Chapter 21:

The Cardiovascular System: Blood Vessels

Vessel Structure - General All vessels same basic

structure 3 wall layers (or tunics)

Tunica adventitia (externa) - elastic and laminar fibers

Tunica media thickest layer elastic fibers and

smooth muscle fibers Tunica interna (intima)

endothelium – non-stick layer

basement membrane internal elastic lamina

Lumen - opening

Structure/function relationship changes as move through cardiovascular tree

Tunic thickness and composition variable throughout cardiovascular tree

Elastic (conducting) arteries Near heart Thick walls More elastic

fiber, less smooth muscle

Lose elasticity with aging

Vessel Structure – Elastic Arteries

Vessel Structure - Elastic Arteries Aorta and elastic

arteries Can vasoconstrict or

vasodilate Large arteries expand,

absorb pressure wave then release it with elastic recoil - Windkessel effect

Help to push blood along during diastole

With aging have less expansion and recoil

Muscular (distributing) arteries Deliver blood to

organs More smooth

muscle Less elastic

fibers

Vessel Structure – Muscular Arteries

Arterioles Distribution of

blood in organs Composition

varies depending on position - more muscle, less elasticity nearer heart

Regulate flow from arteries to capillaries Flow = ΔP/R vary resistance

by changing vessels size

Site of blood pressure regulation

Vessel Structure - Arterioles

Vessel Structure - Capillaries Microcirculation

connects arteries and veins

Found in nearly every tissue in body Higher the metabolic

rate, more capillaries in tissue

Muscle many caps (>600/mm2)

Cartilage none

Vessel Structure - Capillaries

Allow exchange of nutrients and wastes between blood and tissue

Capillary structure - simple Basal lamina - connective tissue Endothelial cells

Structure/function

Flow Regulation Regulation by vessels

with smooth muscle Metarterioles

connect arterioles to venules through capillary bed

allows flow through capillary bed w/out flow through caps

Flow Regulation True capillaries

Pre-capillary sphincter ring of smooth muscle open/close to control

flow regulated by

chemicals Intermittent vasomotion

– caps open for flow 5-10X min

Types of Capillaries

3 types of capillaries

1. Continuous capillaries continuous

endothelial cells except for cleft between cells

tight junctions between endothelial cells prevent most things from leaving caps

most capillaries in body

Types of Capillaries

2. Fenestrated capillaries fenestrations (slits)

allow for filtration of small substances

glomerular capillaries in kidney

Types of Capillaries

3. Sinusoid capillaries wider gaps between

endothelial cells allowing RBC’s to exit the caps

found in liver

Vessel Structure - Veins Venules

Collect blood from caps carry it to veins

Structure changes with position

Become more vessel-like (walls) as move from capillaries

Vessel Structure – Veins Veins

Interna thicker than arteries

Media thinner, less muscle

Externa thick Valves Pressure low High compliance

- change volume easily with small change in pressure

Varicose veins

Vessel Structure - Histology

Very different morphology under light microscopy

Tunica media thickness differentiates artery from vein

Artery VeinVein Artery

Vessel Structure/Function At rest

60% of blood located in veins and venules

Serve as reservoirs for blood, “storing” it until needed

Particularly veins of abdominal organs, skin

ANS regulates volume distribution Vasoconstrict Vasodilate Open areas of

circulation to be supplied with blood veins at rest caps during exercise

Can “shift” volumes to other areas as needed

Vessel Structure/Function

Rest

CO = 5 L/min

0.75 L/min

Vessel Structure/Function

HeavyExercise

Rest

CO = 25 L/min

CO = 5 L/min

20 L/min

0.75 L/min

Physiology of Circulation

Flow = ΔP/R or CO = MAP/R MAP - mean arterial pressure Higher pressure to lower pressure with

resistance (R) factor

Blood pressure Pressure of blood on vessel wall Measurement of pressure of a volume in a

space Systole/diastole - 120/80 BP falls progressively from aorta to O mm Hg at

RA

Regulation of Blood Flow Resistance - opposition to blood flow from blood

and vessel wall friction Factors that affect resistance (R)

Viscosity - V R thickness of blood dehydration, polycythemia

R proportional to vessel length garden hose vs. straw obesity

Vessel diameter changes in diameter affect flow

vessel wall drag – blood cells dragging against the wall laminar flow – layers of flow

R inverse proportional to radius4

decrease in r by 1/2 R 16X only important in vessels that can change their size actively

Regulation of Pressure, Resistance Systemic vascular resistance

(Total Peripheral Resistance - TPR) All vascular resistance

offered by systemic vessels Which vessels change size? Resistance highest in

arterioles Largest pressure drop

occurs in arterioles Relationship of radius to

resistance in arterioles important due to smooth muscle in walls

Systemic Blood Pressure Arterial Blood Pressure

Pulsatile in arteries due to pumping of heart

Systolic/diastolic Pulse pressure = systolic

(minus) diastolicQ - Windkessel effect on

pulse pressure?A - Decreases pulse

pressure

Q - What is the effect of hardening of the arteries on pulse pressure?A - Increases pulse

pressure

Systemic Blood Pressure Capillary Blood

Pressure Relatively low blood

pressure Low pressure good for

caps because: caps are fragile - hi

pressure tears them up

caps are very permeable - hi pressure forces a lot of fluid out

Systemic Blood Pressure Venous return

Volume of blood flowing back to heart from systemic veins

Depends on pressure difference (ΔP) from beginning of venules (16 mmHg) to heart (0 mmHg)

Any change in RA pressure changes venous return

Help for venous return Skeletal muscle pump

muscles squeeze veins force blood back to heart valves prevent back flow

Respiratory pump inhaling pulls air into lungs helps to pull blood back into

thorax

Velocity of Blood Flow Velocity of blood flow -

inversely related to total cross sectional area (CSA) of vessels

Aorta Total CSA 3-5 cm2

Velocity 40 cm/sec Capillaries

Total CSA 4500-6000 cm2

Velocity 0.1 cm/sec Vena Cava

Total CSA 14 cm2 in vena cava

Velocity 5-20 cm/sec

Vessel Structure - Function

Capillary Function Site of exchange between blood and tissues Delivery of nutrients and removal of wastes Slow flow allows time for exchange

Mechanisms of nutrient exchange Diffusion - O2, CO2, glucose, AA's, hormones

diffuse down [ ] gradients If lipid soluble, can travel through cell If water soluble, between cells

Capillary Fluid Exchange

Forces driving the movement of fluid Hydrostatic pressure capillary (HPc)

Hydrostatic pressure interstitial fluid (HPif)

Osmotic pressure capillary(OPc)

Osmotic pressure interstitial fluid (OPif)

Net filtration pressure (NFP) is a sum of all

Fluid movement Fluid filtered and

reabsorbed across capillary wall

Starling’s law of the capillaries

Capillary Fluid Exchange

On average 85% of fluid filtered at arteriole end is reabsorbed at venular end

Maintaining Blood Pressure - Short Term Mechanisms - CNS Neural Control - Cardiac centers in medulla

Vasomotor center medullary area dedicated to control of blood vessels sends sympathetic output to blood vessels

Vasoconstricts or vasodilates as needed tone - normal amount of vasoconstriction or vasodilation can vary tone which varies delivery of blood

receives input from different sources baroreceptors chemoreceptors

Maintaining Blood Pressure – Short term mechanisms – CNS reflexes

Baroreceptor initiated reflex Located at carotid sinus

and aortic arch Monitor changes in blood

pressure Regulate activity of

Sympathetic Nervous System (vascular tone)

Maintaining Blood Pressure – Short term mechanisms – CNS reflexes

Chemoreceptor initiated reflexes Carotid bodies,

aortic bodies Monitors changes

in chemicals (O2, CO2, [H+])

CO2, H+, O2 (stresses) result in sympathetic activity and BP

Maintaining Blood Pressure – Short term mechanisms – CNS reflexes Influence of Higher Brain Centers (areas

above medulla) - Cortex and Hypothalamus Not involved in minute to minute regulation Influence vasomotor center depending on

conditions public speaking temperature regulation

Maintaining Blood Pressure - Short Term Mechanisms - Hormones

Renin - Angiotensin - Aldosterone Renin

enzyme from kidney results in formation of

Angiotensin II (AII) AII

vasoconstrictor stimulates ADH, thirst stimulates

aldosterone - Na+ reabsorption

Why/how would these things affect blood pressure?

Maintaining Blood Pressure - Short Term Mechanisms - Hormones

Adrenal medulla - Epi and Norepi CO (HR, SV) Constrict abdominal, cutaneous

arterioles/venules Dilate cardiac, skeletal muscle beds

Why/how would this affect blood pressure?

Antidiuretic Hormone (ADH) Osmoreceptors in

hypothalamus Retains fluid (inhibited by

alcohol) Vasoconstriction at high

levels

Why/how would this affect blood pressure?

Maintaining Blood Pressure - Short Term Mechanisms - Hormones

Maintaining Blood Pressure - Short Term Mechanisms - Hormones

ANP (atrial natriuretic peptide) Released from atrial cells in response to BP Vasodilator, Na+ and water loss, opposes Aldosterone

Why/how would this affect blood pressure?

Maintaining Blood Pressure - Long Term Regulation Renal mechanism

Volume in a space Regulate space in the

short term – we just talked about it! nervous control hormones

Regulate volume in the long term

The kidneys! BP, urine flow to

BP BP, urine flow to

BP

Control of Blood Flow Autoregulation (local control) - local

automatic adjustment of blood flow to match tissue needs Physical changes

Warming - vasodilation Cooling - vasoconstriction

Chemical changes - metabolic products Vasodilators Vasoconstrictors

Myogenic control smooth muscle controls resistance stretch contraction, stretch relaxation

Blood Flow in Special Areas Skeletal Muscle

Wide variability in amount of flow Sympathetic regulation from brain in response to level of

activity α receptors - vasoconstrict β receptors - vasodilate

Metabolic regulation in tissue low O2 vasodilate to increase flow

hi O2 vasoconstrict to decrease flow

Brain Very little variability in flow Stores few nutrients so flow must be maintained! Metabolic regulation

Blood Flow in Special Areas Skin

Supplies nutrients, aids in temperature regulation, provides a blood reservoir

Metabolic and sympathetic regulation

Lungs Low pressure (25/10), low resistance Flow regulated by O2 availability in the lungs

hi O2 vasodilate to increase flow – opposite to muscle

low O2 vasoconstrict to decrease flow – opposite to muscle

Heart Variable flow depending on activity Metabolic and sympathetic regulation

CO = MAP/RCO = MAP/R

Regulation of Blood Pressure

MAP = CO x RMAP = CO x R