Transcript Blood Vessels

Cardiovascular System:
Blood Vessels
Chapter 19
Blood Vessels
• Closed system starting and ending at heart
• 3 major types
– Arteries carry blood away from the heart
• Systemic are O2 rich
• Pulmonary are O2 poor
– Capillaries have direct contact with tissues
– Veins carry blood toward the heart
• Systemic are O2 poor
• Pulmonary are O2 rich
Blood Vessel Walls
• 3 layers or tunics
– Tunica intima
• Simple squamous endothelia layer
– Tunica media
• Smooth muscle and elastin fibers
• Sympathetic NS controls vasodilation and -constriction
• Thicker in arteries to regulate pressure and flow
– Tunica externa
• Collagen fibers for support and reinforcement
• Larger vessels with vasa vasorum
• Central lumen contains the blood
Arteries
• Elastic arteries are largest and nearest the heart
– Large lumen why?
– Tunica media w/ more elastin than smooth  why?
– Continuous, constant blood flow
• Muscular arteries deliver blood to organs
– Tunica media primarily smooth muscle  why?
• Arterioles lead to capillary beds
– Tunica media scattered smooth muscle and little elastin
– Vasodilation and –constriction alter capillary flow
Capillaries
• Thin tunica intima w/basement membrane
– Pericytes, similar to smooth muscle, to stabilize
– Cells move single file
– Accessible to most tissues for exchange
• Types
– Continuous abundant in skin and muscle
• Endothelium uninterrupted
• Tight junctions link cells
– Fenestrated where rapid fluid exchange occurs
• Endothelium with numerous pores
• Pores thinly covered, but more permeable
– Sinusoids in liver, bone marrow, and lymphoid tissue
• Larger pores and lumens
• Slower blood flow
Capillary Beds
• Vascular shunts
– Terminal arteriole
– Metateriole
– Thoroughfare channel
– Postcapillary venule
• True capillaries
– Precapillary spinchters at junction with shunts
• Rings of smooth muscle
• Regulate flow based on chemical input and needs
– Digestive tract: before and after a meal
– Skeletal muscles: exercising and relaxing
Veins
• Venules
– Smallest are entirely endothelia and extremely porous
– Larger with thin tunica media and tunica externa
• Veins
– 3 thinner tunicas
• Media has little smooth muscle or elastin
• Externa thickest
– Larger lumens offer little blood flow resistance
• Pressure lower
• Hold ~65% of blood volume
– Venous valves in limbs
• Formed by tunica intima
• Prevent blood backflow due to gravity
Vascular Anastomoses
• Fusing of blood vessels in a given region
• Provide multiple paths to/from organs/tissue
– Near joints, also in heart and brain
– Lacking in retina, kidneys, and spleen
• Ensures uninterrupted blood flow
– Metarteriole-thoroughfare channel is an example
Physiology of Circulation Terminology
• Blood flow (ml/min)
– Amount of blood moving at a given time
– Equivalent to cardiac output (CO)
– Constant at rest; varies w/organ needs
• Blood pressure (mm Hg)
– Force exerted on a vascular wall
• Systemic arteries used when measured
– Keeps blood moving from high to low
Physiology of Circulation Terminology
(cont.)
• Peripheral resistance
– Opposition to blood flow from vessel friction
– Sources
• Viscosity: thickness of blood
– Changes in RBC numbers can increase/decrease,
• Vessel length
– Longer vessel = more resistance
– More vessels = more total length
• Vessel diameter
– Smaller arterioles can constrict/enlarge
– Flow is slowed along walls  larger diameter = less wall
contact
– Turbulence, from additional wall resistance, increases
Basic Circulation Physiology
• Blood flow = blood pressure/resistance
SO
– Increase pressure = increase flow
BUT
– Increase resistance = decrease flow
• Resistance affects local flow more than
pressure
– Vasoconstriction/-dilation in an organ
– Pressure basically unchanged overall
Systemic Blood Pressure
• Highest at heart
– Decreases
w/increased
distance
• Steepest change
in arterioles
– Resistance
highest
• Flow maintained
by pressure
gradient
Arterial Pressure
• Pressure near heart is pulsatile
– Systolic pressure is max during ventricular contraction
• Added blood supply stretches arteries
– Diastolic pressure is minimum during ventricular
relaxation
• Recoil of arteries to maintain pressure
– Difference creates pulse pressure, measured as our
pulse
• Increases w/ arteriosclerosis b/c elasticity decreases
• Mean arterial pressure (MAP) is pressure moving
blood to tissues
– MAP = diastolic + 1/3 pulse pressure
– Accounts for changes in arterial BP and longer diastole
Capillary Pressure
• Significantly lower than arterial
• Beneficial to capillary structure
– Vessels are thin = fragile
– Minimum needed to force filtrates out
Venous Pressure
• Minimal changes in a cardiac cycle
• Adaptations to compensate
– Muscular pump: skeletal muscles
• ‘Cankles’ and mats for standing
– Respiratory pump: pressure created
from inhalation
– Valves
– Smooth muscle in tunics
Monitoring Blood Pressure
• Short-term
– Cardiac output (CO)
• Increase CO = increase BP
• Decrease CO = decrease BP
• See fig. 19.7
– Peripheral resistance (R)
• Increase vasoconstriction = increase BP
• Increase vasodilation = decrease BP
• Long-term
– Blood volume
• Increase blood volume = increase BP
• Decrease blood volume = decrease BP
Vasomotor Center
• Monitors blood vessel diameter from medulla
– SNS innervation (NE and ACh)
– Minimizes moment specific changes to BP
• Types of neural controls
– Baroreceptors respond to vessel stretch
• Carotid arteries and aortic arch
• Inhibits vasomotor center  vasodilation
– Chemoreceptors respond to O2 and pH drop (CO2 rise)
• Close to baroreceptors
• Stimulates vasomotor center  vasoconstriction
– Hypothalamus and cerebral cortex input to medulla
Hormonal Control of BP
• Vasodilators
– Atrial natriuretic peptide (ANP): increase Na+ and H2O
excretion
– Nitric oxide (NO): brief and localized
– Inflammatory: histamines and prostocyclin
– Alcohol: inhibits ADH
• Vasoconstrictors
– Adrenal medulla hormones: NE and Epi
– Antidiuretic hormone (ADH): stimulates H2O conservation
– Angiotensin II: renin from kidneys catalyzes production
Renal Regulation of BP
• Monitors blood volume from kidneys
– Increased BP stimulates H2O loss = decrease BP
– Decreased BP stimulates H2O retention = increase BP
• Excessive salt intake
• Indirect mechanisms
– Renin-angiotensin mechanism
– Angiotensin II: vasoconstriction, aldosterone to resorb
Na+/H2O, ADH release to resorb water
• Direct mechanisms
– Fluid speed increased to kidneys
– Less absorption = more excretion
• Figs 19.10 and 19.11
Measuring Circulation
• Pulse
– Measured at pressure points
– Can be used to slow/halt distal blood flow
• Blood pressure
– Ausculatory method at brachial artery
– First sound = systolic, no sounds = diastolic
• Along with respiratory rate and body temp
constitute vital signs
Blood Flow
• Functions
–
–
–
–
Deliver O2 and nutrients; remove CO2 and wastes
Exchange gases
Absorb nutrients
Form urine
• Rate dictated by needs (rest)
–
–
–
–
–
–
Brain (13%)
Heart (4%)
Kidneys (20%)
Abdominal organs (24%)
Skeletal muscles (20%)
All others (19%)
Blood Flow Velocity
• Inversely related to
cross-sectional
area
– More area (more
vessels) = slower
flow
• Slowest in
capillaries = more
exchange time
Short-Tem Autoregulation
• Adjustments of blood flow to tissue needs
– Local, short-term intrinsic control
– MAP and CO unchanged, but diameter of arterioles
not
• Metabolic controls (vasodilation)
– Hb carries O2 and NO to tissues when O2 levels drop
– K+, H+, adenosine, histamine too
• Myogenic controls
– Stretch receptors in smooth muscle
– Less pressure signals vasodilation (how change flow?)
• Fig 19.15
Long-Term Autoregulation
• Angiogenesis increases blood vessel number
and diameter
– Short-term fails to meet needs
– Occurs over weeks or months
• Coronary occlusions or high-altitude living
conditions
Capillary Diffusion
• Via [gradient]
• Nutrients and gases
• Means vary based
on molecule
properties
Capillary Fluid Flow
• Capillary hydrostatic pressure (HPc) force fluid out, more at
arteriole than venule end
• Capillary colloid osmotic pressure (OPc) draws fluid in, constant at
both ends
• Net filtration pressure (NFP) tells of net fluid loss or gain
– NFP = (HPc – HPif) – (OPc – OPif)
• More fluid enter tissue than return to blood, but lymphatic system
returns
Circulatory Shock
• Hypovolemic
– From large loss of blood volume
– HR up, vasoconstriction to increase venous return
– Temporary fix; fluid replacement needed ASAP
• Vascular
– Extreme vasodilation drops resistance
• Anaphylactic shock, septicemia, or ANS failure
– Blood volume normal, but poor circulation
• Cardiac
– Heart fails
Blood Vessel Imbalances
• Varicose veins: stretching veins due to leaky valves
• Arteriosclerosis: hardening of arteries
– Contributes to hypertension: BP > 140/90 mmHg
– Most common is atherosclerosis: plaque build up of tunica
intima
• Hypotension: BP < 100/80 mm Hg
– Less serious and often good
– Can increase fainting/dizziness
• Aneurysm: ballooning of a blood vessel
• Phlebitis: inflammation of a vein