blood vessels

• blood vessels: delivery system of dynamic structures that begins and ends at heart

  -work with lymphatic system to circulate fluids

  arteries: carry blood away from heart; oxygenated except for pulmonary circulation and umbilical vessels of fetus

  capillaries: direct contact with tissue cell; directly serve cellular needs

  veins: carry blood toward heart; deoxygenated except for pulmonary circulation and umbilical vessels of fetus
• -all vessels consist of a lumen, central blood-containing space, surrounded by a wall

  -walls of all vessels, except capillaries, have three layers or tunics:

  1. tunica intima
  2. tunica media
  3. tunica externa

  -capillaries: endothelium with sparse basal lamina
• tunica intima

  -innermost layer that is in "intimate" contact with blood

  -endothelium: simple squamous epithelium that lines lumen of all vessels, continuous with endocardium, slick surface reduces friction

  -sub-endothelial layer connective tissue basement membrane; found only in vessels larger than 1mm
• tunica media

  -middle layer composed mostly of smooth muscle and sheets of elastin

  -sympathetic vasomotor nerve fibers innervate this layer controlling:

  • vasoconstriction: decreased lumen diameter
  • vasodilation: increased lumen diameter

  -bulkiest layer responsible for maintaining blood flow and blood pressure
• arteries divided into three groups based on size and function:

  -elastic arteries

  -muscular arteries

  -arterioles
• elastic arteries: thick walled with large, low-resistance lumen

  -aorta and its major branches, also called conducting arteries because they conduct blood from heart to medium sized vessels

  -elastin found in all three tunics, mostly tunica media

  -contain substantial smooth muscle, but inactive in vasoconstriction

  -act as pressure reservoirs that expand and recoil as blood is ejected from heart, allows for continuous blood flow downstream even between heartbeats
• muscular arteries


  -elastic arteries give rise to muscular arteries

  -also called disturbing arteries because they deliver blood to body organs; diameters range from pinky-finger size to pencil-lead size

  -account for most of named arteries

  -have thickest tunica media with more smooth muscle, but less elastic tissue

  -tunica media sandwiched between elastic membranes

  -active in vasoconstriction
• arterioles: smallest of all arteries

  -larger arterioles contain all three tunics

  -smaller arterioles are mostly single layer of smooth muscle surrounding endothelial cells

  -control flow into capillary beds via vasodilation and vasoconstriction of smooth muscle

  -also called resistance arteries because changing diameters change resistance to blood flow

  -lead to capillary beds
• capillaries


  -microscopic vessels; diameters so small only single RBC can pass through at a time

  -walls just thin tunica intima; in smallest vessels, one cell forms entire circumference

  -pericytes: spider-shaped stem cells help stabilize capillary walls, control permeability, and play a role in vessel repair

  -supply almost every cell, except for cartilage, epithelia, cornea, and lens of eye

  -functions: exchange of gases, nutrients, wastes, hormones, etc., between blood and intestinal fluid
• types of capillaries


  -all capillary endothelial cells are joined by tight junctions with gaps called intercellular clefts (allow passage of fluids and small solutes)

  -three types: continuous, fenestrated, and sinusoidal.
• continuous capillaries


  -abundant in skin, muscles, lungs, and CNS

  -continuous capillaries of brain are unique; form blood brain barrier, totally enclosed with tight junctions and no intercellular clefts
• fenestrated capillary

  -found in areas involved in active filtration (kidneys), absorption (intestines), or endocrine hormone secretion

  -endothelial cells contain Swiss cheese-like pores called fenestrations;

  • allow for increased permeability
  • fenestrations usually covered with thin glycoprotein diaphragm
• sinusoidal capillaries

  -fewer tight junctions; usually fenestrated with large intercellular clefts; incomplete basement membranes (usually have larger lumens)

  -found only in the liver, bone marrow, spleen, and adrenal medulla

  -blood flow is sluggish, allows time for modification of large molecules and blood cells that pass between blood and tissue

  -contain macrophages in lining to capture and destroy foreign invaders
• capillary beds:

  -interwoven network of capillaries between arterioles and venules

  microcirculation: flow of blood through bed

  -capillary beds consist of two types of vessels

  1. vascular shunt: channel that connects arteriole directly with venule (metarteriole-thoroughfare channel)
  2. true capillaries: actual vessels involved in exchange
• vascular shunt: metarteriole-thoroughfare channel starts with:

  -terminal arteriole that feeds into → metarteriole (intermediate between arteriole and capillary) that is continuous with → thoroughfare channel (intermediate between capillary and venule) that feeds into → postcapillary venule that drains bed
• true capillaries

  -10 to 100 exchange vessels per capillary bed

  -branch off metarteriole or terminal arteriole

  -true capillaries normally branch from metarteriole and return to thoroughfare channel

  -precapillary sphincters regulate blood flow into true capillaries (blood may go into true capillaries or to shunt)

  -regulated by local chemical conditions and vasomotor nerves
• veins


  -carry blood toward the heart

  -formation begins when capillary beds unite in postcapillary venules and merge into larger and larger veins

  -have all tunics but thinner walls with large lumens compared with corresponding arteries

  -tunica media is thin, but tunica externa is thick (contain collagen fibers and elastic networks

  -large lumen and thin walls make veins good storage vessels (called capacitance vessels (blood reservoirs) because they contain up to 65% of blood supply)
• venules

  -capillaries unite to form postcapillary venules

  • consist of endothelium and a few pericytes
  • very porous; allow fluids and WBCs into tissues

  -larger venules have one or two layers of smooth muscle cells
• veins (cont.)


  -blood pressure lower than in arteries, so adaptations ensure return of blood to heart

  • large-diameter lumens offer little resistance

  -other adaptations:

  • venous valves; prevent backflow of blood, most abundant in veins of limbs
  • venous sinuses; flattened veins with extremely thin walls, composed only of endothelium, ex. coronary sinus of the heart and dural sinuses of brain
• varicose veins: dilated and painful veins due to incompetent (leaky) valves

  -factors that contribute include heredity and conditions that hinder venous return

  -ex. prolonged standing in one position, obesity, or pregnancy; blood pools in lower limbs, weakening valves; affects more than 15% of adults
• -elevated venous pressure can cause varicose veins

  • ex. straining to deliver a baby or have a bowel movement raises intra-abdominal pressure, resulting in varicosities in anal veins called hemorrhoids
• anastomoses


  -vascular anastomoses: interconnections of blood vessels

  -arterial anastomoses: provide alternate pathways (collateral channels) to ensure continuous flow, even if one artery is blocked, common in joints, abdominal organs, brain, and heart; none in retina, kidneys, spleen

  -arteriovenous anastomoses: shunts in capillaries, ex. metarteriole-thoroughfare channel

  -venous anastomoses: so abundant that occluded veins rarely block blood flow
• blood flow: volume of blood flowing through vessel, organ, or entire circulation in given period

  • measured in ml/min, it is equivalent to cardiac output (CO) for entire vascular system
  • overall is relatively constant when at rest, but at any given moment, varies at individual organ level based on needs
• -blood pressure (BP): force per unit area exerted on wall of blood vessel by blood

  • expressed in mm HG
  • measured as systemic arterial BP in large arteries near heart
  • pressure gradient provides driving force that keeps blood moving from higher-to-lower pressure areas
• resistance (peripheral resistance): opposition to flow

  -measurement of amount of friction blood encounters with vessel walls, generally in peripheral (systemic) circulation

  -three important sources of resistance

  • blood viscosity
  • total blood vessel length
  • blood vessel diameter
• -blood viscosity: the thickness or "stickiness" of blood due to formed elements and plasma proteins, the greater the viscosity, the less easily molecules are able to slide past each other, increased viscosity equals increased resistance

  -total blood vessel length: the longer the vessel, the greater the resistance encountered
• blood vessel diameter:

  -has greatest influence on resistance

  -frequent changes alter peripheral resistance; viscosity and blood vessel length are relatively constant

  -fluid close to walls moves more slowly than in middle of tube (called laminar flow)

  -resistance varies inversely with fourth power of vessel radius

  • if radius increases, resistance decreases, and vice-versa
  • ex. if radius is doubled, resistance drops to 1/16 as much
• blood vessel diameter cont.

  -small-diameter arterioles are major determinants of peripheral resistance; radius changes frequently, in contrast to larger arteries that do not change often.

  -abrupt changes in vessel diameter or obstacles such as fatty plaques from atherosclerosis dramatically increase resistance

  • lamar flow is disrupted and becomes turbulent flow, irregular flow that causes increased resistance
• -blood flow (F) is directly proportional to blood pressure gradient (ΔP), if ΔP increases, blood flow speeds up

  -blood flow is inversely proportional to peripheral resistance (R), if R increases, blood flow decreases, so F = ΔP/R

  -R is more important in influencing local blood flow because it is easily changed by altering blood vessel diameter
• systemic blood pressure

  -pumping action of heart generates blood flow

  -pressure results when flow is opposed by resistance

  -systemic pressure is highest in aorta and declines throughout pathway

  • steepest drop occurs in arterioles
• arterial blood pressure

  -determined by two factors:

  1. elasticity (compliance or distensibility) of arteries close to heart
  2. volume of blood forced into them at any time.

  -blood pressure near heart is pulsatile

  • rises and falls with each heartbeat
• systolic pressure: pressure exerted in aorta during ventricular contraction; left ventricle pumps blood into aorta, imparting kinetic energy that stretched aorta; average 120 mm Hg in normal adult

  diastolic pressure: lowest level of aortic pressure when heart is at rest

  pulse pressure: lowest level of aortic pressure when heart is at rest

  pulse: throbbing of arteries due to difference in pulse pressures, which can be felt under skin
• mean arterial pressure (MAP): pressure that propels blood to tissues

  • pulse pressure phases out near end of arterial tree
  • flow is nonpulsatile with a steady MAP pressure

  -heart spends more time in diastole, so not just a simple average of diastole and systole
• MAP is calculated by adding diastolic pressure + 1/3 pulse pressure

  -ex. BP= 120/80; Pulse pressure= 120-80 = 40; so MAP = 80 + (1/3)(40) = 80 + ~13 = 93 mm Hg

  -pulse pressure and MAP both decline with increasing distance from heart
• clinical monitoring of circulatory efficiency

  -vital signs: pulse and blood pressure, along with respiratory rate and body temperature

  -taking a pulse:

  • radial pulse (taken at wrist): most routinely used, but there are other clinically important pulse points
  • pressure points: areas where arteries are close to body surface, can be compressed to stop blood flow in event of hemorrhaging
• measuring blood pressure

  -systemic arterial BP is measure indirectly by auscultatory methods using a sphygmomanometer

  1. wrap cuff around arm superior to elbow
  2. increase pressure in cuff until it exceeds systolic pressure in brachial artery
  3. pressure is released slowly and examiner listens for sounds of Korotoff with stethoscope

  -systolic pressure: normally less than 120 mm HG, pressure when sounds first occur as blood starts to spurt through artery

  -diastolic pressure: normally less than 80 mm Hg, pressure when sounds disappear because artery no longer constricted; blood flowing freely
• capillary blood pressure

  -ranges from 35 mm Hg at beginning of capillary bed to ~17 mm Hg at the end of the bed

  -low capillary pressure is desirable because:

  1. high BP would rupture fragile, thin-walled capillaries
  2. most capillaries are very permeable, so low pressure forces filtrate into interstitial spaces
• venous blood pressure:

  -changes little during cardiac cycle

  -small pressure gradient, only about 15 mm Hg

  • if vein is cut, low pressure of venous system causes blood to flow out smoothly
  • if artery cut, blood spurts out because pressure is higher

  -low pressure is die to cumulative effects of peripheral resistance; energy of blood pressure is lost as heat during each circuit

  -low pressure of venous side requires adaptations to help with venous return
• factors aiding venous return

  1. muscular pump: contraction of skeletal muscles "milks" blood back toward heart; valves prevent back flow
  2. respiratory pump: pressure changes during breathing move blood toward heart by squeezing abdominal veins as thoracic veins expand
  3. sympathetic venoconstriction: under sympathetic control, smooth muscles constriction, pushing blood back toward heart
• regulation of blood pressure

  -maintaining blood pressure requires cooperation of heart, blood vessels, and kidneys (all supervised by brain)

  -three main factors regulation blood pressure: cardiac output (CO), peripheral resistance (PR), and blood volume

  -blood pressure varies directly with CO, PR, and blood volume