Circulation 2 Physiology II PDF

Circulation 2 Circulation Blood flow to every part of body Blood flow Blood flow means the quantity of blood that passes a given point in the circulation in a given period of time. Ordinarily, blood flow is expressed in milliliters per minute or liters per minute. The heart and blood vessels, a...

Circulation 2 Circulation Blood flow to every part of body Blood flow Blood flow means the quantity of blood that passes a given point in the circulation in a given period of time. Ordinarily, blood flow is expressed in milliliters per minute or liters per minute. The heart and blood vessels, are controlled to provide the necessary cardiac output and arterial pressure to cause the needed tissue blood flow Uninterrupted, continuous, laminar flow circulation … Soon after the initial bolus of the ejected blood volume passes a particular point in an artery, there is a release of the elastic energy generated during the process of active distension, leading to a contraction of the arterial walls. This sequence of distension followed by contraction facilitates the continuous flow of blood. This is achieved by accommodating the rapid increase of blood volume during systole, and then contracting to sustain sufficient pressure to maintain the velocity of blood flow during diastole, when blood is no longer being pumped by the heart. Blood flow There are several important factors to facilitate uninterrupted, continuous circulation and to prevent stagnation at any point. Blood Flow Laminar Flow of Blood in Vessels: When blood flows at a steady rate through a long, smooth blood vessel, it flows in streamlines, with each layer of blood remaining the same distance from the vessel wall Turbulent Flow of Blood under Some Conditions: When the rate of blood flow becomes too great, when it passes by an obstruction in a vessel, when it makes a sharp turn, or when it passes over a rough surface, the flow may then become turbulent, or disorderly.  Regulation of Circulatory Function The rate of blood flow to each tissue of the body is in relation to the tissue need. PROBLEM : When tissues are active, they need a greatly increased supply of nutrients and therefore much more blood flow than when at rest—occasionally as much as 20 to 30 times the resting level. Yet the heart normally cannot increase its cardiac output more than four to seven times greater than resting levels. Therefore, it is not possible simply to increase blood flow everywhere in the body when a particular tissue demands increased flow.  Basic Principles of regulation of Circulatory Function There are three basic principles that underlie all functions of the system. 1. Local blood flow to each tissue of the body 2. Cardiac output 3. Blood pressure  Basic Principles of Circulatory Function There are three basic principles that underlie all functions of the system. 1. The rate of LOCAL blood flow to each tissue of the body is almost always precisely controlled in relation to the tissue need. The microvessels of each tissue continuously monitor tissue needs, such as the availability of oxygen and other nutrients and the accumulation of carbon dioxide and other tissue waste products. These in turn act directly on the local blood vessels, dilating or constricting them, to control local blood flow precisely to that level required for the tissue activity. Also, nervous control of the circulation from the central nervous system and hormones provide additional help in controlling tissue blood flow. Control of Tissue Blood Flow by Endothelial- Derived Relaxing or Constricting Factors The endothelial cells lining the blood vessels synthesize several substances that, when released, can affect the degree of relaxation or contraction of the arterial wall. Nitric Oxide—A Vasodilator Released from Healthy Endothelial Cells. Nitric Oxide—A Vasodilator Released from Healthy Endothelial Cells, affect the degree of relaxation or contraction of the arterial wall.  Basic Principles of Circulatory Function There are three basic principles that underlie all functions of the system. 1. Local blood flow to each tissue of the body 2. Cardiac output 3. Blood pressure Basic Principles of Circulatory Function There are three basic principles that underlie all functions of the system. 2. The cardiac output is controlled mainly by the sum of all the local tissue flows. When blood flows through a tissue, it immediately returns via the veins to the heart. Basic Principles of Circulatory Function There are three basic principles that underlie all functions of the system. 2. The cardiac output is controlled mainly by the sum of all the local tissue flows. If there is vasodilatation - increased return of blood via veins – The heart responds automatically to this increased inflow of blood by pumping it immediately back into the arteries (INCREASED CARDIAC OUTPUT).  Basic Principles of Circulatory Function There are three basic principles that underlie all functions of the system. 1. Local blood flow to each tissue of the body 2. Cardiac output 3. Blood pressure Basic Principles of Circulatory Function There are three basic principles that underlie all functions of the system. 3. Arterial pressure regulation is generally independent of either local blood flow control or cardiac output control. The circulatory system is provided with an extensive system for controlling the arterial blood pressure. For instance, if at any time the pressure falls significantly below the normal level of about 100 mm Hg, within seconds a barrage of nervous reflexes elicits a series of circulatory changes to raise the pressure back toward normal.  Basic Principles of Circulatory Function There are three basic principles that underlie all functions of the system. 3. Arterial pressure regulation is generally independent of either local blood flow control or cardiac output control. The nervous signals especially (a) increase the force of heart pumping, (b) cause contraction of the large venous reservoirs to provide more blood to the heart, and (c) cause generalized constriction of most of the arterioles throughout the body so that more blood accumulates in the large arteries to increase the arterial pressure. Then, over more prolonged periods, hours and days, the kidneys play an additional major role in pressure control both by secreting pressure- controlling hormones and by regulating the blood volume. Thus, in summary, the needs of the individual tissues are served specifically by the circulation. Blood Pressure Methods for Measuring Systolic and Diastolic Pressures Blood pressure almost always is measured in millimeters of mercury (mm Hg) and rare in kPa (divide by 7.5) Normal < 140/90 mmHg Regulation of the Circulation Role of : Humoral and Nervous system Kidney function Control of Tissue Blood Flow by Endothelial- Derived Relaxing or Constricting Factors The endothelial cells lining the blood vessels synthesize several substances that, when released, can affect the degree of relaxation or contraction of the arterial wall. Nitric Oxide—A Vasodilator Released from Healthy Endothelial Cells. Humoral and Nervous System Regulation of the Circulation Humoral Control of the Circulation vasoconstriction (vasopressin, angiotensin II, nor/epinephrine) vasodilatation (histamine, Calcium, bradykinin). Most Vasodilators or Vasoconstrictors Have Little Effect on Long- Term Blood Flow Nervous system control of the circulation has more global functions, such as redistributing blood flow to different areas of the body, increasing or decreasing pumping activity by the heart, and providing very rapid control of systemic arterial pressure. The nervous system controls the circulation almost entirely through the autonomic nervous system Autonomic nervous system The autonomic nervous system regulates certain body processes, such as circulation, blood pressure and the rate of breathing. This system works automatically (autonomously), without a person's conscious effort Autonomic nervous system Sympathetic Nervous System – vasoconstriction, tachycardia Parasympathetic Nervous System – vasodilatation, bradycardia ( Control of Heart Function, Especially Heart Rate – VASOVAGAL REACTION) Role of the Nervous System in Rapid Control of Arterial Pressure One of the most important functions of nervous control of the circulation is its capability to cause rapid increases in arterial pressure. For this purpose, the entire vasoconstrictor and cardioaccelerator functions of the sympathetic nervous system are stimulated together. At the same time, there is reciprocal inhibition of parasympathetic vagal inhibitory signals to the heart What happened during Nervous System Rapid Control of Arterial Pressure 1. Most arterioles of the systemic circulation are constricted. This greatly increases the total peripheral resistance, thereby increasing the arterial pressure. 2. The veins especially (but the other large vessels of the circulation as well) are strongly constricted. This displaces blood out of the large peripheral blood vessels toward the heart, thus increasing the volume of blood in the heart chambers. The stretch of the heart then causes the heart to beat with far greater force and therefore to pump increased quantities of blood. This, too, increases the arterial pressure. 3. Finally, the heart itself is directly stimulated by the autonomic nervous system, further enhancing cardiac pumping Baroreceptor Arterial Pressure Control System Baroreceptor Reflexes are initiated by stretch receptors, called either baroreceptors or pressoreceptors, located at specific points in the walls of several large systemic arteries. A rise in arterial pressure stretches the baroreceptors and causes them to transmit signals into the central nervous system. “Feedback” signals are then sent back through the autonomic nervous system to the circulation to reduce arterial pressure downward toward the normal level. Baroreceptor Arterial Pressure Control System Baroreceptors are spray-type nerve endings that lie in the walls of the arteries; they are stimulated when stretched. A few baroreceptors are located in the wall of almost every large artery of the thoracic and neck regions. Ex. Carotid baroreceptors (effect of massage of neck or tie closure) What happens during putting tie ?? Role of the Kidneys in Long-Term Control of Arterial Pressure and in Hypertension Role of the Kidneys in Long-Term Control of Arterial Pressure and in Hypertension Short-term control of arterial pressure by the sympathetic nervous system, occurs primarily through the effects of the nervous system on total peripheral vascular resistance and capacitance, as well as on cardiac pumping ability. The body, however, also has powerful mechanisms for regulating arterial pressure week after week and month after month. This long-term control of arterial pressure is closely intertwined with homeostasis of body fluid volume, which is determined by the balance between the fluid intake and output.  homeostasis of body fluid volume, which is determined by the balance between the fluid intake and output. Role of the Kidneys in Long-Term Control of Arterial Pressure and in Hypertension For long-term survival, fluid intake and output must be precisely balanced, a task that is performed by multiple nervous and hormonal controls, and by local control systems within the kidneys that regulate their excretion of salt and water. Angiotensin II - peptide endocrine hormone Effect of Angiotensin II in the Kidneys to Cause Renal Retention of Salt and Water—An Important Means for Long-Term Control of Arterial Pressure Angiotensin II causes the kidneys to retain both salt and water in two major ways: 1. Angiotensin II acts directly on the kidneys to cause salt and water retention. 2. Angiotensin II causes the adrenal glands to secrete aldosterone, and the aldosterone in turn increases salt and water reabsorption by the kidney tubules. Thus, whenever excess amounts of angiotensin II circulate in the blood, the entire long-term renal–body fluid mechanism for arterial pressure control automatically becomes set to a higher arterial pressure level than normal.