Chapter 21:

Blood 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.

The types of blood vessels, their structures

and functions.

6 Classes of Blood Vessels

1. Arteries:– carry blood away from heart– Branch and decrease in diameter

2. Arterioles:– Are smallest branches of arteries– Connect to capillaries

3. Capillaries:– are smallest blood vessels– location of exchange between blood and interstitial

fluid

6 Classes of Blood Vessels

4. Venules:– Smallest veins– collect blood from capillaries

5. Veins:– return blood to heart– Converge and increase in diameter

6. Anastomoses:- Bypass connection between vessels

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

The Smallest Blood Vessels

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

across walls

The Anatomy of Blood Vessels

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

Arteries and Veins

• Walls have 3 layers:1. tunica intima2. tunica media3. tunica externa

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

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

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

Vasa Vasorum

• Small arteries and veins• Found:

– in walls of large arteries and veins

• Function:– Supply cells of tunica media and

tunica externa

Structure of Vessel Walls

Figure 21-1

Structure of Blood Vessels

Figure 21-2

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

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

Artery Characteristics

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

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

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

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

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

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

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

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

Capillary Structure

Figure 21-4

Capillary Function

• Location of all exchange functions of cardiovascular system

• Materials diffuse between blood and interstitial fluid

Capillary Structure

• Endothelial tube, inside thin basal lamina

• No tunica media• No tunica externa• Diameter is similar to red blood

cell

Capillaries

Types of Capillaries

1. 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 blood–brain barrier

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

Fenestrated Capillaries

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

Sinusoids

• Areas in:– liver– spleen– bone marrow– endocrine organs

• Have gaps between adjacent endothelial cells

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

Capillary Networks

Figure 21-5

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

Capillary Sphincter

• Guards entrance to each capillary • Opens and closes, causing

capillary blood to flow in pulses

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

Structure of Blood Vessels

Figure 21-2

Veins vs. Arteries

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

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

3 Vein Categories

1. Venules (5th type of vessel):– very small veins

• Average diameter 20 µm

– collect blood from capillary beds– Small venules lack tunica media

2. Medium-sized veins:- Diameter 2-9 mm

3. Large Veins:- Diameters up to 3 cm

Valves in the Venous System

Figure 21-6

Valves in tunicaintima insure oneway movement

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

Health Problems with Veins

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

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

6. Anastomoses

• Bypass routes between vessels– Bypass the capillary bed

• Not present in retina, kidney, or spleen

• More common in veins

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?

A. arteries

B. capillaries

C. arterioles

D. veins

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

A. venous blood pressure is lower

B. venous blood pressure is higher

C. venous walls are more muscular

D. venous lumens are larger

Where in the body would you find fenestrated capillaries?

A. absorptive areas of intestine

B. filtration areas of kidney

C. choroid plexus of brain

D. all of the above are correct

Figure 21-7

Blood Distribution

The mechanisms that regulate blood flow through arteries,

capillaries, and veins.

Physiology of Circulation

Figure 21-8

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

Factors that influence blood pressure and its

regulation.

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

Force (F)

• Is proportional to the pressure difference (P)

• Divided by R

Vascular Resistance

• Adult vessel length is constant• Vessel diameter varies by

vasodilation and vasoconstriction• R increases exponentially as vessel

diameter decreases

Vasoconstriction andVasodilation

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

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

Pressure

• Blood pressure changes throughout body– Greatest in arteries leaving heart, lowest

in veins returning to heart

• Person’s BP measured at arteries near heart– Systolic pressure/diastolic pressure (from

ventricles, squeeze/rest)– “Normal” = 110/70 mmHg

Vessel Diameter and Cardiac Pressure

Figure 21-9a

Figure 21-10

Pressures in the Systemic Circuit

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

Abnormal Blood Pressure

• Hypertension: – Arterial pressure > 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

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

In a healthy individual, where would the blood pressure be greater, at the aorta or

at the inferior vena cava?

A. aorta

B. inferior vena cava

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

Explain.

A. Blood has pooled in her lower limbs.

B. Cardiac output has decreased, sending less blood to the brain.

C. Sweating has reduced blood volume.

D. All of the above have occurred.

The mechanisms and pressures involved

in the movement of fluids between capillaries and

interstitial spaces.

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. Diffusion2. Filtration3. Reabsorption

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

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

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

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

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

Figure 21-12

Forces Across Capillary Walls

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

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

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

Edema

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

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

Cardiovascular Regulation

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 Mechanisms1. Autoregulation2. Neural Mechanism3. Hormonal Regulation

1. Autoregulation

1. Autoregulation– causes immediate, localized

homeostatic adjustments– Single capillary bed: action at a

precapillary sphincter

1. Autoregulation

1. 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

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

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

3. Hormonal Regulation1. Antidiuretic Hormone (ADH)

– From pituitary gland in response to low blood volume– Causes vasoconstriction and water conservation at

kidney2. 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

3. Hormonal Regulation3. Erythropoietin

– From kidney in response to low O2

– Stimulates production and maturation of RBCs4. 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

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

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

1.Incr. cardiac output = trigger peripheral vasoconstriction to incr. BP

2.Venoconstrict to moblize venous reserve to incr. blood volume

3.Release NE, ADH, Angiotensin II to incr. BP

Cardiovascular Response to Hemorrhages

• Long term (aimed at restoring normal blood volume after hemorrhage)1. Recall fluid from interstitial spaces2. Release Incr. ADH for fluid retention at kidney3. Increase thirst4. Release EPO to Incr. RBCs

Shock• Low BP and inadequate blood flow• Due to:

– Loss of > 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

Circulatory Collapse

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

• Results in no blood flow = death

Aging and the Cardiovascular System

1. Decreased hematocrit2. Increased blood clots (thrombus)

formation3. Blood-pools in legs

– due to venous valve deterioration

4. Reduction in max Cardiac output5. Increased arteriosclerosis

A blockage of which branch from the aortic arch would interfere with

blood flow to the left arm?

A. left common carotid artery

B. left subclavian artery

C. brachiocephalic trunk

D. right common carotid artery

Why would a compression of the common carotid arteries cause a person

to lose consciousness?

A. Because it would cause a reflexive decrease in heart rate and blood pressure.

B. Because cerebral arteries would dilate in response to pressure.

C. Because increased blood pressure would occur at the carotid sinus.

D. Because rapid fall in blood flow to the brain would occur.

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

Which vein is this?

A. superior vena cava

B. brachiocephalic vein

C. internal jugular vein

D. external jugular vein

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?

A. umbilical artery

B. umbilical vein

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

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

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