Large quantities of fat are stored in two major tissues of the body, the adipose tissue and the liver. The adipose tissue is usually called fat deposits, or simply tissue fat.
The major function of adipose tissue is storage of triglycerides until they are needed to provide energy elsewhere in the body. A subsidiary function is to provide heat insulation for the body, as discussed in Chapter 73.
Fat Cells (Adipocytes). The fat cells (adipocytes) of adipose tissue are modified fibroblasts that store almost pure triglycerides in quantities as great as 80 to 95 per cent of the entire cell volume. Triglycerides inside the fat cells are generally in a liquid form. When the tissues are exposed to prolonged cold, the fatty acid chains of the cell triglycerides, over a period of weeks, become either shorter or more unsaturated to decrease their melting point, thereby always allowing the fat to remain in a liquid state. This is particularly important, because only liquid fat can be hydrolyzed and transported from the cells.
Fat cells can synthesize very small amounts of fatty acids and triglycerides from carbohydrates; this function supplements the synthesis of fat in the liver, as discussed later in the chapter.
Exchange of Fat Between the Adipose Tissue and the Blood— Tissue Lipases. As discussed earlier, large quantities of lipases are present in adipose tissue. Some of these enzymes catalyze the deposition of cell triglycerides from the chylomicrons and lipoproteins. Others, when activated by hormones, cause splitting of the triglycerides of the fat cells to release free fatty acids. Because of the rapid exchange of fatty acids, the triglycerides in fat cells are renewed about once every 2 to 3 weeks, which means that the fat stored in the tissues today is not the same fat that was stored last month, thus emphasizing the dynamic state of storage fat.
The principal functions of the liver in lipid metabolism are to (1) degrade fatty acids into small compounds that can be used for energy; (2) synthesize triglycerides, mainly from carbohydrates, but to a lesser extent from proteins as well; and (3) synthesize other lipids from fatty acids, especially cholesterol and phospholipids.
Large quantities of triglycerides appear in the liver (1) during the early stages of starvation, (2) in diabetes mellitus, and (3) in any other condition in which fat instead of carbohydrates is being used for energy. In these conditions, large quantities of triglycerides are mobilized from the adipose tissue, transported as free fatty acids in the blood, and redeposited as triglycerides in the liver, where the initial stages of much of fat degradation begin. Thus, under normal physiologic conditions, the total amount of triglycerides in the liver is determined to a great extent by the overall rate at which lipids are being used for energy.
The liver cells, in addition to containing triglycerides, contain large quantities of phospholipids and cholesterol, which are continually synthesized by the liver. Also, the liver cells are much more capable than other tissues of desaturating fatty acids, so that liver triglycerides normally are much more unsaturated than the triglycerides of adipose tissue. This capability of the liver to desaturate fatty acids is functionally important to all tissues of the body, because many structural elements of all cells contain reasonable quantities of unsaturated fats, and their principal source is the liver. This desaturation is accomplished by a dehydrogenase in the liver cells.
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