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Unformatted text preview: lOMoARcPSD|3704854 Summary - The Cardiovascular System - Blood Vessels (Ch19) Introduction to Anatomy and Physiology I (University of Wollongong) StuDocu is not sponsored or endorsed by any college or university Downloaded by Lauren Myburgh ([email protected]) lOMoARcPSD|3704854 CH 19 THE CARDIOVASCULAR SYSTEM: BLOOD VESSELS PART 1: OVERVIEW OF BLOOD VESSEL STRUCTURE AND FUNCTION Three major types of blood vessels: • Arteries • Capillaries • Veins As the heart contracts, it forces blood into the large arteries leaving the ventricles. The blood then moves into successively smaller arteries, finally reaching their smallest branches, the arterioles, which feed into the capillary beds of body organs and tissues. Blood drains from the capillaries into the venules, the smallest veins, and then on into larger and larger veins that merge to form the large veins that ultimately empty into the heart. Altogether, the blood vessels in the adult human body stretch for about 100,000 km through the internal body landscape. Arteries: carry blood AWAY from the heart (systemic circulation: carry oxygentated blood, pulmonary circulation: carry deoxygenated blood) Veins: carry blood TOWARD the heart (systemic circulation: carry deoxygenated blood, pulmonary circulation: carry oxygenated blood) Capillaries: contact tissue cells and directly serve cellular needs STRUCTURE OF BLOOD VESSEL WALLS Lumen: central blood-­‐containing canal The walls of blood vessels (except the very smallest) have three distinct layers, or tunics: • Tunica intima -­‐ Endothelium lines the lumen of all vessels (minimizes friction) -­‐ In vessels larger than 1 mm, a subendothelial connective tissue basement membrane is present (supports the endothelium) • Tunica media -­‐ Smooth muscle and sheets of elastin -­‐ The activity of the smooth muscle is regulated by sympathetic vasomotor nerve fibers (control vasoconstriction and vasodilation of vessels) -­‐ Critical in regulating circulatory dynamics because small changes in vessel diameter greatly influence blood flow and blood pressure • Tunica externa -­‐ Composed of loosely woven collagen fibers that protect and reinforce the vessel and anchor it to surrounding structures -­‐ Infiltrated with nerve fibers, lymphatic vessels and, in larger veins, a network of elastin fibers -­‐ Larger vessels contain vasa vasorum to nourish the external layers of the blood vessel wall SHS111 Anatomy & Physiology I -­‐ 1 Downloaded by Lauren Myburgh ([email protected]) lOMoARcPSD|3704854 SHS111 Anatomy & Physiology I -­‐ 2 Downloaded by Lauren Myburgh ([email protected]) lOMoARcPSD|3704854 Arterial System Arteries can be divided into three groups: • Elastic arteries • Muscular arteries • Arterioles Elastic (Conducting) Arteries • Large thick-­‐walled arteries near the heart • Elastin in all three tunics – the elastin constructs concentric laminae (sheets) of elastic connective tissue interspersed between the layers of smooth muscle cells • Aorta and its major branches • Large lumens offers low-­‐resistance • Relatively inactive in vasoconstriction • Act as pressure reservoirs – expand and recoil as blood is ejected from the heart à blood flows continuously (rather than starting and stopping with the pulsating rhythm of the heartbeat) • Play a major role in dampening the pulsatile pressure of heart contractions • In arteriosclerosis (blood vessels become hard and unyielding) blood flows more intermittently • The walls of the arteries throughout the body experience higher pressures Muscular (Distributing) Arteries • Distal to elastic arteries à deliver blood to body organs • Have thick tunica media with more smooth muscle (less elastic tissue) • Active in vasoconstriction Arterioles • Smallest arteries • Larger arterioles – have all three tunica (tunica media is chiefly smooth muscle) • Smaller arterioles – little more than a single layer of smooth muscle cells spiralling around the endothelial lining • Lead to capillary beds • Control flow into capillary beds via vasodilatation and vasoconstriction -­‐ When arterioles constrict, the tissues served are largely bypassed -­‐ When arterioles dilate, blood flow into the local capillaries increases dramatically • Diameter of arterioles varies in response to neural, hormonal and local chemical influences Capillaries • Microscopic blood vessels • Walls of tunica intima (one cell thick) • Pericytes (smooth muscle like cells) help stabilize their walls and control permeability • Size allows only a single RBC to pass at a time • Tendons and ligaments – poorly vascularised • Cartilage, epithelia, cornea and lens of eye – lack capillaries • Functions: exchange of gases, nutrients, wastes, hormones etc between blood and the interstitial fluid Venous System Moves blood back to the heart as fast as possible. Venules • Formed when capillary beds unite • Very porous – allow fluids and WBCs into tissues • Postcapillary venules (the smallest venules) consist of endothelium and a few pericytes • Larger venules have one or two layers of smooth muscle cells Veins • Formed when venules converge • Have thinner walls and larger lumens compared with corresponding arteries SHS111 Anatomy & Physiology I -­‐ 3 Downloaded by Lauren Myburgh ([email protected]) lOMoARcPSD|3704854 • • • • • • • Three distinct tunics Thin tunica media – relatively little smooth muscle or elastin thick tunica externa consisting of collagen fibers and elastic networks called capacitance vessels and blood reservoirs because up to 65% of the body’s blood supply is found in the veins at any time Blood pressure is lower than in arteries – so walls of veins can be a lot thinner without bursting Adaptations that ensure return of blood to the heart at the same rate it was pumped into circulation: -­‐ Large-­‐diameter lumens offer little resistance -­‐ Valves prevent backflow of blood (formed from folds of the tunica intima. Most abundant in veins of the limb and absent in veins of the thoracic and abdominal body cavities) Venous sinuses: flattened veins with extremely thin walls (eg. coronary sinus of the heart and dural sinuses of the brain) à coordinated dumping points before returning blood to heart Vascular anastomoses • Vascular anastomoses: interconnections formed where vascular channels unite • Arterial anastomoses provide alternate pathways (collateral channels) for blood to reach a given body region -­‐ Common at joints (where active movement may hinder blood flow through one channel), in abdominal organs, brain and heart • Vascular shunts of capillaries are examples of arteriovenous anastomoses • Venous anastomoses are common (veins interconnect much more freely than arteries) • PHYSIOLOGY OF CIRCULATION SUMMARY: • Heart is a pump • Arteries are pressure reservoirs and conduits • Arterioles are resistance vessels that control distribution • Capillaries are gas exchange sites • Veins are conduits and blood reservoirs Introduction to Blood Flow, Blood Pressure and Resistance Definitions • Blood Flow: volume of blood flowing through a vessel, an organ or the entire circulation in a given period -­‐ Measured in ml/min -­‐ Equivalent to cardiac output for entire vascular system -­‐ Relatively constant when at rest -­‐ Varies widely though individual organs, based on needs • Blood pressure (BP): force per unit are exerted on the wall of a blood vessel by the blood -­‐ Expressed in mm Hg -­‐ Measured as systemic arterial BP in large arteries near the heart -­‐ The pressure gradient provides the driving force that keeps blood moving from higher pressure to lower pressure areas • Resistance: opposition to flow – a measure of the amount of friction blood encounters as it passes through the vessels (most friction is encountered in the peripheral (systemic) circulation so we use the term peripheral resistance) -­‐ Three important sources of resistance: o Blood viscosity o Total blood vessel length o Blood vessel diameter SHS111 Anatomy & Physiology I -­‐ 4 Downloaded by Lauren Myburgh ([email protected]) lOMoARcPSD|3704854 -­‐ -­‐ -­‐ Blood viscosity o the ‘stickiness’ of the blood due to formed elements and plasma proteins Total blood vessel length o the longer the vessel, the greater the resistance encountered Blood vessel diameter o changes are frequent and significantly alter peripheral resistance o Radius is the more powerful way to control blood flow o Varies inversely with the fourth power of vessel radius Eg. if the radius is doubled, the resistance drops to 1/16 of its original value o Small-­‐diameter arterioles are the major determinants of peripheral resistance o Abrupt changes in diameter or fatty plaques from atherosclerosis dramatically increase resistance (disrupt lamina flow and cause turbulence) Relationship between flow, 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 -­‐ Eg. when the arterioles serving a particular tissue dilate (thus decreasing the resistance), blood flow to that tissue increases (even though the systemic pressure is unchanged) Systemic Blood Pressure • Blood flows through the blood vessels along a pressure gradient, always moving from higher to lower pressure areas • The pumping action of the heart generates blood flow • Pressure results when flow is opposed by resistance • Systemic pressure -­‐ Is highest in the aorta -­‐ Declines throughout the pathway -­‐ Is 0 mm Hg in the right atrium • The steepest drop occurs in the arterioles Arterial blood pressure • Reflect two factors of the arteries close to the heart: -­‐ How much the elastic arteries to the heart can be stretched (elasticity -­‐ compliance and distensibility) -­‐ Volume of blood forced into them at any time • Blood pressure near the heart is pulsatile • Systolic pressure: pressure exerted during ventricular contraction -­‐ averages 120 mm Hg in healthy adults -­‐ highest level of pressure • Diastolic pressure: lowest level of arterial pressure -­‐ Averages 70 to 80 mm Hg in healthy adults • Pulse pressure: difference between systolic pressure and diastolic pressure -­‐ Felt as a throbbing pulsation (a pulse) in an artery during systole, as the elastic arteries are expanded by the blood being forced into them by ventricular contraction Pulse pressure = SBP -­‐ DBP • Because aortic pressure fluctuates up and down with each heartbeat, the important pressure figure to consider is the mean arterial pressure (MAP) – the pressure that propels the blood to the tissues -­‐ Diastole lasts longer than systole (1/3 of time spent in contracting, 2/3 of time spent in relaxing/refilling) MAP = DBP + 1/3(pulse pressure) • Pulse pressure and MAP both decline with increasing distance from the heart SHS111 Anatomy & Physiology I -­‐ 5 Downloaded by Lauren Myburgh ([email protected]) lOMoARcPSD|3704854 Capillary pressure • Ranges from 15 – 35 mm Hg • Low capillary pressure is desirable -­‐ High BP would rupture fragile, thin-­‐walled capillaries -­‐ Most are very permeable, so low pressure forces filtrate into interstitial spaces • Spread blood over a very large area so pressure drops Venous blood pressure • Changes little during cardiac cycle • Small pressure gradient, about 15 mm Hg • Low pressure due to cumulative effects of peripheral resistance • Factors aiding venous return: -­‐ Respiratory pump: pressure changes occurring in the ventral body cavity created during breathing move blood toward the heart by squeezing abdominal veins as thoracic veins expand -­‐ Muscular pump: contraction of skeletal muscles ‘milk’ blood toward the heart and valves prevent backflow -­‐ Layer of smooth muscle around the veins that constricts under sympathetic control, increasing venous return Maintaining Blood Pressure ONE OF THE MOST CRITICAL THINGS TO MAINTAIN IN THE BODY IS BLOOD PRESSURE – a steady flow of blood from the heart to the toes is vital for organs to function properly. Requires: • Cooperation of the heart, blood vessels and kidneys • Supervision by the brain Main factors influencing blood pressure: • Cardiac output • Peripheral resistance • Blood volume Cardiac output is blood flow of the entire circulation so: CO = ∆P/R ∆P = CO x R (CO also depends on blood volume) • Blood pressure varies directly with CO, PR and blood volume • Changes in one variable are quickly compensated for by changes in the other variables Cardiac Output (CO): • Determined by venous return and neural and hormonal controls • Resting heart rate is maintained by the cardioinhibitory centre via the parasympathetic vagus nerves • Stroke volume is controlled by venous return (EDV) • During stress, the Cardioacceleratory centre increases heart rate and stroke volume via sympathetic stimulation (ESV decreases and MAP increases) SHS111 Anatomy & Physiology I -­‐ 6 Downloaded by Lauren Myburgh ([email protected]) lOMoARcPSD|3704854 Control of Blood pressure • Short-­‐term neural and hormonal controls -­‐ Counteract fluctuations in blood pressure by altering peripheral resistance • Long-­‐term renal regulation -­‐ Counteracts fluctuations in blood pressure by altering blood volume Short-­‐term mechanisms: neural controls • Counteract moment-­‐to-­‐moment fluctuations in blood pressure by altering peripheral resistance and CO • Neural controls of peripheral resistance are directed at two main goals: 1) Maintain MAP by altering blood vessel diameter 2) Alter blood distribution in response to specific demands • Neural controls operate via reflex arcs that involve: -­‐ Baroreceptors (primary receptor for monitoring MAP) -­‐ Vasomotor centres and vasomotor fibers -­‐ Vascular smooth muscle Vasomotor centre: • A cluster of sympathetic neurons in the medulla that oversee changes in blood vessel diameter • Part of the cardiovascular centre, along with the cardiac centres • Maintains vasomotor tone (moderate constriction of arterioles) • Receives inputs from Baroreceptors, chemoreceptors, and higher brain centres • Transmits impulses along sympathetic efferents called vasomotor fibers (exit from the T1 through L2 levels of the spinal cord and run to innervate the smooth muscle of the blood vessels) Baroreceptor-­‐initiated reflexes (VIP EXAM): • Baroreceptors are located in: -­‐ Carotid sinuses -­‐ Aortic arch -­‐ Walls of large arteries of the neck and thorax • Increased blood pressure stimulates baroreceptors to increase input to the vasomotor centre -­‐ Inhibits the vasomotor centre, causing arteriole dilations and venodilation -­‐ Stimulates the cardioinhibitory centre • Afferent impulses from the baroreceptors also reach the cardiac centres, where the impulses stimulate parasympathetic activity and inhibit the cardioacceleratory centre, reducing heart rate and contractile force • The function of rapidly responding baroreceptors is to protect the circulation against short-­‐term (acute) changes in blood pressure (such as those occurring when you change your posture) SHS111 Anatomy & Physiology I -­‐ 7 Downloaded by Lauren Myburgh ([email protected]) lOMoARcPSD|3704854 • • Baroreceptors taking part in the carotid sinus reflex protect the blood supply to the brain Baroreceptors taking part in the aortic reflex help maintain adequate blood pressure in the systemic circuit INCREASE IN BP _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ DECREASE IN BP: Standing up from a lying position (this is what happens in the first 10 seconds when you get out of bed) Lying down and quickly standing up – • BP starts to fall • Blood will fall to legs • HR will increase in 10-­‐20 secs • Next 30 secs PR changes to maintain pressure • CO increases à All to maintain BP (MAP has to remain constant) 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 content -­‐ Transmit impulses to the cardioacceleratory centre, which then increases cardiac output -­‐ Also transmit impulses to the vasomotor centre , which causes reflex vasoconstriction -­‐ The rise in BP that follows speeds the return of blood to the heart and lungs • BUT chemoreceptors are more important in the regulation of respiratory rate Influence of higher brain centres: • Reflexes that regulate blood pressure are integrated in the medulla oblongata of the brain stem • The cerebral cortex and hypothalamus can modify arterial pressure via relays to the medullary centres (even though they are not involved in routine controls of blood pressure) • Example: the fight-­‐or-­‐flight response mediated by the hypothalamus has profound effects on blood pressure • Hypothalamus also mediates the distribution of blood flow and other cardiovascular responses that occur during exercise and changes in body temperature SHS111 Anatomy & Physiology I -­‐ 8 Downloaded by Lauren Myburgh ([email protected]) lOMoARcPSD|3704854 Short term mechanisms: Hormonal controls of BP • Hormones help regulate blood pressure, both in the short term via changes in peripheral resistance and in the long term via changes in blood volume • Adrenal medulla hormones: -­‐ Norepinephrine (NE) and epinephrine (released from the adrenal gland) enhance the sympathetic fight-­‐or-­‐ flight response -­‐ NE and epinephrine cause generalized vasoconstriction and increase cardiac output • Angiotensin II: -­‐ When blood pressure or blood volume are low, the kidneys release the hormone renin which acts as an enzyme, generating angiotensin II -­‐ Stimulates vasoconstriction, promoting a rapid rise in systemic blood pressure • Atrial natriuretic peptide: -­‐ Produced by the atria of the heart -­‐ Causes blood volume and blood pressure to decline -­‐ Causes generalised vasodilation • Antidiuretic hormone (ADH/vasopressin): -­‐ Produced by the hypothalamus -­‐ Stimulates the kidneys to conserve water -­‐ Causes intense vasoconstriction in cases of extremely low BP Long-­‐term mechanisms: renal regulation • Long-­‐term controls of BP counteract fluctuations in blood pressure not by altering peripheral resistance but rather by altering blood volume • Baroreceptors respond to short-­‐term changes but also quickly adapt to prolonged or chronic episodes of high or low pressure • Increased blood volume = increased BP, decreased blood volume = decreased BP • Kidneys act directly and indirectly to regulate arterial blood pressure: 1) Direct renal mechanism -­‐ Alters blood volume independently of hormones -­‐ Increased BP or blood volume causes the kidneys to eliminate more urine, thus reducing BP -­‐ Decreased BP or blood volume causes the kidneys to conserve water (and return it to the blood stream), and BP rises 2) Indirect renal mechanism (Renin-­‐angiotensin mechanism) -­‐ Decreased BP, kidneys release hormone renin into the blood -­‐ Renin à production of angiotensin II -­‐ Angiotensin II increase BP in three main ways: o It is a potent vasoconstrictor (increases BP by increasing peripheral resistance) o Stimulates the adrenal cortex to secrete aldosterone (a hormone that enchances renal absorption + of Na , conserving water and increasing blood volume) o Makes the posterior pituitary release ADH (which promotes more water reabsorption) SHS111 Anatomy & Physiology I -­‐ 9 Downloaded by Lauren Myburgh ([email protected]) lOMoARcPSD|3704854 PART 3: CIRCULATORY PATHWAYS: BLOOD VESSELS OF THE BODY The Two Main Circulations of the Body Vascular system: • Pulmonary circulation – short loop that runs from the heart to the lungs and back to the heart • Systemic circulation – routes blood through a long loop of all part of the body before returning it to the heart Systemic Arteries and Veins: Differences in Pathw...
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