Blood Vessels

I. Structure of Blood Vessels Walls

A. Three layers

1. Tunica intera

a. Inner most layer

b. Endothelium

i. Simple squamous

c. Some larger vessels have subendothelium

i. Loose connective tissue

ii. Basement membrane

2. Tunica media

a. Middle layer

b. Circularly arranged smooth muscle

c. Chemical and nervous control of degree of contraction

i. Sympathetic NS

d. Change in diameter

i. Vasoconstriction

ii. Vasodilation

3. Tunica externa

a. Made of collagen fibers

b. Function

i. Protection

ii. Reinforcement

iii. Anchor to surrounding tissue

c. Accessory tissues

i. Nerve fibers

ii. Lymphatic vessels

iii. Elastic network

iv. Tiny blood vessels within layer-vasa vasorum


II. Arterial System

A. Classification based on size and function

1. Elastic (conducting) arteries

a. Characteristics

i. Thick-walled

ii. Near heart

iii. Largest diameter

iv. More elastic

v. Large lumen

b. Properties

i. Dampen BP changes associated with heart contraction

ii. Passive accommodation results in smooth flow of blood

c. 1.0 - 2.5 cm

2. Muscular arteries-distributing arteries

a. Distal to elastic arteries

b. Deliver blood to specific organs

c. Thick media layer

i. More smooth muscle

d. 0.3 - 1.0 cm

3. Arterioles

a. Determine flow into capillary beds

b. Mostly smooth muscle

c. 10 m - 0.3 cm

4. Capillaries

a. Smallest blood vessels

i. 8 - 10 m

b. Tunica interna only

c. Exchange of materials

B. Types of capillary

1. Continuous

a. Uninterrupted endothelial cells

b. Incomplete tight junctions

i. Intercellular clefts

2. Fenestrated

a. Endothelial cells have oval pores

i. Fenestrations

b. Pores permit greater permeability

3. Sinusoidal

a. Modified, leaky capillaries

b. Large molecules can pass through

C. Capillary beds

1. Capillaries act as networks-capillary beds

2. Microcirculation

a. Arteriole to venule

3. Parts of a capillary bed

a. Vascular shunt

i. Connects arteriole with venule

b. True capillaries

D. Sequence of blood movement through capillary bed

1. Terminal arteriole

2. Metateriole

a. True capillaries branch off

i. Pre-capillary sphincter controls blood flow into capillary

3. Thoroughfare channel

a. Capillaries rejoin

4. Post-capillary venule


III. Venous System

A. Types of vessels

1. Venules

a. 8 - 100 m

b. Characteristics vary with size

i. Little muscle

ii. Thin externa

2. Veins

a. Formed from venules

b. Thinner walls and less muscle than arteries

c. Little muscle in media

i. Mostly elastin

d. Externa is thickest wall layer

B. Capacitance vessels

1. Veins act as reservoirs

a. Large lumens

b. Low blood pressure allows walls to thin

2. Venous valves

a. Prevent backflow

b. Folds of interna


IV. Physiology of Circulation

A. Terms

1. Blood flow-volume flowing through a given structure per unit time (ml/min)

2. Blood pressure-force per unit area (mm Hg)

3. Resistance-opposition to flow; generally encountered in the systemic circuit-peripheral resistance (PR)

a. Sources of resistance

i. Blood viscosity-thickness related to formed elements

ii. Total blood vessels length-longer the vessels, the greater the resistance

iii. Blood vessel diameter-flow is inversely related to diameter; the larger the diameter, the less resistance (1/r4)

b. In healthy humans, diameter is the greatest source of resistance

B. Relationship between flow, pressure and resistance



Blood Flow (F) = ∆P/PR (Difference in blood pressure between two points/peripheral resistance


V. Systemic Blood Pressure

A. Background

1. Heart pumping generates blood flow

2. Pressure results when flow opposed by resistance

3. Blood flows along a pressure gradient

a. From higher to lower pressure

i. Highest in aorta

ii. Lowest in right atrium

B. Arterial blood pressure

1. Factors affecting arterial pressure

a. Stretching of arteries near heart

i. Compliance

ii. Distensibility

b. Volume of blood forced into the arteries near heart

2. Changes associated with systole

a. Aorta is stretched by blood leaving left ventricle

i. Kinetic energy

b. Blood moves toward periphery because peripheral pressure is lower than aortic pressure

i. Systolic pressure: 120 mm Hg

3. Changes associated with diastole

a. Semilunar valve closes

b. Aorta recoils

c. Pressure is maintained by reducing volume

i. Diastolic pressure: 70 - 80 mm Hg

4. Pulse pressure

a. Difference between systolic and diastolic pressure

5. Mean arterial pressure (MAP)

a. Diastolic pressure + 1/3 pulse pressure

C. Capillary blood pressure

1. 40 mm Hg entering

2. 20 mm Hg exiting

D. Venous blood pressure

1. Characteristics

a. Relatively steady throughout cardiac cycle

b. Gradient from venules to vena cava

i. 20 mm HG (60 from aorta to arterioles)

2. Venous return

a. Venous pressure is too low to promote adequate return

b. Need additional functional modifications

3. Functional modification

a. Respiratory pump

i. Abdominal (ventral body cavity) pressure increases squeeze local veins

ii. Backflow is prevented by valves

iii. Blood is forced toward the heart

iv. Chest cavity pressure decreases

v. Thoracic veins expand

vi. Blood enters right atrium

b. Muscular pump (more important)

i. Contraction of skeletal muscle surrounding veins compress vein

ii. Backflow is prevented by valves

iii. Blood moves in direction of heart


VI. Regulation of Blood Pressure

A. Factors influences blood pressure

1. Cardiac output

2. Peripheral resistance

3. Blood volume

B. Blood pressure = Cardiac output X Peripheral resistance

1. Cardiac output is directly related to blood volume

2. Blood pressure is directly related to CO, BV and PR

C. CO = Stroke volume X HR

D. Factors that enhance CO


1. Reduce parasympathetic control

a. Reduce effect of vagus nerve

i. HR increases

2. Increase sympathetic activity

a. Increases contractility of heart

i. Reduces ESV

ii. Increases stroke volume

b. Releases Epi into blood stream from adrenal medulla

i. Increases heart rate

3. Increase activity of respiratory and muscular pumps

a. Increases venous return

i. Increases EDV

ii. Increases stroke volume

E. Neural control of blood pressure

1. Short-term mechanisms

2. Nervous control of peripheral resistance

a. Alter blood distribution

b. Alter blood vessel diameter

3. Vasomotor center


a. Regulation of blood vessel diameter

b. Vasomotor fibers

i. Sympathetic efferents

ii. Innervate smooth muscle of blood vessels

iii. Primarily arterioles

iv. Release NE

v. Vasoconstrictor

c. Vasomotor tone

i. Tonic vasoconstriction

4. Baroreceptors


a. Detect changes in arterial blood pressure

i. Pressure sensitive mechanoreceptors

ii. When BP rises, receptors are stretched

b. Located in carotid sinuses, aortic arch and walls of all large vessels

c. Stretching increases signaling to vasomotor center

i. Inhibits vasomotor center

ii. Causes dilation of arteries and veins

d. Arteriole dilation reduces peripheral resistance

e. Venodilation shifts blood to venous reservoirs

i. Venous return decreases

ii. Cardiac output declines

f. Baroreceptors also send efferent signals to cardiac centers in the medulla

i. Inhibit sympathetic NS

ii. Stimulate parasympathetic NS

iii. HR and contractile force decrease

g. Respond to acute changes in blood pressure

i. Carotid sinus reflex protects blood supply to brain

ii. Aortic reflex maintains supply to systemic circuit

5. Chemoreceptors


a. Respond to changes in O2 and CO2 concentrations and pH

b. Located in carotid and aortic arch and carotid sinus

c. Primarily involved in control of respiratory rate and depth (see Respiration Lecture)

F. Chemical control of blood pressure

1. Short-term

2. Levels of O2 and CO2 (see Respiration Lecture)

3. Blood-borne chemicals


a. Adrenal medulla hormones

i. NE and EPI (nicotine is a monoamine agonist)

ii. NE is a vasoconstrictor

iii. EPI increase cardiac output by increasing cardiac muscle contractility

b. Atrial natriuretic peptide (ANP)

i. Atrial peptide hormone

ii. Reduces blood pressure by antagonizing aldosterone

iii. Increases water excretion from kidney

c. Antidiuretic hormone (ADH)

i. Posterior pituitary hormone

ii. Increases blood pressure by increasing water absorption by distal tubule

iii. At high concentrations, causes vasoconstriction

d. Angiotensin II

i. Mediated by release of renin by JGA of kidney tubule

ii. When amount of blood entering kidney tubule is too low, renin is released

iii. Renin catalyzes the conversion of angiotensinogen into angiotensin II

iv. Angiotensin II causes vasoconstriction of systemic arterioles

v. Increases BP

vi. Angiotensin II also causes release of aldosterone from adrenal cortex

vii. Aldosterone increases absorption of water by kidney tubules

e. Endothelium-derived factors

i. Endothelin-vasoconstrictor

ii. Prostaglandin-derived growth factor (PDGF)-vasoconstrictor

iii. Nitrous oxide (NO)-fast acting, local vasodilator

f. Inflammatory chemicals-vasodilators

i. Histamine, etc. (see Immune Lecture)

ii. Increase capillary permeability

g. Alcohol

i. Reduces blood pressure

ii. Inhibits ADH release-increases loss of water in urine

iii. Increases vasodilation (skin) by depressing vasomotor center

G. Renal regulation of blood pressure

1. Long-term mechanisms for blood pressure regulation

2. Kidney controls blood volume by regulating water loss in urine

3. Blood volume affects cardiac output via:

a. Venous pressure

b. Venous return

c. EDV

d. Stroke volume

4. Blood pressure change parallels change in blood volume

a. Increase in volume increases BP

i. Kidney responds by eliminating water to reduce volume

b. Decrease in volume decreases BP

i. Kidney responds by absorbing water to increase volume

5. Direct action of the kidney

a. Alteration to rate of fluid filtration from blood stream to kidney tubules

i. Increased BP increases amount of filtrate entering tubules

ii. Filtrate entering is greater than the amount that can be processed

iii. Fluid leaves body in the form of urine

iv. Blood volume decreases and therefore BP

b. Indirect renal mechanisms


i. Renin-angiotensin mechanism (see above)

ii. Aldosterone also causes Post. Pituitary to release ADH

iii. ADH promotes water reabsorption from by kidney


VII. Tissue Perfusion

A. Blood flow is distributed to body tissue in an exacting fashion

1. At rest

a. Brain: 13%

b. Heart 4%

c. Kidney: 20%

d. Abdominal organs: 24%

2. During exercise:

a. Skin, muscles and heart increase

b. Remaining tissues either remain same or decrease

B. Blood flow velocity

1. Inversely proportionate to cross-sectional area of blood vessels to be filled

a. Flow is fastest through vessels with smallest cross-sectional area

i. Aorta has a cross-sectional area (2.5 cm2) and an average velocity of 40-50 cm/s

ii. Capillaries have a total cross-sectional area of 4500 cm2 and a very slow flow (0.03 cm/s)

C. Blood flow through individual organs is intrinsically controlled (i.e., autoregulation)

1. Diameter of arterioles feeding a given organ is controlled by that organ

D. Intrinsic control mechanisms

1. Metabolic controls-Levels of nutrients, particularly oxygen, act as autoregulation stimuli

2. Myogenic controls

a. Excessive or inadequate blood pressure can damage or cause the death of an organ

b. Such blood pressure changes stimulate myogenic responses


VIII. Capillary Dynamics


A. Gases and nutrients diffuse from capillary to interstitial fluid

1. Water-soluble solutes pass through clefts and fenestrations

2. Lipid-soluble diffuse through the plasma membranes of capillary epithelial cells

B. Forces responsible for the direction and amount of fluid crossing capillary walls

1. Hydrostatic and osmotic pressure

a. Forces oppose

C. Hydrostatic pressure

1. Force exerted by a fluid against a vessel wall

a. In the capillary bed

i. Hydrostatic pressure is the same as capillary blood pressure

2. Capillary hydrostatic pressure (HPc) forces fluid through capillary wall

a. Greater at arterial end (35 mm Hg)

b. Lower at venous end (17 mm Hg)

3. HPc is opposed by interstitial fluid hydrostatic pressure (Hpif)

a. Hpif is assumed to be zero

i. Interstitial fluid is withdrawn by lymphatic tissue

4. Net effective hydrostatic pressure is equal to HPc (HPc - Hpif)

D. Osmotic pressure

1. Net movement of water from an area of low to high solute concentration

a. Solute concentration in relatively high in capillary blood

i. High concentration of plasma proteins

ii. Capillary colloid osmotic pressure (OPc)

iii. 26 mm Hg

b. Interstitial osmotic pressure (OPif) is much lower

i. 0.1 to 5 mm Hg

c. Net osmotic pressure

i. Approximately 25 mm Hg

E. Net filtration pressure (NFP)

1. Reflects interaction between hydrostatic and osmotic pressure

2. Arterial end

a. NFP = (HPc - Hpif) - (OPc - OPif)

i. 35 - 25 = 10 mm Hg

3. Venous end

a. NFP = (HPc - Hpif) - (OPc - OPif)

i. 17 - 25 = -8 mm Hg



Circulatory System

I. Pulmonary Circulation

A. Function

1. Gas exchange only

B. Sequence

1. Pulmonary trunk

a. Bifurcates into rt. and lt. pulmonary arteries

2. Pulmonary arteries


a. In the lungs, arteries subdivide into lobar arteries

i. Three in right

ii. Two in left

3. Lobar arteries branch to form arterioles

4. Further branching to form pulmonary capillaries

5. Capillaries drain into venules

6. Venules join to form two pulmonary veins per lung

7. Four pulmonary veins drain into left atrium


II. Overview of Systemic Circulation

A. Aorta and Major Arteries of the Systemic Circulation

B. Aortic arch (branches in sequence relative to lt. ventricle)

1. Coronary arteries

2. Brachiocephalic

a. R. common carotid

i. R. internal carotid

ii. R. external carotid

b. R. subclavian

i. R. vertebral

ii. R. axillary

3. L. common carotid

a. L. internal carotid

b. L. external carotid

4. R. subclavian

a. L. vertebral

b. L. axillary

C. Thoracic aorta (above the diaphragm)

1. Parietal branches

2. Visceral branches

D. Abdominal aorta (below diaphragm)

1. Parietal branches

2. Visceral branches

3. R. common iliac

4. L. common iliac


III. Arteries of Head and Neck

A. Brachiocephalic artery branches off aortic arch

1. R. subclavian artery branches off brachiocephalic artery

2. R. vertebral artery branches off R. subclavian artery

3. R. vertebral joins with L. vertebral to form Basilar artery

a. Basilar artery is part of Circle of Willis (see below)

4. Basilar artery divides to form R. and L. posterior cerebral arteries

a. Supply occipital and inferior temporal lobes of brain

B. After R. subclavian artery branches, Brachiocephalic artery is the R. common carotid artery

C. R. common carotid bifurcates to form R. external and internal carotid arteries

D. R. external carotid branches as it runs superiorly

1. Superior thyroid

a. Supplies thyroid and larynx

2. Lingual

a. Supplies tongue

3. Facial

a. Supplies skin and muscles of anterior face

4. Occipital

a. Supplies posterior scalp

5. R. external carotid splits

a. Maxillary

i. Supplies upper and lower jaw

b. Superficial temporal

a. Supplies most of scalp

E. R. Internal carotid

1. Enters the skull and services the brain

2. Opthalmic branches off

a. Supplies eyes, orbits, forehead and nose

3. R. internal carotid divides to form

a. R. anterior cerebral artery

i. Supplies medial surface of brain

b. R. middle cerebral artery

i. Supplies lateral parts of temporal and parietal lobes

F. Circle of Willis

1. R. and L. posterior communicating arteries connect posterior cerebral arteries with R. and L. anterior cerebral arteries

2. Anterior communicating artery connects R. and L. anterior cerebral arteriors

IV. Arteries of Upper Limb and Thorax


A. After giving rise to the R. vertebral artery, R. subclavian courses laterally and gives off branches to neck (do not need to know these vessels)

B. R. subclavian artery passes under clavicle

1. Name changes to axillary artery

C. Axillary artery gives off a number of branches

1. Thoracoacromial

a. Supplies superior shoulder and pectoral region

2. Lateral thoracic

a. Supplies lateral chest wall and breast

3. Subscapular

a. Supplies scapula, latissimus dorsi and thorax wall

4. Anterior and posterior circumflex arteries

a. Supply deltoid and shoulder joint

D. As axillary artery enters arm, name changes to brachial artery

E. Brachial artery gives off a branch

1. Deep brachial artery

a. Supplies triceps brachii (posterior arm)

2. Brachial artery supplies anterior flexor muscles of arm

F. Brachial artery splits to form two arteries

1. Radial artery

a. Supplies lateral muscles of forearm

2. Ulnar artery

a. Supplies medial muscles of forearm

(Do not need to know arteries of wrist and hand)

G. Arteries of the thorax wall

1. Internal thoracic artery branches off the subclavian artery

a. As the internal thoracic artery descends it gives off anterior intercostals arterires

i. Supplies anterior intercostals spaces

2. Costocervical trunk gives rise to the first two posterior intercostals arteries

3. The thoracic aorta gives rise to the next nine pairs

a. Supplies posterior intercostals spaces and deep muscles of back, vertebral columns and spinal cord


V. Arteries of the Abdomen

A. Abdominal artery lies below the level of the diaphragm

B. As abdominal aorta descends it gives the inferior phrenic arteries

1. Supply the diaphragm

C. Abdominal aorta descends and gives off the celiac trunk

D. Celiac trunk divides into three branches

1. Common hepatic

a. Gives off branches to stomach, small intestine and pancreas

b. After giving off gastroduodenal artery, common hepatic becomes hepatic artery

i. Right gastroepiploic branches off gastroduodenal artery

ii. Supplies stomach

c. Hepatic splits into right and left branches

i. Supplies liver

2. Splenic

a. Sends branches to stomach and pancreas

b. Splenic terminates in the spleen

c. Left gastroepiploic branches of splenic artery

i. Supplies stomach

3. L. gastric artery

a. Supplies stomach and inferior esophagus

E. Abdominal aorta descends and gives off the suprarenal arteries


1. Supply adrenal glands

F. Abdominal aorta descends and gives off the superior mesenteric artery

1. Superior mesenteric gives off branches that supply mesenteric organs

a. Intestinal

i. Supply large intestine

b. Ileocolic

i. Supply appendix, colon

c. R. and middle colic

i. Supply transverse colon

G. Abdominal aorta descends and gives off the paired renal arteries

1. Supply kidneys on each side of the body

H. Abdominal aorta descends and gives off the gonadal arteries (Testicular or ovarian)

I. Abdominal aorta descends and gives off the inferior mesenteric artery

1. Inferior mesenteric gives off branches that supply distal part of the colon

a. L. colic

b. Sigmoidal arteries

c. Superior rectal arteries

J. Abdominal aorta descends and gives off the lumbar arteries

1. Supply posterior abdominal wall

K. Aorta terminates giving rise to three arteries

1. Median sacral

2. R. and L. common iliacs


VI. Arteries of Pelvis and Leg

A. Common iliac divides into two branches

1. Internal iliac

a. Supply pelvis and visceral organs

i. Bladder, rectum, uterus and vagina (prostate and ductus deferns)

b. Divides to serve muscles of gluteal muscles and external genitalia

2. External iliac

B. As external iliac enters thigh it becomes the femoral artery

C. Femoral artery gives off branches as it descends down the thigh

1. Deep femoral artery

a. Serves posterior thigh

b. Deep femoral artery gives off branches that supply head and neck of femur

i. Lateral and medial circumflex arteries

D. Femoral artery descends and passes through adductor hiatus and enters popliteal fossa

E. Femoral artery becomes the popliteal artery

1. Supplies knee region

F. Popliteal artery divides

1. Posterior tibial artery

a. Gives off peroneal artery

i. Supplies lateral muscles of the leg

2. Anterior tibial artery

a. Supplies extensor muscles

(Do not need to know arteries of ankle or foot)


VII. Major Veins of the Systemic Circulation


A. Superior vena cava runs from union of brachiocephalic veins (L. and R.) to R. atrium

B. Veins that drain into R. brachiocephalic vein

1. R. internal jugular vein

2. R. vertebral vein

3. R. subclavian vein

a. R. external jugular vein empties into R. subclavian vein

*Left side corresponds to right side

C. Inferior vena cava runs from junction of common iliac veins to R. atrium

D. Veins that drain into inferior vena cava

1. Hepatic veins (R. and L.)

2. R. suprarenal vein

3. Renal veins (R. and L.)

4. R. gonadal vein

5. Lumbar veins


VIII. Veins of Head and Neck


A. Drainage of blood from brain

1. Most veins drain into dural (meningial) sinuses

a. Superior sagittal

b. Straight

c. Cavernous

d. Transverse

2. Most blood from the brain drains into internal jugular veins

B. Deep veins of the face drain into inferior jugular veins


1. Facial

2. Superficial temporal


IX. Veins of Upper Limbs and Thorax


A. Azygous system drains thoracic tissues

1. Azygous vein drains into superior vena cava

a. Posterior intercostal veins drain into azygous

2. Azygous veins that drain into azygous vein

a. Hemiazygous

b. Accessory hemiazygous vein

B. Deep drainage of upper limb

1. Distal veins of the arm drain into ulnar and radial veins

2. Ulnar and radial veins unite to form brachial vein

3. As brachial vein enters shoulder, it becomes axillary vein

4. At level of first rib, axillary vein becomes subclavian vein

C. Superficial drainage of upper limb

1. Median vein of the forearm lies between ulna and radius

a. Connects either to basilic or cephalic veins

2. Cephalic vein joins with axillary vein

3. Basilic vein joins with brachial vein

4. Median cubital vein connects basilic and cephalic veins

a. Commonly used to obtain blood samples


X. Veins of Abdomen


A. Hepatic portal system

1. Multiple hepatic veins carry blood from liver to inferior vena cava

2. Cystic veins drain gall bladder and join the hepatic veins

B. Hepatic portal vein receives drainage from digestive viscera

a. Hepatic portal vein carries blood to liver

i. Nutrients are removed

C. Visceral veins draining into hepatic portal vein

1. Superior mesenteric

2. Inferior mesenteric

a. Drains large intestine and rectum

i. Joins splenic

3. Splenic

a. Drains spleen, parts of the stomach, and pancreas

i. Joins superior mesenteric

D. Other veins draining into inferior vena cava

1. Lumbar veins

a. R. and L. ascending lumbar veins

2. Gonadal veins

a. R.: ovariaries or testes on right side of body

i. Drains directly into vena cava

b. L.: ovariaries or testes on left side of body

i. Drains into L. renal vein

3. Suprarenal

a. R: right adrenal gland

i. Drains directly into vena cava

b. L: left adrenal gland

i. Drains into left renal vein

4. Renal veins

a. Drain kidneys


E. Common iliacs join to form inferior vena cava


XI. Veins of Pelvis and Lower Limbs


A. Anterior and posterior tibial veins joins to form popliteal

B. Above the knee the popliteal becomes the femoral vein

C. As the femoral vein enters the pelvis it becomes the external iliac

D. External iliac joins with internal iliac to form common iliac

E. Saphenous veins

a. Great

i. Drains medial aspects of the leg

ii. Longest vein in the body

iii. Empties into femoral vein

b. Small

i. Drains deep fascia of calf

ii. Empties into popliteal vein