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Chapter 21: The Cardiovascular System: Blood Vessels Slide 2 Vessel Structure - General All vessels same basic structure 3 wall layers (or tunics) Tunica adventitia (externa) - elastic and laminar fibers Tunica media thickest layer elastic fibers and smooth muscle fibers Tunica interna (intima) endothelium non-stick layer basement membrane internal elastic lamina Lumen - opening Slide 3 Structure/function relationship changes as move through cardiovascular tree Tunic thickness and composition variable throughout cardiovascular tree Slide 4 Elastic (conducting) arteries Near heart Thick walls More elastic fiber, less smooth muscle Lose elasticity with aging Vessel Structure Elastic Arteries Slide 5 Vessel Structure - Elastic Arteries Aorta and elastic arteries Can vasoconstrict or vasodilate Large arteries expand, absorb pressure wave then release it with elastic recoil - Windkessel effect Help to push blood along during diastole With aging have less expansion and recoil Slide 6 Muscular (distributing) arteries Deliver blood to organs More smooth muscle Less elastic fibers Vessel Structure Muscular Arteries Slide 7 Arterioles Distribution of blood in organs Composition varies depending on position - more muscle, less elasticity nearer heart Regulate flow from arteries to capillaries Flow = P/R vary resistance by changing vessels size Site of blood pressure regulation Vessel Structure - Arterioles Slide 8 Vessel Structure - Capillaries Microcirculation connects arteries and veins Found in nearly every tissue in body Higher the metabolic rate, more capillaries in tissue Muscle many caps (>600/mm 2 ) Cartilage none Slide 9 Vessel Structure - Capillaries Allow exchange of nutrients and wastes between blood and tissue Capillary structure - simple Basal lamina - connective tissue Endothelial cells Structure/function Slide 10 Flow Regulation Regulation by vessels with smooth muscle Metarterioles connect arterioles to venules through capillary bed allows flow through capillary bed w/out flow through caps Slide 11 Flow Regulation True capillaries Pre-capillary sphincter ring of smooth muscle open/close to control flow regulated by chemicals Intermittent vasomotion caps open for flow 5- 10X min Slide 12 Types of Capillaries 3 types of capillaries 1. Continuous capillaries continuous endothelial cells except for cleft between cells tight junctions between endothelial cells prevent most things from leaving caps most capillaries in body Slide 13 Types of Capillaries 2. Fenestrated capillaries fenestrations (slits) allow for filtration of small substances glomerular capillaries in kidney Slide 14 Types of Capillaries 3. Sinusoid capillaries wider gaps between endothelial cells allowing RBCs to exit the caps found in liver Slide 15 Vessel Structure - Veins Venules Collect blood from caps carry it to veins Structure changes with position Become more vessel-like (walls) as move from capillaries Slide 16 Vessel Structure Veins Veins Interna thicker than arteries Media thinner, less muscle Externa thick Valves Pressure low High compliance - change volume easily with small change in pressure Varicose veins Slide 17 Vessel Structure - Histology Very different morphology under light microscopy Tunica media thickness differentiates artery from vein Slide 18 Artery Vein Vein Artery Slide 19 Vessel Structure/Function At rest 60% of blood located in veins and venules Serve as reservoirs for blood, storing it until needed Particularly veins of abdominal organs, skin ANS regulates volume distribution Vasoconstrict Vasodilate Open areas of circulation to be supplied with blood veins at rest caps during exercise Can shift volumes to other areas as needed Slide 20 Vessel Structure/Function Rest CO = 5 L/min 0.75 L/min Slide 21 Vessel Structure/Function Heavy Exercise Rest CO = 25 L/min CO = 5 L/min 20 L/min 0.75 L/min Slide 22 Physiology of Circulation Flow = P/R or CO = MAP/R MAP - mean arterial pressure Higher pressure to lower pressure with resistance (R) factor Blood pressure Pressure of blood on vessel wall Measurement of pressure of a volume in a space Systole/diastole - 120/80 BP falls progressively from aorta to O mm Hg at RA Slide 23 Regulation of Blood Flow Resistance - opposition to blood flow from blood and vessel wall friction Factors that affect resistance (R) Viscosity - V R thickness of blood dehydration, polycythemia R proportional to vessel length garden hose vs. straw obesity Vessel diameter changes in diameter affect flow vessel wall drag blood cells dragging against the wall laminar flow layers of flow R inverse proportional to radius 4 decrease in r by 1 / 2 R 16X only important in vessels that can change their size actively Slide 24 Regulation of Pressure, Resistance Systemic vascular resistance (Total Peripheral Resistance - TPR) All vascular resistance offered by systemic vessels Which vessels change size? Resistance highest in arterioles Largest pressure drop occurs in arterioles Relationship of radius to resistance in arterioles important due to smooth muscle in walls Slide 25 Systemic Blood Pressure Arterial Blood Pressure Pulsatile in arteries due to pumping of heart Systolic/diastolic Pulse pressure = systolic (minus) diastolic Q - Windkessel effect on pulse pressure? A - Decreases pulse pressure Q - What is the effect of hardening of the arteries on pulse pressure? A - Increases pulse pressure Slide 26 Systemic Blood Pressure Capillary Blood Pressure Relatively low blood pressure Low pressure good for caps because: caps are fragile - hi pressure tears them up caps are very permeable - hi pressure forces a lot of fluid out Slide 27 Systemic Blood Pressure Venous return Volume of blood flowing back to heart from systemic veins Depends on pressure difference (P) from beginning of venules (16 mmHg) to heart (0 mmHg) Any change in RA pressure changes venous return Slide 28 Help for venous return Skeletal muscle pump muscles squeeze veins force blood back to heart valves prevent back flow Respiratory pump inhaling pulls air into lungs helps to pull blood back into thorax Slide 29 Velocity of Blood Flow Velocity of blood flow - inversely related to total cross sectional area (CSA) of vessels Aorta Total CSA 3-5 cm 2 Velocity 40 cm/sec Capillaries Total CSA 4500-6000 cm 2 Velocity 0.1 cm/sec Vena Cava Total CSA 14 cm 2 in vena cava Velocity 5-20 cm/sec Slide 30 Vessel Structure - Function Capillary Function Site of exchange between blood and tissues Delivery of nutrients and removal of wastes Slow flow allows time for exchange Mechanisms of nutrient exchange Diffusion - O 2, CO 2, glucose, AA's, hormones diffuse down [ ] gradients If lipid soluble, can travel through cell If water soluble, between cells Slide 31 Capillary Fluid Exchange Forces driving the movement of fluid Hydrostatic pressure capillary (HP c ) Hydrostatic pressure interstitial fluid (HP if ) Osmotic pressure capillary(OP c ) Osmotic pressure interstitial fluid (OP if ) Net filtration pressure (NFP) is a sum of all Fluid movement Fluid filtered and reabsorbed across capillary wall Starlings law of the capillaries Slide 32 Capillary Fluid Exchange On average 85% of fluid filtered at arteriole end is reabsorbed at venular end Slide 33 Maintaining Blood Pressure - Short Term Mechanisms - CNS Neural Control - Cardiac centers in medulla Vasomotor center medullary area dedicated to control of blood vessels sends sympathetic output to blood vessels Vasoconstricts or vasodilates as needed tone - normal amount of vasoconstriction or vasodilation can vary tone which varies delivery of blood receives input from different sources baroreceptors chemoreceptors Slide 34 Maintaining Blood Pressure Short term mechanisms CNS reflexes Baroreceptor initiated reflex Located at carotid sinus and aortic arch Monitor changes in blood pressure Regulate activity of Sympathetic Nervous System (vascular tone) Slide 35 Maintaining Blood Pressure Short term mechanisms CNS reflexes Chemoreceptor initiated reflexes Carotid bodies, aortic bodies Monitors changes in chemicals (O 2, CO 2, [H + ]) CO 2, H +, O 2 (stresses) result in sympathetic activity and BP Slide 36 Maintaining Blood Pressure Short term mechanisms CNS reflexes Influence of Higher Brain Centers (areas above medulla) - Cortex and Hypothalamus Not involved in minute to minute regulation Influence vasomotor center depending on conditions public speaking temperature regulation Slide 37 Maintaining Blood Pressure - Short Term Mechanisms - Hormones Renin - Angiotensin - Aldosterone Renin enzyme from kidney results in formation of Angiotensin II (AII) AII vasoconstrictor stimulates ADH, thirst stimulates aldosterone - Na + reabsorption Why/how would these things affect blood pressure? Slide 38 Maintaining Blood Pressure - Short Term Mechanisms - Hormones Adrenal medulla - Epi and Norepi CO ( HR, SV) Constrict abdominal, cutaneous arterioles/venules Dilate cardiac, skeletal muscle beds Why/how would this affect blood pressure? Slide 39 Antidiuretic Hormone (ADH) Osmoreceptors in hypothalamus Retains fluid (inhibited by alcohol) Vasoconstriction at high levels Why/how would this affect blood pressure? Maintaining Blood Pressure - Short Term Mechanisms - Hormones Slide 40 ANP (atrial natriuretic peptide) Released from atrial cells in response to BP Vasodilator, Na + and water loss, opposes Aldosterone Why/how would this affect blood pressure? Slide 41 Maintaining Blood Pressure - Long Term Regulation Renal mechanism Volume in a space Regulate space in the short term we just talked about it! nervous control hormones Regulate volume in the long term The kidneys! BP, urine flow to BP BP, urine flow to BP Slide 42 Control of Blood Flow Autoregulation (local control) - local automatic adjustment of blood flow to match tissue needs Physical changes Warming - vasodilation Cooling - vasoconstriction Chemical changes - metabolic products Vasodilators Vasoconstrictors Myogenic control smooth muscle controls resistance stretch contraction, stretch relaxation Slide 43 Blood Flow in Special Areas Skeletal Muscle Wide variability in amount of flow Sympathetic regulation from brain in response to level of activity receptors - vasoconstrict receptors - vasodilate Metabolic regulation in tissue low O 2 vasodilate to increase flow hi O 2 vasoconstrict to decrease flow Brain Very little variability in flow Stores few nutrients so flow must be maintained! Metabolic regulation Slide 44 Blood Flow in Special Areas Skin Supplies nutrients, aids in temperature regulation, provides a blood reservoir Metabolic and sympathetic regulation Lungs Low pressure (25/10), low resistance Flow regulated by O 2 availability in the lungs hi O 2 vasodilate to increase flow opposite to muscle low O 2 vasoconstrict to decrease flow opposite to muscle Heart Variable flow depending on activity Metabolic and sympathetic regulation Slide 45 CO = MAP/R Regulation of Blood Pressure MAP = CO x R