CH 20 Reading PDF

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CH 20 READING: BLOOD VESSELS 20.1A THE VESSEL WALL - veins = afforestation vessels that carry blood back to the heart - Arteries = efferent vessels of cardiovascular system - Carry blood away from the heart - Capillaries = microscopic, thin walled vessels - Connect smallest arteries to smallest veins - Three tunics (layers) of blood vessels - 1.) Tunica Interna (tunica intimate) - Lines inside of vessel and is exposed to blood - Simple squamous epithelium - Called endothelium - Continuous with endocardium of heart - Secretes chemicals - Stimulate dilation/constriction - Semi-permeable - Platelets attach when damaged - Can release cell-adhesion molecules - To induce leukocyte adherence - Congregation = better fighting - 2.) Tunica Media - Middle layer - Usually the thickest - Consists of smooth muscle, collagen, sometimes elastic tissue - These vary greatly from vessel to vessel - Strengthens the vessels - Prevents blood pressure from rupturing the, - Regulates blood vessel diameter - 3.) Tunica externa (tunica adventitia) - Outermost layer - Loose connective tissue merging with other vessels, Nerves, organs - Anchors vessels to adjacent tissues - Provides passage from small nerves, lymphatic vessels, smaller blood vessels - All vessels require nutrition, oxygenation, and waste removal - Most big veins cannot have this met simply by blood flow - Too fast and wall is too thick - Small vessels penetrate external surface - Branch into capillaries to supply big vessels - Vasa Vasorum = the network of smaller vessels serving large ones 20.1B ARTERIES - arteries = sometimes called resistance vessels

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- Strong, resilient structure - Must withstand each surge of the heart to push blood to body Retain round shape when empty (in slides) due to muscles Three classes of arteries: - 1) Conducting (elastic or large) arteries - Biggest - Aorta, pulmonary trunk, common iliac arteries - Have elastic tissue layer: Internal elastic lamina - At border between tunica interna and media - Histology = view dominated by elastic tissue layers - With perforations - External elastic lamina = border between media and externa - Too difficult to distinguish - Expand as they receive blood during ventricular systole - Recoil during diastole - Receives smaller arteries downstream - Helps prevent stress O(especially with sclerosis) - 2) Distributing (muscular or medium) arteries - Smaller branches that distribute blood to specific organs - Like exit ramps to interstate highways (conducting arteries) - Most in the first 2 size classes have specific names - Distributing = brachial, femoral, renal, splenic - Smooth muscle - More conspicuous than elastic in slides - 3) Resistance (small arteries) - Too variable in number/location to have names - 25 layers of smooth muscle - Relatively little elastic tissue - Thicker tunica than large arteries - Arterioles = the smallest (with only 1-3 smooth muscle layers) - Metarterioles link arterioles directly to venules - Provide shortcuts for blood to pass through

ARTERIAL SENSE ORGANS - certain major heart arteries have sensory structures - Monitor blood pressure and composition - Transmit info to brainstem - Serves to regulate heartbeat, vessel diameters, respiration - Three kinds of arterial sense organs - 1.) Carotid Sinuses - Baroreceptors - Ascending neck on each side = common carotid artery - Branches near angle of mandible - Located in wall of internal carotid artery

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- Above branch point - Thin tunica media - Roles in baroreflexes (controlling pressure) 2.) Carotid bodies - Located near branch of common carotid arteries - Oval receptors (3X5 mm) - INNERVATED bye sensory receptors in gloospharyngeal nerve - Chemoreceptors - Monitor blood composition - Transmit signal to brainstem respiratory centers - Adjust breathing - Stabilize pH, CO2 and O2 3.) Aortic bodies - 1 to 3 chemoreceptors in the aortic arch near ateries - Near arteries to head and arms - Structurally similar to carotid bodies - Same function - Transmit the same but transmit to brainstem by vagus nerves

20.1C CAPILLARIES - only place where things bass between blood and tissue fluids - They are the “business end” of the cardiovascular system - Capillaries greatly outnumber venules - Exchange vessels - Microvasculature = arterioles, capillaries, venules - Capillaries = only endothelium and basal lamina - Branch along their way - Some blood has to stretch into elongated shapes to get through small ones - Types of capillaries - 1) Continuous capillaries - Occur in most tissues and organs (skeletal muscles) - Lungs, brain - Endothelial cells held together by tight junctions - To form a continuous tube - Basal lamina surrounds endothelium - Separates it from connective tissues - Intercellular clefts = separate endothelium - Can let small solutes (glucose) pass through - Most plasma proteins , platelets, etc. cannot - Pericytes = in some continuous caps. - Lie external to endothelium - Elongated tendrils that wrap around the capillary - Contain contractile proteins - Can contract and regulate blood flow

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2) Fenestrated capillaries - Endothelial cells with riddles patches of filtration pores - Filtration pores (fenestrations) - 20-100nm in diameter - Spanned by glycoproteins membrane - Thinner than cells plasma membrane - Allow rapid passage of small molecules - Retain most proteins and larger particles - Important in organs that rapidly absorb/filtrate - Kidneys, endocrine, intestine, etc. 3) Sinusoids - Irregular blood filled spaces in the: - Liver, bone marrow, spleen, some other organs - Twisted, conform to shape of surrounding tissues - No basal lamina - Especially large fenestrations - Allow even proteins to pass through - This is how protein is made in liver - And is allowed to go into the blood

CAPILLARY BEDS - capillaries are organized into webs called capillary beds - Typically 10-100 capillaries supplied by 1 arterioles ‘ - Normally, a large portion of capillaries do not function at a given time - There isn’t enough blood flow (especially during rest) - During exercise, these recieve more - However others (skin, intestine) lose to compensate - Capillary function = normally regulated by arterioles constriction - Precapillary sphincter = cuff around opening to each capillary - Found in often in beds supplied by metarterioles - Regulates blood flow 20.1D VEINS - veins are regarded as the capacitance vessels of cardiovascular system - Thin walled and flaccid - Expand easily to compensate for increased blood volume - 64% of blood is found in systemic veins - Relatively low blood pressure - Blood flow is continuous (not pulsing with the heart) - Thus the walls are thinner - When empty (histology) they are flaccid, and irregularly shaped - Not a lot of muscle/wall to hold it in place - Arteries constantly split off into different places - Venous constantly branch into each other to form larger ones

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Smaller veins = tributaries Smallest to largest veins: - 1.) Postcapillary veins - Smallest 10-20 micrometers - Receive blood from capillaries directly or through distal ends - Have tunica interna with a few fibroblasts around it (no muscle) - Surrounded by pericytes - 2.) Muscular Venules - Receive blood from postcapillary venules - 1 mm in diameter - Tunica media with 1-2 smooth muscle layers/thin externa - 3.) Medium veins - Up to 10mm in diameter - Includes most veins with individual names - Radial/ulnar veins - Tunica interna with endothelium - Basement membrane, loose CT - Sometimes thin internal elastic lamina - Many exhibit unfolding of tunica interna and meet in middle - Forming venous valves - Venous valves = prevent blood from falling back down - As sometimes pressure can't pull all up at once - Failure of valves = part of varicose veins - 4.) Venous sinuses - Veins with especially thin walls, large lumens no smooth musc. - Coronary sinus of heart/dural sinuses of brain - Not capable of vasoconstriction - 5.) Large veins - Diameters greater than 10 mm - Smooth muscle in all three tunics - Thin media, with moderate smooth muscle - Tunica externa is thickest contains longitudinal bundles of SM - Include venae cavae, pulmonary veins, jugular, renal, etc.

20.1E CIRCULATORY ROUTES - Simplest and most common blood flow route: - Heart - Arteries - Capillaries - Veins - Heart - Blood usually only goes through 1 capillary bed to get back to heart - Exceptions = portal systems/anastomoses - Portal system = blood flows through 2 consecutive capillary networks - Before it returns to the heart - Occur in kidneys, pituitary, intestines to liver - Anastomoses = converging point between 2 vessels (not capillaries) - Arteriovenous Anastomosis (shunts)

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Blood flows from artery directly into vein Totally bypassing any capillaries Occur in fingers, palms, toes, ears, etc. Allow warm blood to bypass exposed surfaces - To warm up the body when cold Venous anastomoses = most common - Vein empties directly into another - Several alternative routes for drainage from organ - Blocking of vein = rarely life threatening Arterial Aanstomoses - 2 arteries merge - Provide collateral routes of blood supply to a tissue - Common around joints , - Where limb movement could compress an artery

20.2 BLOOD PRESSURE, RESISTANCE, AND FLOW - circulatory system must deliver oxygen and nutrients to tissues - Inadequate supply for minutes can cause necrosis - Flow = amount of load flowing through an organ, tissue, or blood vessel - Perfusion = flow per given volume or mass of tissue - Usually expressed in mL of blood per 100 grams of tissue per min - This means larger organs (femur) could has more flow but less perfusion - Because e.g ovaries = more blood bear 100 grams - Total flow is fairly constant in relation to cardiac output - Blood flow to things such as kidneys can slow when digesting - And blood with surge to intestines - Afterwards the opposite happens - The greater the pressure difference between 2 points - The greater the flow - The greater the resistance, the less the flow 20.2A BLOOD PRESSURE - blood pressure = force exerted by blood on vessel wall - Blood pressure measurement = normal part of a physical exam - Typically measured at brachial artery - With sphygmomanometer - Inflatable cuff with rubber build for air pumping - Also mercury types (gold standard for accuracy) - Using a cuff to measure = collapses brachial artery (even during systole) - Then listen while releasing the pressure - Systolic pressure = arterial blood pressure generated by contraction of left ventricle - Diastolic pressure - the minimum to which BP falls when ventricle is in diastole - Both expressed in mm of mercury - Pulse pressure = difference between systolic and diastolic pressure

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Mean arterial pressure (MAP) = mean pressure obtained of measurements at several intervals - Close estimate from adding diastolic and ⅓ of pulse pressure MAP varies with influence of gravity - Most influential on things like syncope, atherosclerosis, kidney failure, etc. Syncope = fainting Blood pressure is partially maintained by fluidity of artery walls - Stiffness would cause high blood pressure - Arteries expand with each systole Blood flow in arteries is pulsatile - Pressure away from the heart decreases - This is why vein bleeding = steady - Artery bleeding = on and off jets Arteriosclerosis = the thickening and stiffening of vessels (arteries) - Common cause of damage is by free radicals - Another common cause is atherosclerosis - Atherosclerosis = buildup on arterial walls Hypertension (high BP) = considered BP higher than 140/90 - Can weaken arteries or cause aneurysms - Blood loss, dehydration, anemia, other factors can approach death

20.2B PERIPHERAL RESISTANCE - peripheral resistance = opposition to flow that blood encounters - In vessel away from the heart - Blood would exert no pressure endless it encountered some downstream resistance - Hinges on three variables - 1.) Blood viscosity - Viscosity stems mainly from plasma proteins/erythrocytes - Reduction of these thins blood/ reduces pressure - 2.) Vessel length - The farther a liquid travels in a tube, the more friction encountered - Pressure/flow decline with distance - Sitting person = higher pressure in arm than leg - 3.) Vessel radius - Peripheral resistance mainly controlled by vasoconstriction - And vasodilation - Vasoconstriction - When smooth muscle contracts - Vasodilation - No muscular effort - Actually muscular passivity (relaxation) - Allows BP to expand the vessel - Vasomotion = vasoconstriction/dilation

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Controlled by part of nucleus in medulla Oblongata - Called vasomotor center - Laminar flow = usual blood flow - Flowing in layers, faster near center - Encountering less friction - Slower near edges - Encountering friction of walls - Flow = functional to 4th power of the radius of vessel - Makes it instrumental in blood pressure Blood pressure drops from aorta to capillaries for 3 reasons - 1.) The blood has travelled a greater distance - Friction has slowed down - 2.) Arterioles/capillaries have smaller radii - Put up more resistance - 3.) Number of vessel area increases - Even though vessels are smaller Blood pressure rises from capillaries to vena cava - Vessel increase in size = less resistance Arterioles are most significant control of peripheral resistance - 1.) On proximal sides of capillary beds - Best positioned to regulate flow into capillaries - And thus perfusion at organs - 2.) Greatly outnumber any other classes of arteries - Provide most numerous control points - 3.) More muscular in proportion to their dieters - Than any other class of blood vessel - Highly capable of changing radius - Account for ½ of total peripheral resistance through - Constriction/dilation

20.2C REGULATION OF BLOOD PRESSURE AND FLOW - Vasoreflexes = quick/powerful means of altering blood pressure/flow - Local control - Autoregulation = ability of tissues to regulate own blood supply - If tissue is not perfused, it becomes hypoxic - And waste builds up from metabolism - These stimulate vasodilation - Increases blood flow - Vasoactive chemicals = released from platelets, endothelial cells, peri vascular tissues - Stimulate vasodilation for trauma, inflammation, etc. - Include histamine, bradykinin, prostaglandins - Drag of blood of vessels creates shear stress - Stimulates release of prostacyclin/nitric oxide (vasodilator)

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Reactive hypermedia = tissue can experience when cut off of blood then let back on - Increase above normal level of blood flow - Due to accumulation of metabolites during ischemia - Skin flushes after coming in from cold - Occur in forearm if cuff i s inflated for too long Angiogenesis = the growth of new blood vessels - Hypoxic tissue can create own perfusion through this - Also embryonic development of blood vessels - Important in regrowth of uterine lining after menstruation - Increase of capillaries during exercise and training - Growth of arterial bypasses around obstructions in coronary circulation Malignant tumors secrete factors to build vessels to supply them - Oncologists are trying to find out how to choke these of supply

NEURAL CONTROL - blood vessels are under remote control by central/autonomic nervous system - Vasomotor center of medulla Oblongata exerts sympathetic control over vessels - Sympathetic fibers = mostly constrict - Allow for vasodilation by reducing nerve firing rate - Vasomotor center = for baroreflexes, chemoreflexes, medullary ischemic reflex - Baroreflexes = negative feedback response to BP changes - Detected in carotid sinuses - Glossopharyngeal fibers continually transmit to brainstem - When BP rises, their signaling rate rises - Inhibit sympathetic cardiac vasomotor neurons - Reduces sympathetic tone - Excites a gal fibers to the heart - Reduces heart rate/Cardiac output - Important solely in short term BP regulation - E.g = when you stand up baroreflexes act - Not good for chronic hypertension - Reset at new level after a few days of continuous change - Chemoreflex - Autonomic response to changes in blood chemistry - Especially pH and O2/CO2 - Initiated by chemoreceptors called aortic bodies and carotid bodies - Mainly adjusts respiratory changes in blood chemistry - Have secondary role in vasoreflexes - E.g - low O2, pH and high CO2 tell vasomotor center - Cause widespread vasoconstriction - Causes increased BP - Increases perfusion and gas exchange in lungs - Medullary ischemic reflex - Autonomic response to reduced perfusion of the brain

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Medulla monitors it’s own blood supply - Activates corrective reflexes when needed - Within seconds, send sympathetic signals to heart/vessels - To accelerate heart and constrict the vessels - Raises BP and ideally restore cerebral perfusion - Also receive input to stress, anger, arousal (raises BP) Hormonal Control - Hormones = another means of control of perfusion - Angiotensin II = POTENT vasoconstrictor - Raises BP - Synthesis requires angiotensin-converting enzyme (ACE)’ - Aldosterone - Salt retaining hormone - Promotes Na retention by kidneys - Water follows sodium ,s o water also retains - Thereby supports blood pressure - Natriuretic Peptides - Hormones secreted by the heart; antagonize aldosterone - Increase Na excretion by the kidneys - Reduce blood volume and pressure - Generalized vasodilator effect (lowers pressure) - Antidiuretic Hormone - ADH promotes water retention - Is a vasoconstrictor at high concentrations - AKA Arginine vasopressin - Raise blood pressure - Epinephrine and Norepinephrine - Adrenal and sympathetic catecholamines - Bind to A-adrenergic receptors on smooth muscle of BV - Stimulates vasoconstriction and raises blood pressure

20.2D TWO PURPOSES OF VASOMOTION - serves 2 purposes: Raising/Lowering BP and modifying perfusion of organs - Generalized increase in BP requires centralized control - Medullary vasomotor center or by hormones - Widespread vasoconstriction raises BP - Supports cerebral perfusion during dehydration/hemorrhaging - Rerouting of blood/perfusion can be controlled by central or local control - Local e.g = metabolic accumulation stimulating local vasodilation/increase perfusion - Most blood will choose the route of least resistance - Done by constricting vessels that aren’t needed - Blood will travel in other, less pressurized vessels - Chemicals pass through capillary wall in 3 routes - The endothelial cell cytoplasm

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- Intercellular clefts between the endothelial cells - Filtration pores of fenestrated capillaries Mechanisms of movement through capillary wall include: - Diffusion - Transcytosis - Filtration - Reabsorption

20.3A DIFFUSION - most important mechanism for exchange - Glucose/oxygen = more concentrated in blood, flow into organs - Waste = more concentrated in organs, flow into blood - Lipid soluble, small molecules, etc. can easily diffuse through membranes - Lipid insoluble cannot 20.3B TRANSCYTOSIS - endothelial cells pick up material on one side of plasma membrane - By Pinocytosis/receptor-mediated endocytosis - Go across the cell and and discharge by exocytosis - Only a small portion of movement - Accounts for albumin, fatty acids, some hormones (insulin) 20.3C FILTRATION AND REABSORPTION - capillary fluid exchange is largely fueled by filtration/osmosis - Fluid exits the arterial end and enters the venous end - Delivers materials to cells and rises away metabolic waste - Typical capillary pressure = 30 mm Hg - Colloid osmotic pressure = portion of osmotic pressure due to protein - Roughly 28 mm Hg in blood (mainly due to albumin) - On optic pressure = difference between COP of blood/tissue fluid - Net reabsorption pressure = oncotic pressure minus net hydrostatic pressure - Capillary blood pressure is only pressure to change greatly from vessel ends - Most capillaries reabsorb 85% of the fluid they filter - If not too large, molecules are dragged along with water through c...