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