Cardiovascular System: Blood Vessels Chapter 19
Feb 24, 2016
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
• 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
Capillary Beds
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/resistanceSO
– Increase pressure = increase flowBUT
– 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
• Via [gradient]
• Nutrients and gases
• Means vary based on molecule properties
Capillary Diffusion
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