Transcript BP
Goal of the Cardiovascular System: deliver blood to all parts of the body • Does so by using different types of tubing, attached to a pulsatile pump • Elastic arteries • Muscular arteries • Arterioles • Capillaries • Venuoles • Veins • Distribution system broken up into areas called vascular beds • Skin • Digestive (splanchnic) • Muscle Structure of vasculature changes in response to different needs Why does blood flow through this closed circuit? • Blood flows down a pressure gradient • The absolute value of the pressure is not important to flow, but the difference in pressure (DP or gradient) is important to determining flow. What happens to pressure if we decrease the volume of a fluid filled compartment (i.e. ventricles during systole)? P directly proportional to F The resulting pressure is called the driving pressure in the vascular system How does the flow differ in these two vessels? Vascular system possesses different mechanisms for promoting continuous flow of blood to the capillaries: Elastic recoil smooth m. regulation of diameter sphincters Muscular arteries valves Substances causing contraction in vascular smooth muscle Chemical Physiologic role Source Type NE (a ) Baroreceptor reflex Sympathetic neurons Neural Endothelin Paracrine Vascular endothelium Local Serotonin Platelet aggregation, smooth muscle contraction Neurons, digestive tract, platelets Local, neural Substance P Pain, increased capillary permeability Neurons, digestive tract Local, neural Vasopressin Increase blood pressure during hemorrhage Posterior pituitary Hormonal Plasma hormone Hormonal endothelium local Angiotensin II Increase blood pressure Prostacyclin Minimize blood loss from damaged vessels before coagulation Substances that mediate vascular smooth muscle relaxation Chemical Physiologic role Source Type Nitric oxide Paracrine mediator Endothelium Local Atrial natriuretic peptide Reduce blood pressure Atrial myocardium, brain Hormonal Vasoactive intestinal peptide Digestive secretion, relax smooth muscle Neurons Neural, hormonal Histamine Increase blood flow Mast cells Local, systemic Epinephrine (b2) Enhance local blood flow to skeletal muscle, heart, liver Adrenal medulla Hormonal Acetylcholine (muscarinic) Erection of clitoris, penis Parasympathetic neurons neural Bradykinin Increase blood flow via nitric oxide Multiple tissues Local Adenosine Enhance blood flow to match metabolism Hypoxic cells local Even though there are many mechanisms for altering the radius of the vascular system, pressure still drops as blood moves further away from the heart. Why? Resistance = tendency of the vascular system to oppose flow; Flow = 1 R • Influenced by: length of the tube (L), radius of the tube (r), and viscosity of the blood (h) Poiseuille’s Law R = Lh/r 4 • In a normal human, length of the system is fixed, so blood viscosity and radius of the blood vessels have the largest effects on resistance All four tubes have the same driving pressure. Which tube has the greatest flow? The least flow? Why? Even with a decrease in overall pressure, the pressure in the vessels is not constant. The pressure in the vessels mirrors the pressures generated in the heart – systolic and diastolic pressures. Systolic = ventricles contracting Diastolic = ventricles filling Why does the diastolic pressure rise between the left ventricle and the arteries? Normal blood pressure = 120/80 High blood pressure = 140/90 What could be happening to increase both the diastolic and systolic blood pressure? Blood Pressure • Reflects the driving pressures produced by the ventricles • Because arterial pressure is pulsatile, a single value is used to represent the overall driving pressure. This is called the mean arterial pressure. MAP = diastolic P + 1/3(systolic P-diastolic P) Why does diastolic pressure account for a greater proportion of the overall value? MAP = Q x Rarterioles Explain how these two equations are equivalent SVR = systemic vascular resistance CO = cardiac output SV = stroke volume What factors influence blood pressure? • Blood volume • Vascular resistance • Autoregulation • Autonomic influences Regulation of Blood Pressure • Main coordinating center is in the medulla oblongata of the brain; medullary cardiovascular control center • Reflex control of blood pressure •Baroreceptor reflex