chapter 19 Flashcards
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 has direct contact with
direct contact with tissue cells; directly serve cellular needs
Veins:
carry blood toward heart; deoxygenated except for pulmonary circulation and umbilical vessels of fetus
order of blood flow through blood vessels
Largest arteries → smaller arteries → arterioles → capillaries → venules → smaller veins → largest veins
All vessels consist of a
Walls of
Capillaries
All vessels consist of a a lumen, central blood-containing space, surrounded by a wall
all vessels, except capillaries, have three layers, or tunics:
Tunica intima
Tunica media
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
- Subendothelial 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
Tunica externa
-Outermost layer of wall
-Also called tunica adventitia
-Composed mostly of loose collagen fibers that protect and reinforce wall and anchor it to surrounding structures
-Infiltrated with nerve fibers, lymphatic vessels
–Large veins also contain elastic fibers in this layer
-Vasa vasorum: system of tiny blood vessels found in larger vessels
Function to nourish outermost external layer
Elastic Arteries
- 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
Elastic arteries give rise to
called distribution arteries because
account for most
muscular arteries
- Also called distributing 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
Supply
Functions:
All capillary endothelial cells are joined by
- 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
Supply almost every cell, except for cartilage, epithelia, cornea, and lens of eye
-exchange of gases, nutrients, wastes, hormones, etc., between blood and interstitial fluid
All capillary endothelial cells are joined by tight junctions with gaps called intercellular clefts
Allow passage of fluids and small solutes
Pericytes:
spider-shaped stem cells help stabilize capillary walls, control permeability, and play a role in vessel repair
Capillary Types
continuous, fenestrated, 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
-least permeable and most common, skin muscles
Fenestrated capillaries
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
-large fenestrations (pores) increase permeability occurs in special locations kidney, small intestine.
Sinusoidal capillaries
Fewer tight junctions; usually fenestrated with larger 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
most permeate occurs in special location liver, bone marrow spleen
Capillary bed
interwoven network of capillaries between arterioles and venules
Microcirculation:
flow of blood through bed from arteriole to venule
Terminal arteriole
branch off arteriole that further branches into 10 to 20 capillaries (exchange vessels) that form capillary bed
Exchange of gases, nutrients and wastes from surrounding tissue takes place in capillaries
after capillary beds Capillaries then drain into
Flow through bed controlled by
Arteriole and terminal arteriole dilated when
constricted to
postcapillary venule
Flow through bed controlled by diameter of terminal arteriole and upstream arterioles
Local chemical conditions and arteriolar vasomotor nerve fibers regulate amount of blood entering capillary bed
–Arteriole and terminal arteriole dilated when blood needed; constricted to shunt blood away from bed when not needed
Capillaries found in serous membranes of intestinal mesenteries have two additional features that form a special arrangement of capillaries:
- Vascular shunt: channel that directly connects arteriole with venule (bypasses true capillaries)
- consists of metarteriole and thoroughfare channel
- Precapillary sphincter: cuff of smooth muscle surrounding each true capillary that branches off metarteriole; acts as valve regulating blood flow into capillary bed
- Controlled by local chemical conditions (not innervated)
Veins formation begins when
capillary beds unite in postcapillary venules and merge into larger and larger veins (venules)
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
Formed when venules converge
- 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
Veins have ____ pressure
Other adaptations
Venous valves
Venous sinuses
Low Pressure
Blood pressure lower than in arteries, so adaptations ensure return of blood to heart
–Large-diameter lumens offer little resistance
Venous valves
- -Prevent backflow of blood
- -Most abundant in veins of limbs
Venous sinuses
- -Flattened veins with extremely thin walls
- -Composed only of endothelium
- -Examples: coronary sinus of the heart and dural sinuses of the brain
Varicose veins
-dilated and painful veins due to incompetent (leaky) valves
- Factors that contribute include heredity and conditions that hinder venous return
- Example: 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
Example: straining to deliver a baby or have a bowel movement raises intra-abdominal pressure, resulting in varicosities in anal veins called hemorrhoids
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; example: metarteriole–thoroughfare channel
Venous anastomoses
: so abundant that obstructed veins rarely block blood flow
Blood flow:
Measured in
Overall is relatively
- 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
Three important sources of 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 greater the viscosity, the
- Increased viscosity equals
- 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
The longer the vessel, the greater the resistance encountered
Blood vessel diameter
If radius increases, resistance
Abrupt changes in vessel diameter or obstacles such as fatty plaques from atherosclerosis dramatically
- 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
- Example: if radius is doubled, resistance drops to 1/16 as muchSmall-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
- -Laminar flow is disrupted and becomes turbulent flow, irregular flow that causes increased resistance
Relationship Between Flow, Pressure, and Resistance
-Blood flow (F) is directly proportional to
Blood flow is inversely proportional to
F=
is more important in influencing local blood flow because it is easily changed by altering blood vessel diameter
-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=changeP/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
- Pressure results when
- Systemic pressure is highest
- 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
Elasticity (compliance or distensibility) of arteries close to heart
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 stretches aorta
- Averages 120 mm Hg in normal adult
Diastolic pressure
lowest level of aortic pressure when heart is at rest
Pulse pressure
difference between systolic and diastolic pressure
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
Clinical monitoring of circulatory efficiency
Vital signs
pulse and blood pressure, along with respiratory rate and body temperature
Taking a pulse
most common is
what are Pressure points
Radial pulse-(taken at the 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 measured indirectly by
Wrap cuff around arm superior to elbow
Increase pressure in cuff until it exceeds ____ pressure in ____ artery
Pressure is released slowly, and examiner listens for
systolic sound when
diastolic sound when
-auscultatory methods using a sphygmomanometer
-Wrap cuff around arm superior to elbow
Increase pressure in cuff until it exceeds systolic pressure in brachial artery
Pressure is released slowly, and examiner listens for for sounds of Korotkoff with a 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 because:
- High BP would rupture fragile, thin-walled capillaries
- Most capillaries are very permeable, so low pressure forces filtrate into interstitial spaces
Venous Blood Pressure
Factors aiding venous return
Low with little change
Factors aiding venous return
- Muscular pump: contraction of skeletal muscles “milks” blood back toward heart; valves prevent backflow
- Respiratory pump: pressure changes during breathing move blood toward heart by squeezing abdominal veins as thoracic veins expand
- Sympathetic venoconstriction: under sympathetic control, smooth muscles constrict, pushing blood back toward heart
Maintaining blood pressure requires cooperation of
Three main factors regulating blood pressure
heart, blood vessels, and kidneys
Cardiac output (CO) Peripheral resistance (PR) Blood volume
the three main factors (CO, PR, BV) can be affected by
Short-term regulation: neural controls
Short-term regulation: hormonal controls
Long-term regulation: renal controls
Short-Term Regulation: Neural Controls
Two main neural mechanisms control peripheral resistance
- MAP is maintained by altering blood vessel diameter, which alters resistance
Example: If blood volume drops, all vessels constrict (except those to heart and brain) - Can alter blood distribution to organs in response to specific demands
Short-Term Regulation: Neural Controls (Reflex Arcs)
- Cardiovascular center of medulla
- Baroreceptors
- Chemoreceptors
- Higher brain centers
(Cardiovascular Center
composed of
Vasomotor center: sends
Cause continuous moderate constriction called
- composed of clusters of sympathetic neurons in medulla
- Consists of:
- -Cardiac centers: cardioinhibitory and cardioacceleratory centers
- -Vasomotor center: sends steady impulses via sympathetic efferents called vasomotor fibers to blood vessels
- -Cause continuous moderate constriction called vasomotor tone
- Receives inputs from baroreceptors, chemoreceptors, and higher brain centers
Baroreceptor reflexes
response to High MAP
- Located in carotid sinuses, aortic arch, and walls of large arteries of neck and thorax
- If MAP is high:
- –Increased blood pressure stimulates baroreceptors to increase input to vasomotor center
- —Inhibits vasomotor and cardioacceleratory centers
- —Stimulates cardioinhibitory center
- Results decrease in blood pressure due to two mechanisms:
1. Vasodilation: decreased output from vasomotor center causes dilation - -Arteriolar vasodilation: reduces peripheral resistance, MAP falls
- –Venodilation: shifts blood to venous reservoirs, decreasing venous return and CO
2. Decreased cardiac output: impulses to cardiac centers inhibit sympathetic activity and stimulate parasympathetic - Reduces heart rate and contractility; CO decrease causes decrease in MAP
Barorreceptors response to low MAP
Carotid sinus reflex:
-Aortic reflex maintains
Baroreceptors are ineffective if altered blood pressure is
- Reflex vasoconstriction is initiated that increases CO and blood pressure
- Example: upon standing, BP falls and triggers:
- -Carotid sinus reflex: baroreceptors that monitor BP to ensure enough blood to brain
- -Aortic reflex maintains BP in systemic circuit
- Baroreceptors are ineffective if altered blood pressure is sustained
- -Become adapted to hypertension, so not triggered by elevated BP levels
Chemoreceptor reflexes
- Aortic arch and large arteries of neck detect increase in CO2, or drop in pH or O2
- Cause increased blood pressure by:
- –Signaling cardioacceleratory center to increase CO
- –Signaling vasomotor center to increase vasoconstriction
influence of Higher Brain Centers
Reflexes that regulate BP are found in
-Hypothalamus and cerebral cortex can modify arterial pressure via
-Hypothalamus increases blood pressure during
-Hypothalamus mediates redistribution of blood flow during
Influence of higher brain centers
- Reflexes that regulate BP are found in medulla
- Hypothalamus and cerebral cortex can modify arterial pressure via relays to medulla
- Hypothalamus increases blood pressure during stress
- Hypothalamus mediates redistribution of blood flow during exercise and changes in body temperature
Hormonal Controls
Hormones regulate BP in short term via changes in __________or long term via changes in _______
Hormones regulate BP in short term via changes in peripheral resistance or long term via changes in blood volume
- Adrenal medulla hormones
- -Epinephrine and norepinephrine from adrenal gland increase CO and vasoconstriction
- Angiotensin II stimulates vasoconstriction
- ADH: high levels can cause vasoconstriction
- Atrial natriuretic peptide decreases BP by antagonizing aldosterone, causing decreased blood volume
Renal Regulation
Baroreceptors cannot provied
Long Term Regulation
Changes
Baroreceptors cannot provide long-term regulation
Why? They adapt to high or low BP
Long Term Regulation
Changes blood volume
Kidneys regulate arterial blood pressure by:
- Direct renal mechanism
- Indirect renal mechanism (renin-angiotensin-aldosterone)
Direct renal mechanism
1. Increased blood pressure or blood volume
- Decreased blood pressure or blood volume
- Alters blood volume independently of hormones
1. Increased blood pressure or blood volume - elimination of more urine
- reduces blood pressure
2. Decreased blood pressure or blood volume - kidneys conserve water
- Blood pressure rises
Indirect renal mechanism
The renin-angiotensin-aldosterone mechanism
- Decreased arterial blood pressure causes release of renin from kidneys
- Renin enters blood and catalyzes conversion of angiotensinogen from liver to angiotensin I
- Angiotensin-converting enzyme, especially from lungs, converts angiotensin I to angiotensin II
- Angiotensin II (4 actions):
- -Stimulates aldosterone secretion
- -Causes ADH release from posterior pituitary
- -Triggers hypothalamic thirst center to drink more water
- -Acts as a potent vasoconstrictor, directly increasing blood pressure
Hypertension
Sustained elevated arterial pressure of 140/90 mm Hg or higher
Prehypertension
if values elevated but not yet in hypertension range
- May be transient adaptations during fever, physical exertion, and emotional upset
- Often persistent in obese people
Prolonged hypertension is major cause of
heart failure, vascular disease, renal failure, and stroke
- Heart must work harder; myocardium enlarges, weakens, and becomes flabby
- Also accelerates atherosclerosis
Hypotension
- Low blood pressure below 90/60 mm Hg
- Usually not a concern unless it causes inadequate blood flow to tissues
- Often associated with long life and lack of cardiovascular illness
Orthostatic hypotension
temporary low BP and dizziness when suddenly rising from sitting or reclining position
Chronic hypotension
hint of poor nutrition and warning sign for Addison’s disease or hypothyroidism
Acute hypotension
important sign of circulatory shock
Circulatory shock
Condition where blood vessels inadequately fill and cannot circulate blood normally
-Inadequate blood flow cannot meet tissue needs
Hypovolemic shock
Due to large-scale blood loss
Vascular shock
Caused by extreme vasodilation and decreased peripheral resistance
Cardiogenic shock
inefficient heart not pumping enough blood
Tissue perfusion
: blood flow through body tissues; involved in:
- Delivery of O2 and nutrients to, and removal of wastes from, tissue cells
- Gas exchange (lungs)
- Absorption of nutrients (digestive tract)
- Urine formation (kidneys)
Rate of flow is precisely right amount to provide proper function to that tissue or organ
extrinsic control
intrinsic control
Extrinsic control: sympathetic nervous system and hormones control blood flow through whole body
–Act on arteriolar smooth muscle to reduce flow to regions that need it the least
Intrinsic control: Autoregulation (local) control of blood flow: blood flow is adjusted locally to meet specific tissue’s requirements
- -Local arterioles that feed capillaries can undergo modification of their diameters
- -Organs regulate own blood flow by varying resistance of own arterioles
Example: redistribution of blood during exercise
- At rest, skeletal muscles receive about 20% of total blood in body, but during exercise, skeletal muscle can receive over 70% of blood
- ntrinsic controls: skeletal muscle arterioles dilate, increasing blood flow to muscle
- Extrinsic controls decrease blood flow to other organs such as kidneys and digestive organs
- MAP is maintained despite dilation of skeletal muscle arterioles
Velocity of flow changes as
blood travels through systemic circulation
Fastest in aorta, slowest in capillaries, then increases again in veins
Speed is inversely related
to total cross-sectional area
Capillaries have largest area so slowest flow
Slow capillary flow allows adequate time for exchange between blood and tissues
Vasomotion
intermittent flow of blood through capillaries
Due to on/off opening and closing of precapillary sphincters
Diffusion between blood and interstitial fluid
Move down their concentration gradients
Molecules use four different routes to cross capillary:
- Diffuse directly through endothelial membranes
Example: lipid-soluble molecules such as respiratory gases - Pass through clefts
Example: water-soluble solutes - Pass through fenestrations
Example: water-soluble solutes - Active transport via pinocytotic vesicles or caveolae
Example: larger molecules, such as proteins
Fluid is forced out clefts of capillaries at arterial end, most returns to blood at venous end
Determines relative fluid volumes in blood and interstitial space
2 Forces involved
- Hydrostatic pressures (HP)
- force exerted by fluid pressing against wall; two types
- Capillary hydrostatic pressure: capillary blood pressure
- -Greater at arterial end (35mmHg) of bed than at venule end (17mmHg)
- Interstitial fluid hydrostatic pressure: pressure pushing fluid back into vessel; usually assumed to be zero because lymphatic vessels drain interstitial fluid - Colloid osmotic pressures
-Capillary colloid osmotic pressure
–“Sucking” pressure created by nondiffusible plasma proteins pulling water back in to capillary
–Opc ∼26 mm Hg
-Interstitial fluid colloid osmotic pressure
Pressure is inconsequential because interstitial fluid has very low protein content
around only 1mmHg
Hydrostatic-osmotic pressure interactions
- Net filtration pressure –(NFP): comprises all forces acting on
- Net fluid flow out at arterial end
- Net fluid flow in at venous end
- More fluid leaves at than is returned at
-Net filtration pressure –(NFP): comprises all forces acting on capillary bed
NFP = (HPc + OPif) − (HPif + OPc)
-Net fluid flow out at arterial end (filtration)
-Net fluid flow in at venous end (reabsorption)
-More fluid leaves at arterial end than is returned at venous end
–Excess interstitial fluid is returned to blood via lymphatic system
Edema:
Caused by either an increase in
or a decrease in
-An increase in interstitial fluid osmotic pressure can result from an
abnormal increase in amount of interstitial fluid
- Caused by either an increase in outward pressure (driving fluid out of the capillaries) or a decrease in inward pressure
- -An increase in capillary hydrostatic pressure accelerates fluid loss from blood
- -Could result from incompetent venous valves, localized blood vessel blockage, congestive heart failure, or high blood volume
- An increase in interstitial fluid osmotic pressure can result from an inflammatory response
- -Inflammation increases capillary permeability and allows proteins to leak into interstitial fluid
- -Causes large amounts of fluid to be pulled into interstitial space
- A decrease in capillary colloid osmotic pressure hinders fluid return to blood
- -Can be caused by hypoproteinemia, low levels of plasma proteins caused by malnutrition, liver disease, or glomerulonephritis (loss of plasma proteins from kidneys)
Edema also can be caused by decreased
drainage of interstitial fluid through lymphatic vessels that have been blocked by disease or surgically removed
pitting edema
Excess interstitial fluid in subcutaneous tissues generally causes
- Edema can impair tissue function as a result of increased distance for diffusion of gases, nutrients and wastes between blood and cells
- Slow fluid losses can be compensated for by renal mechanisms, but rapid onset may have serious effects on the circulation
Vascular system consists of two main circulations:
Heart pumps blood out to system via
Blood returning to heart is delivered via
Pulmonary circulation: short loop that runs from heart to lungs and back to heart
Systemic circulation: long loop to all parts of body and back to heart
- -Heart pumps blood out to system via single systemic artery, the aorta
- Blood returning to heart is delivered via terminal systemic veins, superior and inferior vena cava, as well as coronary sinus
Important differences between systemic arteries and veins:
which ones run deep and are superficial?
which ones correspond?
which ones are more interconnected?
which systems have unique venous drainage system
- Arteries run deep, whereas veins are both deep and superficial
- -Deep veins share same name with corresponding artery
- -Superficial veins do not correspond to names of any arteries
- Venous pathways are more interconnected
- -Unlike arterial pathways, venous pathways have more interconnections
- —Veins can have more than one name, making venous pathways harder to follow
-The brain and digestive systems have unique –venous drainage systems
Brain contains dural venous sinuses
–Venous system of the digestive system drains into hepatic portal system which goes to the liver
