically high concentrations, vasoconstriction
• Epinephrine and norepinephrine effects
– most blood vessels
• binds to α-adrenergic receptors, vasoconstriction
– skeletal and cardiac muscle blood vessels
• binds to β-adrenergic receptors, vasodilation

53. 20-53
Routing of Blood Flow
• Localized vasoconstriction
– pressure downstream drops, pressure
upstream rises
– enables routing blood to different organs as
needed
• Arterioles - most control over peripheral
resistance
– located on proximal side of capillary beds
– most numerous
– more muscular by diameter

54. 20-54
Blood Flow in Response to Needs
• Arterioles shift blood flow with changing priorities

55. 20-55
Blood Flow Comparison
• During exercise
↑ perfusion of lungs, myocardium and skeletal
muscles ↓ perfusion of kidneys and digestive tract

56. 20-56
Capillary Exchange
• Only occurs across capillary walls
between blood and surrounding tissues
• 3 routes across endothelial cells
– intercellular clefts
– fenestrations
– through cytoplasm
• Mechanisms involved
– diffusion, transcytosis, filtration and
reabsorption

57. 20-57
Capillary Exchange - Diffusion
• Most important mechanism
• Lipid soluble substances
– steroid hormones, O2 and CO2 diffuse easily
• Insoluble substances
– glucose and electrolytes must pass through
channels, fenestrations or intercellular clefts
• Large particles - proteins, held back

58. 20-58
Capillary Exchange - Transcytosis
• Pinocytosis - transport vesicles across cell -
exocytosis
• Important for fatty acids, albumin and some
hormones (insulin)

59. 20-59
Capillary Exchange -
Filtration and Reabsorption
• Opposing forces
– blood (hydrostatic) pressure drives fluid out of
capillary
• high on arterial end of capillary, low on venous end
– colloid osmotic pressure (COP) draws fluid
into capillary
• results from plasma proteins (albumin)- more in
blood
• oncotic pressure = net COP (blood COP - tissue COP)
• Hydrostatic pressure
– physical force exerted against a surface by a
liquid, (BP is an example)

60. 20-60
Capillary Filtration and Reabsorption
• Capillary filtration at arterial end
• Capillary reabsorption at venous end
• Variations
– location
(glomeruli- devoted to filtration
alveolar cap.- devoted to absorption)
– activity or trauma
(↑ filtration)

61. 20-61
Causes of Edema
∀ ↑ Capillary filtration (↑ capillary BP or permeability)
– poor venous return
• congestive heart failure - pulmonary edema
• insufficient muscular activity
– kidney failure (water retention, hypertension)
– histamine makes capillaries more permeable
∀ ↓ Capillary reabsorption
– hypoproteinemia (oncotic pressure ∝ blood albumin)
cirrhosis, famine, burns, kidney disease
• Obstructed lymphatic drainage

62. 20-62
Consequences of Edema
• Tissue necrosis
– oxygen delivery and waste removal impaired
• Pulmonary edema
– suffocation
• Cerebral edema
– headaches, nausea, seizures and coma
• Circulatory shock
– excess fluid in tissue spaces causes low
blood volume and low BP

63. 20-63
Mechanisms of Venous Return
• Pressure gradient
– 7-13 mm Hg venous pressure towards heart
• venules (12-18 mm Hg) to central venous pressure
(~5 mm Hg)
• Gravity drains blood from head and neck
• Skeletal muscle pump in the limbs
• Thoracic pump
– inhalation - thoracic cavity expands (pressure
↓) abdominal pressure ↑, forcing blood
upward
• Cardiac suction of expanding atrial space

64. 20-64
Skeletal Muscle Pump

65. 20-65
Venous Return and Physical Activity
• Exercise ↑ venous return in many ways
– heart beats faster, harder - ↑ CO and BP
– vessels of skeletal muscles, lungs and heart dilate ↑
flow
↑ respiratory rate ↑ action of thoracic pump
↑ skeletal muscle pump
• Venous pooling occurs with inactivity
– venous pressure not enough force blood upward
– with prolonged standing, CO may be low enough to
cause dizziness or syncope
• prevented by tensing leg muscles, activate skeletal m. pump
– jet pilots wear pressure suits

66. 20-66
Circulatory Shock
• Any state where cardiac output
insufficient to meet metabolic needs
– cardiogenic shock - inadequate pumping of
heart (MI)
– low venous return (LVR) shock - 3 principle
forms
1. hypovolemic shock - most common
– loss of blood volume: trauma, burns, dehydration
2. obstructed venous return shock
– tumor or aneurysm
3. venous pooling (vascular) shock
– next slide

67. 20-67
LVR Shock
• Venous pooling (vascular) shock
– long periods of standing, sitting or widespread
vasodilation
– neurogenic shock - loss of vasomotor tone,
vasodilation
• causes from emotional shock to brainstem injury
• Septic shock
– bacterial toxins trigger vasodilation and ↑ capillary
permeability
• Anaphylactic shock
– severe immune reaction to antigen, histamine release,
generalized vasodilation, ↑ capillary permeability

68. 20-68
Responses to Circulatory Shock
• Compensated shock
• Decompensated shock

69. 20-69
Compensated shock
• Homeostatic mechanisms bring about
recovery
∀↓ BP triggers baroreflex and production of
angiotensin II, both stimulate
vasoconstriction
• If person faints and falls to horizontal
position, gravity restores blood flow to
brain; quicker if feet are raised

70. 20-70
Decompensated shock
• Life threatening positive feedback loops occur
↓ CO → myocardial ischemia and infarction →
↓ CO
– slow circulation → disseminated intravascular
coagulation → slow circulation
– ischemia and acidosis of brainstem → ↓
vasomotor tone, vasodilation → ↓ CO →
ischemia and acidosis of brainstem

71. 20-71
Special Circulatory Routes- Brain
• Total perfusion kept constant
– seconds of deprivation causes loss of consciousness
– 4-5 minutes causes irreversible brain damage
– flow can be shifted from one active region to another
• Responds to changes in BP and chemistry
– cerebral arteries: dilate as BP ↓, constrict as BP rises
– main chemical stimulus: pH
• CO2 + H2O → H2 CO3 → H+
+ (HCO3)-
• hypercapnia (CO2 ↑) in brain, pH ↓, triggers vasodilation
• hypocapnia, ↑ pH, vasoconstriction
– occurs with hyperventilation, may lead to ischemia,
dizziness and sometimes syncope

72. 20-72
TIA’s and CVA’s
• TIA’s - transient ischemic attacks
– dizziness, loss of vision, weakness, paralysis,
headache or aphasia; lasts from a moment to a few
hours, often early warning of impending stroke
• CVA - cerebral vascular accident (stroke)
– brain infarction caused by ischemia
• atherosclerosis, thrombosis, ruptured aneurysm
– effects range from unnoticeable to fatal
• blindness, paralysis, loss of sensation, loss of speech
common
– recovery depends on surrounding neurons, collateral
circulation

73. 20-73
Special Circulatory Routes -
Skeletal Muscle
• Highly variable flow
• At rest
– arterioles constrict, total flow about 1L/min
• During exercise
– arterioles dilate in response to epinephrine and
sympathetic nerves
– precapillary sphincters dilate due to lactic acid, CO2
– blood flow can increase 20 fold
• Muscular contraction impedes flow
– isometric contraction causes fatigue faster than
isotonic

74. 20-74
Special Circulatory Routes -
Lungs
• Low pulmonary blood pressure
– flow slower, more time for gas exchange
– capillary fluid absorption
• oncotic pressure overrides hydrostatic pressure
• Unique response to hypoxia
– pulmonary arteries constrict, redirects flow to
better ventilated region

75. 20-75
Pulmonary Circulation
• Pulmonary trunk to pulmonary arteries to lungs
– lobar branches for each lobe (3 right, 2 left)
• Pulmonary veins return to left atrium
– increased O2 and reduced CO2 levels

76. 20-76
Pulmonary Capillaries Near Alveoli
• Basketlike
capillary beds
surround
alveoli
• Exchange of
gases with air
at alveoli

77. 20-77
Major Systemic Arteries
• Supplies oxygen and nutrients to all organs

78. 20-78
Major Branches of Aorta
• Ascending aorta
– right and left coronary arteries supply heart
• Aortic arch
– brachiocephalic
• right common carotid supplying right side of head
• right subclavian supplying right shoulder and upper limb
– left common carotid supplying left side of head
– left subclavian supplying shoulder and upper limb
• Descending aorta
– thoracic aorta above diaphragm
– abdominal aorta below diaphragm

79. 20-79
Major Branches of the Aorta

80. 20-80
Arteries of the Head and Neck
• Common carotid to internal and external carotids
– external carotid supplies most external head structures

81. 20-81
Arterial Supply of Brain
• Paired vertebral aa. combine to form basilar artery on pons
• Circle of Willis on base of brain formed from anastomosis of
basilar and internal carotid aa
• Supplies brain, internal ear and orbital structures
– anterior, middle and posterior cerebral
– superior, anterior and posterior cerebellar

82. 20-82
Arteries of the Upper Limb
• Subclavian
passes between
clavicle and 1st
rib
• Vessel changes
names as passes
to different
regions
– subclavian to
axillary to brachial
to radial and ulnar
– brachial used for
BP and radial
artery for pulse

83. 20-83
Arteries of the Thorax
• Thoracic aorta supplies viscera and body wall
– bronchial, esophageal and mediastinal branches
– posterior intercostal and phrenic arteries
• Internal thoracic, anterior intercostal and
pericardiophrenic arise from subclavian artery

84. 20-84
Major Branches of Abdominal Aorta

85. 20-85
Celiac Trunk Branches
• Branches of celiac trunk supply upper
abdominal viscera -- stomach, spleen, liver and
pancreas

86. 20-86
Mesenteric Arteries

87. 20-87
Arteries of the Lower Limb
• Branches to the lower limb arise from external
iliac branch of the common iliac artery

88. 20-88
Arterial Pressure Points
• Some major arteries close to surface -- allows
palpation for pulse and serve as pressure
points to reduce arterial bleeding

89. 20-89
Major Systemic Veins
• Deep veins run parallel to arteries while
superficial veins have many anastomoses

90. 20-90
Deep Veins of Head and Neck
• Large, thin-walled dural sinuses form in between
layers of dura mater (drain brain to internal
jugular vein)

91. 20-91
Superficial Veins of Head and Neck
• Branches of internal and external jugular veins
drain the external structures of the head
• Upper limb is drained by subclavian vein

92. 20-92
Superficial and Deep Veins of Upper
Limb

93. 20-93
Inferior Vena Cava and Branches
• Notice absence of veins draining the viscera ---
stomach, spleen, pancreas and intestines

94. 20-94
Veins of Hepatic Portal System
• Drains blood from viscera (stomach, spleen and
intestines) to liver so that nutrients are absorbed

95. 20-95
Superficial and Deep Veins of Lower Limb