Copyright © 2010 Pearson Education, Inc. Classes of Blood Vessels Arteries Carry blood away from heart Arterioles Are smallest branches of arteries Capillaries Are smallest blood vessels Location of exchange between blood and interstitial fluid Venules Collect blood from capillaries Veins Return blood to heart

Copyright © 2011 Pearson Education, Inc. Oxygenated blood from heart Deoxygenated blood to heart Connective tissue Smooth muscle Endothelium Lumen Microcirculation Valve Network of capillaries Basement membrane Endothelium Arteriole Artery Vein Venule Capillary Vascular Components

Copyright © 2010 Pearson Education, Inc. Generalized Structure of Blood Vessels Arteries and veins are composed of three tunics tunica interna, tunica media tunica externa Capillaries are composed of endothelium with sparse basal lamina

Copyright © 2010 Pearson Education, Inc. Structure of vessel walls The walls of blood vessels are too thick to allow diffusion between blood stream and surrounding tissues or the tissues of the blood vessels. The walls of large vessels contain small blood vessels that supply both tunica media and externa – vasa vasorum

Copyright © 2010 Pearson Education, Inc. Capillaries Capillaries are the smallest blood vessels Walls consisting of a thin tunica interna, one cell thick Allow only a single RBC to pass at a time Pericytes on the outer surface stabilize their walls There are three structural types of capillaries: continuous, fenestrated, and sinusoids

Copyright © 2010 Pearson Education, Inc. Continuous Capillaries Continuous capillaries are abundant in the skin and muscles Endothelial cells provide an uninterrupted lining Adjacent cells are connected with tight junctions Intercellular clefts allow the passage of fluids Continuous capillaries of the brain: Have tight junctions completely around the endothelium Constitute the blood-brain barrier

Copyright © 2010 Pearson Education, Inc. Fenestrated Capillaries Found wherever active capillary absorption or filtrate formation occurs (e.g., small intestines, endocrine glands, and kidneys) Characterized by: An endothelium riddled with pores (fenestrations) Greater permeability than other capillaries

Copyright © 2011 Pearson Education, Inc.

Copyright © 2010 Pearson Education, Inc. Sinusoids Highly modified, leaky, fenestrated capillaries with large lumens Found in the liver, bone marrow, lymphoid tissue, and in some endocrine organs Allow large molecules (proteins and blood cells) to pass between the blood and surrounding tissues

Copyright © 2010 Pearson Education, Inc. Capillary Beds Vascular shunts – Metarteriole--is a vessel that emerges from an arteriole, passes through the capillary network and empties into a venule. Proximal portions are surrounded by scattered smooth muscle cells whose contraction and relaxation help regulate the amount and force of the blood. Distal portion has no smooth muscle fibers and is called a thoroughfare channel. True capillaries – 10 to 100 per capillary bed, capillaries branch off the metarteriole and return to the thoroughfare channel at the distal end of the bed At their site of origin, there is a ring of smooth muscle fibers called a precapillary sphincter that controls the flow of blood entering a true capillary

Copyright © 2010 Pearson Education, Inc. Capillary Beds Figure 19.4a

Copyright © 2010 Pearson Education, Inc. Capillary Beds Figure 19.4b

Copyright © 2010 Pearson Education, Inc. Blood Flow The purpose of cardiovascular regulation is to maintain adequate blood flow through the capillaries to the tissues Actual volume of blood flowing through a vessel, an organ, or the entire circulation in a given period: Is measured in ml/min. Is equivalent to cardiac output (CO), considering the entire vascular system Is relatively constant when at rest Varies widely through individual organs

Copyright © 2010 Pearson Education, Inc. Blood Flow Through Tissues Blood flow, or tissue perfusion, is involved in: Delivery of oxygen and nutrients to, and removal of wastes from, tissue cells Gas exchange in the lungs Absorption of nutrients from the digestive tract Urine formation by the kidneys Blood flow is precisely the right amount to provide proper tissue function

Copyright © 2010 Pearson Education, Inc. Circulatory Shock Circulatory shock – any condition in which blood vessels are inadequately filled and blood cannot circulate normally Results in inadequate blood flow to meet tissue needs Three types include: Hypovolemic shock – results from large-scale blood loss or dehydration Vascular shock – normal blood volume but too much accumulate in the limbs (long period of standing/sitting) Cardiogenic shock – the heart cannot sustain adequate circulation

Copyright © 2010 Pearson Education, Inc. Blood flow Capillary blood flow is determined by the interplay between: Pressure Resistance The heart must generate sufficient pressure to overcome resistance

Copyright © 2010 Pearson Education, Inc. Physiology of Circulation: Blood Pressure (BP) Force per unit area exerted on the wall of a blood vessel by its contained blood Expressed in millimeters of mercury (mm Hg) Measured in reference to systemic arterial BP in large arteries near the heart The differences in BP within the vascular system provide the driving force that keeps blood moving from higher to lower pressure areas Absolute pressure is less important than pressure gradient

Copyright © 2011 Pearson Education, Inc. A model that relates blood flow to the pressure gradient

Copyright © 2011 Pearson Education, Inc. A model that relates blood flow to the pressure gradient

Copyright © 2011 Pearson Education, Inc. A model that relates blood flow to the pressure gradient

Copyright © 2010 Pearson Education, Inc. Physiology of Circulation: Resistance Opposition to flow Measure of the amount of friction blood encounters Generally encountered in the peripheral systemic circulation Because the resistance of the venous system is very low (why?) usually the focus is on the resistance of the arterial system: Peripheral resistance (PR) is the resistance of the arterial system

Copyright © 2010 Pearson Education, Inc. Physiology of Circulation: Resistance Three important sources of resistance Blood viscosity Total blood vessel length Blood vessel diameter

Copyright © 2010 Pearson Education, Inc. Resistance – constant factors Blood viscosity The “stickiness” of the blood due to formed elements and plasma proteins Blood vessel length The longer the vessel, the greater the resistance encountered

Copyright © 2010 Pearson Education, Inc. Resistance – frequently changed factors Changes in vessel diameter are frequent and significantly alter peripheral resistance Small-diameter arterioles are the major determinants of peripheral resistance Frequent changes alter peripheral resistance Fatty plaques from atherosclerosis: Cause turbulent blood flow Dramatically increase resistance due to turbulence

Copyright © 2011 Pearson Education, Inc. The effect of resistance on flow

Copyright © 2010 Pearson Education, Inc. Systemic Blood Pressure The pumping action of the heart generates blood flow through the vessels along a pressure gradient, always moving from higher- to lower-pressure areas Pressure results when flow is opposed by resistance Systemic pressure: Is highest in the aorta Declines throughout the length of the pathway Is ~0 mm Hg in the right atrium The steepest change in blood pressure occurs between arteries to arterioles

Copyright © 2010 Pearson Education, Inc. Arterial Blood Pressure Arterial pressure is important because it maintains blood flow through capillaries Blood pressure near the heart is pulsatile Systolic pressure: pressure exerted during ventricular contraction Diastolic pressure: lowest level of arterial pressure

Copyright © 2010 Pearson Education, Inc. Arterial Blood Pressure A pulse is rhythmic pressure oscillation that accompanies every heartbeat Pulse pressure = difference between systolic and diastolic pressure Mean arterial pressure (MAP): pressure that propels the blood to the tissues MAP = diastolic pressure + 1/3 pulse pressure Pulse pressure and MAP both decline with increasing distance from the heart Efficiency of the circulation can be assessed by taking pulse and blood pressure measurements

Copyright © 2010 Pearson Education, Inc. Factors that Influence Mean Arterial Pressure Figure 15-10

Copyright © 2011 Pearson Education, Inc. How increases in cardiac output and total peripheral resistance increase mean arterial pressure

Copyright © 2010 Pearson Education, Inc. Alterations in Blood Pressure Hypotension – low BP in which systolic pressure is below 100 mm Hg Hypertension – condition of sustained elevated arterial pressure of 140/90 or higher Transient elevations are normal and can be caused by fever, physical exertion, and emotional upset Chronic elevation is a major cause of heart failure, vascular disease, renal failure, and stroke

Copyright © 2010 Pearson Education, Inc. Capillary Blood Pressure Ranges from 15 to 35 mm Hg Low capillary pressure is desirable High BP would rupture fragile, thin-walled capillaries Low BP is sufficient to force filtrate out into interstitial space and distribute nutrients, gases, and hormones between blood and tissues Capillary blood pressure is one of the driving forces for transport through the capillary wall (filtration)

Copyright © 2011 Pearson Education, Inc. Exchange of materials across the wall of a capillary Endothelial cell Capillary O 2, CO 2, fatty acids, Steroid hormones Pores Lumen PlasmaInterstitial fluid Transcytosis of exchangeable proteins across cells Diffusion through cells (lipid-soluble substances) Diffusion through pores (water-soluble substances) Restricted movement of most plasma proteins (cannot cross capillary wall) Na +, K +, glucose Water-filled pore Proteins Plasma membrane Cytoplasm Endothelial cell

Copyright © 2010 Pearson Education, Inc. Net Filtration Pressure (NFP) NFP—comprises all the forces acting on a capillary bed NFP = (HP c —HP if )—(OP c —OP if ) At the arterial end of a bed, hydrostatic forces dominate At the venous end, osmotic forces dominate Excess fluid is returned to the blood via the lymphatic system

Copyright © 2010 Pearson Education, Inc. Hydrostatic Pressures Capillary hydrostatic pressure (HP c ) (capillary blood pressure) Tends to force fluids through the capillary walls Is greater at the arterial end (35 mm Hg) of a bed than at the venule end (17 mm Hg) Interstitial fluid hydrostatic pressure (HP if ) Usually assumed to be zero because of lymphatic vessels

Copyright © 2010 Pearson Education, Inc. Colloid Osmotic Pressures Capillary colloid osmotic pressure (oncotic pressure) (OP c ) Created by non-diffusible plasma proteins, which draw water toward themselves ~26 mm Hg Interstitial fluid osmotic pressure (OP if ) Low (~1 mm Hg) due to low protein content

Copyright © 2010 Pearson Education, Inc. Figure HP = hydrostatic pressure Due to fluid pressing against a wall “Pushes” In capillary (HP c ) Pushes fluid out of capillary 35 mm Hg at arterial end and 17 mm Hg at venous end of capillary in this example In interstitial fluid (HP if ) Pushes fluid into capillary 0 mm Hg in this example OP = osmotic pressure Due to presence of nondiffusible solutes (e.g., plasma proteins) “Sucks” In capillary (OP c ) Pulls fluid into capillary 26 mm Hg in this example In interstitial fluid (OP if ) Pulls fluid out of capillary 1 mm Hg in this example Arteriole Capillary Interstitial fluid Net HP—Net OP (35—0)—(26—1) Net HP—Net OP (17—0)—(26—1) Venule NFP (net filtration pressure) is 10 mm Hg; fluid moves out NFP is ~8 mm Hg; fluid moves in Net HP 35 mm Net OP 25 mm Net HP 17 mm Net OP 25 mm CHP – mmHg IHP – 0 mmHg BCOP – 25 mmHg ICOP – 0 mmHg

Copyright © 2010 Pearson Education, Inc. Fluid Exchange at a Capillary Hydrostatic pressure and osmotic pressure regulate bulk flow Figure 15-19a

Copyright © 2010 Pearson Education, Inc. Venous Blood Pressure Changes little during the cardiac cycle Small pressure gradient, about 15 mm Hg

Copyright © 2010 Pearson Education, Inc. Factors Aiding Venous Return Venous BP alone is too low to promote adequate blood return and is aided by the: Respiratory “pump” – pressure changes created during breathing suck blood toward the heart by squeezing local veins Muscular “pump” – contraction of skeletal muscles “milk” blood toward the heart Valves prevent backflow during venous return

Copyright © 2010 Pearson Education, Inc. Controls of Blood Pressure Short-term controls: Counteract moment-to-moment fluctuations in blood pressure by altering peripheral resistance Long-term controls regulate blood volume

Copyright © 2010 Pearson Education, Inc. Central Regulation Stimulation of receptors sensitive to changes in systemic blood pressure or chemistry Endocrine mechanisms HOMEOSTASIS RESTORED Neural mechanisms Activation of cardiovascular centers Stimulation of endocrine response Long-term increase in blood volume and blood pressure Short-term elevation of blood pressure by sympathetic stimulation of the heart and peripheral vasoconstriction Central regulation involves neuroendocrine mechanisms that control the total systemic circulation. This regulation involves both the cardiovascular centers and the vasomotor centers. If autoregulation is ineffective

Copyright © 2010 Pearson Education, Inc. Short-Term Mechanisms to controls of Blood Pressure Autoregulation Causes immediate, localized homeostatic adjustments Neural mechanisms Respond quickly to changes at specific sites

Copyright © 2010 Pearson Education, Inc. Autoregulation – automatic adjustment of blood flow to each tissue in proportion to its requirements at any given point in time Local vasodilators accelerate blood flow in response to: Decreased tissue O 2 levels or increased CO 2 levels Generation of lactic acid Rising H + concentrations in interstitial fluid Local inflammation Elevated temperature Vasoconstrictors: Injured vessels constrict strongly (why?) Drop in tissue temperature (why?) Autoregulation of blood flow within tissues

Copyright © 2010 Pearson Education, Inc. Myogenic (myo =muscle; gen=origin) Controls Inadequate blood perfusion (tissue might die) or excessively high arterial pressure (rupture of vessels) may interfere with the function of the tissue Vascular smooth muscle can prevent these problems by responding directly to passive stretch (increased intravascular pressure) The muscle response is by resist to the stretch and that results in vasoconstriction The opposite happens when there is a reduced stretch The myogenic mechanisms keeps the tissue perforation relatively constant.

Copyright © 2011 Pearson Education, Inc. The myogenic response to changes in perfusion pressure

Copyright © 2010 Pearson Education, Inc. Short-Term Mechanisms: Neural Controls Neural controls of peripheral resistance: Alter blood distribution in response to demands Maintain MAP by altering blood vessel diameter Neural controls operate via reflex arcs involving: Vasomotor centers and vasomotor fibers Baroreceptors Vascular smooth muscle

Copyright © 2010 Pearson Education, Inc. Short-Term Mechanisms: Vasomotor Center Vasomotor center – a cluster of sympathetic neurons in the medulla that oversees changes in blood vessel diameter Maintains blood vessel tone by innervating smooth muscles of blood vessels, especially arterioles Vasomotor activity is modified by: Baroreceptors (pressure-sensitive) chemoreceptors (O 2, CO 2, and H + sensitive) bloodborne chemicals

Copyright © 2011 Pearson Education, Inc. Carotid bifurcation Carotid sinus Common carotid artery Aortic arch Arterial baroreceptors Short-Term Mechanisms: Baroreceptor-Initiated Reflexes

Copyright © 2010 Pearson Education, Inc. Short-Term Mechanisms: Baroreceptor-Initiated Reflexes Increased blood pressure stimulates the cardioinhibitory center to: Increase vessel diameter Decrease heart rate, cardiac output, peripheral resistance, and blood pressure Declining blood pressure stimulates the cardioacceleratory center to: Increase cardiac output and peripheral resistance Low blood pressure also stimulates the vasomotor center to constrict blood vessels

Copyright © 2010 Pearson Education, Inc. Medullary cardiovascular control center Carotid and aortic baroreceptors Change in blood pressure Parasympathetic neurons Sympathetic neurons Veins Arterioles Ventricles SA node Integrating center Stimulus Efferent path Effector Sensory receptor KEY Blood Pressure Figure (10 of 10)

Copyright © 2011 Pearson Education, Inc. Response of arterial baroreceptors to changes in arterial pressure

Copyright © 2010 Pearson Education, Inc. Responses to Increased Baroreceptor Stimulation Baroreceptors stimulated HOMEOSTASIS DISTURBED Rising blood pressure Cardioinhibitory centers stimulated Cardioacceleratory centers inhibited Vasomotor centers inhibited Decreased cardiac output Vasodilation occurs HOMEOSTASIS RESTORED Blood pressure declines HOMEOSTASIS Normal range of blood pressure Start

Copyright © 2010 Pearson Education, Inc. HOMEOSTASIS Normal range of blood pressure HOMEOSTASIS DISTURBED HOMEOSTASIS RESTORED Falling blood pressure Blood pressure rises Baroreceptors inhibited Vasoconstriction occurs Increased cardiac output Vasomotor centers stimulated Cardioacceleratory centers stimulated Cardioinhibitory centers inhibited Responses to Decreased Baroreceptor Stimulation Start

Copyright © 2010 Pearson Education, Inc. Short-Term Mechanisms: Chemoreceptor-Initiated Reflexes Chemoreceptors are located in the Carotid sinus Aortic arch Large arteries of the neck Chemoreceptors respond to rise in CO 2, drop in pH or O 2 Increase blood pressure via the vasomotor center and the cardioacceleratory center

Copyright © 2010 Pearson Education, Inc. Increasing CO 2 levels, decreasing pH and O 2 levels Respiratory centers in the medulla oblongata stimulated Respiratory rate increases Increased cardiac output and blood pressure Cardioacceleratory centers stimulated Cardioinhibitory centers inhibited Respiratory Response Cardiovascular Responses Effects on Cardiovascular Centers Vasomotor centers stimulated Vasoconstriction occurs Normal pH, O 2, and CO 2 levels in blood and CSF HOMEOSTASIS Elevated CO 2 levels, decreased pH and O 2 levels in blood and CSF HOMEOSTASIS DISTURBED Start Decreased CO 2 levels, increased pH and O 2 levels in blood and CSF HOMEOSTASIS RESTORED Reflex Response Chemoreceptors stimulated

Copyright © 2010 Pearson Education, Inc. Chemicals that Increase Blood Pressure Adrenal medulla hormones – norepinephrine and epinephrine increase blood pressure Antidiuretic hormone (ADH) – causes intense vasoconstriction in cases of extremely low BP Angiotensin II – kidney release of renin generates angiotensin II, which causes vasoconstriction Endothelium-derived factors – endothelin and prostaglandin-derived growth factor (PDGF) are both vasoconstrictors

Copyright © 2010 Pearson Education, Inc. Chemicals that Decrease Blood Pressure Atrial natriuretic peptide (ANP) – causes blood volume and pressure to decline Nitric oxide (NO) – is a brief but potent vasodilator Inflammatory chemicals – histamine, prostacyclin, and kinins are potent vasodilators Alcohol – causes BP to drop by inhibiting ADH

Copyright © 2010 Pearson Education, Inc. Long-Term Mechanisms: Renal Regulation Long-term mechanisms control BP by altering blood volume Increased BP stimulates the kidneys to eliminate water, thus reducing BP Decreased BP stimulates the kidneys to increase blood volume and BP Kidneys act directly and indirectly to maintain long-term blood pressure Direct renal mechanism alters blood volume (changes in urine volume) Indirect renal mechanism involves the renin-angiotensin mechanism

Copyright © 2010 Pearson Education, Inc. Indirect Mechanism: renin-angiotensin The renin-angiotensin mechanism  Arterial blood pressure  release of renin Renin  production of angiotensin II Angiotensin II is a potent vasoconstrictor Angiotensin II  aldosterone secretion Aldosterone  renal reabsorption of Na + and  urine formation Angiotensin II stimulates ADH release