The Wall of the Alimentary Canal SEROSA Is composed of visceral peritoneum of epithelium on the outside and connective tissue beneath; secrete serous fluid MUCOSA Inner layer that is lined with epithelium attached to connective tissue; carries on secretion and absorption SUBMUCOSA Consists of loose connective tissue housing blood and lymph vessels and nerves; nourishes the layers of the canal MUSCULAR LAYER Consists of two coats of smooth muscle tissue; produces the tube’s movement a. Mucosa, or mucous membrane. Surface epithelium, underlying connective tissue, and a small amount of smooth muscle form this layer. In some regions, the mucosa is folded, with tiny projections that extend into the passageway, or lumen of the digestive tube, which increase the absorptive surface areas. The mucosa also has glands that are tubular invaginations into which the lining cells secrete mucus and digestive enzymes. The mucosa protects the tissues beneath it and carries on secretion and absorption. b. Submucosa – The submucosa consists of considerable loose connective tissue as well as glands, blood vessels, lymphatic vessels, and nerves. Its vessels nourish surrounding tissues and carry away absorbed materials. c. Muscular layer – this layer, which produces movement so the tube, consists of two coats of smooth muscle tissue. The fibers of the inner coat encircle the tube. When these circular fibers contract, the tube’s diameter decreases. The fibers of the outer muscular coat run lengthwise. When these longitudinal fibers contract, the tube shortens. d. Serosa, or second layer – the visceral peritoneum of epithelium on the outside and the connective tissue beneath compose the serous layer, or outer covering, of the tube. The cells of the serosa protect underlying tissues and secrete serous fluid, which moistens and lubricates the tube’s outer surface so that organs within the abdominal cavity slide freely against one another.

SWALLOWING Tongue rolls this mixture into a bolus and forces it into the pharynx Food is chewed and mixed with saliva Food stimulates sensors around the pharyngeal opening triggering the swallowing reflex which briefly inibits breathing Swallowing Reflex Soft palate rises to prevent food from entering the nasal cavity Hyoid bone and larynx elevate; epiglottis closes to prevent food from entering trachea Tongue presses against soft palate to seal off oral cavity from the pharynx Longitudinal muscles in the pharyngeal wall contract, pulling the pharynx upward toward the food Muscles in the lower position of the pharynx relax to open the esophagus A peristaltic wave begins in the pharyngeal muscles and forces food into the esophagus Swallowing has three stages. In the first stage, which is voluntary, food is chewed and mixed with saliva. Then the tongue rolls this mixture into a mass, or bolus, and forces it into the pharynx. The second stage of swallowing begins as food stimulates sensory receptors around the pharyngeal opening. This triggers the swallowing reflex, which includes the following actions: The soft palate raise, preventing food from entering the nasal cavity. The hyoid bone and larynx are elevated. A flaplike structure attached to the larynx, called the epiglottis, closes off the top of the larynx so that food is less likely to enter the trachea. The tongue is pressed against the soft palate, sealing off the oral cavity from the pharynx. The longitudinal muscles in the pharyngeal wall contract, pulling the pharynx upward toward the food. Muscles into the lower position of the pharynx relax, opening the esophagus. A peristaltic wave begins in the pharyngeal muscles and forces food into the esophagus. The swallowing reflex momentarily inhibits breathing. Then, during the third stage of swallowing, peristalsis transports the food in the esophagus to the stomach. Peristalsis transports food in the esophagus to the stomach 1st Stage 2nd Stage 3rd Stage

Absorption in the Small Intestine MONOSACCHARIDES Absorbed by villi through active transport or facilitated diffusion. Then they enter blood. AMINO ACIDS Absorbed by villi through active transport. Then they are carried away in the blood. FATTY ACIDS Fatty acid molecules dissolve into the cell membranes of the villi. The endoplasmic reticula of the cells reconstruct the lipids. These lipids collect in clusters that become encased in chylomicrons. Chylomicrons are carried away in lymphatic lacteals until they join the bloodstream. WATER & ELECTROLYTES Water is absorbed by the intestinal villi through osmosis. Electrolytes are absorbed by the intestinal wall through active transport. CREATE GRAPHIC A negative feedback system sensitive to blood glucose concentration regulates insulin secretion. When blood glucose concentration is high, as after a meal, beta cells release insulin. Insulin helps prevent too high a blood glucose concentration by promoting glycogen formation in the liver and entrance of glucose into adipose and muscle cells. When glucose concentration falls, as occurs between meals or during the night, insulin secretion decreases. At the same time that insulin is decreasing, glucagon secretion is increasing. Therefore, insulin and glucagon function coordinately to maintain a relatively stable blood glucose concentration, despite great variation in the amount of carbohydrates a person eats (see figure 11.17, slide 62).