|MadSci Network: Medicine|
The compensatory mechanisms are many and happen almost at the same time after the initial reflex response by the body. Below is a "quickie" in shock education. The mechanisms that occur are far beyond the scope of information I am able to present here. The "trigger" factor in hypovolemic shock is decreased circulatory volume and inadequate oxygenation (hypoxia) to tissue/organs. Decreased volume initiates the early phase or reflex reaction and begin with arteriolar constriction, increased heart rate, and increased contractile force of the heart. These mechanisms can maintain a stable blood pressure as well as adequate tissue oxygenation for up to as much as 10% volume loss. For how long? The time factor is partially dependent on the type of injury or bodily dysfunction, the age and preexisting health of the person before the assult/alarm occurs. A healthy young person may be successfully compensated in the initial phase for longer periods of time than someone that has a disease process in progress or has -- lets say a small ulcer bleed vs internal bleed from a car accident. As the volume percentage lost increases to 15% -- 25% these mechanisms lose their ability to compensate and rapidly become inadequate. As blood pressure begins to drop baroreceptors (pressure/streach receptors) in the heart, and the great vessels "feel" the pressure drop and initiate the sympathatic/autonomic nervous system (fight or flight reaction) to kick in. This system releases norepinephrine and epinephrine (a powerful vasoconstrictor and heart rate increaser). In other words these two squeeze the vessels and kick up the speed of the heart to work even harder to oxygenate the "life saving organs" (heart, brain, deep large muscles) with the little amount of blood that is left. Blood is redirected away from peripheral tissue/vessels and non vital organs (your skin, gut, liver, lungs and kidneys will not save your life if you are running from a grizzly bear -- thus the name fight or flight) in an effort to support the needs for energy and oxygen for the heart, brain and "running" muscle. In hypovolemic you would be lying down (passed out) as this is the first response to equalize or redistribute oxygenated blood to the brain. At the same time a hormonal response by the pituitary, hypothalamus and adrenal cortex release adrenocortical, mineralocorticoid and glucocorticoids which increase intravascular volume, via the kidney, by decreasing urine output and retaining sodium/water which in turn increases blood pressure; they stimulate a rapid increase in glucogenesis (increasing blood sugar to fuel the heart to beat faster and muscles to run faster/diaphram to breath deeper/faster); they attempt to maintain a sodium/water/electrolyte balance. As dehydration increases so does the concentration of the blood (osmolality) which triggers osmoreceptors to release yet another fluid saving chemical, antidiuretic hormone (ADH) which in turn retains more sodium/water. At the same time everything else is changing, all the tissue and organs that have previously had oxygen to maintain aerobic cellular metabolism are now in anaerobic mode. As a result the body is building up lactic acid (the chemical that make our muscles so sore when we over exert our bodies) this leads to metabolic acidosis causing our respiratory rate to increase in rate and depth to try and "blow" off some of CO2 to decrease the acidosis. This requires even more energy to maintain a rapid deep breath and in turn throws us into metabolic alkalosis making the hormonal responses that have been in progress work even harder to maintain an electrolyte level that is compatible with life. Eventually the acid-base balance mechanisms fail causing cellular damage and death. Without life support and drugs that are given IV immediately the body gets to the irreversible stage and body death occurs. The following references have charts and diagrams to help you understand how all these things can interact to attempt compensatory maintenance of the body in hypovolemic shock. Leavelle, D. Professional Guide to Diseases (3rd ed.). Cardiac Complications. (pp. 1083). Springhouse. Luckmann and Sorensen (1987), Medical Surgical Nursing (3rd ed.). Physiologic Shock: Basic Concepts and Intervention. (pp. 220-225). W.B. Saunders Company.
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