Hemodynamics II PDF

Hemodynamics II Microcirculation, veins, lymphatics & venous return Ian Dixon Molecular Cardiology Lab, R3010 St. Boniface Hospital Research Centre Institute of Cardiovascular Sciences Note – there is some overlap between this lectur...

Hemodynamics II Microcirculation, veins, lymphatics & venous return Ian Dixon Molecular Cardiology Lab, R3010 St. Boniface Hospital Research Centre Institute of Cardiovascular Sciences Note – there is some overlap between this lecture and others.  The Circulating system Supply blood - O 2 and nutrients – to tissues Arterioles – resistance vessels Regulate regional blood flow Capillaries – exchange of gases, water, solutes with interstitial fluid Venules and veins – collecting and storage vessels  Blood vessels Blood moves rapidly through the aorta and its arterial branches. At the periphery, the branches narrow, and their walls become thinner. The aorta is a predominantly elastic structure, but the peripheral arteries become more muscular. At the arterioles, the muscular layer predominates. Capillaries – single layer of endothelium Fig. 15.2. Berne and Levy Physiology  Microcirculation The circulation of blood through the smallest vessels of the body: arterioles, capillaries, and venules. Arterioles (5 to 100 µm in diameter) have a thin adventitial layer, a thick smooth muscle layer and an endothelial lining. Arterioles à capillaries (5 to 10 µm in diameter) or metarterioles (10 to 20 µm in diameter), which then give rise to capillaries. Guyton and Hall, 13th edition  Metarterioles can bypass the capillary bed and connect to venules, or they can connect directly to the capillary bed. Precapillary sphincter: single smooth muscle cell forms a ring around metarteriole-capillary junctions, regulates the blood flow into the capillaries. Thoroughfare channel: if all precapillary sphincters contract and close, blood flow directly from metarteriole to venule. Metabolic activity determines blood delivery to a particular tissue. When metabolic activity increases in the tissues, more precapillary vessels open to allow capillary perfusion, and the reverse happens during low metabolic activity.  In metabolically active organs, such as the heart, skeletal muscle, and glands, capillary density is high. In less active tissues, such as subcutaneous tissue or cartilage, capillary density is low. Blood flow through the capillaries à nutritional flow. Blood flow that bypasses the capillaries via metarterioles à nonnutritional, or shunt flow AV shunts not present everywhere; found in fingertips, ears. Muscles lack anatomic shunts. The capillaries form an interconnecting network of tubes with an average length of 0.5 to 1 mm. Vasoactive Role of the Capillary Endothelium Chemicals that cause vasodilation: Prostacyclin (PGI2), formed from arachidonic acid (AA) by the action of cyclooxygenase and prostacyclin synthase à relaxation via increases in cAMP. NO stimulates guanylyl cyclase (G Cyc) to increase cGMP. The vasodilator nitroprusside (NP) acts directly on vascular smooth muscle. Adenosine stimulates cAMP levels which also relaxes SM.  Passive Role of the Capillary Endothelium Transcapillary Exchange Solvent and solute move across the capillary endothelial wall by three processes: diffusion, filtration, and pinocytosis. Diffusion is the most important process for transcapillary exchange, and pinocytosis is the least important. Diffusion 300 mL of water per minute per 100 g of tissue moves across the capillary wall. Thus diffusion of nutrients is the key factor in providing exchange of gases, substrates, and waste products between capillaries and tissue cells. In capillaries, diffusion of lipid-insoluble molecules is restricted to water-filled channels or pores/fenestrations.  The Lymphatic system Collects the fluid and proteins that have escaped from blood and transports them back into the veins for recirculation in blood. Lymphatic capillaries intercalate with blood containing capillaries. Composed of: Lymphatic vessels Lymphatic nodes Lymphatic tissue  The Lymphatic system Present: Almost all tissues contain lymph channels Not present: Only cartilage, bone, epithelia, and tissues of the central nervous system lack lymphatic vessels. Why is lymphatic circulation important? Substances of high molecular weight, such as proteins, cannot be absorbed from the tissues in any other way. Without this continuous return of the filtered proteins and fluid to the blood, the plasma volume would be rapidly depleted, and interstitial edema would occur. Mechanism of lymphatic action: Accomplished by tissue pressure, intermittent skeletal muscle activity, lymphatic vessel contractions, and one-way (cellular) valves. The endothelial cells of the lymphatic capillary are attached by anchoring filaments to the surrounding connective tissue. The edge of one endothelial cell overlaps the edge of the adjacent cell – forming a valve Interstitial fluid can enter the lymphatic vessels by pushing the valve; once inside, the backflow closes the flap valve. Figure 16-8 Special structure of the lymphatic capillaries that permits passage of substances of high molecular weight into the lymph. Lymphatic system – additional points Lymph flow varies - resting skeletal muscle is almost nil, and it increases during exercise in proportion to the degree of activity. Is increased by increased blood capillary filtration; such mechanisms include increased capillary hydrostatic pressure or permeability and decreased plasma oncotic pressure. When interstitial fluid volume > drainage by lymphatics (or if lymphatic vessels become blocked) interstitial fluid accumulates and gives rise to clinical edema.  The Venous System Capacitance and Resistance Veins are elements of the circulatory system that return blood to the heart from tissues; a reservoir that contains up to 70% of the blood in the circulation. The reservoir function of veins (and smooth muscle) allows the volume of blood returning to the heart to be adjusted (eg, preload) to match dynamic changes to cardiac output. Thus, capacitance is an important property of veins. Venous Return Pulmonary RV LV Venous Return Systemic Cardiac output Blood flows back to the heart. Venous return relies on pressure gradients, muscle pumps, and valves. Pressure gradients dominate in the supine position, but in the upright position, gravity is counteracted by an efficient system of muscle pumps and valves. Unlike arteries, the volume of blood carried in a vein fluctuates considerably without concomitant changes in venous pressure. This large capacitance maintains cardiovascular stability.  Pressure-Flow Relationships and Venous Return The pressure generated by cardiac contraction is termed dynamic pressure. Supine position - blood flow is determined by dynamic pressure gradients, with arterial pressure > venous pressure. Dynamic pressure is dissipated in the arterial system before it reaches the capillary bed. At the venous end of the capillary bed, it ranges from 12 to 18 mm Hg. Atrial pressure averages 4 to 7 mm Hg under normal conditions. Hence, blood flows along this gradient and is returned to the heart.  In the upright position, venous flow in the lower extremities is dominated by the effects of hydrostatic pressure , and a small effect of dynamic pressure. HP is determined by the density of blood and the acceleration of gravity; expressed as a constant multiplier (0.77 mm Hg/cm) of the centimetres below the atrium. Muscle pumps along with venous valves help in overcoming hydrostatic pressure. During relaxation, valves close, and blood is prevented from refluxing Varicose veins – Occurs when venous valves are dysfunctional, and appear as swollen, bulbous, twisted veins on the legs Capacitance and Compliance The relationship between pressure and volume at a given level of smooth muscle tone in the venous system is termed capacitance. Veins = little change in pressure with big changes in volume. Compliance is the change in blood volume that occurs for each unit of change in transmural pressure in a segment of vein; to put it another way, it is the slope of the capacitance curve.  Venous Capacitance Venous capacitance is controlled by the collapsible nature of the venous wall. A low volume of blood in a vein results in an elliptical shape, low pressure. A large increase in volume can occur with only a minimal increase in pressure. Transmural pressure: intraluminal pressure / tissue pressure An increase in venous transmural pressure corresponds to a change in shape from elliptical to circular. However, once the vein is circular, much higher pressure is required to stretch the venous wall to add additional volume. Figure 11-5 Venous Physiology, Rutherford’s Vascular Surgery A, Cross-section of a venous lumen at various transmural pressures. At lower pressures the vein is elliptical, whereas at high pressures it is circular. B, Relationship of venous volume to transmural pressure. At low pressures, veins are compliant and change shape easily to accommodate large increases in volume. At high pressures, they become stiff and cannot accommodate large changes in volume.  CO = VR Cardiac output is the quantity of blood pumped into the aorta each minute by the heart. This is also the quantity of blood that flows through the circulation. Venous return is the quantity of blood flowing from the veins into the right atrium each minute. The venous return and the cardiac output must equal each other except for a few heartbeats at a time when blood is temporarily stored in or removed from the heart and lungs. Figure 20-2 Guyton and Hall Control of Cardiac Output by Venous Return—The Frank- Starling Mechanism of the Heart various factors of the peripheral circulation affect venous return Peripheral because heart has built-in automatic mechanism to pump Frank-Starling law of the heart Bainbridge reflex – reflex increase in heart rate with increased venous return. Three principal factors that affect venous return to the heart from the systemic circulation: 1. Right atrial pressure, which exerts a backward force on the veins to impede flow of blood from the veins into the right atrium. 2. Degree of filling of the systemic circulation, which forces the systemic blood toward the heart (volume). 3. Resistance to blood flow between the peripheral vessels and the right atrium – in this case if less volume is moving through the arterioles, then venous filling (and return will also be reduced. (Alternatively, in a separate scenario, if the smooth muscle contraction in the great veins is activated, then more blood will be shunted to the RA).  References Venous Physiology, Lori L. Pounds and Lois A. Killewich, Rutherford's Vascular Surgery, Chapter 11, 150-162 Cardiac Output, Venous Return, and Their Regulation, John E. Hall PhD, Guyton and Hall Textbook of Medical Physiology, Chapter 20, 245-258 Properties of the Vasculature, Bruce M. Koeppen MD, PhD and Bruce A. Stanton PhD, Berne and Levy Physiology, 17, 345-385