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Which of the following is NOT lined by epithelial tissue?
Skin
Lung alveoli
Kidney tubules
Blood
Blood
Epithelial tissue is a primary tissue type that lines the surfaces, cavities, and vessels of the body. Blood is classified as a specialized connective tissue, not an epithelial tissue, because it consists of formed elements (cells) suspended in a liquid matrix called plasma.
Epithelial tissue is a primary tissue type that lines the surfaces, cavities, and vessels of the body. Blood is classified as a specialized connective tissue, not an epithelial tissue, because it consists of formed elements (cells) suspended in a liquid matrix called plasma.
Think of epithelial tissue like the skin on a fruit or the lining inside a pipe, which protects or encloses. Blood is more like a delivery truck service driving inside those pipes; it connects various body systems rather than forming the walls of the pipes themselves.
Epithelial tissues are characterized by cells closely packed together with little intercellular matrix, forming continuous sheets. In contrast, connective tissues, such as blood, are characterized by cells separated by an abundant extracellular matrix. Blood's primary role is transport, facilitated by its fluid state, which is distinct from the protective and secretory functions of epithelial tissues.
Epithelial tissue covers external surfaces and internal cavities.
Connective tissue binds, supports, and transports; blood is a liquid connective tissue.
Blood cells are derived from mesoderm, whereas many epithelia are derived from ectoderm or endoderm.
Used in histology to distinguish between tissue types in medical diagnostics.
Fundamental for understanding blood donation and physiological transport.
| Feature | Tissue A | Tissue B |
|---|---|---|
Tissue Type | Epithelial | Connective (Blood) |
Blood is the only liquid connective tissue in the human body.
Option A (Skin) has an epidermis made of stratified squamous epithelium.
Option B (Lung alveoli) is lined by simple squamous epithelium for gas exchange.
Option C (Kidney tubules) are lined by cuboidal epithelium for reabsorption.
D is correct — Blood is a specialized connective tissue, not an epithelial tissue.
Always remember the 'Four Basic Tissue Types': Epithelial, Connective, Muscular, and Nervous. Identifying which category a structure belongs to is a frequent question in RRB biology exams.