Anatomy & Physiology

Anatomy & Physiology Study Guide angiotensinogen, a plasma protein produced by the liver, to angiotensin I. In the capillaries of the lungs, angiotensin-converting enzyme (ACE) then modifies angiotensin I to angiotensin II, an active hormone with diverse effects. Angiotensin II has four important functions: (1) it stimulates the secretion of ADH, in turn stimulating water reabsorption at the kidneys and complementing the effects of aldosterone; (2) It stimulates the suprarenal production of aldosterone, causing Na + retention and K + loss at the kidneys; (3) it stimulates thirst, resulting in increased fluid consumption (the presence of ADH and aldosterone ensures that the additional water consumed will be retained, elevating blood volume); and (4) it stimulates cardiac output and triggers the constriction of arterioles, in turn elevating the systemic blood pressure. The effect of angiotensin II on blood pressure is four to eight times greater than that produced by norepinephrine. Erythropoietin If blood pressure falls or if the oxygen content of the blood becomes abnormally low, erythropoietin (EPO) is released at the kidneys. EPO stimulates the production and maturation of red blood cells, thereby increasing the volume and viscosity of the blood and improving its oxygen-carrying capacity. Natriuretic Peptides Atrial natriuretic peptide or ANP is produced by cardiac muscle cells in the wall of the right atrium in response to excessive stretching during diastole. A related hormone, called brain natriuretic peptide, or BNP, is produced by ventricular muscle cells exposed to comparable stimuli. These peptide hormones reduce blood volume and blood pressure by increasing sodium ion excretion at the kidneys; promoting water losses by increasing the volume of urine produced; reducing thirst; blocking the release of ADH, aldosterone, epinephrine, and norepinephrine; and stimulating peripheral vasodilation. As blood volume and blood pressure diminish, the stress on the walls of the heart is removed, and natriuretic peptide production ceases. 20.10 Vascular Supply to Special Regions The vasoconstriction that occurs in response to a fall in blood pressure or a rise in CO2 levels affects multiple tissues and organs simultaneously. The term special circulation refers to the vascular supply through organs in which blood flow is controlled by separate mechanisms. Three important examples are the blood flow to the brain, the heart, and the lungs. Blood Flow to the Brain The brain has a high demand for oxygen and receives a substantial supply of blood. Under a variety of conditions, blood flow to the brain remains steady at about 750 mL/min—roughly 12 percent of the cardiac output delivered to an organ that represents less than two percent of body weight. Neurons do not maintain significant energy reserves, and in functional terms, most of the adjustments made by the cardiovascular system treat blood flow to the brain as the top priority. Even during a cardiovascular crisis, blood flow through the brain remains as near normal as possible: While the cardiovascular centers are calling for widespread peripheral vasoconstriction, the cerebral vessels are instructed to dilate. Although total blood flow to the brain remains relatively constant, blood flow to specific regions of the brain changes from moment to moment. These changes occur in Achieve Page 267 of 368 ©2018