NCERT Solutions for Biology Class 11 – Structure of the small intestine
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In this article, we are discussing ‘the structure of the small intestine’ from Class 11 Biology in order to understand the function of the small intestine in digestion and mainly absorption.
STRUCTURE OF SMALL INTESTINE
The majority of digestion and absorption of nutrients takes place in the small intestine.
Absorption is the passage of digested food material through the lining of the intestine into the blood or lymph. Lymph is a colorless fluid containing white blood cells (WBC) that circulate through our body tissues. The WBC present in lymph helps to fight any infection that enters our body.
The small intestine has four tissue layers:
- The serosa is the outermost layer of the intestine. The serosa is a smooth membrane consisting of a thin layer of cells that secrete serous fluid and a thin layer of connective tissue. Serous fluid is a lubricating fluid that reduces friction from the movement of the muscularis.
- The muscularis is a region of muscle adjacent to the submucosa membrane. It is responsible for gut movement, or peristalsis. It usually has two distinct layers of smooth muscle: circular and longitudinal.
- Thesubmucosa is the layer of dense, irregular connective tissue or loose connective tissue that supports the mucosa, as well as joins the mucosa to the bulk of underlying smooth muscle.
Visit another article on ‘Pancreas’ by clicking Biology class 11.
Figure showing four layers of the small intestine
Brunner’s glands (or duodenal glands) are compound tubular submucosal glands found in the duodenum. The main function of these glands is to produce a mucus-rich, alkaline secretion (containing bicarbonate) in order to neutralize the acidic content of chyme. The chyme is introduced into the duodenum from the stomach. These glands provide an alkaline condition for optimal intestinal enzyme activity, thus enabling absorption to take place and lubricate the intestinal walls.
Peyer’s patches are organized lymph nodules. They are aggregations of lymphoid tissue that are found in the lowest portion of the small intestine i.e. ileum. This differentiates the ileum from the duodenum and jejunum.
The lumen of the gastrointestinal tract is exposed to the external environment;somuch of it is populated with potentially pathogenic microorganisms. Peyer’s patches function as the immune surveillance system of the intestinal lumen and facilitate the generation of the immune response within the mucosa.
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Intestinal villi (singular: villus) are tiny, finger-like projections that protrude from the epithelial lining of the mucosa. Each villus is approximately 0.5–1.6 mm in length and has many microvilli (singular: microvillus), each of which is much smaller than a single villus.So, it’s almost like the villi are fingers protruding from the small intestine wall, and microvilli are hairs on those fingers. Both works to increase surface area so more nutrients can be absorbed. Within each villus, we find plentiful capillary beds, along with lacteals. Lacteals are lymphatic capillaries, which are filled with lymph that absorb dietary fats.
The increased surface area allows for more intestinal wall area to be available for absorption. An increased absorptive area is useful because digested nutrients (including sugars and amino acids) pass into the villi, which is semi-permeable, through diffusion, which is effective only at short distances.
In other words, the increased surface area (in contact with the fluid in the lumen) decreases the average distance traveled by the nutrient molecules, so the effectiveness of diffusion increases. The villi are connected to blood vessels that carry the nutrients away in the circulating blood.
Practically all absorption takes place in the small intestine, little food remains to be absorbed by the time the contents of the small intestine reach the large intestine.
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