5) Absorption : the entrance of ingested and secreted fluids, ions, and products of digestion into the epithelial cells lining the lumen of the GI tract. Absorbed substances pass into blood or lymph and circulate to cells throughout the body. 6) Defecation : Wastes, indigestible substances, bacteria, cells sloughed from the lining of the GI tract and digested materials that were not absorbed through the digestive tract leave the body through the anus as Feces.
Layers of the GI Tract
From the lower esophagus to the anal canal, the wall of the GI tract has the same 4-layered arrangement of tissues. From deep to superficial they are:
A) Mucosa
This is the inner lining of the GI tract. It is composed of a layer of epithelium (inner most in contact with contents of the GI tract), a layer of connective tissue called Lamina Propria , and a thin layer of smooth muscle called Muscularis Mucosae)
Epithelium
Rapid rate of renewal of GI tract epithelial cells (every 5-7 days). They are sloughed off and replaced by new cells. Exocrine and Endocrine cells collectively called Enteroendocrine Cells are located among the epithelial cells. Exocrine secrete mucus and fluid into the lumen of the tract, and the endocrine release hormones.
Lamina Propria
Areolar connective tissue containing many blood and lymphatic vessels. These are routes by which nutrients absorbed into the GI tract reach other tissues of the body. Contains the majority of the cells of MALT . These are prominent lymphatic nodules that contain immune system cells protecting against disease. MALT is present along the GI tract especially in the pharynx, small intestine and large intestine .
Muscularis Mucosae
Thin layer of smooth muscle fibres oriented into many small folds in the stomach and small intestine. This increases surface area.
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