Wilderness and Rescue Medicine 8th Edition

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Section II: Critical Body Systems

body (systemic circulation). As the blood leaves the right ventricle of the heart, it flows into the pulmonary circulation in the lung, where oxygen- ation of the blood occurs and carbon dioxide is released to be exhaled. Oxygenated blood return- ing from the lungs enters the left side of the heart where the left ventricle pumps it to the rest of the body.

Considerable pressure is required to overcome the natural resistance to flow and to ensure perfusion of all body tissues. We can measure the perfusion pressure generated by the circulatory system in the form of arterial blood pressure. Most of this perfusion pressure is generated by the pumping action of the heart and the contraction of smooth muscle in artery walls. Blood circulation is augmented by the elasticity of arteries, the system of one-way valves in the veins and the contraction of skeletal muscles as you move about. In a healthy individual, these work together to keep blood pressure relatively constant throughout a variety of activities and environmental conditions. Shock Shock is the term for inadequate perfusion pres- sure in the circulatory system resulting in inad- equate oxygenation of body cells. While even a small drop in pressure can cause poor oxygen- ation of tissues in the extremities and skin, com- pensatory mechanisms will usually adjust blood flow to preserve the oxygenation and perfusion of vital organs. If pressure continues to drop, even vital body tissues will suffer. Shock that is not cor- rected will inevitably result in circulatory system failure and death.

General Principles

Blood Vessels

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Arteries: • High-pressure flow from the heart to the capillary beds • Smooth muscle in artery walls Capillaries: • Tiny, thin - walled vessels bringing blood close enough to body cells to allow for exchange of gases, nutrients, and waste Veins: • Low-pressure flow of blood back to heart from the capillaries

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From the thoracic aorta, which is about the diameter of a garden hose, the blood flows through progressively smaller vessels into the cap- illary beds. The capillary beds consist of a dense matrix of tiny vessels about the diameter of a red blood cell. This is how blood is brought into very close proximity to individual body cells, allow- ing for cellular oxygenation and the removal of carbon dioxide. Exiting the capillaries, the blood enters the veins to be returned to the right side of the heart.

General Principles

Shock Inadequate perfusion pressure in the circulatory system

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General Principles

Perfusion

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Low blood volume Poor vascular tone Poor cardiac output

Volume Shock Vascular Shock Cardiogenic Shock

• Oxygenation of the blood in the lungs • Oxygenation of the cells in the capillary beds • Requires adequate pressure to perfuse lungs and capillary beds

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Low Risk

High Risk

© 2018 WMA

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“Most of this perfusion pressure is generated by the pumping action of the heart and the contraction of smooth muscle in artery walls.”

There are three basic types of shock that cor- respond to the three major components of the circulatory system. Volume shock (also called hypovolemic shock) is caused by blood or fluid loss from blood vessels. Vascular shock results

© 2018 WMA

To keep you alive, your 5 liters of blood are pumped through approximately 14 kilometers of blood vessels about a thousand times per day.