Blood Vessels and Circulation PDF

Blood vessels and circulation Chapter 20 JM Johnston Ph.D.  Key features Closed circulatory system – heart (pump) – vessels (arteries, arterioles, veins, venules and capillaries) Blood flow rate depends on pressure gradient and vessel resistance ...

Blood vessels and circulation Chapter 20 JM Johnston Ph.D.  Key features Closed circulatory system – heart (pump) – vessels (arteries, arterioles, veins, venules and capillaries) Blood flow rate depends on pressure gradient and vessel resistance  Angiogram :Internal carotid artery Carotid Rt. AP Lat. sinus view view Photos © McGraw-Hill Education  General anatomy of the blood vessels Copyright © The McGraw-Hill Education. Permission required for reproduction or display. Capillaries Artery: Tunica interna Tunica media Tunica externa Nerve Vein Figure 20.1a (a) © The McGraw-Hill Companies, Inc./Dennis Strete, photographer 1 mm Arteries carry blood away from heart Veins carry blood back to heart Capillaries connect smallest arteries to smallest veins 20-4  Circulatory routes Common route – heart arteries arterioles capillaries venules veins Portal system – blood flows through two consecutive capillary networks before returning to heart eg hypothalamus -pituitary, intestines - liver  Anastomoses Anastomosis = point where 2 blood vessels merge Arteriovenous shunt – artery flows directly into vein eg fingers, toes, ears;  heat loss, (blood to bypass cold exposed) Venous anastomosis – most common – alternate drainage of organs Arterial supply of the brain Circle of Willis on base of brain is formed from anastomosis of basilar & paired internal carotid aa Supplies brain, internal ear and orbital structures  Blood vessel structure - walls Tunica externa – outermost layer, loose connective tissue Tunica media – middle thickest layer – smooth muscle (vasoconstriction/vasodilation) Tunica intima (interna) – smooth inner layer of endothelium  The vessel wall Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Conducting (large) artery Large vein Lumen Lumen Tunica interna: Tunica interna: Endothelium Endothelium Basement Basement membrane membrane Tunica media Tunica externa Tunica media Tunica externa Vasa Vasa vasorum vasorum Nerve Nerve Inferior Medium vein vena Distributing (medium) artery cava Aorta Tunica interna: Tunica interna: Endothelium Endothelium Basement Basement membrane membrane Internal elastic lamina Valve Tunica media External elastic lamina Tunica media Tunica externa Tunica externa Direction of blood flow Venule Arteriole Tunica interna: Tunica interna: Endothelium Endothelium Basement Basement membrane membrane Tunica media Tunica media Tunica externa Tunica externa Endothelium Basement membrane Capillary Large vessels  Arteries Conducting (elastic) arteries - largest – eg aorta – tunica media consists of elastic tissue, alternating with smooth muscle and collagen – expand during systole, recoil during diastole; lessens fluctuations in BP Distributing arteries – deliver blood to specific organs eg pulmonary artery Arteries as a pressure reservoir Arteries LM view  Major systemic arteries Supplies oxygen and nutrients to all organs Medium vessels  Arteries and metarterioles Resistance (small) arteries – arterioles - control amount of blood to various organs Metarterioles – Shorter - connect arterioles to capillaries – muscle cells form a precapillary sphincter at entrance Smooth muscle in arteriole wall (LM) Small vessels Control of capillary perfusion  Capillaries Metarteriole connects arteriole to venule Precapillary sphincters control capillary perfusion – only 1/4 of the capillaries are open at a time Structure Endothelial cells Pericytes wrap around capillaries and function to and regulate blood flow Types of capillaries 1. Continuous (common) - endothelial cells with tight junctions 2. Fenestrated (kidneys, small intestine) - endothelial cells with pores 3. Sinusoids (liver, bone marrow, spleen) – irregular blood-filled spaces allow proteins and blood cells to enter Blood cells in a single capillary Continuous capillary Fenestrated capillary Sinusoids  Veins Venules – quite porous, like a capillary Veins – thin walls, large lumens, little smooth muscle – expand easily, have high capacitance – venous valves aid in upward blood flow – Low blood pressure Cross section of vein (LM)  Major systemic veins Deep veins run parallel to arteries Comparison of artery and vein (LM)  Principles of blood flow Delivers nutrients and oxygen, and removes metabolic wastes Blood flow: amount of blood flowing through a tissue in a given time (ml/min) Perfusion: rate of blood flow per mass of tissue (ml/min/g) Hemodynamics: principles of blood flow based on pressure and resistance F  P/R F = flow P = difference in pressure R = resistance to flow Blood flow and pressure gradient  Blood pressure Blood pressure (BP)—the force that blood exerts against a vessel wall – Measured at brachial artery of arm Systolic pressure – BP during ventricular systole Diastolic pressure – BP during ventricular diastole Normal value, young adult: 120/75 mm Hg Measuring blood pressure Sinauer Associates Inc, 1999  Arterial blood pressure Pulse pressure – Difference between systolic and diastolic pressure – Important measure of force on circulation by heart Mean arterial pressure (MAP) –The mean pressure obtained by taking measurements at several intervals throughout the cardiac cycle diastolic pressure + one-third of pulse pressure –Average BP that most influences risk level for edema, fainting (syncope), atherosclerosis, kidney failure, and aneurysm Systemic circulatory pressures  Blood pressure Since pressure varies across the cardiac cycle, blood flow in arteries is pulsatile – Speed surges from 40 cm/s to 120 cm/s – Blood spurts intermittently from an open artery In capillaries and veins, blood flows at steady speed – Bleeding from veins tends to be slow and steady BP tends to rise with age – Arteriosclerosis—stiffening of arteries due to deterioration of elastic tissues of artery walls – Atherosclerosis—build up of lipid deposits that become plaques  Blood pressure Importance of arterial elasticity – expansion and recoil maintains steady flow of blood smooths out pressure fluctuations Other factors – cardiac output – blood volume – peripheral resistance  Peripheral resistance Blood viscosity - by RBC’s and albumin –  viscosity with anemia, hypoproteinemia –  viscosity with polycythemia, dehydration Vessel length – pressure and flow decline with distance Vessel radius – most adjustable variable, controls resistance quickly – vasoconstriction, vasodilation Arteriole diameter regulates blood flow  Flow at different points From aorta to capillaries, flow  – greater distance traveled, more friction – smaller radii of arterioles and capillaries – farther from the heart, greater the total cross sectional area From capillaries to vena cava, flow  again – large amount of blood forced into smaller channels – never regains velocity of large arteries Blood flow and velocity  Blood pressure abnormalities Hypertension – chronic resting BP > 140/90 – can weaken small arteries and cause aneurysms Hypotension – chronic low resting BP