Chapter 19: Blood Vessels - Anatomy and Physiology PDF

Chapter 19 Blood Vessels  Blood Vessels The blood vessels (arteries, arterioles, capillaries, venules, and veins) form a closed tube that carries blood away from the heart, to the cells, and back again.  Blood Vessels Arteries: carry blood away from heart;...

Chapter 19 Blood Vessels  Blood Vessels The blood vessels (arteries, arterioles, capillaries, venules, and veins) form a closed tube that carries blood away from the heart, to the cells, and back again.  Blood Vessels Arteries: carry blood away from heart; oxygenated except for pulmonary circulation and umbilical vessels of fetus Capillaries: contact tissue cells; directly serve cellular needs Veins: carry blood toward heart  Figure 19.1a Generalized structure of arteries, veins, and capillaries. Arter y Vein © 2013 Pearson Education, Inc.  Structure of Blood Vessel Walls Lumen – Central blood-containing space Three wall layers in arteries and veins – Tunica intima, tunica media, and tunica externa Capillaries – Endothelium with sparse basal lamina Figure 19.1b Generalized structure of arteries, veins, and capillaries. Tunica intima Endothelium Subendothelial layer Internal elastic membrane Tunica media (smooth muscle and Valve elastic fibers) External elastic membrane Tunica externa (collagen fibers) Vasa vasorum Lumen Lumen Artery Capillary network Vein Basement membrane Endothelial cells © 2013 Pearson Capillary Education, Inc.  Arteries and Arterioles Arteries are strong, elastic vessels adapted for carrying high-pressure blood. Arteries become smaller as they divide and give rise to arterioles (lead to capillary beds). Arteries are capable of vasoconstriction as directed by the sympathetic impulses; when impulses are inhibited, vasodilation results.  Capillaries Capillaries are the smallest vessels, consisting only of a layer of endothelium through which substances are exchanged with tissue cells. – Diameter allows only 1 RBC to pass through Capillary permeability varies from one tissue to the next, generally with more permeability in the liver, intestines, and certain glands, and less in muscle and considerably less in the brain (blood- brain barrier).  Capillaries The pattern of capillary density also varies from one body part to the next – Areas with a great deal of metabolic activity (leg muscles, for example) have higher densities of capillaries. Precapillary sphincters can regulate the amount of blood entering a capillary bed and are controlled by oxygen concentration in the area. – If blood is needed elsewhere in the body, the capillary beds in less important areas are shut down.  Capillaries In all tissues except for cartilage, epithelia, cornea and lens of eye Provide direct access to almost every cell Functions – Exchange of gases, nutrients, wastes, hormones, etc., between blood and interstitial fluid  Capillaries Blood entering the capillaries contain high concentrations of oxygen and nutrients that diffuse out of the capillary wall and into the tissue – Plasma proteins remain in blood due to their large size  Capillaries Hydrostatic pressure drives the passage of fluids and very small molecules out of the capillary at this point. At the venule end, osmosis, due to the osmotic pressure of the blood, causes much of the tissue fluid to return to the bloodstream. Figure 19.4 Anatomy of a capillary bed. Vascular shunt Precapillary sphincters Metarteriol Thoroughfare e channel True capillarie s Terminal arteriole Postcapillary venule Sphincters open—blood flows through true capillaries. Terminal arteriole Postcapillary venule Sphincters closed—blood flows through metarteriole – thoroughfare © 2013 Pearson channel and bypasses true capillaries. Education, Inc. Capillary System  Venules and Veins Venules leading from capillaries merge to form veins that return blood to the heart. Veins have the same three layers as arteries have and have a flap-like valve inside to prevent backflow of blood.  Venules and Veins Veins are thinner and less muscular than arteries; they do not carry high-pressure blood. Veins also function as blood reservoirs. Physiology of Circulation: Definition of Terms Blood flow – Volume of blood flowing through vessel, organ, or entire circulation in given period Measured as ml/min Equivalent to cardiac output (CO) for entire vascular system Relatively constant when at rest Varies widely through individual organs, based on needs Physiology of Circulation: Definition of Terms 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 Physiology of Circulation: Definition of Terms Resistance (peripheral resistance) – Opposition to flow – Measure 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  Resistance Factors that remain relatively constant: – Blood viscosity The "stickiness" of blood due to formed elements and plasma proteins Increased viscosity = increased resistance – Blood vessel length Longer vessel = greater resistance encountered  Resistance Blood vessel diameter – Greatest influence on resistance Frequent changes alter peripheral resistance Varies inversely with fourth power of vessel radius – E.g., if radius is doubled, the resistance is 1/16 as much – E.g., Vasoconstriction → increased resistance  Resistance Small-diameter arterioles major determinants of peripheral resistance Abrupt changes in diameter or fatty plaques from atherosclerosis dramatically increase resistance – Disrupt laminar flow and cause turbulent flow Irregular fluid motion → increased resistance  Relationship Between Blood Flow, Blood Pressure, and Resistance Blood flow (F) directly proportional to blood pressure gradient (Δ P) – If Δ P increases, blood flow speeds up Blood flow inversely proportional to peripheral resistance (R) – If R increases, blood flow decreases: F = Δ P/R R more important in influencing local blood flow because easily changed by altering blood vessel diameter  Systemic Blood Pressure Pumping action of heart generates blood flow Pressure results when flow is opposed by resistance Systemic pressure – Highest in aorta – Declines throughout pathway – 0 mm Hg in right atrium Steepest drop occurs in arterioles  Figure 19.6 Blood pressure in various blood vessels of the systemic circulation. 120 Blood pressure (mm Hg) Systolic pressure 100 Mean pressure 80 60 Diastolic 40 pressure 20 0 e ies s e le s e a ins iol i ava rt ter lar nu Ao ter Ve Ar s ec pil Ve Ar na Ca Ve © 2013 Pearson Education, Inc.  Arterial Blood Pressure Reflects two factors of arteries close to heart – Elasticity (compliance or distensibility) – Volume of blood forced into them at any time Blood pressure near heart is pulsatile  Arterial Blood Pressure Systolic pressure: pressure exerted in aorta during ventricular contraction – Averages 120 mm Hg in normal adult Diastolic pressure: lowest level of aortic pressure Pulse pressure = difference between systolic and diastolic pressure – Throbbing of arteries (pulse)  Arterial Blood Pressure Mean arterial pressure (MAP): pressure that propels blood to tissues – Not average of 2 because diastole lasts longer MAP = diastolic pressure + 1/3 pulse pressure Pulse pressure and MAP both decline with increasing distance from heart Ex. BP = 120/80; MAP = 93 mm Hg  Capillary Blood Pressure Ranges from 17 to 35 mm Hg Low capillary pressure is desirable – High BP would rupture fragile, thin-walled capillaries – Most very permeable, so low pressure forces filtrate into interstitial spaces  Venous Blood Pressure Changes little during cardiac cycle – Does not pulsate Small pressure gradient; about 15 mm Hg (60 mm Hg pressure change in arteries) Low pressure due to cumulative effects of peripheral resistance – Energy of blood pressure lost as heat during each circuit  Factors Aiding Venous Return 1. Muscular pump: contraction of skeletal muscles "milks" blood toward heart; valves prevent backflow 2. Respiratory pump: pressure changes during breathing move blood toward heart by squeezing abdominal veins as thoracic veins expand 3. Venoconstriction under sympathetic control pushes blood toward heart Figure 19.7 The muscular pump. Venous valve (open) Contracted skeletal muscle Venous valve (closed) Vein Direction of blood flow © 2013 Pearson Education, Inc.  Maintaining Blood Pressure Requires – Cooperation of heart, blood vessels, and kidneys – Supervision by brain Main factors influencing blood pressure – Cardiac output (CO) – Peripheral resistance (PR) – Blood volume  Control of Blood Pressure Short-term neural and hormonal controls – Counteract fluctuations in blood pressure by altering peripheral resistance and CO Long-term renal regulation – Counteracts fluctuations in blood pressure by altering blood volume Short-term Mechanisms: Neural Controls Neural controls of peripheral resistance – Maintain MAP by altering blood vessel diameter If low blood volume all vessels constricted except those to heart and brain – Alter blood distribution to organs in response to specific demands Short-term Mechanisms: Neural Controls Neural controls operate via reflex arcs that involve – Baroreceptors – Cardiovascular center of medulla – Vasomotor fibers to heart and vascular smooth muscle – Sometimes input from chemoreceptors and higher brain centers  The Cardiovascular Center Clusters of sympathetic neurons in medulla oversee changes in CO and blood vessel diameter Consists of cardiac centers and vasomotor center Vasomotor center sends steady impulses via sympathetic efferents to blood vessels → moderate constriction called vasomotor tone Receives inputs from baroreceptors, chemoreceptors, and higher brain centers  Short-term Mechanisms: Baroreceptor Reflexes Baroreceptors located in – Carotid sinuses – Aortic arch – Walls of large arteries of neck and thorax  Short-term Mechanisms: Baroreceptor Reflexes Increased blood pressure stimulates baroreceptors to increase input to vasomotor center – Inhibits vasomotor and cardioacceleratory centers, causing arteriolar dilation and venodilation – Stimulates cardioinhibitory center – → decreased blood pressure  Short-term Mechanisms: Baroreceptor Reflexes Decrease in blood pressure due to – Arteriolar vasodilation – Venodilation – Decreased cardiac output  Short-term Mechanisms: Baroreceptor Reflexes If MAP low – → Reflex vasoconstriction → increased CO → increased blood pressure – Ex. Upon standing baroreceptors of carotid sinus reflex protect blood to brain; in systemic circuit as whole aortic reflex maintains blood pressure Baroreceptors ineffective if altered blood pressure sustained  Figure 19.9 Baroreceptor reflexes that help maintain blood pressure homeostasis. Slide 2 IMB ALA 1 Stimulus: NC E Blood pressure (arterial blood pres- sure rises above Homeostasis: Blood pressure in normal range normal range). IMB ALA NC © 2013 Pearson E Education, Inc.  Figure 19.9 Baroreceptor reflexes that help maintain blood pressure homeostasis. Slide 3 2 Baroreceptors in carotid sinuses and aortic arch are stimulated. IMB ALA 1 Stimulus: NC E Blood pressure (arterial blood pres- sure rises above Homeostasis: Blood pressure in normal range normal range). IMB ALA NC © 2013 Pearson E Education, Inc.  Figure 19.9 Baroreceptor reflexes that help maintain blood pressure homeostasis. Slide 4 3 Impulses from baroreceptors stimulate cardioinhibitory center (and inhibit cardioacceleratory center) and inhibit vasomotor center. 2 Baroreceptors in carotid sinuses and aortic arch are stimulated. IMB ALA 1 Stimulus: NC E Blood pressure (arterial blood pres- sure rises above Homeostasis: Blood pressure in normal range normal range). IMB ALA NC © 2013 Pearson E Education, Inc.  Figure 19.9 Baroreceptor reflexes that help maintain blood pressure homeostasis. Slide 5 3 Impulses from baroreceptors stimulate cardioinhibitory center (and inhibit cardioacceleratory center) and inhibit vasomotor center. 4a Sympathetic impulses to heart cause HR, contractility, and CO. 2 Baroreceptors in carotid sinuses and aortic arch are stimulated. IMB ALA 1 Stimulus: NC E Blood pressure (arterial blood pres- sure rises above Homeostasis: Blood pressure in normal range normal range). IMB ALA NC © 2013 Pearson E Education, Inc.  Figure 19.9 Baroreceptor reflexes that help maintain blood pressure homeostasis. Slide 6 3 Impulses from baroreceptors stimulate cardioinhibitory center (and inhibit cardioacceleratory center) and inhibit vasomotor center. 4a Sympathetic impulses to heart cause HR, contractility, and CO. 2 Baroreceptors in carotid sinuses and aortic arch are stimulated. 4b Rate of vasomotor impulses allows vasodilation, IMB causing R. ALA 1 Stimulus: NC E Blood pressure (arterial blood pres- sure rises above Homeostasis: Blood pressure in normal range normal range). IMB ALA NC © 2013 Pearson E Education, Inc.  Figure 19.9 Baroreceptor reflexes that help maintain blood pressure homeostasis. Slide 7 3 Impulses from baroreceptors stimulate cardioinhibitory center (and inhibit cardioacceleratory center) and inhibit vasomotor center. 4a Sympathetic impulses to heart cause HR, contractility, and CO. 2 Baroreceptors in carotid sinuses and aortic arch are stimulated. 4b Rate of vasomotor impulses allows vasodilation, IMB causing R. 5 CO and R ALA 1 Stimulus: NC E return blood Blood pressure pressure to (arterial blood pres- homeostatic range. sure rises above Homeostasis: Blood pressure in normal range normal range). IMB ALA NC © 2013 Pearson E Education, Inc.  Figure 19.9 Baroreceptor reflexes that help maintain blood pressure homeostasis. Slide IMB 8 ALA NC E Homeostasis: Blood pressure in normal range 1 Stimulus: Blood pressure IMB (arterial blood ALA NC pressure falls below E normal range). © 2013 Pearson Education, Inc.  Figure 19.9 Baroreceptor reflexes that help maintain blood pressure homeostasis. Slide IMB 9 ALA NC E Homeostasis: Blood pressure in normal range 1 Stimulus: Blood pressure IMB (arterial blood ALA NC pressure falls below E normal range). 2 Baroreceptors in carotid sinuses and aortic arch are inhibited. © 2013 Pearson Education, Inc.  Figure 19.9 Baroreceptor reflexes that help maintain blood pressure homeostasis. Slide IMB 10 ALA NC E Homeostasis: Blood pressure in normal range 1 Stimulus: Blood pressure IMB (arterial blood ALA NC pressure falls below E normal range). 2 Baroreceptors in carotid sinuses and aortic arch are inhibited. 3 Impulses from baroreceptors activate cardioacceleratory center (and inhibit cardioinhibitory center) and stimulate vasomotor center. © 2013 Pearson Education, Inc.  Figure 19.9 Baroreceptor reflexes that help maintain blood pressure homeostasis. Slide IMB 11 ALA NC E Homeostasis: Blood pressure in normal range 1 Stimulus: Blood pressure IMB (arterial blood ALA NC pressure falls below E normal range). 2 Baroreceptors in carotid sinuses and aortic arch are inhibited. 4a Sympathetic impulses to heart cause HR, contractility, and 3 Impulses from baroreceptors CO. activate cardioacceleratory center (and inhibit cardioinhibitory center) and stimulate vasomotor center. © 2013 Pearson Education, Inc.  Figure 19.9 Baroreceptor reflexes that help maintain blood pressure homeostasis. Slide IMB 12 ALA NC E Homeostasis: Blood pressure in normal range 1 Stimulus: Blood pressure IMB (arterial blood ALA NC pressure falls below E normal range). 4b Vasomotor fibers stimulate vasoconstriction, causing R. 2 Baroreceptors in carotid sinuses and aortic arch are inhibited. 4a Sympathetic impulses to heart cause HR, contractility, and 3 Impulses from baroreceptors CO. activate cardioacceleratory center (and inhibit cardioinhibitory center) and stimulate vasomotor center. © 2013 Pearson Education, Inc.  Figure 19.9 Baroreceptor reflexes that help maintain blood pressure homeostasis. Slide IMB 13 ALA NC E Homeostasis: Blood pressure in normal range 1 Stimulus: Blood pressure IMB (arterial blood ALA NC pressure falls below E normal range). 5 CO and R return blood pressure to 4b Vasomotor homeostatic range. fibers stimulate vasoconstriction, causing R. 2 Baroreceptors in carotid sinuses and aortic arch are inhibited. 4a Sympathetic impulses to heart cause HR, contractility, and 3 Impulses from baroreceptors CO. activate cardioacceleratory center (and inhibit cardioinhibitory center) and stimulate vasomotor center. © 2013 Pearson Education, Inc.  Figure 19.9 Baroreceptor reflexes that help maintain blood pressure homeostasis. Slide 3 Impulses from baroreceptors 14 stimulate cardioinhibitory center (and inhibit cardioacceleratory center) and inhibit vasomotor center. 4a Sympathetic impulses to heart cause HR, contractility, and CO. 2 Baroreceptors in carotid sinuses and aortic arch are stimulated. 4b Rate of vasomotor impulses allows vasodilation, I MB causing R. AL A 5 CO and R 1 Stimulus: NC E return blood Blood pressure pressure to (arterial blood homeostatic range. pressure rises above Homeostasis: Blood pressure in normal range normal range). 1 Stimulus: Blood pressure IMB (arterial blood AL A pressure falls below NC E normal range). 5 CO and R return blood pressure to 4b Vasomotor homeostatic range. fibers stimulate vasoconstriction, causing R. 2 Baroreceptors in carotid sinuses and aortic arch are inhibited. 4a Sympathetic impulses to heart cause HR, contractility, and 3 Impulses from baroreceptors CO. activate cardioacceleratory center © 2013 Pearson (and inhibit cardioinhibitory center) Education, Inc. and stimulate vasomotor center.  Short-term Mechanisms: Chemoreceptor Reflexes Chemoreceptors in 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 → increase CO – Signaling vasomotor center → increase vasoconstriction Short-term Mechanisms: Influence of Higher Brain Centers Reflexes in medulla 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 Short-term Mechanisms: Hormonal Controls Short term regulation via changes in peripheral resistance Long term regulation via changes in blood volume  Short-term Mechanisms: Hormonal Controls Cause increased blood pressure – Epinephrine and norepinephrine from adrenal gland → increased CO and vasoconstriction – Angiotensin II stimulates vasoconstriction – High ADH levels cause vasoconstriction Cause lowered blood pressure – Atrial natriuretic peptide causes decreased blood volume by antagonizing aldosterone  Long-term Mechanisms: Renal Regulation Baroreceptors quickly adapt to chronic high or low BP so are ineffective Long-term mechanisms control BP by altering blood volume via kidneys Kidneys regulate arterial blood pressure 1. Direct renal mechanism 2. Indirect renal (renin-angiotensin-aldosterone) mechanism  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  Indirect Mechanism The renin-angiotensin-aldosterone mechanism – ↓ Arterial blood pressure → release of renin – Renin catalyzes conversion of angiotensinogen from liver to angiotensin I – Angiotensin converting enzyme, especially from lungs, converts angiotensin I to angiotensin II  Functions of Angiotensin II Increases blood volume – Stimulates aldosterone secretion – Causes ADH release – Triggers hypothalamic thirst center Causes vasoconstriction directly increasing blood pressure Figure 19.10 Direct and indirect (hormonal) mechanisms for renal control of blood pressure. Direct renal mechanism Indirect renal mechanism (renin-angiotensin-aldosterone) Arterial pressure Initial stimulus Arterial pressure Physiological response Result Inhibits baroreceptors Sympathetic nervous system activity Filtration by kidneys Angiotensinoge n Renin release from kidneys Angiotensin I Angiotensin converting enzyme (ACE) Angiotensin II Urine formation Vasoconstriction; ADH release by Thirst via Adrenal cortex peripheral posterior pituitary hypothalamus resistance Secretes Aldosteron e Blood volume Sodium Water reabsorption Water intake reabsorption by kidneys by kidneys Blood volume Mean arterial pressure Mean arterial pressure © 2013 Pearson Education, Inc. Chapter 19 Part II Monitoring Circulatory Efficiency Vital signs: pulse and blood pressure, along with respiratory rate and body temperature Pulse: pressure wave caused by expansion and recoil of arteries Radial pulse (taken at the wrist) routinely used Pressure points where arteries close to body surface – Can be compressed to stop blood flow  Measuring Blood Pressure Systemic arterial BP – Measured indirectly by auscultatory method using a sphygmomanometer – Pressure increased in cuff until it exceeds systolic pressure in brachial artery – Pressure released slowly and examiner listens for sounds of Korotkoff with a stethoscope  Measuring Blood Pressure Systolic pressure, normally less than 120 mm Hg, is pressure when sounds first occur as blood starts to spurt through artery Diastolic pressure, normally less than 80 mm Hg, is pressure when sounds disappear because artery no longer constricted; blood flowing freely  Variations in Blood Pressure Transient elevations occur during changes in posture, physical exertion, emotional upset, fever. Age, sex, weight, race, mood, and posture may cause BP to vary  Alterations in Blood Pressure Hypertension: high blood pressure – Sustained elevated arterial pressure of 140/90 or higher – Prehypertension if values elevated but not yet in hypertension range May be transient adaptations during fever, physical exertion, and emotional upset Often persistent in obese people  Homeostatic Imbalance: Hypertension Prolonged hypertension major cause of heart failure, vascular disease, renal failure, and stroke – Heart must work harder → myocardium enlarges, weakens, becomes flabby – Also accelerates atherosclerosis  Primary or Essential Hypertension 90% of hypertensive conditions No underlying cause identified – Risk factors include heredity, diet, obesity, age, diabetes mellitus, stress, and smoking No cure but can be controlled – Restrict salt, fat, cholesterol intake – Increase exercise, lose weight, stop smoking – Antihypertensive drugs  Homeostatic Imbalance: Hypertension Secondary hypertension less common – Due to identifiable disorders including obstructed renal arteries, kidney disease, and endocrine disorders such as hyperthyroidism and Cushing's syndrome – Treatment focuses on correcting underlying cause  Alterations in Blood Pressure Hypotension: low blood pressure – Blood pressure below 90/60 mm Hg – Usually not a concern Only if leads to inadequate blood flow to tissues – Often associated with long life and lack of cardiovascular illness  Homeostatic Imbalance: Hypotension Orthostatic hypotension: temporary low BP and dizziness when suddenly rising from sitting or reclining position Chronic hypotension: hint of poor nutrition and warning sign for Addison's disease or hypothyroidism Acute hypotension: important sign of circulatory shock; threat for surgical patients and those in ICU Blood Flow Through Body Tissues Tissue perfusion involved in – Delivery of O2 and nutrients to, and removal of wastes from, tissue cells – Gas exchange (lungs) – Absorption of nutrients (digestive tract) – Urine formation (kidneys) Rate of flow is precisely right amount to provide proper function Figure 19.13 Distribution of blood flow at rest and during strenuous exercise. 750 750 Brain 750 Hear 12,500 250 t Skeletal 1200 muscle s Skin 500 Kidneys 1100 Abdomen 1400 1900 Other 600 Total blood 600 flow at rest 5800 600 ml/min 400 Total blood flow during strenuous exercise 17,500 ml/min  Velocity of Blood Flow Changes as travels through systemic circulation Inversely related to total cross-sectional area Fastest in aorta; slowest in capillaries; increases in veins Slow capillary flow allows adequate time for exchange between blood and tissues Figure 19.14 Blood flow velocity and total cross-sectional area of vessels. Relative cross- sectional area of different vessels of the vascular bed 5000 Total area 4000 (cm2) of the 3000 vascular 2000 bed 1000 0 50 40 Velocity of 30 blood flow (cm/s) 20 10 0 Ca ioles Ve es ies les r ta ins ae i av l ar ter Ao nu Ve ter ec pil Ar Ar na © 2013 Pearson Ve Education, Inc.  Autoregulation Automatic adjustment of blood flow to each tissue relative to its varying requirements Controlled intrinsically by modifying diameter of local arterioles feeding capillaries – Independent of MAP, which is controlled as needed to maintain constant pressure Organs regulate own blood flow by varying resistance of own arterioles  Blood Flow: Skeletal Muscles Varies with fiber type and activity – At rest, myogenic and general neural mechanisms predominate - maintain ~ 1L /minute – During muscle activity Active or exercise hyperemia - blood flow increases in direct proportion to metabolic activity Local controls override sympathetic vasoconstriction Muscle blood flow can increase 10×  Blood Flow: Brain Blood flow to brain constant as neurons intolerant of ischemia; averages 750 ml/min Metabolic controls – Decreased pH of increased carbon dioxide cause marked vasodilation Myogenic controls – Decreased MAP causes cerebral vessels to dilate – Increased MAP causes cerebral vessels to constrict  Blood Flow: Brain Brain vulnerable under extreme systemic pressure changes – MAP below 60 mm Hg can cause syncope (fainting) – MAP above 160 can result in cerebral edema  Blood Flow: Skin Blood flow through skin – Supplies nutrients to cells (autoregulation in response to O2 need) – Helps regulate body temperature (neurally controlled) – primary function – Provides a blood reservoir (neurally controlled)  Blood Flow: Skin Blood flow to venous plexuses below skin surface regulates body temperature – Varies from 50 ml/min to 2500 ml/min, depending on body temperature – Controlled by sympathetic nervous system reflexes initiated by temperature receptors and central nervous system  Temperature Regulation As temperature rises (e.g., heat exposure, fever, vigorous exercise) – Hypothalamic signals reduce vasomotor stimulation of skin vessels → – Warm blood flushes into capillary beds → – Heat radiates from skin  Temperature Regulation Sweat also causes vasodilation via bradykinin in perspiration – Bradykinin stimulates NO release As temperature decreases, blood is shunted to deeper, more vital organs  Blood Flow: Lungs Pulmonary circuit unusual – Pathway short – Arteries/arterioles more like veins/venules (thin walled, with large lumens) – Arterial resistance and pressure are low (24/10 mm Hg)  Blood Flow: Lungs Autoregulatory mechanism opposite that in most tissues – Low O2 levels cause vasoconstriction; high levels promote vasodilation Allows blood flow to O2-rich areas of lung  Blood Flow: Heart During ventricular systole – Coronary vessels are compressed Myocardial blood flow ceases Stored myoglobin supplies sufficient oxygen During diastole high aortic pressure forces blood through coronary circulation At rest ~ 250 ml/min; control probably myogenic  Blood Flow: Heart During strenuous exercise – Coronary vessels dilate in response to local accumulation of vasodilators – Blood flow may increase three to four times Important–cardiac cells use 65% of O2 delivered so increased blood flow provides more O2 Blood Flow Through Capillaries Vasomotion – Slow, intermittent flow – Reflects on/off opening and closing of precapillary sphincters  Capillary Exchange of Respiratory Gases and Nutrients Diffusion down concentration gradients – O2 and nutrients from blood to tissues – CO2 and metabolic wastes from tissues to blood Lipid-soluble molecules diffuse directly through endothelial membranes Water-soluble solutes pass through clefts and fenestrations Larger molecules, such as proteins, are actively transported in pinocytotic vesicles or caveolae Figure 19.16 Capillary transport mechanisms. (1 of 2) Pinocytotic vesicles Red blood cell in lumen Endothelial cell Fenestration Endothelial (pore) cell nucleus Basement membran e Tight junction Intercellular © 2013 Pearson cleft Education, Inc. Figure 19.16 Capillary transport mechanisms. (2 of 2) Lumen Caveolae Pinocytotic vesicles Endothelial Intercellular fenestration cleft (pore) 4 Transport via vesicles or caveolae (large substances) 3 Movement Basement through membrane fenestrations (water-soluble 2 Movement substances) 1 Diffusion through through intercellular membrane clefts (water- (lipid-soluble soluble substances) substances) © 2013 Pearson Education, Inc.  Fluid Movements: Bulk Flow Fluid leaves capillaries at arterial end; most returns to blood at venous end – Extremely important in determining relative fluid volumes in blood and interstitial space Direction and amount of fluid flow depend on two opposing forces: hydrostatic and colloid osmotic pressures  Hydrostatic Pressures Capillary hydrostatic pressure (HPc) (capillary blood pressure) – Tends to force fluids through capillary walls – Greater at arterial end (35 mm Hg) of bed than at venule end (17 mm Hg) Interstitial fluid hydrostatic pressure (HPif) – Pressure that would push fluid into vessel – Usually assumed to be zero because of lymphatic vessels  Colloid Osmotic Pressures Capillary colloid osmotic pressure (oncotic pressure) (OPc) – Created by nondiffusible plasma proteins, which draw water toward themselves – ~26 mm Hg Interstitial fluid osmotic pressure (OPif) – Low (~1 mm Hg) due to low protein content  Hydrostatic-osmotic Pressure Interactions: Net Filtration Pressure (NFP) NFP—comprises all forces acting on capillary bed – NFP = (HPc + OPif) – (HPif + OPc) Net fluid flow out at arterial end Net fluid flow in at venous end More leaves than is returned – Excess fluid returned to blood via lymphatic system Figure 19.17 Bulk fluid flow across capillary walls causes continuous mixing of fluid between the plasma and the interstitial fluid compartments, and maintains the interstitial environment. (1 of 5) The big picture Fluid filters from capillaries at their arteriolar end and flows through the interstitial space. Most is reabsorbed at the venous end. Arteriol e Fluid moves through the interstitial space. For all capillary beds, 20 L of fluid is filtered out per day—almost 7 times the total plasma volume! Net filtration pressure (NFP) determines the direction of fluid movement. Two kinds of pressure drive fluid flow: Hydrostatic pressure Osmotic pressure (HP) (OP) Due to fluid pressing against a Due to nondiffusible solutes that boundary cannot cross the boundary HP “pushes” fluid across the OP “pulls” fluid across the boundary boundary In blood vessels, is due to blood In blood vessels, is due to pressure plasma proteins Pisto n 17 L of fluid per day is reabsorbed into the Solute capillaries molecule at the venous About 3 L per day s end. of fluid (and any Boundar (proteins) Boundar leaked proteins) y y are removed by the lymphatic system “Pushes “Pulls (see Chapter 20). ” ” Lymphatic capillary Venule © 2013 Pearson Education, Inc. Figure 19.17 Bulk fluid flow across capillary walls causes continuous mixing of fluid between the plasma and the interstitial fluid compartments, and maintains the interstitial environment. (3 of 5) Net filtration pressure (NFP) determines the direction of fluid movement. Two kinds of pressure drive fluid flow: Hydrostatic pressure Osmotic pressure (HP) (OP) Due to fluid pressing against Due to nondiffusible solutes a that boundary HP “pushes” fluid across OP “pulls” cannot crossfluid theacross boundary the the boundary In blood vessels, is due to boundary In blood vessels, is due to blood plasma proteins pressure Piston Solute molecule s Boundar (proteins) Boundar y y “Pushes” “Pulls” © 2013 Pearson Education, Inc.  Figure 19.17 Bulk fluid flow across capillary walls causes continuous mixing of fluid between the plasma and the interstitial fluid compartments, and maintains the interstitial environment. (4 of 5) How do the pressures drive fluid flow across a capillary? Net filtration occurs at the arteriolar end of a capillary. Capillar Boundary Interstitial fluid y (capillary wall) Hydrostatic pressure in capillary “pushes” HPc = 35 mm Hg fluid out of capillary. To determine the pressure driving Osmotic pressure in the fluid out of the capillary at any capillary “pulls” fluid OPc = 26 mm Hg given point, we calculate the net into capillary. filtration pressure (NFP)––the outward pressures (HPc and OPif) minus the inward pressures Hydrostatic pressure in (HPif and OPc). So, HPif = 0 mm Hg interstitial fluid “pushes” fluid into capillary. NFP = (HPc + OPif) – (HPif + OPc) OPif = 1 mm Hg Osmotic pressure = (35 + 1) – (0 + 26) in interstitial fluid = 10 mm Hg (net outward “pulls” fluid out pressure) of capillary. As a result, fluid moves from the NFP = 10 mm capillary into the interstitial space. Hg © 2013 Pearson Education, Inc.  Figure 19.17 Bulk fluid flow across capillary walls causes continuous mixing of fluid between the plasma and the interstitial fluid compartments, and maintains the interstitial environment. (5 of 5) Net reabsorption occurs at the venous end of a capillary. Capillar Boundary (capillary Interstitial fluid y Hydrostatic pressure in capillary wall) “pushes” fluid out of capillary. HPc = 17 mm Hg The pressure has dropped because of resistance encountered along the capillaries. Osmotic pressure in capillary OPc = 26 mm Hg “pulls” fluid into capillary. Again, we calculate the NFP: NFP = (HPc + OPif) – (HPif + OPc) = (17 + 1) – (0 + 26) HPif = 0 mm Hg Hydrostatic pressure in = –8 mm Hg (net inward interstitial fluid “pushes” pressure) fluid into capillary. OPif = 1 mm Hg Osmotic pressure in Notice that the NFP at the venous interstitial fluid “pulls” end is a negative number. This fluid out of capillary. means that reabsorption, not filtration, is occurring and so fluid moves from the interstitial space NFP= –8 mm Hg into the capillary. © 2013 Pearson Education, Inc.  Circulatory Shock Any condition in which – Blood vessels inadequately filled – Blood cannot circulate normally Results in inadequate blood flow to meet tissue needs  Circulatory Shock Hypovolemic shock: results from large- scale blood loss Vascular shock: results from extreme vasodilation and decreased peripheral resistance Cardiogenic shock results when an inefficient heart cannot sustain adequate circulation Figure 19.18 Events and signs of hypovolemic shock. Initial stimulus Acute bleeding (or other events that reduce Physiological blood volume) leads to: response 1. Inadequate tissue perfusion resulting in O2 and nutrients to cells Signs and symptoms 2. Anaerobic metabolism by cells, so lactic Result acid accumulates 3. Movement of interstitial fluid into blood, so tissues dehydrate Chemoreceptors activated Baroreceptor firing reduced Hypothalamus activated Brain (by in blood pH) (by blood volume and pressure) (by blood pressure) Major effect Minor effect Respiratory centers Cardioacceleratory and Sympathetic nervous ADH Neurons activated vasomotor centers activated system activated released depressed by pH Intense vasoconstriction Heart rate (only heart and brain spared) Central nervous system depressed Renal blood flow Kidneys Renin released Adrenal cortex Angiotensin II produced in blood Aldosterone Kidneys retain Water released salt and water retention Rate and Tachycardia; Skin becomes Urine output Thirst Restlessness Coma depth of weak, thready cold, clammy, (early sign) (late sign) breathing pulse and cyanotic CO2 blown Blood pressure maintained; if fluid volume continues to off; blood decrease, BP ultimately pH rises drops. BP is a late sign. © 2013 Pearson Education, Inc. Circulatory Pathways: Blood Vessels of the Body Two main circulations – Pulmonary circulation: short loop that runs from heart to lungs and back to heart – Systemic circulation: long loop to all parts of body and back to heart Figure 19.19a Pulmonary circulation. Pulmonary Pulmonary capillaries R. pulmonary L. pulmonary capillaries of the artery artery of the R. lung L. lung To Pulmonary systemic trunk circulation R. pulmonary veins From systemic RA LA circulation L. pulmonary veins RV LV © 2013 Pearson Education, Inc. Schematic flowchart. Figure 19.19 Pulmonary circulation. Pulmonary Pulmonary capillaries R. pulmonary L. pulmonary capillaries of the artery artery of the R. lung L. lung To Pulmonary systemic trunk circulation R. pulmonary veins From systemic RA LA circulation L. pulmonary veins RV LV Schematic flowchart. Left pulmonary artery Air-filled alveolus Aortic arch of lung Pulmonary trunk Right pulmonary artery Three lobar arteries to right lung Pulmonary capillary Gas exchange Two lobar arteries Pulmonary to left lung veins Pulmonary Right veins atrium Left atrium Right ventricle Left ventricle Illustration. The pulmonary arterial system is shown in blue to indicate that the blood it carries is oxygen-poor. © 2013 Pearson The pulmonary venous drainage is shown in red to indicate that the blood it transports is oxygen-rich. Education, Inc. Figure 19.20 Schematic flowchart showing an overview of the systemic circulation. Common Capillary beds carotid of arteries head and to head and upper limbs subclavian arteries to Superior upper limbs vena cava Aortic arch Aort a RA LA RV LV Azygo Thoracic s aorta system Arteria Venous l drainage blood Inferio r Capillary beds of vena mediastinal cava structures and thorax walls Diaphrag m Abdomina l aorta Capillary beds of Inferio digestive viscera, r spleen, vena pancreas, cava kidneys Capillary beds of © 2013 Pearson gonads, pelvis, and Education, Inc. lower limbs  Differences Between Arteries and Veins Arteries Veins Delivery Blood pumped into single Blood returns via superior systemic artery—the aorta and interior venae cavae and the coronary sinus Location Deep, and protected by tissues Both deep and superficial Pathways Fairly distinct Numerous interconnections Supply/drainage Predictable supply Usually similar to arteries, except dural sinuses and hepatic portal circulation © 2013 Pearson Education, Inc. Figure 19.21b Major arteries of the systemic circulation. Arteries of the head and trunk Internal carotid artery External carotid arterycarotid Arteries that supply the upper limb Common Subclavian arteries Vertebral artery Axillary artery Subclavian artery artery Brachiocephalic Brachial Aortictrunk artery arch Radial Ascending artery Ulnar aorta Coronary artery artery Celiac Deep palmar trunk Abdominal arch palmar Superficial aorta Superior arch Digital mesenteric arteries Renal artery Arteries that supply the lower artery limb External iliac Gonadal artery mesenteric artery Femoral Inferior artery artery Common iliac Popliteal artery Internal iliac artery tibial Anterior artery arterytibial Posterior artery Arcuate Illustration, anterior view artery © 2013 Pearson Education, Inc. Figure 19.22b Arteries of the head, neck, and brain. Ophthalmic artery Branches of Basilar the external artery carotid artery Superficial Vertebral temporal artery artery Internal Maxillary carotid artery Occipital artery External artery Facial carotid artery Lingual artery Common artery carotid Superior thyroid artery artery Thyrocervical trunk Laryn Costocervical x Thyroid gland trunk (overlying trachea) Subclavian Clavicle (cut) artery Brachiocephalic Axillary trunk artery Internal thoracic artery © 2013 Pearson Arteries of the head and neck, right aspect Education, Inc. Figure 19.22c Arteries of the head, neck, and brain. Colorized arteriograph of the arterial supply © 2013 Pearson of the brain Education, Inc. Figure 19.22d Arteries of the head, neck, and brain. Anterior Cerebral arterial Frontal lobe circle Optic chiasma (circle of Willis) Anterior Middle communicating cerebral artery artery Anterior cerebral Internal artery carotid Posterior artery communicating Mammillar artery y Posterior body cerebral artery Basilar Temporal artery lobe Vertebral artery Pons Occipital lobe Cerebellu m Posterior Major arteries serving the brain (inferior view, right side of cerebellum and part of right temporal lobe removed) © 2013 Pearson Education, Inc. Figure 19.23a Arteries of the right upper limb and thorax. R. common L. common carotid carotid artery R. vertebral artery artery Thyrocervical trunk L. vertebral artery Suprascapular artery L. subclavian artery R. subclavian artery. Axillary artery Thoracoacromial artery Thoracoacromial artery Costocervical Aortic arch (pectoral trunk branch) Anterior Brachiocephalic and posterior trunk circumflex humeral Internal arteries thoracic artery Brachial artery Anterior intercostal arteries Deep Lateral artery thoracic artery Thoracic Posterior of arm intercostal aorta Subscapular arteries artery Anastomosis Common interosseus artery Radial Ulnar artery artery Deep palmar arch Metacarpal arteries Superficial palmar arch Digital arteries © 2013 Pearson Education, Inc. Schematic flowchart Figure 19.23b Arteries of the right upper limb and thorax. Vertebral Common carotid Thyrocervical artery arteries Right subclavian trunk Costocervical artery Suprascapular trunk Left subclavian