Pulmonary Circulation - Pulmonary Circulation Hypoxemia...

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Hypoxemia Hypoxemia, or reduced oxygen in the blood, can be caused by: 1. Low partial pressure of atmospheric oxygen (e.g., high altitudes) 2. Inadequate pulmonary ventilation (e.g., chronic obstructive pulmonary disease) 3. Carbon monoxide poisoning 4. Reduced hemoglobin content in erythrocytes 5. Decreased hematocrit Tissue Edema and General Principles of Transcapillary Fluid Exchange Edema refers to the swelling of tissues that result from excessive accumulation of fluid within the tissue. Edema can be highly localized as occurs in a small region of the skin subjected to a bee sting. Edema, however, can also comprise an entire limb, specific organs such as the lungs (e.g., pulmonary edema ) or the whole body. General principles To understand how edema occurs, it is first necessary to explain the concept of tissue compartments. There are two primary fluid compartments in the body between which fluid is exchanged - the intravascular and extravascular compartments. The intravascular compartment contains fluid (i.e., blood) within the cardiac chambers and vascular system of the body. The extravascular system is everything outside of the intravascular compartment. Fluid and electrolytes readily move between these two compartments. The extravascular compartment is made up of many subcompartments such as the cellular, interstitial, and lymphatic subcompartments, and a specialized system containing cerebrospinal fluid. The movement of fluid and accompanying solutes between compartments (mostly water, electrolytes, and smaller molecular weight solutes) is governed by physical factors such as hydrostatic and oncotic forces. These forces are normally balanced in such a manner the fluid volume remains relatively constant between the compartments. If the physical forces or barriers to fluid movement are altered, the volume of fluid may increase in one compartment and decrease in another. In some cases, total fluid volume increases in the body so that both intravascular and extravascular compartments increase in volume. This can occur, for example, when the kidneys fail to excrete sufficient amounts of sodium and water. When the fluid volume within the interstitial compartment increases, this compartment will increase in size leading to tissue swelling (i.e., edema ). When excess fluid accumulates within the peritoneal space (space between the abdominal wall and organs), this is termed " ascites ." Pulmonary congestion, which can occur in heart failure as the left atrial pressure increases and blood backs up in the pulmonary circuit, causes pulmonary edema . Pulmonary Circulation 1
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A model that helps us to understand what causes edema is shown to the right. Filtration is the movement of fluid out of the capillary and reabsorption is the movement of fluid back into the distal end of the capillary and small venules. In most capillary systems of the body, there is a small net filtration (typically about 1% of plasma) of fluid from the intravascular to the extravascular compartment. In other words, capillary fluid filtration exceeds reabsorption.
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