Circulatory System I PDF

Circulatory System I © McGraw Hill 1  Introduction Route taken by blood caused confusion for many centuries Chinese emperor Huang Ti (2697 to 2597 BC) correctly believed that blood flowed around the body and back to the heart...

Circulatory System I © McGraw Hill 1  Introduction Route taken by blood caused confusion for many centuries Chinese emperor Huang Ti (2697 to 2597 BC) correctly believed that blood flowed around the body and back to the heart Seriously Dude? Roman physician Galen (129 to c.199) thought blood flowed back and forth; liver created blood and organs consumed it © McGraw Hill 2 © McGraw Hill 3  General Anatomy of the Blood Vessels Three principal categories of blood vessels: Arteries Carry blood away from heart Veins Carry blood back to heart Capillaries Connect smallest arteries to smallest veins to create a circuit © McGraw Hill 4  Micrographs of Blood Vessels Figure 20.1 © McGraw Hill a: Dennis Strete/McGraw-Hill Education;b: Susumu Nishinaga/Science Source 5  The Vessel Wall 1 Walls of arteries and veins have three layers (tunics): tunica interna, tunica media, tunica externa © McGraw Hill 6 © McGraw Hill 7  The Vessel Wall 2 Tunica media Middle layer Consists of smooth muscle, collagen, and elastic tissue Strengthens vessels and prevents blood pressure from rupturing them Contraction of muscle controls blood vessel diameter © McGraw Hill 8  The Vessel Wall 3 Tunica externa (tunica adventitia) Outermost layer Consists of loose connective tissue that often merges with that of neighboring blood vessels, nerves, or other organs Anchors the vessel and provides passage for small nerves, lymphatic vessels Vasa vasorum: small vessels that supply blood to outer part of the larger vessels © McGraw Hill 9  Histology of the Blood Vessels Figure 20.2 © McGraw Hill 10  ARTERIES ARE CLASSIFIED BY SIZE 1 Arteries are sometimes called resistance vessels because of their strong, resilient tissue structure Conducting (elastic or large) arteries Biggest arteries Examples: aorta, common carotid, subclavian, pulmonary trunk, and common iliac arteries Internal elastic lamina at the border between interna and media External elastic lamina at the border between media and externa Expand during systole, recoil during diastole Expansion takes pressure of smaller downstream vessels © McGraw Hill 11  ARTERIES ARE CLASSIFIED BY SIZE 2 Distributing (muscular or medium) arteries Distribute blood to specific organs Examples: brachial, femoral, renal, and splenic arteries Smooth muscle layers constitute three-fourths of wall thickness Resistance (small) arteries Thicker tunica media in proportion to their lumen than large arteries and very little tunica externa Arterioles: smallest of the resistance arteries 200 mm diameter; only 1-3 layers of smooth muscle Control amount of blood to various organs © McGraw Hill 12  ARTERIES ARE CLASSIFIED BY SIZE 3 Metarterioles (thoroughfare channels) Short vessels that link arterioles directly to venules in some places (for example, in mesenteries) Provide shortcuts allowing blood to bypass capillary beds © McGraw Hill 13  Aneurysm Aneurysm—weak point in artery or heart wall Forms a thin-walled, bulging sac that pulsates with each heartbeat and may rupture at any time Dissecting aneurysm: blood accumulates between tunics of artery and separates them, usually because of degeneration of the tunica media Most common sites: abdominal aorta, renal arteries, and arterial circle at base of brain Can cause pain by putting pressure on other structures Can rupture causing hemorrhage Result from congenital weakness of blood vessels, trauma, or bacterial infections Most common cause is atherosclerosis and hypertension © McGraw Hill 14  Aneurysms Figure 20.3 © McGraw Hill Susumu Nishinaga/Science Source 15  Arterial Sense Organs 1 Sensory structures in walls of major vessels monitor blood pressure and chemistry Transmit information to brainstem to regulate heart rate, blood vessel diameter, and respiration Carotid sinuses: baroreceptors In walls of internal carotid artery Monitor blood pressure Transmit signals through glossopharyngeal nerve Allow for baroreflex © McGraw Hill 16  ARTERIAL SENSE ORGANS 2 Carotid bodies: chemoreceptors Oval bodies near branch of common carotids Monitor blood chemistry Transmit signals through glossopharyngeal nerve to brainstem respiratory centers Adjust respiratory rate to stabilize pH, CO2, and O2 Aortic bodies: chemoreceptors One to three bodies in walls of aortic arch Same structure and function as carotid bodies, but innervation is by vagus nerve © McGraw Hill 17  Baroreceptors and Chemoreceptors in the Arteries Superior to the Heart Figure 20.4 © McGraw Hill 18 © McGraw Hill 19  Types of Capillaries 1  Continuous capillaries Occur in most tissues Endothelial cells have tight junctions Form a continuous tube with intercellular clefts Allow passage of solutes such as glucose Pericytes wrap around the capillaries and contain the same contractile protein as muscle Contract and regulate blood flow © McGraw Hill 20  Fenestrated capillaries © McGraw Hill 21  Sinusoids © McGraw Hill 22  Capillary Beds Capillary beds are networks of 10 to 100 capillaries Usually supplied by a single arteriole or metarteriole Drain into venule or distal end of metarteriole At any given time, 75% of body’s capillaries are shut down Most control involves constriction of upstream arterioles Precapillary sphincters control flow in capillary beds supplied with metarterioles When sphincters are relaxed, capillaries are well perfused with blood When sphincters contract, they constrict the entry to the capillary and blood bypasses the capillary © McGraw Hill 23  Perfusion of a Capillary Bed Figure 20.7 © McGraw Hill 24  Veins Veins are the capacitance vessels Thin-walled and flaccid Collapse when empty, expand easily Greater capacity for blood containment than arteries At rest, about 64% of blood is in veins, 15% in arteries Have steady blood flow (unlike pulses in arteries) Subjected to relatively low blood pressure Averages 10 mm Hg with little fluctuation © McGraw Hill 25  Typical Blood Distribution in a Resting Adult Figure 20.8 © McGraw Hill 26  Types of Veins 1 Postcapillary venules Smallest veins (10 to 20 mm diameter) Even more porous than capillaries Also exchange fluid with surrounding tissues Tunica interna with only a few fibroblasts around it No muscle Leukocytes leave bloodstream through venule walls © McGraw Hill 27  Types of Veins 2  Muscular venules Receive blood from postcapillary venules Up to 1 mm in diameter One or 2 layers of smooth muscle in tunica media Thin tunica externa  Medium veins Up to 10 mm in diameter Thin tunica media and thick tunica externa Tunica interna forms venous valves Varicose veins may result from failure of these valves Skeletal muscle pump propels venous blood back to heart © McGraw Hill 28  Types of Veins 3  Venous sinuses Especially thin walls, large lumens, no smooth muscle Not capable of vasoconstriction Examples include dural sinuses and coronary sinus  Large veins Diameter >10 mm Smooth muscle in all three tunics Relatively thin tunica media with moderate amount of smooth muscle Tunica externa is thickest layer © McGraw Hill Contains longitudinal bundles of smooth muscle 29  Varicose Veins 1 Blood pools in the lower legs of people who stand for long periods stretching the veins Cusps of the valves pull apart in enlarged superficial veins, further weakening vessels Blood backflows and further distends the vessels, their walls grow weak and develop into varicose veins Hereditary weakness, obesity, and pregnancy promote problems Hemorrhoids are varicose veins of the anal canal © McGraw Hill 30  Varicose Veins 2 Figure 20.9 © McGraw Hill Jaroslav Moravcik/Shutterstock 31  Circulatory Routes 1 Simplest and most common route for blood Heart → arteries → arterioles → capillaries → venules → veins In this route, blood passes through only one network of capillaries from the time it leaves the heart until it returns Alternate pathways may involve two capillary beds or no capillary beds © McGraw Hill 32  Circulatory Routes 2 Portal system Blood flows through two consecutive capillary networks before returning to heart Examples: Between hypothalamus and anterior pituitary In kidneys Between intestines to liver © McGraw Hill 33 © McGraw Hill 34  Circulatory Routes 3 Anastomosis Convergence between two vessels other than capillaries Arteriovenous anastomosis (shunt) Artery flows directly into vein, bypassing capillaries Venous anastomosis Most common One vein empties directly into another Reason vein blockage is less serious than arterial blockage Arterial anastomosis Two arteries merge Provides collateral (alternative) routes of blood supply Coronary circulation and common around joints © McGraw Hill 35  Variations in Circulatory Pathways Figure 20.10 © McGraw Hill 36  Blood Pressure, Resistance, and Flow Blood supply to a tissue expressed in terms of flow and perfusion Blood flow: amount of blood flowing through an organ, tissue, or blood vessel in a given time (mL/min) Perfusion: flow per given volume or mass of tissue in a given time (mL/min/g) At rest, total flow is quite constant and is equal to the cardiac output (5.25 L/min) © McGraw Hill 37  Blood Pressure, Resistance, and Flow 2 Important for delivery of nutrients and oxygen and removal of metabolic wastes Hemodynamics Physical principles of blood flow based on pressure and resistance 𝐹 α ∆𝑃/𝑅 (F = flow, ∆𝑃 = difference in pressure, R = resistance) The greater the pressure difference between two points, the greater the flow; the greater the resistance, the less the flow © McGraw Hill 38 © McGraw Hill 39  Blood Pressure 1 Blood pressure (BP) Force that blood exerts against a vessel wall Measured at brachial artery using sphygmomanometer A close approximation of pressure at exit of left ventricle Two pressures are recorded: Systolic pressure Peak arterial BP taken during ventricular contraction (ventricular systole) Diastolic pressure Minimum arterial BP taken during ventricular relaxation (diastole) between heart beats Normal value for young adult: 120/75 mm Hg © McGraw Hill 40  Blood Pressure 2 Pulse pressure Difference between systolic and diastolic pressure Important measure of driving force on circulation and of stress exerted on small arteries by pressure surges generated by the heart Mean arterial pressure (MAP) Diastolic pressure + (one-third of pulse pressure) Average blood pressure that most influences risk level for edema, fainting (syncope), atherosclerosis, kidney failure, and aneurysm © McGraw Hill 41  DDS/3 80 +80+120/3 = 93 © McGraw Hill 42  Blood Pressure 3 Since pressure varies across the cardiac cycle, blood flow in arteries is pulsatile Speed surges from 40 cm/s to 120 cm/s Blood spurts intermittently from an open artery In capillaries and veins, blood flows at steady speed Bleeding from veins tends to be slow and steady BP tends to rise with age Arteriosclerosis—stiffening of arteries due to deterioration of elastic tissues of artery walls Atherosclerosis—build up of lipid deposits that become plaques © McGraw Hill 43  Changes in Blood Pressure Relative to Distance from the Heart Figure 20.11 © McGraw Hill 44  Blood Pressure 4 Hypertension High blood pressure Chronic resting BP >140/90 Consequences: Can weaken arteries, cause aneurysms, promote atherosclerosis Hypotension Chronic low resting BP Caused by blood loss, dehydration, anemia © McGraw Hill 45  Blood Pressure 5 BP determined by three variables: Cardiac output Blood volume Resistance to flow Blood volume is regulated mainly by kidneys Except for beating of the heart, kidneys have the largest influence on blood pressure of any organ © McGraw Hill 46  Peripheral Resistance 1 Peripheral resistance Opposition to flow that blood encounters in vessels away from the heart Resistance hinges on three variables: Blood viscosity Vessel length Vessel radius © McGraw Hill 47  Peripheral Resistance 2 Blood viscosity (“thickness”) RBC count and albumin concentration raise viscosity the most Low viscosity with anemia and hypoproteinemia speeds flow High viscosity with polycythemia and dehydration slows flow Vessel length Farther liquid travels through tube, the more cumulative friction it encounters Pressure and flow decline with distance © McGraw Hill 48  Peripheral Resistance 3 Vessel radius Vessel radius markedly affects blood velocity Most powerful influence on blood flow Only significant way of controlling resistance Laminar flow: flows in layers, faster in center Blood flow (F) proportional to fourth power of radius (r), 𝐹 α 𝑟4 Small changes in blood vessel radius can cause large changes in flow (mL/min) © McGraw Hill 49 © McGraw Hill 50  Peripheral Resistance 4 Vasomotion © McGraw Hill 51  Laminar Flow and the Effect of Vessel Radius Figure 20.12 © McGraw Hill 52  Peripheral Resistance 5 From aorta to capillaries, blood velocity (speed) decreases for three reasons: Blood has traveled a greater distance Friction has reduced speed Smaller radii of arterioles and capillaries More resistance Farther from heart Number of vessels and their total cross-sectional area becomes greater and greater © McGraw Hill 53  Peripheral Resistance 6 From capillaries to vena cava, velocity increases again Since veins are larger, they create less resistance than capillaries Large amount of blood forced into smaller channels Blood in veins never regains velocity it had in large arteries Veins are further from the pumping heart Veins are more compliant (they stretch more) than arteries © McGraw Hill 54  Peripheral Resistance 7  Arterioles are most significant point of control over peripheral resistance and flow On proximal side of capillary beds and best positioned to regulate flow into the capillaries Outnumber any other type of artery, providing the most numerous control points More muscular in proportion to their diameter Highly capable of changing radius  Arterioles produce half of the total peripheral resistance © McGraw Hill 55  Capacity for Vasoconstriction in an Arteriole Figure 20.13 © McGraw Hill 56  Regulation of Blood Pressure and Flow  Vasomotion is a quick and powerful way of altering blood pressure and flow  Three ways of controlling vasomotor activity: Local control Neural control Hormonal control © McGraw Hill 57  LOCAL CONTROL 1 Autoregulation Ability of tissues to regulate their own blood supply Metabolic theory of autoregulation If tissue is inadequately perfused, wastes accumulate, stimulating vasodilation which increases perfusion Bloodstream delivers oxygen and removes metabolites When wastes are removed, vessels constrict © McGraw Hill 58  Local Control 2 Vasoactive chemicals Substances secreted by platelets, endothelial cells, and perivascular tissue to stimulate vasomotor responses Vasodilators include histamine, bradykinin, prostaglandins Shear stress Drag of blood flowing against the endothelial cells Like rubbing your palms together Stimulates endothelial cells to secrete vasodilators Prostacyclin and nitric oxide © McGraw Hill 59  Local Control 3 Reactive hyperemia If blood supply cut off then restored, flow increases above normal Angiogenesis Growth of new blood vessels Occurs in regrowth of uterine lining, around coronary artery obstructions, in exercised muscle, and malignant tumors Controlled by several growth factors and inhibitors © McGraw Hill 60  Neural Control 1  The central and autonomic nervous systems also exert control over blood vessel size  Vasomotor center of medulla exerts sympathetic control over blood vessels throughout the body Stimulates most vessels to constrict, but dilates vessels in cardiac muscle to meet demands of exercise Vasomotor center is the integrating center for three autonomic reflexes Baroreflexes Chemoreflexes Medullary ischemic reflex © McGraw Hill 61  Neural Control 2 Baroreflex Automatic, negative feedback response to change in blood pressure Govern short-term regulation of BP Adjustments for rapid changes in posture Not helpful in correcting chronic hypertension After 2 days or less, they adjust their set point © McGraw Hill 62  Neural Control 3 Baroreflex involves negative feedback Increases in BP detected by carotid sinuses Glossopharyngeal nerve sends signals to brainstem Results in: Inhibition of sympathetic cardiac and vasomotor neurons Excitation of vagal fibers Slow heart rate and thus reduce BP Decreases in BP have the opposite effect © McGraw Hill 63  Negative Feedback Control of Blood Pressure Figure 20.14 © McGraw Hill 64  Neural Control Chemoreflex Response to changes in blood chemistry Especially pH and concentrations of O2 and CO2 Chemoreceptors: Aortic bodies Located in aortic arch and subclavian arteries Carotid bodies Located in external carotid arteries © McGraw Hill 65 © McGraw Hill 66  Neural Control 5 Primary role of chemoreflexes is to adjust respiration to changes in blood chemistry Secondary role is vasomotion Hypoxemia, hypercapnia, and acidosis stimulate chemoreceptors Act through vasomotor center to cause widespread vasoconstriction Increases BP and lung perfusion Increased gas exchange Chemoreceptors also stimulate breathing so that increased blood flow matches increased perfusion © McGraw Hill 67  Neural Control 6 Medullary ischemic reflex Automatic response to a drop in perfusion of the brain Medulla oblongata monitors its own blood supply Ischemia (insufficient perfusion) triggers corrective reflexes Cardiac and vasomotor centers send sympathetic signals to heart and blood vessels Increases heart rate and contraction force Causes widespread vasoconstriction Raises BP and restores normal perfusion to the brain Other brain centers can affect vasomotor center Stress, anger, arousal can also increase BP © McGraw Hill 68  HORMONAL CONTROL 1  Hormones influence blood pressure Some through their vasoactive effects Some by regulating water balance  Angiotensin II Raises blood pressure Potent vasoconstrictor  Synthesis of Angiotensin II requires angiotensin- converting enzyme (ACE) Hypertension often treated with ACE inhibitors which block production © McGraw Hill 69 © McGraw Hill 70  Hormonal Control 2 Aldosterone Promotes Na+ retention by the kidneys “Salt-retaining hormone” Supports blood pressure Water follows sodium osmotically Na+ retention promotes water retention © McGraw Hill 71  Hormonal Control 3  Natriuretic peptides Secreted by the heart Lower blood pressure by: Antagonizing aldosterone Increase Na+ excretion by the kidneys Reduces blood volume Promoting vasodilation © McGraw Hill 72  Hormonal Control 4 Antidiuretic hormone (ADH) Promotes water retention and raises BP At pathologically high concentrations, it also acts as a vasoconstrictor Epinephrine and norepinephrine Adrenal and sympathetic catecholamines In most blood vessels: Bind to 𝛼-adrenergic receptors on smooth muscle Cause vasoconstriction © McGraw Hill 73  TWO PURPOSES OF VASOMOTION 1 Two purposes of vasodilation and vasoconstriction: General control of BP Routing blood from one body region to another © McGraw Hill 74  Two Purposes of Vasomotion 2 Method of rerouting blood from one region to another for perfusion of individual organs Either centrally or locally controlled During exercise, sympathetic system reduces blood flow to kidneys and digestive tract and increases blood flow to skeletal muscles Metabolite accumulation in a tissue affects local circulation without affecting circulation elsewhere in the body If a specific artery constricts, the pressure downstream drops, pressure upstream rises © McGraw Hill 75  Two Purposes of Vasomotion 3 Examples: Vigorous exercise dilates arteries in lungs, heart, and muscles Vasoconstriction occurs in kidneys and digestive tract Dozing in armchair after big meal Vasoconstriction in lower limbs raises BP above the limbs, redirecting blood to intestinal arteries © McGraw Hill 76  Redirection of Blood Flow in Response to Changing Metabolic Needs Figure 20.15 © McGraw Hill 77  Differences in Systemic Blood Flow According to States of Physical Activity Figure 20.16 © McGraw Hill 78 © McGraw Hill 79