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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 - PowerPoint PPT Presentation
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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
pressureQ - 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/R
Regulation of Blood Pressure
MAP = CO x R
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