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Copyright © 2010 Pearson Education, Inc. Chapter 19 Blood Vessels
Copyright © 2010 Pearson Education, Inc. Blood Vessels Three major blood vessels Arteries: carry blood away from the heart; oxygenated except for pulmonary circulation and umbilical vessels of a fetus Capillaries: contact tissue cells and directly serve cellular needs Veins: carry blood toward the heart
Copyright © 2010 Pearson Education, Inc. Structure of Blood Vessel Walls Arteries and veins Tunica intima, tunica media, and tunica externa Lumen Central blood- containing space
Copyright © 2010 Pearson Education, Inc. Tunics Tunica intima Endothelium lines the lumen of all vessels In vessels larger than 1 mm, a subendothelial connective tissue basement membrane is present
Copyright © 2010 Pearson Education, Inc. Tunics Tunica media Smooth muscle and sheets of elastin Sympathetic vasomotor nerve fibers control vasoconstriction and vasodilation of vessels Tunica externa (tunica adventitia) Collagen fibers protect and reinforce Larger vessels contain vasa vasorum to nourish the external layer
Copyright © 2010 Pearson Education, Inc. Elastic (Conducting) Arteries Large thick-walled arteries with elastin in all three tunics Aorta and its major branches Large lumen offers low-resistance Act as pressure reservoirs—expand and recoil as blood is ejected from the heart
Copyright © 2010 Pearson Education, Inc. Muscular (Distributing) Arteries and Arterioles Distal to elastic arteries; deliver blood to body organs Have thick tunica media with more smooth muscle More responsible for vasoconstriction-less distentible Account for most of the named arteries
Copyright © 2010 Pearson Education, Inc. Arterioles Smallest arteries Lead to capillary beds Tunica media is mostly smooth muscle Control flow into capillary beds via vasodilation and vasoconstriction
Copyright © 2010 Pearson Education, Inc. Capillaries Smallest blood vessels Contain just a thin tunica intima, one cell thick Pericytes help stabilize their walls and control permeability Size allows only a single RBC to pass at a time In all tissues except for cartilage, epithelia, cornea and lens of eye – poorly vascularized Functions: exchange of gases, nutrients, wastes, hormones, etc.
Copyright © 2010 Pearson Education, Inc. Continuous Capillaries Abundant in the skin and muscles Tight junctions connect endothelial cells Intercellular clefts allow the passage of fluids and small solutes Continuous capillaries of the brain Tight junctions are complete, forming the blood-brain barrier
Copyright © 2010 Pearson Education, Inc. Figure 19.3a Red blood cell in lumen Intercellular cleft Endothelial cell Endothelial nucleus Tight junction Pinocytotic vesicles Pericyte Basement membrane (a) Continuous capillary. Least permeable, and most common (e.g., skin, muscle).
Copyright © 2010 Pearson Education, Inc. Fenestrated Capillaries Some endothelial cells contain pores (fenestrations) More permeable than continuous capillaries Function in absorption or filtrate formation (small intestines, endocrine glands, and kidneys)
Copyright © 2010 Pearson Education, Inc. Figure 19.3b Red blood cell in lumen Intercellular cleft Fenestrations (pores) Endothelial cell Endothelial nucleus Basement membrane Tight junction Pinocytotic vesicles (b) Fenestrated capillary. Large fenestrations (pores) increase permeability. Occurs in special locations (e.g., kidney, small intestine).
Copyright © 2010 Pearson Education, Inc. Sinusoidal Capillaries (Sinusoids) Fewer tight junctions, larger intercellular clefts, large lumens Usually fenestrated Allow large molecules and blood cells to pass between the blood and surrounding tissues Found in the liver, bone marrow, spleen Kupffer cells – macrophages in liver vessels
Copyright © 2010 Pearson Education, Inc. Figure 19.3c Nucleus of endothelial cell Red blood cell in lumen Endothelial cell Tight junction Incomplete basement membrane Large intercellular cleft (c) Sinusoidal capillary. Most permeable. Occurs in special locations (e.g., liver, bone marrow, spleen).
Copyright © 2010 Pearson Education, Inc. Capillary Beds Interwoven networks of capillaries Microcirculation – Flow of blood from arterioles to venules Consist of two types of vessels 1.Vascular shunt (metarteriole—thoroughfare channel): Directly connects the terminal arteriole and a postcapillary venule 2.True capillaries 10 to 100 exchange vessels per capillary bed Branch off the metarteriole or terminal arteriole
Copyright © 2010 Pearson Education, Inc. Figure 19.4 (a) Sphincters open —blood flows through true capillaries. (b) Sphincters closed —blood flows through metarteriole thoroughfare channel and bypasses true capillaries. Precapillary sphincters Metarteriole Vascular shunt Terminal arteriolePostcapillary venule Terminal arteriolePostcapillary venule Thoroughfare channel True capillaries
Copyright © 2010 Pearson Education, Inc. Venules Formed when capillary beds unite Very porous; allow fluids and WBCs into tissues Postcapillary venules consist of endothelium and a few pericytes Larger venules have one or two layers of smooth muscle cells
Copyright © 2010 Pearson Education, Inc. Veins Formed when venules converge Have thinner walls, larger lumens compared with corresponding arteries Blood pressure is lower than in arteries Thin tunica media and a thick tunica externa consisting of collagen fibers and elastic networks Called capacitance vessels (blood reservoirs); contain up to 65% of the blood supply
Copyright © 2010 Pearson Education, Inc. Figure 19.1a Artery Vein (a)
Copyright © 2010 Pearson Education, Inc. Veins Adaptations that ensure return of blood to the heart 1.Large-diameter lumens offer little resistance 2.Valves prevent backflow of blood Most abundant in veins of the limbs Varicose veins, hemorrhoids Venous sinuses: flattened veins with extremely thin walls (e.g., coronary sinus of the heart and dural sinuses of the brain)
Copyright © 2010 Pearson Education, Inc. Vascular Anastomoses Interconnections of blood vessels Arterial anastomoses provide alternate pathways (collateral channels) to a given body region Common at joints, in abdominal organs, brain, and heart Vascular shunts of capillaries are examples of arteriovenous anastomoses Venous anastomoses are common in skin
Copyright © 2010 Pearson Education, Inc. Atherosclerosis Arteriosclerosis Thickening and stiffening of arteries Results in hypertension Atherosclerosis – most common Development of plaque deposits Most often in aorta and carotid arteries MI, stroke, aneurysm
Copyright © 2010 Pearson Education, Inc. Physiology of Circulation: Definition of Terms Blood flow Volume of blood flowing through a vessel, an organ, or the entire circulation in a 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
Copyright © 2010 Pearson Education, Inc. Physiology of Circulation: Definition of Terms Blood pressure (BP) Force per unit area 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 to lower pressure areas
Copyright © 2010 Pearson Education, Inc. Physiology of Circulation: Definition of Terms Resistance (peripheral resistance) Opposition to flow Measure of the amount of friction blood encounters Generally encountered in the peripheral systemic circulation Three important sources of resistance Blood viscosity Total blood vessel length Blood vessel diameter
Copyright © 2010 Pearson Education, Inc. Resistance Factors that remain relatively constant: 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 Blood vessel diameter can change frequently increasing or decreasing diameter
Copyright © 2010 Pearson Education, Inc. Resistance Small-diameter arterioles are the major determinants of peripheral resistance Abrupt changes in diameter or fatty plaques from atherosclerosis dramatically increase resistance Disrupt laminar flow and cause turbulence
Copyright © 2010 Pearson Education, Inc. Relationship Between Blood Flow, Blood Pressure, and Resistance Blood flow (F) is directly proportional to the blood 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
Copyright © 2010 Pearson Education, Inc. Systemic Blood Pressure 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 arterioles
Copyright © 2010 Pearson Education, Inc. Arterial Blood Pressure Reflects two factors of the arteries close to the heart Elasticity (compliance or distensibility) Volume of blood forced into them at any time Blood pressure near the heart is pulsatile
Copyright © 2010 Pearson Education, Inc. Arterial Blood Pressure Systolic pressure: pressure exerted during ventricular contraction Diastolic pressure: lowest level of arterial pressure 120/80 mm Hg Pulse pressure = difference between systolic and diastolic pressure Feel in the wrist and neck to assess pulse
Copyright © 2010 Pearson Education, Inc. Arterial Blood Pressure Mean arterial pressure (MAP): pressure that propels the blood to the tissues MAP = diastolic pressure + 1/3 pulse pressure Diastole lasts longer than systole so MAP is not just halfway between the two Pulse pressure and MAP both decline with increasing distance from the heart
Copyright © 2010 Pearson Education, Inc. Capillary Blood Pressure Ranges from 35 (at beginning of capillaries) to 15 mm Hg (at capillary beds) 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
Copyright © 2010 Pearson Education, Inc. Venous Blood Pressure Changes little during the cardiac cycle Small pressure gradient, about 15 mm Hg Low pressure due to cumulative effects of peripheral resistance
Copyright © 2010 Pearson Education, Inc. Factors Aiding Venous Return 1.Respiratory “pump”: pressure changes created during breathing move blood toward the heart by squeezing abdominal veins as thoracic veins expand 2.Muscular “pump”: contraction of skeletal muscles “milk” blood toward the heart and valves prevent backflow 3.Vasoconstriction of veins under sympathetic control
Copyright © 2010 Pearson Education, Inc. Maintaining Blood Pressure Requires Cooperation of the heart, blood vessels, and kidneys Supervision by the brain The main factors influencing blood pressure: Cardiac output (CO) Peripheral resistance (PR) Blood volume
Copyright © 2010 Pearson Education, Inc. Figure 19.8 Venous return Exercise Contractility of cardiac muscle Sympathetic activity Parasympathetic activity Epinephrine in blood EDVESV Stroke volume (SV) Heart rate (HR) Cardiac output (CO = SV x HR Activity of respiratory pump (ventral body cavity pressure) Activity of muscular pump (skeletal muscles) Sympathetic venoconstriction BP activates cardiac centers in medulla Initial stimulus Result Physiological response
Copyright © 2010 Pearson Education, Inc. Control of Blood Pressure Short-term neural and hormonal controls Neuronal controls Maintain MAP by altering blood vessel diameter Alter blood distribution in response to specific demands (i.e. exercise) Reflex arcs that involve Baroreceptors Vasomotor centers and vasomotor fibers Vascular smooth muscle
Copyright © 2010 Pearson Education, Inc. Short-Term Mechanisms: Baroreceptor- Initiated Reflexes Baroreceptors are located in Carotid sinuses Aortic arch Walls of large arteries of the neck and thorax Message to vasomotor center when stretched – vasodialation of arterioles and veins - ↓BP ↓MAP initiates reflexive vascostriction, ↑BP
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
Copyright © 2010 Pearson Education, Inc. Short-Term Mechanisms: Chemoreceptor-Initiated Reflexes Chemoreceptors respond to rise in CO 2, drop in pH or O 2 Increase blood pressure, increase CO 2 Increase vasoconstriction Are more important in the regulation of respiratory rate
Copyright © 2010 Pearson Education, Inc. Short-Term Mechanisms: Hormonal Controls Adrenal medulla hormones norepinephrine (NE) and epinephrine cause generalized vasoconstriction and increase cardiac output Angiotensin II, generated by kidney release of renin, causes vasoconstriction Atrial natriuretic peptide (ANP) causes blood volume and blood pressure to decline, causes generalized vasodilation Antidiuretic hormone (ADH)(vasopressin) causes intense vasoconstriction in cases of extremely low BP
Copyright © 2010 Pearson Education, Inc. Long-Term Mechanisms: Renal Regulation Long-term mechanisms step in to control BP by altering blood volume Kidneys act directly and indirectly to regulate arterial blood pressure 1.Direct renal mechanism 2.Indirect renal (renin-angiotensin) mechanism
Copyright © 2010 Pearson Education, Inc. 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 BP rises
Copyright © 2010 Pearson Education, Inc. Indirect Mechanism The renin-angiotensin mechanism Arterial blood pressure release of renin Renin production of angiotensin II 1.Angiotensin II is a potent vasoconstrictor 2.Angiotensin II aldosterone secretion Aldosterone renal reabsorption of Na + and urine formation 3.Angiotensin II stimulates ADH release
Copyright © 2010 Pearson Education, Inc. Monitoring Circulatory Efficiency Vital signs: pulse and blood pressure, along with respiratory rate and body temperature Pulse: pressure wave caused by the expansion and recoil of arteries Radial pulse (taken at the wrist) routinely used
Copyright © 2010 Pearson Education, Inc. Measuring Blood Pressure Systemic arterial BP Measured indirectly by the auscultatory method using a sphygmomanometer 1.Pressure is increased in the cuff until it exceeds systolic pressure in the brachial artery 2.Pressure is released slowly and the examiner listens for sounds of Korotkoff with a stethoscope 3.Sounds first occur as blood starts to spurt through the artery (systolic pressure, normally 110–140 mm Hg) 4.Sounds disappear when the artery is no longer constricted and blood is flowing freely (diastolic pressure, normally 70– 80 mm Hg)
Copyright © 2010 Pearson Education, Inc. Variations in Blood Pressure Blood pressure cycles over a 24-hour period BP peaks in the morning due to levels of hormones Age, sex, weight, race, mood, and posture may vary BP
Copyright © 2010 Pearson Education, Inc. Alterations in Blood Pressure Hypotension: low blood pressure Systolic pressure below 100 mm Hg Often associated with long life and lack of cardiovascular illness
Copyright © 2010 Pearson Education, Inc. Alterations in Blood Pressure Hypertension: high blood pressure Sustained elevated arterial pressure of 140/90 or higher May be transient adaptations during fever, physical exertion, and emotional upset Often persistent in obese people
Copyright © 2010 Pearson Education, Inc. Blood Flow Through Body Tissues Blood flow (tissue perfusion) is involved in Delivery of O 2 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 the right amount to provide for proper function
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 Is controlled intrinsically by modifying the diameter of local arterioles feeding the capillaries Is independent of MAP, which is controlled as needed to maintain constant pressure
Copyright © 2010 Pearson Education, Inc. Blood Flow Skeletal muscle – varies with fiber type and activity (greater in red fiber than white fiber) Brain – averages 750 ml/min and relatively constant Skin – supplies nutrients, aids in temperature regulation, also a blood reservoir Lungs – opposite the systemic circuit in many ways Heart – averages 250 ml/min and fairly constant
Copyright © 2010 Pearson Education, Inc. Capillary Exchange of Respiratory Gases and Nutrients Diffusion of O 2 and nutrients from the blood to tissues CO 2 and metabolic wastes from tissues to the 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
Copyright © 2010 Pearson Education, Inc. Circulatory Shock Any condition in which Blood vessels are inadequately filled Blood cannot circulate normally Results in inadequate blood flow to meet tissue needs
Copyright © 2010 Pearson Education, Inc. Circulatory Pathways Two main circulations Pulmonary circulation: short loop that runs from the heart to the lungs and back to the heart Systemic circulation: long loop to all parts of the body and back to the heart
Copyright © 2010 Pearson Education, Inc. Figure 19.19a R. pulmon- ary veins Pulmonary trunk Pulmonary capillaries of the R. lung Pulmonary capillaries of the L. lung R. pulmonary artery L. pulmonary artery To systemic circulation L. pulmonary veins (a) Schematic flowchart. From systemic circulation RA RVLV LA
Copyright © 2010 Pearson Education, Inc. ArteriesVeins DeliveryBlood pumped into single systemic artery—the aorta Blood returns via superior and interior venae cavae and the coronary sinus LocationDeep, and protected by tissuesBoth deep and superficial PathwaysFairly distinctNumerous interconnections Supply/drainagePredictable supplyUsually similar to arteries, except dural sinuses and hepatic portal circulation Differences Between Arteries and Veins
Copyright © 2010 Pearson Education, Inc. Arteries of the body
Copyright © 2010 Pearson Education, Inc. Veins of the Body
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