Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Cell Bio and Physio – Lecture 18: Intro to the Cardiovascular System 5/3/12 Definitions o Cardiovascular system = heart and blood vessels o Circulatory system = heart, blood vessels, blood, and lymphatics Purpose of Circulatory System o Primay role: distribution of gases, nutrients, and waste o Secondary roles: Circulation of hormones Distribution of heat Transport of immune cells, antibodies, and clotting factors Components o Heart Background Weighs less than 1 pound and ~ fist-sized Pump Detail R heart receives deoxygenated blood from the body and pumps it to the lungs L heart receives oxygenated blood from the lungs and pumps it to the body 2 Circuits Pulmonary Circuit o Only way for blood from the R side to get to the L side Systemic Circuit o Provides multiple pathways for blood to get from L to R heart o Vessels Arteries Transport blood from the heart to the tissues under high pressure Have strong walls w/ significant amts of smooth muscle & elastic tissue Arterioles Control conduits through which blood is released into the capillaries Have strong muscular walls which can completely constrict or dilate several flod o 75% of arterioles are empty at any given moment Have 1st – 4th order sizes Capillaries Only vessels allowing exchange of substances b/t blood & interstitial fluid Essentially an endothelial tube w/in a basement membrane Venules Collect blood from the capillaries and gradually merge to form veins Have 1st – 4th order sizes Veins Return blood to the heart Low BP o Have thin, collapsible walls o Blood Blood volume: 5-6 liters (~ 6 quarts) in adult male Distribution Total blood volume is unevenly distributed Vast majority of blood is in the systemic circulation (84%) o Most is in the veins (64%) – blood reservoir o Relatively little in capillaries (7%) Foundations of Circulation o Premise Circulation exists to handle the metabolic needs of working tissue Supply nutrients and remove waste 2 defining aspects All tissues need blood The amt of blood needed is usu. not static o Blood Supply There is more vasculature than there is blood Not all tissues need blood at every moment (efficiency issue) Dynamics Active tissue may require 20-30 times more blood than at rest CV system must be able to quickly adjust and direct the flow of blood to tissues in need Major Circulatory Concepts Blood Flow Control o Mainly controlled locally, in relation to tissue need o Tissue microvasculature monitors extracellular fluid composition (gases, nutrients, waste) and local blood vessels constrict or dilate accordingly Blood Pressure Control o Regulation of systemic BP is generally independent of control of local blood flow, or cardiac output Autonomic control o Systemic BP must remain at a certain level (~100 mm Hg) at all times to maintain adequate blood flow to all tissues Cardiac Output Control o CO is the amt of blood pumped by the ventricles (ml/min) o CO = HR x stroke volume (SV) Stroke volume is the volume of blood pumped out by a ventricle during a single contraction (~70 ml/min) o Control CO is mainly determined by peripheral factors What blood the heart receives (venous return) it pumps back out A stretched heart ( venous return) contracts more forcefully and HR Blood Flow o Usu. measured in ml/min o Human Blood Flow Blood flow to the entire body is equal to cardiac output At rest, CO ~ 5 L/min Thus, blood flow at the aorta = 5 L/min o Context At rest, the entire blood volume is pumped by the heart in 1 minute CO = HR x SV = 75 bpm x 70 ml/min = 5.25 L/min o Blood flows b/c it’s not compressible like a gas It must go somewhere – travels to area of lower pressure – when pressure is applied o Factors of flow Ohm’s Law: F = P/R Pressure BP = force per unit area on a vessel wall by the blood Pressure head – created by beating heart at the “start” of circulation o Remains constant over time Pressure gradient o Pressure generated by the heart must be sufficient to overcome the resistance to blood flow in the pulmonary and systemic circuits o BP ranges systemically from ~100 to 2 mm Hg Resistance Allows pressure to exist in a moving fluid Total Peripheral Resistance o The resistance of the entire circulation Varies up to 20-fold o Resistance in the pulmonary circuit is a/b 1/7th that in the systemic circuit o Measured in peripheral resistance units (PRU) o . o . o . Poiseuille’s equation and Ohm’s Law explains: o Resistance is directly proportional to viscosity and length o Resistance is inversely related to the diameter to the 4th power The er the diameter, the er the resistance Diameter/Radius Rationale o The narrower the blood vessel, the greater the proportion of blood therein that is in contact with the vessel wall The outer fluid lamina has a flow velocity of 0 o Thus overall, the narrower the blood vessel the slower the flow of blood Laminar Flow o Blood tends to flow through long tubes in concentric layers (lamina) Viscosity is a measure of the slipperiness b/t layers o Outmost layers in contact w/ the wall has a flow velocity of zero o The more inward layers move more quickly Shearing o A more viscous fluid experiences greater shear (more resistance to sliding) b/t the fluid layers This resistance (or velocity) reduces flow at any given pressure o Causes of blood viscosity Almost entirely related to the hematocrit Proportion of blood cells Circuit Resistance From the aorta to the vena cava, blood flows through arteries, veins, and capillaries o These are arranged in series Total resistance = sum of each individual vessel Total peripheral resistance = sum of the resistances of all the arteries, arterioles, capillaries, venules, and veins All of the above vessels branch extensively o Arterioles,capillaries, venules Branches are arranged in parallel Vessels have a lower total resistance than those arranged in series Conductance o Each additional vessel provides another pathway, making it easier for blood flow through the circuit o Practical Implications Most of our tissues contribute to the overall conductance of the systemic circulation Removal of a limb or organ eliminates a parallel circuit Increased TPR Velocity of Blood Flow Blood flow = volume through any pt in a given time period (L/min) The same volume of blood must flow through each segment of the circulation each minute o Principle of continuity/conservation of mass Velocity Requirement o B/c total vessel cross-sectional area changes throughout circulation, the velocity of blood flow must change in order to maintain constant flow (~5L/min) in each segment Ex: capillary velocity is slower Turbulent Flow Though blood flow is generally laminar, it can become turbulent in certain conditions Reynold’s Number o The measure of the propensity for turbulence to occur o Flow velocity is the main modifiable factor Causes the parabolic profile of the linear velocity to become blunted Practical Meaning o Turbulence is energetically inefficient as it vastly resistance Heart must produce pressure head to maintain flow o Clinical Turbulent flow is heard as murmurs – Korotkoff sounds