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Chapter 21The Cardiovascular System
Blood Vessels and Circulation
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21.1 Functions of the Circulatory System
1. Carry blood2. Exchange nutrients, waste products, and gases3. Transport of hormones, components of the
immune system, molecules required for coagulation, enzymes, nutrients, gases, waste products, etc.
4. Regulate blood pressure5. Directs blood flow
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21.2 Structural Features of Blood Vessels
• Arteries– Elastic– Muscular– Arterioles
• Capillaries: site of exchange with tissues• Veins: thinner walls than arteries, contain less
elastic tissue and fewer smooth muscle cells– Venules– Small veins– Medium or large veins
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Capillaries• Capillary wall consists of endothelial cells (simple squamous
epithelium), basement membrane and a delicate layer of loose C.T. Scattered pericapillary cells that are fibroblasts, macrophages or undifferentiated smooth muscle cells.
• Substances move through capillaries by diffusion through – Lipid-soluble and small water-soluble molecules through plasma
membrane– Larger water-soluble molecules pass through fenestrae or gaps
between endothelial cells.
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Types of Capillaries• Continuous. No gaps between endothelial cells. No
fenestrae. Less permeable to large molecules than other capillary types. E.g., muscle, nervous tissue.
• Fenestrated. Have pores. Endothelial cells have numerous fenestrae. Fenestrae are areas where cytoplasm is absent and plasma membrane is made of a thin, porous diaphragm. Highly permeable. E.g., intestinal villi, ciliary process of eye, choroid plexus, glomeruli of kidney
• Sinusoidal. Large diameter with large fenestrae. Less basement membrane. E.g., endocrine glands (large molecules cross their walls).
• Sinusoids. Large diameter sinusoidal capillaries. Sparse basement membrane. E.g., liver, bone marrow.
• Venous sinuses are similar in structure but even larger. E.g., spleen
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Structure of Capillary Walls
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Capillary Network
• Blood flows from arterioles through metarterioles, then through capillary network
• Flow through thoroughfare channel fairly consistent while flow through arterial capillaries is intermittent
– Smooth muscle allows vessels to regulate blood supply by constricting or dilating
– Most of the smaller unnamed arteries– Thick walls due to 25-40 layers of smooth muscle.– Also called distributing arteries because smooth muscle allows vessels to
partially regulate blood supply to different regions of the body.• Smaller muscular arteries
– Adapted for vasodilation and vasoconstriction.
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Arterioles
• Transport blood from small arteries to capillaries
• Smallest arteries where the three tunics can be differentiated
• Like small arteries, capable of vasoconstriction and dilation
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Venules and Small Veins
• Venules drain capillary network. Endothelial cells and basement membrane with a few smooth muscle cells. As diameter of venules increases, amount of smooth muscle increases.
• Small veins. Smooth muscle cells form a continuous layer. Addition of tunica adventitia made of collagenous connective tissue
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Medium and Large Veins• Medium veins. Go-between between small veins and large
veins.• Large veins. Tunica intima is thin: endothelial cells, relatively
thin layer of C.T and a few scattered elastic fibers. Tunica media has circularly arranged smooth muscle cells. Adventitia is predominant layer.
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Valves
• Valves found in all veins greater than 2 mm in diameter.• Folds in intima form two flaps that overlap.• More valves in veins of lower extremities than in veins
of upper extremities.
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Other Vessel Types
• Vasa vasorum: blood vessels that supply the walls of arteries and veins. Penetrate vessel walls from the exterior. Branches of arteries.
• Portal veins: veins that begin in a primary capillary network, extend some distance and end in a secondary capillary network without a pumping mechanism, such as the heart, between.
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Neural Innervation of Blood Vessels
• Unmyelinated sympathetic nerve fibers form plexi in tunica adventitia: vasoconstriction
• Small arteries and arterioles innervated to greatest extent
• Vessels of penis and clitoris innervated by parasympathetic
• Some blood vessels innervated by myelinated fibers and act as baroreceptors that monitor stretch and detect changes in blood pressure
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Aging of the Arteries
• Arteriosclerosis: general term for degeneration changes in arteries making them less elastic
• Atherosclerosis: deposition of plaque on walls
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21.3 Pulmonary Circulation
• From right ventricle into pulmonary trunk
• Pulmonary trunk divides into left and right pulmonary arteries.
• Two pulmonary veins exit each lung and enter left atrium
Veins of the Upper LimbRt & Lt Subclavian v. Lt Subclavian v.
Rt Subclavian v. Lt Axillary v.
Veins of the Upper LimbAxillary v.
Brachial v.
Deep Veins of the Upper Limb
Radial v. Ulnar v.
Deep Forearm Veins
Radial v. Ulnar v.Interosseous v.
Hand Veins
Common palmar
digital v.
Deep venous palmar
arch
Proper palmar digital
Superior venous palmar
arch
Superficial Arm Veins
Basilic v. Cephalic v.
Median cubital v.
Basilic v. Cephalic v.Median cubital v.
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Veins of the Upper Limb
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Veins of the Thorax
Veins of Thorax
Azygos v.
Post. Intercostal v.
Internal Thoracic v.
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Inferior Vena Cava and Its Tributaries
Veins of Abdomen and Pelvis
Inferior Vena Cava
Major Tributaries of the Inferior Vena Cava
Common iliac v.
gonadal v. hepatic v.
Inferior mesenteric v.
Hepatic Portal v.Inferior
phrenic v.
Major Tributaries of the Inferior Vena Cava
Right renal v. Left renal v.
Superior mesenteric v. Splenic v.
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Hepatic Portal System• Portal system: vascular system that begins and ends at a capillary
bed with no pumping mechanism in between.• Hepatic portal- liver; renal portal- kidney;
hypothalamohypophyseal portal between hypothalamus and pituitary.
• Blood entering the hepatic portal vein is rich with nutrients collected from the intestines, but may also contain toxic substances. Both nutrients and toxic substances will be regulated by the liver– Nutrients: either taken up and stored or modified chemically and used by
other parts of the body– Biotransformation: Toxic substances can be broken down by hepatocytes
or can be made water soluble. To be transported in blood and excreted by the kidneys.
Splenic v.
Inferior mesenteric v.
Superior mesenteric v.
Hepatic portal v.
Liver
Hepatic Portal System
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Veins of Abdomen and Pelvis
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Veins of the Lower Limb
Veins of the Pelvic Organs and Lower Limb
Common iliac v. External iliac v. Internal iliac v.
Veins of the Pelvic Organs and Lower Limb
Femoral v. Deep thigh vein Great saphenous v.
Veins of the Lower Limb
Femoral v. Deep thigh vein
Popliteal vein
Great saphenous
vein
Veins of the Lower LimbAnterior tibial v. Malleolar v. Lateral tarsal
v.Dorsal vein
Veins of the Lower LimbDorsal venous arch Dorsal metatarsal Dorsal digital
Veins of the Lower LimbPosterior tibial Posterior fibular malleolar
Veins of the Lower LimbMedian plantar
Lateral plantar
Plantar arch
Plantar metatarsal
Plantar digital
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Veins of Lower Limb
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21.6 Dynamics of Blood Circulation
• Interrelationships between– Pressure– Flow– Resistance– Control mechanisms that regulate
blood pressure and blood flow
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Blood Pressure• Measure of force exerted by blood against the wall• Blood moves through vessels because of blood
pressure• Measured by listening for Korotkoff sounds
produced by turbulent flow in arteries as pressure released from blood pressure cuff
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Blood Pressure
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Laminar and Turbulent Flow• Laminar flow
– Streamlined; interior of blood vessel is smooth and of equal diameter along its length
– Outermost layer moving slowest and center moving fastest
• Turbulent flow– Interrupted– Rate of flow exceeds critical velocity– Fluid passes a constriction, sharp turn, rough surface– Partially responsible for heart sounds– Sounds due to turbulence not normal in arteries and is
probably due to some constriction; increases the probability of thrombosis
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Laminar and Turbulent Flow
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Blood Flow• Rate of flow through a tube is expressed as the
volume that passes a specific point per unit of time. E.g.; cardiac output at rest is 5L/min, thus blood flow through the aorta is 5L/min
• Flow = (P1 – P2/R)• P1 and P2 are pressures in the vessel at points one
and two; R is the resistance to flow• Directly proportional to pressure differences,
inversely proportional to resistance• Resistance = 128vl/D4
• v is viscosity, l = length of the vessel, r is the radius of the vessel
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Poiseuille’s Law• Flow decreases when resistance increases and vice
versa. Since resistance is proportional to blood vessel diameter, constriction of a blood vessel increases resistance and thus decreases flow
• Flow =(P1 –P2)/R - (P1 –P2)D4/128vl• During exercise, heart beats with greater force
increasing pressure in the aorta. Capillaries to skeletal muscle increase in diameter decreasing resistance and increasing flow. Increased flow in aorta can go from 5L/min to 5 times that amount
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Viscosity• Measure of resistance of liquid to flow• Resistance proportionate to flow• As viscosity increases, pressure required to
flow increases• Viscosity influenced largely by hematocrit
(percentage of the total blood volume composed of red blood cells).
• Dehydration and/or uncontrolled production of RBCs can lead to increased viscosity which increases the workload on the heart.
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Critical Closing Pressure, Laplace’s Law and Compliance
• Critical closing pressure: Pressure at which a blood vessel collapses and blood flow stops
• Laplace’s Law– Force acting on blood vessel
wall is proportional to diameter of the vessel times blood pressure
– F= D X P; thus as diameter of a vessel increases, force on the wall increases. Weakened part of a vessel wall bulges out and is an aneurysm.
• Vascular compliance: Tendency for blood vessel volume to increase as blood pressure increases
• Compliance = increase in volume/Increase in pressure
– More easily the vessel wall stretches, the greater its compliance
– Venous system has a large compliance (24 times greater than that of arteries) and acts as a blood reservoir
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21.7 Physiology of the Systemic Circulation
• Determined by– Anatomy of circulatory system– Dynamics of blood flow– Regulatory mechanisms that control heart and
blood vessels• Blood volume
– Most in the veins (greater compliance)– Smaller volumes in arteries and capillaries
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Cross-Sectional Area• As diameter of vessels
decreases, the total cross-sectional area increases and velocity of blood flow decreases. Only one aorta with a cross-sectional area of 5 cm2. Total cross-sectional area of the millions of capillaries is 2500 cm2.
• Much like a stream that flows rapidly through a narrow gorge but flows slowly through a broad plane
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Pressure and Resistance• Blood pressure averages 100 mm
Hg in aorta and drops to 0 mm Hg by the time the blood gets to the right atrium. Due to decreased resistance to flow as cross-sectional area increases.
• Greatest drop in pressure occurs in arterioles which regulate blood flow through tissues
• No large fluctuations in capillaries and veins
• Muscular arteries and arterioles are capable of constricting or dilating in response to autonomic and hormonal stimulation. Muscular arteries regulate flow into a region of the body; arterioles regulate flow into a specific tissue.
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Pulse Pressure• Difference between systolic and
diastolic pressures• Increases when stroke volume
increases or vascular compliance decreases. Compliance tends to decrease with age (arteriosclerosis) and pressure rises.
• Pulse pressure can be used to take a pulse to determine heart rate and rhythmicity
• Most frequent site used to measure pulse rate is in the carpus with the radial artery- the radial pulse.
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Capillary Exchange and Regulation of Interstitial Fluid Volume
• Capillary exchange: the movement of substances into and out of capillaries
• Most important means of exchange: diffusion. – Lipid soluble cross capillary walls diffusing through plasma
membrane. E.g., O2, CO2, steroid hormones, fatty acids.– Water soluble diffuse through intercellular spaces or through
fenestrations of capillaries. E.g., glucose, amino acids.• Blood pressure, capillary permeability, and osmosis affect
movement of fluid from capillaries• Fluid moves out of capillaries at arterial end and most but
not all returns to capillaries at venous end. That which remains in tissues is picked up by the lymphatic system then returned to venous circulation.
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Fluid Exchange Across Capillary Walls
• Net filtration pressure (NFP)- force responsible for moving fluid across capillary walls. Two forces affect pressure– Hydrostatic pressure: physical pressure of blood flowing
through the vessels or of fluid in interstitial spaces– Osmotic pressure: movement of solutes (plasma or tissue
fluid) through a membrane (plasma membrane) in the presence of a non-diffusible solute (large proteins). Large proteins do not freely pass through the capillary walls and the difference in protein concentrations between the blood and interstitial fluid is responsible for osmosis
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Pressures Involved• NFP = Net hydrostatic pressure minus net osmotic
pressure• Net hydrostatic pressure = BP-IFP• Net osmotic pressure = BCOP-ICOP where
BP = blood pressureIFP = interstitial fluid pressure= (lymphatic vessels are
Edema and Capillary Exchange• If capillaries become more permeable, proteins can
leak into the interstitial fluid increasing ICOP. More fluid moves from the capillaries into the interstitial fluid: edema.– Chemicals of inflammation increase permeability– Decreases in plasma concentration of protein reduces
BCOP; more fluid moves into interstitial fluid• Liver disease resulting in fewer plasma proteins• Loss of plasma proteins through the kidneys• Protein starvation
– Blockage of veins increases capillary BP; reduced venous return due to gravity
– Blockage or removal of lymphatic vessels (blockage: elephantiasis; removal: cancer)
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Functional Characteristics of Veins
• Venous return to heart increases due to increase in blood volume, venous tone, and arteriole dilation
• Venous tone: continual state of partial contraction of the veins as a result of sympathetic stimulation
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Blood Pressure and the Effect of Gravity
• In a standing position, hydrostatic pressure caused by gravity increases blood pressure below the heart and decreases pressure above the heart.
• Muscular movement improves venous return.
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21.8 Control of Blood Flow in Tissues
• Local control: in most tissues, blood flow is proportional to metabolic needs of tissues
• Nervous System: responsible for routing blood flow and maintaining blood pressure
• Hormonal Control: sympathetic action potentials stimulate epinephrine and norepinephrine
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Local Control of Blood Flow in Tissues
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Local Control of Blood Flow• Blood flow can increase 7-8 times as a result of
vasodilation of metarterioles and precapillary sphincters in response to increased rate of metabolism
• Vasomotion: periodic contraction and relaxation of precapillary sphincters. Autoregulation.
• Long-term local control: capillaries become more dense in a region that regularly has an increased metabolic rate.
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Nervous Control of Blood Flow in the Tissues• Important in minute-to-minute regulation of local
circulation• Provides a means by which blood can be shunted
from one large area of the peripheral circulatory system to another area by increasing resistance
• Sympathetic division most important. Innervates all vessels except capillaries, precapillary sphincters, and most metarterioles.
• Vasomotor center in lower pons and upper medulla oblongata. – Excitatory part is tonically active. Causes
vasomotor tone. Norepinephrine– Inhibitory part can cause vasodilation by
decreasing sympathetic output
•Sympathetic stimulation of adrenal medulla causes output of norepinephrine and epinephrine into circulation. Causes vasoconstriction in vessels (α-adrenergic receptors) except in skeletal muscle where vasodilation takes place (ß-adrenergic receptors)
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21.9 Regulation of Mean Arterial Pressure
• Mechanisms that maintain arterial blood pressure within a normal range of values
• Mean arterial pressure (MAP): slightly less than the average of systolic and diastolic pressures because diastole lasts longer than systole.
• 70 mmHg at birth, 100 mmHg from adolescence to middle age, 110 mmHg in healthy older individuals.
• MAP = CO(PR), MAP = HR(SV)(PR) • MAP α HR, SV, and PR. If any of these go up, so does
MAP.• Two systems to regulate pressure: short-term and long-term
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Short-Term Regulation ofBlood Pressure
• Baroreceptor reflexes: change peripheral resistance, heart rate, and stroke volume in response to changes in blood pressure
• Chemoreceptor reflexes: sensory receptors sensitive to oxygen, carbon dioxide, and pH levels of blood
• Central nervous system ischemic response: results from high carbon dioxide or low pH levels in medulla and increases peripheral resistance
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Baroreceptor Reflex Control
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Adrenal Medullary Mechanism• Activated when stimuli
result in a substantial increase in sympathetic stimulation of heart and blood vessels (large decrease in blood pressure, sudden and substantial increase in physical activity, stress)
• Adrenal releases epinephrine and norepinephrine
• Hormones mimic sympathetic stimulation of heart and blood vessels.
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Baroreceptor Effects
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Chemoreceptor Reflex Control
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Effects of pH and Gases
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Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.
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CNS Ischemic Response• Elevation of BP in response to a lack of blood
flow to the medulla oblongata. • Functions in response to emergency situations and
BP falls below 50 mmHg• Neurons of vasomotor center strongly stimulated;
increases blood flow to brain if vessels are intact but at the same time, decreases oxygenation of blood because blood does not go to lungs.
• Lack of oxygen causes vasomotor center to become inactive; extensive vasodilation follows with concomitant drop in BP. Death if CNS ischemic response lasts longer than a few minutes.
• Atrial natriuretic hormone: released from cardiac muscle cells when atrial blood pressure increases, simulating an increase in urinary production, causing a decrease in blood volume and blood pressure
• Fluid shift: movement of fluid from interstitial spaces into capillaries in response to decrease in blood pressure to maintain blood volume and vice versa.
• Stress-relaxation response: adjustment of blood vessel smooth muscle to respond to change in blood volume. When blood volume suddenly declines and pressure drops, smooth muscles contract and vice versa.
