Chapter 21: The Cardiovascular System: Blood Vessels and Hemodynamics

Copyright 2009, John Wiley & Sons, Inc.

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

1.

Tunica interna (intima) 2.

Tunica media



3.

Tunica externa

Modifications account for 5 types of blood vessels and their structural/ functional differences Copyright 2009, John Wiley & Sons, Inc.

Histological Structure of Blood Vessels

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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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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Copyright 2009, John Wiley & Sons, Inc.

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

contractility

1.

elasticity

and

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.

2.

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.

21_01d

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

Arteries

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

Arterioles

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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.

21_01e

Types of Capillaries

 1.

2.

3.

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

   

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!

Arterioles → Capillaries → Venule

Copyright 2009, John Wiley & Sons, Inc.

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.

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

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.

 

Circulatory Routes

(a) Simplest pathway (1 capillary bed)

simplest and most common route  heart  arteries capillaries   arterioles  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

(c) Arteriovenous anastomosis (shunt)

before returning to heart   between hypothalamus and anterior pituitary in kidneys  between intestines to liver

(d) Venous anastomoses (b) Portal system (2 capillary beds) (e) Arterial anastomoses

  

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

(a) Simplest pathway (1 capillary bed) (b) Portal system (2 capillary beds) (c) Arteriovenous anastomosis (shunt)

   two arteries merge provides collateral (alternative) routes of blood supply to a tissue coronary circulation and around joints

(d) Venous anastomoses (e) Arterial anastomoses



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.

Diffusion -

Most important method

 

Substances move down their concentration gradient

 O 2 and nutrients from blood to interstitial fluid to body cells  CO 2 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.

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.

Capillary Filtration and Reabsorption

  capillary filtration - at arterial end capillary reabsorption - at venous end

Arteriole

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Net filtration pressure: 13 out 33 out 20 in Net reabsorption pressure: 7 in 13 out 20 in

 variations  location    glomeruli - devoted to filtration alveolar capillary -devoted to absorption activity or trauma  increases filtration

Capillary Blood flow Arterial end 30 out +3 out 33 out 28 in –8 out 20 in 13 out Forces (mm Hg) Hydrostatic pressures Blood hydrostatic pressure Interstitial hydrostatic pressure Net hydrostatic pressure Colloid osmotic pressures (COP) Blood Tissue fluid Oncotic pressure (net COP) Venous end 10 out +3 out 13 out 28 in –8 out 20 in Net filtration or reabsorption pressure 7 in Venule

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.

Dynamics of Capillary Exchange

10

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

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

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 Copyright 2009, John Wiley & Sons, Inc.

Vascular resistance

  Opposition to blood flow due to friction between blood b.v. & walls 1.

Depends on Size of lumen – vasoconstriction males lumen smaller meaning greater resistance 2.

3.

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.

     

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 - 7-13 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.

3

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.

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.

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.

CV Center

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.

Chemoreceptors & Baroreceptors

Carotid body Aortic body Aortic body

40

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright 2009, John Wiley & Sons, Inc.

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.

21_table_02

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

 

Autoregulation of blood flow within tissues

Local vasodilators accelerate blood flow in response to:      Decreased tissue O 2 levels or increased CO 2 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

Autoregulation of blood pressure

  

2 general types of stimuli

1.

2.

Physical

temperature

changes: warming promotes vasodilation and cooling vasoconstriction

myogenic

response: arteriole contracts more forcefully when stretched 1.

2.

3.

Chemicals

alter blood vessel diameter

WBC, platelets, smooth muscle, macrophages endothelial cells release vasoactive chemicals and

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

Copyright 2009, John Wiley & Sons, Inc.

Circulation

   Important difference between autoregulatory response

pulmonary

and

systemic circulation

in  

Systemic blood vessel walls increase O 2 delivery dilate in response to low O Walls of pulmonary blood vessels constrict under low O 2 ensure most blood flows to better ventilated areas of lung 2 The brain



Four arteries which anastomose insuring constant blood flow The heart



Coronary arteries arising from the ascending aorta to to

Copyright 2009, John Wiley & Sons, Inc.

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 Copyright 2009, John Wiley & Sons, Inc.

21_16

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

Copyright 2009, John Wiley & Sons, Inc.

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