Anatomy & Physiology

Anatomy & Physiology Study Guide

©2018 Achieve Page 266 20.9 Autoregulation, Neural Mechanisms, and Endocrine Responses Cardiovascular activity must be regulated by homeostatic mechanisms to ensure that tissue perfusion, or blood flow through tissues, meets the demand for oxygen and nutrients. The factors that affect tissue perfusion are cardiac output, peripheral resistance, and blood pressure. Most cells are relatively close to capillaries. When a group of cells becomes energetic, the circulation to that region must increase to deliver the necessary oxygen and nutrients and to carry away the waste products and carbon dioxide they generate. The purpose of cardiovascular regulation is to ensure that these blood flow changes occur at an appropriate time, in the right area, and without drastically changing blood flow and blood pressure to vital organs. The regulatory mechanisms focus on controlling cardiac output and blood pressure to restore adequate blood flow after a fall in blood pressure. These mechanisms can be broadly categorized as follows: • Autoregulation: Local factors change the pattern of blood flow within capillary beds as precapillary sphincters open and close in response to chemical changes in interstitial fluids. This is an example of autoregulation at the tissue level. Autoregulation causes immediate, localized homeostatic adjustments. If autoregulation fails to normalize conditions at the tissue level, neural mechanisms and endocrine factors are activated. • Neural mechanisms: Neural mechanisms respond to changes in arterial pressure or blood gas levels sensed at specific sites. When those changes occur, the cardiovascular centers of the autonomic nervous system adjust cardiac output and peripheral resistance to maintain blood pressure and ensure adequate blood flow. • Endocrine mechanisms: The endocrine system releases hormones that enhance short-term adjustments and that direct long-term changes in cardiovascular performance. Hormones and Cardiovascular Regulation The endocrine system provides both long and short-term regulation of cardiovascular activity. Epinephrine and NE from the suprarenal medullae stimulate cardiac output and peripheral vasoconstriction. Other hormones important in regulating cardiovascular function include antidiuretic hormone (ADH), angiotensin II, erythropoietin (EPO), and natriuretic peptides (ANP and BNP). Although ADH and angiotensin II also affect blood pressure, all four are concerned primarily with the long-term regulation of blood volume. An idiuretic Hormone Antidiuretic hormone (ADH) is released at the neurohypophysis in response to a decrease in blood volume, to an increase in the osmotic concentration of the plasma, or (secondarily) to circulating angiotensin II. In response to ADH, peripheral vasoconstriction occurs to elevate blood pressure. This hormone also stimulates the conservation of water at the kidneys, thus preventing a reduction in blood volume that would further reduce blood pressure. Angiotensin II As a response to a drop in renal blood pressure, renin is released by juxtaglomerular cells. Once in the bloodstream, renin starts an enzymatic chain reaction. In the first step, renin converts of 368