1. Chapter 21: The Cardiovascular System: Blood Vessels and Hemodynamics
Copyright 2009, John Wiley & Sons, Inc.

2. Structure and function of blood vessels
The circulatory system forms a CLOSED system of tubes that carries blood
5 main types
Arteries – carry blood AWAY from the heart
Arterioles (muscular + regular)
Capillaries – site of exchange ; in general conects arteries to veins
Venules (muscular + regular)
Veins – carry blood TO the heart
3 layers or tunics
Tunica interna (intima)
Tunica media
Tunica externa
Modifications account for 5 types of blood vessels and their structural/ functional differences
Copyright 2009, John Wiley & Sons, Inc.

3. Histological Structure of Blood Vessels

4. Structure of Vessel Wall
tunica interna (tunica intima)- lines the blood vessel ; exposed to blood
endothelium – simple squamous epithelium overlying a basement membrane and a sparse layer of loose connective tissue
acts as a selectively permeable barrier
secrete chemicals that stimulate dilation or constriction of the vessel
normally repels blood cells & platelets that may adhere to it and form a clot
when tissue around vessel is inflamed, the endothelial cells produce cell-adhesion molecules that induce leukocytes to adhere to the surface
causes leukocytes to congregate in tissues where their defensive actions are needed
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5. Structure of Vessel Wall
tunica media - middle layer
consists of smooth muscle, collagen, and elastic tissue
strengthens vessel and prevents blood pressure from rupturing them
vasomotion – changes in diameter of the blood vessel brought about by smooth muscle
tunica externa (tunica adventitia)-outermost layer
consists of loose connective tissue that often merges with that of neighboring blood vessels, nerves, or other organs
anchors the vessel and provides passage for small nerves, lymphatic vessels
vasa vasorum – small vessels that supply blood to at least the outer half of the larger vessels
blood from the lumen is thought to nourish the inner half of the vessel by diffusion
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Arteries
3 layers of typical blood vessel
Thick muscular-to-elastic tunica media
High compliance – walls stretch and expand in response to pressure without tearing
The functional properties of arteries are elasticity and contractility
Elasticity, due to the elastic tissue in the tunica internal and media, allows arteries to accept blood under great pressure from the contraction of the ventricles and to send it on through the system.
Contractility, due to the smooth muscle in the tunica media, allows arteries to increase or decrease lumen
Vasoconstriction – decrease in lumen diameter
Vasodilation – increase in lumen diameter
Copyright 2009, John Wiley & Sons, Inc.

9. 21_01d
21_01d

10. Compared to veins, arteries have thicker walls, more smooth muscle
and elastic fibers and are more resilient

11. Arteries- Undergo changes in diameter – Vasoconstriction /Vasodilation
General Types:
1. Elastic (conducting) Arteries:esrvoiroducting (elastic) arteries – largest
Tunica media has lots of elastic fibers, example: the aorta helps propel blood away from the heart ; pulmonary ; carotid
expand during systole, recoil during diastole; lessens fluctuations in BP
Function as pressure reservoir
2.) Distributing (muscular) arteries
medium sized, tunica media has more smooth muscle fibers, they can vasoconstrict and vasodilate, example radial artery
3.) Resistance (small) arteries, example: arterioles
arterioles control amount of blood to various organs
4.) Metarterioles – short vessels connect arterioles to capillaries
-have precapillary sphincters→monitor capillary flow; constriction of
sphincter reduces bloodflow through their capillaries→redirects
bloodflow to other beds
**Anastomoses- union of branches of two or more arteries supplying the same body region →provides alternate routes/ collateral circulation
Arteries

12. Arterioles
21-12

13. Capillaries - An endothelial tube inside a basal lamina
Smallest blood vessels that connect arterial outflow and venous return
Microcirculation – flow from metarteriole through capillaries and into postcapillary venule
Exchange vessels – primary function is exchange between blood and interstitial fluid
Lack tunica media and tunica externa
Substances pass through just one layer of endothelial cells and basement membrane
Capillary beds – arise from single metarteriole
Vasomotion – intermittent contraction and relaxation
Thoroughfare channel – bypasses capillary bed
Copyright 2009, John Wiley & Sons, Inc.

14. 21_01e
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15. Types of Capillaries 3 types Continuous
Endothelial cell membranes from continuous tube
Fenestrated
Have fenestrations or pores
Sinusoids(Discontinuous)
Wider and more winding
Unusually large fenestrations
Allow proteins, clotting factors and blood cells to enter circulation

16. Capillary Beds
capillaries organized into networks & usually supplied by single metarteriole
thoroughfare channel - metarteriole that continues through capillary bed to venule
precapillary sphincters control which beds are well perfused
when sphincters open
capillaries are well perfused with blood and engage in exchanges with the tissue fluid
when sphincters closed
blood bypasses the capillaries
flows through thoroughfare channel to venule
three-fourths of the bodies capillaries are shut down at a given time!

17. Arterioles → Capillaries → Venule
Copyright 2009, John Wiley & Sons, Inc.

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Veins
Structural changes not as distinct as in arteries
Very thin walls in relation to total diameter
Same 3 layers
Tunica interna thinner than arteries
Tunica interna thinner w/ little smooth muscle
Tunica externa thickest layer
Not designed to withstand high pressure
Valves – folds on tunica interna forming cusps
Aid in venous return by preventing backflow
Three Categories:
Venules: very small veins; collect blood from capillaries
Medium-sized veins: thin tunica media & few smooth muscle cells
: tunica externa with longitudinal bundles of elastic fibers
Large veins: have all 3 tunica layers; thick tunica externa; thin tunica media
Copyright 2009, John Wiley & Sons, Inc.

19. Valves in the Venous System
Vein Valves are folds of tunica intima → Prevent blood from flowing backward
-Compression pushes blood toward heart
Figure 21-6

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Blood Distribution
Largest portion of blood at rest is in systemic veins and venules
Blood reservoir
1/3 of venous blood is in the large venous networks of the liver, bone marrow, and skin
Venoconstriction reduces volume of blood in reservoirs and allows greater blood volume to flow where needed
Copyright 2009, John Wiley & Sons, Inc.

21. Circulatory Routes simplest and most common route
(a) Simplest pathway
(1 capillary bed)
simplest and most common route
heart  arteries  arterioles  capillaries  venules  veins
passes through only one network of capillaries from the time it leaves the heart until the time it returns
portal system
blood flows through two consecutive capillary networks before returning to heart
between hypothalamus and anterior pituitary
in kidneys
between intestines to liver
(b) Portal system
(2 capillary beds)
(c) Arteriovenous
anastomosis
(shunt)
(d) Venous
anastomoses
(e) Arterial
anastomoses

22. Anastomoses - point where 2 blood vessels merge
arteriovenous anastomosis (shunt)
artery flows directly into vein bypassing capillaries
venous anastomosis
most common
one vein empties directly into another
reason vein blockage less serious than an arterial blockage
arterial anastomosis
two arteries merge
provides collateral (alternative) routes of blood supply to a tissue
coronary circulation and around joints
(a) Simplest pathway
(1 capillary bed)
(b) Portal system
(2 capillary beds)
(c) Arteriovenous
anastomosis
(shunt)
(d) Venous
anastomoses
(e) Arterial
anastomoses

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Capillary exchange - Movement of substances between blood and interstitial fluid
3 basic methods : 1. Diffusion 2. Transcytosis 3. Bulk flow
Copyright 2009, John Wiley & Sons, Inc.

24. Diffusion - Most important method
Substances move down their concentration gradient
O2 and nutrients from blood to interstitial fluid to body cells
CO2 and wastes move from body cells to interstitial fluid to blood
Can cross capillary wall through intracellular clefts, fenestrations or through endothelial cells
Most plasma proteins cannot cross
Except in sinusoids – proteins and even blood cells leave
Blood-brain barrier – tight junctions limit diffusion
Transcytosis
- Small quantity of material
Substances in blood plasma become enclosed within pinocytotic vesicles
that enter endothelial cells by endocytosis and leave by exocytosis
-Important mainly for large, lipid-insoluble molecules that cannot cross
capillary walls any other way
Copyright 2009, John Wiley & Sons, Inc.

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Bulk Flow
Passive process in which large numbers of ions, molecules, or particles in a fluid move together in the same direction
Based on pressure gradient
Diffusion is more important for solute exchange !!
Bulk flow more important for regulation of relative volumes of blood and interstitial fluid !!
Filtration – from capillaries into interstitial fluid
Reabsorption – from interstitial fluid into capillaries
Copyright 2009, John Wiley & Sons, Inc.

26. Capillary Filtration and Reabsorption
- at arterial end
capillary reabsorption
- at venous end
variations
location
glomeruli
- devoted to filtration
alveolar capillary
-devoted to absorption
activity or trauma
increases filtration
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Arteriole
Venule
Net
filtration
pressure:
13 out
Net
reabsorption
pressure:
7 in
33 out
13 out
20 in
Capillary
20 in
Blood flow
Arterial end
Forces (mm Hg)
Venous end
Hydrostatic pressures
30 out
Blood hydrostatic pressure
10 out
+3 out
Interstitial hydrostatic pressure
+3 out
33 out
Net hydrostatic pressure
13 out
Colloid osmotic pressures (COP)
28 in
Blood
28 in
–8 out
Tissue fluid
–8 out
20 in
Oncotic pressure (net COP)
20 in
13 out
Net filtration or reabsorption pressure
7 in

27. NFP = (BHP + IFOP) – (BCOP + IFHP)
Net filtration pressure (NFP) balance of 2 pressures
2 pressures promote filtration :
Blood hydrostatic pressure (BHP) generated by pumping action of heart
Falls over capillary bed from 35 to 16 mmHg
Interstitial fluid osmotic pressure (IFOP)
1 mmHg
2 pressures promote reabsorption:
Blood colloid osmotic pressure (BCOP) promotes reabsorption
-Due to blood plasma proteins too large to cross walls
- Averages 36 mmHg
Interstitial fluid hydrostatic pressure (IFHP)
- Close to zero mmHg
Copyright 2009, John Wiley & Sons, Inc.

28. Dynamics of Capillary Exchange10 9
Starling’s law of the capillaries- volume of fluid & solutes reabsorbed is almost as large as the volume filtered
On average, about 85% of fluid filtered in reabsorpbed
Excess enters lymphatic capillaries (about 3L/ day) to be eventually returned to blood

29. Principles of Blood Flow
Blood supply to a tissue can be expressed in terms of flow & perfusion
Blood flow – the amount of blood flowing through an organ, tissue, or blood vessel in a given time (ml/min)
Perfusion – flow/given volume or mass of tissue in a given time (ml/min/g)
At rest, total flow is quite constant & is equal to cardiac output (5.25 L/min)
Volume of blood that circulates through systemic (or pulmonary) blood vessels each minute→important for delivery of nutrients & oxygen, & removal of metabolic wastes
CO = heart rate (HR) x stroke volume (SV)
Distribution of CO depends on
Pressure differences that drive blood through tissue
- Flows from higher to lower pressure
Resistance to blood flow in specific blood vessels
- Higher resistance means smaller blood flow
F is proportional to 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 the less the flow

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Blood Pressure
Contraction of ventricles generates blood pressure
Systolic BP – highest pressure attained in arteries during systole
Diastolic BP – lowest arterial pressure during diastole
Pressure falls progressively with distance from left ventricle
Blood pressure also depends on total volume of blood
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Vascular resistance
Opposition to blood flow due to friction between blood b.v. & walls
Depends on
Size of lumen – vasoconstriction males lumen smaller meaning greater resistance
Blood viscosity – ratio of RBCs to plasma and protein concentration, higher viscosity means higher resistance
Total blood vessel length – resistance directly proportional to length of vessel
400 miles of additional blood vessels for each 2.2lb. of fat
Copyright 2009, John Wiley & Sons, Inc.

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Venous return - Volume of blood flowing back to heart through systemic veins (due to pressure generated by Left ventricle constriction)
Mechanisms of Venous Return:
pressure gradient - BP is the most important force in venous return
mm Hg venous pressure towards heart
venules (12-18 mm Hg) to central venous pressure – point where the venae cavae enter the heart (~5 mm Hg)
gravity drains blood from head and neck
skeletal muscle pump in the limbs
- contracting muscle squeezed out of the compressed part of the vein
thoracic (respiratory) pump
inhalation - thoracic cavity expands and thoracic pressure decreases, abdominal pressure increases forcing blood upward
CVP fluctuates : 2mm Hg- inhalation, 6mm Hg-exhalation
blood flows faster with inhalation
cardiac suction of expanding atrial space
Copyright 2009, John Wiley & Sons, Inc.

33. Proximal
valve
Distal 1 2 3
Proximal
valve
Distal 1 2
Proximal
valve
Distal 1

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Velocity of blood flow
Speed in cm/sec in inversely related to cross-sectional area
Velocity is slowest where total cross sectional area is greatest
Blood flow becomes slower farther from the heart
Slowest in capillaries→ Aids in exchange
Circulation time – time required for a drop of blood to pass from right atrium, through pulmonary and systemic circulation and back to right atrium→ Normally 1 minute at rest
Copyright 2009, John Wiley & Sons, Inc.

36. Control of blood pressure and blood flow
Interconnected negative feedback systems control blood pressure by adjusting heart rate, stroke volume, systemic vascular resistance, and blood volume
Some act faster that others ; Some shorter- or longer-term
Mechanisms:
Neural mechanisms
Endocrine mechanisms
Autoregulation
Copyright 2009, John Wiley & Sons, Inc.

37. Role of cardiovascular center (CV)
In medulla oblongata
Helps regulate heart rate and stroke volume
Also controls neural, hormonal, and local negative feedback systems that regulate blood pressure and blood flow to specific tissues
Groups of neurons regulate heart rate, contractility of ventricles, and blood vessel diameter
Cardiac (cardiostimulatory and cardioinhibitory) centers
Vasomotor center control blood vessel diameter
Vasoconstriction via adrenergic release of NE
Vasodilation via direct or indirect release of NO
Receives input from both higher brain regions and sensory receptors:
Baroreceptors reflexes - monitor stretch
Atrial baroreceptors monitor blood pressure
Chemoreceptor reflexes monitor CO2, O2, or pH levels
Output from CV center flows along neurons of ANS
Sympathetic (stimulatory) vs. parasympathetic (inhibitory)
Copyright 2009, John Wiley & Sons, Inc.

38. CV Center

39. Neural regulation of blood pressure
Negative feedback loops from 2 types of reflexes
1. Baroreceptor reflexes
Pressure-sensitive receptors in internal carotid arteries and other large arteries in neck and chest
Carotid sinus reflex helps regulate blood pressure in brain
Aortic reflex regulates systemic blood pressure
When blood pressure falls, baroreceptors stretched less, slower rate of impulses to CV which then decreases parasympathetic stimulation and increases sympathetic stimulation
2.Chemoreceptor reflexes
Receptors located close to baroreceptors of carotid sinus (carotid bodies) and aortic arch (aortic bodies)
Detect hypoxia (low O2), hypercapnia (high CO2), acidosis (high H+) and send signals to CV → increases sympathetic stimulation to arterioles and veins→vasoconstriction & increase in blood pressure
Receptors also provide input to respiratory center to adjust breathing rate
Copyright 2009, John Wiley & Sons, Inc.

40. Chemoreceptors &
Baroreceptors
Carotid body
Aortic body

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44. Hormonal regulation of blood pressure
1. Renin-angiotensin-aldosterone (RAA) system
Renin (released by kidney when blood volume falls or blood flow decreases) and angiotensin converting enzyme (ACE) act on substrates to produce active hormone angiotensin II
Raises BP by vasoconstriction and secretion of aldosterone
- increases water reabsorption in kidneys ≈ ↑blood volume & pressure)
2. Epinephrine and norepinephrine
Adrenal medulla releases in response to sympathetic stimulation
Increase CO by increasing rate and force of heart contractions
3. Antidiuretic hormone (ADH) or vasopressin
Produced by hypothalamus, released by posterior pituitary
Response to dehydration or decreased blood volume
Causes vasoconstriction which increases BP
4. Atrial Natriuretic Peptide (ANP)- Released by cells of atria
Lowers blood pressure by causing vasodilation and promoting loss of salt and water in urine→ Reduces blood volume
Copyright 2009, John Wiley & Sons, Inc.

45. 21_table_02
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48. Autoregulation of blood flow within tissues
Local vasodilators accelerate blood flow in response to:
Decreased tissue O2 levels or increased CO2 levels
Generation of lactic acid
Release of nitric acid
Rising K+ or H+ concentrations in interstitial fluid
Local inflammation
Elevated temperature
2 general types of stimuli:
Physical
Temp. changes: warming→vasodilation ; cooling→vasoconstriction
myogenic response: arteriole contracts more forcefully when stretched
Chemicals - alter blood vessel diameter
Released by WBC, platelets, smooth muscle, macrophages and endothelial cells release vasoactive chemicals
Vasodilation: K+, H+, lactic acid, adenosine (ATP), NO, kinins, histamine
Vasoconstriction: thromboxane A2, superoxide, serotonin, endothelins

49. Autoregulation of blood pressure
2 general types of stimuli
Physical
temperature changes: warming promotes vasodilation and cooling vasoconstriction
myogenic response: arteriole contracts more forcefully when stretched
Chemicals alter blood vessel diameter
WBC, platelets, smooth muscle, macrophages and endothelial cells release vasoactive chemicals
Vasodilation: K+, H+, lactic acid, adenosine (ATP), NO, and tissue injury releases kinins and histamine
Vasoconstriction: thromboxane A2, superoxide radicals, serotonin from platelets, and endothelins from endothelial cells
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Circulation
Important difference between pulmonary and systemic circulation in autoregulatory response
Systemic blood vessel walls dilate in response to low O2 to increase O2 delivery
Walls of pulmonary blood vessels constrict under low O2 to ensure most blood flows to better ventilated areas of lung
The brain
Four arteries which anastomose insuring constant blood flow
The heart
Coronary arteries arising from the ascending aorta
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Shock
Types of Shock
Hypovolemic: acute trauma
Cardiogenic MI
Ischemia
Heart valve disease
Arrhythmias
Normal blood volume and cardiac output
Anaphylactic
Neurogenic
Septic
Obstructive
Pulmonary embolism
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52. 21_16
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53. Signs and symptoms of shock
Systolic < 90
Rapid heart rate
Pulse weak and rapid
Skin cool, pale and clammy
Altered mental state
Urine output reduced
Thirsty
pH is low
nausea
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54. 21_17
21_17