Chapter 9 Circulatory Systems Sections 9.8-9.16

9.11 Circulatory Vessels: Arteries Arteries provide rapid passage of blood from the heart to the tissues Large radii offer little resistance to flow Blood flows at high velocity Arteries serve as pressure reservoirs Arteries’ elasticity enables them to expand during ventricular systole Elastic recoil is the driving force for continued flow of blood during diastole 2

9.11 Circulatory Vessels: Arteries 3

Elastin fibers FIGURE 9-42 Elastin fibers in an artery. Light micrograph of a portion of the aorta wall in cross section, showing numerous wavy elastin fibers, common to all arteries. Figure 9-42 p430 4

9.11 Circulatory Vessels: Arteries Arterial blood pressure Maximum pressure exerted on the arteries (systolic blood pressure) averages 120 mmHg in humans Minimum pressure (diastolic blood pressure) averages 80 mmHg Arterial blood pressure is expressed as a fraction (e.g. 120/80 mmHg) Mean arterial pressure is the main driving force of blood flow Mean arterial pressure = diastolic pressure + 1/3 (systolic pressure - diastolic pressure) 5

Arterial pressure (mm Hg) v 120 Systolic pressure Pulse pressure Arterial pressure (mm Hg) Mean pressure 93 FIGURE 9-44 Arterial blood pressure. Values are for an average human. The systolic pressure is the peak pressure exerted in the arteries when blood is pumped into them during ventricular systole. The diastolic pressure is the lowest pressure exerted in the arteries when blood is draining off into the vessels downstream during ventricular diastole. The pulse pressure is the difference between systolic and diastolic pressure. The mean pressure is the average pressure throughout the cardiac cycle. 80 Diastolic pressure Time (msec) Figure 9-44 p432 6

9.12 Circulatory Vessels: Arterioles Arterioles are the major resistance vessels Radii of arterioles are small enough to offer considerable resistance to flow Large drop in blood pressure through the arterioles Mean arterial pressure of 93 mmHg drops to 37 mmHg where blood enters the capillaries Eliminates pulsatile pressure swings Thick layer of smooth muscle is innervated by sympathetic nerve fibers Vasoconstriction results from smooth muscle contraction —-> decreased radius, increased resistance Vasodilation results from smooth muscle relaxation —-> increased radius, decreased resistance 7

9.12 Circulatory Vessels: Arterioles Distribution of blood flow Amount of blood flow received by each organ is determined by the number and diameter of its arterioles Local (intrinsic) controls are changes within a tissue that alter the radii of arterioles Local metabolic changes produce relaxation of arteriolar smooth muscle to increase blood flow to the organ (active hyperemia) Histamine release causes vasodilation as an inflammatory response Exposure to heat or cold Stretch produces myogenic vasoconstriction 8

9.12 Circulatory Vessels: Arterioles Extrinsic control includes neural and hormonal influences with the sympathetic nervous system dominating Sympathetic stimulation redistributes blood flow to the heart and skeletal muscle at the expense of other organs Vasoconstriction of most arterioles due to activation of α1-adrenergic receptors by NE or epinephrine Lung and brain arterioles do not have α1-receptors and do not constrict Epinephrine activates β2-adrenergic receptors in arterioles of skeletal muscle and heart to cause vasodilation Mean arterial pressure is the product of cardiac output and total peripheral resistance 9

Total peripheral resistance Arteriolar radius Blood viscosity Number of red blood cells Extrinsic control (important in regulation of blood pressure) Response to shear stress (compensates for changes in longitudinal Force to blood flow) Local (intrinsic) control (local changes acting on arteriolar smooth muscle in the vicinity) Vasopressin (hormone important in fluid balance; exerts vasoconstrictor effect) Myogenic responses to stretch (important in autoregulation) Angiotensin II (hormone important in fluid balance; exerts vasoconstrictor effect) FIGURE 9-48 Factors affecting total peripheral resistance. The primary determinant of total peripheral resistance is the adjustable arteriolar radius. Two major categories of factors influence arteriolar radius: (1) local (intrinsic) control, which is primarily important in matching blood flow through a tissue with the tissue’s metabolic needs and is mediated by local factors acting on the arteriolar smooth muscle, and (2) extrinsic control, which is important in the regulation of blood pressure and is mediated primarily by sympathetic influence on arteriolar smooth muscle. Heat, cold application (therapeutic use) Epinephrine (hormone that generally reinforce sympathetic nervous system) Histamine release (involved with injuries and allergic responses) Local metabolic changes In O2 and other Metabolites (important in matching blood flow with metabolic needs) Sympathetic activity (exerts generalized vasoconstrictor effect) Figure 9-48 p438 Major factors affecting arteriolar radius 10

9.13 Circulatory Vessels: Capillaries Capillaries maximize diffusion rates for exchange of materials Diffusion distance is short Capillary walls are thin Capillaries are narrow Capillaries branch extensively Surface area is large Permeability is high Pores between capillary endothelial cells Tight junctions in brain capillaries form protective blood-brain barrier Large pores in liver capillaries allow passage of proteins 11

Endothelial cell nucleus Capillary lumen FIGURE 9-49 Capillary anatomy. (a) Electron micrograph of a cross section of a capillary. The capillary wall consists of a single layer of endothelial cells. The nucleus of one of these cells is shown. (b) Photograph of a capillary bed. The capillaries are so narrow that the red blood cells must pass through single file. (a) Cross section of a capillary Figure 9-49a p439 12

Capillary Red blood cell (b) Capillary bed FIGURE 9-49 Capillary anatomy. (a) Electron micrograph of a cross section of a capillary. The capillary wall consists of a single layer of endothelial cells. The nucleus of one of these cells is shown. (b) Photograph of a capillary bed. The capillaries are so narrow that the red blood cells must pass through single file. (b) Capillary bed Figure 9-49b p439 13

9.13 Circulatory Vessels: Capillaries 14

(b) Transport across a typical capillary wall Interstitial fluid Endothelial cell Water-filled pore Plasma proteins generally cannot cross the capillary wall Plasma Plasma proteins Plasma membrane Lipid-soluble substances pass through the endothelial cells typical Cytoplasm O2, CO2 Exchangeable proteins Na+, K+, glucose, amino acids FIGURE 9-50 Exchanges across a continuous capillary wall, the most common type of capillary. (a) Slitlike gaps between adjacent endothelial cells form pores within the capillary wall. (b) As depicted in this cross section of a capillary wall, small water-soluble substances are exchanged between the plasma and the interstitial fluid by passing through the water-filled pores, whereas lipid-soluble substances are exchanged across the capillary wall by passing through the endothelial cells. Proteins to be moved across are exchanged by vesicular transport. Plasma proteins generally cannot escape from the plasma across the capillary wall. Exchangeable proteins are moved across by vesicular transport Small water-soluble substances pass through the pores (b) Transport across a typical capillary wall Figure 9-50 p439 15

9.13 Circulatory Vessels: Capillaries Diffusion follows concentration gradients between blood and the surrounding cells Cells consume glucose and oxygen Cells produce CO2 and other metabolic wastes Precapillary sphincters controlling flow through individual capillaries are sensitive to local metabolic changes Blood velocity is slowest due to the very large cross-sectional area of the capillaries 16

Precapillary sphincter Metarteriole (thoroughfare channel) Smooth muscle Precapillary sphincter Metarteriole (thoroughfare channel) Capillary FIGURE 9-52 Capillary bed. Capillaries branch either directly from an arteriole or from a metarteriole, a thoroughfare channel between an arteriole and venule. Capillaries rejoin at either a venule or a metarteriole. Smooth muscle cells form precapillary sphincters that encircle capillaries as they arise from a metarteriole. (a) When the precapillary sphincters are relaxed, blood flows through the entire capillary bed. (b) When the precapillary sphincters are contracted, blood flows only through the metarteriole, bypassing the capillary bed. Venule (a) Sphincters relaxed Figure 9-52 p440 17

9.14 Circulatory Vessels: Lymphatic System The lymphatic system returns excess filtered fluid to the blood. Network of one-way vessels Interstitial fluid enters small blind-ended terminal vessels (initial lymphatics) to form lymph Lymph vessels flow into the venous system near the heart Passage of lymph through the lymph nodes aids in immune defense against disease Lymph vessels absorb fat form the digestive tract 18

9.14 Circulatory Vessels: Lymphatic System 19

(a) Relationship between initial lymphatics and blood capillaries To venous system Arteriole Tissue cells Interstitial fluid Venule FIGURE 9-58 Initial lymphatics. (a) Blind-ended initial lymphatics pick up excess fluid filtered by blood capillaries and return it to the venous system in the chest. (b) Note that the overlapping edges of the endothelial cells create valvelike openings in the vessel wall. Blood capillary Initial lymphatic (a) Relationship between initial lymphatics and blood capillaries Figure 9-58a p445 20

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9.15 Circulatory Vessels: Venules and Veins Veins serve as a blood reservoir and return blood to the heart. Veins are large in diameter with little elasticity and low myogenic tone. Easily distend to accommodate additional volumes of blood with no recoil (capacitance vessels) Mammalian veins contain more than 60% of the total blood volume Blood velocity accelerates on return from capillaries 22

9.15 Circulatory Vessels: Venules and Veins 23

9.15 Circulatory Vessels: Venules and Veins Factors enhancing venous return to the heart Driving pressure of cardiac contraction Sympathetic activity produces venous vasoconstriction Repeated contraction of skeletal muscles compresses veins (skeletal muscle pump) Venous valves prevent blood from flowing backward (away from the heart) Respiratory activity reduces pressure near the heart (respiratory pump) Cardiac suction due to low ventricular pressures during diastole 24

(b) Action of venous valves, permitting flow of blood toward Open venous valve permits flow of blood toward heart Vein Contracted skeletal muscle Closed venous valve prevents backflow of blood FIGURE 9-61 Function of venous valves. (a) When a tube is squeezed in the middle, fluid is pushed in both directions. (b) Venous valves permit the flow of blood only toward the heart. (b) Action of venous valves, permitting flow of blood toward heart and preventing backflow of blood Figure 9-61b p448 25

Mean arterial blood pressure 1 1 Total peripheral resistance Cardiac output 2 2 15 15 Heart rate Stroke volume Arteriolar radius Blood viscosity 3 6 17 16 4 5 20 Para-sympathetic activity Sympathetic activity and epinephrine 7 10 Cardiac- suction effect Local metabolic control Extrinsic vaso-constrictor control Number of red blood cells Venous return FIGURE 9-67 Determinants of mean arterial blood pressure. Note that this figure is basically a composite of Figure 9-29, p. 417, “Control of cardiac output”; Figure 9-48, p. 438, “Factors affecting total peripheral resistance”; and Figure 9-60, p. 447, “Factors that facilitate venous return.” See the text for a discussion of the circled numbers. 11 9 8 18 19 21 Skeletal muscle activity Sympathetic activity and epinephrine Vasopressin and angiotensin II Blood volume Respiratory activity 22 12 13 Salt and water balance Vasopressin, renin–angiotensin– aldosterone system (Chapters 14 and 15) Passive bulk-flow fluid shifts between vascular and interstitial fluid compartments 14 Figure 9-67 p454 26

9.16 Integrated Cardiovascular Function Cardiovascular function is regulated in an integrated fashion Two major goals: Proper gas and heat transport Maintaining arterial blood pressure Cardiac output is homeostatically regulated when an animal is at rest to maintain consistent delivery of oxygen to key organs. During activity, cardiac output is reset higher to boost blood flow to skeletal muscles and skin. 27

9.16 Integrated Cardiovascular Function Regulation of arterial blood pressure Pressure must be high enough to overcome gravity, friction and other resistance factors Pressure must be high enough for ultrafiltration in the kidneys Pressure must not be so high that it creates extra work for the heart and increases the risk of vascular damage. Chronic high blood pressure (hypertension) is due to excess resistance 28

9.16 Integrated Cardiovascular Function 29

(a) Baroreceptor reflex in response to an elevation in blood pressure When blood pressure becomes elevated above normal Carotid sinus and aortic arch receptor potential Rate of firing in afferent nerves Cardiovascular center Heart rate and stroke volume arteriolar and venous vasodilation Blood pressure decreased toward normal Sympathetic cardiac nerve activity and sympathetic vasoconstrictor nerve activity parasympathetic nerve activity Cardiac output and total peripheral resistance (a) Baroreceptor reflex in response to an elevation in blood pressure Carotid sinus and aortic arch receptor potential When blood pressure falls below normal Rate of firing in afferent nerves Cardiovascular center FIGURE 9-65 Baroreflexes to restore the blood pressure to normal. Heart rate and stroke volume arteriolar and venous vasoconstriction Blood pressure increased toward normal Sympathetic cardiac nerve activity and sympathetic vasoconstrictor nerve activity parasympathetic nerve activity Cardiac output and total peripheral resistance (b) Baroreceptor reflex in response to a fall in blood pressure Figure 9-65 p451 30

9.16 Integrated Cardiovascular Function Response to increased locomotor activity Increased heart rate and stroke volume result in increased cardiac output Blood flow to skeletal muscle, heart and skin increases Anticipatory responses Increased locomotor activity induces cardiac and vascular changes before that activity leads to disturbances in blood gases. Perception of stress can trigger the fight-or-flight response 31