Intro to the Cardiovascular System - Vasculature II (Jahn).pdf

Intro to the Cardiovascular System – Vasculature II Stephan C. Jahn, Ph.D. Blood Pressure, Resistance, and Flow Blood supply to a tissue can be expressed in terms of flow and perfusion  Blood flow: amount of blood flowing through an organ, tissue, or blood vessel (mL/min)  P...

Intro to the Cardiovascular System – Vasculature II Stephan C. Jahn, Ph.D. Blood Pressure, Resistance, and Flow Blood supply to a tissue can be expressed in terms of flow and perfusion  Blood flow: amount of blood flowing through an organ, tissue, or blood vessel (mL/min)  Perfusion: the flow per given volume or mass of tissue in a given time (mL/min/g) At rest, total flow is quite constant, and is equal to the cardiac output (5.25 L/min) 20-2  Blood Pressure Blood pressure (BP)—the force that blood exerts against a vessel wall Measured at brachial artery of arm using sphygmomanometer  A close approximation of pressure at exit of left ventricle Two pressures are recorded  Systolic pressure: peak arterial BP taken during ventricular contraction (ventricular systole)  Diastolic pressure: minimum arterial BP taken during ventricular relaxation (diastole) between heart beats Normal value, young adult: 120/75 mm Hg 20-3  Blood Pressure Pulse pressure—difference between systolic and diastolic pressure  Measure of driving force on circulation  Stress exerted on small arteries by pressure surges generated by the heart Mean arterial pressure (MAP)  Diastolic pressure + 1/3 of pulse pressure  Average blood pressure that most influences risk level for edema, fainting (syncope), atherosclerosis, kidney failure, and aneurysm 20-4  Blood Pressure Since pressure varies across the cardiac cycle, blood flow in arteries is pulsatile In capillaries and veins, blood flows at steady speed 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 20-5 20-6  Blood Pressure Hypertension—high blood pressure  Chronicresting BP > 140/90  Consequences Can weaken arteries, cause aneurysms, promote atherosclerosis Hypotension—chronic low resting BP  Caused by blood loss, dehydration, anemia 20-7 Blood Pressure BP determined by  cardiac output  blood volume Regulated mainly by kidneys  resistance to flow 20-8 Regulation of Blood Pressure and Flow Vasoreflexes are quick and powerful ways of altering blood pressure and flow Three ways of controlling vasomotor activity  Localcontrol  Neural control  Hormonal control 20-9  Local Control Autoregulation—the ability of tissues to regulate their own blood supply  Metabolic theory of autoregulation If tissue is inadequately perfused, wastes accumulate, stimulating vasodilation which increases perfusion  Bloodstream delivers oxygen and removes metabolites  When wastes are removed, vessels constrict 20-10  Local Control Vasoactive chemicals—substances secreted by platelets, endothelial cells, and perivascular tissue to stimulate vasomotor responses  Histamine, bradykinin, and prostaglandins stimulate vasodilation  Endothelial cells secrete prostacyclin and nitric oxide (vasodilators) 20-11  Neural Control The central and autonomic nervous systems also exert control over blood vessel size Vasomotor center of medulla exerts sympathetic control over blood vessels throughout the body  Stimulates most vessels to constrict, but dilates vessels in cardiac muscle to meet demands of exercise 20-12  Neural Control Vasomotor center is the integrating center for three autonomic reflexes Baroreflexes Chemoreflexes Medullary ischemic reflex 20-13  Neural Control Baroreflex—automatic, negative feedback response to change in blood pressure  Increases in BP detected by carotid sinuses  Results in 1) inhibition of sympathetic cardiac and vasomotor neurons, and 2) excitation of vagal fibers that slow heart rate and thus reduce BP  Decreases in BP have the opposite effect Baroreflexes govern short-term regulation of BP  Adjustments for rapid changes in posture  Not helpful in correcting chronic hypertension  After 2 days or less they adjust their set point 20-14  Neural Control Chemoreflex—an automatic response to changes in blood chemistry  Especially pH, and concentrations of O2 and CO2 Chemoreceptors called aortic bodies and carotid bodies  Located in aortic arch, subclavian arteries, external carotid arteries 20-15 Negative Feedback Control of BP Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Elevated Reduced blood pressure blood pressure Vasodilation Arteries Reduced stretched heart rate Reduced vasomotor tone Baroreceptors Increased increase firing rate vagal tone Cardioinhibitory Reduced neurons stimulated sympathetic tone Vasomotor center is inhibited Figure 20.13 20-16