Blood Vessels Part II PDF

Blood Vessels Part II Physiology of Circulation Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Flow, Pressure, and Resistance Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Definition of Terms Blood flow: volume of blood flowing...

Blood Vessels Part II Physiology of Circulation Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Flow, Pressure, and Resistance Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Definition of Terms Blood flow: volume of blood flowing through vessel, organ, or entire circulation in given period – Measured in ml/min, it is equivalent to cardiac output (CO) for entire vascular system – Overall is relatively constant when at rest, but at any given moment, varies at individual organ level, based on needs Blood pressure (BP): force per unit area exerted on wall of blood vessel by blood – Expressed in mm Hg – Measured as systemic arterial BP in large arteries near heart – Pressure gradient provides driving force that keeps blood moving from higher- to lower-pressure areas Resistance (peripheral resistance): opposition to flow – Measurement of amount of friction blood encounters with vessel walls, generally in peripheral (systemic) circulation – Three important sources of resistance  Blood viscosity  Total blood vessel length  Blood vessel diameter Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved – Blood viscosity  The thickness or “stickiness” of blood due to formed elements and plasma proteins – The greater the viscosity, the less easily molecules are able to slide past each other  Increased viscosity equals increased resistance – Total blood vessel length  The longer the vessel, the greater the resistance encountered Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved – Blood vessel diameter  Has greatest influence on resistance  Small-diameter arterioles are major determinants of peripheral resistance – Radius changes frequently, in contrast to larger arteries that do not change often  Abrupt changes in vessel diameter or obstacles such as fatty plaques from atherosclerosis dramatically increase resistance Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Milk Shake and 2 Different Straws Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Relationship Between Flow, Pressure, and Resistance Blood flow is directly proportional to blood pressure gradient – If blood pressure gradient increases, blood flow speeds up Blood flow is inversely proportional to peripheral resistance – If peripheral resistance increases, blood flow decreases Peripheral resistance is more important in influencing local blood flow because it is easily changed by altering blood vessel diameter Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Systemic Blood Pressure Pumping action of heart generates blood flow Pressure results when flow is opposed by resistance Systemic pressure is highest in aorta and declines throughout pathway – Steepest drop occurs in arterioles Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Blood Pressure in Various Blood Vessels of the Systemic Circulation Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Arterial Blood Pressure Determined by two factors: 1. Elasticity (compliance or distensibility) of arteries close to heart 2. Volume of blood forced into them at any time Blood pressure near heart is pulsatile – Rises and falls with each heartbeat Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Arterial Blood Pressure cont. Systolic pressure: pressure exerted in aorta during ventricular contraction – Left ventricle pumps blood into aorta, imparting kinetic energy that stretches aorta – Averages 120 mm Hg in normal adult Diastolic pressure: lowest level of aortic pressure when heart is at rest Pulse pressure: difference between systolic and diastolic pressure Pulse: throbbing of arteries due to difference in pulse pressures, which can be felt under skin Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Arterial Blood Pressure cont. Mean arterial pressure (MAP): pressure that propels blood to tissues – Pulse pressure phases out near end of arterial tree – Flow is nonpulsatile with a steady MAP pressure – Measure of tissue perfusion Heart spends more time in diastole, so not just a simple average of diastole and systole MAP is calculated by adding diastolic pressure + 1/3 pulse pressure – Example:  BP = 120/80  Pulse Pressure = 120 − 80 = 40  So MAP = 80 + (1/3)*40 = 80 + ~13 = 93 mm Hg Pulse pressure and MAP both decline with increasing distance from heart Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Arterial Blood Pressure cont. Clinical monitoring of circulatory efficiency – Vital signs: pulse and blood pressure, along with respiratory rate and body temperature – Taking a pulse  Radial pulse (taken at the wrist): most routinely used, but there are other clinically important pulse points  Pressure points: areas where arteries are close to body surface – Can be compressed to stop blood flow in event of hemorrhaging Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Body Sites Where the Pulse is Most Easily Palpated Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Arterial Blood Pressure – Measuring blood pressure  Systemic arterial BP is measured indirectly by auscultatory methods using a sphygmomanometer 1. Wrap cuff around arm superior to elbow 2. Increase pressure in cuff until it exceeds systolic pressure in brachial artery 3. Pressure is released slowly, and examiner listens for sounds of Korotkoff with a stethoscope Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved – Measuring blood pressure (cont.)  Systolic pressure: normally less than 120 mm Hg – Pressure when sounds first occur as blood starts to spurt through artery  Diastolic pressure: normally less than 80 mm Hg – Pressure when sounds disappear because artery no longer constricted; blood flowing freely Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Capillary Blood Pressure Ranges from 35 mm Hg at beginning of capillary bed to ∼17 mm Hg at the end of the bed Low capillary pressure is desirable because: 1. High BP would rupture fragile, thin-walled capillaries 2. Most capillaries are very permeable, so low pressure forces filtrate into interstitial spaces Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Venous Blood Pressure Changes little during cardiac cycle Small pressure gradient, only about 15 mm Hg – If vein is cut, low pressure of venous system causes blood to flow out smoothly – If artery cut, blood spurts out because pressure is higher Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Factors Aiding Venous Return – The pressure gradient: blood flows from higher pressure to lower pressure (venules to veins) – Gravity: pulls blood downward toward veins – Respiratory “pump”: pressure changes created during breathing suck blood toward the heart by squeezing local veins – Skeletal muscular “pump” : contraction of skeletal muscles “milk” blood toward the heart – Cardiac Suction: chordae tendineae pulls AV valves downward during ventricular ejection, which sucks blood into atria – Valves prevent backflow during venous return Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Muscular Pump Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Regulation of Blood Pressure Maintaining blood pressure requires cooperation of heart, blood vessels, and kidneys – All supervised by brain Three main factors regulating blood pressure – Cardiac output (CO) – Peripheral resistance (PR) – Blood volume Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Regulation of Blood Pressure Anything that increases SV, HR, or R will also increase MAP – SV is effected by venous return (EDV) – HR is maintained by medullary centers – R is effected mostly by vessel diameter Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Major Factors That Increase MAP Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Regulation of Blood Pressure Factors can be affected by: – Short-term regulation: neural controls – Short-term regulation: hormonal controls – Long-term regulation: renal controls Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Short-Term Regulation: Neural Controls Two main neural mechanisms control peripheral resistance 1. MAP is maintained by altering blood vessel diameter, which alters resistance  Example: If blood volume drops, all vessels constrict (except those to heart and brain) 2. Can alter blood distribution to organs in response to specific demands Neural controls operate via reflex arcs that involve: – Cardiovascular center of medulla – Baroreceptors – Chemoreceptors – Higher brain centers Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Short-Term Regulation: Neural Controls cont Role of the cardiovascular center – Cardiovascular center: composed of clusters of sympathetic neurons in medulla – Consists of:  Cardiac centers: cardioinhibitory and cardioacceleratory centers  Vasomotor center: sends steady impulses via sympathetic efferents called vasomotor fibers to blood vessels – Cause continuous moderate constriction called vasomotor tone – Receives inputs from baroreceptors, chemoreceptors, and higher brain centers Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Short-Term Regulation: Neural Controls cont. Baroreceptor reflexes – Located in carotid sinuses, aortic arch, and walls of large arteries of neck and thorax – If MAP is high:  Increased blood pressure stimulates baroreceptors to increase input to vasomotor center  Inhibits vasomotor and cardioacceleratory centers  Stimulates cardioinhibitory center  Results in decreased blood pressure Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Short-Term Regulation: Neural Controls cont. Baroreceptor reflexes (cont.) – If MAP is low:  Reflex vasoconstriction is initiated that increases CO and blood pressure  Example: upon standing, BP falls and triggers: – Carotid sinus reflex: baroreceptors that monitor BP to ensure enough blood to brain – Aortic reflex maintains BP in systemic circuit  Baroreceptors are ineffective if altered blood pressure is sustained – Become adapted to hypertension, so not triggered by elevated BP levels Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Short-Term Regulation: Neural Controls cont. Chemoreceptor reflexes – Aortic arch and large arteries of neck detect increase in CO2, or drop in pH or O2 – Cause increased blood pressure by:  Signaling cardioacceleratory center to increase CO  Signaling vasomotor center to increase vasoconstriction Influence of higher brain centers – Hypothalamus and cerebral cortex can modify arterial pressure via relays to medulla – Hypothalamus increases blood pressure during stress – Hypothalamus mediates redistribution of blood flow during exercise and changes in body temperature Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved  Short-Term Mechanisms: Hormonal Controls Adrenal medulla hormones – Epinephrine and norepinephrine from adrenal gland increase CO and vasoconstriction Angiotensin II stimulates vasoconstriction ADH: high levels can cause vasoconstriction Atrial natriuretic peptide decreases BP by antagonizing aldosterone, causing decreased blood volume Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Long-Term Mechanisms: Renal Regulation Baroreceptors quickly adapt to chronic high or low BP so are ineffective for long-term regulation Long-term mechanisms control BP by altering blood volume via kidneys Kidneys regulate arterial blood pressure by: 1. Direct renal mechanism 2. Indirect renal mechanism (renin-angiotensin- aldosterone) Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Long-Term Mechanisms: Renal Regulation Direct renal mechanism – Alters blood volume independently of hormones  Increased BP or blood volume causes elimination of more urine, thus reducing BP  Decreased BP or blood volume causes kidneys to conserve water, and BP rises Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved  Long-Term Mechanisms: Renal Regulation Indirect mechanism – The renin-angiotensin-aldosterone mechanism  Decreased arterial blood pressure causes release of renin from kidneys  Renin enters blood and catalyzes conversion of angiotensinogen from liver to angiotensin I  Angiotensin-converting enzyme, especially from lungs, converts angiotensin I to angiotensin II – Angiotensin II acts in four ways to stabilize arterial BP and ECF:  Stimulates aldosterone secretion  Causes ADH release from posterior pituitary  Triggers hypothalamic thirst center to drink more water  Acts as a potent vasoconstrictor, directly increasing blood pressure Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved RAAS (Renin-Angiotensis-Aldosterone-System) Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Summary of Blood Pressure Regulation Goal of blood pressure regulation is to keep blood pressure high enough to provide adequate tissue perfusion, but not so high that blood vessels are damaged – Example: If BP to brain is too low, perfusion is inadequate, and person loses consciousness – If BP to brain is too high, person could have stroke Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved