Blood Flow 2023 PDF

Blood Flow and its Regulation Dr K Thaxter Nesbeth UWI. Mona  Blood Vessel Structure Arteries – Elastic, muscular, arterioles Capillaries – Blood flows from arterioles to capillaries – Most of exchange between blood and interstitial spaces occurs across the...

Blood Flow and its Regulation Dr K Thaxter Nesbeth UWI. Mona  Blood Vessel Structure Arteries – Elastic, muscular, arterioles Capillaries – Blood flows from arterioles to capillaries – Most of exchange between blood and interstitial spaces occurs across the walls – Blood flows from capillaries to venous system Veins – Venules, small veins, medium or large veins Physiology of Systemic Circulation Determined by – Anatomy of circulatory system – Dynamics of blood flow – Regulatory mechanisms that control heart and blood vessels Blood volume – Most in the veins – Smaller volumes in arteries and capillaries  Blood Flow Quantity of blood passing a given point in the circulation in a given period Normally expressed in ml/min Blood flow in the total circulation of an adult is about 5000 ml/min….The cardiac output Blood flows from areas of high pressure to areas of low pressure Poiseuille’s Law Describes Blood Flow Poiseuille's Law describes factors affecting blood flow – Flow rate is inversely proportional to resistance – Flow decreases when resistance increases – Flow = ΔP/R (change in pressure gradient over resistance) – Resistance is directly proportional to length of vessel (L) & viscosity of blood (η) – Flow resistance decreases when vessel diameter increases – Blood flow = ΔPπr4 8ηL  Blood flow and Vessel Diameter As diameter of vessels decreases, the total cross-sectional area increases and velocity of blood flow decreases Much like a stream that flows rapidly through a narrow Big decrease in blood flow gorge but flows slowly velocity controlled by arterioles – through a broad plane facilitates capillary exchange later  Summary: Blood Flow, Poiseuille’s Law and Viscosity Poiseuille’s Law Blood flow – Flow decreases when – Amount of blood moving resistance increases through a vessel in a – Flow resistance given time period decreases when vessel – Directly proportional to diameter increases pressure differences, Viscosity inversely proportional to – Measure of resistance resistance of liquid to flow – As viscosity increases, pressure required to flow increases Laminar and Turbulent Flow Laminar flow – Streamlined – Outermost layer moving slowest and center moving fastest Turbulent flow – Interrupted – Fluid passes a constriction, sharp turn, rough surface Control of Blood Flow by Tissues Local control – In most tissues, blood flow is proportional to metabolic needs of tissues Nervous System – Responsible for routing blood flow and maintaining blood pressure Hormonal Control – Sympathetic action potentials stimulate epinephrine and norepinephrine  Local Control of Blood Flow by Tissues Blood flow can increase 7-8 times as a result of vasodilation of metarterioles and precapillary sphincters in response to increased rate of metabolism – Vasodilator substances produced as metabolism increases – Vasomotion is periodic contraction and relaxation of precapillary sphincters Blood Flow, Capillary Exchange and Interstitial Fluid Volume Regulation Blood pressure, capillary permeability, and osmosis affect movement of fluid from capillaries A net movement of fluid occurs from blood into tissues. Fluid gained by tissues is removed by lymphatic system. Fluid Exchange Across Capillary Walls  Local Control of Blood Flow Each tissue regulates its own local blood flow based on its needs, which include: - Deliver O2, glucose, amino acids, and fatty acids. - Remove CO2 and H+ ions. - Maintain proper [ion]s. - Transport hormones and other nutrients.  Local and Humoral Control of Blood Flow Local Control - Acute control – rapid (seconds to minutes) – changes in vasodilation or vasoconstriction. - Long-term local control – – change in the physical size of blood vessels – Change in numbers of blood vessels – occurs over days to months  Long-term Local Regulation of Blood Flow Works by changing the vascularity (number & size of arterioles and capillaries) Aims to match the needs of a tissue. Degree of vascularity is determined by the maximum blood flow needed. Important peptides that increase vascularity are vascular endothelial growth factor (VEGF), fibroblast growth factor, and angiogenin.  Humoral Vasoconstriction Sympathetic and adrenal release of norepinephrine and epinephrine. Angiotensin II (more on this when we discuss renal mechanisms). Vasopressin (ADH) – very potent vasoconstrictor secreted by the posterior pituitary. Also increases renal H2O reabsorption. Endothelin A – released from damaged vessels.  Humoral Vasodilation Bradykinin – powerful arteriolar dilation and increased permeability of the capillaries. Histamine – released from damaged or inflamed tissue; also during an allergic reaction. Also cases arteriolar dilation and increased permeability of the capillaries.  Humoral Control - Substances secreted or absorbed into the body fluids that cause vasoconstriction or vasodilation e.g., hormones, peptides and ions.  Ions and Other Chemical Factors Ca2+ ions – vasoconstriction. K+ ions – vasodilation. Mg2+ ions – vasodilation (often inhibits the actions of Ca2+ ions). H+ ions – increase cause vasodilation, decrease causes constriction. Anions – acetate and citrate cause vasodilation. CO2 – vasodilation, particularly important in the brain.  Nervous Regulation of Circulation More global control, such as: - Redistribution of blood flow - Regulating heart rate - Rapid control of arterial pressure Autonomic nervous system provides the main nervous control of CV function. - Sympathetic NS innervates vessels and heart Parasympathetic NS primarily acts through heart rate changes by direct innervation via vagus nerve Sympathetic Control  Sympathetic Neurotransmitters and Hormones Sympathetic nerve endings release almost entirely norepinephrine (alpha adrenergic receptors). Sympathetic NS stimulates the adrenal medulla to release norepinephrine and epinephrine. In some tissues (skeletal muscle), epinephrine causes vasodilation through beta adrenergic receptors.  Critical Closing Pressure, Compliance Critical closing pressure Vascular compliance – Pressure at which a blood – Tendency for blood vessel collapses and blood vessel volume to flow stops increase as blood pressure increases – More easily the vessel Laplace’s Law wall stretches, the – Force acting on blood greater its compliance vessel wall is proportional – Venous system has a to diameter of the vessel large compliance and times blood pressure acts as a blood reservoir Relationship Between Metabolism and Blood Flow Relationship Between Metabolism and Blood Flow  Reactive and Active Hyperemia Reactive hyperemia is an increase of blood flow after the flow to a tissue has been blocked Active hyperemia is an increase in blood flow in response to increased activity.  Effects of pH and Gases Example: blood pH increase – Often decrease in blood CO2 Detected in medulla (chemoreceptor reflex) Decrease in sympathetic stimulation and increases parasympathetic outflow Vasodilation of blood vessels, decreased peripheral resistance Decrease in heart rate and cardiac output, slower flow through pulmonary circulation Increase in CO2, resulting in a decrease in pH  Shock Inadequate blood flow throughout body Three stages – Compensated: Blood pressure decreases only a moderate amount and mechanisms able to reestablish normal blood pressure and flow – Progressive: Compensatory mechanisms inadequate and positive feedback cycle develops; cycle proceeds to next stage or medical treatment reestablishes adequate blood flow to tissues – Irreversible: Leads to death, regardless of medical treatment