101 The Digestive System
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Key Idea: The digestive tract is specialised to maximise the digestion of food, absorption of nutrients, and elimination of undigested material. The human digestive system (gut) is a tubular tract, which is regionally specialised into a complex series of organs and glands that work in sequence to maximise the efficiency with which food is processed. Collectively, the organs of the
digestive tract carry out the physical and chemical breakdown (digestion) of food, absorption of nutrients, and elimination of undigested material. The gut is a hollow, open-ended, muscular tube, and the food within it is essentially outside the body, having contact only with the cells lining the tract. External to the digestive tract are several accessory organs and glands, which add enzymes to the food to aid digestion.
PR E O V N IE LY W
Gastric gland In the stomach, gastric glands contain parietal cells, which produce hydrochloric acid, and chief cells, which produce enzymes to break down protein.
Salivary glands
Cells lining the walls on the small intestine (the intestinal epithelium) have microscopic extensions of the plasma membrane called microvilli. These form a brush border that increases the surface area for absorption of food molecules. Under lower power microscopy, it appears as a fuzzy edge.
Microvilli
Oesophagus
Louisa Howard, Katherine Connollly Dartmouth College
Villus
Liver
ach
G
m
Gall bladder
In the small intestine, the intestinal epithelial cells (E) and mucus-producing goblet cells (G) make up the epithelium lining the gut wall. The wall is folded into finger like projections called villi (sing. villus). These further increase the surface area of the intestine.
o St
E
Pancreas
La
rge intestine
Nephron
Small intestine
Villus
SM
Lumen
The intestinal epithelium is supported by underlying connective tissue. Two layers of smooth muscle (SM), one running lengthwise and one running around the gut, encircle the tube, contracting in waves to move food through the intestine.
EII
3. What is the purpose of the smooth muscle surrounding the intestine?
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CL
2. How are villi formed?
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1. Name three cell types of the digestive system and state their function:
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