Transitions allows the ureter wall to stretch as a

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“transitions”) allows the ureter wall to stretch as a greater volume of urine flows through that tube- like organ. In the bladder, it allows more urine to be stored. Glandular Epithelium A gland consists of one or more cells that make and secrete a particular product. This product, called a secretion, typically contains protein mol- ecules in an aqueous (water-based) fluid. The term secretion also indicates an active process in which the glandular cells obtain needed materials from the blood and use them to make their secretion, which they then discharge. Two major types of glands develop from epithelial sheets. Endocrine (en do-krin) glands lose their connection to the surface (duct); thus they are often called ductless glands. Their secretions (all hormones) diffuse directly into the blood vessels that weave through the glands. Examples of en- docrine glands include the thyroid, adrenals, and pituitary. Exocrine (ek so-krin) glands retain their ducts, and their secretions empty through the ducts to the epithelial surface. Exocrine glands, which include the sweat and oil glands, liver, and pancreas, are both internal and external. They are discussed with the organ systems to which their products are related. Connective Tissue Connective tissue, as its name suggests, connects body parts. It is found everywhere in the body. It is the most abundant and widely distributed of the tissue types. Connective tissues perform many functions but they are primarily involved in pro- tecting, supporting, and binding together other body tissues. Common Characteristics of Connective Tissue The characteristics of connective tissue include the following: Variations in blood supply. Most connective tis- sues are well vascularized (that is, they have a good blood supply), but there are exceptions. Tendons and ligaments have a poor blood sup- ply, and cartilages are avascular. Consequently, all these structures heal very slowly when in- jured. (This is why some people say that, given a choice, they would rather have a broken bone than a torn ligament.) Extracellular matrix. Connective tissues are made up of many different types of cells plus varying amounts of a nonliving substance found outside the cells, called the extracellular matrix. Extracellular Matrix The extracellular matrix deserves a bit more ex- planation because it is what makes connective tis- sue so different from the other tissue types. The matrix, which is produced by the connective tissue cells and then secreted to their exterior, has two main elements, a structureless ground substance and fibers. The ground substance of the matrix is
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composed largely of water plus some adhesion proteins and large, charged polysaccharide mole- cules. The cell adhesion proteins serve as a glue that allows the connective tissue cells to attach themselves to the matrix fibers embedded in the ground substance. The charged polysaccharide mol- ecules trap water as they intertwine. As the relative abundance of these polysaccharides increases, they cause the matrix to vary from fluid to gel-like to firm to rock-hard in its consistency. The ability of
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