PowerPoint® Lecture Slides prepared by Janice Meeking, Mount Royal College
C H A P T E R
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19
The Cardiovascular System: Blood Vessels: Part A
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Blood Flow
• The purpose of cardiovascular regulation is to maintain adequate blood flow through the capillaries to the tissues
• Actual volume of blood flowing through a vessel, an organ, or the entire circulation in a given period:
• Is measured in ml/min.
• Is equivalent to cardiac output (CO), considering the entire vascular system
• Is relatively constant when at rest
• Varies widely through individual organs
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The distribution of blood
• The blood volume is unevenly distributed among arteries, veins and capillaries
• The heart, arteries and capillaries contain about 30-35%
• The venous system contains the rest – 65-70%
• About 1/3 of the venous blood is circulating in the liver, bone marrow and skin
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Blood flow
• Capillary blood flow is determined by the interplay between:
• Pressure
• Resistance
• For the blood to keep flowing the heart must generate sufficient pressure to overcome resistance
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Physiology of Circulation: Blood Pressure (BP)
• Force per unit area exerted on the wall of a blood vessel by its contained blood
• Expressed in millimeters of mercury (mm Hg)
• Measured in reference to systemic arterial BP in large arteries near the heart
• The differences in BP within the vascular system provide the driving force that keeps blood moving from higher to lower pressure areas
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Physiology of Circulation: Resistance
• Opposition to flow
• Measure of the amount of friction blood encounters
• Generally encountered in the peripheral systemic circulation
• Because the resistance of the venous system is very low (why?) usually the focus is on the resistance of the arterial system:
• Peripheral resistance (PR) is the resistance of the arterial system
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Physiology of Circulation: Resistance
• Three important sources of resistance
• Blood viscosity
• Total blood vessel length
• Blood vessel diameter
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Resistance – constant factors
• Blood viscosity
• The “stickiness” of the blood due to formed elements and plasma proteins
• Blood vessel length
• The longer the vessel, the greater the resistance encountered
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Resistance – frequently changed factors
• Changes in vessel diameter are frequent and significantly alter peripheral resistance
• Small-diameter arterioles are the major determinants of peripheral resistance
• Frequent changes alter peripheral resistance
• Varies inversely with the fourth power of vessel radius
• Fatty plaques from atherosclerosis:
• Cause turbulent blood flow
• Dramatically increase resistance due to turbulence
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Relationship Between Blood Flow, Blood Pressure, and Resistance
• Blood flow (F) is directly proportional to the blood (hydrostatic) pressure gradient (P)
• If P increases, blood flow speeds up
• Blood flow is inversely proportional to peripheral resistance (R)
• If R increases, blood flow decreases: F = P/R
• R is more important in influencing local blood flow because it is easily changed by altering blood vessel diameter
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Systemic Blood Pressure
• The pumping action of the heart generates blood flow through the vessels along a pressure gradient, always moving from higher- to lower-pressure areas
• Pressure results when flow is opposed by resistance
• Systemic pressure:
• Is highest in the aorta
• Declines throughout the length of the pathway
• Is ~0 mm Hg in the right atrium
• The steepest change in blood pressure occurs between arteries to arterioles
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Arterial Blood Pressure
• Arterial pressure is important because it maintains blood flow through capillaries
• Blood pressure near the heart is pulsatile
• Systolic pressure: pressure exerted during ventricular contraction
• Diastolic pressure: lowest level of arterial pressure
• A pulse is rhythmic pressure oscillation that accompanies every heartbeat
• Pulse pressure = difference between systolic and diastolic pressure
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Arterial Blood Pressure
• Mean arterial pressure (MAP): pressure that propels the blood to the tissues
MAP = diastolic pressure + 1/3 pulse pressure
• Pulse pressure and MAP both decline with increasing distance from the heart
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Capillary Blood Pressure
• Ranges from 15 to 35 mm Hg
• Low capillary pressure is desirable
• High BP would rupture fragile, thin-walled capillaries
• Low BP is sufficient to force filtrate out into interstitial space and distribute nutrients, gases, and hormones between blood and tissues
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Venous Blood Pressure
• Changes little during the cardiac cycle
• Small pressure gradient, about 15 mm Hg
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Factors Aiding Venous Return
• Venous BP alone is too low to promote adequate blood return and is aided by the:
• Respiratory “pump” – pressure changes created during breathing suck blood toward the heart by squeezing local veins
• Muscular “pump” – contraction of skeletal muscles “milk” blood toward the heart
• Valves prevent backflow during venous return
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Monitoring Circulatory Efficiency
• Efficiency of the circulation can be assessed by taking pulse and blood pressure measurements
• Vital signs – pulse and blood pressure, along with respiratory rate and body temperature
• Pulse – pressure wave caused by the expansion and recoil of elastic arteries
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Measuring Blood Pressure
• Systemic arterial BP is measured indirectly with the auscultatory method
• A sphygmomanometer is placed on the arm superior to the elbow
• Pressure is increased in the cuff until it is greater than systolic pressure in the brachial artery
• Pressure is released slowly and the examiner listens with a stethoscope
• The first sound heard is recorded as the systolic pressure
• The pressure when sound disappears is recorded as the diastolic pressure
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Alterations in Blood Pressure
• Hypotension – low BP in which systolic pressure is below 100 mm Hg
• Hypertension – condition of sustained elevated arterial pressure of 140/90 or higher
• Transient elevations are normal and can be caused by fever, physical exertion, and emotional upset
• Chronic elevation is a major cause of heart failure, vascular disease, renal failure, and stroke
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Maintaining Blood Pressure• Requires
• Cooperation of the heart, blood vessels, and kidneys
• Supervision by the brain
• The main factors influencing blood pressure:
• Cardiac output (CO)
• Peripheral resistance (PR)
• Blood volume
Blood pressure = CO x PR
• Blood pressure varies directly with CO, PR, and blood volume
• Changes in one variable are quickly compensated for by changes in the other variables
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Controls of Blood Pressure
• Short-term controls:
• Are mediated by the nervous system and bloodborne chemicals
• Counteract moment-to-moment fluctuations in blood pressure by altering peripheral resistance
• Long-term controls regulate blood volume
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Short-Term Mechanisms: Neural Controls
• Neural controls of peripheral resistance:
• Alter blood distribution in response to demands
• Maintain MAP by altering blood vessel diameter
• Neural controls operate via reflex arcs involving:
• Vasomotor centers and vasomotor fibers
• Baroreceptors
• Vascular smooth muscle
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Short-Term Mechanisms: Vasomotor Center
• Vasomotor center – a cluster of sympathetic neurons in the medulla that oversees changes in blood vessel diameter
• Maintains blood vessel tone by innervating smooth muscles of blood vessels, especially arterioles
• Vasomotor activity is modified by:
• Baroreceptors (pressure-sensitive)
• chemoreceptors (O2, CO2, and H+ sensitive)
• bloodborne chemicals
• hormones
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Short-Term Mechanisms: Baroreceptor-Initiated Reflexes
• Baroreceptors are located in
• Carotid sinuses
• Aortic arch
• Walls of large arteries of the neck and thorax
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Short-Term Mechanisms: Baroreceptor-Initiated Reflexes
• Increased blood pressure stimulates the cardioinhibitory center to:
• Increase vessel diameter
• Decrease heart rate, cardiac output, peripheral resistance, and blood pressure
• Declining blood pressure stimulates the cardioacceleratory center to:
• Increase cardiac output and peripheral resistance
• Low blood pressure also stimulates the vasomotor center to constrict blood vessels
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Short-Term Mechanisms: Chemoreceptor-Initiated Reflexes
• Chemoreceptors are located in the
• Carotid sinus
• Aortic arch
• Large arteries of the neck
• Chemoreceptors respond to rise in CO2, drop in pH or O2
• Increase blood pressure via the vasomotor center and the cardioacceleratory center
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Chemicals that Increase Blood Pressure
• Adrenal medulla hormones – norepinephrine and epinephrine increase blood pressure
• Antidiuretic hormone (ADH) – causes intense vasoconstriction in cases of extremely low BP
• Angiotensin II – kidney release of renin generates angiotensin II, which causes vasoconstriction
• Endothelium-derived factors – endothelin and prostaglandin-derived growth factor (PDGF) are both vasoconstrictors
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Chemicals that Decrease Blood Pressure
• Atrial natriuretic peptide (ANP) – causes blood volume and pressure to decline
• Nitric oxide (NO) – is a brief but potent vasodilator
• Inflammatory chemicals – histamine, prostacyclin, and kinins are potent vasodilators
• Alcohol – causes BP to drop by inhibiting ADH
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Long-Term Mechanisms: Renal Regulation
• Long-term mechanisms control BP by altering blood volume
• Increased BP stimulates the kidneys to eliminate water, thus reducing BP
• Decreased BP stimulates the kidneys to increase blood volume and BP
• Kidneys act directly and indirectly to maintain long-term blood pressure
• Direct renal mechanism alters blood volume (changes in urine volume)
• Indirect renal mechanism involves the renin-angiotensin mechanism
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Indirect Mechanism: renin-angiotensin
• The renin-angiotensin mechanism
• Arterial blood pressure release of renin
• Renin production of angiotensin II
• Angiotensin II is a potent vasoconstrictor
• Angiotensin II aldosterone secretion
• Aldosterone renal reabsorption of Na+ and urine formation
• Angiotensin II stimulates ADH release
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Blood Flow Through Tissues
• Blood flow, or tissue perfusion, is involved in:
• Delivery of oxygen and nutrients to, and removal of wastes from, tissue cells
• Gas exchange in the lungs
• Absorption of nutrients from the digestive tract
• Urine formation by the kidneys
• Blood flow is precisely the right amount to provide proper tissue function
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Velocity of Blood Flow
• Changes as it travels through the systemic circulation
• Is inversely related to the total cross-sectional area
• Is fastest in the aorta, slowest in the capillaries, increases again in veins
• Slow capillary flow allows adequate time for exchange between blood and tissues
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• Autoregulation – automatic adjustment of blood flow to each tissue in proportion to its requirements at any given point in time
• Local vasodilators accelerate blood flow in response to:
• Decreased tissue O2 levels or increased CO2 levels
• Generation of lactic acid
• Rising K+ or H+ concentrations in interstitial fluid
• Local inflammation
• Elevated temperature
• Vasoconstrictors:
• Injured vessels constrict strongly (why?)
• Drop in tissue temperature (why?)
Autoregulation of blood flow within tissues
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Myogenic (myo =muscle; gen=origin) Controls
• Inadequate blood perfusion (tissue might die) or excessively high arterial pressure (rupture of vessels) may interfere with the function of the tissue
• Vascular smooth muscle can prevent these problems by responding directly to passive stretch (increased intravascular pressure)
• The muscle response is by resist to the stretch and that results in vasoconstriction
• The opposite happens when there is a reduced stretch
• The myogenic mechanisms keeps the tissue perforation relatively constant.
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Homeostatic Adjustments that Compensate for a Reduction in Blood Pressure and Blood Flow - autoregulation
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Long-Term Autoregulation
• Angiogenesis
• Occurs when short-term autoregulation cannot meet tissue nutrient requirements
• The number of vessels to a region increases and existing vessels enlarge
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Circulatory Shock
• Circulatory shock – any condition in which blood vessels are inadequately filled and blood cannot circulate normally
• Results in inadequate blood flow to meet tissue needs
• Three types include:
• Hypovolemic shock – results from large-scale blood loss or dehydration
• Vascular shock – normal blood volume but too much accumulate in the limbs (long period of standing/sitting)
• Cardiogenic shock – the heart cannot sustain adequate circulation
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Capillary Exchange of Respiratory Gases and Nutrients
• Diffusion of
• O2 and nutrients from the blood to tissues
• CO2 and metabolic wastes from tissues to the blood
• Lipid-soluble molecules diffuse directly through endothelial membranes
• Water-soluble solutes pass through clefts and fenestrations
• Larger molecules, such as proteins, are actively transported
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Hydrostatic Pressures
• Capillary hydrostatic pressure (HPc) (capillary blood pressure)
• Tends to force fluids through the capillary walls
• Is greater at the arterial end (35 mm Hg) of a bed than at the venule end (17 mm Hg)
• Interstitial fluid hydrostatic pressure (HPif)
• Usually assumed to be zero because of lymphatic vessels
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Colloid Osmotic Pressures
• Capillary colloid osmotic pressure (oncotic pressure) (OPc)
• Created by non-diffusible plasma proteins, which draw water toward themselves
• ~26 mm Hg
• Interstitial fluid osmotic pressure (OPif)
• Low (~1 mm Hg) due to low protein content
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Net Filtration Pressure (NFP)
• NFP—comprises all the forces acting on a capillary bed
• NFP = (HPc—HPif)—(OPc—OPif)
• At the arterial end of a bed, hydrostatic forces dominate
• At the venous end, osmotic forces dominate
• Excess fluid is returned to the blood via the lymphatic system