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Chapter 20 Cardiovascular: Blood Vessels Blood Vessels Blood is carried in a closed system of vessels that begins and ends at the heart Three major types of vessels: – Arteries: carry blood away from the heart – Capillaries: contact tissue cells and directly serve cellular needs – Veins: carry blood...

Chapter 20 Cardiovascular: Blood Vessels Blood Vessels Blood is carried in a closed system of vessels that begins and ends at the heart Three major types of vessels: – Arteries: carry blood away from the heart – Capillaries: contact tissue cells and directly serve cellular needs – Veins: carry blood toward the heart Circulatory System 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 Artery and Vein Muscular (Distributing) Arteries and Arterioles Distal to elastic arteries; deliver blood to body organs Have thick tunica media with more smooth muscle Active in vasoconstriction Arterioles – Smallest arteries – Lead to capillary beds – Control flow into capillary beds via vasodilation and vasoconstriction Elastic Arteries vs Muscular Arteries vs Arterioles Circulatory System Arterial system Venous system Large veins (capacitance vessels) Heart Large lymphatic vessels Lymph node Lymphatic system Elastic arteries (conducting vessels) Muscular arteries (distributing vessels) Small veins (capacitance vessels) Postcapillary venule Arterioles (resistance vessels) Terminal arteriole Precapillary sphincter Capillaries (exchange vessels) Figure 19.2 Capillaries Microscopic blood vessels Size allows only a single RBC to pass at a time In all tissues except for – – – – cartilage epithelia cornea of eye lens of eye Functions: exchange of gases, nutrients, wastes, hormones, etc. Types of Capillaries Three structural types 1. Continuous capillaries 2. Fenestrated capillaries 3. Sinusoidal capillaries (sinusoids) Types of Capillaries Continuous Capillaries Abundant in the CNS, 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 bloodbrain barrier 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) Sinusoidal Capillaries Specialized “leaky” capillaries Fewer tight junctions, larger intercellular clefts, large lumens Function: allow large molecules and blood cells to pass between the blood and surrounding tissues Found in the liver, bone marrow, spleen Capillary Beds Networks of capillaries that form the microcirculation between arterioles and venules Consist of two types of vessels 1. Vascular shunt Directly connects the terminal arteriole and a postcapillary venule 2. True capillaries 10 to 100 exchange vessels per capillary bed Branch off the terminal arteriole Blood Flow Through Capillary Beds Precapillary sphincters regulate blood flow into true capillaries Regulated by local chemical conditions and vasomotor nerves Capillary Beds Venules Smallest of the venous system Very porous; allow fluids and WBCs into tissues Larger venules have one or two layers of smooth muscle cells Veins Formed when venules converge to form the larger veins Have thinner walls, larger lumens compared with corresponding arteries Blood pressure is lower than in arteries Blood reservoirs- contain up to 65% of the blood supply 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 Veins 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 Physiology of Circulation: Definition of Terms Blood pressure (BP) – The pressure gradient provides the driving force that keeps blood moving from higher to lower pressure areas – 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 Physiology of Circulation: Definition of Terms Resistance (peripheral resistance) – Measure of the amount of friction blood encounters – Generally encountered in the peripheral systemic circulation – Referred to as peripheral resistance (PR) Three important sources of resistance 1. Blood viscosity 2. Total blood vessel length 3. Blood vessel diameter Sources of Resistance Factors affecting resistance: 1. Blood viscosity: The “stickiness” of the blood due to formed elements and plasma proteins Greater viscosity = greater resistance 2. Total blood vessel length: Longer vessel = greater resistance 3. Blood vessel diameter: Smaller tube = greater resistance – Fluid is slower at walls, faster in center – Small-diameter arterioles are major factor in resistance Relationship Between Blood Flow, Blood Pressure, and Resistance Blood flow is directly proportional to the pressure gradient – If blood flow increases, pressure increases Blood flow is inversely proportional to peripheral resistance – If resistance increases, blood flow decreases Systemic Blood Pressure The pumping action of the heart generates blood flow Pressure results when flow is opposed by resistance Blood pressure near the heart is pulsatile (BP rises and falls) Systemic pressure – Is highest in the aorta – Declines throughout the pathway – Is 0 mm Hg when returning to heart at the right atrium The steepest drop occurs in arterioles Blood Pressure throughout the Circulatory System Arterial Blood Pressure Systolic pressure: pressure exerted during ventricular contraction Diastolic pressure: lowest level of arterial pressure during ventricular relaxation Pulse pressure = difference between systolic and diastolic pressure Mean arterial pressure (MAP): pressure that propels the blood to the tissues Pulse Points Capillary Blood Pressure 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 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 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 Muscular Pump Maintaining Blood Pressure Requires – Cooperation of the: Heart – Cardiac Output (CO) Blood vessels – Peripheral Resistance (PR) Kidneys – Blood volume – Supervision by the brain Maintaining Blood Pressure Blood pressure = CO x PR (and CO depends on blood volume) Blood pressure varies directly with CO, PR, and blood volume Changes in one variable are quickly compensated for by changes in the other variables Control of Blood Pressure Short-term controls – neural and hormonal controls Counteract fluctuations in blood pressure by altering peripheral resistance Long-term controls – renal regulation – Counteracts fluctuations in blood pressure by altering blood volume Short-Term Mechanisms: Neural Controls of Blood Pressure Neural controls operate via reflex arcs involving: – Baroreceptors – Vasomotor centers and vasomotor fibers – Vascular smooth muscle Baroreceptors are located in – Carotid sinuses – Aortic arch – Walls of large arteries of the neck and thorax Short-Term Mechanisms: Neural Control of Blood Pressure Neural Controls: – Vasomotor center – a cluster of sympathetic neurons in the medulla that oversees changes in blood vessel diameter Maintains vasomotor tone (moderate constriction of arterioles) Receives inputs from baroreceptors – Cardiovascular center – vasomotor center plus the cardiac centers that integrate blood pressure control by altering cardiac output and blood vessel diameter Short-Term Mechanisms: Hormone Controls of Blood Pressure Hormonal Controls: – Adrenal medulla hormones Norepinephrine and epinephrine Cause generalized vasoconstriction and increased cardiac output – Antidiuretic hormone (ADH): causes intense vasoconstriction in cases of extremely low BP – Atrial natriuretic peptide (ANP): causes blood volume and blood pressure to decline, causes generalized vasodilation Long-Term Mechanisms: Renal Regulation of Blood Pressure Long-term mechanisms - control BP by altering blood volume – Kidneys act directly and indirectly to regulate arterial blood pressure 1. Direct renal mechanism 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 2. Indirect renal (renin-angiotensin) mechanism Blood Flow Through Tissues Tissue perfusion: blood flow through body tissues Purposes: – – – – – a. delivery of oxygen and nutrients to tissue cells b. removal of wastes from tissue cells c. gas exchange in the lungs d. absorption of nutrients from the digestive tract e. urine formation in the kidneys Blood flow is precisely the right amount to provide proper tissue function Velocity of Blood Flow Blood velocity changes as it travels through the systemic circulation Capillary blood flow is slow to allow adequate time for diffusion and exchange between blood and tissues Diameter, Cross-Sectional Area, BP, and Velocity Autoregulation: Autoregulation: the automatic adjustments of blood flow to each tissue in proportion to its requirements at any instant – Blood flow to individual organs is controlled by modifying local arteriole diameters – Metabolic controls: decrease O₂ or increased metabolic wastes will increase blood flow – Angiogenesis: increase in number of blood vessels Increased numbers of vessels Enlargement of existing vessels Major Arteries Major Arteries branching off the aorta Major Arteries of the head/neck Major Arteries of the brain Major Arteries of the torso Major Arteries of the upper extremity Major Arteries of the lower extremity Major Veins Major Veins of the torso Major Veins of the head/neck Major veins of the upper extremity Major veins of the lower extremity Hepatic Portal System